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Review

sp2/sp3 framework from diamond nanocrystals: A keybridge of carbonaceous structure to carbocatalysis

Xiaoguang Duan, Wenjie Tian, Huayang Zhang, HongqiSun, Zhimin Ao, Zongping Shao, and Shaobin Wang

ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01565 • Publication Date (Web): 12 Jul 2019

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sp2/sp3 framework from diamond nanocrystals: A key bridge of

carbonaceous structure to carbocatalysis

Xiaoguang Duana, Wenjie Tian a, Huayang Zhang a, Hongqi Sun b, Zhimin Aoc, Zongping Shaod,e,

Shaobin Wang a,e*

a School of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australiab School of Engineering, Edith Cowan University, Joondalup, WA 6027, Australiac School of Environmental Science and Engineering, Institute of Environmental Health and Pollution

Control, Guangdong University of Technology, Guangzhou 510006, China d State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry &

Chemical Engineering, Nanjing University of Technology, Nanjing 210009, Jiangsu, Chinae Department of Chemical Engineering, Curtin University, Perth, WA 6102, Australia

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Abstract

Diamond nanocrystals in robust sp3 hybridization are appealing carbonaceous materials in the material

community, whose structure can be transformed into unique sp2/sp3 nanohybrids as bulky

nanodiamonds (NDs) and sp2 concentric onion-like carbons (OLC). Functionalized NDs have been

used as carbocatalysts to drive a diversity of heterogeneous reactions, presenting promising catalytic

performances, great stability/durability, and low toxicity compared with other carbonaceous and metal

materials. More importantly, the tuneable configurations of NDs-related materials from sp3 to sp2/sp3

and sp2 carbons endow them as ideal chemical probes to elucidate the intrinsic nature toward metal-

free catalysis. Herein, a comprehensive overview is presented in the synthesis, properties,

functionalization and characterization of NDs-based materials as well as their recent applications in

fuel cell reactions, carbon dioxide reduction, photocatalysis, organic synthesis, oxidative

dehydrogenation reactions, and advanced oxidation processes. More importantly, we provide an

insightful discussion on unveiling the intrinsic catalytic centers and structure-reactivity chemistry of

NDs in redox reactions from an atomic level. Advanced protocols were proposed for regulating the

electronic structures of NDs by surface and structural engineering toward better carbocatalysis, which

assists to provide valuable guidance for the rational design of ND-based materials toward target

catalytic processes. Finally, future research opportunities were proposed to address the current

dilemmas in materials synthesis to facilitate mechanistic studies by theoretical computations, to enable

structural/surface functionalization of NDs for advanced catalysis, and to expand the NDs-based

materials toward other promising chemical reactions.

Keywords: nanodiamond, sp2/sp3 hybrids, carbocatalysis, structure-performance regime, redox

reaction

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1. Introduction

Recently, nanoscaled carbonaceous materials, namely nanocarbons, have thrust into the limelight as

rising stars in material communities and significantly renovated the catalytic processes in a green and

sustainable manner, attributing to earth abundance of carbon, tunable structure, high robustness, rich

functionality, and featured electronic configurations. Of particular interest, nanocarbon allotropes such

as fullerene, carbon nanotubes, and graphene are hybridized with sp2 configuration and perfectly

packed in the honeycomb lattice with a conjugated π system to be constructed into different dimensions.

These nanocarbons demonstrate as promising metal-free catalysts to drive various chemical reactions

with outstanding catalytic efficiency, desirable selectivity, high durability and stability.1-3 The catalytic

sites of the nanocarbons have been unveiled to be the heteroatom dopants, oxygen functionalities, edge

geometry and structural defects, making it a complicated system to identify the intrinsic role of carbon

in catalysis. 4-7

Figure 1. (a) A schematic model illustrating the structure of detonation nanodiamond. (b) Closer view

of surface region of nanodiamond covered with surface functional groups and sp2 carbon and (c)

illustration of the sp3 carbon framework in the core. Reproduced with permission from ref 23.

Copyright 2012 Springer Nature.

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On the other hand, diamond nanocrystals stand out among the carbon community due to the unique

sp3 hybridization in tetrahedral bonding units, which can be expanded from a zero-dimensional cluster

to a three-dimensional framework with versatile face-centered terminations. Bulk diamond can be

downsized from microscope to nanoscale and engaged in a diversity of applications due to their

exceptional physical features. For instance, diamonds are the hardest materials in nature and their

extreme mechanical robustness has been commercially applied for cutting, drilling, and polishing

materials.8-11 Benefited from the excellent thermal conductivity (22 W/(cm·K)) and stability, diamond

can be utilized in semiconductor manufacturing to prevent the substrate from overheating. Compared

with other nanocarbon allotropes, nanodiamonds (NDs) have manifested superior chemical stability,

ultralow toxicity and great biocompatibility, which can be leveraged in bioimaging and biosensing

techniques as well as drug delivery to grift large synthetic molecules in targeting therapy.12-14

Figure 2. Transformation from nanodiamonds (sp3) to hybrids (sp2/sp3) and carbon onions (sp2) by

thermal annealing and summary of their physical properties. Reproduced with permissions from ref

17. Copyright 2016 The Royal Society of Chemistry and ref 152. Copyright 2014 Wiley.

Distinct from the bulk scale, diamond nanocrystals typically present as spherical nanoparticles with a

large surface-to-volume ratio. The unique morphology enables the exposure of high fraction of

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diamond carbon atoms at the surface and subsurface regions. As shown in Figure 1, the dangling bonds

of unsaturated carbons at the grain boundary of nanodiamond would typically undergo significant

surface relaxation to be fabricated into partially sp2 hybridized carbon domains or be stabilized with

hydrogen and oxygen atoms to reduce the surface energy.15-16 More intriguingly, thermal treatment

can decompose the surface functionalities and facilitate the phase transition from diamond

terminations into fullerene-like graphitic shells. This feature affords the conversion of sp3-hybridized

NDs into either a uniform core/shell (sp3/sp2) hybrid or concentric graphitic carbon onion (sp2 carbons)

regulated by the annealing ambience (Figure 2).17-18 Therefore, engineered NDs can provide a

promising platform for investigating the intrinsic nature of sp2, sp3 carbons and their hybrid in metal-

free catalysis. More importantly, the relative proportions of the sp2 shell and sp3 core can be

deliberately governed in the self-assembled nanohybrids. Similar to graphene-based materials, the

graphitic shell can inherit all the fascinating properties of sp2 carbons with a conjugated π electron

system, which can be further engineered with a diversity of defects, heteroatom dopants, and

functionalities.19-21 In addition, the inner diamond core is capable of tailoring the surface electron states

of the fullerene shell by directly injecting electrons via the covalent bonds at the graphene/diamond

interfaces.22 Thanks to the sp2/sp3 hybrid structure, the ND triggered catalysis can deliver many new

features that cannot be realized in the sole graphene/diamond-based catalysis.

To the best of our knowledge, a few excellent reviews have been reported focusing on the structure,

and applications of NDs in biotechnology, batteries and electronics.17, 23-27 In contrast, this review will

be dedicated to the recent progress on the applications of NDs and their derivatives in diverse catalytic

processes, with a specific emphasis on unveiling the structure-performance relationship in ND-based

catalysis. In this review, we will firstly provide a brief introduction of synthesis, characterization, and

properties of NDs for readers’ easy access. Then, we will present the successful engagement of

nanodiamond in different atomic configurations of sp2, sp3, and sp2/sp3 for carbocatalytic reactions in

electrochemistry, photocatalysis, organic synthesis, hydrocarbon conversion, and environmental

remediation. Lastly, we will contribute to some insights into the impacts of surface modification

(defects and heteroatom doping) and structure engineering (hybrids) on the electronic features and

reactivity of NDs to unveil the intrinsic structure-performance properties in carbocatalysis. Therefore,

this review will assist to elucidate the design protocols of efficient ND-based catalysts and propose

future prospects for applying the advanced carbocatalysis in sustainable chemical/fuel production and

environmental purification.

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2. Synthesis and properties of nanodiamond

Physically milling Micro/natural diamond

Chemical vapor deposition CH4, H2, camphor

Detonation TNT, RDX, HMX

Figure 3 Illustration of the nanodiamond synthesis approaches.

2.1 Synthesis of nanodiamond

Nano/microscopic diamond was first discovered in the meteorite and detonation soot.28-29 Since then

tremendous efforts were inspired to synthesize artificial diamond via state-of-the-art techniques. As

displayed in Figure 3, diamond nanocrystals can be manufactured by a few techniques from either bulk

structure or carbon-rich precursors. For instance, NDs can be derived by directly physical milling of

the microscale synthetic or natural diamond crystals into nanoparticles in the presence of ceramic

beads.30-31 Additionally, the polycrystalline nanodiamond can be fabricated by in situ growth on a

metallic or silicon substrate by a chemical vapor deposition (CVD) approach under carbon sources

(e.g. CH4) and hydrogen (H2).32-33 The presence of chemisorbed atomic hydrogen can stabilize the

diamond carbons during the nucleation and growth. However, most of the commercial nanodiamonds

are manufactured via the detonation approach, which converts the carbon-rich explosives such as

trinitrotoluene (TNT), hexogen (RDX) and octogen (HMX) to NDs by the instantaneously generated

high temperature and pressure during the explosion in an oxygen-deficient and adiabatic chamber.34-

35 The free carbon atoms under the extreme conditions can be closely packed into the densest

arrangement of a cubic phase in sp3 hybridization to form diamond nanocrystals. The derived

explosives are composed of single-crystal NDs covered with a certain amount of non-diamond carbons

(graphite, amorphous carbons or soot) because of the incomplete transition of precursors and

spontaneous graphitization of carbon deposits over the diamond surface.36 The detonation soot will

undergo a series of complex post-synthesis treatment to obtain high-quality NDs. These processes

include removing the non-diamond (amorphous or sp2) carbons and incombustible impurities (metals

from the reactor chamber), and controlling the particle size and aggregation degree by using liquid

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oxidizers (HNO3/H2O2, H2SO4/H3PO4 etc.), ozonation and/or sonication.37-40 Taking the ozonation

process for example, the raw denotation soot will be pre-treated with acid washing to remove the metal

impurities, and then undergone a gas-phase oxidation in a fluidized bed reactor fed with an ozone/air

mixture to decompose the nondiamond carbons (amorphous and/or graphitic carbons) into CO2 and

CO at elevated temperatures.38, 41 After the purification, the typical chemical compositions of NDs

include C (~80-90%), N (~2-3%), H (~0.5-1.5%), O (<10 %), and trace level of metal residue.42

2.2 Physical properties and functionalities

Natural diamonds are transparent crystals while some of them are featured with colors due to the

presence of lattice imperfections (pink) and alien-atom dopants such as B (blue) and N

(yellow/brown).43-44 In contrast, nanoscaled diamond crystals are typically grey due to the hydrogen

terminations and surface coverage. NDs have a broad bandgap of 5.5 eV, and only respond to UV

irradiations below 225 nm.45-46 The commercial NDs can be graded by the particle size into different

ranges as nanocrystalline (over 10 nm), ultra-nanocrystalline (3-10 nm), and diamondoid (1-2 nm)

particles. The diamondoids are the smallest diamond clusters with over tens of sp3 carbons terminated

with hydrogen atoms.47 These diamonds are varied not only in the size and specific surface area (SSA),

but also in the proportion and re-arrangement of exposed surface atoms, functionality, and

corresponding physicochemical properties. Notably, most of the nanodiamonds discussed in this

review are commercial ultra-nanocrystalline diamonds with a diameter between 4-8 nm.

The diamond nanocrystals are characterized for their extreme mechanical robustness and spherical

morphology, affording NDs with an ultrahigh surface-to-volume ratio to facilitate feasible access to

the reactants in chemical reactions. The grain boundaries of diamond carbons are thermodynamically

unstable, and usually undergo significant self-relaxation to form sp2 terminations to saturate the

dangling bonds and to lower the overall surface energy (Figure 4a-f).48-50 Additionally, the diamond

surface is of high chemical activity to bond with atmospheric gases (oxygen and hydrogen), moisture

and other chemicals to form diverse surface moieties, because the presence of oxygen functionalities

and hydrogen bonds are beneficial to stabilize the diamond surface. Treatment by gas or liquid

oxidizers can increase the oxygen contents and acidity of the diamond surface. These surface oxygen

groups can assist a further functionalization of the diamond surface with other diverse functional

groups and even graft with proteins and long-alkyl chains via chemical bonds or π-π interactions.51-53

The presence of the functionalities will dramatically alter the properties of ND in aggregation,

stability/solubility and graphitization degree, affording the NDs with multifunctional roles in chemical

reactions and bio-applications.23, 54-55 However, post-treatment of NDs by thermal annealing under

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inert or reductive gaseous atmospheres can partially remove the oxygen groups and regulate the surface

acidity/basicity.

(a) (c)

(b) (d)

(e)

(f)

(g) (h) (i)

Figure 4. Structural relaxation of octahedral (a), cuboctahedra (c), and cubic (e) diamond to form

corresponding sp2/sp3 hybrid structures of (b, d, f). (g) Illustration of [001], [011], and [111]

terminations of a simulated near-spherical nanodiamond morphology. (h) and (i) HRTEM images of

nanodiamond in different crystalline terminations. Reproduced with permissions from ref 38.

Copyright 2011 and ref 48. Copyright 2017 American Chemical Society.

2.3 Graphitization into nanohybrids

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As aforementioned, the most appealing characteristic of NDs is that the carbons on the diamond surface

can be de-attached and re-fabricated into sp2-hybridized carbon sphere under certain circumstances.

The transformation tends to initiate at the imperfect sites such as dangling bonds and defects of NDs

surface, which are first converted into isolated graphite islands, and then gradually formed into

nanosheets and ultimately closed as a fullerene shell.18, 56 Thus, the diamond nanocrystals can be

converted into a self-constructed and an uniform core(sp3)/shell(sp2) nanocomposite, namely bulky

nanodiamond. As shown in Figure 2, the relative proportions of sp2 and sp3 carbons in the hybrids can

be feasibly regulated by controlling the graphitization conditions. NDs can be completely transformed

into graphitic nano-onions and hollow carbons at ultrahigh annealing temperature (> 1700 ºC).57 It has

been unveiled that the diamond particle size, surface index (Figures 4g-i), annealing ambience (e.g.

gaseous atmosphere and pyrolysis temperature), as well as the presence of a metallic catalyst can

impact the graphitization rate and degree during the phase transformation of nanodiamonds.17-18 For

instance, thermal decomposition of oxygen moieties will lead to massive structural defect and dangling

bonds that would increase the reactivity of the diamond surface and facilitate the re-fabrication of these

unsaturated carbon atoms into a graphite phase.58 Additionally, the presence of metal impurities (Fe,

Co, Ni) can behave as catalysts to remarkably speed up the surface graphitization.59-60

Therefore, the graphitic shells of bulky NDs can manifest all the physicochemical and electronic

properties of graphene-based materials, meanwhile the encapsulated diamond core enables the

graphitic shell with new properties via the intimate interactions at the interfaces. Theoretical

computations have witnessed the formation of strong covalent bonds between the diamond grain

boundary and covered graphitic lattice, which afford tunnels for electron shuttling from underlying

diamond core to graphene sphere.22, 61 Thus, this tunnelling effect results in a denser electron

population on the outer graphitic shell, thanks to the excited electrons from the diamond. Moreover,

similar to carbon nanotubes and fullerene, the curvature of the graphitic shell will enable the bending

of the π system, giving rise to unbalanced electron distribution in the carbon outer sphere with extra

electrons from excited s orbital.62-63 Notably, the volume of the graphitized NDs will be slightly

expanded due to the exfoliation of the fullerene shells. These shells can be instantaneously

functionalized with vacancies and topological defects arisen from the insufficiency of surface diamond

atoms to form an expanded and closed fullerene sphere.18

2.4 Characterizations of sp2/sp3 NDs

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There are a handful of classic and advanced characterization techniques that can be harnessed to

evaluate the surface chemistry and crystal texture of nanodiamonds and their derivatives. For instance,

diffuse-reflectance infrared Fourier transform (DRIFT), Fourier transform infrared spectroscopy

(FTIR), temperature programed desorption (TPD), and X-ray photoelectron spectroscopy (XPS) are

sensitive tools to investigate the surface elemental compositions, chemical states, and functionalities

of NDs-based materials.64-66 One challenge in characterization of bulky NDs is to verify the existence

and to quantify the contents of sp2 and sp3 carbons in the hybrids. In this regard, X-ray diffraction

(XRD) can detect the [002] peak of well-ordered graphite and the [111] facet of diamond crystals in

the nanohybrids67, which can be directly visualized by the high-resolution transmission electron

microscopy (HRTEM) imaging with distinguished lattice spacings of 0.34 and 0.21 nm accordingly68.

Moreover, nondestructive Raman spectroscopy can well identify the fingerprints of diamond and

graphite carbons at different locations in the spectra.69-71 Other state-of-the-art techniques such as

electron energy-loss spectroscopy (EELS)61, 72 and X-ray absorption near-edge structure (XANES)

spectroscopy73-74, and solid-state 13C NMR75 have also been utilized to qualitatively and/or

quantitatively estimate the proportions of graphitic and diamond carbons in the sp2/sp3 hybrids.

3. Catalytic applications of nanodiamonds

3.1. Green fuel cell reactions

The energy crisis arising from the dilemma between increasing energy consumption and depletion of

natural resources has posed imperative demands for developing high-end technologies for sustainable

energy production. The modern energy storage and conversion are realized by fuel cell technology,

which involves several key electrocatalytic processes, such as hydrogen evolution reactions (HER),

oxygen evolution (OER) or reduction reactions (ORR). Noble metals (Pt, Pd, Ir, Ru etc.) have been

demonstrated as the most efficient catalysts in those reactions, whereas the applications are limited by

their scarcity, poor stability in long-term operation and potential secondary contamination.76 The

emerging nanocarbons are metal-free in nature and are the promising alternatives, which can be

fabricated as the electrodes to initiate the aforementioned electrochemical reactions without a

compromise in reactivity.77

Swain et al. first evaluated the electrochemical reactivity of B-doped diamond (BDD) film.78 The

electron-deficient boron was incorporated to lower the Fermi level of sp3 hybridized carbons and to

endow the diamond with semiconducting property by increasing the population of charge carriers. As

a result, the BDD electrode possessed a low double layer capacitance with great polarization resistance

against surface contamination, exhibiting an ultra-stable activity in FeII/FeIII(CN)6 transition over a

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two-month operation. The diamond electrode bearing heavy boron loading was evaluated for hydrogen

and oxygen evolution reactions.79 The high-quality BDD (with fewer sp2 carbons) manifested a broad

potential window (-1.25 ~ + 2.3 V vs SHE) without occurrence of water splitting. A redox couple was

witnessed at +1.7 V ascribed to the diamond surface oxidation prior to water oxidation. However, the

corresponding kinetics toward OER and HER are relatively sluggish with low current densities

compared with those of pyrolytic graphite and metal electrodes.

Figure 5. (a) CV curves (dashed line for N2-saturated solution), (b) calculated Jk (at −0.30 and −0.35

V), (c) durability evaluation in O2-saturated 0.1 M KOH solution, (d) illustration of assembled zinc-

air battery for record of (e) polarization and power density curves and (f) recharge test at 16.0 mA

cm–2. Reproduced with permissions from ref 80. Copyright 2013 American Chemical Society.

Quan and co-workers exploited B and N dual-doped nanodiamond (BND) with the [111] dominated

facet as a cathode material for oxygen reduction reactions.80 As shown in Figure 5a, BND2 exhibits a

more positive peak potential and a higher current density than the N-doped diamond (NDD) and BND1

(with a lower nitrogen content). The ORR process on BND2 is evidenced to be a direct four-electron

process and manifested a great kinetic current density (Jk, 51.6 mA cm-2), which is 6.2 and 2.2 folds

of NDD and Pt/C accordingly (Figure 5b). Therefore, B-, N- co-doping in diamond exhibits a synergy

in promoted electrocatalysis, which not only helps increase the density of carriers and Hall mobility

of diamond, but also optimizes the electronic structure of sp3 carbons to introduce more catalytic sites.

The negative electron affinity of the diamond surface upon B and N modifications would facilitate the

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oxygen chemisorption, hence promoting the reduction process.81-82 Benefited from the robust structure

of diamond, BND2 presents greater durability and better resistance to methanol than the commercial

Pt/C electrode for continuous ORR (Figure 5c). Equipped as the cathode in the zinc-air battery (ZAB,

Figure 5d), BND delivers a higher battery voltage and a greater power density than Pt/C as depicted

in Figure 5e. The battery can maintain a higher voltage for 30 h discharge at a steady current density

of 30 mA cm-2. The recharge test in Figure 5f of ZAB-BND showed stable charge-discharge capacities

for 80 cycles without significant fluctuations in the potential.

Diamond nanocrystals covered by graphitic layers exhibited good performances in electrocatalytic

water splitting, whereas the active centers were still ambiguous due to the chemical and structural

complexity of carbon hybrids. To differentiate the roles of oxygen functionalities, Lin et al.

functionalized the graphitic surface of onion-like carbons (OLC) with a series of model aromatic

molecules via strong π - π interactions by a hydrothermal approach (Figure 6a).83 These highly

dispersive chemical probes with simplified oxygen functionality can assist to identify the catalytic

activity of a target oxygen group in OER reactions from a molecular level. As depicted in Figure 6b,

the quinone group is the intrinsic active site among the oxygen species regardless of the edging

configurations (zigzag/armchair). The 9-penantherenol with hydroxyl groups manifests the similar

redox feature to those with ketonic groups, due to the reversible redox cycle of the two groups by

losing/deriving an electron at the electrode (Figure 6c). Moreover, the carbon host with extended

aromatic units is beneficial for improving the onset potential and turnover frequencies (TOF) in OER.

Electron migration was also expected from OLC to adsorbed quinones via covalent bonds to induce a

denser electron population of the oxygen atom and enhance the carbocatalysis.

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Figure 6. (a) Synthesis of aromatic molecules-modified OLC by a solvothermal method to graft

different individual oxygen functionalities (e.g., C═O, C–OH, −COOH, and C–O–C). (b) Intrinsic

OER reactivity of different aromatic organic molecules-modified OLC catalysts. (c) Illustration of

reversible proton-coupled electron transfer between phenolic and quinone groups to mediate radical

intermediates during the electrochemical process. Reproduced with permissions from ref 83. Copyright

2018 American Chemical Society.

Moreover, the proportion of graphitic carbon in sp2/sp3 hybrids of annealed nanodiamond (AND) can

be controlled in a range of 12 to 71 at.% by regulating the pyrolysis temperature from 500 to 1300

ºC.84 ANDs with more graphitic shells deliver a better reactivity in ORR via a two-electron process,

due to the increased electron-transport capacity within the sp2-hybridized network. Further decoration

of the fullerene shells with nitrogen dopants (N-AND) impressively enhances the carbocatalysis and

facilitates the oxygen reduction as a four-electron pathway. The catalytic performance of N-AND is

gradually enhanced after 10000 cycling scans with a higher onset potential and a better limiting current

density. TEM images and energy loss spectra witness the partial conversion of the sp3 diamond core

to sp2 shells under constant electric simulations on the cathode, which secures the excellent stability

in a long-term operation. Additionally, the intrinsic catalytic centers of the nitrogen-modified bulky

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NDs in ORR can be probed by a selectively masking strategy.85 As illustrated in Figure 7a, the curved

graphitic shell over diamond is simultaneously decorated with positively-charged quaternary nitrogen

and negatively charged species such as pyridinic N, pyrrolic N and oxygen moieties. To identify the

intrinsic catalytic sites, two ionic surfactants of positively charged CTAB (cetyltrimethylammonium

bromide) and negatively charged SDS (sodium dodecyl sulfate) are selected to mask the oppositely-

charged sites by Columbic interactions (Figure 7a). As a result, the cationic CTAB treated diamond

exhibits no impacts on either the reaction pathway (Figure 7b) or the onset potential during oxygen

reduction (Figure 7d). However, the anionic SDS not only changes the oxygen reduction from a one-

step current (four-electron process) to two-step current (two-electron process) (Figure 7c), but also

lowers the onset potentials with the dosing SDS (Figure 7e). This suggests that the positively charged

graphitic nitrogens are the intrinsic catalytic centers of carbocatalysis in ORR.

Figure 7. (a) Schematic diagram depicting the idea of specific Coulombic interactions of ionic

surfactants with charged N-sites on NND (nitrogen-doped nanodiamond). (b, c) SCV-RRDE profiles,

(d, e) the first-derivative voltammograms (disk) of NND(+CB) in O2-saturated 0.1 M KOH containing

different surfactant concentrations. Reproduced with permissions from ref 85. Copyright 2017

American Chemical Society.

In summary, boron-doping can lower the Fermi level of sp3 carbons and increase the density of charge

carriers in NDs with improved conductivity, meanwhile the intrinsic features of diamond with wide

potential window and great resistance to polarization can be preserved. Co-doping of N into BDD can

boost the catalytic activity of the sp3 carbons in ORR due to the optimized electronic structure,

increased populations of charge carriers as well as the synergistic effect between B and N to facilitate

the chemisorption of oxygen. Partially graphitization of the diamond surface into fullerene shells with

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nitrogen modification can promote the ORR performance via a two-electron process and the graphitic

nitrogen is the primary reactive species validated by chemical probes. Moreover, the graphitized

diamond surface can provide an intact platform to graft diverse small-molecule catalysts via π-π

interactions to introduce specific edges or oxygen functionality to elucidate the origins of

carbocatalysis in OER.

3.2. Electrochemical carbon dioxide reduction

Carbon dioxide is the key component of greenhouse gases and its excessive emission from combustion

of fossil fuels has raised great concerns in global climate change. Therefore, the commitment of carbon

control by converting CO2 into value-added products such as fuels and chemicals would be a promising

strategy to maintain a sustainable carbon cycle. Because of the highly thermodynamic and chemical

stability of carbon dioxide, direct transformation of CO2 into C1 and C2 chemicals in aqueous solution

suffers from high-energy thresholds with poor selectivity toward diverse products as well as low

Faradaic efficiency due to the competing hydrogen evolution reactions. Recent studies on catalytic

CO2 reduction reactions (CO2RR) still rely on noble and transition metals/oxides (Au, Ag, Cu, Ni, Mo

etc). 86-89 The diamonds are robust carbon crystals and can tolerate high overpotential, rendering it an

ideal metal-free cathode for electrocatalytic reduction reactions.

Figure 8. (a) SEM image of NDDL/Si RA. N 1s XPS spectra of (b) NDDL/Si RA and (c) NDDH/Si RA

electrodes. (d) Electrochemical reduction of CO2 for acetate and formate productions on NDDL/Si RA

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and NDDH/Si RA electrodes in CO2 saturated 0.5 M NaHCO3. (e) Production rates on NDDL/Si RA

electrodes with different N doping level (−1.0 V). (f) Schematic pathway for electrocatalytic CO2

reduction on NDDL/Si RA electrode. Reproduced with permissions from ref 95. Copyright 2015

American Chemical Society.

The heteroatom modification with boron in diamond skeleton can enhance the conductivity of sp3

carbons as a p-type semiconductor, meanwhile optimizing the electronic culture and spin density of

the neighbouring carbon atoms as the catalytic centers.90-92 Yao et al. employed B-doped diamond

(BDD) as a conductive substrate to graft the Co-porphyrin, and the resulting surface functionalized

diamond can electrochemically convert CO2 to CO in acetonitrile with superb stability.93 Moreover

the bare BDD can yield a high selectivity in CO2RR to produce formaldehyde (HCHO) other than

formic acid (HCOOH) in methanol, surpassing grassy carbon (GC) and metal electrodes.94 Thanks to

the broad electrochemical window (EW) and robust structure of BDD, hydrogen evolution is

suppressed and a high Faradaic efficiency (85%) is achieved at -1.5 V (Ag/Ag+) during the CO2

reduction. Since the Faradaic efficiency and selectivity of sp2-carbon-terminated BDD and GC are

low, the intrinsic reactivity of pristine BDD is determined to stem from the pristine sp3 carbons.

Moreover, such a process can take place in the salty solution (Table 1), taking leverage of the abundant

electrons and protons in seawater.

Quan and co-workers coated N-doped nanodiamond (NDD, Figure 8a) onto Si arrays for

electrochemical conversion of CO2.95 Two samples, NDDL and NDDH, were synthesized by a CVD

approach at 450 and 500 ºC, respectively, and their chemical compositions are illustrated in Figure 8b

and 8c. Apart from their similarities in crystallinity, morphology, and total N-doping level, the

NDDL/Si with a larger proportion of graphitic N dopant leads to a better catalytic performance in CO2

conversion to both CH3COOH and HCOOH (Figure 8d). It is speculated that the N-doped sp3 carbons

are more reactive than the N-sp2 carbons toward CO2RR. The reaction on NDDH exhibits a high

selectivity for acetate production over formic acid under a low onset potential of -0.36 V (vs RHE). A

superb Faradic efficiency (over 91%) is achieved at a potential of -0.8 ~ -1.0 V. As illustrated in Figure

8e, the production rates of both acetate and formic acid are favored at more N dopants from 0.93 to

2.12 at.%, where a further increase in N-doping level (3.68 at.%) shows limited improvement. This is

because the overabundant nitrogen species may dramatically affect the diamond structure and disturb

the intrinsic electronic property of sp3 carbons, giving rise to a deteriorated activity in electrocatalysis.

Additionally, a high nitrogen content may also enhance the proton reduction (hydrogen evolution),

which will compete with CO2 reduction. As illustrated in Figure 8f, CO2 will firstly get a proton to

form a surface-confined CO2•− as the rate-determination step (RDS), and then the intermediate follows

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two routes to generate formate (pathway I) and acetate (pathway II) separately. Since the coupling of

two CO2•− to form -O2C-CO2- is thermodynamically more favorable than the protonation of single

CO2•−, the NDDH manifests a better selectivity to acetate.

Figure 9. (a) SEM images of a) BND1, b) BND2 and c) BND3 along with their corresponding d) XRD 

and (e) Faradaic efficiencies for CO2 reduction at −1.0 V. Reproduced with permissions from ref 96.

Copyright 2017 Wiley.

A subsequent study indicated that B-, N- dual-doped nanodiamonds (BND, Figure 9 a-c) outperformed

the sole-doped BDD and NDD for CO2RR at a range of more negative potential (-1.0 ~ -1.1 V) without

the involvement of proton reduction for hydrogen evolution.96 Increasing the N-doping level in the

BND can transfer the dominating grain boundary of cubic diamond from the [111] facet in BND1 (3.1

at.%) to the [220] facet in BND2 (3.6 at.%) and BND3 (4.9 at.%) in Figure 9d. The desirable onset

potential, current density, and selectivity can be simultaneously achieved following the sequence of

BND3>BND2>BND1. BND3 manifested the best performance in ethanol production with the

maximum Faradaic efficiency of 93.2% at -1 V (Figure 9e). Meanwhile, excellent stability can be

maintained for multiple runs without a noticeable change in Faradaic efficiency. However, density

functional theory (DFT) computations unveiled that the [111] diamond plane is more favorable to

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CO2RR with lower energy barriers other than the [020] facet. Therefore, the intrinsic enhancement in

the electrochemical activity of BND3 over BND1, BDD, and NDD may be attributed to the synergistic

effect of B and N co-doping and the improved conductivity, rather than the altered diamond

termination.

Table 1. Summary of the reaction conditions and Faradaic yields of the products from

electrochemical CO2 reduction over different nanodiamonds.

Catalyst Faradaic yields of products of

CO2 reduction

Applied potential Electrolyte ref

Glassy carbon

electrode (sp2

carbon)

Formaldehyde (19%) -1.5 V vs. Ag/Ag+ MeOH (0.1 M,

TBAP)

94

BDD (with large

amounts of sp2

carbons)

Formaldehyde (15%) -1.5 V vs. Ag/Ag+ MeOH (0.1 M,

TBAP)

94

BDD (sp3 carbon) Formaldehyde (65%),

Formic acid (14%),

Hydrogen (5.2%)

-1.5 V vs. Ag/Ag+ MeOH (0.1 M,

TBAP)

94

BDD (sp3 carbon) Formaldehyde (62%),

Formic acid (3.2%),

Hydrogen (22%)

-1.5 V vs. Ag/Ag+ Water (0.1 M,

NaCl)

94

BDD (sp3 carbon) Formaldehyde (36%),

Formic acid (1.5%),

Hydrogen (58%)

-1.5 V vs. Ag/Ag+ Sea water 94

NDDL/Si

(sp3 carbon)

Formate (13.6-14.6%),

Acetate (77.3-77.6%)

-0.8~-1.0 V vs. RHE 0.5 M

NaHCO3

solution

95

BDD (sp3 carbon) Formaldehyde (53.9%), Formic

acid (26.1%)

-1.0 V vs. RHE 0.1M NaHCO3

solution

96

NDD (sp3 carbon) Formaldehyde (62.4%), Formic

acid (24.7%)

-1.0 V vs. RHE 0.1M NaHCO3

solution

96

BND (sp3 carbon) Ethanol (93.2%) -1.0 V vs. RHE 0.1M NaHCO3

solution

96

In diamond-driven CO2RR, sp3 carbons are determined as the intrinsic active centers, which can be

regulated by introducing B and/or N dopants to improve the electrocatalytic performances. The

intriguing aspects of the system are that the category of products exhibit high dependence on the

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chemical compositions of the diamond. Specifically, the BDD, NDD, and BND delivered great

selectivity to produce HCHO, HCOOH, and CH3CH2OH accordingly. It can be deduced that the

chemical states of sp3 carbons tailored by alien dopants may determine the reaction pathways in

electrochemical CO2 reduction. Moreover, the tuneable N-doping level renders the nanodiamond with

great tolerance at a highly negative potential (-1.1 V) to outcompete H2 evolution, leading to high

Faradic efficiency. Therefore, doping level and species (B and/or N), carbon configuration (sp2 or sp3),

and diamond terminations should be rationalized in fabrication of ideal diamond electrodes to realize

a high activity, Faradic efficiency, and desired product distributions. The reaction conditions such as

applied potential and reaction media (solvent) need to be carefully selected in the electrochemical CO2

reduction processes.

3.3. Photocatalytic reduction reactions

Distinct from the sp2 hybridized graphene and carbon nanotubes with a zero bandgap, the diamond

constructed with sp3 carbons is a semiconductor with a wide bandgap of 5.5 eV. Pristine diamond

nanocrystals are terminated with dangling bonds and partially stabilized with hydrogen/oxygen atoms

at the grain boundaries, which can simultaneously regulate the energy levels, conductivity and

reactivity together with shaping and heteroatom doping (e.g. B and N).97-99 However, hydrogen-

terminated diamond can only harvest energy upon illumination of intensive light (hv > 5.5 eV). Since

the conduction band locates at 0.8 – 1.3 eV above the vacuum level, the excited electrons can further

migrate to the diamond surface with a trivial energy threshold.100-105 This phenomenon features H-

diamond to be a solid-state electron reservoir with a negative electron affinity (NEA). These high

energy electrons possess a very low redox potential of -5.5 V (NHE) and may initiate reduction

reactions in a liquid phase, which can be hardly reached by the fellow semiconductors.106

Ammonium (NH3) is a significant feedstock in the chemical industry and a green energy carrier, which

is commercially produced from nitrogen reduction. The traditional Harber-Bosch process usually

requires harsh reaction conditions under high temperature and pressure and is unfavorable to

experience on conventional solid catalysts due to the weak bonding with N2 as well as the participation

of high-energy-barrier intermediates.107-108 Hamers and co-workers managed to use H-terminated

natural diamond powders as photocatalysts to in situ reduce N2 to NH3 under ambient conditions.109

As illustrated in Figure 10a, the excited electron from diamond will first combine with water to

generate hydrated electron (eaq, -2.86 V NHE), which subsequently attacks a proton (H+) to generate

hydrogen radical (H•). The atomic hydrogen would rapidly react with the dissolved N2 to generate

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N2H. This step needs to overcome a very high energy threshold (RDS) in conventional homogeneous

catalysis.110 Then the intermediate (N2H) undergoes continuous hydrogenation to be eventually

converted to the product (NH3). This reaction is more favored in a dual-compartment H-cell with a

separate Pt electrode in KI solution (Figure 10b and c), which alleviated the sluggish water oxidation

at the diamond surface due to the shallow valence band of the diamond. More interesting, a moderate

NH3 production can be yielded at above 225 nm irradiation (< 5.5 eV, Figure 10d), attributed to the

structural defects (e.g. B or N doping or sp2 defects) which induces a smaller bandgap. However, the

oxygen-terminated diamonds demonstrated a much lower photoactivity than the H-terminated ones

(Figure 10e), because the positive electron affinity (PEA) of oxygen functionalities can confine or

prohibit the excitation of electrons in the conduction band from adjacent sp3 carbons.111 As shown in

Figure 10f, the H-diamond suffers from slight surface oxidation and nitrogen contamination after long-

term operation.

Figure 10. (a) The potentials of the VB and CB of H-terminated diamond toward some important

electrochemical reduction reactions. (b) Schematic diagram of reaction vessels. (c) Total ammonia

yield from ECG diamond in N2-saturated water using single-compartment and dual-compartment

cells. (d) Influence of illumination wavelength (e) Comparison of NH3 yield from H-terminated and

O-terminated diamond samples. (f) Positions of VB and CB of ECG diamond as determined by

ultraviolet photoemission spectroscopy measurements, showing transition from NEA to PEA.

Reproduced with permission from ref 109. Copyright 2013 Springer Nature.

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Jang et al. functionalized detonated nanodiamonds with hydrogen (HND) and oxygen (OND) and

evaluated the materials for hydrogen evolution under 532 nm laser irradiation.112 As illustrated in

Figure 11a, HND exhibits the highest photocatalytic activity in H2 production in water, followed by

pristine nanodiamond (PND) and OND. Addition of methanol impressively speeded up H2 evolution

rate by 40% on HND. Figure 11b displays the energy diagram of HND and OND with a similar

bandgap of 5.5 eV. The CB and VB of H-terminated nanodiamond are estimated to be 0.3 and -0.52

eV accordingly, which are 1 eV more positive than the CB (-0.7 eV) and VB (-6.2 eV) of O-terminated

nanodiamond.113 Upon laser stimulation, the electrons will be ejected into the surface adsorbed water

molecules to form a layer of hydrated electrons, leaving the accumulated holes to yield surface

positively charged hydrogens.114 Such an electric double layer near the diamond surface facilitates the

continuous electron diffusion and reaction with solution protons to produce hydrogen. Moreover, the

presence of methanol could serve as a sacrificing agent to donate electrons and compensate the

accumulated holes on the valence band for a more efficient charge-carrier separation to drive H2

evolution. However, oxygen terminations in OND manifest a higher ionization potential and limit the

electron transfer processes. As a result, the low redox potential of holes in the negative CB can be

hardly consumed for water or methanol oxidation, giving rise to the poor photocatalysis of OND.

Additionally, the nonlinear relationship between laser intensity and H2 evolution (Figure 11c) implies

the involvement of multiphoton processes, due to the greater bandgap than single photon energy (2.6

eV). Moreover, HND can be well dispersed onto the surface of graphene oxide (GO) and eliminates

the excessive oxygen functionalities of the GO into reduced graphene oxide (rGO) under the laser

pulse (Figure 11d). This reduction process will in situ generate a uniform ND/graphene hybrid (Figure

11 e-f), which substantially extends the visible light absorbance of NDs and improves the stability

against photo-corrosion of rGO toward applications in photovoltaic cells and optoelectronic

nanodevices.

Similar to ammonia synthesis, photocatalytic reduction of carbon dioxide is always hard to achieve,

which not only competes with hydroxyl evolution but also involves the multiple electrons and protons

to form intermediates via the proton-coupled and electron-transfer mechanisms. The first electron

transfer from a photocatalyst to CO2 requires a highly negative redox potential of -1.9 V, which lies

above the CB of most semiconductors.115-117 This rate-determination stage prohibits the initiation of

CO2RR by one-electron transfer from CO2 to CO2•− in photocatalysis. However, the CB of H-

terminated diamond can be as low as –5.2 V vs. SHE (Figure 11h), making CO2 reduction realistic by

photocatalysis under ultraviolet light irradiation. Hamers et al. performed CO2 reduction in a two-

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compartment cell using commercial BDD as the photocathode connected to a Pt photoanode in

Na2SO3/Na2SO4 solution to migrate the oxidative holes for overall charge balance.118 Figure 11i

illustrates that the electrons can be effectively excited from diamond surface to the nearby solvent

layer under the wavelength of 225 nm, which combined with water to yield a rich population of

solvated electrons, e(aq). The e(aq) subsequently attacks CO2 to form CO2•− via an one-electron transfer

process, which is rapidly decomposed to CO and O− over the photocatalysts.119-120 The system

manifested a high selectivity to CO with undetectable H2 from H• coupling, because the saturated

[CO2(aq)] is three-order magnitudes higher than that of [H+] in the acid solution (pH = 3.2), leading to

the low amount of H•. Since the produced CO2•− (-1.9 V) is less negative than H• (-2.3 V), the

accumulation and coupling of H• are insignificant. Herein, CO2•− serves as the electron reservoir and

determines the high selectivity toward CO. This is different from the electrochemical reduction process

where (CO2•-)ad needs to overcome a high energy barrier and suffers from proton/bimolecular coupling

to form hydrated C1 and C2 products.121

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(a) (b)

(d)

(e)

(f) HND-RGO

(g)

(c)

(h) (i)

Figure 11. (a) Hydrogen evolution over PND, OND, and HND in different solutions under 532 nm

Nd-YAG laser pulse irradiation (80 mJ per pulse). (b) A proposed energy diagram of H- and O-

terminated ND for photocatalytic H2 generation. (c) Laser power-dependent H2 evolution for the HND.

(d) I–V characteristics of the RGO and HND–RGO under 514 nm Ar ion laser irradiation and in dark.

(e) Photograph of the GO, HND, GO-HND (1:5) hybrids solutions before and after laser irradiation.

(f) TEM image and (g) AFM image (height profile) of HND–RGO composites. (h) Energy diagram of

HND compared with relevant redox potentials on absolute energy scale (left) and electrochemical

energy scale (right). (i) FTIR spectra of gaseous headspace demonstrating reduction of CO2 to CO by

illuminated diamond, along with control samples. Reproduced with permission from ref 112.

Copyright 2012 The Royal Society of Chemistry and ref 118. Copyright 2014 Wiley.

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It has been widely accepted that, due to the semiconducting property of the photocatalysts, the rapid

recombination of photogenerated charge carriers (electrons and holes) to generate useless heat severely

limits the efficient utilization of solar energy toward target reactions. In this regard, building

heterojunction with other semiconductors with matched band structures or constructing composite

structures with other conductive materials can substantially facilitate the spatial separation of

photoinduced high-energy electrons and holes to extend their lifetime to react with the adsorbed

reactants and enhance the quantum yield in photocatalysis.122-123 Silva and co-workers coupled TiO2

with oxidized NDs (NDox) as novel composites for photodegradation of an aqueous pharmaceutical

contaminant (diphenhydramine) under near-UV/Vis light irradiation.124 TiO2 was in situ grown on

NDox to generate the hybrids with a smaller particle size and homogeneous dispersion of the two phases,

which contributed to the promoted photocatalysis than the bare TiO2 and composites with

microdiamond. The TiO2-NDox was further reported to be able to drive photooxidation of microcystin-

LA in water under simulated solar light.125 The IR diffuse reflectance spectroscopy (DRIFT) witnessed

the formation of Ti-O-C bond at the interfaces of the composite, which facilitates the electron transfer

in the CB from NDox to CB of TiO2 and hole migration in VB from TiO2 to NDox. Yu et al. deposited

n-type TiO2 onto the p-type BDD to generate a high-performance heterostructure for oxidation of an

azo dye or reduction of Cr(VI) under UV irradiation.126 The formation of p-n heterojunction

significantly facilitated the separation of charge carriers in the redox reactions, and more importantly,

a synergistic effect could be observed in the co-presence of the metal and organic compounds to

simultaneously consume the generated electrons and holes, respectively. The TiO2-NDs composites

were also explored for photocatalytic oxidation of other substances such as toluene and ethanol.127-128

More recently, Zhou et al. combined NDs with graphitic carbon nitride (g-C3N4) via a hydrothermal

approach.129 The resulting NDs@g-C3N4 hybrids demonstrated a higher photocurrent than the single

counterpart under UV-visible light irradiation and produced more hydroxyl radicals for decolorization

of methylene blue (MB).

Therefore, despite of the large band-gap of diamond (5.5 eV), the hydrogen-functionalized grain

boundaries can simultaneously down-shift the CB and VB of HNDs and enable the excitation of

electrons by laser into the vacuum region to form the hydrated electrons in aqueous solution. The high-

energy eaq possesses a very negative potential (- 2.86 V NHE) to reduce the inert molecules such as

CO2 and N2, which cannot be realized by conventional semiconducting materials. In contrast, coupling

oxygen-terminated NDs with semiconductors (TiO2 or g-C3N4) can dramatically narrow the band gap

of the composites for photocatalytic purification of organic contaminants in water under UV or UV/Vis

light irradiations. Further tailoring the diamond and semiconductor by atomic modification (doping or

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functionalization) or structure engineering (size, facet, or morphology control) may optimize the

interface interactions and optimal property of the hybrids toward reinforced photocatalysis.

3.4. Chemistry in organic synthesis

Metal-based homogenous and heterogeneous catalytic systems have significantly innovated the

protocols for diverse organic synthesis and boosted the development of chemical community. However,

the scarcity, high-cost and potential toxicity of transition/noble metal systems cannot offer a green and

sustainable route to a large-scale chemical production. Recently, graphene-based materials have been

demonstrated as metal-free catalysts for selective oxidation, catalytic reduction, coupling and

acid/basic reactions, taking advantage of the abundant defects and functional groups as well as the

tuneable acidity/basicity of carbon surface.130 The annealed nanodiamond with curved graphitic shells

can present a similar catalytic activity to sp2 carbon materials meanwhile it may deliver new properties

from the sp3 carbon in the diamond core assembled in the hybrid structure.

Su and co-workers refluxed ND in concentrated mixed acids to functionalize the diamond surface with

abundant versatile oxygen groups, which was used as a mild solid catalyst to prepare N-rich

heterocyclic organics.131 It was found that the densities of –C=O and –O–C=O exhibit positive

proportions to the product yields, suggesting these two groups may serve as the dominant catalytic

centers among the oxygen minorities. The control experiment using small organic molecule catalysts

to mimic different oxygen groups on ND further confirmed the crucial roles of the acidic and nonacidic

carbonyl groups as the intrinsic active sites. The catalytic pathways are illustrated in Figure 12. In the

catalytic cycle, hydrazine is first decomposed over –C=O to generate a reactive imidogene (HN**).

Then, the HN** would attack benzonitrile to produce a N-centred N, N’-benzamidinediyl radical,

which is the key species to trigger the two possible pathways to yield the desired N-rich heterocyclic

products.

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Figure 12 Reaction mechanism and catalytic cycle hydrazine activation and reaction with benzonitrile

to mediate N-containing free radicals over the ketonic groups of nanodiamond surface to generate of

N-rich heterocyclic compounds. Reproduced with permission from ref 131. Copyright 2018 Elsevier.

Moreover, rational regulation of the sp2/sp3 ratios of graphitized NDs delivers an intriguing structure-

activity relationship in catalytic reduction reactions. A comparative study (Figure 13a) illustrated that

alteration of sp2 proportion can tune the work function (Φ) of bulky nanodiamonds.132 The Φ decreases

with the enhanced graphitization degree of NDs and OLC manifests an even lower Φ than graphene

and carbon nanotubes, suggesting that the sp2 hybridization, curvature, and surface group

synergistically account for exiting the π electrons from the Fermi level to the vacuum layer to realize

a high-energy surface. Therefore, the work function reflects the barrier for an inner electron excitation

to the catalyst surface and is determined by both surface functionalities and electronic configurations,

which can be used as an indicator to predict the catalytic capacity in a redox reaction.133-136 In Figure

13b and 13c, NDs with a lower work function attain a greater yield for selective reduction of

nitrobenzene (NB, phenyl-NO2) to aniline (AN, phenyl-NH2). Additionally, the surface defects and

oxygen groups may facilitate the adsorption of reactants and the curvature of fullerene sphere will lead

to delocalized π electron with lower binding energy and high surface energy to host the active surface

complex in the catalytic process. Moreover, the yield and selectivity in NB reduction can be promoted

by the presence of external electronic field using N-doped ND as the electrode.137 In such a process,

NB would be first transferred to nitrosobenzene (phenyl-NO), and then is rapidly reduced to phenyl

hydroxylamine (phenyl-NHOH), which is detected as the key intermediate to AN. Moreover, the

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vertically aligned NDD/Si electrode exhibits a low hydrogen evolution potential of -1.95 V (vs

Hg/Hg2SO4), which can be leveraged for electrochemically reductive debromination of

polybrominated diphenyl ethers to less nontoxic diphenyl ethers.138

NDs can also be applied as a robust metal-free catalyst for C-H bond activation and selective oxidation

reactions. For instance, the bulky nanodiamond can activate tert-butyl hydroperoxide (TBHP or tBuOOH) for selective oxidation of a wide array of alcohols to their aldehydes without using an organic

solvent.139A structure-activity relationship is established in the populations of defects and oxygen

functionalities with the catalytic reactivity. As a result, the edging sites, especially for zigzag edges

with localized electrons at vacancies and periphery of external fullerene, are unveiled to be

catalytically active for TBHP decomposition to evolve highly oxidative intermediates. Furthermore,

Lin et al. prepared functionalized AND with nitrogen dopants for enhanced selective oxidation of

benzyl alcohol (BzOH) with tBuOOH.140 The N-doped AND stands out among the carbonaceous

materials for catalytic oxidation of BzOH to benzaldehyde with the satisfactory conversion, yield, and

selectivity. The metal-free system can maintain the superb performance for multiple cycles with a

negligible decline in reactivity and the N-doping impressively promotes the catalytic oxygenation of

4-methlbenzeyl and 4-nitrobenzyl alcohols. In the N-doped AND, the pyridinic nitrogen is identified

as the primary catalytic centre, whose amount manifests good linearity with the BzOH oxidation rate

(Figure 13d). The catalytic process is that TBHP is first decomposed over the carbocatalyst to generate

a hydroxyl radical (via tBuOOH → tBuO• + •OH), which presents an intimate affinity to the electron-

rich pyridinic N to form a surface-confined complex (Figure 13e). The diamond-N-OH complex will

subsequently attack the hydroxyl group of –CH2–OH in BzOH to produce –CH2O and H2O. Finally,

the tBuOO• (from the coupling of tBuOOH and tBuO•) will abstract another hydrogen atom to yield

the product of benzaldehyde.

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Figure 13. (a) Relationship between the fraction of sp2 carbon and the work function over different

carbon materials, the dependence of the work function on the catalytic activity for nitrobenzene

reduction (b) and (c) selective oxidation of 2,3,6-trimethylphenol, (d) correlation of benzyl alcohols

conversion with the contents of pyridinic N1/C, (e) Mechanistic illustration of catalytic activation of tBuOOH for oxidation of benzylic alcohols over the N-doped annealed nanodiamond. Reproduced

with permissions from ref 132. Copyright 2017 The Royal Society of Chemistry and ref 140. Copyright

2014 American Chemical Society.

Likewise, AND was further carbonized into OLC and engineered with N-decorations (N-OLC), the

derived carbocatalyst can demonstrate great catalytic performance in the epoxidation of styrene (ST)

to styrene oxide (SO).141 The N-dopants are unveiled to be catalytically more reactive than the oxygen

functionalities on N-OLC. Distinctively, the graphitic N is identified as the primary active site among

the N species, as the contents demonstrate a positive linear relationship with the styrene oxide yield.

The N-dopant can tailor the electronic configurations of surrounding carbons with metal-like d-band

states. The activated carbon sites are capable of reaction with TBHP to generate oxidative Carbon-OH,

subsequently transferred to Carbon-OOtBu. Then, the α-O will attack the venerable olefines bond to

produce the epoxide products. Meanwhile, the N-dopants can decrease the adsorption energy of the

reactants with an optimal energy barrier for intermediate dissociation, which is advantageous to

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improve the selectivity of SO. The oxidized NDs was also reported to be engaged for direct

dehydrogenation and dehydration of low-molecular alcohols (C2-C3).142

Thus, sp3 carbons are catalytically inactive in chemical synthesis. After graphitization of pristine NDs

into sp2/sp3 hybrids, the generated fullerene shell affords the diamond the catalytic features of

graphene-based materials with potential active sites such as curvature, defects, edges, functionality

and heteroatom dopants. More importantly, the presence of the diamond core can excite electrons to

the graphitic shell to enrich the charge density and work function. The two properties can be

deliberately controlled by the sp2/sp3 ratio in the hybrids and are significant in tailoring the electronic

structure of the surface active sites to attain desirable reactivity and selectivity in catalytic

reduction/oxidation reactions.

3.5. Oxidative dehydrogenation (ODH) reactions

Styrene is an important chemical stock for polymer manufacturing industry, which is typically

produced from ODH of ethylbenzene (EB) with steam over a K-Fe2O3 (K-Fe) catalyst.143-144 However,

this process is suffering from the massive energy consumption due to the endothermic nature and

severe catalyst passivation by surface contamination.

Inspired by the impressive promotion of deposited coke in oxidative dehydrogenation of ethylbenzene

to styrene, Keller et al. first employed nanocarbons such as carbon nanofragments, bulky NDs and

OLC as metal-free catalysts for styrene production, which surpassed the commercial K-Fe catalyst in

both yield and selectivity over the long-term operation (Figure 14a and Table 2).145-147 The oxygen

functionalities have been identified to play critical roles during the ODH. The nucleophilic attack of

hydrogen in EB over quinone groups (C=O) on sp2 carbons is more reactive than that over C=O on

nanodiamond carbons. This is because the edges and kinks of curved graphitic (0001) facets can host

the ketonic oxygen to produce electron-rich Lewis basic centers with partially delocalized π electrons

to activate saturated hydrocarbons.145 Graphitic carbons are beneficial for dissociation of molecular

oxygen to produce basic ketonic groups at the periphery, which are the intrinsic active sites to

accommodate the hydrogen atom from EB as the rate-limitation step to produce hydroxyl groups and

styrene. Additionally, the highly conjugated π network can contribute as a buffering region for

reversely storing/donating elections during the redox process.148 Moreover, the NDs outperform other

carbonaceous materials in both conversion efficiency and selectivity (Figure 14b), due to the defective

carbon sphere in activating oxygen molecules and localizing the catalytic oxygen functionalities.149-150

The spherical morphology with a loose internal density of NDs facilitates the styrene desorption and

prevents the polymerization of the products. Additionally, the well-defined graphitic shell can achieve

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self-purification by in situ gasification of the cokes to maintain a clean catalytic surface for a steam-

free ODH.145 Notably, the thermally conducted sp3 diamond core is also advantageous for the heat

diffusion between the diamond bulk and surface reactive regions to further promote the exothermic

ODH reaction.147, 149

Activation energy (eV)

(d) (e)

(f) (g)

(a) (b) (c)

Figure 14. (a) Comparison of long-term performance for EB dehydrogenation over nanodiamond and

commercial catalyst (K-Fe2O3). (b) Steady‐state activities of various carbocatalysts. (c) Illustration of

promoted selectivity for propane ODH over phosphate modified nanodiamond. Activation barriers of

first C–H bond breakage in isobutane molecules (d) over cuboctahedral NDs in different diameters

and (e) on CNT and ND, (f) energy profile and (g) reaction pathway of dehydrogenation of isobutane

at a quinone group on CNT and ND. Reproduced with permissions from ref 48. Copyright 2017 and

ref 151. Copyright 2015 American Chemical Society and ref 149. Copyright 2010 Wiley.

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As shown in Figure 14c, Sun et al. decorated the AND surface with phosphate ions to selectively

eliminate the inactive phenol groups and suppress the combustion.151 Such modification prevents the

formation of electrophilic oxygen species (phenols), which account for the overoxidation of light

olefins to COx, whereas the active ketonic groups remain intact. Therefore, the selectivity in ODH

conversion of propane to propene can be substantially improved. Additionally, the quitonic groups

anchored on zigzag edges are evidenced to be selective than that on armchair terminations.148 More

interestingly, the structure-function sensitivity was also discovered in the AND-based catalysis and

the regulation of the sp2-to-sp3 ratio in AND will impressively promote the selectivity in the production

of light alkenes such as propene and butenes.150, 152-153 The high-temperature pyrolysis not only tunes

the surface acidity/basicity to leave more ketonic groups due to their better thermal stability, but also

induces significant lattice re-arrangement with a greater fraction of sp2 carbons. The electronic culture

of the outermost fullerene sphere would be simultaneously tailored by the increased number of

graphitic layers, which in turn optimize the nucleophilicity of the attached ketonic groups toward a

better selectivity.57, 154-155

Apart from the hybrid structure, density functional theory studies unveiled that both particle size

(Figure 14e) and exposed grain boundaries (Figure 14e) of bulky diamond can impact the electron

density and work function of graphitic shell.48 The electronic states of the anchored ketone group will

be synergistically regulated to determine the activation energy of C-H bond in ODH reactions. The

ketonic groups on diamond clusters manifest lower atomic charges compared with the pure sp2-

hybridized CNT, which helps lower the energy barriers in both dehydrogenation of isobutane (Figure

14f and g) and recovery of hydrogenated quinontic sites. Therefore, the key to the carbocataytic ODH

is to regulate the electronic states of the ketonic groups on nanodiamond. As an electron-withdrawn

functionality, the electron density of ketonic group can be determined by the electron configuration of

the adjacent surface carbons in different hybridizaitons, work function, and crystal structure (size and

termination). Furthermore, fine-tuning the oxygen species to remove the acidic groups and develop

the proper population of the quinone group can improve the selectivity.

Table 2. Summary of catalytic performances of nanodiamond and other catalysts in ODH reactions.

Catalyst ODH reaction Conversion (X ,%) or

Yield (Y, %)

Selectivity

(S, %)

Condition Ref

K-Fe catalyst EB to styrene X = 50 - 790K on steam 145

Graphite EB to styrene X = ~45 - 790K on steam 145

CNT EB to styrene - SST = 97.0 823K, 2.8% EB in He 149

Activated carbon EB to styrene - SST = 77.0 823K, 2.8% EB in He 149

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Mesoporous carbon EB to styrene - SST = 55.7 823K, 2.8% EB in He 149

UDD EB to styrene X =25 (38 after 10 h) - 788K on steam 147

OLC EB to styrene X = 40 (62 after 10 h) - 788K on steam 147

OLC EB to styrene X = 62 SST = 68 790K on steam 145

NDs EB to styrene Y = 20.5 SST = 97.3 823K, 2.8% EB in He 149

ND-900 EB to styrene X = 31.7 SST = 99.8 823K, 2.8% EB in He 156

ND@NMC-700 EB to styrene X = 37.7 SST= 99.6 823K, 2.8% EB in He 156

ND@CNx EB to styrene - SST = ~99.0 873K, 2.8% EB in Ar 157

N-RGO@ND EB to styrene - SST = ~98.0 823K, 2.8% EB in Ar 158

SWCNT n-butane to butenes X = 9, Y = 1 S1-C4H8 = 2

S2-C4H8 = 1

SC4H6 = 10

723K, 2.64 vol% n-butane

and 1.32 vol% O2 in He

150

UDD n-butane to butenes X = 1, Y = 6 S1-C4H8 = 33

S2-C4H8 = 9

SC4H6 = 15

723K, 2.64 vol% n-butane

and 1.32 vol% O2 in He

150

15P-AND Propane to propene X = ~ 10, Y > 20 SC3H6 > 45 773K, C3H8:O2:He =

1:1:31.3

151

ND-1000 Propane to propene X = ~ 21 SC3H6 = ~ 45 723K, 2%C3H8/O2/He 152

ND-1000 Propane to propene X = ~ 11.6, Y = ~ 9.5 SC3H6 = ~ 84 823K, 2%C3H8/He 153

3.6. Advanced oxidation processes (AOPs)

The critical environmental issues urge advanced and green technologies for purification of aquatic

systems contaminated by diverse hazardous organic substances from human activities. As promising

candidates, dimensional carbons such as fullerene, graphene and carbon nanotubes (CNT) have been

studied as green catalysts to replace toxic transition/noble metal catalysts in heterogeneous catalysis.6,

159 Wang’s group discovered that the surface engineered nanocarbons can be used for effective

activation of persulfates such as peroxymonosulfate (PMS) and peroxydisulfate (PDS) in aqueous

solution.160-162 The catalytic processes produced versatile highly reactive radicals (SO4•−, •OH and O2

•−)

in oxidizing and mineralizing a wide array of aqueous organic pollutants.163-164 More importantly, the

metal-free system would not induce secondary contaminations by metal leaching in Fenton reaction

and transition metal-based AOPs. However, nanocarbon catalysts (such as graphene and carbon

nanotubes) suffer from slight bio-toxicity and unsatisfactory resistance to the strong oxidative

environment in AOPs. Thus, the robust and environmental-benign nanodiamonds stand out as

promising candidates.

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Figure 15. (a) PDS activation by diverse carbonaceous materials in the presence and absence of phenol.

(b) Phenol oxidation by G-NDs/persulfate systems with different ROS scavengers. (c) Records of

linear sweep voltammograms in G-NDs/PDS system with/without phenol. (d) Effect of annealing

temperature on the catalytic performance of nanodiamond in PMS activation and phenol degradation.

(e) Radical quenching effects on phenol oxidation rate decrement with different loadings of radical

scavenger (methanol, MeOH) in S-ND-900 (1-2 layer graphene)/PMS and S-ND-1100 (2-4 layer

graphene)/PMS systems. (g) Mechanistic illustration of the core-shell layer dependence for tuning

radical and nonradical pathways. Reproduced with permission from ref 61. Copyright 2018 Elsevier

and ref 166. Copyright 2016 American Chemical Society.

NDs were first explored to activate persulfate (PDS) for phenol oxidation, whereas the catalytic

performances of both pristine and oxidized NDs were poor and not comparable to CNT and defective

graphene.165-166 This is because the sp3 carbon framework is chemically inert compared with sp2

carbons with the electron-rich ketonic groups and defective sites on the grain boundaries to coordinate

the redox process in AOPs. Therefore, thermal treatment has been applied to NDs to partially convert

the outer region of sp3 carbons into a graphitic shell over the diamond surface. The graphitized or

annealed nanodiamond (GND/AND) can effectively catalyze PDS for organic oxidation.166-168 Figure

15a illustrates the superior catalytic performance of GND in PDS decomposition, which surpasses

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other dimensional nanocarbons including fullerene (0D), carbon nanotubes (1D), graphene (2D) and

graphite (3D). Competitive radical quenching tests (Figure 15b) and in situ electron paramagnetic

resonance (EPR) technique evidence that the GND/PDS system involves both free radicals (minor

role) and nonradical species (dominant ROS). The quinone groups on the fullerene shell of AND are

supposed to deliver charges to persulfate to produce sulfate radicals. In contrast, the peripheral carbons

present a strong adsorption tendency to persulfate, giving rise to a surface activated intermediate in

attacking the organic compounds without releasing free radicals.168 In this process, the presence of

organic substrates can further enhance the PDS activation by serving as electron donators (Figure 15c).

Such nonradical oxidation can be reinforced by completely graphitized the diamond core into OLC,

which improves the graphitic degree and conductivity of carbon surface to mediate the continuously

electron shuttling from the electron-rich organic to the co-adsorbed and metastable persulfate/carbon

complex.166

The graphitic degree of AND was further manipulated by adjusting the carbonization temperatures

(600 – 1100 ºC) to tailor the core/shell ratios in the composites. The corresponding catalytic

performances are illustrated in Figure 15d. As aforementioned, the higher temperature will gradually

decay the diamond grain boundary and increase the proportion of graphitic carbons as well as the

conductivity of the curved graphene surface.167 Interestingly, AND-900 with 1-2 shells in sp2/sp3

hybrid structure manifests the best reactivity among the NDs, suggesting that there exists an optimal

diamond configuration for peroxymonosulfate (PMS) activation. Similar to the AND/PDS system,

radical screening and EPR trapping tests suggest that the AND/PMS system also involves both free

radicals (•OH and SO4•-) and nonradical pathway. More importantly, radical quenching experiment in

Figure 15e suggests that AND with a higher ratio of sp2 carbons (AND-1100 vs AND-900) in the

hybrid configuration possesses a greater proportion of nonradical pathway with better resistance to

alcohols and background matters in water.61, 167 Therefore, by rationally regulating the thickness of the

graphitic shells in bulky NDs, it is possible to tune the oxidative system from radical oxidation to a

nonradical pathway as shown in Figure 15f.

This phenomenon is further investigated by theoretical computations to mimic PMS interacting with

graphene/diamond hybrids.22, 61 It is witnessed in Figure 16 that covalent bonds are formed between

diamond termination and graphene cluster, implying the strong interactions at the interfaces in the

hybrids. The Bader charge analysis suggested the electron density of graphene cluster is populated

upon coupling with diamond due to the electron excitation and migration from sp3 diamond carbons

to the graphitic sphere. This feature can be attributed to the inherent differences between sp2 and sp3

carbons in work functions.132 Increasing the graphene layers (from one to three) gradually diminishes

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the electron migration to reach the outermost surface, because the electrons cannot penetrate the

multiple carbon layers via the van der Waals interactions. As a result, sp2/sp3 hybrids with more

graphene layers enhanced PMS adsorption with fewer electron transport. The DFT results well

illustrate the transformation from a radical reaction (fast electron transfer with moderate adsorption)

to a nonradical pathway (less electron transfer with strong adsorption) over AND with higher

graphitized degrees.163, 169 Moreover, modification of the outer fullerene shell of sp2/sp3 hybrids with

nitrogen can further activate graphitic carbon network and boost PMS decomposition.22, 170 Theoretical

computations have illustrated that N-doping can break the homogeneous π system of graphene and

create a dipole moment between the nitrogen dopant and its adjacent carbon atoms.81, 171 The positively

charged carbons in graphene not only attract more electrons excited from the diamond base, but also

enhance PMS adsorption and activation to form metastable ROS/carbon complexes for organic

degradation in a nonradical manner.

Figure 16. Theoretical calculations PMS adsorption onto different hybrid models (a) 1-layer

graphene@diamond, (b) 1-layer N-doped graphene@diamond, (c) 2-layer graphene@diamond, and (d)

3-layer graphene@diamond. Reproduced with permissions from ref 22. Copyright 2016 and ref 61.

Copyright 2018 Elsevier.

Such a structure-activity sensitivity is also observed in electrochemical processes using BDD as the

anode for direct oxidation of organic contaminants without the addition of peroxides.172-175 Thanks to

the robust sp3 carbon framework, BDD is promising for practical wastewater treatment due to its

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superb stability, low accumulation of organics, high intrinsic reactivity and broad potential window.

The abundant boron dopants in BDD secure the excellent electron conductivity and contribute to

providing variable states in the bandgap of diamond toward the electrocatalytic oxidation.172, 176 Thus,

the pristine BDD can serve as a ‘non-active’ anode to produce highly reactive hydroxyl radicals from

water oxidation, which is capable of completely mineralizing the microcontaminants into CO2 and

inorganic salts.177-180 However, partial surface relaxation or graphitization of BDD with a certain

degree of sp2 carbons afford the BDD to be an 'active' electrode and transform the degradation pathway

into a nonradical pathway.181-182 The sp2 carbons are much favorable for organic adsorption, which

leads to direct oxidation of the contaminants via outer/inner-sphere electron transfer over the anode

surface. Meanwhile, the intermediates can be accumulated onto the diamond surface and prevent the

deeper mineralization due to the insufficient redox potential at the anode, giving rise to fast oxidation

of the target organics but a low removal efficiency in total organic carbon.

Our recent study witnessed that singlet oxygen (1O2), a high-energy and excited state of oxygen

molecule, was also generated in a GND/PMS system which outperformed the sulfate radical based

metal systems (Fe2+/PMS, Co2+/PMS) in 4-chlorophenol degradation.183 Singlet oxygen production

and organic decomposition rate can be feasibly regulated by tuning the population of ketonic groups

on diamond surface regardless of pH conditions, suggesting that the ketonic groups are the intrinsic

catalytic centers in PMS activation. It is worth noting that, similar to surface reactive species, singlet

oxygen can also be classified as a nonradical ROS, which presents a moderate oxidative capacity and

exhibits specific selectivity to the target organics.184-186 Distinct from the pure radical systems induced

by transition metals and nonradical systems by nanocarbons (such as graphene and carbon nanotubes),

the oxidative capacity of persulfate/nanodiamond is tuneable by rationally regulating the contributions

from the radical/nonradical pathways.61 Therefore, it is much beneficial for a fast and selective

decomposition of persistent organic contaminants without being limited by the natural radical

scavengers of background matters (e.g. inorganic ions or natural organic materials) in remediation of

practical water matrixes.

As displayed in Table 3, the diamond-based catalytic AOPs are structure-performance dependent. The

pristine sp3 carbons can induce a pure radical pathway in electrochemical oxidation, whereas the

nonradical pathways emerge and gradually dominate the organic degradation with the increased

proportion of graphitic carbons in the bulky NDs to drive the anodic oxidation and persulfate activation.

The reaction pathways are governed by carbon atomic configuration (sp2 or sp3) and electronic density

of outer graphitic carbons. These features ultimately determine the redox and electron-transfer

capacities of the surface carbons (sp2 or sp3) to react with water or persulfates for directly producing

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free radicals or generating a metastable intermediate for nonradical oxidation via an electron-shuttle

mechanism. Therefore, it is crucial to rationally controlling the structure and surface chemistry of NDs

to regulate the oxidation regimes via radical/nonradical degradation or singlet oxygenation in

wastewater purification.

Table 3. Summary of oxidation of organic contaminants by diamond-catalyzed persulfates via

different reaction pathways.

Catalyst Persulfate Reactive oxygen species/mechanism

Target organics Ref

AND-1000 PDS •OH, SO4•−, surface

bounded SO4•−

Phenol 167

G-NDs (graphitized at 1200 ºC)

PDS Electron-transfer pathway Phenol, aniline, bisphenol-A, acetaminophen, sulfamethoxazole, ranitidine, carbamazepine

166

N-ND, N-ND/PDDA (graphitized at 500 ºC with N-doping or PDDA coating)

PMS Electron-transfer pathway 4-chlorophenol 170

N-AND (graphitized at 700 ºC with N-doping or PDDA coating)

PMS •OH, SO4•− Phenol 22

AND-900 PMS •OH, SO4•−, nonradical

oxidation on carbon surface

Phenol, methylene blue, catechol, Sulfachloropyridazine (SCP)

168

S-ND-900 and S-ND-1100

PMS •OH, SO4•−, nonradical

oxidation on carbon surface

Phenol 61

AND/550, /700, and /800 PMS singlet oxygen 4-chlorophenol 183

4. Origins of carbocatalysis in nanodiamonds

Figure 17 illustrates that the nanodiamonds can be tailored by both structure control and surface

engineering. Nanodiamonds in sp3 hybridization can be graphitized into uniform core/shell structures

with tuneable sp3/sp2 proportions or ultimately converted into a concentric nanocarbon onion by high-

temperature pyrolysis under inert ambience. During the transformation, pristine and annealed NDs can

demonstrate different carbon architectures and properties of sp2, sp3 or their hybrids, providing an ideal

platform to unveil the intrinsic differences of carbon configurations in carbocatalysis. Additionally,

thermal pyrolysis of carbon-rich precursors (e.g. D-glucose, polydopamine or melamine) with sp3

diamond nanocrystals can also generate similar core/shell hybrids with desirable surface

functionalization.156-157, 187 NDs and the hybrid derivatives deliver outstanding catalytic reactivity in

diverse reactions, covering chemical and clean fuel production, energy conversion and environmental

remediation. More importantly, the well-defined and robust crystal structure of NDs with a partially

graphitized carbon surface secures superb stability to resist oxidation, carbon decomposition and

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extreme acidic/basic conditions for a long-term operation, outperforming other popular carbonaceous

materials such as biochar, activated carbon, graphene and carbon nanotubes. Particularly, the NDs-

based materials present spherical structures with a high surface-volume ratio in nanoscale, which not

only facilitates easy access and diffusion of reactants to surface active sites from both kinetic and

thermodynamic aspects, but also maximizes the atomic utilization efficiency of the carbocatalyst in

catalysis.

Zigzag/armchair Edges

Vacancy &Functionality

Defects

Dopants

Structure Control Surface Engineering

Figure 17. Illustration of structural and surface engineering of nanodiamonds.

With a wide band gap of 5.5 eV, the H-terminated NDs (HND) can be excited by laser irradiation (λ

< 225 nm) to emit electrons from the high-energy conduction band (~ 0.8-1.3 eV) above the vacuum

level to the surrounding environment.109 These high-energy electrons will directly combine with the

protons in solution to form reactive hydrogen radicals for hydrogen evolution or directly attacking the

inert molecules such as N2 and CO2 to generate the corresponding reductive products.109, 112, 118

Therefore, HND is a solid-state electron reservoir for photocatalytic reduction reactions. However, the

pristine NDs are chemically inert and semiconducting and require elemental doping (B and N) to

introduce more charge carriers to improve the conductivity, and more importantly to manipulate the

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reactivity of sp3 carbons to drive electrocatalytic reactions. In the tailored conducting nanodiamond,

the doping species, doping level, and facet engineering synergistically determine the conductivity of

the bulky diamond and optimal electronic configuration of the surface sp3 carbons for interacting with

the reactants toward enhanced electrocatalysis in CO2RR, ORR, and water splitting.79-80, 94-96 However,

the emergence of graphitic carbons will give rise to a poor Faradic efficiency in CO2RR.

As illustrated in Figure 18, the sp3 carbons prone to mediate photocatalysis and electrochemical

reactions, while the sp2/sp3 hybrid and sp2 carbon onions prefer to drive other heterogeneously catalytic

processes such as oxidative dehydrogenation, advanced oxidation, and selective oxidation/reduction

reactions. In these bulky diamond-based processes, the surface sp2 carbons or functionalities are

typically the intrinsic reactive sites and the catalytic behaviors are much similar to graphene-based

materials. Notably, during the thermal pyrolysis and phase transition of the diamond nanocrystals,

diverse defective sites such as edges, vacancies and topological defects (non-six-atom carbon rings)

will be simultaneously created on the graphitic shells (Figure 17) due to the expanded volume and

insufficiency of carbon atoms to form a closed fullerene sphere. These structural defects have been

witnessed by advanced microscopy techniques and unveiled by theoretical computations to possess

unique electronic configurations with partially localized electrons, which manifest high chemical

potentials to stabilize the reactive intermediates and coordinate the redox processes in oxygen

reduction/evolution or persulfate activation.19, 139, 188-191 Moreover, the defects/edges in the sp2 carbon

sphere are typically the hosting sites to accommodate oxygen functional groups and heteroatom

dopants. The oxygen moieties, especially ketonic/quinone groups (C=O), can serve as basic Lewis

sites with extra lone-pair electrons to be the intrinsic catalytic sites in oxygen evolution/reduction83,

192, dehydrogenation149, 151, 153, and organic synthesis131-132, and advanced oxidation reactions165, 168, 183.

Notably, the electron configurations of the oxygen groups (e.g. C=O) are dynamic, which can be

rationally tailored by the local environments of the adjacent carbons. In this regard, the hybridization

(sp2, sp2@sp3, sp3), defects (zigzag/armchair edges and topological defects), dopants (B and/or N),

facet terminations,48, 193 and size of NDs can fine-tune the work function and atomic arrangement of

the surface carbons on bulky NDs, which can simultaneously regulate the electron density and

reactivity of the grafted oxygen functionalities in chemical reactions. Moreover, incorporating metal-

free alien atoms (e.g. B, N) into the sp3 diamond framework or sp2 shells can promote the electron

conductivity of the carbon substrate and substantially enhance the activity of adjacent carbons due to

the optimized charge and spin density distributions, driven by the inherent differences in

electronegativities or valence electron numbers.22, 95-96 Nevertheless, the activated carbon atoms are

believed as the intrinsic active centers in carbocatalysis, and the dopants can also be directly involved

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in the catalytic processes to mediate/stabilize the intermediates and promote the conductivity.94, 140-141

It should be pointed out that the relationship between the work function and oxygen groups in diamond-

based catalysis has already been established in catalytic oxidation/reduction reactions,132, 140, 183

whereas the tremendous efforts are still lacking to establish the structure-performance regimes to

correlate them and other properties (e.g. defects and doping) in nanodiamond with the corresponding

catalytic performances in other catalytic applications. Also, it should be taken into consideration that

multi-factors may cooperatively determine the intrinsic reactivity of nanodiamond in different

reactions.

Figure 18. Transformation of sp3 to sp2 carbons and their corresponding applications in heterogeneous

catalysis.

The advantages of nanodiamonds as advanced carbocatalysts lie in their superior flexibility in both the

structure (from sp3 to controlled sp2/sp3, or sp2) and surface chemistry (dopants, functionality and

defects), which endow the surface carbon with manoeuvrable electronic structure to catalyze versatile

chemical reactions in photocatalysis, electrochemistry and other heterogeneous catalysis. Different

from the graphene basal planes, the graphitic shell of NDs presents a certain degree of curvature, which

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may induce distortion/strain of the π system with partial sp3-hybridization due to the participation of

σ electrons, herein bringing in new properties to the sp2 carbons.62, 150 Moreover, the presence of the

diamond core in the sp2/sp3 hybrid can afford fascinating features to the graphitic shells. DFT

calculations illustrated that strong covalent bonds were formed between the interfaces of diamond

surface and graphitized carbon lattice, which could serve as electron tunnels to mediate charge

transport from diamond slab to graphene sphere to enrich the electron population of the sp2 carbons.26

Incorporating nitrogen dopants into the graphitic sphere can further promote the electron migration

(Figure 16) to bring in much advantageous to the carbon-based redox processes.22 Both the

nanodiamond size and terminations (facet) can impact the electron density of the grafted oxygen

groups on the sp2 sphere, to modulate the energy barriers in oxidative dehydrogenation reactions

(Figure 14).48 The catalytic activities of bulky NDs have been elucidated to be significantly impacted

by sp2/sp3 ratios in terms of the work function as well as the electron shuttling from diamond core to

the outermost shell.61, 132, 152 Therefore, a rational design of sp2/sp3 hybrids by controlling the graphitic

degree of bulky ND is necessary to manipulate the charge density and work function of external carbon

(including the carbons in defects and near heteroatom dopants) and electronic properties of oxygen

functionalities on the hybrids in carbocatalysis.

In NDs, the sp2 layers play different roles in electrochemistry and can provide the adsorption and/or

catalytic sites, while the internal fullerene serves as a conducting channel for high-speed charge

transport.154 Herein, altering the thickness of graphitic layers not only governs the electron density of

the outer carbon sphere but also the conductivity of the bulky NDs. Therefore, fabrication of high-

performance hybrid carbocatalysts requires deliberate structure and surface engineering to achieve

favorable surface chemistry and secure the effective communications between the core and shells to

attain satisfactory electronic configurations and redox potential in carbocatalysis. In addition, sp3

carbons are excellent thermal conductors, therefore, the diamond core can facilitate the heat transfer

to the surface catalytic centres in heterogeneous reactions. Such a feature is particularly favorable for

endothermic reactions such as ODH and AOPs.149, 167

5. Perspective and concluding remarks

In summary, nanodiamond and its functional derivatives have stood out among the nanocarbon

community as green and high-performance catalysts. The unique sp3 hybridized carbon framework is

highly robust in extreme working conditions, and more importantly, can be converted into graphitic

carbons to drive carbocatalytic reactions. The bulky NDs are inherent from most of the fascinating

properties of graphene-based materials, meanwhile deliver many new features in architecture and

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surface chemistry from embedded diamond nanocrystals. The electronic structure of the diamond can

be feasibly regulated by surface modifications with heteroatom functionalization/doping as well as

structural regulation of number graphitic shells and diamond size/terminations. Therefore, a

satisfactory catalytic reactivity can be always expected thanks to the optimized electronic

configurations of the sp2/sp3 hybrid, which cannot be achieved by other carbon materials.

However, compared with the intensive and extensive studies on graphene- and carbon nanotubes-

based materials in catalytic reactions, the research on nanodiamond as metal-free catalysts are still

lacking. This is partially due to the expensiveness of nanodiamond from detonation synthesis and

tedious purification processes, which involve huge consumptions of energy and chemicals. Therefore,

novel strategies are urged to manufacture high-quality and low-cost nanodiamond for scaling-up

applications. Moreover, due to the inherent complexity of nanodiamonds in both structure and surface

properties, there are tremendous research opportunities to explore the intrinsic roles of sp2, sp3 carbons

and their hybrids in catalytic reactions. The vital roles of engineered microstructure, defects and

vacancies, lattice strain, and surface functionalities should also be comprehensively understood.

To this end, state-of-the-art theoretical computations can be leveraged as a handy tool to gain insights

in reaction mechanisms as well as guiding the material design and optimization. For instance, as

aforementioned, B-doping can increase the electron conductivity of the diamond substrate in

electrochemical processes, and N-doped sp3 carbons is catalytically more reactive than N-doped sp2

carbons.78-79, 94 However, the understandings of B (or N) doping in tailoring the electronic states of the

diamond carbons as well as the synergy of B-, N- co-doping in diamond for promoted carbocatalysis

(especially compared with sp2 carbons) are still limited. Elemental doping/surface functionalization of

diamond with other metal-free atoms (e.g. S, P, F, Cl) has rarely been investigated in catalysis. In this

regard, quantum chemistry can predict the regulated electronic structures of sp3 carbons and their

catalytic behaviors in chemical reactions upon heteroatom(s) doping. In addition, the curved and

strained graphitic surface contains carbon atoms with a certain degree of sp3 hybridization. The

curvature of carbon sphere would cause an imbalanced electron distribution on both sides, which is

expected to exhibit high catalytic activity due to the partial delocalized π electrons as well as the

possible excitation of electrons from s into p orbital. Such the intriguing electronic configuration of

the curved graphitic carbons coupled with diamond can be directly visualized by DFT approaches for

the implications on catalysis. Moreover, theoretical computations have witnessed the covalent bonds

between diamond termination and the adjacent sp2 carbon layer which act as tunnels for efficient

electron migration at the interface. However, only van der Waals interactions exist between the

graphitic spheres in the sp2/sp3 hybrids. Thus, the electrons excited from diamond carbons can reach

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the innermost fullerene, yet may not be able to penetrate multiple layers to the outermost graphene,

which have been evidenced by Bader/Mulliken charge analysis. The structure-function relations

(sp2/sp3 ratio and catalysis) may vary in different catalytic scenarios and can be predicted by DFT

calculations.

It should be noted that, during the graphitization, heteroatom dopants (N and B) in NDs can be in situ

fabricated into the inner graphitic layers. The impacts of the internal doped graphitic layers on catalytic

activity have not yet been researched. Post-treatment of denotated NDs by thorough acid pickling

removes most of the metal residues on the surface of denotated NDs, whereas a trace level of metal

impurities may still exist in the diamond framework, which may impact the graphitization process and

chemical potential of the derivatives in diamond-based catalysis. A handful of studies have illustrated

that transition and noble metal atoms (Fe, Co, Ni, Pt etc.) can be incorporated into the carbon latticed

coordinated by nitrogen atoms.194-196 Such single-atom catalysts have demonstrated superior

performances and stability in catalytic reactions compared to pure carbon and metal catalysts. In this

regard, functional NDs can serve as potential open substrates to host the metal dopants to further

regulate the electronic configuration of NDs and introduce new features to the catalytic behaviors. As

suggested by Lin and co-workers, advanced in situ/operando characterization and evaluation strategies

including chemical titration, isotopic trapping, X-ray absorption spectroscopy can help identify the

intrinsic catalytic centres in complicated diamond-driven catalysis.

Moreover, nanodiamonds in different carbon configurations can be coupled with other materials to

fabricate novel heterostructure materials. For example, NDs have been leveraged as versatile carbon

supports (in sp2, sp3, sp2/sp3 configurations) to load and tailor the electronic structures of

transition/noble metal nanoparticles (e.g. Au, Pt, Mo) for enhanced catalysis in organic synthesis and

ODH reactions.197-200 Coupling NDs with semiconductors (e.g. TiO2, g-C3N4) as nanocomposites

impressively enhance the light-harvest efficiency and facilitate charge carriers separation, leading to

the promoted photocatalytic activities. Therefore, NDs and graphitized derivatives can be used as both

a carbon support and co-catalyst to build novel and high-performance catalysts. The enhanced

activities of the composites may be benefited from modulated catalytic sites by the tunnelling or

coupling effects and the emerging metal-carbon (or carbon-carbon) interfaces. Notably, some key

points or technique difficulties for preparation of these composites should be considered. For instance,

the nanosized diamond crystals are prone to aggregate due to the high surface energy, rational surface

modification and good solvents are necessary to disperse the NDs for homogeneously

coating/depositing the metal nanoparticles onto NDs or formation of intimately interacted nanohybrids.

Efforts to coupling ND with other metal/metal-free nanomaterials are encouraged to construct novel

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composite materials with appealing new features. Furthermore, the textural characteristics of the

composites can be optimized by fine-tuned crystalline phases and particle sizes of the supported metal

species with advanced synthesis protocols, in pursuit of favorable electronic and/or geometric

structures in heterogeneous catalysis.

In conclusion, diamond nanocrystals are appealing metal-free materials with tunable structure and

surface chemistry, showing outstanding reactivity, selectivity and stability in metal-free catalytic

reactions. The green and robust nature of nanodiamonds render them to be promising carbocatalysts

to drive industrial processes in chemical and fuel production, energy storage and conversion, as well

as environmental remediation. Fascinating diamond-based materials not only provide a perfect

platform to probe the nature of carbocatalysis in chemical reactions but also afford tremendous

opportunities to extend their applications in new catalytic systems and to address the bottlenecks in

modern chemistry toward sustainable production.

AUTHOR INFORMATION

Corresponding Author.

*Email: shaobin.wang@adelaide.edu.au

Notes

The authors declare no competing financial interest.

ACKNOWLEDGEMENT

We thank the Australian Research Council for the financial support (DP190103548). We also

acknowledge the supports from the Fundamental Research Funds for National Natural Science

Foundation of China (Grant No: 21777033), Science and Technology Program of Guangdong Province

(2017B020216003).

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