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0D/2D Au25(Cys)18 Nanoclusters/g-C3N4 Nanosheets Composites for Enhanced Photocatalytic Hydrogen Production under Visible Light Chaoyue Wang, Peng Lv, Daxiang Xue, Yun Cai, Xiaoxiao Yan, Lin Xu, Jun Fang, and Yang Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00643 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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ACS Sustainable Chemistry & Engineering

0D/2D Au25(Cys)18 Nanoclusters/g-C3N4 Nanosheets Composites for Enhanced Photocatalytic Hydrogen Production under Visible Light Chaoyue Wang, † Peng Lv, † Daxiang Xue, † Yun Cai, † Xiaoxiao Yan, ‡ Lin Xu, ‡ Jun Fang*, † and Yang Yang*, † †

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, No. 5 Xinmofan Road, Nanjing 210009, China

‡

School of Chemistry and Materials Science, Nanjing Normal University, No. 1 Wenyuan Road, Nanjing 210023, China

*Corresponding Authors： Prof. Dr. Jun Fang, E-mail: [email protected] Prof. Dr. Yang Yang, E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Au25(Cys)18 nanoclusters/g-C3N4 nanosheets composite photocatalysts were fabricated via a facile wet impregnation method. Modifying g-C3N4 nanosheets with Au25(Cys)18 nanoclusters led to the enhancement of photocatalytic hydrogen evolution under visible light. Such an improvement of activity is attributed to a vast number of junctions formed at the interface between the Au nanoclusters and the g-C3N4 nanosheets of the hybrid photocatalysts. The efficient separation of photo-generated charges was thus facilitated by their appropriate interfacial band structures. More importantly, the ultra-small size Au25 nanoclusters exhibited the resistance to aggregation during the photocatalytic reactions. Thus, the hybrid photocatalysts are not only more active, but also more stable than photo-reduced Au nanoparicles/g-C3N4 composite photocatalysts in the hydrogen production reaction, which make these novel catalysts promising in the application of solar fuels production. The combination of noble metal clusters with g-C3N4 materials opens a new window for the design of novel photocatalysts.

KEYWORDS: Photocatalysis, H2 production, Au25 nanoclusters, Carbon nitride, Heterostructure


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INTRODUCTION Photocatalytic H2 production from water splitting has been recognized as a potential alternative way to produce sustainable clean energy, which could replace the traditional fossil fuels to solve the world-wide energy crisis the human society is facing.1–3 Since Fujishima and Honda in 1970s first found H2 evolved from water splitting on TiO2 electrode in the photoelectrochemical (PEC) cell under UV light,4 researches on photocatalytic H2 production catalyzed by such semiconductors were rapidly thrived.5–8 Recently, graphitic carbon nitride (g-C3N4), as a novel metal free semiconductor photocatalyst, has drawn intensive attention due to low cost from its earth-abundant components C and N, low toxicity and high chemical stability.9–14 The band gap of g-C3N4 (GCN) is ~2.7 eV, which allows the light absorption in the visible range (≤ 460 nm). Besides, a suitable conduction band position (Ecb = –0.9 V vs. NHE) enables the reduction of water to produce H2 and the reduction of CO2 to hydrocarbon fuels by GCN under visible light.9,15 However, such photocatalytic conversion efficiencies are quite limited due to its fast recombination of photo-generated electron-hole pairs.16–18 It has been well demonstrated that noble metal deposition onto semiconductors could remarkably enhance the photocatalytic activity due to the efficient charge transfer and separation at the interface between the metal and semiconductor nanomaterials, either resulting from the formation of Schottky junctions at the interface, or the surface plasmon resonance (SPR) effect for certain metals (Au, Ag, Cu).19–21 Among these noble metals, Au has been widely investigated due to its good performance and extensively tuneable catalytic properties in photocatalysis.22–25 Whereas, fine controlling of the particle size in sub-nano range (< 2 nm) for such Au nanoparticles (NPs), denoted as Au nanoclusters (NCs), in heterocatalysis is still a great challenge.26,

27

As it is all known that

monodispersed fine NCs with identical molecular structure on support are significant for


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catalysis process. On one hand, with the unique physic-chemical features distinguished from traditional metallic Au NPs, they could provide a large quantity of active sites; on the other hand, we could easily correlate the overall activity to each uniformed single particle.28–30 Thiol ligandsprotected atomically precise Au NCs are potential candidates for the fabrication of such hybrid photocatalysts.31 In the past decade, it has drawn increasing research interests due to their unique properties arising from the special atomic packing structures (usually core–shell structures) and strong electron energy quantization resulted from the ultra small particle size. Broad applications of these NCs have been found in bio-sensing, optoelectronic, catalysis and so on.32–37 Fine tuning of the atomic numbers for the clusters and the types of the ligands, allows a great flexible adjustment of the catalytic properties of these clusters. Among them, the family of Au25(SR)18 (– SR denoted for thiol ligands) is well-studied mostly for its crystalline structure, and recently, it has been utilized as the photosensitizer in photocatalytic reactions.38–41 In general, the Au25(SR)18 cluster has an ultra small core size (~1 nm) from Au13 core, and the other 12 Au atoms bond with thiol ligands forming a shell. The strong electron energy quantization leads to a band gap of ~1.2 eV.32 Both the above features presented in photocatalytic systems are similar to those of semiconductor quantum dots. Furthermore, such Au25(SR)18 NCs are also shown to directly generate photo-induced electrons for the reduction reactions in photocatalytic process.41 Recently, it is revealed that the composites of zero-dimensional (0D) materials (single atoms, nanoclusters, etc.) and 2D materials (e.g. semiconductor nanosheets) show remarkable photocatalytic performance due to their maximized reactive sites and superior charge mobility to lower down the recombination rate of the charge carriers.42–44 According to these reports, the combination of Au25(SR)18 NCs with GCN semiconductor nanosheets (NSs) as the hybrid


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catalysts is supposed to present great potential in photocatalytic applications, for example, the enhanced photocatalytic H2 evolution from water splitting. Herein, we report the fabrication of Au25(Cys)18/GCN heterostructures by a facile wetimpregnation method. The Au25 NCs were highly dispersed on the surface of GCN NSs. Such 0D-2D structure guaranteed the intimate contact between the two components, and the appropriate band structures between Au25 NCs and GCN allowed rapid charge transfer at the interface between NCs and semiconductors, which led to drastic enhancement of the activity for photocatalytic H2 production. Besides, the hybrid catalysts exhibited favorable stability during the H2 production reaction, which is superior to photo-reduced Au NPs/GCN catalysts. Such combination of noble metal clusters with GCN materials could act as a model photocatalyst for better understanding the fundamentals of photocatalysis, such as size dependence, molecular activation, and active centers. Studying on such hybrid materials could better correlate the general activity to the structure of each NC, which is very difficult to achieve in traditional metal/semiconductor system. It opens a new window for the design of novel photocatalysts with superior catalytic performance.

EXPERIMENTAL SECTION Materials. Sodium hydroxide (NaOH) and ammonium chloride (NH4Cl) were purchased from Xilong Scientific Co., Ltd. Tetrachloroauric (III) acid hydrate (HAuCl4·4H2O), chloroplatinic acid hexahydrate (H2PtCl6·6H2O), dicyandiamide (C2H4N4), triethanolamine (C6H15NO3) and sodium borohydride (NaBH4) were procured from Sinopharm Chemical Reagent Co., Ltd. Lcysteine (Cys) was bought from Shanghai Huixing Biochemical Reagent Co., Ltd. All chemicals


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were analytical-grade reagents and used without further purification. Deionized (DI) water (18.2 MΩ) was used throughout the experiment. Synthesis of Au25(Cys)18 Nanoclusters. The Au25(Cys)18 NCs were synthesized according to the reported method.45 Briefly, 0.04 mmol of HAuCl4 and 0.08 mmol of L-cysteine (Cys) were added into 18.4 mL deionized water under vigorous stirring. Then NaOH solution (1 M, 1.2 mL) was introduced to the reaction solution, followed by the addition of NaBH4 solution (112 mM, 0.4 mL) which was prepared by dissolving 43 mg of NaBH4 powder in 10 mL NaOH aqueous solution (0.2 M). The mixed solution was kept stirring for 3 h, and then the Au25(Cys)18 nanoclusters in the solution were dialyzed by using a RC membrane (cutoff 7000 g/mol) in an ice-water bath to obtain purified clusters. Preparation of GCN nanosheets. The GCN NSs were synthesized based on previously reported literature.46 In detail, 1 g of dicyandiamide powder and 5 g of ammonium chloride were dissolved in 30 ml water. Then the mixture was put in a vacuum freeze-drying system to remove the water. Finally, the obtained dried mixture was polymerized in a crucible with a cover at 550 o

C for 4 h, where the temperature was ramping at a rate of 3 oC/min. After cooling down, the

resultant yellow materials were milled into powder in a mortar for further use. Loading of Au25(Cys)18 nanoclusters onto GCN. In a typical synthesis, 50 mg GCN and various amounts of Au25(Cys)18 nanoclusters were mixed in 10 mL ethanol and kept stirring for 1 h. The mixing ratio of Au25(Cys)18 clusters to GCN was fixed at 0.2, 0.5, and 1 wt % by Au vs. GCN (denoted as xAu/GCN, x represents the weight ratio between Au and GCN). Then the suspension was centrifuged and the precipitate was dried in vacuum oven at room temperature. Characterizations. The morphologies and structures of the samples were examined by scanning electron microscope (SEM, Hitachi S-4800 microscope). Transmission electron


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microscope (TEM) images were obtained by using a JEOL 2010 microscope at an accelerating voltage of 200 kV. High-angle annular dark field scanning transmission electron microscope (HAADF-STEM) was performed on a JEOL 2010F microscope at an accelerating voltage of 200 kV. Atomic Force Microscope (AFM) images of GCN were obtained on a Bruker Icon microscope. X-ray diffraction (XRD) patterns were acquired on a SmartLab diffractometer (Rigaku Corporation) equipped with a 9 kW rotating anode Cu source operating at 40 kV and 100 mA. The chemical structure of the composites was characterized by Fourier transform infrared spectroscopy (Thermo). Ultraviolet−visible (UV−vis) absorption spectra and the UV−vis diffuse reflectance spectra were recorded on a Shimadzu UV-3600 spectrophotometer. The latter was equipped with an integrating sphere with BaSO4 as the reference. X-ray photoelectron spectroscopy (XPS) measurements were recorded on an ESCALAB 250 highperformance electron spectrometer with monochromatized Al Kα (hν = 1486.7 eV) source; the likely charging of all the catalysts was calibrated by setting the binding energy of adventitious carbon (C 1s) to 284.6 eV. The photoluminescence (PL) measurements were performed on RF5301PC spectrofluorophotometer with an excitation wavelength at 400 nm. The exact weight ratio of Au in the composite was determined using an inductively coupled plasma optical emission spectroscope (ICP-OES; Perkin Elmer, Optima 7000 DV). Photocatalytic activity measurement. Photocatalytic water-splitting reactions were carried out at room temperature using a top-irradiation quartz reactor. In a typical run, 20 mg composite photocatalysts were dispersed into 20 mL aqueous solution containing 2 mL triethanolamine scavengers. In control experiments, 1 wt% Au or Pt was loaded on GCN by in situ photoreduction technique using HAuCl4 or H2PtCl6 (samples are denoted as PR 1Au/GCN and PR 1Pt/GCN, respectively). Before irradiation, the solution was purged with nitrogen gas to


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remove dissolved oxygen. Then the suspension was exposed under a 300 W Xe lamp (PLS-SXE 300, Beijing Perfectlight Co. Ltd.) coupling with a UV cutoff filter (λ > 420 nm) to evaluate the photocatalytic efficiencies under visible light. The output light intensity was controlled at 100 mW cm-2. The evolved gases were analyzed by gas chromatography (Shimadzu GC-2014 equipped with a thermal conductive detector (TCD) and a 5A molecular sieve column; N2 carrier). Electrochemical measurement. The photoelectrochemical properties were investigated on an electrochemical workstation (CHI660E, Shanghai Chenhua Co. Ltd.) using a standard threeelectrode cell with a working electrode, a Pt wire counter electrode, and a saturated Ag/AgCl reference electrode. 0.5 M Na2SO4 was used as the electrolyte. The working electrodes were prepared as follows: 2 mg of the sample was suspended in 400 µL ethanol containing 40 µL nafion to produce a slurry, and then dip-coated on a 10 mm × 30 mm fluorine-doped tin oxide (FTO) glass, which was cleaned by sonication in acetone, ethanol and water for 30 min, respectively. The electrodes were dried in air at 60 oC. The 300 W Xe lamp (PLS-SXE 300, Beijing Perfectlight Co. Ltd.) was used as the light source with a light intensity of 100 mW cm-2. RESULTS AND DISCUSSION Preparation of Au/GCN nanocomposite photocatalysts. Au25(Cys)18 NCs were synthesized by a modified aqueous reduction method according to our previous work.45 The production of the NCs was monitored by UV-vis absorption spectrum. As shown in Figure 2A, the product of NCs displayed a typical absorption curve of Au25(Cys)18 NCs with the major peak centered at 672 nm, as expected. The TEM image of the NCs was presented in Figure 1A, and the size distribution of the NCs was also shown (Figure 1B), from which a mean particle size of 1.7 nm for the Au25 NCs was obtained (note in Supporting Information). The GCN NSs were


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synthesized by a modified copolymerization method according to previous reports.46 The morphology of GCN NSs features curved sheets in the SEM image (Figure 1C). The thickness of the as-prepared NSs was characterized by AFM, as shown in Figure 1D. The value of 0.8 nm suggested an ultra-thin morphology of the obtained GCN NSs. Afterwards different amounts of the obtained NCs were deposited onto the GCN NSs in ethanol solution via a wet impregnation method according to the weight ratio between Au and GCN. The completed deposition of NCs onto GCN NSs was firstly evidenced by the colorless supernatant after the deposition process. Secondly, the UV-vis absorption spectra of the supernatant were recorded (Figure 2A), and it was shown that all the characteristic peaks of Au25(Cys)18 NCs vanished in the supernatant layer, further confirming the completed deposition of NCs onto the GCN NSs. Moreover, the exact weight ratio between Au and GCN in various composite samples was determined by ICP-OES spectra, with the value of 0.18%, 0.49% and 0.96% for 0.2Au/GCN, 0.5Au/GCN and 1Au/GCN, respectively. The exact weight ratios were very close to the theoretical ones, indicating that all the free NCs with given amounts in the solution were completely deposited onto the GCN NSs.


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Figure 1. (A) TEM image of free Au25(Cys)18 NCs; (B) their mean size distribution; (C) SEM image of as-prepared GCN NSs; (D) AFM image of as-prepared GCN NSs and inset is the corresponding thickness analysis taken around the white line Structure and morphology of Au/GCN nanocomposite photocatalysts. The crystalline structure of Au/GCN nanocomposite photocatalysts was characterized by XRD, as shown in Figure 2B. Pure GCN NSs exhibited a typical diffraction pattern of g-C3N4, with one diffraction peak at 27.4o, which was assigned to (002) plane, represented as the inter-layer stacking of conjugated aromatic systems. Another diffraction peak was at 13.1o, which was assigned to (100) plane, corresponding to the in-planar structural packing of aromatic systems.15 After the deposition of Au25(Cys)18 NCs onto the GCN NSs with different weight ratios, the as-prepared Au/GCN nanocomposites showed similar diffraction patterns to pure GCN NSs, which had two characteristic peaks, without any other additional pattern. For the Au25(Cys)18 NC itself, it has an atomic packing structure quite different from the traditional face centered cubic (fcc) structure, adopted by Au NPs or bulk Au.31 Since there were no typical diffraction patterns related to Au observed in the XRD results, it is reasonable to deduce that all the NCs were highly dispersed on the surface of GCN NSs without aggregation. FT-IR analysis was further conducted for revealing the microstructures of all the Au/GCN nanocomposite photocatalysts. As shown in Figure S1, all the samples displayed typical g-C3N4 IR absorption features. The strong band at 810 cm-1 was assigned to tristriazine ring sextant out of plane bending vibration modes, and the consecutive bands at 1243 cm-1, 1320 cm-1 and 1406 cm-1 were assigned to the aromatic C–N stretching. The band at 1636 cm-1 was due to the C=N stretching vibration mode.47 The graphitic C-N structure was well-preserved after the deposition of Au25 NCs onto GCN NSs for all the photocatalysts. No vibrational bands related to Au25 NCs


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or their thiol ligands were presented in IR spectra, probably due to the limited amount of NCs loaded onto the GCN NSs.

Figure 2. (A) UV-vis absorption spectra of the as-prepared Au25(Cys)18 NC solution before (black line) and after (color lines) the addition of GCN. Inset image is the ball-stick model of a Au25(Cys)18 NC; (B) XRD patterns of various Au/GCN photocatalysts; SEM images for (C) 0.2Au/GCN, (D) 0.5Au/GCN, and (E) 1Au/GCN. The morphology and microstructures of the Au/GCN nanocomposite photocatalysts were observed by SEM (Figure 2C–E). The Au NCs and GCN hybrid photocatalysts exhibited morphologies similar to that of pure GCN NSs (Figure 1C), and none of the Au25 NCs could be observed on the NSs due to their ultra-small size. HAADF-STEM was conducted to confirm the existence of Au25 NCs after the deposition. Unfortunately, the hybrid catalysts displayed severe curling and aggregation of the NSs under the exposure of high-energy electron beam in the TEM chamber in a very short time. Nevertheless, some of the original Au25 clusters were still present in the STEM images, as denoted in Figure S2.


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Figure 3. XPS spectra of various Au/GCN photocatalysts with the high resolution spectra for (A) C 1s, (B) N 1s, (C)Au 4f, and (D) S 2p. The surface chemical compositions of various Au/GCN photocatalysts were analyzed by XPS, which only probed the elements C, N, Au and S. Figure 3 showed the high-resolution XPS spectra of C 1s, N 1s, Au 4f and S 2p. For the detected C 1s and N 1s signals, all the hybrid photocatalysts exhibited similar binding energy (BE) with those in the pristine GCN NSs. The BE of 284.6 eV and 288. 4 eV in C 1s for all the catalysts were attributable to the sp2 C–C bonds and sp2 C atoms bonded to N in the aromatic rings (N–C=N).10 As shown in Figure 3B, N 1s spectra for all the catalysts could be deconvoluted into three components, which were N in the aromatic rings (N–C=N, 398.8 eV), tertiary bridging nitrogen (C3–N, 400.3 eV), and N bonded with H (C–N–H, 401.4 eV).10 From the results of C 1s and N 1s spectra, it was indicated that the original graphitic C–N backbones were well-retained for GCN NSs after Au NCs deposition. Au 4f and S 2p spectra for the hybrid photocatalysts were also investigated. In principle, due to the partial electron transfer from core Au to S in thiol ligand, the BE of Au 4f in Au25(Cys)18 NCs always displayed a slight positive shift than that of metallic gold (83.8 eV).45 As shown in Figure


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3C, the BE of Au in various hybrid photocatalysts were centered at 84.1 eV, indicating the intact loading of Au NCs on the surface of GCN NSs, without any aggregations. For Au/GCN photocatalysts, the BE of S 2p showed a red shift, from 163.6 eV in 0.2Au/GCN to 162.8 eV in 1Au/GCN (Figure 3D). All of these BE positions could be assigned to the characteristic feature of the thiolate group in cysteine ligand for the NCs.45 Nevertheless, such a red shift indicated the interaction between the Au NCs and GCN NSs. This kind of shift was only observed in the XPS spectra of S than any other elements (Figure 3), showing that S could act as the feature element to illustrate the interaction between the Au NCs and GCN NSs, because of its uniqueness at the interface of the Au NCs and GCN NSs. Furthermore, with the increasing deposition amount of the NCs, the peak intensity of S 1s became more pronounced.

Figure 4. UV–vis diffuse reflectance spectra of various Au/GCN nanocomposite photocatalysts. The inset is the corresponding plots of (αhν)2 vs. hν for GCN and 1Au/GCN samples. The electronic band structure of various Au/GCN nanocomposite photocatalysts was studied by UV-vis diffuse reflectance spectra (DRS), and the results were shown in Figure 4. The samples of 0.2Au/GCN and 0.5Au/GCN exhibited similar absorption features with pure GCN NSs, with the absorption band edge at 460 nm.9 Meanwhile, for the sample of 1Au/GCN, when the deposition amount of Au NCs reached 1 wt%, the absorption edge of the hybrid materials


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showed a red shift, to the value of 467 nm. The inset in Figure 4 showed the estimated band gap of GCN and 1Au/GCN, which were 2.84 eV and 2.79 eV from the plots of (αhν)2 vs. photo energy (hν) according to the Tauc’s method, derived from the DRS spectra. The hybrid of Au25 NCs with GCN NSs led to a small band gap narrowing (~0.05 eV) for GCN NSs, which implied a slight tuning of the band structure of GCN NSs. This might be due to the interaction between Au25 NCs and GCN NSs by thiol ligand bonding with surface atoms of GCN, as illustrated in XPS analysis. Besides, the characteristic surface plasmon resonance (SPR) peak for Au NPs was absent in the spectra for all samples, indicating the preservation of Au NCs after deposition process.

Figure 5. (A) Plots for photocatalytic H2 production under visible light irradiation by Au or Pt/GCN photocatalysts ; (B) Rate of H2 evolution by various Au/GCN photocatalysts under visible light; (C) Recycling test of photocatalytic H2 evolution for 1Au/GCN and PR 1Au/GCN. Photocatalytic hydrogen evolution and stability under visible light. The photocatalytic performance of Au/GCN samples was evaluated by H2 production from water splitting under visible light irradiation using triethanolamine as the electron donor to consume photo-generated holes. As shown in Figure 5A, pure GCN sample exhibited negligible activity for H2 evolution. This probably resulted from the rapid recombination rate of photo- generated electron–hole pairs in single component of semiconductors, leading to the limited H2 evolution efficiency. Whereas,


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the combination of Au25 NCs with the GCN NSs remarkably improved the photocatalytic activity, which was illustrated by the increased slopes from the plots of H2 evolution by different hybrid catalysts with increased loading amount of Au25 NCs. The H2 evolution rate for 0.2Au/GCN, 0.5Au/GCN and 1Au/GCN are 93 µmol h-1 g-1, 177 µmol h-1 g-1, 320 µmol h-1 g-1, respectively (Figure 5B). 1Au/GCN showed the optimal photocatalytic activity, which was 1.7 times higher than that of PR 1Au/GCN, 3 times higher than that of PR 1Pt/GCN, both as control samples. The stability of the photocatalysts was examined by recycling tests under the identical photo-reaction condition for 1Au/GCN and PR 1Au/GCN samples (Figure 5C). It was shown that for 1Au/GCN, after three cycles, the H2 evolution rate kept at 272 µmol h-1 g-1, only with a 15% decrease from the original rate. Meanwhile, for PR 1Au/GCN, after three cycles, the H2 evolution rate declined to 111 µmol h-1 g-1, with a 40.5% decrease from the original rate. This result indicated the favorable stability of the hybrid photocatalysts during the reactions. To better evaluate the H2 production activity for 1Au/GCN catalyst, we also summarized recent published reports on metal/g-C3N4 photocatalysts applied for H2 production, and the major information was listed in Table 1. Compared with the NPs hybrid catalysts in other reports via various modification methods, Au25 NCs/GCN exhibited moderate reactivity for H2 production. This indicated the study on this binary photocatlyst is still at very early stage. We need more efforts on the deep understanding of such NCs’ photocatalysis and further reactivity improvement. To investigate the reasons for such stability of the hybrid photocatalysts after the photocatalytic hydrogen evolution reaction, the used catalyst 1Au/GCN-AR (denoted for 1Au/GCN after reaction) was characterized by XRD, DRS, SEM and XPS, as shown in Figure S3 and Figure S4. The XRD pattern kept unchanged for 1Au/GCN-AR, compared with that of 1Au/GCN before photo-reaction (Figure S3A). This indicated the intact preservation of graphite


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Table 1 Representative summary of the H2 production from water splitting by metal/g-C3N4 based photocatalysts. Composite type

Co-catalysts

Coupling methods for metal with g-C3N4

Weight of catalysts used

Reactant solution and sacrificial agent

Light source

Activity (unit)

Stability

Ref (Year)

Au/SnO2/g-C3N4

Au (2 wt%) SnO2 (6 wt%)

Photodeposition

0.1 g

Mixture of distilled water (80 mL) and methanol (20 mL)

300 W Xe lamp (λ > 420 nm)

H2: 770 µmol g−1 h−1

>30 h

48 (2016)

Au/g-C3N4

Au (1 wt%)

Deposition–precipitation

0.02 g

TEOA (10 vol%)

125W medium pressure visible-light Hg lamp (λ > 400 nm)

H2: 532 µmol for 3h

>12 h

49 (2014)

(Au, Ag, or Pt)decorated g-C3N4/TiO2 nanofibers

Au (1 wt%) Ag (1 wt%) Pt (1 wt%)

Electrospinning

0.005 g

10 mL of TEOA (15 vol%)

300 W Xe lamp with AM 1.5 solar simulation filter 300 W Xe lamp (λ > 400 nm)

Ag/g-C3N4/TiO2 H2 : 1.50 µmol h−1

>8 h

50 (2016)

g-C3N4/Au/P3HT/ Pt

Au (1 wt%) P3HT(0.5 wt%) Pt (0.5 wt%)

Photodeposition

N/A

200 mL of TEA (10 vol%)

300 W Xe lamp (λ > 420 nm)

H2 : 320 µmol h−1

N/A

51 (2015)

Single atom Pt/g-C3N4

Pt (0.16 wt%)

Chemical deposition

0.05 g


300 W Xe lamp

H2: 318 µmol h−1

>16 h

52 (2016)

Au NPs/g-C3N4

Au (2.1 wt%)

Thermal annealing

0.01 g

a mixture of 80 mL DI water and 20 mL TEOA aqueous solution

300 W Xe lamp (λ > 400 nm)

H2: 3308.3 µmol h−1 g−1

>18 h

53 (2017)

AuPd/g-C3N4

AuPd(0.5 wt%)

Reduction by NaBH4

0.05 g


300 W Xe lamp (λ > 400 nm)

H2: 326 µmol h−1 g−1

>24 h

54 (2015)

Au-TiO2/ g-C3N4

Au (0.75 wt%) C3N4:TiO2 1:10

Chemical reduction

0.03 g

Reaction mixture (water: methanol, 4:1)

Direct solar irradiation with an average solar flux of (330–750 W m−1) and average 25 oC

H2: 2933 µmol h−1 g−1

>15 h

55 (2017)


Ag/g-C3N4/TiO2 H2: 0.61 µmol for 2h

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Table 1 continued Composite type

Co-catalysts



Core–shell Ag@C3N4 nanocomposites

Ag (10 wt%)

Ultrasonication and heating

0.1 g

PtAu/g-C3N4

Pt50Au50 (3.0 wt%)

Impregnation and precipitation

UiO-66/g-C3N4/Pt

UiO-66 (50 wt%) Pt (0.5 wt%)

CdS/Au/g-C3N4


Light source

Activity (unit)

Stability

Ref (Year)


300 W Xe lamp (λ > 420 nm)

H2: 25.2 µmol for 10 h

N/A

56 (2014)

0.04 g

TEOA (10 vol%)

300 W Xe lamp (λ > 420 nm)

H2: 1.6025 mmol h−1 g−1

>10 h

57 (2018)

Photodeposition

0.01 g

20 mL L -ascorbic acid aqueous solution (0.1 M , pH 4.0)

300 W Xe lamp (λ > 420 nm)

H2: 14.11 × 10-6 M h−1

N/A

58 (2015)

Pt (1.0 wt%) Au/C3N4 (0.5 g) S8 (2.0 mmol) Cd(ClO4)2·xH2O (4.0 mmol)

Chemical reduction

0.1 g

10 mL methanol and 50 mL water

300 W Xe lamp (λ > 420 nm)

H2: 19.02 µmol h−1 g−1

N/A

59 (2015)

Pd-Ag/ g-C3N4

Pd (0.7 wt%) Ag (0.3 wt%)

Chemical reduction

0.01 g


Direct solar irradiation

H2: 1250 µmol g−1 h−1

>12 h

60 (2018)

Au-PtO/ g-C3N4

Au (0.4 wt%) Pt (0.6 wt%)

Photodeposition

0.05 g

80 mL of 25 vol% methanol aqueous solution

300 W Xe lamp (λ > 400 nm)

H2: 16.9 µmol h−1

>9 h

61 (2016)

PtCo/ g-C3N4

PtCo (1 wt%)

Solvothermal reaction

0.05 g


300 W Xe lamp (λ > 400 nm)

H2: 960 µmol g−1 h−1

>27 h

62 (2015)

K- g-C3N4

K (10 wt%) Pt (0.5 wt%)

KCl-template method

0.1 g


300 W Xe lamp (λ > 420 nm)

H2: 102.8 µmol g−1 h−1

>16 h

63 (2014)

Ag/ g-C3N4

Ag (3 wt%)

Chemical reduction

0.005 g


300 W Xe lamp

H2: 344.51 µmol g−1 h−1

>20 h

64 (2016)

g-C3N4

Pt (3 wt%)

Photodeposition

0.05 g


300 W Xe lamp (λ >420 nm)

H2: 14.99 µmol g−1 h−1

>12 h

65 (2016)


17

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Page 18 of 42

Table 1 continued Composite type

Co-catalysts




Light source

Activity (unit)

Stability

Ref (Year)

g-C3N4

Pt (3 wt%)

Photodeposition

0.1 g


300W Xe lamp (16.1 mW/cm−2) (λ > 420 nm)

H2: 248 µmol g−1 h−1

N/A

66 (2014)

CQDs/g-C3N4

Pt (3 wt%)

Photodeposition

0.05 g


300 W Xe lamp (100 mW/cm−2) (λ > 420 nm)

H2: 116.1 µmol h−1

N/A

67 (2016)

C-ZIF/g-C3N4

Pt (3 wt%)

Thermal condensation

0.1 g


300 W Xe lamp (λ > 400 nm)

H2: 11.6 µmol h−1

>20 h

68 (2016)

CoTiO3/g-C3N4

Pt (3 wt%)

Photodeposition

0.02 g

Ethanol solution (10 vol %).

300 W Xe lamp (190 mW/cm−2) (λ > 420 nm)

H2: 858 µmol g−1 h−1

>16 h

69 (2016)

W18O49/g-C3N4

Pt (3 wt%)

Photodeposition

0.005 g


300 W Xe lamp (190 mW/cm−2) (λ > 420 nm)

H2: 3.69 µmol h−1

>5 h

70 (2016)

Au25(Cys)18/g-C3N4

Au (1 wt%)

Impregnation and precipitation

0.02 g


300 W Xe lamp (λ > 420 nm)

H2: 320 µmol g−1 h−1

> 12 h

This work


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C–N backbones of GCN NSs after reaction. Besides, no additional diffraction peak assigned to Au NPs was observed, which showed that severe aggregation of Au NCs was avoided during the reaction. DRS spectra exhibited the similar information (Figure S3B). The absorption curve of 1Au/GCN-AR was still retained as that of g-C3N4, with the absorption edge at 467 nm. Meanwhile, the characteristic peak of SPR for Au NPs which usually at 530 nm was absent, which also indicated the avoiding of aggregation of Au NCs after reactions. Same conclusion could also be drawn from SEM images (Figure S3C and D). 1Au/GCN-AR sample was also composed of wrinkled g-C3N4 nanosheets. The morphology was similar to that of 1Au/GCN, and Au NPs were not present in the image of 1Au/GCN-AR sample. Moreover, XPS spectra of C 1s and N 1s were in good agreement with above results. The binding energy and the chemical composition for these two elements were the same between these two samples before and after the photo-reaction (Figure S4A and B). There were attenuation of the peak intensities for the BE of Au 4f and S 2p for 1Au/GCN-AR sample, comparing to those of 1Au/GCN (Figure S4C and D). This may indicate that some of the weakly adsorbed Au25 NCs on GCN NSs were detached from the catalyst surface during the photocatalytic reaction process, and then the loss of NCs on the catalyst may lead to the slight decrease of the activity for 1Au/GCN catalyst in the recycling test. Nevertheless, most of the NCs were still kept close contact with GCN NSs and thus the hybrid catalyst exhibited superior activity and stability over PR 1Au/GCN catalyst. Furthermore, from the XPS spectra of S 2p, it is known that the thoil ligands were preserved (Figure S4D), indicating the Au25 NCs were resistant to aggregate during the reaction, which was demonstrated by various characterization techniques. The stable interfacial structure of such Au/GCN nanocomposite photocatalysts resulted in the favorable stability for photocatalytic H2 production.


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Figure 6. Schematic illustration of the photocatalytic H2 evolution by Au/GCN photocatalysts. (A) Energy band diagram; (B) Schematic diagram.

Figure 7. (A) Transient photocurrent responses for GCN, 0.2Au/GCN and 1Au/GCN; (B) Photoluminescence spectra of various Au/GCN photocatalysts at an excitation wavelength of 400 nm. Mechanism of the enhanced photocatalytic H2 production. The enhanced photocatalytic H2 production could be reasonably attributed to the effective interfacial charge transfer between Au25 NCs and GCN NSs via the formation of a vast number of heterojunctions. The ultra-small size of Au25 NCs dramatically increased the reactive surface area-to-mass ratio of the hybrid photocatalysts, thus, sufficient heterojunctions as the active sites could be formed at the interface, which number was far more than that of traditional NPs/semiconductors. The precise weight ratio of Au or Pt NPs on PR 1Au/GCN or PR 1Pt/GCN were determined via ICP-OES technique, and the values are 1.17 wt% and 0.86 wt%, which were very close to the theoretical values and the weight ratio of Au NCs on 1Au/GCN (0.96 wt%). This indicated that the dramatic differences of the H2 production rates by these three samples were not due to the distinguished


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weight ratios of the metal in these samples. Furthermore, the BET surface areas of pristine GCN, 1Au/GCN and PR 1Au/GCN were also calculated from the isotherm curves in Figure S5A, which were 42.1 m2/g, 44.9 m2/g and 48.0 m2/g. In Figure S5B, all the three samples showed similar pore size distributions, and the average pore size was 1.8 nm, 1.9 nm and 2.0 nm for GCN, 1Au/GCN and PR 1Au/GCN, respectively. This implied that loading metal NCs or NPs onto GCN had little affection on the micro-texture of GCN and the differences of the H2 production rates between 1Au/GCN and PR 1Au/GCN were not due to the dramatic changes in specific surface area of GCN. Thus, it is reasonable to deduce that the enhanced H2 production by 1Au/GCN was due to the ultra small size of Au25 NCs, which lead to the formation of vast reaction sites at the interface between NCs and GCN NSs. Besides, the appropriate band structure of this binary nanocomposite facilitated the charge separation and transfer across the interface. As shown in Figure 6A, the CB edge of GCN NSs (–0.9 V vs. NHE, obtained from Mott-Schottky measurements) is more negative than the lowest unoccupied molecular orbital (LUMO) of Au25 NCs (–0.1 ~ –0.5 V vs. NHE, depending on the type of the ligand).32, 40 After the illumination of visible light, the photo-induced electrons on GCN surface could rapidly transfer to Au25 NCs across the interfacial junctions. Then these electrons were further involved in the reduction of water to produce H2, even if in neutral aqueous solution (EH+/H = –0.4 V vs. 2

NHE, pH=7) in our experiments. Meanwhile, the VB edge of GCN shows the similar band energy position to the highest occupied molecular orbital 1(HOMO1) of Au25 NCs. The vast numbers of photogenerated holes were just involved in the oxidation of electron donor TEOA on GCN surface, without transferring to Au25. Such differences in energy band position between these two components in this binary composite shaped the behavior of photogenerated charge carriers, which led to the effective suppression of the recombination of electron–hole pairs, and


21


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rapid charge transfer at the interface to facilitate the H2 production reaction. The schematic diagram describing the possible photocatalytic process by this Au/GCN nanocomposite photocatalysts was illustrated in Figure 6B. The enhancement of charge separation was further proven by the transient photocurrent responses of various hybrid catalysts via PEC measurements. As shown in Figure 7A, 1Au/GCN exhibited the highest photocurrent intensity of all the tested samples. This result indicated the enhancement of photo-generated charge separation by the formation of Au25 NCs/GCN NSs heterojunctions, which was also close-related to the number of the junctions.9, 15 Besides, the effective interfacial charge transfer was evidenced by the decreased photoluminescence (PL) intensity of the hybrid photocatalysts with the increased loading amount of Au25 NCs (Figure 7B), where the 1Au/GCN showed the lowest PL intensity. The quenching of PL signal of 1Au/GCN sample demonstrated the effective photo-induced charge separation in 1Au/GCN after the photo excitation.13, 15 It was reported that besides accepting photo-generated electrons from excited semiconductors by light as the active site, the Au clusters may also act as semiconductors which could generate photo induced electrons and become catalytic center.40 To test whether Au NCs themselves could absorb visible light to generate photo-electrons which would further be involved in the H2 generation reactions, such reactions were conducted under the light with selective mono-wavelength (550 ± 10 nm and 650 ± 10 nm) by the 1Au/GCN photocatalyst. It is known that the typical SPR absorption band for Au NPs is around 550 nm, and the feature absorption band for Au25(Cys)18 NCs is around 650 nm. Negligible amount of H2 was generated under both conditions, which indicated that Au25(Cys)18 NCs did not make significant contributions to the photocatalytic activity from its own excitation by visible light to generate photo induced charges. This result may be due to such a limited amount of Au25(Cys)18 NCs with


22

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rapid charge recombination in themselves excited under the light with the wavelength around 650 nm, which could not excite the major component of GCN in this binary composite photocatalyst. Besides, the apparent quantum yield (AQY) of this photocatalytic system was estimated by another monochromic light (420 ± 10 nm), and the AQY could reach 0.2%.

CONCLUSION To summarize, Au25(Cys)18/GCN nanocomposite photocatalysts were synthesized via a facile wet impregnation method. Au25 NCs were found highly dispersed on the surface of GCN. The introduction of Au25 NCs onto GCN could remarkably improve the photocatalytic H2 production under visible light irradiation by the nanocomposite photocatalysts. A vast number of junctions formed at the metal NC/semiconductor interfaces, and the maximized distribution of the active sites originating from the highly dispersion of Au25 NCs, led to the considerable enhancement of the photocatalytic activity. The enhancement could be attributed to the appropriate band structures between these two components in the binary nanocomposites, which led to the efficient separation of photo-generated electron-hole pairs across the interface junctions. Such hybrid photocatalysts retained good stability in the recycling test. The combination of metal NCs with GCN NSs provides new insights in the design of novel photocatalysts with superior catalytic performance for efficient solar energy conversion.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge.


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FT-IR spectra, HAADF-STEM image of Au/GCN; BET surface area and pore size distribution of Au/GCN, XRD pattern, UV-vis DRS, SEM image and XPS spectra of 1Au/GCN after photocatalytic reactions. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (No. 21706130), the Jiangsu Natural Science Funds (BK20150943), grants from the Key Programs (15KJA150005) and the General Programs (15KJB150011) of Educational Commission of Jiangsu Province, the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and research grants of the State Key Laboratory of MaterialsOriented Chemical Engineering (ZK201410, ZK201714). Financial support from “the Youth Thousand Talents Plan” of China and “the Shuang Chuang Plan” of Jiangsu province for Y. Y. is also gratefully acknowledged. REFERENCES (1) Kudo, A.; Miseki, Y. Heterogeneous photocatalyst materials for water splitting. Chem. Soc. Rev. 2009, 38, 253–278.


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0D/2D Au25 NCs/GCN nano-composites photocatalysts exhibited enhanced H2 production under visible light and favorable recycling stability, compared to Au NPs/GCN photocatalysts. 70x43mm (300 x 300 DPI)



Figure 1. (A) TEM image of free Au25(Cys)18 NCs; (B) their mean size distribution; (C) SEM image of asprepared GCN NSs; (D) AFM image of as-prepared GCN NSs and inset is the corresponding thickness analysis taken around the white line. 66x54mm (300 x 300 DPI)


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Figure 2. (A) UV-vis absorption spectra of the as-prepared Au25(Cys)18 NC solution before (black line) and after (color lines) the addition of GCN. Inset image is the ball-stick model of a Au25(Cys)18 NC; (B) XRD patterns of various Au/GCN photocatalysts; SEM images for (C) 0.2Au/GCN, (D) 0.5Au/GCN, and (E) 1Au/GCN. 73x54mm (300 x 300 DPI)



Figure 3. XPS spectra of various Au/GCN photocatalysts with the high resolution spectra for (A) C 1s, (B) N 1s, (C)Au 4f, and (D) S 2p. 71x63mm (300 x 300 DPI)


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Figure 4. UV–vis diffuse reflectance spectra of various Au/GCN nanocomposite photocatalysts. The inset is the corresponding plots of (αhν)2 vs. hν for GCN and 1Au/GCN samples. 52x46mm (300 x 300 DPI)



Figure 5. (A) Plots for photocatalytic H2 production under visible light irradiation by Au or Pt/GCN photocatalysts; (B) Rate of H2 evolution by various Au/GCN photocatalysts under visible light; (C) Recycling test of photocatalytic H2 evolution for 1Au/GCN and PR 1Au/GCN. 41x12mm (300 x 300 DPI)


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Figure 6. Schematic illustration of the photocatalytic H2 evolution by Au/GCN photocatalysts. (A) Energy band diagram; (B) Schematic diagram. 29x10mm (300 x 300 DPI)



Figure 7. (A) Transient photocurrent responses for GCN, 0.2Au/GCN and 1Au/GCN; (B) Photoluminescence spectra of various Au/GCN photocatalysts at an excitation wavelength of 400 nm. 27x12mm (300 x 300 DPI)


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