& Photocatalysis | Hot Paper |

Advantageous Interfacial Effects of AgPd/g-C3N4 forPhotocatalytic Hydrogen Evolution: Electronic Structure and H2ODissociation

Weixin Zou,[a, b, c] Lixia Xu,[a, c] Yu Pu,[b, c] Haojie Cai,[a, c] Xiaoqian Wei,[b, c] Yidan Luo,[b, c]

Lulu Li,[b, c] Bin Gao,[d] Haiqin Wan,*[a, c] and Lin Dong[a, b, c]

Abstract: &&Please add academic titles for authors&&Bi-metallic AgPd nanoparticles have been synthesized before,but the interfacial electronic effects of AgPd on the photoca-talytic performance have been investigated less. In thiswork, the results of hydrogen evolution suggest that the bi-metallic AgPd/g-C3N4 sample has superior activity to Ag/g-C3N4 and Pd/g-C3N4 photocatalysts. The UV/Vis diffuse reflec-tance spectroscopy, X-ray photoelectron spectroscopy, COadsorption diffuse reflectance FTIR spectroscopy, and FTIRresults demonstrate that in the AgPd/g-C3N4, the surface

electronic structures of Pd and Ag are changed, which isbeneficial for faster photogenerated electron transfer andgreater H2O molecule adsorption. In situ ESR spectra suggestthat, under visible light irradiation, there is more H2O dissoci-ation to radical species on the AgPd/g-C3N4 photocatalyst.Furthermore, DFT calculations confirm the interfacial elec-tronic effects of AgPd/g-C3N4, that is, Pdd�···Agd+ , and the ac-tivation energy of H2O molecule dissociation on AgPd/g-C3N4 is the lowest, which is the main contributor to the en-hanced photocatalytic H2 evolution.

1. Introduction

With the increasing demand for energy and diminishing con-ventional fossil fuel resources, the development of alternative,clean energy sources has become a very significant issue.[1]

Solar-driven hydrogen (H2) production by water splitting hasbeen considered a promising alternative to meet the challengeof the energy crisis. Generally, there are three possible solar-H2

conversion systems: photocatalysis, photoelectrochemical(PEC) cells, and photovoltaic (PV)-EC cells.[2, 3] The conversion ef-ficiency relies significantly on materials that can utilize photonenergy from the wide solar spectrum, separate electron–holepairs, and promote charge carrier diffusion.[4–6] It has been re-ported that the fabrication of organic conjugated polymers is

one effective strategy, thanks to the advantages of stability inaqueous solution, visible light absorption, intramolecularcharge transfer, low cost, and so on.[5–12]

Graphitic carbon nitride (g-C3N4), with its well-known physio-chemical stability, narrow bandgap, attractive electronic prop-erties, and environmental friendliness, has become a promisingtwo-dimensional nonmetal material for many photocatalyticapplications such as water splitting, CO2 reduction, and envi-ronmental contaminant removal.[13–16] In addition, g-C3N4 canbe used as a flexible support to encapsulate nanocrystals, re-sulting in the stabilization of nanocrystals and enhanced pho-tocatalytic performance.[17–20] However, there are some short-comings of g-C3N4, such as low visible light absorption, fastelectron–hole recombination, and low quantum efficiency,which limit its application in photocatalysis.[20]

Several attempts have been made to mitigate these chal-lenges, and it has been suggested that anchoring metal nano-particles such as Au,[21] Ag,[22] Pd,[23] and so on is an effectivemethod to prolong the lifetime of photogenerated electrons.Ma et al.[24] showed that Ag metal on g-C3N4 can extend thevisible light absorption owing to the inherent surface plasmonresonance (SPR) effect, which is beneficial for the generation ofmore active radical species, and thus, the photocatalytic disin-fection efficiency is improved. Ni et al.[23] proposed that theSchottky barrier is formed in Pd/g-C3N4, and Pd nanoparticlescan shuttle the photoexcited electrons from g-C3N4, which im-proves the photocatalytic activity of NO oxidation. Comparedwith monometal-based nanocatalysts, bimetals with uniqueelectronic structures display greater potential in photocatalyticapplications. Various bimetallic alloy nanoparticles have been

[a] W. Zou, L. Xu, H. Cai, H. Wan, L. DongState Key Laboratory of Pollution Control and Resource ReuseSchool of the Environment, Nanjing University, Nanjing 210093 (PR China)E-mail : wanhq@nju.edu.cn

[b] W. Zou, Y. Pu, X. Wei, Y. Luo, L. Li, L. DongKey Laboratory of Mesoscopic Chemistry of MOESchool of Chemistry and Chemical Engineering, Nanjing UniversityNanjing 210093 (PR China)

[c] W. Zou, L. Xu, Y. Pu, H. Cai, X. Wei, Y. Luo, L. Li, H. Wan, L. DongJiangsu Key Laboratory of Vehicle Emissions ControlCenter of Modern Analysis, Nanjing University, Nanjing 210093 (PR China)

[d] B. GaoDepartment of Agricultural and Biological EngineeringUniversity of Florida, Gainesville, FL, 32611 (USA)

Supporting information and the ORCID identification number(s) for the au-thor(s) of this article can be found under :https ://doi.org/10.1002/chem.201806074.

Chem. Eur. J. 2019, 25, 1 – 8 � 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim1

These are not the final page numbers! ��

Full PaperDOI: 10.1002/chem.201806074

1 12 23 34 45 56 67 78 89 9

10 1011 1112 1213 1314 1415 1516 1617 1718 1819 1920 2021 2122 2223 2324 2425 2526 2627 2728 2829 2930 3031 3132 3233 3334 3435 3536 3637 3738 3839 3940 4041 4142 4243 4344 4445 4546 4647 4748 4849 4950 5051 5152 5253 5354 5455 5556 5657 57

reported, such as AuPd,[25, 26] AgPd,[27, 28] FePd,[29] AgCu,[30]

PtNi,[31] and so forth. They exhibit better photocatalytic activi-ties than monometallic catalysts, and possible factors in the ac-tivity enhancement have been investigated. Majeed et al.[28]

considered that there is a “synergism effect” between Pd andAg atoms, that is, Pd can quench photogenerated electronsthrough the Schottky barrier formation mechanism, and thecharacteristic SPR property of Ag enhances visible light absorp-tion, resulting in the better activity of the Pd-Ag/g-C3N4 photo-catalyst. Zhu et al.[30] suggested that there is a stronger metal–support interaction in Ag-Cu/g-C3N4 than in its monometalliccounterparts, leading to efficient photoinduced electron–holeseparation, and thus, Ag-Cu/g-C3N4 achieves better photocata-lytic H2 evolution activity. However, the interfacial electronic ef-fects of bimetallic nanoparticles on photocatalytic water split-ting are not very clear.

In this work, bimetallic AgPd nanoparticles were loaded ong-C3N4 for photocatalytic H2 evolution. The interfacial electron-ic effects were studied by XPS, CO-adsorption DRIFTS, UV/VisDRS, FTIR spectroscopy, and DFT calculations. The results dem-onstrate that the strong electronic interactions between Ag,Pd, and g-C3N4 are beneficial for the photogenerated electron–hole separation and visible light absorption, leading to a lowerdissociation energy of H2O molecules and higher H2 evolution.The work investigated the interactions of interfacial electronicstructures in photocatalysis, which provides a scientific basisfor the development of desirable functional photocatalysts.

2. Experimental Section

2.1. Catalyst preparation

Noble metal nanoparticles were synthesized using the method ofpolyol reduction.[32] Poly(vinylpyrrolidone) (PVP, MW = 58 000,50 mg) was dissolved in ethylene glycol (15 mL) in a flask underreflux with magnetic stirring and the temperature was increased to150 8C in an oil bath (methyl silicone oil). Subsequently, the metalprecursor (NaPdCl4 or AgNO3) (0.05 mmol) dissolved in the ethyl-ene glycol (5 mL) solution was injected into the flask and reactedat this temperature for 30 min. Then, the mixture was cooled rapid-ly by quenching in an ice-water bath. The resulting metal nanopar-ticles were washed with acetone to remove the PVP. The obtainedsamples were denoted as Pd and Ag. AgPd bimetallic nanoparticleswere synthesized through the sequential reduction method. Agnanoparticles (0.04 mmol) were obtained by using the abovemethod. The Ag nanoparticles were then dispersed in the flaskwith ethylene glycol (15 mL) and PVP (50 mg). As the temperaturewas increased to 150 8C, NaPdCl4 (0.01 mmol) in ethylene glycol so-lution was injected into the flask. The other steps were the sameas for the preparation of Pd nanoparticles. In the AgPd bimetallicnanoparticles, the molar ratio of Pd to Ag was 1:4.

Ag/g-C3N4, Pd/g-C3N4, and AgPd/g-C3N4 catalysts were preparedthrough the impregnation method. The support g-C3N4 was madeby calcining urea under air in a muffle furnace; the temperaturewas increased to 550 8C at a rate of 5 8C min�1 and maintained atthat temperature for 4 h. In a typical synthesis, g-C3N4 powder(200 mg) was dispersed in ethanol/H2O (20 mL; 1:1) solution, andthe obtained noble metal nanoparticles were dispersed in ethanol/H2O (10 mL; 1:1) solution. Then, the two kinds of slurries weremixed, sonicated for 20 min, and stirred for 1 h. Finally, the samples

were dried overnight at 70 8C in an oven. The prepared sampleswere denoted as Ag/CN, Pd/CN, and AgPd/CN, respectively.

2.2. Catalyst characterization

Powder X-ray diffraction (XRD) patterns of Ag/CN, Pd/CN, AgPd/CN, and CN were recorded with a Philips X’pert Pro diffractometerusing a Ni-filtered CuKa source (1.5418 �) operated at 40 kV and40 mA. Transmission electron microscopy (TEM), high-resolutionTEM (HRTEM), and high-angle annular dark field (HAADF)-STEMimages were obtained with a JEM-2100F microscope equippedwith an Oxford energy-dispersive X-ray analysis (EDX) instrumentoperating at 200 kV. X-ray photoelectron spectroscopy (XPS) wasperformed on an Axis Ultra DLD photoelectron spectrometer, usinga monochromatic AlKa (1486.6 eV) source with a pass energy of50 eV (0.1 eV per step). The binding energies were calibrated usingthe C 1s peak at 284.6 eV. UV/Vis diffuse reflectance spectroscopy(UV/Vis DRS) was recorded in the range 200–800 nm with the refer-ence of BaSO4 with a Shimadzu UV-2401 spectrophotometer. Pho-toluminescence (PL) spectra were measured on a FluoroMax-4spectroscope with an excitation wavelength of 320 nm. Electro-chemical measurements on a CHI660E electrochemical workstationwere performed with a standard three-electrode cell. The photoca-talyst deposited on fluorine-doped tin oxide was used as the work-ing electrode, and the reference and counter electrodes were Hg/Hg2Cl2 and platinum wire (in saturated KCl), respectively. The elec-trolyte was 0.1 m Na2SO4 solution, and a Xe lamp was employed forirradiation. The FTIR spectra were collected from 400 to 4000 cm�1

on a Nicolet 5700 FTIR spectrometer at the spectral resolution of4 cm�1. For CO adsorption diffuse reflectance infrared Fourier trans-form spectra (DRIFTS), the Praying Mantis diffuse reflection acces-sory with a highly efficient diffuse collection system was used tostudy the adsorption processes on the samples in the controlledreaction conditions with the appropriate diffuse reflectance reac-tion chamber. First, the samples were pretreated in a pure N2

stream at 50 8C for 1 h. After cooling to room temperature, thebackground spectrum was collected, and then CO/Ar (10 % CO byvolume) was switched on for 1 h until adsorption saturation. TheN2 stream was switched on to blow off CO gas, and the spectrawere obtained by subtraction of the background spectra. Thein situ electron spin resonance (ESR) signal was examined at 77 Kon a Bruker ESP-300E spectrometer under visible light, and anaqueous mixture containing the photocatalyst and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was saturated with N2 gas to eliminateO2 in the EPR flat cell.

2.3. Catalytic performance measurements

The photocatalytic hydrogen evolution reaction under visible lightwas performed under 300 W Xe lamp irradiation with a 420 nmcutoff filter. The sample (40 mg) was dispersed in a solution(100 mL) of triethanolamine (10 mL) and deionized water (90 mL).The temperature of the reaction solution was kept at around 6 8Cwith a circulation pump. The system was sealed and evacuated for30 min before illumination, the generated hydrogen was purgedwith N2 into a gas chromatograph (GC), and a thermal conductivitydetector (TCD) in the GC was used to measure the concentrationof hydrogen. In addition, to ensure the reproducibility of the pho-tocatalytic activity, the H2 evolution of AgPd/CN under visible lightwas performed again, and the catalytic performance measurementwas the same as before.

Chem. Eur. J. 2019, 25, 1 – 8 www.chemeurj.org � 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2

�� These are not the final page numbers!

Full Paper

1 12 23 34 45 56 67 78 89 9

10 1011 1112 1213 1314 1415 1516 1617 1718 1819 1920 2021 2122 2223 2324 2425 2526 2627 2728 2829 2930 3031 3132 3233 3334 3435 3536 3637 3738 3839 3940 4041 4142 4243 4344 4445 4546 4647 4748 4849 4950 5051 5152 5253 5354 5455 5556 5657 57

2.4. Computational details

The Vienna Ab Initio Simulation Package (VASP)[33, 34] for all thespin-polarized DFT calculations within the generalized gradient ap-proximation (GGA) using the PBE functional formulation[35] was ap-plied in this work. Projected augmented wave (PAW)[36] pseudopo-tentials were employed to describe the interactions between ioniccores and valence electrons. 1 (H), 4 (C), 5 (N), and 6 (O) valenceelectrons were explicitly taken into account. The valence electronicstates were expanded in plane wave basis sets with a cutoffenergy of 450 eV. The DFT-D3(BJ)[37] method was used to describethe dispersion effects in the system. Partial occupancies of elec-tronic bands were allowed with the Gaussian smearing methodand a width of 0.01 eV. A p(3 � 3) supercell with a single-layeredcorrugated g-C3N4 sheet was used, and the vacuum between slabswas 25 �, in which a Monkhorst-Pack K-point mesh of 2 � 2 � 1 wasincluded. Ag22, Pd22, and Ag12Pd10 clusters chosen as models of Ag,Pd, and AgPd NPs supported on the g-C3N4 sheet, respectively (Fig-ure S1, Supporting Information).

The adsorption energy, Eads, of the water on the M/g-C3N4 sheetwas calculated as Eads = Esurf + Ewater�Ewater/surf, in which Ewater/surf andEsurf were the total energy of the M/g-C3N4 sheet with and withoutthe water adsorption, respectively, and Ewater was the energy of thewater molecule in vacuum.

3. Results and discussion

3.1. Morphology and phase structure

The morphology and particle size of the as-synthesized AgPd/CN sample were observed by TEM. In Figure 1 a, the ultrafineAgPd nanoparticles with high dispersion were successfullyanchored onto the g-C3N4 sheets. The size of the AgPd nano-particles was in the range 4–6 nm. For further confirmation ofthe bimetallic nanoparticles, HAADF-STEM was conducted toexplore the elemental distributions of AgPd/CN. The mappingresults of Pd, Ag, and C elements are displayed in Figure 1 d–f,respectively. The results reveal that there are bimetallic AgPdnanoparticles on the g-C3N4 sheets.

In addition, the phase structures of AgPd/CN, Pd/CN, Ag/CN,and CN were determined by XRD characterization. In Figure S2(Supporting Information), all AgPd/CN, Pd/CN, Ag/CN, and CNsamples displayed the typical diffraction peaks (13.18 and27.38) of g-C3N4, which are attributed to the (100) and (002)planes of g-C3N4, respectively.[38] For the Ag/CN sample, therewere no clear diffraction peaks of Ag, and on the Pd/CNsample, a peak at 41.28 was observed and indexed as the Pd0

(111) plane.[28] However, no diffraction peaks ascribed to Agand Pd species were obtained on AgPd/CN, which may be at-tributed to the low loading content of Pd on the AgPd/CNsample.

3.2. Photocatalytic H2 evolution

The photocatalytic performance of H2 evolution was deter-mined under visible-light (l�420 nm) irradiation, with 10 vol %triethanolamine (TEOA) as the scavenger in the aqueous solu-tion. The activities of AgPd/CN, Pd/CN, Ag/CN, and CN sampleswere compared. In Figure 2, the pure g-C3N4 sample without

metallic nanoparticle loading showed a distinctlylower H2 evolution than the other samples. Upon g-C3N4 loading with Ag and Pd nanoparticles, the H2

evolution increased significantly. AgPd/CN exhibiteda better photocatalytic performance than the others,in the order AgPd/CN>Pd/CN>Ag/CN>CN. Toensure the reproducibility of the photocatalytic ac-tivity, we repeated the H2 evolution experiment forAgPd/CN under visible light (Figure S3, SupportingInformation). The photocatalytic H2 evolution in therepeated experiment was almost the same asbefore, which suggests that the reproducibility ofthe phenomena described in the work is believable.Generally, for the metal-based photocatalyst, the re-action activity is closely related to the surface elec-tronic structure; on this basis, we employed XPS,UV/Vis DRS, CO adsorption DRIFTS, FTIR, and DFTmethods to explore the interfacial electronic struc-

Figure 1. a) TEM image of AgPd/CN, b) size distributions of AgPd NPs, c) STEM-HAADFimage, and d–f) EDS mapping images of Pd, Ag, and C elements of AgPd/CN, respective-ly.

Figure 2. Photocatalytic H2 production of AgPd/CN, Pd/CN, Ag/CN, and CNunder visible light irradiation.

Chem. Eur. J. 2019, 25, 1 – 8 www.chemeurj.org � 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3

These are not the final page numbers! ��

Full Paper

1 12 23 34 45 56 67 78 89 9

10 1011 1112 1213 1314 1415 1516 1617 1718 1819 1920 2021 2122 2223 2324 2425 2526 2627 2728 2829 2930 3031 3132 3233 3334 3435 3536 3637 3738 3839 3940 4041 4142 4243 4344 4445 4546 4647 4748 4849 4950 5051 5152 5253 5354 5455 5556 5657 57

tures of AgPd/CN in comparison with those of Pd/CN, Ag/CN,and CN.

3.3. Interfacial electronic state

Information about the surface electronic states can be ob-tained from UV/Vis DRS measurements. In Figure 3 a, the UV/

Vis DRS results of AgPd/CN, Pd/CN, Ag/CN, and CN are com-pared. In the UV absorption region, it is reported that thepeaks at approximately 265 and 290 nm are ascribed to theelectron transitions of �C=N� of g-C3N4 and the charged clus-ters of silver (Agm

d+), respectively.[39, 40] In comparing the rela-tive intensity of I290 to I265, the peak intensity at 290 nm wasstronger on AgPd/CN and Ag/CN, which suggests that onAgPd/CN and Ag/CN, the Ag species has the positive charge.Furthermore, for the Pd- and AgPd-loaded g-C3N4 samples, asignificant increase in visible light absorption was obtained be-cause of the localized surface plasmon resonance (LSPR), andthe shape and position of the plasmonic band are affected bythe size of the metallic nanoparticles.[41] However, the Ag/CNsample exhibited no clear SPR spectra, possibly because of thesmall particle size and low loading content of Ag, and the XRDresult of no Ag diffraction peaks on the Ag/CN sample alsoconfirmed this phenomenon.

The AgPd/CN, Pd/CN, Ag/CN, and CN samples were subject-ed to XPS characterization to explore the interfacial electronicstates. In the XPS spectra of Pd 3d (Figure 3 b), the two peaksat approximately 334.7 and 340.0 eV are attributed to Pd 3d5/2

and Pd 3d3/2 of metallic Pd0, respectively, and the asymmetry ofthe above peak indicates that Pdd+ species is present in Pd/CN. Furthermore, in AgPd/CN, the binding energies of thePd 3d peaks were shifted to lower values relative to those inPd/CN. The phenomenon suggests that, in bimetallic particlesAgPd/CN, the interfacial electronic interaction is enhanced,leading to electron enrichment on Pd. CO adsorption in situDRIFTS of AgPd/CN and Pd/CN also illustrates the differentelectronic state of the Pd species (Figure S4, Supporting Infor-mation). The bands in the region 2300–2400 cm�1 are ascribedto gaseous CO2, the peak at around 2150 cm�1 is the linearlybonded CO on Pdd+ species, and the peaks at approximately2076 and 1943 cm�1 are assigned to the linear and bridge-bonded CO on Pd0.[42, 43] On this basis, AgPd/CN has more Pd

species with the low valence than Pd/CN. Therefore, combinedwith the UV/Vis DRS and XPS results of AgPd/CN, it is deducedthat there is a strong interfacial electronic interaction betweenbimetallic Ag and Pd, that is, the electrons transfer from Ag toPd.

Furthermore, the FTIR results suggest that the electronicstructures of the s-triazine ring on the g-C3N4 are changed withthe introduction of AgPd, Pd, and Ag nanoparticles. In Fig-ure S5 (Supporting Information), the peaks in the range 1800–800 cm�1 are attributed to the typical stretching vibrations ofthe s-triazine ring system, and the relative intensities of thetwo peaks at 1250 and 1200 cm�1 were different on AgPd/CN,Pd/CN, and Ag/CN samples. In general, the relative peak inten-sity in the FTIR spectra is attributed mainly to the factor ofdipole moment, resulting from the vibration model, electrone-gativity, and so on. Therefore, it is proposed that the load ofAgPd, Pd, and Ag has various interfacial electronic effects ong-C3N4. In addition, the XPS spectra of C 1s and N 1s were usedto investigate the electronic states (Figure 4 a,b). According to

the literature, in the C 1s spectra, the peaks centered at around284.6 and 287.9 eV are related to carbon contamination andthe sp2-bonded carbon atom (N�C=N) in aromatic rings of g-C3N4, respectively. As for the N 1s spectra, the peaks at approxi-mately 398.5, 399.7, 401.0, and 404.2 eV are ascribed to thesp2-hybridized nitrogen (C=N�C), tertiary nitrogen (H�N�C2 orN�C3), amino functional group, and charging effect localizationin the heterocycle, respectively.[44] Interestingly, compared withthe pure g-C3N4, there was a shift in the binding energies ofcarbon and sp2-bonded nitrogen in the s-triazine ring of theAgPd/CN, Pd/CN, and Ag/CN samples. For the N�C=N peaks ofthe C 1s spectra, the peak position shifted to the lower bindingenergy, in the order AgPd/CN�Pd/CN<Ag/CN<CN, suggest-ing that C atoms of g-C3N4 obtain electrons upon loading ofmetal nanoparticles. As for the N atoms of C=N�C, the peakposition shifted to higher binding energy, in the order AgPd/CN>Pd/CN>Ag/CN�CN, suggesting that N atoms of the s-triazine on the g-C3N4 lose electrons after loading metals. Onthe basis of the above UV/Vis DRS, XPS, and FTIR spectroscopyresults, it is deduced that the electronic interaction is relativelystronger on the AgPd/CN interface, leading to the increasedelectrons on C atoms and decreased electrons on N atoms.

Figure 3. a) UV/Vis DRS results, and b) XPS spectra of Pd 3d.

Figure 4. XPS spectra of a) C 1s, and b) N 1s of AgPd/CN, Pd/CN, Ag/CN, andCN.

Chem. Eur. J. 2019, 25, 1 – 8 www.chemeurj.org � 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim4

�� These are not the final page numbers!

Full Paper

1 12 23 34 45 56 67 78 89 9

10 1011 1112 1213 1314 1415 1516 1617 1718 1819 1920 2021 2122 2223 2324 2425 2526 2627 2728 2829 2930 3031 3132 3233 3334 3435 3536 3637 3738 3839 3940 4041 4142 4243 4344 4445 4546 4647 4748 4849 4950 5051 5152 5253 5354 5455 5556 5657 57

To describe and confirm the above interfacial electronic in-teractions accurately, we calculated the electron density differ-ences of Ag/CN, Pd/CN, and AgPd/CN. The discrepancy of in-teractions of metal (M) clusters and g-C3N4 could be visualizedby the three-dimensional electron density difference of D1=

1(M/CN)�1(g-C3N4)�1(M), in which 1(M/CN) is the electrondensity of the total metal clusters and g-C3N4 system, and 1(M)and 1(g-C3N4) are the unperturbed electron densities of themetal clusters and g-C3N4 sheet, respectively. In the schematicdiagram (Figure 5), the cyan region suggests electron deple-

tion and the yellow region indicates electron accumulation. Onthis basis, the average electron densities on the Ag, Pd, C, andN atoms of Ag/CN, Pd/CN, and AgPd/CN were calculated. InTable 1, for Ag/CN, Pd/CN, AgPd/CN, upon loading of g-C3N4

with metals, the average electron densities of the C and Natoms of g-C3N4 increased and decreased, respectively. Con-versely, the changes in electron densities on the metals weredifferent. Ag and Pd had positive electricity on Ag/CN and Pd/CN, whereas for AgPd, Pd possessed the negative electricity,owing to the donation of electrons from Ag (i.e. , Pdd�···Agd+).Thus, the interfacial electronic states of AgPd/CN are differentfrom those of Ag/CN and Pd/CN, which have important effectson the photogenerated charge transfer and H2O activation.

3.4. Photogenerated charge transfer

Photoluminescence (PL) spectroscopy is a method for investi-gation of the recombination process involved with the photo-generated electrons and holes of Ag/CN, Pd/CN, AgPd/CN, andCN. In Figure 6 a. A broad PL peak centered at around 480 nm

with a tail spreading to 700 nm was displayed on all the sam-ples, which is explained as down-transfer of the photoexcitedcharge carriers of g-C3N4.[45] Upon loading with metallic nano-particles, the PL emission intensities decreased, suggestingfaster photogenerated electron transfer. The PL intensities ofthe metal-based samples were in the order AgPd/CN<Pd/CN<Ag/CN. This result demonstrates that, compared withmonometallic nanoparticles, bimetallic AgPd/CN shows fasterphotogenerated electron transfer, leading to a decreased re-combination rate of electrons and holes.

To confirm that AgPd/CN could improve the photogeneratedelectron transfer process, we performed photoelectrochemicalmeasurements. The corresponding transient photocurrent re-sponses of AgPd/CN, Pd/CN, and Ag/CN are shown in Fig-ure 6 b. AgPd/CN showed the highest photocurrent response,Pd/CN was second, and Ag/CN had the poorest photocurrentresponse. Because of the advantageous electronic structures,the photoinduced electron transfer of the AgPd/CN sample ismore efficient than those of the Pd/CN and Ag/CN samples,

Figure 5. 3 D Charge density differences for Ag/CN, Pd/CN, and AgPd/CN.The cyan and yellow areas indicate electron depletion and accumulation, re-spectively.

Table 1. Average electron density on Ag, Pd, C, and N atoms of Ag/CN,Pd/CN, and AgPd/CN.

Ag Pd C N

Ag 0g�C3N4 + 1.473 �1.105Ag/CN + 0.064 + 1.446 �1.104D (Ag/CN) + 0.064 �0.027 + 0.001Pd 0g-C3N4 + 1.473 �1.105Pd/CN + 0.031 + 1.444 �1.093D (Pd/CN) + 0.031 -0.029 + 0.012AgPd + 0.039 �0.046g-C3N4 + 1.473 �1.105AgPd/CN + 0.126 �0.053 + 1.443 �1.095D (AgPd/CN) + 0.087 �0.007 �0.030 + 0.010

Figure 6. a) PL spectra, and b) transient photocurrent responses under visi-ble light irradiation.

Chem. Eur. J. 2019, 25, 1 – 8 www.chemeurj.org � 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim5

These are not the final page numbers! ��

Full Paper

1 12 23 34 45 56 67 78 89 9

10 1011 1112 1213 1314 1415 1516 1617 1718 1819 1920 2021 2122 2223 2324 2425 2526 2627 2728 2829 2930 3031 3132 3233 3334 3435 3536 3637 3738 3839 3940 4041 4142 4243 4344 4445 4546 4647 4748 4849 4950 5051 5152 5253 5354 5455 5556 5657 57

leading to the superior photocatalytic performance in H2 evo-lution.

3.5. H2O molecule dissociation

For photocatalytic water splitting to H2, the adsorption anddissociation processes of the H2O reactant are of great impor-tance and worthy of investigation. Therefore, the XPS spectraof O 1s, in situ ESR, and DFT methods were employed.

The high-resolution XPS spectra of O 1s are displayed in Fig-ure 7 a. On AgPd/CN, Pd/CN, and Ag/CN samples, there was abroad peak in the region 534–529 eV, ascribed to the adsorbedoxygen, and this broad peak could be fitted into two peaks:one centered at around 532.5 eV, attributed to the chemisor-bed H2O, and another at approximately 531.0 eV, belonging to

the adsorbed OH groups.[46, 47] On the AgPd/CN sample, thesignal of chemisorbed H2O was clearer than that of Ag/CN andPd/CN, indicating that H2O molecules are more easily adsorbedon the AgPd/CN surface, which is beneficial for the next stepof H2O activation.

The in situ ESR spin-trap technique can detect efficiently thereactive radical species in photocatalytic processes. Herein, thewater splitting of CH and COH radicals were determined withthe help of DMPO. It has been reported that the signal of theDMPO-H radical species shows nine peaks with approximateintensity ratios of 1:1:2:1:2:1:2:1:1, and the DMPO-OH speciesexhibits four peaks (1:2:2:1).[48, 49] In this study, Figure 7 b sug-gests that AgPd/CN, Pd/CN, and Ag/CN samples have mixturesignals attributed to CH and COH radicals. The signal on AgPd/CN was clearer than that on Ag/CN, and slightly more intensivethan that on Pd/CN. The different signal intensities of CH andCOH radicals indicate that on the AgPd/CN photocatalyst, pho-togenerated electrons are obtained more efficiently and H2Omolecules are more easily dissociated, owing to the advanta-geous interfacial electronic structures.

The dissociation processes of H2O molecules on Ag/CN, Pd/CN, and AgPd/CN samples were considered further throughtheoretical calculations, namely H2O (g)!H2O (ads)!H + OH.In Figure 8, on the Ag/CN sample, H2O was adsorbed on theAg cluster with an adsorption energy of �30.88 kJ mol�1, andsubsequently, the adsorbed H2O dissociated to OH and H, and

in the state OH still coupled with the Ag site, H migrated andbridged with two other neighboring top Ag sites with an acti-vation energy of 141.83 kJ mol�1. The dissociation processes ofH2O molecules on Pd/CN and AgPd/CN samples were similarto that on Ag/CN, but the activation of adsorbed H2O dissoci-ating to OH and H was easier. The AgPd/CN and Pd/CN sam-ples had lower activation energies of 76.22 and 87.80 kJ mol�1,respectively. Therefore, it is clear that AgPd/CN has the lowestactivation energy for H2O dissociation, and the dissociationprocess of H2O to OH and H on AgPd/CN occurs easily, whichis helpful for the superior photocatalytic activity in H2 evolu-tion.

4. Conclusions

Bimetallic AgPd nanoparticles were loaded on g-C3N4 for pho-tocatalytic H2 evolution, and the resultant AgPd/g-C3N4

showed better activity than Ag/g-C3N4 and Pd/g-C3N4. The in-terfacial electronic structures of AgPd/g-C3N4 demonstratedthat electrons of Ag were able to donate to Pd in the form ofPdd�···Agd+ . This interfacial interaction was beneficial in accel-erating photogenerated electron transfer, H2O molecule ad-sorption, and H2O dissociation, and thus, gave rise to en-hanced photocatalytic H2O reduction to H2.

Acknowledgements

The National Key Research and Development Program ofChina (2016YFC0204301) and the National Natural ScienceFoundation of China (Nos. 21707066, 21677069, 21573105) aregratefully acknowledged.

Conflict of interest

The authors declare no conflict of interest.

Figure 7. a) XPS spectra of O 1s, and b) in situ ESR results under visible lightirradiation.

Figure 8. Activation energies of water dissociation to OH and H on Ag/CN,Pd/CN, and AgPd/CN, with the corresponding optimized geometries for re-lated states.

Chem. Eur. J. 2019, 25, 1 – 8 www.chemeurj.org � 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim6

�� These are not the final page numbers!

Full Paper

1 12 23 34 45 56 67 78 89 9

10 1011 1112 1213 1314 1415 1516 1617 1718 1819 1920 2021 2122 2223 2324 2425 2526 2627 2728 2829 2930 3031 3132 3233 3334 3435 3536 3637 3738 3839 3940 4041 4142 4243 4344 4445 4546 4647 4748 4849 4950 5051 5152 5253 5354 5455 5556 5657 57

Keywords: AgPd/g-C3N4 · bimetallic · interfacial electroniceffect · nanoparticles · photocatalytic hydrogen evolution ·water splitting

[1] S. Chu, A. Majumdar, Nature 2012, 488, 294 – 303.[2] H. Tong, S. X. Ouyang, Y. P. Bi, N. Umezawa, M. Oshikiri, J. H. Ye, Adv.

Mater. 2012, 24, 229 – 252.[3] W. J. Chang, K. H. Lee, J. I. Ha, K. T. Nam, IEEE Electrific. Mag. 2018, 6,

19 – 25.[4] H. W. Yuan, Y. Y. Zhao, Y. D. Wang, J. L. Duan, B. L. He, Q. W. Tang, J.

Power Sources 2019, 410 – 411, 53 – 58.[5] S. Galliano, F. Bella, G. Piana, G. Giacona, G. Viscardi, C. Gerbaldi, M.

Gr�tzel, C. Barolo, Sol. Energy 2018, 163, 251 – 255.[6] Z. Wang, Y. D. Xia, L. F. Chao, Y. F. Pan, M. J. Li, R. Z. Li, B. Du, Y. H. Chen,

W. Huang, J. Energy Chem. 2019, 28, 107 – 110.[7] A. Carella, R. Centore, F. Borbone, M. Toscanesi, M. Trifuoggi, F. Bella, C.

Gerbaldi, S. Galliano, E. Schiavo, A. Massaro, A. B. MuÇoz-Garc�a, M.Pavone, Electrochim. Acta 2018, 292, 805 – 816.

[8] F. Bella, A. Verna, C. Gerbaldi, Mater. Sci. Semicond. Process. 2018, 73,92 – 98.

[9] D. Pintossi, G. Iannaccone, A. Colombo, F. Bella, M. V�lim�ki, K. V�is�nen,J. Hast, M. Levi, C. Gerbaldi, C. Dragonetti, S. Turri, G. Griffini, Adv. Elec-tron. Mater. 2016, 2, 1600288 – 1600298.

[10] P. Lang, J. Habermehl, Sergey I. Troyanov, S. Rau, M. Schwalbe, Chem.Eur. J. 2018, 24, 3225 – 3233.

[11] X. L. Zhai, J. Liu, L. Y. Hu, J. C. Bao, Y. Q. Lan, Chem. Eur. J. 2018, 24,15930 – 15936.

[12] Y. T. Pan, D. D. Li, H. L. Jiang, Chem. Eur. J. 2018, 24, 18403 – 18407.[13] W. X. Zou, B. Deng, X. X. Hu, Y. P. Zhou, Y. Pu, S. H. Yu, K. L. Ma, J. F. Sun,

H. Q. Wan, L. Dong, Appl. Catal. B 2018, 238, 111 – 118.[14] C. Ye, J. X. Li, Z. J. Li, L. X. Bing, X. Fan, L. P. Zhang, B. Chen, C. H. Tung,

L. Z. Wu, ACS Catal. 2015, 5, 6973 – 6979.[15] Z. F. Jiang, H. L. Sun, T. Q. Wang, B. Wang, W. Wei, H. M. Li, S. Q. Yuan,

T. C. An, H. J. Zhao, J. G. Yu, P. K. Wong, Energy Environ. Sci. 2018, 11,2382 – 2389.

[16] N. Sun, Y. Liang, X. J. Ma, F. Chen, Chem. Eur. J. 2017, 23, 15466 – 15473.[17] J. Li, Y. C. Yin, E. Z. Liu, Y. N. Ma, J. Wan, J. Fan, X. Y. Hu, J. Hazard. Mater.

2017, 321, 183 – 192.[18] Y. F. Li, K. Li, Y. Yang, L. J. Li, Y. Xing, S. Y. Song, R. C. Jin, M. Li, Chem. Eur.

J. 2015, 21, 17739 – 17747.[19] L. L. Bi, X. P. Gao, L. J. Zhang, D. J. Wang, X. X. Zou, T. F. Xie, ChemSu-

sChem 2018, 11, 276 – 284.[20] N. N. Meng, J. Ren, Y. Liu, Y. Huang, T. Petit, B. Zhang, Energy Environ.

Sci. 2018, 11, 566 – 571.[21] L. Chen, X. Zeng, P. Si, Y. Chen, Y. Chi, D. H. Kim, G. Chen, Anal. Chem.

2014, 86, 4188 – 4195.[22] K. Sridharan, E. Jang, J. H. Park, J. Kim, J. Lee, T. J. Park, Chem. Eur. J.

2015, 21, 9126 – 9132.[23] Z. L. Ni, F. Dong, H. W. Huang, Y. X. Zhang, Catal. Sci. Technol. 2016, 6,

6448 – 6458.[24] S. L. Ma, S. H. Zhan, Y. N. Jia, Q. Shi, Q. X. Zhou, Appl. Catal. B 2016, 186,

77 – 87.

[25] C. C. Han, L. N. Wu, L. Ge, Y. J. Li, Z. Zhao, Carbon 2015, 92, 31 – 40.[26] G. Darabdhara, M. R. Das, Chemosphere 2018, 197, 817 – 829.[27] D. X. Tan, J. L. Zhang, J. B. Shi, S. P. Li, B. X. Zhang, X. N. Tan, F. Y. Zhang,

L. F. Liu, D. Shao, B. X. Han, ACS Appl. Mater. Interfaces 2018, 10, 24516 –24522.

[28] I. Majeed, U. Manzoor, F. K. Kanodarwala, M. A. Nadeem, E. Hussain, H.Ali, A. Badshah, J. A. Stride, M. A. Nadeem, Catal. Sci. Technol. 2018, 8,1183 – 1193.

[29] B. W. Sun, H. J. Li, H. Y. Yu, D. J. Qian, M. Chen, Carbon 2017, 117, 1 – 11.[30] Y. X. Zhu, A. Marianov, H. M. Xu, C. Lang, Y. J. Jiang, ACS Appl. Mater.

Inter. 2018, 10, 9468 – 9477.[31] G. F. Liang, L. M. He, M. Arai, F. Y. Zhao, ChemSusChem 2014, 7, 1415 –

1421.[32] S. Garc�a, L. Zhang, G. W. Piburn, G. Henkelman, S. M. Humphrey, ACS

Nano 2014, 8, 11512 – 11521.[33] G. Kresse, J. Furthm�ller, Phys. Rev. B 1996, 54, 11169 – 11186.[34] G. Kresse, J. Furthm�ller, Comput. Mater. Sci. 1996, 6, 15 – 50.[35] J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865 –

3868.[36] G. Kresse, D. Joubert, Phys. Rev. B 1999, 59, 1758 – 1775.[37] S. Grimme, S. Ehrlich, L. Goerigk, J. Comput. Chem. 2011, 32, 1456 –

1465.[38] X. F. Yang, Z. P. Chen, J. S. Xu, H. Tang, K. M. Chen, Y. Jiang, ACS Appl.

Mater. Interfaces 2015, 7, 15285 – 15293.[39] R. Bartolomeu, B. Azambre, A. Westermann, A. Fernandes, R. B�rtolo,

H. I. Hamoud, C. Henriques, P. D. Costa, F. Ribeiro, Appl. Catal. B 2014,150 – 151, 204 – 217.

[40] J. Shibata, K. Shimizu, Y. Takada, A. Shichi, H. Yoshida, S. Satokawa, A.Satsuma, T. Hattori, J. Catal. 2004, 227, 367 – 374.

[41] T. Pakizeh, C. Langhammer, I. Zoric, P. Apell, M. K�ll, Nano Lett. 2009, 9,882 – 886.

[42] K. Mori, T. Sano, H. Kobayashi, H. Yamashita, J. Am. Chem. Soc. 2018,140, 8902 – 8909.

[43] J. S. Zou, Z. C. Si, Y. D. Cao, R. Ran, X. D. Wu, D. Weng, J. Phys. Chem. C2016, 120, 29116 – 29125.

[44] D. Gao, Q. Xu, J. Zhang, Z. Yang, M. Si, Z. Yan, D. Xue, Nanoscale 2014,6, 2577.

[45] X. Wang, S. Blechert, M. Antonietti, ACS Catal. 2012, 2, 1596 – 1606.[46] K. Tabata, Y. Hirano, E. Suzuki, Appl. Catal. A 1998, 170, 245 – 254.[47] J. Carrasco, D. Lpez-Durn, Z. Y. Liu, T. Duchonn, J. Evans, S. D. Sena-

nayake, E. J. Crumlin, V. Matol�n, J. A. Rodr�guez, M. V. Ganduglia-Pirova-no, Angew. Chem. Int. Ed. 2015, 54, 3917 – 3921; Angew. Chem. 2015,127, 3989 – 3993.

[48] G. Liu, J. Zhao, H. Hidaka, J. Photochem. Photobiol. A 2000, 133, 83 – 88.[49] J. J. Wang, Z. J. Li, X. B. Li, X. B. Fan, Q. Y. Meng, S. Yu, C. B. Li, J. X. Li,

C. H. Tung, L. Z. Wu, ChemSusChem 2014, 7, 1468 – 1475.

Manuscript received: December 6, 2018

Revised manuscript received: January 31, 2019

Accepted manuscript online: February 4, 2019

Version of record online: && &&, 0000

Chem. Eur. J. 2019, 25, 1 – 8 www.chemeurj.org � 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7

These are not the final page numbers! ��

Full Paper

1 12 23 34 45 56 67 78 89 9

10 1011 1112 1213 1314 1415 1516 1617 1718 1819 1920 2021 2122 2223 2324 2425 2526 2627 2728 2829 2930 3031 3132 3233 3334 3435 3536 3637 3738 3839 3940 4041 4142 4243 4344 4445 4546 4647 4748 4849 4950 5051 5152 5253 5354 5455 5556 5657 57

FULL PAPER

& Photocatalysis

W. Zou, L. Xu, Y. Pu, H. Cai, X. Wei, Y. Luo,L. Li, B. Gao, H. Wan,* L. Dong

&& –&&

Advantageous Interfacial Effects ofAgPd/g-C3N4 for PhotocatalyticHydrogen Evolution: ElectronicStructure and H2O Dissociation

Bimetallic photocatalysts : BimetallicAgPd nanoparticles are loaded on g-C3N4 for photocatalytic H2 evolution.The resultant AgPd/g-C3N4 shows supe-rior activity to the monometallic Ag/g-C3N4 and Pd/g-C3N4. The interfacial inter-action between the metals is beneficialin accelerating photogenerated electrontransfer, H2O molecule adsorption, andH2O dissociation, leading to enhancedphotocatalytic H2O reduction to H2.&&text and graphic ok?&&

Researchers at @njuniversity and @UF investigate water splitting with a bimetallic photocatalyst. SPACERESERVED FOR IMAGE AND LINK

Share your work on social media! Chemistry - A European Journal has added Twitter as a means to promote yourarticle. Twitter is an online microblogging service that enables its users to send and read text-based messages of up to140 characters, known as “tweets”. Please check the pre-written tweet in the galley proofs for accuracy. Should you oryour institute have a Twitter account, please let us know the appropriate username (i.e., @accountname), and we willdo our best to include this information in the tweet. This tweet will be posted to the journal�s Twitter account@ChemEurJ (follow us!) upon online publication of your article, and we recommended you to repost (“retweet”) it toalert other researchers about your publication.

Please check that the ORCID identifiers listed below are correct. We encourage all authors to provide an ORCID identifierfor each coauthor. ORCID is a registry that provides researchers with a unique digital identifier. Some funding agenciesrecommend or even require the inclusion of ORCID IDs in all published articles, and authors should consult their fundingagency guidelines for details. Registration is easy and free; for further information, see http://orcid.org/.

Weixin ZouLixia XuYu PuHaojie CaiXiaoqian WeiYidan LuoLulu LiBin GaoHaiqin Wan http://orcid.org/0000-0003-0639-4576Lin Dong

Chem. Eur. J. 2019, 25, 1 – 8 www.chemeurj.org � 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim8

�� These are not the final page numbers!

Full Paper

1 12 23 34 45 56 67 78 89 9

10 1011 1112 1213 1314 1415 1516 1617 1718 1819 1920 2021 2122 2223 2324 2425 2526 2627 2728 2829 2930 3031 3132 3233 3334 3435 3536 3637 3738 3839 3940 4041 4142 4243 4344 4445 4546 4647 4748 4849 4950 5051 5152 5253 5354 5455 5556 5657 57