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Letter
Insight into Electrocatalysts as Co-Catalysts in Efficient Photocatalytic Hydrogen Evolution Wentuan Bi, Lei Zhang, Zhongti Sun, Xiaogang Li, Tao Jin, Xiaojun Wu, Qun Zhang, Yi Luo, Changzheng Wu, and Yi Xie ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00913 • Publication Date (Web): 01 Jun 2016 Downloaded from http://pubs.acs.org on June 3, 2016
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Insight into Electrocatalysts as Co-Catalysts in Efficient Photocatalytic Hydrogen Evolution Wentuan Bi||,†, Lei Zhang||,‡, Zhongti Sun||,§, Xiaogang Li†, Tao Jin†, Xiaojun Wu§, Qun Zhang*,‡, Yi Luo‡, Changzheng Wu*,† and Yi Xie† †
Hefei National Laboratory for Physical Science at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Hefei Science Center (CAS) and CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, Hefei, Anhui 230026 (P. R. China) ‡
Hefei National Laboratory for Physical Sciences at the Microscale, Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026 (P. R. China)
§
CAS Key Laboratory of Materials for Energy Conversion and Department of Material Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026 (P. R. China)
ABSTRACT: Recently the implantation of non-noble metal electrocatalysts into photocatalysts has brought dramatically improved hydrogen evolution activities, yet the mechanistic details are still under debate due to the poor understanding of interfacial charge carrier dynamics. Here for the first time we unravel that it is the electrocatalytic process that plays the critical role in these hetero-structured systems. Spectroscopic characterizations combined with theoretical calculations give a clear physical picture that the photoexcited electrons transfer from photocatalysts to phosphides electrocatalysts, then driving H2 evolution reaction like electrocatalysis; and also reveal the Fermi level of electrocatalysts as a feasible descriptor for the photocatalytic activity. KEYWORDS: photocatalysis, hydrogen evolution reaction, electrocatalysts, co-catalysts, electron transfer
Environmental crisis, along with sustained growth in global energy demand has triggered keen interest in the conversion of solar energy to fuels. Among them, direct utilization of sunlight to create the driving force needed for water splitting is especially appealing because it potentially enables much simpler and more economically competitive systems to achieve H2 production.1-4 However, currently photocatalysts for direct water splitting still suffer from severe charge recombination, insufficient reaction sites, and the resulting low conversion efficiency.5,6 As is known, loading co-catalysts shows promise for resolving these problems.7-9 Nevertheless, the most efficient co-catalysts till now remain dominated by noble metals which suffer from high cost in terms of upscaling. Recently the rapid emergence of earth-abundant alternatives in electrocatalysis, such as transition metal chalcogenides,10-12 nitrides,13 carbides14,15 and phosphides,1618 brings new opportunities to this field with dramatically improved H2 evolution activities.19-21 As is already known, for these non-noble metal electrocatalysts, effective charge separation is critical to photocatalytic H2 evolution. However, the direction of photoexcited electron transfer across the interface is still poorly understood, directly causing the confusion of identification of the reaction sites. One demonstrates that, driven by the interfacial p–n junctions,
photoinduced electrons transfer from electrocatalysts to semiconductors, and then participate in the H+ reduction reaction at the surface of the semiconductors.22-25 On the contrary, the other assumes that electrocatalysts accept the photoinduced electrons from semiconductors and provide the additional active sites for H2 evolution.20,26,27 The current controversy has obviously hindered the comprehensive understanding of the general design principles of hetero-structured photocatalytic systems. In this regard, uncovering the mechanism of non-noble metal electrocatalysts becomes an urgent yet inevitable task for advancing photocatalytic H2 evolution. In this work, we demonstrate the critical role of nonnoble metal electrocatalysts in photocatalytic H2 evolution using the most efficient non-noble metal electrocatalysts (phosphides) coupled with classic photocatalysts (cadmium sulfide CdS) as the model systems. We give a clear physical picture that the photoexcited electrons transfer from photocatalyst to electrocatalysts, driving H2 evolution reaction like electrocatalysis. The synergic effect of CdS nanorods and electrocatalysts brings a remarkable H2 evolution rate of 10.8 mmol g-1 h-1, up to 16 times with respect to that of bare CdS nanorods. The design and implantation of nonnoble metal electrocatalysts provide a tangible guideline to develop low-cost, high-efficiency hetero-structured photocatalytic systems.
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Figure 1. Photocatalytic performance and the charge carrier dynamics of Co2P/CdS catalysts. (a) Photocatalytic activity of the hybrid catalysts with different ratios of Co2P/CdS. (b) TA spectra of CdS NRs (excitation at 400 nm), showing the evolution of the dominant 1Ʃ excitonic bleach of CdS. The signal (that is, the absorbance changes, or ΔAbs.) is given in mOD where OD stands for optical density. (c) The relaxation kinetics of the CdS 1Ʃ excitonic bleach. (d) Schematic illustration of the mechanisms involved. VBM and CBM stand for valence band maximum and conduction band minimum, respectively. (e) Steady-state PL spectra (excitation at 400 nm). (f) Time-resolved PL spectra (excitation at 400 nm, emission at 525 nm).
As a representative visible-light-response photocatalyst for H2 production, one-dimensional CdS nanostructures provide an ideal platform for artificial photocatalysis as they can shorten the diffusion path of charge carriers, thus facilitating charge transfer and separation.28 In this study, CdS nanorods (NRs) with lengths of ~100-500 nm and diameters of ~30-40 nm were synthesized via a solvothermal method, as shown by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images (Figure S1 in the Supporting Information). Given the superior electrocatalytic performance, metalrich phosphides are chosen as the model electrocatalysts. Upon loading Co2P nanoparticles, an additional diffraction peak at 2θ = 40.7 degrees was detected in the X-ray diffraction (XRD) pattern (Figure S2a), corresponding to the (121) plane of Co2P. The typical morphological features revealed by both SEM and TEM images showed that Co2P nanoparticles with sizes of ~5– 10 nm were well dispersed on the CdS NRs. The wellresolved lattice fringes in Figure S2d revealed the high crystallinity of CdS NRs, and the lattice fringe spacings coincided well with those of the (110) and (002) facets. Additionally, the 0.18-nm spacing in the nanoparticle can be assigned to the (001) plane of Co2P. Moreover, energy dispersive X-ray spectrum (EDS) shown in Figure S2e confirmed the presence of Co and P elements with an atom ratio of 2:1. Taken together, the morphology and microstructure characterizations above clearly confirmed that the CdS/Co2P hybrid catalysts have been successfully obtained.
The photocatalytic H2 production activities of the CdS nanorods with different amounts of Co2P were measured and compared in Figure 1a. In a typical photocatalytic H2 evolution reaction, 50 mg catalysts were dispersed in 200 mL aqueous solution containing 10 vol% lactic acid as sacrificial reagents, and the amount of H2 was monitored by gas chromatography. CdS nanorods alone showed relatively low photocatalytic H2 evolution activity (~650 µmol g-1 h-1). Notably, when Co2P was loaded, the H2 evolution activity was dramatically enhanced as expected. A substantial increase of H2 evolution rate was observed with the Co2P content ranging from 0.5 to 6 wt%, reaching a maximum value of 10.8 mmol g-1 h-1, up to 16 times with respect to that of bare CdS nanorods. Notably, further increasing the Co2P content brought about slight decline in photocatalytic performance, which is probably due to the strong light absorption of Co2P itself that consequently reduces the light reaching CdS.19 We also performed recycling tests to evaluate the photocatalytic stability of the CdS/Co2P hybrids. The reaction system was evacuated every four hours and the process was repeated four consecutive cycles. As shown in Figure S3, CdS NRs with optimal loading amount of Co2P (6 wt%) showed a near-linear increase in the amount of hydrogen in the continued 16 h reaction under irradiation, and no noticeable degradation of H2 evolution rate was detected during four consecutive cycles, indicating the high stability of the hybrid catalysts with no obvious photocorrosion of CdS during the photocatalytic process. Moreover, TEM images in Figure S4 verified the
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morphological maintenance of CdS/Co2P hybrids after the reaction. The optical properties of the samples were examined with ultraviolet–visible (UV–Vis) absorption spectra in the spectral region 300–800 nm. As shown in Figure S5, the absorption edge of pure CdS NRs was observed at approximately 530 nm, corresponding to a band gap of ~2.34 eV. When Co2P was loaded onto CdS NRs, the color of the catalysts turned from yellow to dark green. A weak absorption enhancement was observed in the visible region beyond 530 nm, which arose probably from the absorption of Co2P; nevertheless, the absorption edge of CdS showed no noticeable shift. Reasonably, the improved photocatalytic performance mentioned above cannot be attributed to the slightly enhanced light absorption in the presence of Co2P. In an attempt to decode the direction of photogenerated electron migration across the interface, we interrogated the CdS/Co2P(6%) hybrid systems with reference to the bare CdS NRs by means of transient absorption (TA) spectroscopy, a useful tool proven quite robust for tracking the specific charge carrier dynamics of nanosystems.29-31 In our TA measurements, a scheme featuring ultraviolet pump/white-light continuum (WLC) probe was employed. The center wavelength of the femtosecond pump pulses was chosen at 400 nm, which was proven effective for promoting electrons from the valence band to the conduction band of CdS (refer to Figure S5). The subsequent WLC in the 460–630 nm region monitored the spectral evolution of the wellknown 1Ʃ excitonic band of CdS32 that manifests as probe bleach peaking at ~490 nm (material size-dependent), as displayed in Figure 1b. The loading of Co2P onto CdS NRs turned out to bring about negligible variation in the TA spectral profiles of CdS, but result in changes in the TA kinetics. We noticed that the relaxation kinetics of the CdS 1Ʃ excitonic bleach depend on the probing wavelength. In order to properly retrieve the characteristic relaxation time constants, we applied a global fitting procedure to a set of kinetic traces acquired in a broad range of the probing wavelengths 470–570 nm (51 traces with a 2-nm interval), as shown in Figure 1c. The bi-exponential global fitting results are τ1 = 42 ± 1 ps (68%) and τ2 = 610 ± 36 ps (32%) for bare CdS while τ1 = 42 ± 1 ps (72%) and τ2 = 498 ± 24 ps (28%) for CdS/Co2P. The mean relaxation lifetimes are 538 ± 34 and 417 ± 22 ps for bare CdS and CdS/Co2P, respectively. It is known that the emergence of two decay times usually points to two consecutive pathways of exciton relaxation to two trap states with different trap depths.33 In this particular case, the exciton relaxation manifests as electron transfer (ET) from the conduction band minimum to a shallow trap state (labeled TS1 in Figure 1d) and then to a deep trap state (labeled TS2 in Figure 1d), featuring different ET rates of 1/τ1 and 1/τ2, respectively. Interestingly, the loading of Co2P onto CdS NRs barely affected τ1 (i.e., 42 ps for both CdS/Co2P and bare CdS) but gave rise to a pronounced change in τ2 (cf. 498 ps for CdS/Co2P versus 610 ps for bare CdS). Such a change should be attributed
to the opening of an additional ET channel from the shallow trap state TS1 to the Co2P part, as depicted in Figure 1d. Following a similar treatment documented in the literature,34 we can readily estimate the ET rate associated with this additional channel, i.e., kET = 1/τ2(CdS/Co2P) – 1/τ2(bare CdS) = 3.69 × 108 s-1. According to the interfacial electron dynamics extracted from TA spectroscopy, it can be safely inferred that the excited electrons transfer from CdS to Co2P, realizing the spatial separation of charge carriers. Also, we interrogated the CdS/Co2P hybrids by means of PL spectroscopy. Figure 1e compares the steady-state PL spectra for CdS/Co2P(1%), CdS/Co2P(6%), and bare CdS NRs. The two PL bands centered at ~500 and 700 nm correspond to the 1Ʃ excitonic band-edge emission and the trap-state emission, respectively (refer to Figure 1d). It is seen that the loading of Co2P onto CdS NRs resulted in PL quenching of CdS NRs and that the increase of loading amount led to more severe quenching. In terms of the extent of quenching, the PL intensity of band-edge emission turned out to be smaller than that of trap-state emission. This is understandable because when Co2P is added, the new ET channel from TS1 to Co2P will aid in vacating the electron occupation of TS1, which will lead to substantial quenching of the trap-state PL and meanwhile leave room for accommodating more electrons from CBM, thereby quenching the band-edge PL to a certain extent. On the other hand, time-resolved PL kinetics (excitation at 400 nm and emission at 525 nm) were recorded on CdS/Co2P (6%) and bare CdS NRs for comparison, as shown in Figure 1f. It is worth mentioning here that the trap-state PL lifetime cannot be given as it turned out to exceed the measurement limit of our PL spectrometer using a picosecond excitation light source. The loading of Co2P resulted in shortening of the mean PL lifetime by a factor of 1.65 (cf. 2.83 ns for CdS/Co2P versus 4.66 ns for bare CdS). The observations of both PL quenching and acceleration of the band-edge PL decay kinetics demonstrate that the introduction of Co2P resulted in significant suppression of the photoexcited charge recombination in CdS NRs due to the opening of an additional ET channel from the CdS part to the Co2P part. The acceleration of the cascading ET processes (CBM→ TS1→TS2 and Co2P) facilitates the decrease of the CBM electrons and hence, to a certain extent, quenches the band-edge PL emissions. Clearly, the above TA and PL results are consistent with one another and nicely commensurate with the mechanistic picture illustrated in Figure 1d, which in turn verifies the direction of the electron migration and the efficient charge separation in the CdS/Co2P hybrid system. Furthermore, we elucidate the mechanistic details of co-catalysts in an electrochemical way with the aid of theoretical calculations. As illustrated in Figure 2a, an external bias is required to raise the Fermi level (EF) of Co2P, thus providing the driving force for water splitting in electrocatalysis.35 While in the case of photocatalysis, since EF of Co2P is lower than the CBM of CdS, photogenerated electrons can be transferred from CdS to
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Figure 2. Schematic illustration of metallic phosphides electrocatalysts as co-catalysts for photocatalytic H2 production. (a) The comparison of Fermi level of Co2P and + EH /H2, depicting the need of an external bias to drive the electrocatalysis H2 production. (b) Representation of the proposed mechanism for photocatalytic H2 production with the CdS/Co2P hybrid system described in this work.
Co2P, as confirmed above. More importantly, theoretical calculations show that the injection of electrons into phosphides upshifts the EF, as in the case of noble metals.36,37 Remarkably, when only 0.01 electron per unit cell is injected, the EF of Co2P is levitated by 0.25 eV (from –4.63 to –4.38 eV), thereby breaking the hydrogen electrode potential EH+/H2 (–4.60 eV), as illustrated in Figure 2b. Moreover, the more the injected electrons are, the larger the magnitude of EF up-shifting is, thus providing a larger driving force for H2 evolution. Theoretical analysis above discloses the underlying correlations between electrocatalysis and photocatalysis, confirming that metallic phosphides can indeed facilitate charge separation and drive H2 evolution reaction like electrocatalysis, in other words, meeting the requirements of co-catalysts. Besides Co2P, other metallic phosphides such as CoP and Ni2P (Figure S6) were also investigated for comparison. As shown in Figure 3a, they also remarkably improved the H2 production performance of CdS NRs, 12 and 10 times higher than that of bare CdS NRs catalysts, respectively. These observations revealed the general applicability of phosphides electrocatalysts as efficient cocatalysts for photocatalytic H2 production. Additionally, theoretical calculations enable an in-depth understanding on the photocatalytic activities among different electrocatalysts. In principle, a higher Fermi level can be expected to provide a larger driving force for H2 evolution, thus leading to a relatively higher photocatalytic activity. As shown in Figure 3b and Supporting Table S1, the Co2P system exhibits the highest Fermi level. Moreover, theoretical calculations indicate that when the same amount of electrons are injected, the magnitudes of the
Figure 3. General applicability of phosphides electrocatalysts as co-catalysts for photocatalytic H2 production. (a) Comparison of the photocatalytic H2 production activity of CdS NRs with different phosphides. (b) The magnitude of Fermi level upshifting induced by injecting the same amount of electron.
Fermi-level upshifting for the three phosphides follow the order of Co2P > CoP > Ni2P, which is consistent with their photocatalytic activities. Taking together the above results, we can safely conclude that the electrocatalytic process is the close-up of hetero-structured photocatalytic H2 evolution systems and the photocatalytic activity of phosphides in this case is closely related to both the Fermi-level position and the magnitude of the Fermi-level upshifting induced by electron injection. Meanwhile, it is worth mentioning that semiconductor-type electrocatalysts also raise their Fermi levels when accepting photoexcited electrons,35 thus driving the H2 evolution reaction. In conclusion, for the first time we have unraveled that it is the electrocatalytic process that plays a critical role in the hetero-structured photocatalytic H2 evolution systems. Electrocatalysts serve as the superior co-catalysts to facilitate charge separation and provide the dominant reaction sites for H2 evolution, thus addressing the longstanding debate on the role of electrocatalysts in photocatalysis. Additionally, insights gained from the underlying correlations between electrocatalysis and photocatalysis reveal the Fermi level of phosphide as a feasible descriptor for the photocatalytic activity: the high
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Fermi level and the efficient Fermi-level upshifting of Co2P endow CdS nanorods with a remarkable H2 evolution rate of 10.8 mmol g-1 h-1, up to 16 times with respect to that of bare CdS nanorods, which is among the best non-platinum photocatalysts. We anticipate that this general mechanism will provide a rational guideline to develop more efficient hetero-structured photocatalytic H2 evolution systems.
ASSOCIATED CONTENT Supporting Information. Experimental section, supporting characterizations of CdS/Co2P hybrid catalysts and other metal phosphides. This material is available free of charge via the Internet at http://pubs.acs.org.
Corresponding Author * C.Z.W. (email:
[email protected]); Q. Z. (email:
[email protected]).
Author Contributions ||
These authors contributed equally to this work.
(11) Benck, J. D.; Hellstern, T. R.; Kibsgaard, J.; Chakthranont, P.; Jaramillo, T. F., ACS Catal 2014, 4, 3957-3971. (12) Kong, D.; Wang, H.; Lu, Z.; Cui, Y., J. Am. Chem. Soc. 2014, 136, 4897-4900. (13) Chen, W. F.; Sasaki, K.; Ma, C.; Frenkel, A. I.; Marinkovic, N.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R., Angew. Chem. Int. Ed. 2012, 51, 6131-6135. (14) Liu, Y.; Yu, G.; Li, G. D.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X., Angew. Chem. Int. Ed. 2015, 54, 10752-10757. (15) Michalsky, R.; Zhang, Y. J.; Peterson, A. A., ACS Catal 2014, 4, 1274-1278. (16) Liu, Q.; Tian, J.; Cui, W.; Jiang, P.; Cheng, N.; Asiri, A. M.; Sun, X., Angew. Chem. Int. Ed. 2014, 53, 6710-6714. (17) Xing, Z.; Liu, Q.; Asiri, A. M.; Sun, X., ACS Catal 2015, 5, 145-149. (18) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E., J. Am. Chem. Soc. 2013, 135, 9267-9270. (19) Vesborg, P. C. K.; Seger, B.; Chorkendorff, I., J. Phys. Chem. Lett. 2015, 6, 951-957. (20) Sun, Z.; Zheng, H.; Li, J.; Du, P., Energy Environ. Sci. 2015, 8, 2668-2676. (21) Cao, S.; Chen, Y.; Hou, C. C.; Lv, X. J.; Fu, W.-F., J. Mater. Chem. A 2015, 3, 6096-6101.
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT This work was financially supported by the National Basic Research Program of China (2015CB932302), National Natural Science Foundation of China (U1432133, 21331005, 11321503, U1532265, J1030412, 21173205, 91127042, 21573211, 21421063), National Young Top-Notch Talent Support Program, Chinese Academy of Sciences (XDB01020000), the Fok Ying-Tong Education Foundation, China (141042), the China Postdoctoral Science Foundation (2015M581999), and the Fundamental Research Funds for the Central Universities (WK2060190027, WK2340000063, WK2060190058).
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SYNOPSIS TOC
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Figure 1. Photocatalytic performance and the charge carrier dynamics of Co2P/CdS catalysts. (a) Photocatalytic activity of the hybrid catalysts with different ratios of Co2P/CdS. (b) TA spectra of CdS NRs (excitation at 400 nm), showing the evolution of the dominant 1Ʃ excitonic bleach of CdS. The signal (that is, the absorbance changes, or ∆Abs.) is given in mOD where OD stands for optical density. (c) The relaxation kinetics of the CdS 1Ʃ excitonic bleach. (d) Schematic illustration of the mechanisms involved. VBM and CBM stand for valence band maximum and conduction band minimum, respectively. (e) Steadystate PL spectra (excitation at 400 nm). (f) Time-resolved PL spectra (excitation at 400 nm, emission at 525 nm). 631x331mm (300 x 300 DPI)
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Figure 2. Schematic illustration of metallic phosphides electrocatalysts as co-catalysts for photocatalytic H2 production. (a) The comparison of Fermi level of Co2P and EH+/H2, depicting the need of an external bias to drive the electrocatalysis H2 production. (b) Representation of the proposed mechanism for photocatalytic H2 production with the CdS/Co2P hybrid system described in this work. 182x155mm (300 x 300 DPI)
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Figure 3. General applicability of phosphides electrocatalysts as co-catalysts for photocatalytic H2 production. (a) Comparison of the photocatalytic H2 production activity of CdS NRs with different phosphides. (b) The magnitude of Fermi level upshifting induced by injecting the same amount of electron. 99x157mm (300 x 300 DPI)
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