A Promising Cocatalyst to Boost Photocatalytic Hydrogen Evolution of

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Metallic Co2C: A Promising Cocatalyst to Boost Photocatalytic Hydrogen Evolution of Colloidal Quantum Dots Qing Guo, Fei Liang, Xiao-Ya Gao, Qi-Chao Gan, XuBing Li, Jian Li, Zheshuai Lin, Chen-Ho Tung, and Li-Zhu Wu ACS Catal., Just Accepted Manuscript • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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ACS Catalysis

Metallic Co2C: A Promising Cocatalyst to Boost Photocatalytic Hydrogen Evolution of Colloidal Quantum Dots Qing Guo,†,1,3 Fei Liang,†,2,3 Xiao-Ya Gao,1,3 Qi-Chao Gan,1,3 Xu-Bing Li,1,3,* Jian Li,1,3 Zhe-Shuai Lin,2,3 Chen-Ho Tung1,3 and Li-Zhu Wu1,3,* 1

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China 2 Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China 3 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China. ABSTRACT: Transition-metal carbide (TMC), owing to its electronic conductivity, chemical stability and physical properties, has aroused widespread interests in catalysis. Here, we have systematically studied the photocatalytic hydrogen (H2) evolution of metallic cobalt carbide (Co2C) by a combination of theoretical and experimental investigations. In term of intrinsic proton reduction property of Co2C (020) facet and facile interficial electron transfer, the assembled architecture of QDs/Co2C can give an rate of ~18000 µmol g-1 h-1 (λ = 450 nm) using TMC as cocatalysts and an apparent quantum yield of ∼2.7% of photocatalytic H2 evolution, a ~10-fold enhancement compared with bare QDs under identical conditions. Our results indicate that Co2C with suitable morphology and facet exposure can work as a cocatalyst to achieve photocatalytic H2 evolution.

KEYWORDS: transition-metal carbide, metallic Co2C, photocatalytic H2 evolution, DFT calculations

Production of clean energy from renewable resources remains as one of the greatest challenges to relieve energy crisis and environmental disruption.1 Among various approaches, sustainable production of hydrogen (H2) gas using sunlight as the energy input, especially photocatalytic water splitting by utilizing semiconducting nanocrystals, is regarded as one of the most promising alternatives.2 With the help of cocatalyst, that is able to facilitate the separation of photogeneated electron-hole pairs and act as active sites for proton reduction, there have been numerous advances in the activity and stability of photocatalysts.3 Platinum and its alloys have long been proven to work as the most efficient cocatalysts toward photocatalytic H2 evolution.4 However, its high price and scarcity have seriously restricted the widespread application. In this way, exploring earth-abundant and low-cost cocatalysts to boost the process of H2 evolution is with vital importance.5 Very recently, transition-metal carbide (TMC),6 due to the outstanding electronic conductivity, chemical stability and physical properties, has emerged as a new kind of cocatalysts for H2 production from water,7 either electrochemical or photochemical scenario. For example, Fe-doped Ni3C nanosheets exhibited outstanding bifunctional electrocatalytic performances for both H2 evolution reaction and oxygen (O2) evolution reaction.8 Ti3C2 nanoparticles, terminated by oxygen atom, were synthesized and confirmed as a highly efficient cocatalyst for solar H2 evolution.9 Cobalt carbide (Co2C), which has

long been used in Fischer-Tropsch synthesis (FTS) reaction,10 was recently realized as an electrocatalyst for H2 evolution from water splitting.11 The diverse and promising applications trigger a huge interest in understanding the reaction mechanism and designing exquisite structure of TMC for catalysis. Particular advantages in solar energy conversion prompt us to explore the possibility of making Co2C as a cocatalyst for effective photocatalytic H2 evolution. Toward this end, we firstly utilized density functional theory (DFT) calculations to investigate the abilities of different facets of Co2C on proton reduction.12 The DFT results suggested that Co2C with preferred facet exposure was a promising kind of cocatalyst for proton reduction. Based on the exciting theoretical studies, we designed a rational method to prepare Co2C nanoflakes with favoured facet exposure to work as cocatalysts of H2 evolution under visible light. To our delight, after coupling with Co2C nanoflakes, photocatalytic H2 evolution activities of CdSe QDs, CdS QDs and CdSe/CdS QDs could be significantly enhanced. Taking CdSe/CdS QDs as an example, a ~10-fold enhancement of H2 evolution was observed (λ = 450 nm), indicating the application of Co2C as a promising cocatalyst. Mechanistic insights indicated that the excellent H2 evolution activity not only benefits from the intrinsic privileges in proton reduction of Co2C (020) facet, but also the facile photo-induced interfacial electron transfer from QDs to Co2C in the assembled architecture.

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in X-ray diffraction (XRD) pattern perfectly matched with the standard pattern of Co2C (PDF No. 65-1457) (Figure S5). Transmission electron microscopy (TEM) confirmed the obtained Co2C acquired a shape of nanoflake (Figure 2a). The enlarged TEM image showed a lattice fringe of ~2.18 Å, which could be ascribed to the exposed (020) facet of Co2C, indicating the nanoflake was formed by the stack of nanoparticles (Figure 2b). Moreover, the full X-ray photoelectron spectroscopy (XPS) spectrum verified the coexistence of Co and C elements (Figure S6) and the enlarged XPS spectrum for Co 2p at 778.7 eV could be assigned to typical signal of carbidic Co (Figure S7).10 These results manifested the successful formation of Co2C nanoflakes with favored facet exposure.

Figure 1. Structural models of H atom adsorption on the surface of Co2C (a) (101), (b) (020) and (c) (111) facet. (d) The calculated free-energy diagram of HER at the equilibrium potential (U = 0 V) on the surfaces of (101), (020), (111) facet of Co2C model, respectively.

A typical model of Co2C was established for DFT calculations, see details in Supporting Information (Figure S1). Spin polarized electronic band calculations (Figure S2 and S3) revealed that the Fermi level crossed the conduction band and no band gap existed in Co2C.13 This metallic character of Co2C can facilitate electron transfer from inner sites to exterior active centers, an extreme advantage to proton reduction. As Gibbs free energy ΔGH* is regarded as a major parameter to evaluate the hydrogen evolution reaction (HER) activity of a given material, and a preferred HER cocatalyst should give a |∆GH*| value close to zero,14 ΔGH* values of Co2C (101), (020) and (111) facets are compared. Figure 1a to 1c showed the corresponding structual models of H atom adsorption on the surface of Co2C (101), (020) and (111) facets. Notably, the Gibbs free energy of ∆GH* on (101) and (111) facets were -0.637 eV and -0.482 eV, indicating the strong adsorption of H atom on these surfaces to slow down H2 release. On the contrary, the |∆GH*| of Co2C (020) facet dramatically decreased to +0.013 eV, even much lower than that of Pt (+0.09 eV).14 The metallic property of Co2C (020) facet ensured a large amount of free electrons at the Fermi level. The facts that, (i) the high percentage of carbon atoms on (020) facet leading more valance electrons to Co atoms, and (ii) the largest total DOS for the Co adsorption site in (020) facet among three facets leading to the enhanced electron mobility (Figure S4), indicated the great potential of Co2C for HER application (Figure 1d). Moreover, the Fermi level of Co2C (020) facet is calculated to be -1.2 V versus the normal hydrogen electrode (NHE), suggesting the transfer of photoelectron from QDs to Co2C is thermodynamically feasible (vide infra). With the guidance of DFT calculations, metallic Co2C nanoflakes with preferred facet exposure were synthesized via a solution pyrolysis method with polyhydric alcohols,15 see details in Supporting Information. The narrow peaks of the prepared material

To explore the potential of the obtained Co2C nanoflakes as an effective cocatalyst for photocatalytic H2 evolution, colloidal CdSe QDs, synthesized in water according to our reported methods, were chosen as a model light absorber.4a The as-synthesized CdSe QDs were characterized by UV-vis absorption spectrum (Figure S8) and TEM image (Figure S9). Metallic Co2C nanoflakes were coupled with water soluble QDs via a self-assembled approach, see details in Supporting Information. CdSe QDs with a lattice spacing of about 3.38 Å (inset of Figure 2c) corresponding to the (111) plane were observed along with the Co2C (Figure 2c). Energy dispersive X-ray analysis revealed the co-existence of Cd, Se and Co (Figure S10) and corresponding elemental mapping showed very similar distribution of the three elements in the assembly (Figure 2d), all of them indicated the successful coupling of QDs with Co2C.

Figure 2. (a) TEM image and (b) corresponding high resolution TEM image of Co2C nanoflakes. (c) High resolution TEM image of CdSe QDs/Co2C assembly. Inset is enlarged image of QDs in white circle. (d) TEM image of CdSe QDs/Co2C assembly and corresponding elemental mapping of Co, Cd and Se, respectively.

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ACS Catalysis evolution of CdSe/CdS QDs as the light absorbers can be attributed to the favored charge separation in the type-II heterostructure, as shown in Figure S11. BET surface areas of these photocatalysts were further measured (Table S1) and the normalized rate of H2 evolution with BET surface area gave an almost 6.5 times improvement compared to bare QDs. All of the results indicated that Co2C could act as a versatile cocatalyst to achieve enhanced H2 evolution.

Figure 3. (a) Control experiments of solar H2 evolution in 1.0 h (λ = 450 nm). (b) The variation of solar H2 evolution with different mass ratios of Co2C to CdSe QDs. (c) Solar H2 evolution under the optimal conditions. (d) Solar H2 production with different QDs. Error bars represent mean ±s.d. of at least three independent experiments.

The initial photocatalytic H2 evolution experiments were performed with triethylamine (TEA) as a sacrificial reagent under visible light (λ = 450 nm). Control experiments implied that CdSe QDs, sacrificial reagent and visible light were all necessary for efficient photocatalytic H2 evolution (Figure 3a). Excitingly, the assembled photocatalysts of CdSe/Co2C could produce H2 gas with a rate of about 4200 µmol g-1 h-1 under visible light irradiation, which was almost ~4-fold to that of bare CdSe QDs (~1000 µmol g-1 h-1) under identical conditions. Moreover, bare Co2C nanoflakes showed no activity toward H2 evolution under light irradiation, suggesting Co2C merely acted as a cocatalyst of proton reduction, rather than a photocatalyst itself. This initial result confirmed our DFT prediction, i.e. the great potential of Co2C as an efficient cocatalyst for photocatalytic H2 evolution. In order to further improve the efficiency of solar H2 evolution, the mass ratio of Co2C to CdSe QDs was optimized. As shown in Figure 3b, gradually increasing the mass ratio of Co2C from o to 3 wt.% significantly improved the rate of H2 generation to give a climax. However, further increasing the mass ratio of Co2C to 4 wt.% led to the decline of H2 production, which was possibly due to the light-screening effect of excessive cocatalysts.[3b] Under the optimal condition, the assembled photocatalysts showed much higher activity of H2 evolution than that of bare CdSe QDs (Figure 3c). In addition, the performances of photocatalytic H2 evolution of other QDs, such as CdS and CdSe/CdS, could also be significantly enhanced (Figure 3d). After coupling with Co2C nanoflakes, a ~10-fold enhancement of H2 evolution activity was achieved by using CdSe/CdS QDs as the light absorbers to give a value of ~18000 µmol g-1 h-1 and an apparent quantum yield (AQY) of ∼2.7 % (see details in Supporting Information). The better activity of H2

Figure 4. (a) The standard conditions of solar H2 evolution. (b) A comparison of the photocatalytic H2-production activities of Co2C with reported cocatalysts under the condition of (a) for 2.5 h irridiation. (c) Long-time photocatalytic H2 evolution of Co2C taking CdSe/CdS QDs as light absorbers and TEA as sacrificial reagent.

Furthermore, we compared Co2C nanoflakes obtained with reported state-of-the-art cocatalysts to evaluate the H2-evolution activity under the standard experimental conditions (Figure 4a), see experimental details in Supporting Information (Figure S12 and Figure S13).16 The Co2C nanoflakes showed much higher activity of H2 evolution than most of the reported cocatalysts under the same condition (Figure 4b). The highest activity of Co3O4 observed was probably due to its smaller size distribution than that of Co2C nanoflakes, which would expose more active sites for proton reduction to H2 (Figure S14). Even after 12 cycles of repetition tests by taking CdSe/CdS QDs as the light absorbers, the Co2C nanoflakes could evolve H2 gas consistently during 60 h visible-light irradiation (Figure 4c). Additionally, the assembly worked well in acidic, neutral and basic solutions by taking ascorbic acid (AA), isopropanol (IPA) and triethylamine (TEA) as the sacrificial reagents (Figure S15), suggesting the wide applicability of this extremely stable cocatalyst. To investigate the driving force of photocatalytic reaction, valence-band (VB) XPS analysis (Figure S16) was utilized to reveal the VB potential of CdSe QDs, in which

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the band position of CdSe QDs was determined to be +1.07 V versus NHE. As shown in Figure S8, the band-gap of CdSe QDs was calculated to be ~2.65 eV by using Tauc plot. Thus, the conduction band (CB) of CdSe QDs was determined to -1.58 V versus NHE. Accordingly, the driving force (~0.38 eV), estimated as the difference between CB of CdSe QDs and Fermi level of Co2C, of photoinduced electron transfer from CdSe QDs to Co2C nanoflakes is thermodynamically feasible (Figure S17). Steady-state and time-resolved spectroscopic techniques were employed to further illucidate the facile process of interfacial eletron transfer process. As shown in Figure 5a, the photoluminescent (PL) intensity of CdSe QDs around 470 nm was dramatically quenched after introducing Co2C. It indicated that Co2C efficiently inhibited the recombination of photogenerated electron-hole pairs through radiative pathway. In addition, the PL lifetime of CdSe QDs was decayed from ~6.4 ns to ~4.9 ns with the couple of Co2C nanoflakes (Figure 5b and see details in Table S2), suggesting the promoted interfacial electron transfer from QDs to metallic Co2C nanoflakes.

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much smaller than that of bare CdSe QDs (16156 Ω), further confirmed the favored interfacial charge transfer (Figure 5d).18 On the basis of above results, it can be concluded that, in the assembled architechture of QDs and Co2C nanoflakes, efficient photo-induced electron transfer can be achieved under visible light, and then the photoelectrons migrate to the active sites of Co2C for facilating the separation of photoexcited electrons and holes and further reducing proton into H2 gas, thus leading to a significantly enhanced activity of H2 evolution. In summary, we have systematically demonstrated that metallic Co2C can be a highly efficient cocatalyst for solar H2 evolution. Under the guidance of DFT calculations, the solar H2 evolution of colloidal QDs, coupled with Co2C nanoflakes with preferred facet exposure, can be dramatically improved to give a ~10-fold enhancement of the activity. The improved activity of solar H2 evolution is a result of the efficient photo-induced electron-hole pairs separation, the facilitated interfacial charge transfer and outstanding proton reduction ability of metallic Co2C. The promising application of Co2C to work as a cocatalyst for photocatalytic H2 evolution is a starting point, the catalytic performance of this material can be further improved by optimizing the size, structure and/or surface property. It's anticipated that this research line would lead to fabrication of smarter, cheaper and more robust artificial photocatalysts for solar H2 evolution, which is actively undergoing in our research group.

ASSOCIATED CONTENT Supporting Information. Experimental procedures, methods, and product characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Figure 5. (a) PL spectrum (excitation: 400 nm laser) and (b) time-resolved PL spectra of bare CdSe QDs and CdSe/Co2C counterparts. (c) Transient photocurrent responses and (d) EIS Nyquist plots of bare CdSe QDs and CdSe /Co2C (3 wt.%) electrodes under the same conditions, inset is the fitted equivalent circuit.

Photoelectrochemical (PEC) measurements were also used to investigate the promoted interfacial charge separation. The experimental details of preparing electrodes were provided in Supporting Information. As shown in Figure 5c, CdSe/Co2C (3 wt.%) electrode showed an enhanced photocurrent transient response compared with bare CdSe QDs counterparts under the identical conditions, suggesting an improved charge separation efficiency in the presence of Co2C.17 The smaller arc radius of electrochemical impedance spectroscopy (EIS) Nyquist plot of CdSe/Co2C (3 wt.%) electrode than that of bare QDs showed a much lower interfacial charge transfer resistance. The charge transfer resistance (Rct) of CdSe/Co2C electrode (9257 Ω) in equivalent circuit was

Corresponding Authors [email protected] [email protected]

Author contributions †

These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are grateful for financial support from the Ministry of Science and Technology of China (2014CB239402 and 2017YFA0206903), the National Natural Science Foundation of China (21390404, 21861132004 and 21603248), the Strategic Priority Research Program of the Chinese Academy of Science (XDB17000000), Key Research Program of Frontier Sciences of the Chinese Academy of Science (QUZDY-SSW-JSC029), and the Youth Innovation Promotion Association of Chinese Academy of Sciences (2018031).

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Nanoparticles as Highly Active and Stable Electrocatalyst for Hydrogen Evolution Reaction. Nano. Res. 2017, 10, 1322-1328. (12) Kohn, W. Nobel Lecture: Electronic Structure of Matter-Wave Functions and Density Functionals. Rev. Mod. Phys. 1999, 71, 1253-1266. (13) Zhao, Y. H.; Su, H. Y.; Sun, K.; Liu, J.; Li, W. X. Structural and Electronic Properties of Cobalt Carbide Co2C and its Surface Stability: Density Functional Theory Study. Surf. Sci. 2012, 606, 598-604. (14) Hinnemann, B.; Moses, P. G.; Bonde, J.; Jorgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendroff, I.; Norskov, J. K. Biomimetic Hydrogen Evolution: MoS2 Nanoparticles on Graphite as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 2005, 127, 5308-5309. (15) Huba, Z. J.; Carpenter, E. E. A Versatile Synthetic Approach for the Synthesis of CoO, CoxC, and Co Based Nanocomposites:

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Tuning Kinetics and Crystal Phase with Different Polyhydric Alcohols. Crystengcomm. 2014, 16, 8000-8007. (16) Qureshi, M.; Takanabe, K. Insights on Measuring and Reporting Heterogeneous Photocatalysis: Efficiency Definitions and Setup Examples. Chem. Mater. 2017, 29, 158-167. (17) Khon, E.; Lambright, K.; Khnayzer, R. S.; Moroz, P.; Perera, D.; Butaeva, E.; Lambright, S.; Castellano, F. N.; Zamkov, M. Improving the Catalytic Activity of Semiconductor Nanocrystals through Selective Domain Etching. Nano Lett. 2013, 13, 2016-2023. (18) Li, X. B.; Gao, Y. J.; Wu, H. L.; Wang, Y.; Guo, Q.; Huang, M. Y.; Chen, B.; Tung, C. H.; Wu, L. Z. Assembling Metallic 1T-MoS2 Nanosheets with Inorganic-Ligand Stabilized Quantum Dots for Exceptional Solar Hydrogen Evolution. Chem. Commun. 2017, 53, 5606-5609.

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