Black Phosphorus/Platinum Heterostructure: A Highly ... · chemistry of Pt/BP. The binding energy of PtNPs on the BP nanosheets is found to be as large as 13.6 eV due to multi-Pt

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Black Phosphorus/Platinum Heterostructure: A Highly Efficient Photocatalyst for Solar-Driven Chemical Reactions

Licheng Bai, Xin Wang, Shaobin Tang, Yihong Kang, Jiahong Wang, Ying Yu, Zhang-Kai Zhou, Chao Ma, Xue Zhang, Jun Jiang, Paul K. Chu, and Xue-Feng Yu*

Dr. S. TangKey Laboratory of Organo-Pharmaceutical Chemistry of Jiangxi ProvinceGannan Normal UniversityGanzhou 341000, P. R. ChinaDr. Y. Yu, Prof. Z.-K. ZhouState Key Laboratory of Optoelectronic Materials and TechnologiesSchool of PhysicsSun Yat-sen UniversityGuangzhou 510275, P. R. ChinaProf. C. MaCollege of Materials Science and EngineeringHunan UniversityChangsha 410082, P. R. ChinaProf. P. K. ChuDepartment of Physics and Department of Materials Science and EngineeringCity University of Hong KongTat Chee Avenue, Kowloon, Hong Kong, China

DOI: 10.1002/adma.201803641

natural resource depletion, pollution, and possible global warming. Solar-driven photocatalysis for chemical reactions can convert solar energy into the chemical bonds of the product molecules thus pro-viding a promising approach to address the energy crisis.[4–6] To fully utilize solar energy in chemical production, there are three goals for photocatalysts: robust har-vesting of solar light spanning a broad spectrum, high charge separation and migration to the catalyst surface, and effective coupling of the photogenerated carrier into chemical reactions.

Black phosphorus (BP), an emerging 2D semiconductor with a narrow bandgap and high charge carrier mobility,[7–9] has attracted a great deal of attention in elec-tronics and optoelectronics.[10–14] The layer-dependent direct bandgap ranging between 0.3 and 2.0 eV makes BP a promising absorber of broad solar light extending into the infrared region.[12,15–18] Furthermore, the 2D structure enhances

charge separation by decreasing the diffusion length[19] and provides numerous reaction sites due to the large surface area.[20] These properties of BP are unique from the perspective of solar energy conversion by photocatalysis and have been demonstrated in H2 evolution and degradation reactions.[21–26]

A 2D black phosphorus/platinum heterostructure (Pt/BP) is developed as a highly efficient photocatalyst for solar-driven chemical reactions. The heterostructure, synthesized by depositing BP nanosheets with ultrasmall (≈1.1 nm) Pt nanoparticles, shows strong Pt–P interactions and excellent stability. The Pt/BP heterostructure exhibits obvious P-type semiconducting characteristics and efficient absorption of solar energy extending into the infrared region. Furthermore, during light illumination, accelerated charge separation is observed from Pt/BP as manifested by the ultrafast electron migration (0.11 ps) detected by a femtosecond pump-probe microscopic optical system as well as efficient electron accumulation on Pt revealed by in situ X-ray photoelectron spectroscopy. These unique properties result in remarkable performance of Pt/BP in typical hydrogenation and oxidation reactions under simulated solar light illumination, and its efficiency is much higher than that of other common Pt catalysts and even much superior to that of conventional thermal catalysis. The 2D Pt/BP heterostructure has enormous potential in photochemical reactions involving solar light and the results of this study provide insights into the design of next-generation high-efficiency photocatalysts.

Black Phosphorus

Catalytic processes are the most efficient ways to produce chemical compounds via chemical reactions.[1–3] However, in most commercial reactions, a significant amount of thermal energy is required and the associated consumption of nonre-newable fossil energy is known to cause problems such as

Dr. L. Bai, Dr. X. Wang, Dr. Y. Kang, Dr. J. Wang, Dr. X. Zhang, Prof. X.-F. YuCenter for Biomedical Materials and InterfacesShenzhen Institutes of Advanced TechnologyChinese Academy of SciencesShenzhen 518055, P. R. ChinaE-mail: [email protected]. S. Tang, Prof. J. JiangHefei National Laboratory for Physical Sciences at the MicroscaleCollaborative Innovation Center of Chemistry for Energy MaterialsCAS Center for Excellence in NanoscienceSchool of Chemistry and Materials ScienceUniversity of Science and Technology of ChinaHefei 230026, P. R. China

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201803641.

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However, the ultrashort charge carrier lifetime of several pico-seconds of BP[13,27] and instability during light illumination[28] are serious issues currently stifling photochemical applications.

It is known that a semiconductor–metal heterostructure can quickly trap photoexcited electrons and holes to prolong the charge carrier lifetime and improve the charge separa-tion and utilization efficiency in photocatalysis by Schottky junctions.[29–32] Herein, a novel P-type semiconductor–metal heterostructure comprising BP nanosheets and ultrasmall Pt nanoparticles (PtNPs) is developed to be a highly efficient photocatalyst for solar-driven chemical reactions. By fabricating ultrasmall (≈1.1 nm) PtNPs on BP nanosheets, the strong Pt–P interaction produces a PtPxOy complex on the surface with greatly enhanced stability. The Pt/BP heterostructure exhibits obvious P-type semiconducting characteristics and efficient absorption of solar energy extending into the infrared region. Under light illumination, the Pt/BP heterostructure shows not only accelerated charge separation with an unprecedented ultrafast electron migration time of 0.11 ps as monitored by a femtosecond pump-probe microscopic optical system, but also efficient accumulation of photogenerated electrons on the

ultrasmall PtNPs demonstrated by in situ X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calcula-tion. In hydrogenation and oxidation reactions driven by sim-ulated solar light, Pt/BP exhibits much higher efficiency than other typical Pt catalysts and the performance is even superior to that of conventional thermally driven catalysis. The proposed Pt/BP photocatalyst thus has great potential in solar-driven chemical reactions.

The Pt/BP heterostructures are synthesized from BP nanosheets, which are exfoliated from ground powders of bulk BP by ultrasonication.[33] By using in situ chemical reduction, PtNPs are grown on the BP surface in ethanol containing a controlled oxygen concentration at an elevated temperature (60 °C).[34] The morphologies of the BP nanosheets (Figure S1, Supporting Information) and Pt/BP heterostructures (Figure 1a,b) examined by transmission electron microscopy (TEM) and atomic force microscopy (AFM) reveal an average size and thickness of about 500 and 23 nm, respectively. As shown by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM, Figure 1c), the PtNPs with a diameter of about 1.1 nm are uniformly distributed

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Figure 1. Structural characterization of the Pt/BP heterostructures. a) TEM image of Pt/BP. Inset: photograph of Pt/BP dispersed in ethanol. b) AFM image of a typical Pt/BP heterostructure. c) HAADF-STEM image of Pt/BP. Inset: corresponding particle size distribution of PtNPs. d) High-resolution HAADF image of Pt/BP. Inset: atomic-resolution HAADF image of Pt/BP. Scale bar, 1 nm. e) High-resolution TEM image of Pt/BP. Scale bar, 1 nm. f,g) STEM-EDS elemental mappings of a selected area on Pt/BP indicated by the dashed line in (c).



on the entire surface of the BP nanosheets. The PtNPs with such a small size give rise to good catalytic activity according to a recent study.[34] The crystal structure of the Pt/BP het-erostructure is characterized by high-resolution STEM, TEM (Figure 1d,e), as well as X-ray diffraction (XRD, Figure S2b, Supporting Information). The PtNPs lack a crystalline struc-ture due to the ultrasmall size as discussed previously.[34–36] The chemical composition of the Pt/BP heterostructure is determined by energy-dispersive X-ray spectroscopy (EDS) which confirms the uniform distribution of PtNPs on the BP nanosheets (Figure 1f,g). For comparison, three other kinds of Pt catalysts including Pt/P25, Pt/SiO2, and commercial Pt/C are prepared (Figures S3–S5, Supporting Information). Pt/P25 and Pt/SiO2 are synthesized by a method similar to that of Pt/BP and the PtNPs dispersed on the surface have the same average size as that of Pt/BP. Commercial Pt/C is a Pt catalyst purchased from Sigma-Aldrich LLC.

The environmental instability of BP nanosheets impacts their application and so the stability of the BP nanosheets before and after Pt decoration is evaluated by examining the morphology and optical absorption. The reduced absorption (Figure S6a, Supporting Information) and appearance of water droplets on the surface indicate oxidization and degra-dation of the BP nanosheets (Figure 2a). As shown by recent studies,[28] the contact between BP nanosheets and oxygen leads to the formation of PxOy on the surface and light illu-mination accelerates oxidation. In the presence of water, the produced PxOy reacts with water to form phosphoric acid, resulting in quick removal of PxOy and continuous oxidation and degradation of BP.[37]

In contrast, the absorbance of the Pt/BP aqueous solution is maintained for 7 d (Figure S6b in the Supporting Information and Figure 2c). The morphology shown by TEM (Figure 2b) and AFM (Figure S7, Supporting Information) clearly indicates the enhanced stability after Pt decoration. In fact, during storage under ambient conditions, the Pt/BP heterostruc-tures are stable for at least 15 days without noticeable degra-dation (Figure S8, Supporting Information). To investigate the mechanism of the enhanced stability, first-principles simula-tion by DFT and XPS are performed to investigate the bonding chemistry of Pt/BP. The binding energy of PtNPs on the BP nanosheets is found to be as large as 13.6 eV due to multi-Pt atom adsorption (Figure S9, Supporting Information) and it is significantly stronger than that of O atoms on BP (2.06 eV).[38] The large binding energy indicates strong Pt–BP interac-tion which facilitates efficient deposition of PtNPs on the BP nanosheets.

The chemical states of the PtNPs are determined from the Pt 4f core-level XPS spectrum which is fitted with the spin-orbit split 4f7/2 and 4f5/2 components (Figure 2d). Comparing the Pt 4f peaks (73.4 eV) obtained from Pt/P25 and Pt/SiO2, the peak at 72.9 eV observed from Pt/BP suggests the formation of an additional bond between Pt and P (Figure S10, Supporting Information). In comparison with the pristine BP, the corre-sponding core-level peak shift of P 2p is observed from Pt/BP (Figure 2e) and the P 2p spectrum at 134 eV is analyzed quan-titatively (Figure S11, Supporting Information). The results corroborate the strong Pt–BP interaction. In addition, the two unchanged O1s peaks near 532.7 and 531.4 eV verify the original state of BP with a native oxide and the weak oxidation

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Figure 2. Stability mechanism of the Pt/BP heterostructures. a) TEM image of a typical BP nanosheet stored in water for 1 d. b) TEM image of a typical Pt/BP heterostructure stored in water for 7 d. c) Visible light absorption of BP and Pt/BP stored in water. d) Pt 4f core level XPS spectra of Pt/BP, Pt/P25, and Pt/SiO2. e) P 2p core level XPS spectra of the Pt/BP heterostructure and BP nanosheets. f) O 1s core level XPS spectra of the Pt/BP heterostructures and BP nanosheets.



peak at 530.5 eV may arise from PtO bond (Figure 2f).[34,36,39] These results reveal the formation of a PtPxOy complex on the surface of Pt/BP. The strong Pt–P interaction and PtPxOy complex formed on the surface prevent degradation of BP nanosheets consequently leading to excellent stability.

Diffuse reflectance UV–vis–NIR spectroscopy is employed to determine the absorption properties of the Pt/BP heterostruc-tures (Figure 3a). Both Pt/BP and pristine BP exhibit strong light absorption extending into the infrared region (>1800 nm) comparable to P25 and Pt/P25. The positive Seebeck coefficient measured from Pt/BP by a thermoelectric parameter testing system (Figure S12, Supporting Information) suggests P-type semiconducting characteristics.[40] Hence, Pt/BP is a P-type semiconductor–metal heterostructure with a broad adsorption range from the ultraviolet to infrared regions.

Accelerated migration of photon-excited electrons in the Pt/BP heterostructures is corroborated by a femtosecond pump-probe dark field microscopic optical system (Figure S13, Supporting Information).[13,41] A single nanosheet is pumped by a 400 nm femtosecond laser and probed by 660 nm. As shown in Figure 3b, the signal from the pristine BP nanosheet increases rapidly upon excitation, followed with a decay of 2.8 ps. The signal detected at 660 nm (1.879 eV) is dominated by the free electrons in the conductive band of BP nanosheets, while the signal of trapped electrons from BP nanosheets would appears around 950 nm.[42] The same measurement is conducted on the Pt/BP heterostruc-ture and a significantly shorter delay time of 0.11 ps is observed, which reveals the existence of a short-cut path for migration of free electrons out of BP nanosheet.

To study the migration and transfer of photogenerated electrons between BP and ultrasmall PtNPs, in situ XPS is performed to confirm injection and accumulation of electrons

on PtNPs in the Pt/BP heterostructures. A 100 mW laser with a wavelength of 521 nm is used to illuminate the Pt/BP through an observation window made of lead glass (Figure S14, Supporting Information). As shown in Figure 3c, the Pt 4f core level shifts by 0.2 eV toward the lower binding energy under illumination, whereas the Pt 4f of Pt/BP in the dark and C1s reference peak show no significant change (Figure S15, Supporting Information), suggesting partial reduction of Pt resulting from the accumulation of excess electrons. The TEM images in Figure S16 (Supporting Information) confirm that the morphology of Pt/BP is intact before and after in situ XPS. Moreover, the Pt 4f core level remains unchanged under light irradiation in the same measurement conducted on Pt/SiO2 and commercial Pt/C (Figure S17, Supporting Information). The charge density difference of Pt/BP with and without the extra carrier is computed and displayed in Figure 3d,e. The transient absorption and in situ XPS experiments indicate that the PtPxOy layer does not impede migration of photogenerated electrons from BP to PtNPs. Therefore, we can simplify the structure model of the Pt/BP heterostructure by neglecting the PtPxOy layer in the calculation. It is noted that when photogen-erated electrons are injected into Pt/BP, they are mainly located at some of the top Pt atoms of Pt NP rather than BP surface. The results thus demonstrate the unique photoresponse of Pt/BP and reveal accumulation of electrons on the PtNPs under illumination.

With unambiguous spectral and calculation verification, the overall electron migration process in the Pt/BP heterostruc-tures is schematically illustrated in Figure 3f. When the PtNPs are in contact with the BP nanosheets, a Schottky barrier forms at the interface and is pinned to about ½ of the bandgap of BP resulting in downward band bending in the BP nanosheets.

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Figure 3. Photoelectric properties of the Pt/BP heterostructures. a) UV–vis–NIR spectra of the BP nanosheets, Pt/BP heterostructures, P25 NPs, and Pt/P25 heterostructures. b) Ultrafast transient signal (probed at 660 nm) as a function of probe delay for Pt/BP heterostructure with BP nanosheet as the reference, recorded with a 400 nm pump. c) Pt 4f core level XPS spectra of Pt/BP under 521 nm light illumination and in darkness. The charge density difference of Pt/BP with d) the neutral condition and e) one extra carrier [one photogenerated electron (e−)], respectively. f) Schematic of the band structure at the Pt/BP interface and corresponding migration of photogenerated electrons.



This downward band bending produces a built-in voltage in the BP facilitating migration of the excited electrons toward the Pt/BP interface.[43] Following migration to the Pt/BP interface, the electrons are injected into the PtNPs and become undetect-able by the probing light, resulting in the ultrafast decay in the pump-probe delay time. The ultrafast migration and accumula-tion of photon-excited electrons suggest a large potential of Pt/BP in electron-assisted photochemical reactions.

Based on the excellent photoelectric properties, the Pt/BP heterostructures are expected to have enhanced catalytic activity in electron-assisted photochemical reactions. A sty-rene hydrogenation reaction is performed under simulated solar light irradiation (Figure S18, Supporting Information). As shown in Figure 4a, Pt/BP possesses remarkable photo-catalytic activity that is not only higher than that of commer-cial Pt/C, but also Pt/P25 and Pt/SiO2 with the same Pt size of 1.1 nm. 100% conversion of styrene is observed from Pt/BP in 25 min at a molar ratio of 4: 104 (Pt: styrene) and with-drawal of Pt/BP from the reactants results in immediate ter-mination of the reaction (Figure S19, Supporting Information) confirming the heterogeneous catalytic process. It is noted that the pristine BP nanosheets show inactivity in this photocatalytic reaction (Figure S20, Supporting Information) and the Pt/BP with thick BP nanosheets also show poor photocatalytic activity (Figure S21, Supporting Information).

The turnover frequencies (TOFs) of Pt/BP in styrene hydro-genation under different illumination and thermal conditions are shown in Figure 4b with reference to other Pt catalysts. At room temperature (r.t. = 28 °C) and without light illumination,

Pt/BP, Pt/P25, and Pt/SiO2 show similar intrinsic TOFs of about 1350/h, which is better than that of commercial Pt/C (TOF is 700 h−1) due to the ultrasmall PtNPs.[34] Under simu-lated solar light irradiation at r.t., Pt/BP shows an approxi-mately fourfold (400%) increase in TOF reaching 5900 h−1 and excellent photocatalytic activity. In contrast, Pt/P25, a widely investigated photocatalyst, only shows an ≈36% increase in TOF (1731 h−1) and no detectable enhancement in TOF can be observed from Pt/C and Pt/SiO2, which have been predicted to have no photocatalytic capability. Even when the temperature is increased to 60 °C without illumination, the enhancement of TOFs is only 28–55% for these four catalysts. It is found that photocatalysis of Pt/BP with simulated solar light exhibits threefold increase in TOF compared to thermally driven catal-ysis at 60 °C, demonstrating its great potential in solar-driven chemical reactions.

The correlation between the photogenerated electrons and activity of the catalyst is further investigated according to the reaction mechanism. Dissociation of molecular H2 occurs easily on the Pt surface[34] and the primary isotope effect is observed with the ratio of the reaction rates (KH/KD) being as large as 3.2 (Figure 4c), suggesting that Had atoms insertion into styrene is the rate-limiting step in hydrogenation.[4,34,44,45] The effects of the photogenerated electrons on hydrogen insertion are evaluated by analyzing the Had atoms insertion barrier on the Pt surface and corresponding reaction pathways are shown in Figure 4d. Had atoms insertion into styrene involves two main processes: hydrogen insertion into β-carbon and α-carbon. The Had atoms insertion barrier of β-carbon and α-carbon is

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Figure 4. Photocatalytic activity and mechanism of Pt catalysts in hydrogenation of styrene. a) Plots of conversion of styrene toward ethylbenzene against reaction time under simulated solar light illumination (Pt: styrene = 4.1 × 10−2 mol%). b) TOFs of various Pt catalysts in hydrogenation of styrene under different illumination and thermal conditions (Pt: styrene = 4.1 × 10−3 mol%). c) Primary isotope effect observed with Pt/BP in the hydrogenation of styrene. d) Had atoms insertion barrier of α-carbon and β-carbon of styrene with and without additional 0.5 electron.



reduced by the injected 0.5 electron by 0.18 and 0.86 eV, respec-tively. The significant decrease in the hydrogen insertion barrier enhances the hydrogenation efficiency due to photogenerated electrons. The photogenerated holes are consumed during oxi-dation in ethanol,[46,47] which is used as a solvent and electron donor to enhance the photocatalytic electron–hole separation. Furthermore, owing to the enhanced stability, the recovered Pt/BP can be used repeatedly at least five times without notice-able structural or morphological changes (Figures S22 and S23, Supporting Information).

Besides CC hydrogenation, Pt/BP shows threefold enhancement of TOFs in hydrogenation of benzalde-hyde under simulated solar light irradiation (Figure S24a, Supporting Information), demonstrating its excellent photocatalytic activity of CO hydrogenation. Further-more, Pt/BP shows a 13-fold enhancement of TOFs in the oxidation of benzyl alcohol under simulated solar light irradiation (Figure S24b, Supporting Information). In photo-oxidation, the photogenerated holes attract hydrogen atoms from CH2OH of benzyl alcohol. The photoinduced benzyl alcohol radicals automatically release another electron to form benzaldehyde due to the current-doubling effect.[48] On the other hand, the photogenerated electrons increase the overall reaction rate through the assisted O2

− dissocia-tion process which is the rate limiting step, thereby facili-tating the oxidation reaction. The photogenerated electrons increase the photo-oxidation reaction rate by the electron-assisted O2 dissociation process.[5] These results confirm the high efficiency of Pt/BP in solar energy conversion for chemical reactions.

In summary, Pt/BP, a new 2D semiconductor–metal het-erostructure, is developed as a solar photocatalyst for chemical reactions. The Pt/BP heterostructure exhibits excellent stability and broad adsorption range extending into the infrared region (>1800 nm). By employing a femtosecond pump-probe micro-scopic optical system and in situ XPS, ultrafast photogenerated electron migration (0.11 ps) in BP and efficient electron accu-mulation in Pt in the semiconductor–metal heterostructure are confirmed. These unique properties result in remarkable photocatalytic performance of Pt/BP in both hydrogenation and oxidation of organic compounds under simulated solar light illumination and its efficiency is much higher than that observed from three other common Pt catalysts. In sty-rene hydrogenation, the Pt/BP photocatalysis shows threefold increase in TOF compared to thermally driven catalysis at 60 °C. This 2D Pt/BP heterostructure with broad solar light adsorption and efficient electron transfer and accumulation is promising in solar-driven chemical reactions. Furthermore, this investigation reveals the migration and accumulation pro-cesses of photogenerated electrons in semiconductor–metal heterostructures. The technique and physical understanding provide insights into the design of future high-efficiency photocatalysts.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsL.B. and X.W. contributed equally to this work. This work was jointly supported by the Key Research Program of Frontier Sciences, CAS (QYZDB-SSW-SLH034), Shenzhen Science and Technology Research Funding Nos. JCYJ20170818163003088 and JCYJ20160229195124187, China Postdoctoral Science Foundation Nos. 2017M612759 and 2017M622824, Leading Talents of Guangdong Province Program No. 00201520, Chinese Academy of Sciences Technology Service Network Program (KFJ-STS-SCYD-102), and Hong Kong Research Grants Council (RGC) General Research Funds (GRF) Nos. CityU 11301215 and 11205617.

Conflict of InterestThe authors declare no conflict of interest.

Keywords2D materials, black phosphorus, heterostructures, photocatalysis, ultrasmall platinum nanoparticles

Received: June 8, 2018Revised: August 2, 2018

Published online: September 2, 2018

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Adv. Mater. 2018, 30, 1803641

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2018.

Supporting Information

for Adv. Mater., DOI: 10.1002/adma.201803641

Black Phosphorus/Platinum Heterostructure: A HighlyEfficient Photocatalyst for Solar-Driven Chemical Reactions

Licheng Bai, Xin Wang, Shaobin Tang, Yihong Kang, JiahongWang, Ying Yu, Zhang-Kai Zhou, Chao Ma, Xue Zhang, JunJiang, Paul K. Chu, and Xue-Feng Yu*

1

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2018.

Supporting Information

Black phosphorus/platinum heterostructure: a highly efficient photocatalyst for solar-

driven chemical reactions

Licheng Bai, Xin Wang, Shaobin Tang, Yihong Kang, Jiahong Wang, Ying Yu, Zhang-Kai

Zhou, Chao Ma, Xue Zhang, Jun Jiang, Paul K Chu, and Xue-Feng Yu*

Dr. L. Bai, Dr. X. Wang, Dr. Y. Kang, Dr. J. Wang, Dr. Xue Zhang, Prof. X-F. Yu

Center for Biomedical Materials and Interfaces, Shenzhen Institutes of Advanced Technology,

Chinese Academy of Sciences, Shenzhen, 518055, P. R. China;

E-mail: [email protected]

Dr. S. Tang, Prof. J. Jiang

Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation

Center of Chemistry for Energy Materials, CAS Center for Excellence in Nanoscience,

School of Chemistry and Materials Science, University of Science and Technology of China,

Hefei, 230026, P. R. China;

Dr. Y. Yu, Prof. Z-K. Zhou

State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, Sun

Yat-sen University, Guangzhou 510275, P. R. China;

Prof. C. Ma

College of Materials Science and Engineering, Hunan University, Changsha, 410082, P. R.

China;

Prof. P. K. Chu

Department of Physics and Department of Materials Science and Engineering, City University

of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

Dr. S. Tang

Key Laboratory of Organo-Pharmaceutical Chemistry of Jiangxi Province, Gannan Normal

University, Ganzhou, 341000, P. R. China

2

Experimental Section

1. Materials

The black phosphorus (BP) crystals were purchased from Mophos (Mophos.cn). NMP

(99.5 %), ethanol (99.8 %), benzyl alcohol (99.5 %), and ammonium hydroxide (28 wt%)

were obtained from Aladdin Reagents. Sodium dodecyl benzene sulfonate (SDBS),

potassium tetrachloroplatinate (II) (K2PtCl4, 99.9 %), formaldehyde (HCHO, 37 wt%), 3-

aminopropyltriethxoysilane (APS), tetraethylorthosilicate (TEOS), styrene (99 %), and

benzaldehyde (99.5 %) were purchased from Sigma-Aldrich. Aeroxide TiO2 (P25) was

purchased from Acros Organics.

All the chemicals were used as received without further

purification.

2. Characterization

High-resolution transmission electron microscopy (HRTEM) and scanning transmission

electron microscopy (STEM) were performed on a JEM-ARM200CF probe aberration

corrected 200 kV STEM/TEM with a cold field emission source and 0.35 eV energy

resolution at 200 kV. The X-ray diffraction (XRD) patterns were recorded on a Rigaku

SmartLab X-ray diffractometer equipped with Cu Kα radiation and D/teX Ultra detector

scanned from 10 ° to 90 ° of 2θ at a rate of 10 °/min. Elemental analysis was conducted by

inductively-coupled plasma mass spectrometry (ICP-MS) on the Agilent 7500CE. X-ray

photoelectron spectroscopy (XPS) was carried out a Thermo Fisher ESCALAB 250i XPS

equipped with monochromatic Al Kα radiation. The Seebeck coefficients of Pt/BP were

measured on a thermoelectric parameter test system (MRS-3, JiaYiTong Company).

3. Catalyst preparation.

Synthesis of BP nanosheets. The BP nanosheets were prepared with an ultrasonic probe

followed by bath sonication of the ground powders of bulk BP[S1]

. In brief, 15 mg of bulk BP

were added to 30 mL of NMP and sonicated with a sonic tip for 3 h with a power of 1200 W.

The mixture was agitated in an ultrasonic ice bath continuously for 10 h at a power of 300 W.

3

After exfoliation, the solution was centrifuged at 4,000 rpm for 10 min to remove non-

exfoliated bulk BP. The supernatant was centrifuged at 7,000 rpm for 20 min, washed with

ethanol twice. The concentration of BP sheets was determined by the adsorption intensity at

808 nm.

Synthesis of Pt/BP heterostructure. 2.7 mg of the BP nanosheets were dispersed in a mixture

containing 16.5 mL of ethanol, 24.6 mL of water, and 0.12 g of SDBS at 28 °C. 4.5 µL of

ammonia hydroxide (28 wt%), 75 µL of HCHO (37 wt%), and 295 µL of K2PtCl4 (0.1 M)

were added to the solution under stirring, followed by purging with nitrogen for another 30

min. After stirring for 1 h, the temperature was raised to 60 °C and maintained at 60 °C for 2

h. The solid was collected by centrifugation and washed three times with ethanol. The

obtained catalyst was kept in 5 mL of ethanol.

Synthesis of Pt/P25 catalyst. 200 mg of the P25 nanoparticles were dispersed in a mixture

containing 10 mL of ethanol, 20 mL of water, and 0.06 g of SDBS at 28 °C. 4 µL of

ammonia hydroxide (28 wt%), 80 µL of HCHO (37 wt%), and 1 mL of K2PtCl4 (0.1 M) were

added to the solution under stirring. After stirring for 24 h, the temperature was raised to

60 °C and maintained at 60 °C for 2 h. The solid was collected by centrifugation and rinsed

with ethanol three times. The obtained catalyst was kept in 5 mL of ethanol.

Synthesis of Pt/SiO2 catalyst. The silica nanospheres (~150 nm) were prepared by a modified

Stöber method. Typically, 3 mL of tetraethylorthosilicate (TEOS) were added to a mixture

containing 59 mL of ethanol, 5 mL of deionized water, and 2.5 mL of ammonia hydroxide (28

wt%). After stirring for 12 h, the silica nanospheres were collected by centrifugation, washed

with ethanol, and re-dispersed in 20 mL of isopropanol. Then, 0.02 mL of APS were added to

the silica sol and refluxed at 70 °C for 10 h. The amino-functionalized silica nanospheres

were isolated by centrifugation, washed with water/ethanol, and re-dispersed in 30 mL of

ethanol. To obtain Pt/SiO2, 6 mL of the amino-functionalized silica sol in ethanol were mixed

with 4 mL of ethanol, 25 mL of water, 0.04 mL of ammonia hydroxide (28 wt%), and 0.06 g

4

of SDBS at 28 °C. Subsequently, 0.08 mL of HCHO (37 wt%) and 0.6 mL of K2PtCl4 (0.1

M) were added to the solution under stirring. After stirring for 24 h, the temperature was

raised to 70 °C and maintained 70 °C for 10 h. The solid was collected by centrifugation and

rinsed with ethanol/water three times. Finally, the materials were dried at 60 °C for 12 h.

4. Catalytic reactions.

Photocatalytic hydrogenation of styrene. In the typical reaction, 0.87 mmol of styrene and

the catalyst (Pt: 4.1*10-2

mol% of styrene) were mixed with 2 mL of ethanol in a 25 mL two-

neck flask. A H2 balloon was tied above the flask and quickly purged with H2. The system

was kept at a certain temperature (r.t. or 60 °C) under magnetic stirring and irradiated with a

Xe light source with a power density of 150 mW cm-2

through simulated solar-light filters. A

syringe was used to collect aliquots of the products at certain time intervals. The products

were identified and quantified by gas chromatography-mass spectrometry (GC-MS, Agilent

5975C with a thermal conductivity detector) to calculate the conversion and selectivity of the

reaction. In calculating TOF, 8.7 mmol of styrene, 2 mL of ethanol, and a certain amount of

catalyst (Pt: 4.1*10-3

mol% of styrene) were used in a single reaction.

Photocatalytic hydrogenation of benzaldehyde. In the typical reaction, 0.2 mmol of

benzaldehyde and the catalyst (Pt: 5.2*10-2

mol% of benzaldehyde) were mixed with 2 mL of

ethanol in a 25 mL two-neck flask capacity. The reaction conditions were similar to those in

styrene hydrogenation. The products were identified and quantified by GC-MS.

Photocatalytic oxidation of benzyl alcohol. In the typical catalytic oxidation reaction, 0.2

mmol of benzyl alcohol and the catalyst (Pt: 5.2*10-2

mol% of benzyl alcohol) were mixed

with 1 mL of toluene in a 20 mL glass vial, with the headspace connected to the air via a

needle. The system was kept at a certain temperature (r.t. or 60 °C) under magnetic stirring

and irradiated with the same Xe light source with simulated solar-light filters. The products

were identified and quantified by GC-MS.

5

Calculation of TOF. In the calculation of TOF, the conversion was controlled to be < 5 %.

50 µL of the solution were collected with a syringe after reacting for 10 min and the quantity

of the reacted substrates was determined by GC-MS. The TOF was calculated by the

following equation:

TOF = (reacted mol substrate) / [(total mol metal) × (reaction time)]

Durability of hydrogenation of styrene using the Pt/BP heterostructure as the catalyst. 0.87

mmol of styrene and the catalyst (Pt: 4.1*10-2

mol% of styrene) were mixed with 2 mL of

ethanol and one cycle of the hydrogenation reaction was carried out. After the reaction, the

catalyst was recovered by centrifugation and rinsed with ethanol twice. The catalyst was

reused in another cycle of the hydrogenation reaction. Five reaction cycles in total were

performed to examine the stability of the catalyst.

5. Computation details

The calculation was performed by the plane-wave technique implemented in the Vienna ab-

initio simulation package.[S2]

The Perdew-Burke-Ernzerhof (PBE)[S3]

functional including the

van der Waals (vdW) correction proposed by Grimme[S4]

was used. The projector augmented

wave method (PAW)[S5]

with a cutoff energy of 400 eV was used to describe the electron-ion

interaction and the two-dimensional (2D) periodic boundary conditions were applied along

the growth directions of black phosphorus (BP) monolayer. A vacuum spacing of 25 Å was

considered in the calculation to prevent the interactions between the sheets in adjacent cells.

The 4×3 supercell of BP composed of 48 phosphorus atoms is used for Pt nanoparticle

deposition and the 2D Brillouin zone was sampled by 3×3×1 k-points within the Monkhorst-

Pack scheme. All the atomic positions were optimized with the converging tolerance of 0.02

eV/Å for the force on all atoms and the convergence criteria for energy was 10-5

eV. The

climbing image nudged elastic band (CI-NEB) method[S6]

was applied to search the transition

state and 4 images were inserted. A frequency analysis was performed on the stable states to

6

confirm that these represented the genuine minima. The transition states were verified

accompanied by a single vibrational frequency along the reaction coordinate.

7

Figures, tables and related discussion

Figure S1. Characterization of the BP nanosheets. (a) TEM image of the BP nanosheets.

Inset: Photograph of the BP nanosheets dispersed in ethanol. (b) AFM image of a typical BP

nanosheet. (c) AFM image of the BP nanosheets. (d) Thickness distribution of the BP

nanosheets.

8

Figure S2. Characterization of the Pt/BP heterostructures. (a) STEM image. (b) XRD

pattern.

9

Figure S3. Characterization of Pt/P25 catalyst. (a) TEM and (b) Enlarged TEM images of

Pt/P25. (c) Size distribution of PtNPs on Pt/P25. (d) XRD pattern of Pt/P25.

10

Figure S4. Characterization of Pt/SiO2 catalyst. (a) TEM and (b) Enlarged TEM images of

Pt/SiO2. (c) Size distribution of PtNPs on Pt/SiO2. (d) XRD pattern of Pt/SiO2.

11

Figure S5. Characterization of commercial Pt/C. (a) TEM image and (b) Enlarged TEM

images of commercial Pt/C. (c) Size distribution of PtNPs on commercial Pt/C. (d) XRD

pattern of Pt/C.

12

Figure S6. Absorption spectra of (a) BP nanosheets and (b) Pt/BP in the aqueous solution

under daylight irradiation for 0−7 days.

13

Figure S7. AFM image of the Pt/BP heterostructures immersed in water for 7 days.

14

Figure S8. STEM image of the BP nanosheets and Pt/BP heterostructures stored in air. (a)

BP nanosheets stored in air for 2 days and (b) Pt/BP heterostructures stored in air for 15 days.

15

Figure S9. Atomic structures of (a) pristine Pt10 cluster and (b) side and (c) top view of Pt10

deposited on BP.

Discussion on Figure S9. The highly stable Pt10 cluster with a pyramidal geometry from

previous theoretical study[S7]

is selected to simulate the metal Pt nanoparticle. After

deposition, the puckered surface of BP is retained. The large binding energy (13.6 eV)

calculated as the difference between the total energies of Pt10 cluster plus BP and that of bare

Pt/BP heterostructure indicates stability of Pt NP deposited on BP surface.

16

Figure S10. Pt 4f core level XPS spectra of the 4 nm PtNPs and 4 nm-Pt/BP.

Discussion on Figure S10. The PtNPs with a size of 4 nm are prepared and loaded on the

surface of BP nanosheets to obtain 4 nm-Pt/BP and their chemical states are determined from

the Pt 4f core-level XPS spectrum (Figure S10). Both the 4 nm PtNPs and 4 nm-Pt/BP show

Pt 4f peaks at 71 eV. The other peak at 72.9 eV observed from the 4 nm-Pt/BP is related to

Pt–P.

17

Figure S11. P 2p core level spectra of the Pt/BP heterostructure. P1 represents phosphorus

bonded to phosphorus and P2, P3 and P4 represent the different oxide species as described in

the text.

Discussion on Figure S11. The oxidized structure of BP consists primarily of P2O5 (P4) and a

small contribution from the intermediate stable oxide p-P4O2 consisting of two distinct

oxygen-phosphorus moieties: O-P=O (P3) and P-O-P (P2). The binding energy separations of

the P2, P3, and P4 oxide components are larger than the P 2p 3/2 of P-P bonds in BP by 1.48,

2.68 and 4.48 eV, respectively.[S8]

Accordingly, the oxidation states of BP are determined

from the P 2p core-level XPS results which are fitted by the P2, P3, and P4 components. By

measuring the areas of the respective peaks, the fractions of P2, P3, and P4 are calculated to be

8%, 29.7%, and 62.3%, respectively.

18

Figure S12. Seebeck coefficient mesurment. The measured Seebeck potential of the Pt/BP

heterostructures under various temperature differences and Seebeck coefficient is derived

from the slope of the fitted line.

19

Figure S13. Schematic of the femtosecond pump-probe dark field microscopic optical

system

Discussion on Figure S13. A mode-locked oscillator (Mai Tai HP) is used to generate

femtosecond pulses at 800 nm with a repetition rate of 80 MHz and pulse duration of 100 fs.

The laser pulses are split into two parts. One of them passes through BBO to generate a 400

nm pump light and then goes through a delay line. The other passes through a photonic

crystal fiber (Newport SCG-800) to generate a supercontinuum white light (400~1200 nm),

then one wavelengths is selected for the probe light by a band-pass filter of 660±10 nm. The

pump and probe laser beams are collinear and focused onto the sample at normal incidence by

a ×50 microscope objective with a spot size of about 2 μm. The probe light reflected from the

samples is detected by a high-sensitivity photomultiplier (Thorlabs PMM02

https://www.thorlabschina.cn/newgrouppage9.cfm?objectgroup_id=2909), while the reflected

https://www.thorlabschina.cn/newgrouppage9.cfm?objectgroup_id=2909

20

pump light is filtered by a long pass filter of 450 nm. Finally, the electronic signal converted

from the probe light is separated and amplified by utilizing a lock-in amplifier with a

chopping frequency of 1 kHz. The differential reflection signals (ΔR=R'-R0, R′and R0 are

the probe reflectance with and without pump excitation, respectively) are recorded by a self-

written Matlab program.

21

Figure S14. Photographs of the in situ XPS setup. (a) and (b) Photographs of the in situ XPS

setup in which the laser is pointed to the sample. Photographs of the unilluminated (c) and

illuminated (d) samples.

Discussion on Figure S14. The XPS spectra are collected when the sample is irradiated with

a green laser (wavelength = 521 nm, power = 100 mW). The laser is mounted on a

siderocradle outside of the XPS analysis chamber to illuminate the sample through the

observation window made of lead glass.

22

Figure S15. C 1s core level XPS spectra of the Pt/BP heterostructures with and without light

illumination.

23

Figure S16. TEM images of the Pt/BP heterostructures before (a) and after (b) in situ XPS

measurements.

24

Figure S17. Pt 4f and C 1s core level XPS spectra of Pt/SiO2 and Pt/C with and without light

illumination. (a) Pt 4f and (b) C 1s core level XPS spectra of Pt/SiO2. (c) Pt 4f and (d) C 1s

core level XPS spectra of Pt/C.

25

Figure S18. Spectra of the simulated solar light. (a) Emission spectra of the Xe light source.

(b) Absorption spectra of the simulated solar light filer.

26

Figure S19. Plots of conversion of styrene versus reaction time with and without catalyst

(Pt/BP) removed at t = 15 min.

Discussion on Figure S19. To confirm the heterogeneous nature of the catalysis, the solid

catalyst (Pt/BP) is removed from the reaction system by centrifugation at a specific stage and

the reaction stops immediately. Therefore, the hydrogenation reaction is confirmed to be a

heterogeneous catalysis which occurs on the surface of the PtNPs.

27

Figure S20. Plots of conversion of styrene to ethylbenzene versus reaction time with Pt/BP

and BP nanosheets as the catalysts under simulated solar light illumination.

28

Figure S21. Characterization and photocatalytic activity of hydrogenation of styrene with the

Pt/BP heterostructures synthesized with thicker (about 180 nm) BP nanosheets. (a, b) TEM

images, inset: size distribution of PtNPs. (c) TOFs of Pt/BP (23 nm) and Pt/BP (180 nm) in

hydrogenation of styrene with and without illumination. (d) AFM image of a typical Pt/BP

(180 nm).

29

Discussion on DFT calculation of styrene hydrogenation. To illustrate the photocatalytic

activity of Pt/BP heterostructure, styrene hydrogenation is examined. Styrene hydrogenation

involves two main processes: hydrogen insertion into β-carbon and α-carbon. Figure 5d

shows the reaction pathways for hydrogen insertion to the Pt10/BP heterostructure under the

neutral condition and photo-generated 0.5 electron (e-) and the geometrical structure of the

related states. The β-insertion requires a barrier of 0.59 eV, which is comparable to that on

pristine Pt cluster (free energy barrier with ～ 0.8 eV). When 0.5 electron is injected into the

Pt/BP system, the barrier for β-insertion is reduced to 0.41 eV, indicating enhanced activity

by the photocatalytic interaction.

In the second hydrogenation step, without effects of external charges, hydrogen insertion

into α-carbon must overcomes an energy barrier as high as 1.54 eV, which is consistent with

that of hydrogen insertion into pristine Pt8 and Pt9 clusters.[S9]

Compared to hydrogen β-

insertion, the higher barrier suggests that hydrogen insertion into α-carbon is the rate

determining step in styrene hydrogenation. More importantly, the injected 0.5 external

electrons reduce the reaction barrier from 1.54 to 0.68 eV for hydrogen α-insertion leading to

improved reduction activity.

30

Figure S22. Change of conversion in 5 cycles of hydrogenation of styrene with the Pt/BP

catalyst. Reduction of conversion may be caused by the loss of catalyst during styrene

reloading.

31

Figure S23. Characterization of the Pt/BP heterostructure after photochemical hydrogenation

of styrene. (a, b) TEM images; (c) HR-TEM image; (d) XRD pattern.

32

Figure S24. TOFs of the Pt/BP catalyst in (a) hydrogenation of benzaldehyde and (b)

oxidation of benzyl alchonol under simulated solar light illumination at room temperature,

without light at 60˚C and room temperature, with the catalyst of 5.2*10-2

mol% of the

substrate.

33

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P. K. Chu, Adv. Funct. Mater. 2015, 25, 6996.

[S2] G. Kresse, J. Hafner, Phys. Rev. B 1993, 47, 558.

[S3] J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865.

[S4] G. Stefan, J. Comput. Chem. 2006, 27, 1787.

[S5] J. P. Perdew, Y. Wang, Phys. Rev. B 1992, 45, 13244.

[S6] G. Henkelman, B. P. Uberuaga, H. Jónsson, J. Chem. Phys. 2000, 113, 9901.

[S7] A. S. Chaves, G. G. Rondina, M. J. Piotrowski, P. Tereshchuk, J. L. F. Da Silva, The J.

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Documents

Black Phosphorus/Platinum Heterostructure: A Highly ... · chemistry of Pt/BP. The binding energy of PtNPs on the BP nanosheets is found to be as large as 13.6 eV due to multi-Pt