Au Nanoparticle and CdS Quantum Dot Codecoration of In2O3

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Au Nanoparticles and CdS Quantum Dots Co-decorated In2O3 Nanosheets for Improved H2 Evolution Resulted from Efficient Light Harvesting and Charge Transfer Dandan Ma, Jian-Wen Shi, Diankun Sun, Yajun Zou, Linhao Cheng, Chi He, Zeyan Wang, and Chunming Niu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04086 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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ACS Sustainable Chemistry & Engineering

Au

Nanoparticles

and

CdS

Quantum

Dots

Co-decorated

In2O3Nanosheets for Improved H2 Evolution Resulted from Efficient Light Harvesting and Charge Transfer Dandan Ma,a Jian-Wen Shi,a,c,* Diankun Sun,a Yajun Zou,a Linhao Cheng,a Chi He,b Zeyan Wang,c Chunming Niua

a

State Key Laboratory of Electrical insulation and Power Equipment, Center of Nanomaterials

for Renewable Energy, School of Electrical Engineering, Xi'an Jiaotong University, Xi'an 710049, China. bDepartment

of Environmental Science and Engineering, School of Energy and Power

Engineering, Xi’an Jiaotong University, Xi’an 710049, China cState

Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, China

Corresponding author: Jian-Wen Shi, E-mail: [email protected]

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ACS Paragon Plus Environment

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ABSTRACT Au nanoparticles (NPs) and CdS quantum dots (QDs) co-decorated In2O3 nanosheets assembled into flower-like structure (In2O3/Au/CdS) are synthesized to facilitate the photocatalytic H2 production. The optimized In2O3/Au4/CdS-12 (4 wt.% Au NPs, and CdS QDs are deposited for 12 cycles) displays obviously promoted photocatalytic hydrogen generation ability of 17.23 μmol/h (10 mg of catalyst), which is 22.97, 5.08 and 5.05 times as high as that of pristine In2O3 (0.75 μmol/h), In2O3/Au4 (3.39 μmol/h) and In2O3/CdS-12 (3.41 μmol/h), respectively. The significant improvement of H2 generation rate can be attributed to several positive factors: the heterojunction at the In2O3-CdS interface and the Schottky barrier at the interface between In2O3-Au and CdS-Au which improves the migration and separation of charge carriers, the surface plasma resonance (SPR) effect of noble metal Au NPs which enhances the light harvesting capacity of In2O3 and boosts the generation of hot electrons, efficiently improving the utilization rate of sunlight.

KEYWORDS: Photocatalysis, H2 evolution, Heterojunction, Schottky barrier, SPR effect

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INTRODUCTION Photocatalytic water splitting hydrogen evolution converts endless solar energy to clean and environmentally friendly hydrogen fuel, thus have becoming an impressive strategy in dressing the problems of both energy and environment.1-3 In2O3, as a well-known moderate band gap semiconductor (2.8 eV), possesses appropriate energy band structure, easily regulated morphology, low toxicity and good stability, thus has received extensive attention in many areas.4-8 Although In2O3 with different morphologies, such as In2O3 nanorods,9,10 microspheres,11 mesoporous films,12 nanowires13,14 and microflowers (MFs) assembled with nanosheets,15-17 have been studied as photocatalysts, their photocatalytic performances for hydrogen evolution are still very poor because of the limited absorption ability of solar light and the serious recombination of charge carriers.15,17 Therefore, it is very eager to seek effective strategies to solve the two main problems simultaneously for obtaining novel In2O3-based photocatalysts with high photocatalytic efficiency. It has been demonstrated that constructing heterostructure by coupling the semiconductors with different band-gaps together is one of effective strategies,18-20 where the narrow band-gap semiconductor can expand the light response region, resulting in the enhanced utilization efficiency of sunlight,21,22 while the heterojunction composed of two semiconductors can drive the directive migration of photogenerated charge carriers , promoting the separation of photoexcited charge carriers.23-26 Among the large number of small band-gap semiconductors, CdS (2.4 eV), especially CdS quantum dots (QDs), is a popular candidate because of its suitable energy band positions and simple production method.27-29 CdS decoration is considered as one of the most efficient

strategy

in

improving

the

photocatalytic

efficiency

3


of

wide

band-gap


semiconductor.27,30,31 Recently, the noble metal decorated heterostructures have attracted numerous research interests,32-34 such as Au decorated TiO2-CdS,32Au decorated TiO2-g-C3N4.33 In our previous report, we also successfully fabricated a g-C3N4/Au/C-TiO2 hollow structure, which exhibited a dramatically improved photocatalytic hydrogen generation ability under visible-light irradiation.35 It has been found that a Schottky barrier can be formed at the interface noble metal and semiconductor, which can induce the generation of inner electric field, facilitating the separation and transfer of photogenerated electrons and holes.33,36 In addition, the noble metals decorated on the surface of semiconductor can lead to an extended light response region owing to the surface plasmon resonance (SPR) effect of noble metals, which is also good for the generation of hydrogen.37,38 Therefore, it can be deduced that the two main problems existed in pristine In2O3 are expected to be solved simultaneously by the co-decoration of Au nanoparticles (NPs)and CdS QDs. However, we have consulted mass of literatures and find there is no report about the CdS QDs and Au NPs co-decorated In2O3 system yet. In current work, we successfully developed a novel photocatalyst, Au NPs and CdS QDs co-decorated In2O3 nanosheets (In2O3/Au/CdS). We firstly prepared In2O3 nanosheets, which assembled into flower-like structures. Secondly, In2O3 surface was decorated with Au NPs through a photo-deposition method, resulting in In2O3/Au. Then CdS QDs were introduced into the system through a successive ionic layer absorption and reaction (SILAR) method, resulting in In2O3/Au/CdS (the synthetic route diagram ofIn2O3/Au/CdS is illustrated in Scheme 1). As a result, the obtained In2O3/Au/CdS displayed dramatically improved photocatalytic hydrogen generation activity. 4


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Scheme 1. The synthetic route diagram of In2O3/Au/CdS.

EXPERIMENTAL SECTION Chemicals and Materials. Indium (III) chloride hydrate (InCl3·4H2O, 99.99% metals basis) and chloroauric acid solution (HAuCl4·4H2O 10 wt. % in H2O) were purchased from Shanghai Macklin. Co., Ltd, China. 8 wt. % Chloroplatinic acid solution was ordered from Shanghai Aladdin Bio-Chem Technology Co., Ltd, China. Urea (CO(NH2)2), 99.0%) and sodium dodecylsulfate (SDS, C12H25NaO4S, 99.0%) were supplied by Tianjin Zhiyuan Chemical Reagents Development Centre. Cadmium acetate dihydrate (Cd(CH3COO)2·2H2O) and sodium sulfide (Na2S·9H2O) were obtained from Tianjin Guangfu Fine Chemical Reagent Co., Ltd and Tianjin Tianli Chemical Reagent Co., Ltd, respectively.

Na2SO4 and Na2SO3 were ordered from Sinopharm Chemical Reagent Co., Ltd,

China. Synthesis of In2O3/AuMFs. In2O3MFs were firstly prepared by calcining In(OH)3MFs at 500 oC in air for 2h, where In(OH)3 MFs were produced following our previous report.14 Afterwards, In2O3 MFs were decorated with Au NPs through a photo-deposition method.39 Typically, 20 mg of In2O3 powder 5



was suspended in 20 mL of water by ultrasonic treatment, and then a certain amount of chloroauric acid solution (HAuCl4·4H2O, 10 wt%) was added in and followed by irradiation with a Xenon lamp (300 W) for 30 min under drastic agitation. The resultant purple gray product was collected by centrifugation, washed with deionized water for three times. Afterwards, the product was dried at 60 oC in air to obtain Au decorated In2O3. The weight ratio of Au toIn2O3 was adjusted by the added amount of chloroauric acid solution. Three Au decorated In2O3 samples with different weight ratio of Au to In2O3 (3, 4, and 5 wt. %) were obtained, and were denoted as In2O3/Aux where x was the weight ratio of Au to In2O3. Synthesis of In2O3/Au/CdS MFs. In2O3/Au/CdS MFs were obtained by the in-situ deposition of tiny CdS QDs on In2O3/Aux MFs through a SILAR method.19,27 One SILAR cycle included two steps: Firstly, In2O3/Aux was added to a Cd(CH3COO)2 solution (0.1 M) under stirring for 5 min, and then was washed with deionized water; Secondly, the dry In2O3/Aux was soaked in a Na2S solution (0.1 M) for 5 min, and then was washed with deionized water. The amount of CdS QDs anchored on In2O3/AuxMFs was adjusted by the number of deposition cycles. For convenience, the as-prepared CdS decorated In2O3/Aux samples were denoted as In2O3/Aux/CdS-y where y stood for the number of deposition cycles. For comparison, CdS decorated In2O3 MFs were also synthesized by the same AILAR method where In2O3 MFs were directly used as the substrate, and the obtained samples were labeled as In2O3/CdS-y. Photocatalytic H2 Production. The photocatalytic H2 production experiment was performed in a closed glass gas circulation system (CEL-SPH2N, Beijing), and a top-irradiated Xenon lamp (225 W) was equipped as light 6


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source. In a typical experiment, 2.1 g of Na2S·9H2O (0.35M) and 0.79 g of Na2SO3 (0.25 M) were dissolved into 25 mL of aqueous solution as sacrificial agent. After then, 10 mg of photocatalyst samples were dispersed into the solution by a mild ultrasound, 1 wt% of Pt was added as co-catalyst. The generated H2 was tested by a gas chromatograph (TCD detector, GC-9720). Characterization. A multipurpose X-ray diffraction system (Philips X’pert) was used to record the X-ray diffraction (XRD) patterns of samples. Raman spectra were performed on a microscope (ReinishawinVia) equipped with CCD array detector and an edge filter. The microstructures of samples were identified by a GeminiSEM 500 scanning electron microscopy (SEM) equipped with energy dispersive spectrometer (EDS) and a JEM-2100 transmission electron microscopy (TEM). Elemental dispersive X-ray (EDX) mapping images were obtained from an FEI Tecnai G2 F30 TEM (S-Twin, USA). The light absorption properties were carried out by a UV-vis-NIR spectrometer (JASCO, V-670). The photoluminescence (PL) spectra were recorded with a fluorescence spectrometer (Edinburgh Instruments FLS 980). X-ray photoelectron spectroscopies (XPS) were studied on a Versa Probe analyzer (PHI 5300, Al Kα radiation). The electrochemical analyses were tested by a standard three-electrode configuration in an electrochemical workstation (CHI 660 E, Shanghai, China). For photocurrent-time curves and electrochemical impedance spectroscopy EIS tests, Ag/AgCl, Pt wire and the catalyst-coated fluorine-doped tin oxide (FTO, 7Ω per square) glass substrate were used as reference, counter and working electrode, respectively. Electrochemical impedance spectroscopies (EIS) was obtained under the frequency range from 0.1 Hz to 105 Hz with a 5-mV bias voltage. Mott-Schottky (M-S) plots were recorded by conducting impedance-potential spectroscopy at 10 kHz over a range of potentials between -0.6 7



and 0.4 V by changing the Ag/AgCl electrode to saturated calomel electrode (SCE). The Na2SO4 solution (0.1 M) was utilized as the electrolyte.

RESULTS AND DISCUSSION Characterizations of heterostructure photocatalysts. SEM images of the In2O3, In2O3/Au4 and In2O3/Au4/CdS-12 catalysts are shown in Figure 1. The pristine In2O3 MFsdisplayed a flowerlike three-dimensional (3D) hierarchical structure assembled by a larger number of In2O3 nanosheets (Figure 1a), and there was no obvious change in this flowerlike morphology after Au decoration (Figure 1c) and even CdS QDs deposition (Figure 1e). By further enlarging, we can observe the changes. For pristine In2O3 (Figure 1b), the surface of In2O3 nanosheets was smooth. After the photo-deposition of Au, many tiny particles (bright dots, marked with black arrows) can be detected on In2O3 surface (Figure 1d), indicating the successful deposition of Au NPs.40 After finishing the deposition of CdS QDs, many newly generated tiny particles were detected on the surface of In2O3 nanosheets, indicating the successful deposition of CdS QDs (Figure 1f). The SEM images of In2O3/Au4/CdS-y are displayed in Figure S1. All the samples shared the same flower-like morphology with the pristine In2O3 (Figure S1a, c and e). From the magnified images (Figure S1b, d and f), it is clearly that the surface of In2O3 became rougher with the deposition cycles of CdS QDs increasing from 8, 12 to 16. No obviously aggregation of CdS QDs was observed, indicating the good dispersibility of CdS QDs due to the mild fabrication procedure of SILAR method, which is contributed to the enhanced photocatalytic efficiency.

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Figure 1. The SEM images of (a, b) In2O3, (c, d) In2O3/Au4 and (e, f) In2O3/Au4/CdS-12.

Figure 2a, Figure S2a and Figure S3a display the XRD patterns of all samples. The recognized diffraction peaks of pristine In2O3 (Figure 2a) were in accord well with the standard card of cubic In2O3 (JCPDS no. 06-0416).41After Au decoration, two characteristic diffraction peaks of Au (JCPDS 65-2870) at 38.2° and 44.3°, corresponding to the (111) and (200) crystal face of Au respectively,34,42 were detected (Figure 2a), and their intensity slightly increased with the enhancement of Au deposition amount (Figure S2a). After further decoration with CdS QDs, a wide diffraction peak with weak intensity appeared at 24-30° (Figure 2a), and its intensity gradually strengthened (Figure S3a) with the increasing amount of deposited CdS QDs, which can be ascribed to the overlap of three XRD peaks resulted from (100), (002), and (101) crystal faces of hexagonal CdS (JCPDS No. 41-1049), and the weak intensity is due to the small amount and good dispersity of CdS QDs.27,43Moreover, the Raman spectra of In2O3/Au4 and In2O3/Au4/CdS-12 were further carried out to investigate the role of CdS QDs (Figure 2b). For In2O3/Au4, the peak centered at about 580 cm-1 comes from the photon mode of the substrate.44 The other three peaks centered at around 306, 367 and 494cm-1 were the characterized vibrations 9



of cubic In2O3, which is belonging to the In-O bending vibration of δ(InO6) units, the stretching vibrations of In-O-In bonds and the A1g vibration mode in In2O3, respectively.44,45 After the decoration of CdS QDs (In2O3/Au4/CdS-12), the vibration at about 367 cm-1 was disappeared (or covered by the peak centered at 494 cm-1), indicating the decreased In-O bending vibration. Moreover, the Raman peaks at 494 cm-1 displayed an obvious shift to the lower wavenumber, which can be ascribed to the effect of decorated CdS QDs on the detected signal of In2O3.

Figure 2. (a) XRD patterns of as synthesized In2O3, In2O3/Au4 and In2O3/Au4/CdS-12; (b) Raman spectra of In2O3/Au4and In2O3/Au4/CdS-12.

Figure 3 is the corresponding TEM images of In2O3/Au4/CdS-12. The In2O3 MFs presented the diameter of about 4.5 μm (Figure 3a) assembled by many thin nanosheets (Figure 3b), which can also be seen from the TEM images of In2O3/Au4 (Figure S4). Furthermore, many black small dots assigned to the decorated Au NPs were observed (Figure 3b and Figure S4b).39 Further magnifying, three recognized lattice fringes with the width of 0.29, 0.24 and 0.36 nm, corresponding to (222),(111)and (100) planes of cubic In2O3, Au and hexagonal CdS, respectively,37,46,47 were found from the HRTEM image (Figure 3c), suggesting the successful construction of In2O3/Au/CdS. Moreover, the intimately contacted In2O3-Au-CdS (Figure 3d1), 10


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Au-CdS (Figure 3d2), In2O3-CdS (Figure 3d3) and In2O3-Au (Figure 3d4) interfaces were observed, which would contribute to the rapid transfer and separation of the photogenerated electrons and holes, facilitating the generation of H2.

Figure 3. (a, b) TEM and (c, d) HRTEM images of In2O3/Au4/CdS-12: (d1) In2O3-Au-CdS interface, (d2) Au-CdS interface, (d3) In2O3-CdS interface, (d4) In2O3-Au interface.

The distribution of elements inIn2O3/Au4/CdS-12 was investigated by using the EDX mappings with nanoscale (Figure 4) and micron magnifications (Figure S5). The distribution of elemental Cd corresponded to the distribution of elemental In, meaning the uniform and dense distribution of CdS QDs. The yellow dots represented Au were much scarcer than the green dots represented Cd, suggesting the loaded amount of Au is much less than that of CdS QDs. The distribution of elemental Au was also very uniform except some observable surface aggregation.The aggregation can be ascribed to the concentrated photogenerated electrons accumulated on In2O3 surface, 11



facilitating the formation and accumulation of Au NPs.48,49 As a counterpart, the distribution of elements in In2O3/Au4 was also studied and the results were displayed in Figure S6, the distribution of elemental Au in In2O3/Au4 is exactly the same as that in In2O3/Au4/CdS-12 (Figure S5). Figure S7 shows the EDS spectra of In2O3/Au4 and In2O3/Au4/CdS-12 (the inserts are the corresponding elemental contents). The measured Au content in the In2O3/Au4 was 6.25% (Figure S7a), higher than the added amount (4%), which can be ascribed to the facts that the Au NPs were mainly distributed on In2O3 surface, and EDS is a surface detection technology, resulting in a higher measured value than the true value. As for In2O3/Au4/CdS-12, the measured value of Au content was drastically declined to 0.62% (Figure S7b), indicating that the Au NPs have been covered by the later deposited CdS QDs.

Figure 4. (a)HAADF STEM image and (b-d) EDX mappingsof the In2O3/Au4/CdS-12; (e) the overlay EDX mapping of In, Cd and Au elements.

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XPS were further measured to study the elemental composition and valence state of In2O3/Au4 and In2O3/Au4/CdS-12. The XPS survey spectra (Figure 5a) confirmed the existence of Au, O and In elements in the In2O3/Au4 and the existence of Au, O, In, as well as Cd and S elements in the In2O3/Au4/CdS-12, respectively. The C 1s peak centered at about 285 eV was observed over the two samples, the detected signal of carbon mainly comes from the carbon tape substrate used in the test and a small amount of adsorbed CO2 on the sample surface.50,51A slightly shift to the lower electron volt was observed over the In 3d peak of In2O3/Au4/CdS-12 in comparison with that of In2O3/Au4 (Figure 5b), that is due to the changed electric field environment of In2O3 caused by the introduction of CdS QDs, indicating the considerable interaction between the two semiconductors. 50,52

The O 1s peaks of the two samples can be fitted into two peaks at 529.2 and 530.5 eV (Figure

5c), which can be ascribed to the lattice oxygen and defective oxygen of In2O3, respectively.38,53,54 The characteristic doublet peaks at 83.6 eV and 88.2 eV were detected in the high-resolution Au 4f XPS spectrum of In2O3/Au4 (Figure 5d), which are the characteristic signals of Au 4f7/2and Au 4f5/2, respectively, demonstrating the presence of noble metal Au.37,42,47 It should also be noticed that In2O3/Au4 displayed much stronger Au 4f signal than that of In2O3/Au4/CdS-12, the decreased Au signal is due to the fact that the Au NPs were covered by the subsequently deposited CdS QDs,47,55 in accord with the result of EDS shown in Figure S7b. A pair of peaks located at 405.6 eV and 412.3 eV (Figure 5e) correspond to the 3d5/2 and 3d3/2 of Cd2+, respectively,47,56 while the XPS peaks centered at 161.5 eV and 162.5 eV (Figure 4e) are in accord to the characterize peaks of S2+,57 confirming the formation of CdS in In2O3/Au4/CdS-12.

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Figure 5. (a) The survey XPS spectra, and high-resolution XPS spectra of (b) In 3d, (c) O 1s, and (d) Au 4f of In2O3/Au4 and In2O3/Au4/CdS-12; (e) and (f) are the Cd 3d and S 2p spectra of In2O3/Au4/CdS-12, respectively.

The corresponding UV-vis absorption spectra of all samples were studied to reveal their optical absorption properties. As displayed in Figure 6a, pure In2O3 can absorb the light source less than 14


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450 nm wavelength because of the large band gap (2.8 eV).12,14 By contrast, narrow band-gap semiconductor CdS (2.4 eV) exhibited a wide absorption edge at about 580 nm.18,27,58 Compared with pristine In2O3, the decoration of Au NPs caused the enhancement on the visible-light absorption (Figure S2b) because of the SPR effect of Au,59-62 and the optical absorption increased with the introduction of Au NPs. The deposition of CdS QDs led to the red-shifted absorption edge, whether for In2O3/CdS-y (Figure S3b) or In2O3/Au/CdS-y (Figure 6a). Moreover, the red-shift increased with the deposition of CdS QDs. It has been confirmed that the red-shift of absorption edge contributes to the enhancement of photocatalytic performance because of the expanded usable light region.22,27,42 Figure 6b is the respective band gap plots of the samples. The band gap (Eg) can be determined from equation (1): (Rhν)2 = A(hν − Eg)

(1)

where R, hν and A represent the absorption coefficient, the photon energy and the corresponding constant for the material, respectively.27,39,40 Table S1 is the obtained Eg values of the photocatalysts.

Figure 6. (a) UV-vis absorption spectra and (b) Tauc plots of (F(R)hν)2 versus photon energy (hν) of the samples.

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Photocatalytic activity study of heterostructure photocatalysts. The hydrogen generation performances of these photocatalysts were tested. The pristine In2O3 showed a relatively low H2 evolution rate (0.75 μmol/h), which can be ascribed to its poor light absorption ability and the serious recombination of photoexcited electron and holes (Figure7a). After the decoration of Au, the resultant In2O3/Aux displayed obviously enhanced photocatalytic activity, which can be attributed to the promoted light absorption ability because of the SPR effect of the noble metal.34,36 In addition, the formed Schottky barrier between semiconductor (In2O3) and metal (Au) drives the directional migration and efficiently separation of charge carriers, facilitating the generation of H2.31,32 The In2O3/Au4 displayed the highest hydrogen evolution efficiency of 3.39 μmol/h, which is 4.5 times as high as that of pristine In2O3. However, further increasing Au to 5% witnessed a decline in the hydrogen evolution rate, this is due to the opacity and light scattering resulted from the excessive Au NPs, decreasing the utilization to sunlight.38,63 Moreover, excessive Au NPs may in turn act as charge recombination centers, which inhibits the efficiently utilization of charge carriers, leading to the decline in photocatalytic efficiency.64,65 The deposition of CdS QDs also played a promoted role on the photocatalytic reaction of In2O3. As observed in Figure S3c, the H2 production rate increased from 0.75μmol/h of pristine In2O3 to 2.61 μmol/h of In2O3/CdS-8, and then to 3.41 μmol/h of In2O3/CdS-12, which is due to the facilitated charge separation caused by the formed heterojunction between In2O3 and CdS.9,15,17 The surplus deposition of CdS QDs resulted in a decline in the photocatalytic hydrogen production rate (from 3.41 μmol/h of In2O3/CdS-12 to 2.83 μmol/h of In2O3/CdS-16), this is because the over-loaded CdS QDs are not conducive to maximizing heterojunction effect. After In2O3 was decorated by Au NPs and CdS QDs successively, the resultant In2O3/Au/CdS exhibited further 16


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enhanced photocatalytic performance (Figure 7b), and the In2O3/Au4/CdS-12 presented the optimized hydrogen generation ability of 17.23 μmol/h among these samples, which is 22.97, 5.08 and 5.05 times higher than that of In2O3 (0.75 μmol/h), In2O3/Au4 (3.39 μmol/h) and In2O3/CdS-12 (3.41 μmol/h), respectively. The significantly promoted photocatalytic efficiency can be primarily ascribed to the intimately contacted In2O3-Au-CdS, In2O3-CdS and In2O3-Au interfaces, which facilitate the fast transfer and efficient separation of electrons and holes. In addition, the enhanced utilization efficiency of simulate sunlight due to the SPR effect is also one of the important reasons. To further investigate the durability and stability of the developed sample, the cyclic hydrogen production experiment was carried out over In2O3/Au4/CdS-12. As observed in Figure S8a, no detected decline in the hydrogen generation ability was observed after the sample was used for four cycles (total time of 20 h). In addition, the results of XRD showed that no distinguished structure change occurred in the used sample (Figure S8b), further demonstrating the good durability and stability of the developed In2O3/Au4/CdS-12.

Figure 7. (a) Photocatalytic hydrogen production over Au decorated In2O3, (b) the comparison of H2 evolution activity over the In2O3, In2O3/Au4, In2O3/CdS-12and In2O3/Au4/CdS-y.

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Charge Separation Studies. The photoluminescence (PL) spectra and photoelectrochemical tests were further carried out to in-depth study the separation of charges. Figure 8a displays the PL spectra of the samples. Pristine In2O3 displayed the strongest PL intensity because of the severe charge recombination. By contrast, the PL intensity of In2O3/Au4 and In2O3/CdS-12 was far lower than that of pristine In2O3. As for the In2O3/Aux/CdS-y, the PL intensity further declined, suggesting that the introduction of Au and CdS can efficiently inhibits the charge recombination.32,37 Figure 8b displays the photocurrent-time curves of the as fabricated samples. The pristine In2O3 and In2O3/Au4 exhibited relatively low photocurrent, indicating the limited photoelectric conversion ability.34,62,66 The In2O3/CdS-12 displayed a higher photocurrent than In2O3 due to the formed heterojunction between In2O3 and CdS. The photocurrent of In2O3/Au4/CdS-y was significantly enhanced, indicating that the combination of the deposited Au NPs and CdS QDs can efficiently facilitate the separation of electron-hole pairs. The EIS Nyquist plots of the photocatalysts are displayed in Figure 8c. In2O3/CdS-12 displayed a smaller radius than pristine In2O3, indicating that the heterostructure can facilitate the migration of charge carriers. The radius of In2O3/Au4/CdS-y was much smaller than that of In2O3/Au4 and In2O3/CdS-12, demonstrating that the synergistic effect of co-decorated Au NPs and CdS QDs can markedly decrease the charge migration resistance, which is benefit to the generation of hydrogen. Figure 8d is the Mott-Schottky (M-S) plots of In2O3, In2O3/Au4 and In2O3/Au4/CdS-12. The measured samples all displayed positive slope, indicating their n-type behavior.17,18,67 Additionally, the density of charge carriers (ND) can be obtained by calculating with the follow equation:17,67 2

𝑁𝐷 = ε𝜀0e𝑘

(2)

18


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where e, ε and ε0 represent the elementary electron charge, dielectric constant and permittivity in vacuum, respectively, k is the slopes corresponding to the linear portions of the M-S plots. According to the equation, the ND of In2O3/Au4/CdS-12 HS was calculated to be 1.46×1020, which is about 13 and 1.7 times as high as that of pure In2O3 (1.13×1019) and In2O3/Au4 (8.81×1019), respectively, suggesting that more electrons-holes pairs are generated in In2O3/Au4/CdS-12 under the same irradiation, which contributes to the improvement of photocatalytic reaction rate.14,20,31.

Figure 8. (a) PL spectra under the irradiation of 370 nm Xe lamp, (b) photocurrent versus time curves, (c) EIS and (d) M-S plots of the samples.

Photocatalytic Mechanism investigation. Based on the above experiments and analyses, we try to propose a possible photocatalytic mechanism over In2O3/Au/CdS. As illustrated in Figure 9. Three contact interfaces existed in the 19



In2O3/Au/CdS:(1) the In2O3-CdS interface where In2O3 directly contacts with CdS QDs, (2) the In2O3-Au-CdS interface where CdS QDs only cover the surface of Au NPs, (3) the In2O3-CdS-Au interface where CdS QDs contact with In2O3 and Au NPs at the same time. The results of XPS and EDS tests indicate that the surface of Au NPs is almost covered by the deposited CdS QDs, thus the pure In2O3-Au interface is nearly inexistent. The conduction band (CB) and valence band (VB) of In2O3 are located at -0.62 and +2.20 eV, respectively, while the corresponding positions of CdS are -0.52 and +1.88 eV, respectively.9,14,68 At the In2O3-CdS interface, the CB of In2O3 is more negative than that of CdS while the VB of In2O3 is more positive than that of CdS, so both the photogenerated electrons and holes transfer from In2O3 to CdS presenting the characteristic type-I heterojunction energy transfer (pathway I) process.10 At the In2O3-Au-CdS interface, the Fermi energy level of Au lies lower than that of CdS and In2O3, the Schottky contact will form between semiconductors and Au NPs, in which the electrons jump from the CB of CdS and In2O3 to Au NPs, and the Au NPs worked as co-catalyst in the generation of hydrogen (pathway II).37,47,69,70At the In2O3-CdS-Au interface, In2O3, CdS and Au contact intimately to each other and form three inner interfaces of In2O3-CdS, In2O3-Au and CdS-Au where one type-I heterojunction and a couple of Schottky contacts exist at the same time, thus the photogenerated electrons not only transfer from In2O3 to CdS and Au but also transfer along the In2O3-Au and CdS-Au interface under the driving forces of double inner electronic field resulted from heterojunction and Schottky barrier, facilitating the separation of charge carriers (pathway III). Moreover, due to the SPR effect of noble metal, Au NPs absorb visible light to produce the photogenerated hot electrons, resulting the improvement of charge carrier density.26,32,34 The synergistic effects of the SPR effect, the heterojunction between semiconductor and semiconductor, and the Schottky barrier 20


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between semiconductor and noble metal, facilitate the utilization of light source and accelerate the generation and separation of electrons and holes, thus boost the generation of H2.

Figure 9. Schematic illustration on the transfer mechanism of photoexcited electrons and holes in In2O3/Au/CdS HS.

To further confirm the availability of the design, In2O3/CdS-12/Au4 was synthesized as a counterpart by photo-depositing Au NPs on the surface of In2O3/CdS-12. As shown in Figure S9, In2O3/CdS-12/Au4 maintained the flowerlike morphology of In2O3. From Figure S10a, it can be observed that In2O3/CdS-12/Au4 displayed a similar UV-vis absorption spectrum to that of In2O3/Au4/CdS-12, but the H2 evolution rate over In2O3/CdS-12/Au4 (14.88 μmol/h) was lower than that over In2O3/Au4/CdS-12 (17.23 μmol/h) (Figure S10b), demonstrating that the construction of In2O3/Au4/CdS-12 is more efficient in the separation of charge carriers and the evolution of H2. To be specific, taking the synthesis procedure of In2O3/CdS-12/Au4 into consideration, Au NPs can only deposit on the surface of In2O3 and CdS QDs, thus there are the absence of pathway II energy transfer process in the counterpart, resulting in a declined H2 21



evolution rate.

CONCLUSION

In conclusion, we fabricated a novel In2O3/Au/CdS photocatalyst by decorating In2O3 nanosheets assembled into flower-like morphology with Au NPs and CdS QDs simultaneously. The well-designed In2O3/Au/CdS exhibited much higher H2 generation ability than that of single decorated In2O3 and pristine In2O3. Among them, In2O3/Au4/CdS-12 displayed the most efficient photocatalytic hydrogen production of 17.23 μmol/h (10 mg of catalyst), which was 22.97, 5.08 and 5.05 times higher than that of In2O3 (0.75 μmol/h), In2O3/Au4 (3.39 μmol/h) and In2O3/CdS-12 (3.41 μmol/h), respectively. The remarkably enhanced H2 evolution ability mainly comes from the following synergistic effects: (1) The combination of In2O3 with narrow band-gap CdS led to the promoted light absorption ability, resulting in the improved utilized efficiency of sunlight ; (2) The SPR effect of Au NPs enhanced the light response of photocatalyst in visible region and boosted the generation of hot electrons, thus enhanced the effective availability of the captured solar energy; (3) Two kinds of inner electronic fields, the heterojunction (In2O3-CdS interface) and the Schottky barrier (In2O3-Au and CdS-Au interfaces), drove the quickly directional migration and efficient separation of photoexcited electrons and holes, facilitating the generation of hydrogen.

ASSOCIATED CONTENT Supporting Information: SEM images of In2O3/Au4/CdS-y; XRD, UV-vis adsorption spectra and TEM images of In2O3, In2O3/Aux and In2O3/CdS-y; UV-vis adsorption spectra and H2evolution 22


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performances of In2O3 and In2O3/CdS-y; EDX mapping and EDS spectra of In2O3/Au4/CdS-12 and In2O3/Au4; Long time H2 evolution performance and XRD patterns of In2O3/Au4/CdS-12 before and after reaction; TEM images and photocatalytic performance of In2O3/CdS-12/Au4; Eg values for different samples.

ACKNOWLEDGMENTS This work was sponsored by the Opening Project of State Key Laboratory of Crystal Materials, Shandong University, China (KF1710), the Opening Project of State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, China (201715), the State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, China (EIPE18311). TEM was carried out at International Center for Dielectric Research (ICDR). EDX mapping, SEM and XPS was carried out at Analysis and Test Center of Xi’an Jiaotong University.We thank Chuansheng Ma, Jiao Li, Zijun Ren and Jiamei Liu for their help in using TEM, EDX mapping, SEM and XPS, respectively.

CONFLICT OF INTEREST The authors declare no conﬂict of interest.

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Au NPs and CdS QDs Co-decorated In2O3 nanosheets and the inner charge carrier transfer mechanisms in photocatalytic H2 Evolution reaction. 326x295mm (300 x 300 DPI)


Au Nanoparticle and CdS Quantum Dot Codecoration of In2O3

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