Site-Selective Deposition of Reductive and Oxidative Dual Cocatalysts

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Energy, Environmental, and Catalysis Applications

Site-selective deposition of reductive and oxidative dual cocatalysts to improve the photocatalytic hydrogen production activity of CaIn2S4 with surface nanostep structure Jianjun Ding, Xiangyang Li, Lin Chen, Xian Zhang, Hao Yin, and Xingyou Tian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17663 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 23, 2018

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ACS Applied Materials & Interfaces

Site-Selective Deposition of Reductive and Oxidative Dual Cocatalysts to Improve the Photocatalytic Hydrogen Production Activity of CaIn2S4 with Surface Nanostep Structure Jianjun Dingab*, Xiangyang Liab, Lin Chenab, Xian Zhangab, Hao Yincd*, and Xingyou Tianab a

Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese

Academy of Sciences, Hefei 230088, People's Republic of China. b

CAS Key Laboratory of Photovoltaic and Energy Conservation Materials, Chinese

Academy of Sciences, Hefei 230031, People's Republic of China. c

CAS Key Laboratory of Crust-Mantle Materials and the Environments, School of

Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, People’s Republic of China d

Mass Spectrometry Lab, Hefei National Laboratory for Physical Sciences at

Microscale, University of Science and Technology of China, Hefei 230026, People’s Republic of China KEYWORDS: Photocatalytic hydrogen production, CaIn2S4, dual cocatalyst, spatial separation, activation energy

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ABSTRACT

Photocatalytic hydrogen production from water exhibits great potential for solar energy conversion. In this work, using monoclinic CaIn2S4 with surface nanostep structure as a model photocatalyst, we demonstrate a facile and efficient strategy for the construction of AOx/AuCu/CaIn2S4 (A=Mn, Ni and Pb) composites by siteselective photo-deposition of reductive cocatalyst AuCu alloy and oxidative cocatalyst AOx on the edge and groove sites of CaIn2S4 nanosteps, respectively. Compared to single-cocatalyst composites (AuCu/CaIn2S4 and AOx/CaIn2S4) and CaIn2S4, the simultaneous deposition of AuCu and AOx spatially separate the photogenerated charges and the photocatalytic reaction sites, therefore effectively improving the separation efficiency of charge carriers. Meanwhile, the synergistic effect of AuCu and AOx dual cocatalysts notably reduces the apparent activation energy for photocatalytic hydrogen production reaction. This novel dual-cocatalyst composites show enhanced performance for hydrogen production under visible light irradiation. A high rate of hydrogen production of 95.75 mmol h-1 g-1 is achieved over MnOx/AuCu/CaIn2S4 composite with the deposition of 0.5 wt% AuCu and 0.2 wt% MnOx. Our work sheds new lights on designing efficient photocatalytic materials with site-selective surface deposition of reductive and oxidative dual cocatalysts for solar energy conversion.


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INTRODUCTION Semiconductor photocatalysis has attracted lots of attention due to its potential application in solar energy conversion and environmental management in the past few decades. To that end, considerable efforts have been made in the development of photocatalysts with high efficiency, especially under visible light irradiation.1-5 According to the semiconductor energy band theory, the photocatalytic reaction mainly includes the following three processes: (1) generation of electron-hole pairs in semiconductor with the irradiated energy greater or equal to the band gap of the semiconductor, (2) separation and transportation of the photogenerated charges before recombination, and (3) redox reactions with the reactants adsorbed on the surface of the photocatalyst. Therefore, the keys of the enhancement of photocatalytic activity are to broaden the light absorption, to promote the separation of the photogenerated charges and to catalyze the surface redox reactions. To date, the efficiency for photocatalytic solar conversion is still unsatisfactory mainly due to the high probability of carrier recombination. The recombination of the photogenerated charges can be effectively reduced through loading of cocatalysts, which facilitate the transportation of the photogenerated charges from the semiconductor photocatalyst to the cocatalysts.6-10 Compared to the homogeneous deposition of cocatalysts that are random distributed on the surface of photocatalysts, the selective deposition of reductive and oxidative dual cocatalysts, respectively, can trap the photogenerated electrons and holes, further improving the separation efficiency and the photocatalytic performance. Past efforts



on the selective loading of dual cocatalysts were mainly been attempted based on semiconductor photocatalysts with different crystal facets, such as TiO2, BiVO4, BiOCl, Cu2O and NaTaO3.11-18 In these dual-cocatalyst composites, reductive and oxidative cocatalysts are in-situ selectively deposited on the reduction and oxidation facets of the photocatalysts. It can be achieved by photo-deposition because the photogenerated electrons and holes can selectively migrate to the different facets to take part in photo-deposition reactions. After the deposition of dual cocatalysts, the photogenerated electrons can directionally migrate from the semiconductor photocatalyst to the reductive cocatalyst where the reduction reaction occurs, while the oxidation reaction takes place on the surface of the oxidative cocatalyst. Therefore, the photocatalytic reduction and oxidation reaction sites are spatially separated. However, there are limited reports on this creative construction due to the lack of suitable photocatalysts to support the selective deposition of dual cocatalysts. Moreover, it’s difficult to obtain photocatalysts with 100% exposure degree of the specified crystal facets, and the influence of coexisting crystal facets is also hard to be excluded. As we know, nanostep structure on the surface of a photocatalyst is beneficial for the separation and transportation of the photogenerated charges because the photogenerated electrons and holes in semiconductor photocatalyst incline to migrate to the edge and groove sites of surface nanostep structure, respectively. The nanostep structure can be used not only to improve the separation efficiency of charge carriers but also to realize the spatial separation of photocatalytic reduction and oxidation


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reaction sites. By taking the advantage of the nanostep structure, a variety of photocatalysts, such as NaTaO3,19-22 (AgIn)xZn2(1-x)S2,23 NaNbO3-xNx,24 La2Ti2O7,25,26 Na2Ca2Nb4O13,27 CdS,28 Cd1-xZnxS 29 and monoclinic CaIn2S4,30 have been developed as efficient photocatalysts with different surface nanostep structures, for hydrogen production and organic decomposition. In our previous work, Au bimetallic nanoparticles (AuPt and AuCu) were site-selectively photo-deposited on the edges of CaIn2S4 nanosteps using the photo-deposition method, showing enhanced performance

for

photocatalytic

hydrogen

production

under

visible

light

illumination.31,32 During the photo-deposition process, almost all metal precursors were in-situ photo-reduced to metal nanoparticles and site-selectively deposited on the edges of the surface nanosteps. The site-selective deposition of Au bimetallic nanoparticles facilitated the transportation of the photogenerated electrons because the photogenerated electrons can directionally transfer from the conduction band of CaIn2S4 to the site of Au bimetallic nanoparticles. Unfortunately, the photogenerated holes remained at the groove sites of CaIn2S4 nanosteps, which still reduced the separation efficiency of the photogenerated charges. However, to the best of our knowledge, no study has been focused on simultaneous deposition of reductive and oxidative dual cocatalysts on the edge and groove sites of the nanostep structure, respectively. Inspired by previous work, for the first time, reductive and oxidative dual cocatalysts were site-selectively deposited on the edge and groove sites of monoclinic CaIn2S4 nanosteps in the absence of any sacrificial reagents during the photo-



deposition process. Reductive cocatalyst AuCu alloy nanoparticles were siteselectively deposited on the edge sites of CaIn2S4 nanosteps operating as the photocatalytic reduction sites for hydrogen production, while oxidative cocatalyst AOx (A=Mn, Ni and Pb) particles were site-selectively deposited on the groove sites of CaIn2S4 nanosteps served as the photocatalytic oxidation sites. Compared to CaIn2S4, the characteristic structure of AOx/AuCu/CaIn2S4 composites shows the following advantages: (1) The simultaneous deposition of AuCu and AOx dual cocatalysts promoted the spatial separation of the photogenerated charges and the photocatalytic reaction sites, which effectively suppressed the recombination of charge carries; (2) The synergistic effect of dual cocatalysts notably reduced the apparent activation energy for photocatalytic hydrogen production; (3) The surface plasmon resonance (SPR) effect of Au in AuCu nanoparticles provides more electrons to take part in photocatalytic hydrogen production reaction because of the characteristic absorption of Au in the visible region; (4) The introduction of secondmetal Cu in AuCu alloy acts as an electron acceptor, further improving the transportation of the photogenerated electrons from CaIn2S4 or Au to Cu. As we expected, AOx/AuCu/CaIn2S4 composites exhibited considerable activity for photocatalytic hydrogen production under visible light irradiation. Our results demonstrated the great potential of construction of efficient photocatalysts with spatial separation of reduction and oxidation sites with the aid of the surface nanostep structure. EXPERIMENTAL SECTION


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Materials All reagents were commercial and used without further purification. Indium nitrate (InN3O9·xH2O, 99.9%), chloroauric acid (HAuCl4·3H2O, 99.9%), lead nitrate (PbN2O6, 99.99%), Manganese nitrate solution (MnN2O6, 50 wt% in H2O), nickel nitrate (NiN2O6·xH2O, 99.99%) and potassium iodate (KIO3, 99.8%) were purchased from Aladdin Industrial Inc. All other reagents were at least of analytical grade supplied by Sinopharm Chemical Reagents Co., Ltd. According to our previous work,32 monoclinic CaIn2S4 was prepared through the high-temperature sulfurization method, and single-reductive-cocatalyst composite Au0.4Cu0.1/CaIn2S4 (abbreviated as AuCu/CaIn2S4) with AuCu alloy structure was synthesized using the photo-deposition method. Site-selective photo-deposition of metals and/or oxides Dual-cocatalyst composites AOx/AuCu/CaIn2S4 (A=Mn, Ni and Pb) were fabricated by simultaneous photo-deposition of AuCu alloy and AOx. A 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfectlight Co., Ltd.) was used as the visible light source (420 nm ≤ λ ≤ 750 nm) with the equipment of UV and IR cutoff filters. In a typical preparation process, 200 mg of monoclinic CaIn2S4 was dispersed into 100 mL water solution containing cocatalysts precursors (such as HAuCl4, Cu(NO3)2, Mn(NO3)2, Ni(NO3)2 and Pb(NO3)2). Before photo reaction for 3 h under stirring, the suspension was degassed completely with pure N2. The obtained powder was filtered, washed and dried in vacuum at 333 K overnight. The loading amount of AuCu alloy nanoparticles was set at 0.5 wt% with an Au/Cu of 4 by the weight of the metallic contents. The



content of metal oxides was changed from 0 to 0.5 wt%. The as-prepared samples were denoted as MnOx/AuCu/CaIn2S4, NiOx/AuCu/CaIn2S4 and PbO2/AuCu/CaIn2S4, respectively, where the value of x in MnOx and NiOx locates between 1.5 and 2.0. Single-oxidative-cocatalyst composite MnOx/CaIn2S4 was also fabricated by the photo-oxidation reaction with the addition of Mn(NO3)2 and electron sacrificial reagent KIO3 through a similar procedure. The experimental conditions of the photo-deposition of metal and/or metal oxides were listed in Table S1. The result of ICP-AES analysis showed that most metal precursors were photoreduced (or photooxidated) to metal (or metal oxides). Photocatalytic Experiments The experiments of photocatalytic hydrogen production were investigated using a gas-closed circulation reactor with an outside cooling-water jacket (to keep the reaction temperature at room temperature). 10 mg of photocatalyst, 0.025 mol of Na2S/Na2SO3, and 100 mL deionized water was first added into the reactor to form a homogeneous suspension under stirring. Then, high pure N2 was used to remove the dissolved air in the suspension. During the photocatalytic reaction under visible light, the evolved gas was intermittently analyzed online using a Kexiao GC-1690 gas chromatograph equipped with a TDX01 column and a thermal conductive detector (TCD) detector. The photocatalytic performance for hydrogen production was also evaluated at different irradiation wavelengths to investigate the synergistic effect of dual cocatalysts. The abovementioned Xe lamp was also used but with regulation of the


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light irradiation wavelength using a series of band-pass filters (λmax=435, 450, 475, 500, 520, 550 and 600 nm, half-width=15 nm, transmissivity≥ 85%, Beijing Perfectlight Co., Ltd.). The photocatalytic stability was carried out using cyclic experiment. After the photocatalytic reaction, the photocatalyst powder was collected and reused in the fresh sacrificial reagents solution. The apparent activation energies of the photocatalytic reaction for hydrogen production were determined at four different temperatures (T=283, 298, 308 and 318 K). The apparent activation energy Ea was calculated from the slope of the plots of lnR (R is the photocatalytic hydrogen production rate, μmol/min) against 1/T, (Ea=slope×8.314, KJ/mol). Structure Characterization The crystal structure of the as-prepared photocatalysts was characterized by powder X-ray diffraction (XRD) technique using a Rigaku TTR III X-ray diffractometer with a Cu Kα radiation source. The microstructure, including SEM, TEM, HRTEM, STEM and line scanning, was studied using a GeminiSEM 500 Schottky-field-emission scanning electron microscope and a JEM-2100F field-emission transmission electron microscope. The absorption spectrum was collected using a SolidSpec 3700 UV/vis spectrometer. The surface composition was investigated by an ESCALAB 250 X-ray photoelectron spectroscopy. The surface areas were estimated using the BrunauerEmmett-Teller (BET) method (Tristar II 3020 M) and listed in Table S1. The contents of Au, Cu and AOx were measured using inductively coupled plasma atomic emission



spectroscopy (ICP-AES, Optima 7300 DV) and listed in Table S1. The photoelectrochemical properties were analyzed using a CHI-760D electrochemical analyzer where a three-electrode configuration (including a Pt wire counter electrode, an Ag/AgCl reference electrode and a working electrode) was employed. The working electrodes with an active area of 0.5 cm2 were fabricated using a dip-coating method as follows: 10 mg of the as-prepared photocatalyst was first dispersed in 3 mL absolute ethanol containing 5% DuPont Nafion solution, then the suspension was directly dip-coated onto a FTO glass electrode. After the volatilization of ethanol, the dip-coating process was repeated two times before being heated in a vacuum drying oven at 80 oC for 1 h. RESULTS AND DISCUSSION Characterization of structure and morphology


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Figure

1.

XRD

patterns

of

CaIn2S4,

MnOx/CaIn2S4,

AuCu/CaIn2S4

and

MnOx/AuCu/CaIn2S4. Figure 1 shows the XRD patterns of CaIn2S4, MnOx/CaIn2S4, AuCu/CaIn2S4 and MnOx/AuCu/CaIn2S4, respectively. For CaIn2S4, all the peaks can be ascribed to the pure phase of monoclinic CaIn2S4 (PCPDF #72-0875). The main diffraction peaks at 13.1o, 17.4o, 24o, 26.7o, 27.5o, 31.5o, 38.5o, 45.1o and 46.4o corresponding to the (002), (220), (111), ( − 112), ( − 312), (712), (805), ( − 1802) and (007) planes of CaIn2S4, respectively. After the deposition of AuCu alloy nanoparticles and/or metal oxide MnOx, the intensities and positions of the diffraction peaks did not change in the XRD patterns of the composites, indicating the phase structure stability of monoclinic CaIn2S4 after the photo-deposition process. No phases of Au, Cu, MnOx nor other metal compounds were observed, mainly due to the low content and the high dispersion of the metal and/or metal oxide.33,34 For the site-selective photo-deposition, three synthesis routes including single photo-reduction, single photo-oxidation as well as simultaneous photo-reduction and photo-oxidation were investigated. In our previous work,32 single photo-reduction deposition of AuCu alloy nanoparticles on the surface of CaIn2S4 was achieved using HAuCl4 and Cu(NO3)2 as metal precursors and Na2S/Na2SO3 (or CH3OH) as hole sacrificial reagents. Almost all AuCu alloy nanoparticles were site-selectively deposited on the edges of CaIn2S4 nanosteps and no metal nanoparticles can be observed outside the nanostep. Single photo-oxidation deposition of Mn2+ ion on CaIn2S4 surface was carried out with KIO3 as an electron sacrificial reagent. From



Figure S1, sponge-like manganese oxides site-selectively deposited on the groove sites of CaIn2S4 nanosteps, indicating that the photogenerated holes migrated to the groove sites to take part in the photo-oxidation reaction. Kudo demonstrated that PbO2 nanoparticles with a diameter of about 5 nm can be clearly distinguished at the groove sites of NaTaO3:La nanosteps through the photo-oxidation of Pb2+.19 However, AOx particles (including MnOx, NiOx and PbO2) with the size from several hundreds of nanometers to several microns were formed after the photo-oxidation reaction in this study. AOx particles exhibit a sponge-like structure which is similar to that in MnOx/BiVO4 composites.13

Figure 2. TEM (A, B), STEM (C) and HRTEM (D for AuCu alloy, E for MnOx) images of MnOx/AuCu/CaIn2S4 composite. The simultaneous photo-reduction and photo-oxidation deposition of metal and metal oxide on the surface of CaIn2S4 were obtained by using HAuCl4, Cu(NO3)2 and Mn(NO3)2 as precursors without any sacrificial reagents. SEM (Figure S2) and TEM (Figure 2) images show that metal and metal oxide particles were site-selectively


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photo-deposited on CaIn2S4 nanosteps. To clearly reveal the microstructure of AOx/AuCu/CaIn2S4 composites, the high-resolution TEM (HRTEM) and high-angle annular dark field scanning TEM (HAADF-STEM) images with line scan measurements were carried out. It can be clearly seen from the STEM image in Figure 2 that metal nanoparticles and metal oxide were successfully deposited on the edge and groove sites of CaIn2S4 nanosteps, respectively. The measured lattice fringe of 0.23 nm is in accordance with the calculated value of Au0.4Cu0.1 alloy structure according to the Vegard’s law (aalloy=aAu × 0.8 + aCu × 0.2=0.2355 × 0.8 + 0.2087 × 0.2=0.2301 nm),35,36 demonstrating that both Au and Cu exist in the form of an alloy structure. The average diameter of AuCu alloy nanoparticles in MnOx/AuCu/CaIn2S4 composite was measured to be 7.2 nm (Figure S3), which is slightly smaller than that in AuCu/CaIn2S4 composite (8.3 nm). The planar spaces of 0.236 and 0.278 nm corresponding to the (121) and (101) planes of MnO2, while the planar space of 0.165 is attributed to the (440) plane of Mn2O3, confirming that the formation of manganese oxides (MnOx, x=1.5~2.0).37-39 The site-selective formation of AuCu alloy and MnOx is an indication that the metal precursors (HAuCl4 and Cu(NO3)2) are in-situ photoreduced to AuCu alloy nanoparticles by the accumulated electrons at the edge sites of CaIn2S4 nanosteps while Mn(NO3)2 is in-situ photo-oxidized to manganese oxides by the accumulated holes at the groove sites of CaIn2S4 nanosteps during the simultaneous photo-reduction and photo-oxidation process. The reaction can be summarized in equation (1) and (2): Au 4   Cu 2   e   AuCu (1)



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Mn 2   H 2O  h   MnO x  H  (2) This was further confirmed by the line scanning profiles, as shown in Figure 3. The spatially resolved element mapping of gold, copper, manganese and oxygen demonstrates that Au and Cu are homogeneous distributed in an individual nanoparticles and that Mn(NO3)2 is photo-oxidized to form a sponge-like shape of MnOx. The TEM and STEM images of NiOx/AuCu/CaIn2S4 and PbO2/AuCu/CaIn2S4 composites were also shown in Figure S4 and S5. Both AuCu alloy nanoparticles and NiOx (or PbO2) particles were also site-selectively photo-deposited on CaIn2S4 nanosteps. Almost no AuCu alloy or AOx can be observed outside CaIn2S4 nanosteps. All these results highly suggested that reductive cocatalyst (AuCu alloy) and oxidative cocatalyst (MnOx, NiOx and PbO2) were site-selectively deposited on the edge and groove sites of CaIn2S4 nanosteps during the simultaneous photo-reduction and photo-oxidation process, respectively. It means that the photogenerated electrons and holes readily migrated and accumulated at the corresponding edge and groove sites and the precursors were in-situ photo-reduced (or photo-oxidized) to metal nanoparticles

(or

metal

oxide

particles).

The

synthesis

approach

of

AOx/AuCu/CaIn2S4 composites proceeds entirely without the usage of any sacrificial reagents and depends on the characteristic structure of surface nanosteps to spatially separate the oxidation and reduction sites.


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Figure 3. STEM images and EDS line scanning profiles of MnOx/AuCu/CaIn2S4 composite. To analyze the composition of MnOx/AuCu/CaIn2S4 composite, high-resolution XPS spectra of Au 4f, Cu 2p and Mn 2p were carried out and shown in Figure 4. For MnOx/CaIn2S4 composite, the two main peaks of Mn 2p at 641.7 and 653.2 eV are the signals of Mn 2p3/2 and Mn 2p1/2 of Mn-O compounds.40-41 The peak of the Mn 2p3/2 was fitted with two peaks located at 641−642 and 642−643 eV (Figure S6), corresponding to Mn3+ and Mn4+ species, respectively. The result confirms that Mn ion exists in Mn3+ and Mn4+ oxidation states, which is consistent with the result of HRTEM. The binding energies of Au 4f and Cu 2p in AuCu/CaIn2S4 composite are



the characteristics of metallic gold and copper, respectively. Compared to the binding energies of bulk Au and Cu, the shift of the binding energies of Au 4f and Cu 2p is attributed to the interaction and electron transfer between AuCu alloy and CaIn2S4.4244

The introduction of Cu in AuCu alloy can serve as an electron acceptor which can

further improve the separation and transportation of the photogenerated electrons from CaIn2S4 or Au to Cu.32 When AuCu alloy and MnOx simultaneously photodeposited on CaIn2S4 nanosteps, the binding energies of Au 4f, Cu 2p and Mn 2p in MnOx/AuCu/CaIn2S4 composite were almost identical to those in AuCu/CaIn2S4 and MnOx/CaIn2S4 composites, revealing that the composition and electronic structure of Au, Cu and Mn were not affected by the synthesis strategies. High resolution XPS spectra of Ni 2p in NiOx/AuCu/CaIn2S4 and Pb 4f in PbO2/AuCu/CaIn2S4 were also investigated and shown in Figure S6. The results indicated that Ni ion and Pb ion existed in the form of NiOx (x=1.5−2) and PbO2, respectively.


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Figure 4. High-resolution XPS spectra of Au 4f, Cu 2p and Mn 2p in MnOx/CaIn2S4 (A), AuCu/CaIn2S4 (B, C) and MnOx/AuCu/CaIn2S4 (D, E and F) composites. Figure 5 shows the UV-visible absorption spectra of CaIn2S4, MnOx/CaIn2S4, AuCu/CaIn2S4 and MnOx/AuCu/CaIn2S4 composites. The absorption band edge and the direct band gap of CaIn2S4 were estimated to be 450 nm and 2.6 eV, respectively. The enhancement of the absorption of MnOx/CaIn2S4 composite in visible region was mainly attributed to the scattering effect of MnOx particles. For AuCu/CaIn2S4 composite, the visible absorption beyond the absorption region of CaIn2S4 was assigned to the characteristic SPR absorption of Au in AuCu alloy nanoparticles, which has been widely proved to be beneficial for the improvement of the photocatalytic activity.45-46 The absorption enhancement of MnOx/AuCu/CaIn2S4



composite in visible region can be considered as the synergy of AuCu alloy and MnOx particles.

Figure 5. UV-visible absorption spectra of CaIn2S4, MnOx/CaIn2S4, AuCu/CaIn2S4 and MnOx/AuCu/CaIn2S4 composites. Photocatalytic performance for hydrogen production The visible-light photocatalytic activities of the as-prepared photocatalysts were evaluated through the photocatalytic hydrogen production reaction in Na2S/Na2SO3 solution. Control experiments showed that no hydrogen was detected in the absence of photocatalyst or light, indicating that hydrogen was entirely produced through the photocatalytic reaction. As shown in Figure 6A, without any cocatalysts, CaIn2S4 can produce hydrogen from Na2S/Na2SO3 solution with a rate of 0.59 mmol h-1 g-1 due to the characteristic structure of surface nanosteps. With the site-selective deposition of


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single cocatalyst on CaIn2S4 nanosteps, the photocatalytic activity for hydrogen production can be improved effectively. For MnOx/CaIn2S4 composite, the hydrogen production rate was increased to 5.52 mmol h-1 g-1, while it was 45.28 mmol h-1 g-1 for AuCu/CaIn2S4 composite. Significant difference in photocatalytic hydrogen production activity between AuCu alloy and MnOx cocatalyst indicates that the siteselective deposition of AuCu reductive cocatalyst is more beneficial for photocatalytic hydrogen production reaction because of the effective separation of the photogenerated charges and the SPR effect of Au. In AuCu/CaIn2S4 composite, the enhancement of photocatalytic hydrogen evolution performance was mainly caused by the synergistic effect of AuCu bimetallic nanoparticles and the SPR effect of Au. The photogenerated electrons on the surface of CaIn2S4 can migrate to AuCu alloy nanoparticles, inhibiting the recombination of the electron-hole pairs. With the addition of second-metal Cu, the accumulated electrons on the surface of Au can further migrate to the adjacent Cu due to the formation of AuCu alloy structure. The SPR effect of Au nanoparticles can absorb more light beyond the absorption of CaIn2S4 and provide more electrons to take part in the photocatalytic hydrogen production reaction. In MnOx/CaIn2S4 composite, the photogenerated holes at the groove sites of CaIn2S4 nanosteps migrate to the surface of MnOx and the photogenerated electrons leave at the edge sites of CaIn2S4 nanosteps where they participate in hydrogen production. Therefore, the separation of the photogenerated charges and the absorption in visible region in AuCu/CaIn2S4 composite are more effective than in the MnOx/CaIn2S4 composite. That’s why the photocatalytic



hydrogen production activity of AuCu/CaIn2S4 composite is much higher than that of MnOx/CaIn2S4 composite.

Figure 6. (A) Photocatalytic hydrogen activity over the obtained photocatalysts with different cocatalysts. (B) Photocatalytic hydrogen activity of MnOx/AuCu/CaIn2S4 composites as a function of MnOx content (the content of Au0.4Cu0.1 alloy was set at 0.5 wt%). (C) Photocatalytic hydrogen activity of MnOx/AuCu/CaIn2S4 composites as a function of AuCu alloy content (the content of MnOx was set at 0.2 wt%). (D)


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Photocatalytic hydrogen activity of MnOx/AuCu/CaIn2S4 composites as a function of Au/Cu ratio (the content of Au, Cu or AuCu alloy was set at 0.5 wt%; the content of MnOx

was

set

at

0.2

wt%).

(E)

Photocatalytic

hydrogen

activity

of

AOx/AuCu/CaIn2S4 (A=Mn, Ni and Pb) composites with different oxidative cocatalysts. (F) Photocatalytic stability of MnOx/AuCu/CaIn2S4 composites. When AuCu alloy and MnOx were simultaneously photo-deposited on the edge and groove sites of CaIn2S4 nanosteps, respectively, the photocatalytic hydrogen production rate of MnOx/AuCu/CaIn2S4 composite was further improved to 95.75 mmol h-1 g-1. This value was 2.1, 17.3 and 162.3 times higher than that of AuCu/CaIn2S4, MnOx/CaIn2S4 and CaIn2S4, respectively. The photocatalytic performance with different values of MnOx content, AuCu content and Au/Cu ratio in MnOx/AuCu/CaIn2S4 composite is presented in Figure 6B, 6C and 6D, respectively. In MnOx/AuCu/CaIn2S4 composite, the optimal content of MnOx was 0.2 wt%, while it was 0.5 wt% in MnOx/CaIn2S4 composite (Figure S7). The optimal content of MnOx in MnOx/AuCu/CaIn2S4 composite was lower than that in MnOx/CaIn2S4 composite due to the co-existence of AuCu alloy nanoparticles. The optimal value of AuCu alloy content and Au/Cu ratio in MnOx/AuCu/CaIn2S4 composite was still 0.5 wt% and 4:1, respectively, indicating the deposition of MnOx did not change the characteristics of AuCu alloy. The difference of the metal oxides in photocatalytic hydrogen production activity over AOx/AuCu/CaIn2S4 composites was investigated. As shown in Figure 6E, MnOx/AuCu/CaIn2S4 composite exhibited the best photocatalytic performance for hydrogen production under visible light irradiation.



All the dual-cocatalyst composites showed better photocatalytic performance than the corresponding single-cocatalyst composites. The result indicated that the synergistic effect of dual cocatalysts is beneficial to the effective separation of the photogenerated charges and the improvement of photocatalytic performance. From Figure 6F, after 40 h photocatalytic reaction in the fourth reaction run, MnOx/AuCu/CaIn2S4 composite retained 95% of the initial photocatalytic activity under visible light irradiation, suggesting good stability of MnOx/AuCu/CaIn2S4 composite. The migration of the photogenerated holes from CaIn2S4 to MnOx should be helpful for the improvement of photocatalytic stability.

Figure 7. Photocatalytic activities of AuCu/CaIn2S4 and MnOx/AuCu/CaIn2S4 composites as a function of the incident light wavelength. The dash line was the absorption spectrum of CaIn2S4.


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The difference in photocatalytic hydrogen activity between single-cocatalyst and dual-cocatalyst composites can be further proved from Figure 7, which plots the hydrogen production rates from AuCu/CaIn2S4 and MnOx/AuCu/CaIn2S4 composites as a function of the irradiation wavelength. Almost no hydrogen was detected for CaIn2S4 and MnOx/CaIn2S4 composite under the irradiation of λ ≥ 475 nm, indicating that MnOx is only an oxidative cocatalyst without SPR effect in visible region. Both AuCu/CaIn2S4 and MnOx/AuCu/CaIn2S4 composites exhibited considerable activity when the irradiation wavelength was beyond the absorption of CaIn2S4, indicating that the SPR effect of Au is very important for the enhancement of photocatalytic activity. The hydrogen production rate from MnOx/AuCu/CaIn2S4 composite was about 4 times higher than that from AuCu/CaIn2S4 composite at all the selected wavelengths. The maximum hydrogen production rate over AuCu/CaIn2S4 composite was 10.06 mmol h-1 g-1 when the irradiation light was located at 475 nm. Surprisingly, the corresponding hydrogen production rate was 43.8 mmol h-1 g-1 at 475 nm for MnOx/AuCu/CaIn2S4 composite, which is comparable to that of AuCu/CaIn2S4 composite under visible light irradiation. The result further confirmed that the coexistence of AuCu alloy and MnOx on the edge and groove sites of CaIn2S4 nanosteps enhanced the photocatalytic performance for hydrogen production. Photoelectrochemical properties To prove the favourable effect of the dual cocatalysts on the separation and transportation of the photogenerated charges and the enhancement of photocatalytic performance under visible light irradiation, photoelectrochemical measurements were



carried out. The photocurrent transient responses of CaIn2S4, MnOx/CaIn2S4, AuCu/CaIn2S4 and MnOx/AuCu/CaIn2S4 were collected, as shown in Figure 8a. Photocurrent is readily generated in all photocatalyst electrodes, with uniformity and reproducibility for four on/off cycles of illumination. Under the irradiation of visible light, the photocurrent is improved step-by-step from CaIn2S4, MnOx/CaIn2S4, AuCu/CaIn2S4 to MnOx/AuCu/CaIn2S4 electrodes. The photocurrent density of MnOx/AuCu/CaIn2S4 electrode was measured to be 0.45 mA/cm2, which is about 11.2, 7.5 and 1.3 times higher than that of CaIn2S4, MnOx/CaIn2S4 and AuCu/CaIn2S4 electrodes, respectively. The enhancement of photocurrent density indicates that dualcocatalyst composite MnOx/AuCu/CaIn2S4 exhibits much higher charge separation efficiency than single-cocatalyst composites MnOx/CaIn2S4 and AuCu/CaIn2S4, and pure CaIn2S4. 47,48

Figure 8. Transient photocurrent density responses (a) and EIS spectra (b) of CaIn2S4 (D), MnOx/CaIn2S4 (C), AuCu/CaIn2S4 (B) and MnOx/AuCu/CaIn2S4 (A). Electrochemical impedance spectra (EIS) analysis is a powerful tool to study the interfacial charge transfer process and the EIS Nynquist plots of the four electrodes are shown in Figure 8b. It can be seen that with the site-selective deposition of


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cocatalysts on CaIn2S4 nanosteps, the semicircles of all composite electrodes in the plots became shorter, which indicated that the charge transfer resistance on the interfaces between CaIn2S4 photocatalyst and cocatalysts decreased. Among all the electrodes, MnOx/AuCu/CaIn2S4 composite electrode has the smallest arc radius, demonstrating more effective charge separation and faster interfacial charge transportation with the site-selective deposition of reductive cocatalyst AuCu alloy and oxidative cocatalyst MnOx on the edge and groove sites of CaIn2S4 nanosteps.49,50 The photoelectrochemical results are in accordance with that of photocatalytic performance. Discussion As abovementioned, the photogenerated electrons and holes can readily migrate to the edge and groove sites of the characteristic nanostep structure on the surface of CaIn2S4 due to the different charge densities between edge and groove sites. Therefore, the precursors should be in-situ photo-reduced to metal nanoparticles and in-situ photo-oxidized to metal oxides by the accumulated electrons and holes at the edge and groove sites, respectively. For single-cocatalyst and dual-cocatalyst composites, the process of migration and transportation of the photogenerated charges under visible light irradiation is different, which is proposed in Figure 9. In singlecocatalyst composite AuCu/CaIn2S4, the photogenerated electrons in the conduction band of CaIn2S4 and the surface of Au will eventually migrate to the surface of Cu where hydrogen will be preferentially produced as indicated by the formation of AuCu alloy structure. Directional migration of electrons to Cu maximizes the atom



utilization efficiency of the metal in photocatalytic hydrogen reaction. However, the photogenerated holes remain at the groove sites of CaIn2S4 nanosteps. Similarly, the photogenerated electrons remain at the edge sites while the photogenerated holes migrate to the surface of MnOx in single-cocatalyst composite MnOx/CaIn2S4. In both single-cocatalyst composite AuCu/CaIn2S4 and MnOx/CaIn2S4, the remaining electrons or holes on the surface of CaIn2S4 will inevitably recombine. Therefore, the site-selective photo-deposition of single reductive or oxidative cocatalyst cannot balance the mobility of the photogenerated electrons and holes. In dual-cocatalyst composite MnOx/AuCu/CaIn2S4, the photogenerated electrons in both Au and CaIn2S4 migrate to the surface of Cu to participate the reduction reaction of water, while the photogenerated holes in CaIn2S4 migrate to the surface of MnOx to be captured by the hole sacrificial reagents when MnOx/AuCu/CaIn2S4 composite is irradiated under visible light. In this case, the directional migration of electrons and holes to the reductive and oxidative cocatalysts promotes the real separation of the photogenerated charges in space, and realize the balance of the photogenerated electrons and holes to the specific reaction sites during the photocatalytic reaction. Therefore, the siteselective photo-deposition of dual cocatalysts on CaIn2S4 nanosteps can accelerate the separation and transportation of the photogenerated charges and accordingly improve the photocatalytic performance for hydrogen production. For single-cocatalyst composites, the deposition of reductive cocatalyst AuCu alloy on the nanostep structure is more favorable for the photocatalytic hydrogen production compared to the deposition of oxidative cocatalyst MnOx. Besides, the slight increase of the


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surface area is also contributed to the improvement of the photocatalytic activity (Table S1).

Figure 9. Proposed mechanism of the charge separation and transportation for CaIn2S4, single-cocatalyst and dual-cocatalyst composites under visible light irradiation. On the other hand, when MnOx/AuCu/CaIn2S4 composite is irradiated under SPR excitation (λ≥ 475 nm), electrons and holes can only generate on the surface of Au because the energy of the irradiation wavelength is lower than the band gap of CaIn2S4. The accumulated electrons on the surface of Au will have higher potential



energy,

51,52

which can transfer to the surface of CaIn2S4 or adjacent Cu. Thus, the

site-selective deposition of oxidative cocatalyst MnOx on the groove sites of CaIn2S4 nanosteps cannot affect the separation and transportation of the photogenerated charges since there is no photogenerated holes on the surface of CaIn2S4. In other words, the deposition of MnOx in MnOx/AuCu/CaIn2S4 composite should not change the photocatalytic activity for hydrogen production, compared to AuCu/CaIn2S4 composite. However, the photocatalytic activity for hydrogen production of MnOx/AuCu/CaIn2S4 composite under SPR excitation was much higher than that of AuCu/CaIn2S4 composite, according to Figure 7. As is known to all, the role of cocatalysts on the surface of photocatalysts can not only facilitate the charge separation and transportation but also reduce the apparent activation energy.53-55 The apparent activation energies for hydrogen production over different photocatalysts were calculated by means of Arrhenius equation, as shown in Figure 10. The photocatalytic rates for hydrogen production at different temperatures were listed at Table S2. The site-selective deposition of reductive cocatalyst AuCu alloy can remarkably lower the apparent activation energy from 48.2 to 26.4 KJ/mol, indicating AuCu alloy is an effective reductive cocatalyst to trigger the photocatalytic hydrogen reaction at lower activation energy and thus greatly enhance photocatalytic hydrogen performance of CaIn2S4. Interestingly, the apparent activation energy for photocatalytic hydrogen production can also be slightly decreased with the loading of oxidative cocatalyst MnOx. As a result, the simultaneous deposition of reductive cocatalyst AuCu alloy and oxidative cocatalyst MnOx can further lower the apparent


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activation energy to 19.4 KJ/mol, compared to AuCu/CaIn2S4 composite. Therefore, decreasing the apparent activation energy is important for promoting the photocatalytic hydrogen production performance of MnOx/AuCu/CaIn2S4 composite under SPR excitation with the site-selective deposition of AuCu alloy and MnOx cocatalysts. Moreover, the decreasing of the AuCu alloy size in MnOx/AuCu/CaIn2S4 composite may induce additional positive effect on the photocatalytic enhancement because decreasing the cocatalyst size increases the number of unsaturated coordination atoms and the active sites for photocatalytic hydrogen production reaction. 56-58

Figure 10. Photocatalytic rates for hydrogen production as a function of temperature under visible light illumination. The apparent activation energies are obtained by fitting the results with an Arrhenius equation. CONCLUSIONS



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In summary, for the first time, we have successfully fabricated AOx/AuCu/CaIn2S4 composites (A=Mn, Ni and Pb) using the sacrificial-reagents-free photo-deposition method. During the photo-deposition process, the metal precursors HAuCl4 and Cu(NO3)2 were in-situ photoreduced to AuCu alloy nanoparticles and site-selectively deposited on the edge sites of CaIn2S4 nanosteps, while the metal oxide precursors (such as Mn(NO3)2, Ni(NO3)2 or Pb(NO3)2) were in-situ photooxidated to metal oxide particles and site-selective deposited on the groove sites of CaIn2S4 nanosteps. The simultaneous deposition of reductive cocatalyst AuCu alloy and oxidative cocatalyst AOx on the edge and groove sites of CaIn2S4 nanosteps spatially separated the photogenerated charges and the photocatalytic reaction sites because the photogenerated electrons and holes can be readily migrate to AuCu alloy and AOx, respectively.

Compared

to

single-cocatalyst

composites

AuCu/CaIn2S4

and

AOx/CaIn2S4, the synergistic effect of AuCu and AOx cocatalysts can not only effectively improve the separation efficiency of the photogenerated charges, but also notably lower the apparent activation energy for photocatalytic hydrogen production reaction. As a result, AOx/AuCu/CaIn2S4 composites exhibited considerable activity for photocatalytic hydrogen production under visible light irradiation. A high hydrogen production rate of 95.75 mmol h-1 g-1 for MnOx/AuCu/CaIn2S4 composite was achieved with 0.5 wt% AuCu and 0.2 wt% MnOx in the composite, which was 2.1, 17.3 and 162.3 times higher than that of AuCu/CaIn2S4, MnOx/CaIn2S4 and CaIn2S4, respectively. The authors believe that site-selective deposition of reductive


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and oxidative cocatalysts on semiconductor photocatalysts with surface nanostep structure provides new prospective to design efficient photocatalytic systems. ASSOCIATED CONTENT Supporting Information. Experimental parameters, atomic concentrations, surface areas and photocatalytic activities for the obtained samples, rates of hydrogen production from different temperature, TEM images of MnOx/CaIn2S4 composite, SEM image of MnOx/AuCu/CaIn2S4 composite, the size distribution of AuCu alloy nanoparticles in MnOx/AuCu/CaIn2S4 composite, TEM and STEM images of NiOx/AuCu/CaIn2S4 and PbO2/AuCu/CaIn2S4 composites, XPS spectra of Mn 2p3/2, Ni 2p and Pb 4f, photocatalytic activity of MnOx/CaIn2S4 composite as a function of MnOx content. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Tel: (+86)551-65591418; Fax: (+86)551-65393564; E-mail: [email protected] (J.D.). * E-mail: [email protected] (H.Y.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT



This work was financially supported by the CASHIPS Director’s Fund (YZJJ201523), Anhui Provincial Natural Science Foundation (1708085MB46), National Natural Science Foundation of China (21774133, 21306181), the Equipment Function Development and Technology Innovation Project of Chinese Academy of Science (YG2012064), Science and Technology Service Network Initiative of Chinese Academy of Sciences (KFZD-SW-416) and National Key Research and Development Project of China (2017YFC0703201-03).


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Site-Selective Deposition of Reductive and Oxidative Dual Cocatalysts

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