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Nano Energy

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Jahn-Teller distortions in molybdenum oxides: An achievement in exploringhigh rate supercapacitor applications and robust photocatalytic potential

Liqi Bai, Yihe Zhang⁎, Likai Zhang, Yuanxing Zhang, Li Sun⁎, Ning Ji, Xiaowei Li, Haochen Si,Yu Zhang, Hongwei Huang⁎

Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science andTechnology, China University of Geosciences, Beijing 100083, PR China

A R T I C L E I N F O

Keywords:Energy storage materialsJahn-Teller effectsDFT calculationsSupercapacitorsPhotocatalysis

A B S T R A C T

Although transition-metal oxides offer the potential for intimate coupling of energy storage application andenvironmental sustainability, how to find the suitable relationship between materials structures and properties isstill a problem to explore further applications. In this work, Jahn-Teller effects proved by Crystal Field Theory(CFT) and Density Functional Theory (DFT) are used to predict the electronic structures of MoO3 and MoO2reasonably, such results guided us to design two more reasonable applications. On account of the metallicitytendency of MoO2 with a stronger electron mobility from band structure, MoO2/rGO/g-C3N4 composite wasperformed as a high-performance electrode in asymmetric supercapacitors (SCs). MoO2 with the assistance ofmesoporous graphitic carbon nitride and high conductivity graphene shows an enhanced capacity at 1700 F g−1

at 1 A g−1 and cycling retention at 84% retention after 3000 cycles for supercapacitors, the corresponding as-semble asymmetric devices shows a maximum power density of 6.25 kW kg−1 at an energy density of16.0Wh kg−1. Furthermore, the proper band structure of MoO3 predicted by DFT calculation has guided MoO3/rGO composite as a photocatalyst to degrade tetracycline, which shows a high elimination efficiency of 90.6%within 2 h illumination of simulated solar light.

1. Introduction

Due to the various physical and chemical properties of advancedmaterials can be adjusted for different applications, tracking thesefeatures is a very difficult task [1]. To explore better properties of ad-vanced materials, theoretical prediction and calculation before experi-ment are essential. For instance, Materials Genome Initiative (MGI) [2]is a project which enhanced the fundamentals of materials science bysharing information and accelerated the development of new materials.In recent years, assisted by the improvements in crystal structure pre-diction, some new materials, especially superconductors [3], lithiumbatteries [4], semiconductors [5] and so forth, have been predicted invirtue of computers or machine-learning tools [6].

Supercapacitors (SCs) [7], a device can achieve fast charge anddischarge results in a high power density and energy density thancommercial secondary batteries, such as Lithium-ion batteries [8], Li-Sulfur batteries [9], Sodium-ion batteries [10], and Potassium-ionbatteries [11], whose principles are mainly work by redox reactions.Thus, SCs is still a substitute for secondary batteries in a short period oftime. Photocatalysis, a technology to produce photo-generated

electrons and holes, excites active radicals with powerful oxidationunder the light irradiation and decomposes environmental hazardousorganic substances such as dyes [12], antibiotics [13], etc..

There is no doubt that molybdenum oxides has such excellentphysical and chemical properties above [14], for example, owing to thelow-spin state electronic structure in 4d electron orbital, molybdenumions can play an important role in many redox reactions as oxidants orreducers, thus the Mo-based electrodes in lithium-ion batteries [15]possess a high electrochemical capacity, cycling retention, and highrecharging rates.

Nevertheless, it is not clear that who is more suitable for SCs be-tween MoO2 and MoO3. And there is much contradiction in literature asto who is more suitable for photocatalysis. Therefore, in order to un-derstand the relationship between the structures and properties of suchtwo molybdenum oxides, it is necessary to make theoretical predictionand calculation. Jahn-Teller effects [16,17], a distortion of the octa-hedral structure happened in MoO3 crystal, which leads to variation inMoO2 crystal lattice parameters when the phase transition occurs andleads to a series of different properties. Density Functional Theory(DFT), namely ab initio, a theory can be used to simulate complex

https://doi.org/10.1016/j.nanoen.2018.09.028Received 19 August 2018; Received in revised form 10 September 2018; Accepted 12 September 2018

⁎ Corresponding author.E-mail addresses: zyh@cugb.edu.cn (Y. Zhang), sunli@cugb.edu.cn (L. Sun), hhw@cugb.edu.cn (H. Huang).

Nano Energy 53 (2018) 982–992

Available online 17 September 20182211-2855/ © 2018 Elsevier Ltd. All rights reserved.

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atomic structures [18]. Stable or metastable structures can be de-termined at given chemical compositions from first principles calcula-tions, which is an efficient way to help us design multifunctional mo-lybdenum oxides materials.

In this paper, we first use the crystal field theory (CFT) and DFT toanalyze the electronic structures of MoO2 and MoO3 reasonably andcalculated the band gap changes caused by the transition of 4d energylevel electrons. The relationships between its structure and its re-spective properties have been found out and proved by experimental.However, the theoretical capacitance of pure MoO2 is less than700 F g−1 in previous literature [19,20], which is still difficult to meetthe urgent demand of high specific capacitance and energy density.Carbon nitride, a burgeoning material that possesses a high nitrogendoping, high activity specific surface area, and a high theoretical spe-cific capacitance above 372 F g−1 [21] was introduced in super-capacitor application to reduce the amount of costly graphene, which isan ideal strategy for synthesizing low-cost and high-performance elec-trochemical storage devices via synergistic effect. Besides, the decom-posing of cheaper melamine precursor is carried out at about 520℃,which is consistent with the temperature of the phase transition fromMoO3 to MoO2, that is to say, the MoO2/rGO/g-C3N4 (GMCN) activematerials can be easily can be easily synthesized by mixing MoO3 andmelamine precursors to anneal directly, such a GMCN electrode ex-hibits an excellent specific capacity at 1700 F g−1 at a density of1 A g−1 whose capacity is almost twice as high as MoO2/rGO (GM2)electrodes because of the specific surface area had been improved di-rectly, the devices shows a maximum power density of 6.25 kW kg−1 atan energy density of 16W h kg−1 and cycling retention at 74.7% over3000 cycles. As for MoO3, it is more suitable for photocatalysis, andMoO3/rGO photocatalyst demonstrates a high degradation efficiency of90.6% for degrading tetracycline (TC) within 2 h simulated solar lightirradiation, and the possible mechanism (GM3) composites have beenproposed. Such combined experimental and theoretical research pro-vides insights into the design and preparation of nanomaterials forenergy storage and environmental applications. And further band en-ergy of rGO and g-C3N4 have been identified depending on the reduc-tion degree of rGO and structure of g-C3N4 and comparatively de-scribed.

2. Experimental details

2.1. Preparation of the agents

Graphene oxide (GO) powder with a diameter of 0.5–5 µm waspurchased from Nanjing XFNANO Materials Tech Co. Ltd, which issynthesized by a modified Hummers method. All of the other chemicalreagents are analytical grade products without further purification.

2.2. Preparation of GM2 and GM3 solids

GM3 solid has been modified from a previous description. For ex-ample, 13mg GO was added to 13ml of deionized water, the solutionwas ultrasonic stirred for 30min. Another 28ml of deionized water wasmixed with 0.84 g Na2MoO4·2H2O and 2.1 g sodium salicylate undervigorously stirred for more than 10min. After fully dissolved, the dis-persions were added with the as-prepared GO dispersions. The con-centrated HCl aqueous solution was dripped into the solutions withstirring and adjusted pH to 2. All of the mixtures were sealed in a100ml Teflon-lined autoclave and then heated at 180℃ for 24 h. Theautoclave was naturally cooled to room temperature, then the obtainedprecipitate was collected by 8000 rpm centrifugation, afterward, wa-shed several times with deionized water and ethanol. The mixture wasdried at 80℃, then the GM3-13 powder was obtained whose color ismidnight blue. (The GM3-8, GM3-14, and GM3-18 are same.) The GM3solids were heated at 520℃ for 100min at a rate of 5℃/min underargon gas (99.9%) flow in a tube furnace, then GM2–13 solid was

formed with a color of black.

2.3. Preparation of g-C3N4 powders and GMCN composite

The synthesis of g-C3N4 powders was based on related reports viaheated melamine directly at 520℃ for 4 h. Herein, we used 640mgmelamine mixed with 150mg to prepare unannealed precursors bygrounding substantially for 15min. And the hybrid precursors wascalcined at 520℃ for 4 h as aforesaid. The obtained GMCN solid color isblack.

2.4. Characterization of GM2, GM3, g-C3N4 and GMCN powders

The crystal diffraction data of the products was recorded by X-raydiffraction (XRD) via a Bruker D8 AdvanceX X-ray diffractometer withCu Kα radiation (λ=1.5418 Å) with the operating conditions at 60 kVand 80mA, ranging from 10° to 70° at room temperature. A JEM 2100 Fhigh-resolution Transmission electron microscopy (TEM) images wereoperating at 200 kV to obtain the morphology, elements distributionand microstructure. Scanning electron microscopy (SEM) image ob-servations were carried out on a JSM-IT300 scanning electron micro-scope to examine the general morphology of these samples. Ramanspectra were characterized by a Renishaw 2000 Raman Spectrometer at1633 nm. Fourier-transform infrared (FTIR) spectra at a frequencyrange of 4000–500 cm−1. The surface properties and the bonding reg-ularity of such active materials were obtained by X-ray photoelectronspectroscopy (XPS) via a Thermo Scientific ESCALAB 250 instrument(USA) operating at 150W with Al Kα X-ray irradiation. UV–vis diffusereflectance (DRS) of the samples were recorded by means of Cary 5000(America Varian) spectrophotometer. BET specific surface area andpore contribution were measured by ASAP 2020 specific surface andaperture analyzer. Atomic Force Microscope (AFM) images were cap-tured on Si substrate by Bruker Dimension Icon.

2.5. Electrochemical measurements

Electrochemical workstation (CHI 760E, CH Instruments Inc.,Shanghai, China) with a three-electrode configuration in a 3M KOHaqueous was applied to record the main data of electrochemical prop-erties, whose reference and counter electrodes were Ag/AgCl and Ptfoil, respectively. The working electrodes were mainly prepared bymixing active materials on Ni form: the samples, acetylene black, andpolyvinylidene fluoride (PVDF) were mixed in N-methyl-2-pyrrolidone(NMP) solvent at a weight ratio of 8: 1: 1 to form a slurry, which waspasted onto the Ni form and dried in a vacuum oven at 80 °C all night toremove the solvent.

Specific capacitance (F g−1) and current rate (A g−1) were calcu-lated based on the total mass of the active materials. Cyclic voltam-metry (CV) curves were performed at a scanning rate of 2–50mV s−1

from 0 to 0.52 V at room temperature. Galvanostatic charging-dis-charging (GCD) measurements were measured from 0 to 0.52 V at adensity of 1–20 A g−1. Electrochemical impedance spectroscopy (EIS)measurement was performed in an alternate current (AC) voltage with5mV amplitude in a frequency range from 0.01 Hz to 100 kHz.

2.6. Fabrication of the GMCN//AC SCs

The GMCN sample was employed as an electrode and the ActivatedCarbon as another electrode. These two active electrodes, 3M KOHelectrolyte and one piece of cellulose paper as the separator were as-sembled in a CR2016-type coin. The mass of the ASC active materialswas calculated via the relationship below: m−/m+ =(C+ × ΔV+)/(C−× ΔV−), where m is the mass of electrode, C is the specific capacitance,ΔV is the potential range in GCD process, the “+” and “−” are differentelectrodes.

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2.7. Photocatalytic evaluation

TC was employed to evaluate the photocatalytic activities of the as-prepared photocatalysts via simulated solar light degradation. 5mgphotocatalyst powders were dropped into quartz tubes, which werecontaining 50ml TC solution (10mg L−1), respectively. Then, theabove solution was ultrasonically dispersed for 10min and stirred for30min in dark to achieve an adsorption-desorption equilibrium be-tween the photocatalyst solid and the TC. After turning the 500Wxenon lamp light on, the photocatalytic reaction systems were exposedto simulated solar light irradiation. Afterward, 4ml of the mixture wasextracted out in the centrifuge tubes, and the catalysts were removedfrom suspensions (8000 rpm, 5min). The centrifuged solution wasanalyzed by recording the maximum absorption band (378 nm) andUV–vis spectra were recorded by Shimadzu UV-5500PC spectro-photometer.

2.8. First-principles calculation

For MoO2 and MoO3, all first-principles calculations were carriedout with the Generalized Gradient Approximation (GGA) by adoptingthe Perdew-Burke-Ernzerhof (PBE) exchange-correlation para-meterization to the Density Functional Theory (DFT) using the ViennaAb initio Simulation Package. A plane-wave basis with a kinetic energycutoff of 500 eV and a Monkhorst-Pack grid with a 5×5×5 k-pointmesh for the integration in the Brillouin zone were used, and the con-vergence criteria for the residual force and energy on each atom duringstructure relaxation were set to− 0.04 eV/Å and 10−5 eV, respectively.

3. Results and discussion

The splitting of energy levels caused by the crystal field will result inadditional energy when filling the electron. According to CFT and theGoodenough's model [22], the fifth periodic element Mo, whose atompossesses 4d5 electron orbital with a high energy level, and is trend tobe low-spin state, the electron configurations and the correspondingcrystal field stabilization energy (CFSE) of MoO3 and MoO2 can be in-ferred in Fig. 1a–b, the Δ0 is the different energy level, the octahedral(Oh) component of the ligand field formed by the molybdenum ionsplits the 4d levels into t2g and eg sets, and the two electrons of the Mo4+

is priority to occupy t2g orbit. the electrons in t2g orbit of MoO3 andMoO2 are different, the two extra electrons in 4d2 orbit of MoO2 lies int// orbit on the top while MoO3 has an empty orbit with no additionalelectrons, thus the energy level difference of Mo6+ (Δ01) is higher thanthat of Mo4+ (Δ02), namely

>Δ Δ01 02 (1)

That is to say, the metallic conductivity may be possessed by MoO2and the MoO3 have semi-conductor tendency because of the higherband gaps. Besides, the t2g level is related to the orbital in the planedefined by the shared octahedral edges [23], on account of the differentCFSE leading to the Jahn-Teller effects, the original Oh componentstructure of [MoO6] units could happen tetragonal distortion, differentcrystal structures are formed as MoO2 and MoO3, which are shown inFig. 1c, for example, the O-Mo-O bond angle of MoO3 is 82.88° and thatof MoO2 is 82.59°, the lattice parameters in our model are shown inTable S1–2, the c/a is less than 1, because the distorted Mo6+ is not atetrahedron but a state between octahedron and tetrahedron. The lat-tice distortion is formed by the destroyed electron cloud symmetry of

Fig. 1. The 4d orbital energy levels of (a) Mo6+ and (b) Mo4+ with Jahn-Teller effects. Bond structure of [MoO6] unit in (c) MoO3 and (d) MoO2. (e) The mechanismof the reduction reactions of MoO3. (f–g) [MoO6] units in MoO3 viewing along different directions. (h–i) [MoO6] units in MoO2 viewing along different directions.

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Fig. 2. The DFT calculation results: (a) calculated band structure of MoO2. (b) 1) Total densities of states of the MoO2 and partial densities of states of 2) O and 3) Mo.(c) Calculated band structure of MoO3. (d) 1) Total densities of states of the MoO3 and partial densities of states of 2) O and 3) Mo.

Fig. 3. (a) Schematic illustration of the fabrication process of the GM3, GM2, and GMCN. Comparison of (b) capacitance performance and (c) photocatalyticdegradation TC evaluation of different content of GM3 and GM2. (d) TEM and SEM (inset picture) morphology of pure g-C3N4 which synthesized at 520℃ with (e)mesoporous characteristics. SEM image of coralloid morphology (inset picture) of (f) GM2 and (g) TEM morphology shows that MoO3 loaded on a single-layeredgraphene. (h) TEM morphology of GMCN, and the EDS of GMCN shows C, N, Mo, and N elements.

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Oh. Fortunately, the formation of MoO2 and MoO3 can be achieved byphase transition. The transformation mechanism is simplified as fol-lows:

MoO3 + H2 → MoO2 + H2O (2)

Actually, the intermediate Mo4O11 during this consecutive reaction[24] is unrelated to this paper. Due to the migration of the electrons,this reduction reaction entropy (ΔH) may connect with the energy gap(ΔEoct=Δ01 - Δ02), which is shown in Fig. 1e. To reflect the [MoO6] cellsand the distribution of chemical bonds, the stacking [MoO6] units be-longed to MoO2 and MoO3 are shown in Fig. 1f–i and Fig. S1. Oh ofMoO3 shows a better symmetry than MoO2, due to the stable octahe-dron coordination from the d° orbit of transition metal ions. And thedissonant structure in MoO2 in Fig. 1d and h-i is well matched to theeffects resulted from Jahn-Teller distortion.

In order to verify whether the prediction of the band gap is rea-sonable, it is necessary to simulate the result by theoretical calculationso as to guide the experiment. Fig. 2 shows the DFT results of the MoO3and MoO2, Fig. 2a shows the band gap of MoO2, the dashed line standsfor the Fermi energy level, two Mo 4d bands are discovered below theFermi energy level, which is not only corresponding to the previouswork [23] but caters to the two electrons in t// orbit in Goodenough'smodel, the metallic tendency possessed by MoO2 has been provedgenerally. Fig. 2c shows the band gap of MoO3 is equals to 1.844 eV,which is consistent with some previous works approximately [25,26],the total density of states (DOS) is mainly due to partial densities ofstates (PDOS) of 4d orbit in Mo and 2p orbit in O, which is conform tovalence electron laws. According to quantum theory, only electronsnear the Fermi level in MoO2 can make a contribution to the currentunder the applied electric field [27], which is beneficial to the related

supercapacitor applications. MoO3 with a higher band gap is the bestcandidate for optoelectronics applications, such as photocatalyst andnonlinear optical crystal.

To enhance the performance of the pure MoO2 and MoO3 crystal, anovel method to synthesize rGO/MoO2/g-C3N4 (GMCN) compositeswhich are on the basis of the rGO/MoO3 (GM3) composites. The pro-cessing is shown in Fig. 3a. The nanostructured GM3 composites couldbe synthesized hydrothermally according to the following reactions[28]:

Na2MoO4 + 2HCl → MoO3 (nanobelts) + H2O + 2NaCl (3)

Then the GM3 composites would be reduced under the Ar protectivegas at 520℃ according to Eq. (2). For the sake of enhancing the elec-trochemical properties of GM2 simultaneously, the melamine precursorof g-C3N4 was adopted to mix with GM3, then reduced at 520℃ forsynthesizing GMCN composites which deliver the double benefit. Toensure the purity and crystallinity of these samples, the tested XRDpatterns of pure g-C3N4, GM2, GM3, and GMCN are demonstrated inFig. S2. The peak located at 28.3° demonstrates the stacking interlayercharacter of g-C3N4, which is well consistent with the previous works[29]. It should be noticed that our synthesized g-C3N4 shows a meso-porous morphology observed from TEM images in Fig. 3d, the SEMimages inset and the Figs. S3–4 shows a commonsensible morphologyon a micron scale. Such porous structures possess a higher specificsurface area which used in electrochemical reactions. SEM image inFig. 3f and TEM image shows a coral-like GM2 morphology, whoseMoO2 nanorods are loaded on single-layered graphene directly inFig. 3g. More evidence of the single layer can be found in AFM images,from Figs. S5a and 5c we can see that a large number of graphene sheetswere observed on the vision, and the thickness measured by the

Fig. 4. XPS spectra of (a) GMCN sample and its energy regions of (d) Mo 3p +N1s and (g) C1s in high-resolution, XPS spectra of (b) GM2 sample and its high-resolution (e) Mo 3d and (h) C 1s energy regions, and XPS spectra of (c) GM3 sample and its high-resolution (f) Mo 3d and (i) C 1s energy regions.

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calibration is ~ 0.336 nm, which is almost equal to the well-knownthickness of a carbon atom (~ 0.34 nm) and proves that they are singlelayer. Besides, from the field at magnification in Figs. S5b and 5d, thegraphene layer has loaded with molybdenum oxide nanorods with aheight of ~ 4 nm. The corresponding XRD patterns of in Fig. S2 isidentical with the related peaks of JCPDS No. 32-0671 card, the strongpeaks at 26.0°, 37.0 and 53.5° matching with (1̄ 1 1) (2̄ 1 1), and (2̄ 2 2)lattice planes of MoO2, respectively. The main calculated lattice para-meters of MoO2 in Table S2 is a= 5.610 Å, b= 4.843 Å, c= 5.526 Åwhich is unanimous with XRD results. And peaks of GM3 sample showsa crystallinity (a= 3.962 Å, b= 13.858 Å, c= 3.697Å) that in ac-cordance with JCPDS No.05–0508 of MoO3 and the lattice parameters(a= 3.962 Å, b= 13.855 Å, c= 3.699Å) in our models in Table S1.Such results lead to not only the crystal perfection of samples despitethey were decorated with rGO but also the consistency of simulationmodel and experiments so that the assumption of different propertiesholds by two composites are persuasive. As a result, the initial

experimental results in exploring the SCs properties of GM2 and thephotocatalysis properties of GM3 are shown in Fig. 3b and c, respec-tively. Fig. 3b shows that the supercapacitance capacity of GM2-14samples is 950.87 F g−1 on first galvanostatic discharge-charge (GCD)at 1 A g−1, which is almost twice as much as GM3–14 samples at408.77 F g−1, besides, the capacitance of pure graphene is almost154.1 F g−1 in previous literature at 1 A g−1 [30], which proved thatMoO2 composites are the best candidate for supercapacitor applicationsrather than MoO3 composites. Fig. 3c shows that GM3 is a much betterphotocatalyst than GM2 in degrading TC in water solutions under thesimulated solar light. The only 1.36% degradation rate after 2 h of GM2shows metallicity of MoO2 from the DFT calculation results, while thedegradation rate of TC after the 1 h is over 40% at a random graphenecontent of GM3, which shows the real photocatalysis function of MoO3heterojunctions instead of MoO2 heterojunctions.

To further uncover the structure and elemental composition of thesesamples, X-ray photoelectron spectroscopy (XPS) and elemental

Fig. 5. GCD curves of (a) GM2-14 and (c) GMCN electrodes at different scan rates from 1–20 A g−1. CV curves of (b) GM2-14 and (d) GMCN at different scan ratesfrom 2 to 50mV s−1. (e) The cycle performance of GMCN electrodes measured at 3 A g−1 for 3000 times. (f) The last a dozen cycles of GMCN electrodes measured at10 A g−1.

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analysis were employed. As shown in Fig. 4a, GMCN is mainly includingMo, O, C, and N, and the peak ratio of C and N is nearly 3:4 because thecontent of graphene is weak, and it is close to the related atomic ratio ofgraphitic carbon nitride molecule. High-resolution Mo 3p +N1s peaksin Fig. 4d show the N1s spectrum deriving from NHx groups in theheptazine framework (N2C) from g-C3N4 at 397.8 eV. The peak locatedat 532.4 eV is attributed to adsorbed oxygen species (water andoxygen), and the 530.6 eV peak shows the Mo–O bonds, which aredemonstrated in Fig. S6c. And the peaks high-resolution Mo 3d in Fig.S6d fits well with that of GM2 in Fig. 4e, for example, the 229.4 eV and232.6 eV strong peaks are attributed to the binding energies of Mo 3d5/2and Mo 3d3/2 of Mo4+ the characteristic peaks [31] of GM2 are229.5 eV and 235.0 eV in Fig. 4e acts in the same way, which shows theMoO2 phase exists in GMCN samples rather than MoO3 phase. Theexistence of the characteristic peaks of the MoO2 and g-C3N4 phasesproves the feasibility of the sample prepared by the precursor blending.The characteristic peaks of GM2 and GM3 are very different, which isshown in Fig. 4e and f. 231.0 eV and 235.0 eV are derived from the Mo3d5/2 and Mo 3d3/2 [32] of Mo5+ due to the partial oxidation of theMoO2 phase. From Fig. 4f we can see that the maximum peak intensityof the Mo 3d5/2 component is found at 233.2 eV of Mo 3d5/2 and236.3 eV of Mo 3d3/2, which is identical to the decomposition of theNa2MoO4 precursor directly [33]. Comparing the peaks in C1s spectrumof three samples in Fig. 4g–i, GMCN, GM2 and GM3 samples possess thesame peak in 284.7 eV, which may stand for the C–C bonds of sp2

carbon from reduced graphene species, the distinct 285.5 eV and288.6 eV in Fig. 4g is the C–N bonds and C˭N bonds of sp3 carbon fromg-C3N4 phase, respectively [34]. From Fig. 4h and i we found that thearea of C˭O peaks is different because of the reduce imparity of the

oxygen-containing functional group, the GM2 sample was synthesizedat 520℃, thus the area of C˭O peaks in 287.8 eV is much lower thanthat in 285.9 eV of the GM3 sample. Besides, the area of Mo–O bonds ofGM2 sample in Fig. S6b is over 23,000, which is much higher than thatof GM3 sample at only 8322 in Fig. S6a, which proves that the goodmetallic performance of MoO2, and accumulate the evidence that GM2is the best candidate for supercapacitor applications rather than pho-tocatalysis applications.

Owing to the weak cycling of most electrode performance at a highcurrent density, GM2 sample is still worth improving, whose capaci-tance decay is shown in Fig. S9b, the retention is only 192 F g−1 re-mained after 1000 cycling numbers at a density of 10 A g−1, even if therate capacitance is much higher than GM3 samples. It is essential toadding electrochemical active materials except for expensive graphene,the introduction of porous g-C3N4 have received a good comprehensiveelectrochemical performance, which is shown in Fig. 5c–f. The corre-sponding capacitance can be calculated according to the equations asfollows:

=×

×

C I Δtm ΔV (4)

where I (A), m (g), Δt (s), and ΔV (V) is the current constant and themass of active materials, discharging time, and potential drop whendischarging, respectively. The mass of the active materials is nearly5mg, the theoretical specific capacity in Fig. 5a is 1042, 873, 731, 652,and 569 F g−1 at a current density of 1, 2, 5, 10, and 20 A g−1 re-spectively, and the calculated specific capacity shown in Fig. 5c can bereached to 1700, 1464, 1220, 1040, and 776 F g−1 at a same partialcurrent density of GMCN. These GCD curves of the ternary electrodes

Fig. 6. (a) Comparison of the capacitance performance of GMCN and GM2-14 electrodes. (b) Schematic structure of GMCN//AC asymmetric SCs devices. (c) GCD and(d) CV of GMCN//AC asymmetric SCs. (e) Nyquist plots of GMCN//AC asymmetric SCs. (f) The long cycle performance of GMCN//AC devices at 4 A g−1 for 3000times. The demonstration of the device: (g) A red LED powered by this device for more than 12min and (h) 21 LEDs powered by this device shows a “CUGB” pattern.(i) Comparison of this work with other devices in literature.

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are highly symmetric, deriving from the ideal capacitance behavior andthe fast Faraday reaction with the distinct surface chemical state andnanostructure of mesoporous g-C3N4. Besides, pyrrolic N defects in g-C3N4 have made a contribution for the high rate capacity [35], g-C3N4possesses a large number of defects in lattice and doubly bonded ni-trogen at the edges of the vacancy, which are beneficial for the ad-sorption and diffusion of ion [36]. On the other side, the g-C3N4 has alower band gap with calculated value 2.7 eV (Fig. S10a) and it is con-ducive to the formation of Mott-Schottky heterojunctions when com-positing with graphene oxide [37], which may improve the charge se-paration performance of this sample. As shown in Fig. 5d, the area ofCV curves is larger than that in Fig. 5b which shows the longest dis-charge lifetime at the scan rate of 2, 5, and 10mV s−1, the improvedcapacitance and electrochemical area may attribute to the g-C3N4 na-noparticles adsorbed closely on the surface of MoO3 nanorods, which isshown in TEM images and the EDX inset of Fig. 3h, besides, the betterelectric conductivity of g-C3N4/rGO that synthesized at 520℃ and theextending delocalized π-electron by activated inert g-C3N4 [38]. Andthe Nyquist plots in Fig. S11 shows there is no significant variation inslope which related to the electrochemical resistance by adding poorconductive bulk g-C3N4. In addition, the relatively low Rct values ofGMCN at 0.708Ω roughly estimated from Nyquist plots implies that theGMCN has smaller internal resistance and faster electrochemical ki-netics compared with GM3. (Table S3) The cycling lifetime is worthy ofsatisfaction that the capacitance retention is 81.5% after a long 3000cycles in Fig. 5e, the last 10 cycles exhibited in Fig. 5f. Thus, the as-prepared GMCN electrode as supercapacitor is competitive with otherMo-based energy storage materials.

Except for the gravimetric theoretical capacitances, the practicaldevices are demonstrated in Fig. 6. On account of the larger surface

area and fast ion diffusion arising from its distinct porous structure, theGMCN samples as the electrode has received almost as twice as thecapacity of GM2 electrode, which is recorded in Fig. 6a, the GMCN andactivated carbon (AC) materials were assembled as a button-likeasymmetric supercapacitor with 3M KOH electrolyte, the interiorstructure schematic is shown in Fig. 6b, whose MoO3 nanorods wereloaded with g-C3N4 nanoparticles. The GCD curves are stable at 1.2 Vindicating good capacitive behavior at a current density of 1, 2, 3, 5,and 10 A g−1 in Fig. 6c, which is corresponding to a practical specificcapacitance of 189.2, 163, 151.1, 119.0, and 76.7 F g−1. The GCDcurves of the GMCN//AC asymmetric supercapacitor shows small IRdrops at the current densities from 1 to 10 A g−1, indicating fast kineticsfor charge-storage processes [39]. Due to the asymmetric active mate-rials contributed in button cells, CV curves in Fig. 6d were not assymmetry as that in Fig. 5d, the scanning rate increases from 2 to100mV s−1 reveals fast charging and discharging characteristics. Asdisplayed in Fig. 6e, the Nyquist plots for in liquid electrolyte, exhibitthe imaginary part versus the real part of impedance [40] of GMCN atinitial status and after 3000 cycles. The semicircle in inset pictureshows a little larger diameter after 3000 cycles which declares a slightincrement of transfer resistance after long recurrence, a small incre-ment of diffusion resistance after cycling can also be observed from themildly reduced slope of the curve after intense cycles [35]. Fig. 6fshows the cycling performance at a current density of 4 A g−1 for over3000 cycles and a retention of 74.7%. The attenuation of the capaci-tance is owing to the decomposition of the non-compensated electrodeand the consumption of the electrolyte caused by an irreversible reac-tion between the active materials and electrolyte in a sealed device. Inorder to reflect the actual discharge effect applications of GMCN//ACdevice, we demonstrate such a power source to power a light-emitting

Fig. 7. UV–vis diffuse reflectance spectra of GM3 and the deduction of band gaps (inset figure). (b) The light absorption contribution of GM3 at (0 0 1), (0 1 0) and (10 0) lattice diffraction direction via DFT calculation. (c–d) The results of different photocatalysts on the photocatalytic degradation of TC under the simulated solarlight. (e) Kinetic rate constants of TC degradation over different contents of GM3. (f) The possible mechanisms of degrading TC under simulated solar light irradiationwith photocatalyst MoO3.

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diode (LED). As shown in the insets in Fig. 6g, two devices powered one3mm diameter red LEDs (1.8 V, 20mA) for more than 12min. At last,by welding 21 different colors of LEDs, and formed the word "CUGB" inparallel on a circuit board, which are powered by two devices for morethan 30 s, which is demonstrated in Fig. 6h and Movie S1. Fig. 6i showsthe measured energy density (E) and power density (P) of the GMCN//AC corresponding to 189.2, 163.0, 151.1, 119.0, and 76.7 F g−1, whichare calculated according to the equation as follows [41]:

Supplementary material related to this article can be found online atdoi:10.1016/j.nanoen.2018.09.028.

Since GM3 is estimated to have a good photocatalytic propertyderiving from its favorable theoretical band gap, the experimental bandgap width is obtained through UV–vis diffuse reflectance absorbancespectra, and the curve is shown in Fig. 7a. The absorption edge of MoO3is located at about 404 nm. The band gap of a semiconductor near theabsorption edge can be obtained according to the Kubelka-Munk (KM)expression [47]:

= −Αhν A hν E( )g n/2 (8)

where α, hν, A, and Eg are optical absorption coefficient, photonic en-ergy, the proportionality constant, and band gap, respectively. The n ofMoO3 is 1, and its band gap determined as 2.9 eV, which is in goodagreement with the previous literature [48]. From the DFT calculationresults in Fig. 7b, we can draw a conclusion that the contribution oflight absorption lies mainly in (1 0 0) plane of MoO3, and the result ofMoO2 structure cannot be drawn because of its low absorption. Thephotocatalytic activities of the as-prepared samples were evaluated viadegradation of tetracycline (TC) under simulated solar light irradiation(Fig. 7c). All the GM3 samples exhibited high photocatalytic perfor-mance for TC degradation, and the photocatalytic activity first in-creases with increasing the loading content of graphene from GM3-8 toGM3-13, and then decreases at GM3-18. It may be due to that the MoO3nanorods were wrapped by excessive rGO in GM3-18 [49], whichshields MoO3 from light irradiation. Significantly, GM3 composite ex-hibits a superior photocatalytic activity for TC degradation with theremoval efficiency of 90.6% of within 2 h irradiation. In contrast, only1.7% of TC were decomposed within the same time period in the pre-sence of GM2, which proved that MoO3/rGO is an efficient photo-catalyst for containment elimination compared to MoO2/rGO. The ab-sorption spectra demonstrate the gradual decomposition of TC withprolonging illumination time (Fig. S13). In order to figure out the re-action kinetics of TC degradation process quantitatively, Langmuir-Hinshelwood (L-H) kinetics model was adopted as shown in the fol-lowing equation [50,51]:

= −C C k tln( / )0 obs (9)

where kobs is the apparent pseudo-first-order rate constant (min−1), C0is the initial TC concentration (mol L−1), and C is the TC concentrationin aqueous solution (mol L−1) at time t. The kobs is less than 10−4

min−1, 0.0110min−1, 0.0186min−1, and 0.0116min−1 for the GM2-13, GM3-8, GM3-13, and GM3-18, respectively. (Fig. 7d and e) Thus,the optimal photocatalytic efficiency was obtained in GM3-13, which isover 180 times higher than that of GM2-13. The conduction band (CB)and valence band (VB) potentials of MoO3 can be deduced by the fol-lowing equations:

= − +E X E E0.5VB e g (10)

= −E E ECB VB g (11)

where EVB is the VB edge potential, X is the Mulliken electronegativityof MoO3, which is the geometric average of the absolute electro-negativity of the constituent atoms (X values of MoO3 is 6.39 eV de-riving from X value of O, and Mo are 7.54 eV and 3.9 eV [52], re-spectively); Ee is the energy of free electrons on the hydrogen scale (Ee≈ 4.5 eV) and Eg is the band gap energy of the MoO3. The EVB and ECBare calculated to be 3.34 eV and 0.44 eV, respectively (Fig. 7f). The

photocatalytic mechanism can be deduced as follows: Under irradia-tion, the photogenerated electrons (e-) jump to the CB of MoO3 andmeanwhile leave the holes (h+) in the VB. As graphene possesses a 2Dconjugated π structure and superior electrical conductivity, it serves asan excellent e- acceptor for swift charge transfer. Then, the h+ re-mained in the VB has a positive enough potential to oxidized H2O orOH- ions to produce abundant powerful hydroxyl radicals (•OH) at2.27 eV. •OH is unselective, and can destroy any organism. Under thesuccessive attack by •OH, TC was decomposed, and finally transformedinto CO2 and H2O through a series of possible transformation pathways[48]. Moreover, GMCN system is also a photocatalytic potential can-didate to discuss with, according to the band structure of g-C3N4 andMoO3 as well as the Fermi level of rGO, they possess overlappedmatchable band energy levels, which are conducive to the formation ofMott-Schottky heterojunctions. (Fig. S10b) rGO here not only serves asan excellent electron transfer platform but also enhances the bandbending of the semiconductor, providing new chemically active statesnear the interfaces. As illustrated in Fig. S10b, under light irradiation,the electrons are produced from MoO3 and g-C3N4. Because of the lowerFermi level of rGO compared to the conduction band position of twoother semiconductors, the electrons will transfer from MoO3 and g-C3N4to rGO. Owing to the super electroconductivity of rGO, the photo-generated electrons migrate swiftly, allowing the efficient charge se-paration and photocatalytic activity. Such evidence can be confirmed inphotoelectrochemical impedance spectroscopy in Fig. S11.

4. Conclusion

In this paper, we predicted the electronic structure and bandstructure of molybdenum oxide by Crystal Field Theory and DensityFunctional Theory, then proved two different suitable properties pos-sessed by MoO3 and MoO2 experimentally, SCs and photocatalytic TCdegradation, respectively. The MoO3/rGO (GM3) synthesized by theone-step hydrothermal method demonstrates robust photocatalytic ac-tivity for degrading tetracycline with a 90.6% removal efficiency within2 h simulated solar light irradiation. MoO2/rGO/g-C3N4(GMCN) com-posites have been synthesized via precursor mixing reduction method,possesses a theoretical capacity of 1700 F g−1 at a current density of1 A g−1, the button-like GMCN//AC SCs have a long-term cycling sta-bility with 74.7% capacitance retention at a current density of 4 A g−1

after 3000 cycles. The maximum energy density of 39.4W h kg−1 at ahigh density of 625W kg−1, as well as a maximum power density of6.25 kW kg−1 at an energy density of 16.0W h kg−1 are achieved at anoperating voltage of 1.2 V, and two devices have powered 21 LEDs.High capacity, satisfactory stability, and demonstrated effect possessedby the nanostructured GMCN may become one of the first choices forMolybdenum-based SCs.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (No. 51572246 and 51672258) and theFundamental Research Funds for the Central Universities (No.2652015425 and 2652017401).

Appendix A. Supplementary material

Supplementary data associated with this article can be found in theonline version at doi:10.1016/j.nanoen.2018.09.028.

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Liqi Bai is now pursuing his Ph.D. degree under the su-pervision of Prof. Yihe Zhang and Prof. Hongwei Huang atthe School of Materials Science and Technology, ChinaUniversity of Geosciences (Beijing). He obtained his B.S.degree in the Taiyuan University of Technology in 2017. Hehad won an honorable mentioned prize in TheInterdisciplinary Contest in Modeling in 2016. Now his re-search interest focuses on the design of defects for photo-catalysis and energy storage applications.

Yihe Zhang is a Professor at School of Materials Scienceand Technology and head of the Beijing Key Laboratory ofMaterials Utilization of Nonmetallic Minerals and SolidWastes, China University of Geosciences (Beijing). He re-ceived his Ph.D. from Technical Institute of Physics andChemistry, Chinese Academy of Sciences in 2005. His cur-rent research fields are focused on nanomaterials, mineralscomposites and their applications for the environment,energy, and biomaterials.

Likai Zhang received his B.S. degree at Taiyuan Universityof Technology, China in 2017. He is currently a mastercandidate of China University of Geosciences (Beijing) su-pervised by Associate Prof. Xiaowei Li. His current researchinterest is computational study of lithium ion batteries withelectrode containing transition elements.

Yuanxing Zhang is a master candidate supervised byAssociate Prof. Li Sun at the Beijing Key Laboratory ofMaterials Utilization of Nonmetallic Minerals and SolidWastes, School of Materials Science and Technology, ChinaUniversity of Geosciences (Beijing). His research field fo-cuses mainly on micro/nano materials for energy storageand conversion.

Li Sun is an Associate Professor at School of MaterialsScience and Technology, China University of Geosciences(Beijing). She received her B.S. and M.S. degree fromTsinghua University and a Ph.D. degree from Hong KongPolytechnic University. Her research fields are mainly fo-cused on the preparation, structure and mechanical prop-erties of electrode materials for advanced batteries such aslithium-ion batteries and lithium-sulfur batteries.

Ning Ji is a master candidate supervised by Prof. HongweiHuang at the Beijing Key Laboratory of MaterialsUtilization of Nonmetallic Minerals and Solid Wastes,School of Materials Science and Technology, ChinaUniversity of Geosciences (Beijing). His research interestsfocus on the design and synthesis of layered photocatalystsfor environmental and energy applications.

Xiaowei Li is an Associate Professor at School of MaterialsScience and Technology, China University of Geosciences(Beijing). She received her Ph.D. degree from NanjingUniversity and had made her postdoctoral research inPeking University. Her main research direction is the designand simulation of low dimension materials and novel pho-toelectronic and magnetic functional materials.

Haochen Si is a master candidate supervised by AssociateProf. Li Sun at the Beijing Key Laboratory of MaterialsUtilization of Nonmetallic Minerals and Solid Wastes,School of Materials Science and Technology, ChinaUniversity of Geosciences (Beijing). His research field fo-cuses mainly on micro-nano materials for energy storageand conversion.

Yu Zhang is a master candidate supervised by Prof. YiheZhang and Associate Prof. Li Sun at the Beijing KeyLaboratory of Materials Utilization of Nonmetallic Mineralsand Solid Wastes, School of Materials Science andTechnology, China University of Geosciences (Beijing). Hisresearch field focuses mainly on nanostructured materialsfor electrochemical energy storage.

Hongwei Huang is a Professor at School of MaterialsScience and Technology, China University of Geosciences(Beijing). He received his Ph.D. in 2012 from TechnicalInstitute of Physics and Chemistry, Chinese Academy ofSciences. His current research fields mainly focus on thedesign and synthesis of layered nanomaterials and func-tional crystals and their applications in environment andenergy.

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https://doi.org/10.1002/adfm.201102306https://doi.org/10.1021/ic00277a030https://doi.org/10.1021/ic00277a030

Jahn-Teller distortions in molybdenum oxides: An achievement in exploring high rate supercapacitor applications and robust photocatalytic potentialIntroductionExperimental detailsPreparation of the agentsPreparation of GM2 and GM3 solidsPreparation of g-C3N4 powders and GMCN compositeCharacterization of GM2, GM3, g-C3N4 and GMCN powdersElectrochemical measurementsFabrication of the GMCN//AC SCsPhotocatalytic evaluationFirst-principles calculation

Results and discussionConclusionAcknowledgmentsSupplementary materialReferences