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.
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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