Earth-Abundant MoS2 and Cobalt Phosphate Dual Cocatalysts on 1D

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Earth-Abundant MoS2 and Cobalt Phosphate Dual Cocatalysts on 1D CdS Nanowires for Boosting Photocatalytic Hydrogen Production Kang-Qiang Lu, Ming-Yu Qi, Zi-Rong Tang, and Yi-Jun Xu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b01409 • Publication Date (Web): 31 Jul 2019 Downloaded from pubs.acs.org on August 6, 2019

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Earth-Abundant MoS2 and Cobalt Phosphate Dual Cocatalysts on 1D CdS Nanowires for Boosting Photocatalytic Hydrogen Production Kang-Qiang Lu,ab Ming-Yu Qi,ab Zi-Rong Tang,b and Yi-Jun Xu*ab

a State

Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, 350116, P. R. China

b College

of Chemistry, New Campus, Fuzhou University, Fuzhou 350116, P. R. China *To whom correspondence should be addressed. E-mail: [email protected]

1

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Abstract Cocatalysts play a significant role in accelerating the catalytic reactions of semiconductor photocatalyst. In particular, a semiconductor assembled with dual-cocatalysts, i.e., reduction and oxidation cocatalysts, can obviously enhance photocatalytic performance because of the synergetic effect of fast consumption of photogenerated electrons and holes simultaneously. However, in most cases, the noble metal cocatalysts are employed, which tremendously increases the cost of the photocatalysts and restricts its large-scale application. Herein, on the platform of one-dimensional (1D) CdS nanowires, we have utilized the earth-abundant dual-cocatalysts, MoS2 and cobalt phosphate (Co-Pi), to construct the CdS@MoS2@Co-Pi (CMC) core-shell hybrid photocatalysts. In this dual-cocatalysts system, Co-Pi is in a position to expedite the migration of holes from CdS, while MoS2 acts as electron transporter as well as active sites to accelerate the surface water reduction reaction. Taking the advantages of the dual-cocatalyst system, the prepared CMC hybrid shows obvious enhancement of both photoactivity and photostability toward hydrogen production compared with bare 1D CdS nanowire and binary hybrids (CdS@MoS2 and CdS@Co-Pi). This work highlights the promising prospects for rational utilization of earth-abundant dual-cocatalysts to design low-cost and efficient hybrids toward boosting photoredox catalysis. Keywords: Dual-cocatalysts; Earth-abundant; Core-shell; Photocatalysts

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Introduction Solar hydrogen evolution is an ideal alternative for fossil fuels because of its eco-friendly nature using abundant resources sun light and water.1-4 In principle, an ideal catalyst for photocatalytic hydrogen evolution should meet the following criteria: (i) conduction band (CB) edge should be more negative than standard H2 reduction potential (H+/H2, -0.41 V vs. NHE at pH = 7); (ii) effective light absorption with an appropriate band gap; (iii) photogenerated electron-hole pairs can be effectively separated and there should be adequate active sites on the surface of photocatalyst (iv) superior photocatalytic activity and anti-photocorrosion ability for long-time H2 evolution.5-8 Cadmium sulfide (CdS) is a hopeful candidate due to its favorable light absorption with band-gap of 2.4 eV and ideal CB edge for hydrogen evolution.9,10 However, the photocatalytic activity of CdS NWs is still relatively low because of the rapid charge carrier recombination and susceptibility to photocorrosion.11,12 In this regard, proper cocatalyst decoration for CdS is a feasible approach to enhance the photocatalytic performance. The introduction of cocatalysts can not only accelerate the separation of electron-hole pairs but also provide active sites to promote the surface redox reactions.13 Specifically, co-loading of both reduction and oxidation cocatalysts on semiconductor can greatly improve the photocatalytic activity for H2 production because of the synergetic effect of effective consumption of photogenerated electron-hole pairs simultaneously.14-17 So far, various dual-cocatalysts systems have been constructed, such as Pt-PdS/CdS18, Pd-IrOx/TiO2,19 Pt-IrO2/Ta3N5,20 and Rh/Cr2O3-Mn3O4/GaN:ZnO.21 However, noble metals are often employed in most cases and the up-scaling application of them is tremendously restricted by the prohibitive costs and scarcity.22,23 Therefore, substituting noble metals and utilizing the low-cost and earth abundant materials to construct dual-cocatalysts hybrid photocatalysts are extremely desired. In recent years, earth abundant transition metal-based cocatalysts have been extensively studied and deemed to be a promising alternative to noble metal.24-26 MoS2, as a typical layered transition metal disulfide, has been considered to be a promising cocatalyst substitute to noble metal because of its earth-abundant nature and similar free energy compared with Pt.27-31 In addition, MoS2 can not only act as electron delivery channels, but also offer abundant active sites for hydrogen production because the S atoms on exposed edges of MoS2 have strong bonds to H+ in the solution.29,32 Impregnation-reduction, thermal decomposition, or hydrothermal methods have been widely used to load MoS2 on semiconductor.33 However, these methods require high temperature or pressure and sometimes these routes require toxic H2S or explosive H2 as a co-sulfurizing reagent.34 Coupling MoS2 with semiconductor via photodeposition with (NH4)2MoS4 as precursor salt has been reported as a simple and effective way.35 In addition, the photodeposition method enables MoS2 to be intimately loaded on electron outlet points of semiconductor, facilitating interfacial charge transfer and shortening electron transfer path from the semiconductor to MoS2.36 On the other hand, earth-abundant cobalt phosphate (Co-Pi), which was firstly reported by Kanan and Noreca in 2008, has been shown excellent ability to transfer photogenerated holes of various light harvesting semiconductors, such as ZnO,37,38 C3N4,39 Fe2O3,40 and BiVO4.41 Electrochemical,42 extended X-ray absorption fine structure (EXAFS),43 and electron paramagnetic resonance (EPR)44 results in previous work indicate that working mechanism of Co-Pi is related to the convenient redox transformation of different chemical states of Co (Co2+, Co3+, and Co4+). In addition, Co-Pi can also be easily synthesized in neutral phosphate buffer solution containing Co2+ by simple photodeposition, by which Co-Pi is deposited where photogenerated holes are the most readily 3


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available.40 The formation mechanism includes oxidation of Co2+ ions to Co3+ ions by holes of semiconductors, while Co3+ ions have limited solubility in neutral phosphate buffer solution, which leads to precipitation of the Co-Pi.37 Consequently, rational deposition of earth-abundant MoS2 and Co-Pi dual-cocatalysts onto CdS is expected to harvest low-cost and efficient hybrid photocatalyst for boosting hydrogen production. Herein, on the platform of 1D CdS NWs, a well-defined CdS@MoS2@Co-Pi (CMC) core-shell hybrid photocatalyst has been synthesized by a simple two-step photodeposition. The obtained 1D core-shell construction provides a compact interfacial interaction between CdS NWs core and MoS2/Co-Pi shell, which is able to effectively promote separation of photoinduced electron-hole pairs. In this hybrid system, Co-Pi cocatalyst acts as a holes transfer accelerator to extract photogenerated holes from CdS NWs while photoexcited electrons from CdS NWs inject to MoS2 which serves as the active sites for facilitating H2 production. Such efficient improvement of charge carriers separation and transfer results in significantly enhanced photocatalytic activity and anti-photocorrosion ability of the hybrid photocatalyst. The as-obtained CMC composites with optimal composition have displayed a H2-evolution rate of about 40.5 mmol g-1 h-1 under visible light irradiation, which is markedly higher than bare CdS NWs and binary counterparts of CdS NWs with MoS2 (CdS@MoS2) or CdS NWs with Co-Pi (CdS@Co-Pi). This work suggests the promising scope of rational utilization of earth-abundant dual-cocatalysts to construct high-performance photocatalysts with boosted photoactivity and photostability. Experimental Section Materials All of chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), including ammonium tetrathiomolybdate ((NH4)2MoS4), sodium phosphate monobasic ((NaH2PO4·2H2O), sodium diethyldithiocarbamate trihydrate (C5H10NNaS2·3H2O), disodium hydrogen phosphate ((Na2HPO4·12H2O), cobalt nitrate (Co(NO3)2·6H2O), cadmium chloride (CdCl2·2.5H2O), alcohol (C2H6O) lactic acid (C3H6O3) and ethylenediamine (C2H8N2). All the reagents were of analytical grade and were used without further purification. Deionized water was used in all experiments. Materials Synthesis Synthesis of CdS nanowires (CdS NWs) CdS NWs were synthesized via a simple solvothermal method, which was reported in our previous works.45,46 Detailed preparation process was demonstrated in supporting information. Synthesis of CdS@MoS2 (CM) hybrids A simple one-step photodeposition method was used to fabricate CM composites.47 Typical, 0.1 g of the prepared CdS NWs were dispersed in a 100 mL mixed solution of distilled water (80 mL) and ethanol (20 mL). Then, (NH4)2MoS4 with different weight was added and the obtained solution was degassed with nitrogen for half an hour. After that, above solution was illuminated by visible light for one hour in nitrogen atmosphere. Subsequently, these samples were collected via filtration and rinsed by distilled water for 3 times. Finally, CM hybrids with various weight percents of MoS2 were blow-dried by nitrogen. Deposition contents of MoS2 in the obtained CM composites can be varied by changing the amount of (NH4)2MoS4. The synthesized products were denoted as 4


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CxM (x = 0.5, 1, 2, 5 and 7), where x is the theoretical weight percentage of loaded MoS2. Preparation of CdS@MoS2@Co-Pi (CMC) composites According to the photocatalytic activity test results, C2M displayed the highest H2-production activity among CM composites. Therefore, C2M was selected to investigate the synergistic effects of MoS2 and Co-Pi. Typically, 40 mg C2M composites were dispersed in an 80 mL aqueous neutral buffer solution containing 0.1 M sodium phosphate and a certain amount of cobalt nitrate hexahydrate (Co(NO3)2·6H2O). After nitrogen degassing for 30 minutes, the suspension was illuminated with visible light for 1 hour in nitrogen atmosphere. After photodepositions, samples were washed drastically with distill water and dried in vacuum. The deposition contents of Co-Pi in the CMC composites can be varied by changing the amount of Co(NO3)2·6H2O. The obtained products were denoted as CMxC (x = 5, 10, 20, and 30), where x is theoretical weight percentage of the loaded Co-Pi. CdS@Co-Pi (CC) hybrids with 20 wt% Co-Pi were synthesized under same conditions without the first photochemical deposition step. In addition, different weight percents of Pt instead of MoS2 were photodeposited on CdS NWs and CC using H2PtCl6 as precursor. CdS/Pt (CP) hybrids with different theoretical weight percent of Pt were recorded as C0.5P, C1P, C1.5P and C2P. Analogously, resultant CdS/Pt/Co-Pi (CPC) hybrids with different theoretical weight percent of Pt were recorded as C0.5PC, C1PC, C1.5PC and C2PC, respectively. Notably, these obtained samples should be preserved in N2 atmosphere. Characterization The powder X-ray diffraction (XRD) spectra of photocatalysts were measured on a Bruker D8 Advance powder X-ray diffractometer with Ni-filtered Cu Kα radiation. The patterns were recorded between 5 to 80° with a scan rate of 0.02° s-1 at 40 kV and 40 mA. UV-vis diffuse reflectance spectra (DRS) were measured on a scanning UV-vis spectrometer (Cary 500, Varian Co.). The transmission electron microscopy (TEM), mapping images and energy dispersive X-ray spectroscopy (EDX) spectra were measured using a JEOL model JEM 2010 EX instrument with operating voltage at 200 kV. The Brunauer-Emmett-Teller (BET) surface area measurements were recorded with N2 adsorption/desorption isotherms by Micromeritics ASAP2010 equipment. X-ray photoelectron spectroscopy (XPS) measurements were conducted on Thermo Scientific ESCA Lab250 spectrometer. Photoluminescence (PL) spectra of obtained photocatalysts were measured on Edinburgh FLS-920 spectrofluorometer (Edinburgh, UK). The Xe lamp emission with 405 nm wavelength was used as the excitation source for the steady-state PL spectra. For the measurement of PL lifetime, a 405 nm laser beam was used as the excitation source and maximum emission wavelength was 520 nm. In order to guarantee the accuracy, conditions of the PL measurement including quality of photocatalyst, slit width and excitation wavelength, were identical. The loading amounts of MoS2 and Co-Pi in CM20C composite and concentration of leached ions (Cd2+, Mo4+ and Co3+/Co2+) in solution after recycling test were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, PerkinElmer Optima 8000). Photoelectrochemical tests were carried out on an electrochemical workstation (Autolab PGSTAT204) in a conventional three-electrode system, where photocatalysts were spread on fluorine-doped tin oxide (FTO) glass as the working electrode, Ag/AgCl electrode was utilized as the reference electrode and Pt wire was applied as the counter electrode. For the working electrode, 5 mg of the as-prepared photocatalyst was dispersed in 0.5 mL of ethanol to obtain slurry by 5


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ultrasonication. Then, 20 μL of catalyst slurry was dropped onto FTO glass with an active area of ca. 1 cm2. After airing at room temperature, working electrode was ulteriorly dried at 373 K for two hours to enhance adhesion. Electrochemical impedance spectroscopy (EIS) was measured in a 0.5 M KCl electrolyte containing 0.01 mM K3[Fe(CN)6]/K4[Fe(CN)6]. Linear sweep voltammetry (LSV) was tested with a scanning speed of 0.2 mV s−1 in Na2SO4 solution (0.1 M). The photocurrent were measured without voltage bias and electrolyte was Na2SO4 solution (0.1 M, pH = 6.8). The open-circuit photovoltage decay (OCPD) was also measured in Na2SO4 solution (0.1 M) by visible light illuminated for 120 s and then supervised the succedent attenuation of photovoltage for 130 s with light turned off. Incident photon-to-current conversion efficiency (IPCE) was conducted by PEC-S20 (Peccell Technology Co. Ltd.) without bias potential. The light was afforded by AM 1.5 solar simulator and the light intension was corrected with a normative solar cell for amorphous silicon solar cell manufactured by Japan Quality Assurance Organization. The value of IPCE was calculated based on the following formula:48 IPCE 

1240  J ph

  I light

100%

(1)

where Jph was photocurrent density, Ilight was wavelength of incident light, and Ilight was power density of incident light for each wavelength. Photocatalytic Hydrogen Evolution Photocatalytic activity test of hydrogen production was implemented in a Pyrex vessel, which was connected to a gas-circulation and vacuum plant. Typically, 20 mg photocatalyst was dispersed in an 80 mL aqueous solution (72 ml H2O and 8 ml lactic acid). Then, the reaction system was drastically deaerated and illuminated by a xenon lamp (Perfect Light, Beijing, China) with a filter to cut off ultraviolet light and incident light power density was 405 mW·cm-2. In addition, circulating refrigerant water system was employed to ensure that temperature of reaction solution can be maintained at 278 K. Generated H2 was monitored termly by a gas chromatograph (Shimadzu GC-2014C, argon as a carrier gas and MS-5A column). To evaluate photocatalytic stability of obtained samples, cyclic tests were measured, and detailed experimental procedures were shown as follows. After photocatalytic reaction of first run, the catalysts were separated and rinsed by distilled water for 3 times. Afterwards, second photocatalytic recycling test was carried out by mixing fresh reaction solution with the used catalyst. The subsequent three runs of cyclic experiment were executed in an analogous pattern. Apparent quantum efficiency (AQE) was measured under identical condition compared with the photocatalytic activity test and illuminated by xenon lamp with various wavelength (400 nm, 420 nm, 450 nm, 500nm, 520 nm) band-pass filters. The photon flux of light was supervised by a PL-MW200 photoradiometer (Perfect Light, Beijing, China). The AQE was calculated based on the formula below: ηAQE =

Ne Np

× 100% =

2

× S

M

×

× P

NA ×

× t

h ×

× λ

c

× 100% (2)

Where, Ne was the amount of reacted electrons, Np was the amount of incident photons, M was the quantity of H2 molecules (mol), NA was Avogadro constant (6.022×1023/mol), h was the Planck constant (6.626×10-34 J•s), c was the speed of light (3×108 m/s), S was the irradiation area (cm2), P 6


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was the irradiation light intensity (W/cm2), t was the photoreaction time (s), λ was the wavelength of the monochromatic light (m). Results and Discussion Figure 1A displays the synthetic process of CdS@MoS2@Co-Pi (CMC) hybrids by two-step photodeposition technique. At first, when CdS NWs have been irradiated with visible light in the presence of inert N2 atmosphere, photoexcited electron of CdS NWs can reduce Mo6+ of (NH4)2MoS4 to form MoS2, and thus a series of CdS@MoS2 (CM) hybrids with various theoretical weight percent of MoS2 can be synthesized.30,47,49 Subsequently, Co2+ ions of cobalt nitrate solution can be oxidized into Co3+ by photogenerated holes of C2M composite. Because the Co3+ has limited solubility at aqueous phosphate buffer solution with pH = 7, amorphous cobalt phosphate (Co-Pi) will be deposited on the surface of CMC composites in situ.50-52 The distinctive advantage of two-step photodeposition method is that Co-Pi can be deposited where photogenerated holes are the most readily available, and MoS2 is loaded in situ on the electron outlet points of semiconductor, which is beneficial to inhibit the stack of MoS2 and Co-Pi.35,36,40 Morphology and microscopic structure information of the dual-cocatalysts system have been checked by transmission electron microscopy (TEM). As displayed in Figure 1B, pure CdS NWs display a one-dimensional (1D) morphology with diameters of about 50 nm and the surface of blank CdS NWs is very smooth. After two-steps photochemical deposition process, as revealed in Figure 1C and D, the CM20C composite still maintains 1D morphology, but the surface becomes coarse along total length, demonstrating a well-defined core-shell structures (taking the photocatalyst with the highest activity as an example, as discussed later). Identified edge area of the CM20C composite indicates that intimate interface interaction between CdS NWs core with MoS2/Co-Pi shell layer has been formed. Since migratory process of photogenerated charge carriers in hybrid photocatalyst intimately relates with interfacial contact, it can be anticipated that such a compact interfacial interaction for the CM20C hybrid photocatalyst will promote the transfer of charge carrier across the interface between MoS2/Co-Pi shell layer and CdS NWs core.53 The core-shell structure of CM20C composite has been further confirmed by elemental mapping images. As demonstrated in Figure 1E, element Cd is confined in core, whereas elements Mo, S, P and Co attributed to MoS2 and Co-Pi are homogenously distributed on the whole shell of nanowire. In addition, energy dispersive X-ray spectroscopy (EDX) measurement on designate district of the CM20C composite has also demonstrated the presence of Cd, Mo, P, S and Co elements (Figure 1F). High-resolution TEM (HRTEM) image of the CM20C composite has also been shown in Figure S1. It is seen that different form CdS NWs which show clear crystal lattice, 0.34 nm for (002) facet of hexagonal CdS, there is no obvious lattice spacing observed from shell layer of MoS2/Co-Pi, which is due to amorphous nature of MoS2 and Co-Pi.39,47,54,55 X-ray photoelectron spectrum (XPS) measurements have also been performed to obtain detailed information of chemical constituent and binding status of elements of CM20C hybrid. The XPS survey spectrum in Figure S2A indicates that elements Cd, Mo, Co, S and O are co-existence in the CM20C composite. As shown in Figure S2B, the Cd 3d XPS spectrum shows two peaks at 405.3 and 412.0 eV, and they can be assigned to Cd2+ in CM20C.47,56 In addition, Figure 2A reveals Mo 3d XPS spectrum, and the binding energies at 228.9 and 232.1 eV are attributed to Mo 3d5/2 and Mo 3d3/2, indicating Mo is in +4 valence state.45 In S 2p high-resolution XPS spectrum shown in Figure 2B, it can be observed that the binding energy at 161.5 and 162.7 eV can be 7


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assigned to S2−.32 In addition, Figure 2C and D show the P 2p and Co 2p XPS spectrum. The peak of P 2p is at 133.3 eV, indicating that P exists in the form of phosphate group.52 In the case of Co 2p XPS spectrum, peaks of Co 2p3/2 and Co 2p1/2 are situated at 781.6 eV and 797.3 eV (satellite peaks at 786.7 eV and 803.3 eV), indicating presence of Co3+ and surface adsorbed Co2+ ions in the sample, which accords well with Co-Pi species reported in the literatures.38,40,50,52 These results confirm the existence of MoS2 and Co-Pi in the CM20C hybrid. Crystal structures of these synthetic samples have been confirmed by X-ray diffraction (XRD). As indicated in Figure 2E, XRD pattern of CdS NWs is in line with the hexagonal phase CdS (PDF#77-2306).57,58 CC, C2M and CM20C composites have exhibited similar XRD patterns as compared with the blank CdS NWs and the diffraction peaks of MoS2 and Co-Pi are not observed in these composites, which confirms the amorphous nature of MoS2 and Co-Pi.39,47 In addition, XRD patterns of CM composites with different MoS2 loading contents (Figure S3A) and CMC composites with different Co-Pi loading contents (Figure S3B) further confirm that the crystal structure of CdS NWs is not changed after deposition of MoS2 or Co-Pi, and the amounts of the cocatalysts have no obvious effect on the crystal texture of the CdS NWs. The optical characters of the prepared photocatalysts have also been measured by diffuse reflectance spectra (DRS) spectra. As indicated in Figure 2F, blank CdS NWs display an absorption edge around 520 nm, which indicates its band-gap is about 2.4 eV.47 Compared with pure CdS NWs, absorption intensities of CC, C2M and CM20C hybrids in visible light range (520-750 nm) are gradually increased, because of intense absorption of MoS2 and Co-Pi.16,30,55 In addition, no obvious absorption edge shift has been observed for CM composites with different MoS2 loading contents (Figure S4A) and CMC composites with different Co-Pi loading contents (Figure S4B), which demonstrates that MoS2 and Co-Pi cocatalysts have been deposited only on surface and not incorporated into lattice of CdS NWs.8,17,59 Photocatalytic hydrogen evolution is used to evaluate photocatalytic performance of blank CdS NWs and composite photocatalysts under visible light irradiation with the addition of lactic acid, which has been well proven to be a superior hole scavenger without obvious influence on the origin of the produced H2.47,60 As displayed in Figure 3A, pure CdS NWs show a reversely low activity with hydrogen evolution rate of 1.5 mmol g-1 h-1, corresponding to an AQE of 0.9% under visible light irradiation, due to poor separation efficiency of photoinduced electron-hole pairs and the lack of surface reactive site.9,22 After introduction of MoS2 cocatalyst, the optimal C2M composite exhibits a higher yield of H2 production (24.8 mmol g-1 h-1, corresponding to an AQE of 26%) compared with blank CdS NWs. This can be attributed to the fact that MoS2 deposited onto CdS NWs can not only act as electronic capture agents to rapid transfer of photogenerated electrons, but also provide active sites to accelerate surface reaction of H2 production.61 In addition, the optimal CC composite also shows higher H2 evolution rate (6.7 mmol g-1 h-1, corresponding to an AQE of 6.3%) compared with blank CdS NWs, which is because deposition of Co-Pi can promote the transfer of photogenerated holes and reduce recombined rate of photogenerated electron-hole pairs.55 In particular, when MoS2 with theoretical weight percentage of 2% and Co-Pi with theoretical weight percentage of 20% have been loaded in the CdS NWs simultaneously, the optimal CM20C composite has achieved hydrogen evolution rate of about 40.5 mmol g-1 h-1 and it is approximately 27 times higher than the H2 evolution rate of pure CdS NWs. The significant improvement of the photocatalytic activity over CM20C composite is due to the synergistic effect of dual-cocatalyst. In the dual-cocatalyst system, Co-Pi cocatalyst can rapidly transport photogenerated holes and MoS2 can effectively migrate photogenerated electrons and provide active 8


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site.62 Thereby, the recombination of photogenerated electron-hole pairs can be effectively inhibited and thus obviously enhancing photocatalytic activity of CM20C composite. The reaction system can produce vast gas bubbles in reaction vessel over the CM20C composite, which are easily observable by naked eye (Movie S1). As shown in Table S1, compared to some representative photocatalytic hydrogen generation activity of CdS-based ternary hybrids reported in recent years, the as-prepared CM20C composite in this work displays superior photocatalytic hydrogen evolution performances. In addition, photocatalytic activity of CdS/Pt (CP) and CdS/Pt/Co-Pi (CPC) composites using noble metal Pt as cocatalyst instead of MoS2 has also been measured for contrast under the same conditions. As shown in Figure S5A and B, the optimal H2 evolution rate of CP or CPC composite is much lower than that of the CM20C composite. In order to further investigate the effect of deposition amount of cocatalyst on photocatalytic performance over the CdS NWs, photocatalytic activity of CM composites with different MoS2 loading contents and CMC composites with different Co-Pi loading contents have been tested. As indicated in Figure S6, when MoS2 with theoretical weight percentage of 2%, C2M composite shows the optimal H2 production rate, and further increasing deposition of MoS2 in the hybrids will lead to a piecemeal decline of photoactivity, which may be ascribed to the fact that enhanced light absorption of increased MoS2 will influence the activation of CdS NWs.32,49 Similar reduction of photocatalytic activity has also been observed for CMC composites when the theoretical weight percentage of Co-Pi exceeds 20% (Figure S7). The probable reason for the negative effect of excess Co-Pi is because higher loading amount of Co-Pi will mask the active reaction sites of the CMC composites.8,62 Therefore, controlling addition ratios of MoS2 and Co-Pi is critical for obtaining the best synergistic interaction between cocatalysts and CdS NWs, and thus achieving the optimal activity of the CMC composites. In addition, the actual loading amounts of MoS2 and Co-Pi in CM20C composite have been measured by inductively coupled plasma optical emission spectroscopy (ICP-OES). As shown in Table S2, actual loading weight percent of MoS2 and Co-Pi in CM20C composite is 0.54% and 4.99%, respectively. To elaborate the influence of optical absorption on photocatalytic activity, wavelength-dependent H2 evolution and AQE measurement of CM20C composite have been performed. As shown in Figure 3B and C, it can be seen that H2 evolution and AQE of the CM20C match well with the UV/Vis absorption spectrum of the CM20C composite and no H2 has been detected when wavelength of incident light exceeds 520nm, indicating that the photocatalytic reaction is indeed driven by light irradiation. The AQE value of CM20C at 420 nm is estimated to be 36%, which is obviously higher than that of the pure CdS NWs, CC and C2M composite. Furthermore, incident photon to current conversion efficiency (IPCE) of pure CdS NWs, CC, C2M and CM20C hybrids has been performed. As shown in Figure 3D, it can be seen that when wavelength is above 520 nm, the IPCE responses values of these samples become to zero, which is due to the fact that the photon energy within this region is lower than the band gap of CdS NWs, and thus the CdS NWs cannot be excited. The IPCE responses values of these samples follow the order CM20C > C2M > CC > CdS NWs within the same wavelength profile, which is consistent with the aforementioned hydrogen evolution performance of these samples, further confirming that decorating of dual-cocatalysts, MoS2 and Co-Pi, is an effective way to improve charge transfer efficiency of CdS NWs. To investigate the photostability of these samples, recycling tests over pure CdS NWs, C2M, CC and CM20C composite have been performed. As shown in Figure 3E, the consequence 9


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indicates that after 20 hours of light illumination, photocatalytic activity of pure CdS NWs has declined about 62%, confirming obvious photocorrosion of blank CdS NWs. After loading cocatalyst MoS2 or Co-Pi, the C2M and CC composites show improved photocatalytic stability to some extent. As shown in Figure S8A and B, the H2 evolution activities of C2M and CC have deteriorated by ca. 33% and 16% after five times recycling test. When MoS2 and Co-Pi load in the CdS NWs simultaneously, the CM20C hybrid photocatalyst shows superior photocatalytic stability, and the decrease of photocatalytic activity is lower than 10% after five cycle tests (Figure 3F). The enhanced photocatalytic stability of CM20C composite is mainly because Co-Pi as hole collectors can promptly migrate photogenerated holes to prevent photocorrosion of CdS NWs, while MoS2 can efficiently migrate photogenerated electrons so that electrons accumulated on CdS NWs can effectively reduce protons.25,50 As shown in Figure S9A and B, the results of XRD and DRS over fresh and used CM20C composites show the same crystalline structure and optical absorption property. The elemental mapping images of used CM20C samples also indicate no obvious change of morphology and structural composition over CM20C after visible light illumination of 20 hours (Figure S10). Furthermore, as shown in Table S3, ICP-OES result of reaction solution of CM20C composite indicates that there is no obvious leaching of the ions (Cd2+, Mo4+ and Co3+/Co2+) during the recycling test. In contrast, significant leaching of Cd2+ has been observed for the reaction solution of blank CdS NWs, confirming the serious photocorrosion of CdS NWs. These complementary characterizations unambiguously confirm that the introduction of dual-cocatalysts can markedly improve anti-photocorrosion ability of CdS NWs. To further explore the synergistic effect of MoS2 and Co-Pi on boosting photocatalytic acticity of CdS NWs, a series of complementary photo- and electrochemical characterizations have been performed. As displayed in Figure 4A, transitory photocurrent-time curves of blank CdS NWs, CC, C2M and CM20C composite are acquired by on-off cycles of intermittent visible light illumination. Blank CdS NWs show the minimum photocurrent intensity, manifesting the slowest transfer rate and a severe recombination of photogenerated charge carriers.62 Compared with blank CdS NWs, the photocurrent density of CC and C2M electrodes has increased after photodeposition of the Co-Pi or MoS2, respectively. This result confirms that introduction of Co-Pi or MoS2 can facilitate transfer and decrease recombination of photogenerated charge carriers.54 Specially, when Co-Pi and MoS2 are loaded onto CdS NWs simultaneously, CM20C composite demonstrates the highest photocurrent intensity, which can be attributed to the synergistic effect of the dual-cocatalysts. In the dual-cocatalysts system, MoS2 is able to efficiently transfer photogenerated electrons accumulated on the CdS NWs and Co-Pi as hole collectors can effectively transfer the photogenerated holes.61 Furthermore, as shown in Figure S11, linear sweep voltammetry (LSV) results exhibit analogous trends, which further confirms the important roles of dual-cocatalysts for boosting hydrogen evolution performance of CdS NWs. Electrochemical impedance spectroscopy (EIS) is another effective way to investigate the charge transfer velocity in the photocatalyst.27,63 As shown in Figure 4B, the four samples all display semicycles at high frequency, and the minimum arc radius of CM20C implies the that its photogenerated electron-hole pairs can be most effectively separated as compared with CC, C2M and blank CdS NWs.16,64 In order to further investigate the transter efficiency of photoinduced charge carriers within the photocatalytic materials, a measurement of open-circuit photovoltage decay (OCPD) has been performed to assess the lifetime of photoelectrons. During light illumination, the sample electrodes 10


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absorbed the photons and decreased their open-circuit voltage (VOC). After equilibration to a steady state, the light has been turned off, and subsequent decay of VOC has been observed. The decay lifetime of the accumulated electrons can be related to the drop in potential using the Supplementary Equation 1.65-67 The calculated photoelectron lifetime as a function of VOC is displayed in Figure 4C, from which it can be found that CC and C2M show longer electron-lifetime than blank CdS NWs. Furthermore, because of the synergistic effect of MoS2 and Co-Pi dual-cocatalysts, CM20C composite demonstrates distinctly prolonged electron lifetime as comparison with CC, C2M and blank CdS NWs. To further investigate the effect of adding MoS2 or Co-Pi for charge carriers of CdS NWs, time-resolved photoluminescence spectra (TRPL) measurement is exhibited, which probes the specific charge carrier dynamics of the systems (Figure 4D).68,69 The emission decay curves of the samples are fitted by exponential decay kinetics function expressed as follows: I(t) = A1 • exp(-t/τ1) + A2 • exp(-t/τ2) + A3 • exp(-t/τ1)

(3)

In addition, the average emission lifetime, reflecting the overall emission decay behavior of samples, has also been calculated through the following equation: Ave.τ =(A1τ12 + A2τ22 + A3τ32)/(A1τ1 + A2τ2 + A3τ3)

(4)

where τ1, τ2, and τ3 are the emission lifetimes, and A1, A2, and A3 are the corresponding amplitudes.70 As listed in Table S4, it can be seen that compared with pure CdS NWs, the average lifetimes of CC, C2M and CM20C decrease from 9.54 ns to 9.08, 8.57, and 6.61 ns, respectively. Furthermore, the steady-state PL spectra in Figure 4E show obvious PL quenching of CC, C2M and CM20C as compared with blank CdS NWs. The corresponding observations of PL quenching and lifetime reduction confirm that photodeposition of MoS2 or Co-Pi on the surface of CdS NWs affords a route for photogenerated charge carriers transfer, which contends with the excited state inactivation of CdS NWs.71 Notably, when MoS2 and Co-Pi have been loaded simultaneously, CM20C composite shows the shortest fluorescent lifetime and most obvious PL quenching, which is attributed the synergistic effect of MoS2 and Co-Pi dual-cocatalysts, rendering the most effective interfacial charge transfer and separation of photogenerated electron-hole pairs in CM20C composite.62,70 Furthermore, Figure S12 displays the N2 adsorption-desorption isothermals of the four samples. It can be found that these photocatalysts show analogous BET surface area, indicating surface area is not primary factor for the obvious photocatalytic activity disparity between CdS, CC, C2M and CM20C composites. On the other hand, diverse separation and migration efficiency of photogenerated charge carrier caused by loading of cocatalysts are the predominant factors affecting the photoactivity between CdS, CC, C2M and CM20C composite. Based on the abovementioned consequences, we have proposed a feasible mechanism of photocatalytic hydrogen evolution under visible light illumination. As displayed in Figure 4F, under visible light irradiation, electrons are excited from valence band (VB) of CdS NWs to conduction band (CB), leaving holes in the VB simultaneously. Then, electrons and holes will randomly shift on the CdS NWs surface. In this course, a part of electrons and holes will recombine, which is an significant factor that restraints the performance of photocatalyst.62 In this work, MoS2 has been deposited on CdS NWs as electron trap and active sites. Consequently, photoexcited 11


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electrons can easily migrate to MoS2 and then directly combine with protons in water to form H2. Furthermore, based on the proposed working mechanism of Co-Pi by Surendranath et al.,51 the holes of CdS NWs will transfer to Co-Pi and drive circular catalytic Co2+/3+  Co4+  Co2+/3+ reactions, accompanied by a fast output of holes from CdS NWs to oxidize sacrificial reagent of lactic acid.50,52 Therefore, the synergistic effects of MoS2 and Co-Pi dual-cocatalyst for electrons extraction and holes consumption guarantee the effective separation of electron-hole pairs, and thus obviously enhancing photocatalytic activity and anti-photocorrosion ability of CM20C composite. Conclusions In summary, a novel one-dimensional core shell CdS@MoS2@Co-Pi (CMC) hybrid photocatalyst has been rationally constructed via a facile two step photodeposition strategy. By adjusting the photodeposition amount of MoS2 and Co-Pi, the optimized CM20C hybrid achieves a high H2-production rate of about 40.5 mmol g-1 h-1, which is significantly higher than bare CdS NWs and binary counterparts of CdS NWs with MoS2 (CdS@MoS2) or CdS NWs with Co-Pi (CdS@Co-Pi). In the dual-cocatalysts system, MoS2 not only promote the transfer of photoinduced electrons but also can provide active sites for hydrogen evolution, while Co-Pi can act as a pump to accelerate hole output from CdS NWs. Therefore, synergistic effect of the dual-cocatalysts efficiently promotes separation of photoinduced charge carrier, and thus enhancing H2 production photoactivity and anti-photocorrosion ability of the hybrid CMC photocatalysts. This work opens a new avenue for construction of low-cost and efficient hybrid composites via rational utilization of earth-abundant dual-cocatalysts for versatile photocatalytic applications with enhanced performance. Conflicts of interest There are no conflicts to declare. Acknowledgements The support from the National Natural Science Foundation of China (21872029, U1463204, 21173045 and 20903023), the first Program of Fujian Province for Top Creative Young Talents, the Independent Research Project of State Key Laboratory of Photocatalysis on Energy and Environment (No. 2014A05), the Award Program for Minjiang Scholar Professorship, the Program for Returned High-Level Overseas Chinese Scholars of Fujian province and the Natural Science Foundation of Fujian Province for Distinguished Young Investigator Rolling Grant (2017J07002) is kindly acknowledged. Supporting Information. Additional characterization and photoactivity results. This material is available free of charge via the Internet at http://pubs.acs.org.

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10430-10441. (63)Moniz, S. J. A.; Shevlin, S. A.; Martin, D. J.; Guo, Z.-X.; Tang, J. Visible-Light Driven Heterojunction Photocatalysts for Water Splitting–a Critical Review. Energy Environ. Sci. 2015, 8, 731-759. (64)Qiu, B.; Xing, M.; Zhang, J. Mesoporous TiO2 Nanocrystals Grown in Situ on Graphene Aerogels for High Photocatalysis and Lithium-Ion Batteries. J. Am. Chem. Soc. 2014, 136, 5852-5855. (65)Xiao, F.-X.; Miao, J.; Wang, H.-Y.; Yang, H.; Chen, J.; Liu, B. Electrochemical Construction of Hierarchically Ordered CdSe-Sensitized TiO2 Nanotube Arrays: Towards Versatile Photoelectrochemical Water Splitting and Photoredox Applications. Nanoscale 2014, 6, 6727-6737. (66)Meekins, B. H.; Kamat, P. V. Got TiO2 Nanotubes? Lithium Ion Intercalation Can Boost Their Photoelectrochemical Performance. ACS Nano 2009, 3, 3437-3446. (67)Hung, S.-F.; Xiao, F.-X.; Hsu, Y.-Y.; Suen, N.-T.; Yang, H.-B.; Chen, H. M.; Liu, B. Iridium Oxide-Assisted Plasmon-Induced Hot Carriers: Improvement on Kinetics and Thermodynamics of Hot Carriers. Adv. Energy Mater. 2016, 6, 1501339. (68)Zhang, Z.; Huang, Y.; Liu, K.; Guo, L.; Yuan, Q.; Dong, B. Multichannel-Improved Charge-Carrier Dynamics in Well-Designed Hetero-nanostructural Plasmonic Photocatalysts toward Highly Efficient Solar-to-Fuels Conversion. Adv. Mater. 2015, 27, 5906-5914. (69)Bi, W.; Zhang, L.; Sun, Z.; Li, X.; Jin, T.; Wu, X.; Zhang, Q.; Luo, Y.; Wu, C.; Xie, Y. Insight into Electrocatalysts as Co-catalysts in Efficient Photocatalytic Hydrogen Evolution. ACS Catal. 2016, 6, 4253-4257. (70)Yang, M.-Q.; Xu, Y.-J.; Lu, W.; Zeng, K.; Zhu, H.; Xu, Q.-H.; Ho, G. W. Self-Surface Charge Exfoliation and Electrostatically Coordinated 2D Hetero-Layered Hybrids. Nat. Commun. 2017, 8, 14224. (71)D'Souza, F.; Smith, P. M.; Zandler, M. E.; McCarty, A. L.; Itou, M.; Araki, Y.; Ito, O. Energy Transfer Followed by Electron Transfer in a Supramolecular Triad Composed of Boron Dipyrrin, Zinc Porphyrin, and Fullerene: a Model for the Photosynthetic Antenna-Reaction Center Complex. J. Am. Chem. Soc. 2004, 126, 7898-7907.

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Figure 1. Schematic diagram for the preparation of CMC composite photocatalysts (A), typical TEM image of as-prepared blank CdS NWs (B), typical TEM images of CM20C composite (C, D), mapping analysis results of CM20C composite (E) and EDX of the CM20C composite (F).

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Figure 2. XPS spectra of Mo 3d (A), S 2p (B), Co 2p (C), and P 2p (D) of CM20C composite; XRD patterns (E) and UV–vis DRS (F) of blank CdS, CC, C2M and CM20C composites.

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Figure 3. Photocatalytic H2 evolution over blank CdS, CC, C2M and CM20C composites (A). Wavelength-dependence of H2 evolution over CM20C composite (B). AQE of the CM20C versus the incident light wavelength (left axis), UV-Vis light absorption spectrum of the CM20C composite (right axis) (C). IPCE spectra of the blank CdS, CC, C2M and CM20C composites under monochromatic light irradiation with wavelength ranging from 300 nm to 540 nm (D). Recycling test of photocatalytic H2 evolution over CdS NWs (E) and CM20C composite (F). Note that the error bars represent the photoactivity s.d. values calculated from triplicate experiments.

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Figure 4. Transient photocurrent responses (A). Electrochemical impedance spectroscopy Nyquist plots (B). Electron lifetime determined from the decay of open circuit potential in dark (C). Time-resolved photoluminescence spectra decay (excitation at 405 nm and emission at 520 nm) (D). Steady-state photoluminescence spectra (E) over CdS, CC, C2M and CM20C composites. Proposed mechanism of the photocatalytic H2 generation over the CMC composite (F).

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TOC Graphic

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Earth-Abundant MoS2 and Cobalt Phosphate Dual Cocatalysts on 1D

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