Yingzhen Zhang,Yonggang Lei,Tianxue Zhu,Zengxing Li,Shen Xu,Jianying Huang,2,,Xiao Li,2,Weilong Cai,2,Yuekun Lai,2,,Xiaojun Bao,2,

1 National Engineering Research Center of Chemical Fertilizer Catalyst (NERC-CFC),College of Chemical Engineering,Fuzhou University,Fuzhou 350116,China

2 Fujian Science &Technology Innovation Laboratory for Chemical Engineering of China,Quanzhou 362114,China

Keywords:Photoelectrochemical Water oxidation Ag-N,S-C/TiO2 Surface plasmon resonance Biomass carbon

ABSTRACT Exploring efficient and stable photoanode materials is a necessary link to realize the practical application of solar-driven photoelectrochemical (PEC) water splitting.Hence,we prepared rutile TiO2 nanorods,with a width of 50 nm,which was growth in situ on carbon cloth (TiO2@CC) by hydrothermal reaction.And then,Ag nanoparticles (NPs) and biomass N,S-C NPs were chosen for the additional modification of the fabricated TiO2 nanorods to produce broccoli-like Ag-N,S-C/TiO2@CC nanocomposites.According to the result of ultraviolet–visible diffuse reflectance spectroscopy(UV–vis)and PEC water splitting performance tests,Ag-N,S-C/TiO2@CC broadens the absorption region of TiO2@CC from the ultraviolet region to the visible region.Under AM 1.5G solar light irradiation,the photocurrent density of Ag-N,S-C/TiO2@CC is 89.8 μA·cm-2,which is 11.8 times higher than TiO2@CC.Under visible light irradiation,the photocurrent density of Ag-N,S-C/TiO2@CC reaches to 12.6 μA·cm-2,which is 21.0 times higher than TiO2@CC.Moreover,Ag-N,S-C/TiO2@CC shows a photocurrent responses in full pH range.It can be found that Ag NPs and N,S-C NPs play key roles in broaden the absorption range of TiO2 nanorods to the visible light region and,promote the occurrence of PEC water oxidation reaction due to the surface plasmon resonance effect of Ag NPs and the synergistic effect of N,S-C NPs.The mechanism demonstrated that Ag-N,S-C/TiO2@CC can separate the photogenerated electron-hole pairs effectively and transfer the photogenerated electrons to the photocathode (Pt plate) in time.This research provides a new strategy for exploration surface plasma metal coupled biomass carbon materials in the field of PEC water splitting.

1.Introduction

Photoelectrochemical (PEC) water splitting provides a green,sustainable and promising strategy for rational utilization of solar energy to produce hydrogen.According to the principle of PEC water splitting,an n-type semiconductor was used as photoanode,holes will be generated on the surface of the photoanode and migrate to the electrode/electrolyte interface instantly,and then water oxidation reaction will occur.At the same time,the free electrons transfer to the counter electrode through the external circuit,and the reduction reaction will occur in contact with water to produce hydrogen[1–3].According to previous reports,from the perspective of the feasibility of commercial promotion,semiconductor materials with a solar hydrogen conversion efficiency (STH) ＞10%are of practical significance[4–6].However,it’s still a big challenge to design a semiconductor photoanode material with broad visible light absorption,high charge separation and transfer efficiency,and good chemical stability simultaneously [7,8].To explore the best catalyst for PEC water splitting,many semiconductor materials have been widely reported in recent decades,such as C3N4[9–11],TiO2[12–14],Ta3N5[15,16],BiVO4[17] and so on.Among them,TiO2exhibits ideal valence band position,excellent photocorrosion resistance,simple preparation method,and low cost,etc.It has been considered to be one of the best candidates for PEC water oxidation reaction[18,19].Unfortunately,the wide band gap of TiO2(3.0 eV for rutile)makes it only absorb ultraviolet light(UV,～5%),and cannot make full use of visible light,accounting for～45% [20,21].Moreover,the difficulty of photo-generated charge separation,slow carrier migration rate and slow oxygen evolution kinetics greatly restrict the practical application of TiO2[22].And thus,to broaden the visible light absorption region and improve the solar energy conversion efficiency of TiO2,many heteroatom modification strategies have been widely reported (e.g.,nitrogen[23],carbon [24] and surface plasmon resonance (SPR) metals)[25,26].

SPR effect is known as a high degree of light scattering occurs on metal NPs and a strong electric field is generated,resulting in strong light absorption and faster surface reaction dynamics [27–29].The SPR effect has been proved to be an effective method in increasing the charge carrier generation rate in semiconductors(e.g.,electron-hole pairs) and promoting the transfer of electrons from metal to semiconductor.It exhibits great potential in the field of photoelectrochemical [30,31],photocatalytic CO2conversion[32],and bioimaging [33,34].As one of the common metals with SPR,Ag has received extensive attention in recent years,owing to relatively high earth abundance [35–37].However,there are few studies on plasma-activated semiconductor/biomass carbon coupling systems at present,and to achieve rapid charge separation at the interface between semiconductor/biomass carbon/electrolyte by localized surface plasmon response effect is still in its infancy.

Biomass materials,due to the rich carbon content and various heteroatoms,are widely favored in the preparation of carbonbased functional materials [38–40].Researchers obtained different types of heteroatom-doped carbon-based materials by pyrolyzing different biomass e.g.,jackfruit peel [41],lotus plumule[42],seaweeds[42,43],sugar cane bagasse[44],etc.As composed by a variety of amino acids and N,S elements,hair can be regarded as one of the ideal precursors for N and S co-doped carbon materials (N,S-C) [45,46].Up to now,human hair-based carbon materials has shown broad prospects in the fields of batteries[47,48],wastewater remediation [45] and sensing [49].However,as far as we know,the research on the PEC water splitting performance of using hair as a biomass carbon material is not thorough enough.

Based on the above considerations,we have developed a multistep synthetic strategy to SPR Ag NPs and N,S-doped carbon NPs(N,S-C NPs) modified TiO2nanorods (broccoli-like Ag-N,S-C/TiO2@CC) for PEC water oxidation reaction.Firstly,the rutile TiO2nanorods were grown on the hydrophilic carbon cloth (TiO2@CC)through a hydrothermal approach.Then,the N,S-C NPs was prepared by hydrothermal method,and human-hair as carbon sources.Importantly,the photocurrent density of Ag-N,S-C/TiO2@CC is 89.8 μA·cm-2,which is higher than Ag/TiO2@CC,N,SC/TiO2@CC and TiO2@CC under AM 1.5G simulated sunlight,at 1.23 V vs.RHE.Under visible light irradiation,the photocurrent density of Ag-N,S-C/TiO2@CC reaches to 12.6 μA·cm-2,which is 21.0 times higher than TiO2@CC.The developed synthesis strategy opens up a new way for the design of photoelectrode materials with good controllable configuration and outstanding catalytic activity.

2.Experimental

2.1.Chemicals and reagents

The carbon cloth (CC,thickness 0.33 mm,density 120 g·m-2)was purchased from Cetech.Titanium butoxide,oleylamine(OAm) and oleic acid (OA) were purchased from Aladdin Bio-Chem Technology Co.,Ltd.Silver nitrate (AgNO3),KOH,were purchased from Sinopharm Chemical Reagent Co.,Ltd.,(China).Hair was provided by the first author.All solutions in the experiments were prepared with ultrapure water (18.2 MΩ) that was obtained by a Millipore Direct-Q 3UV.

2.2.Synthesis of TiO2@CC

Rutile TiO2nanorods were grown on CC(TiO2@CC)via a simple hydrothermal process.The weakly hydrophobic carbon cloth was cleaned by acetone,ethanol and deionized water,respectively.Secondly,the cleaned CC was treated by oxygen plasma for 5 min to make it hydrophilic,which is beneficial to the growth of TiO2nanorods.Next,5 ml HCl was diluted with 5 ml ultrapure water,and mixed with 400 μl titanium butoxide in a 25 ml Teflon-lined stainless-steel autoclave.And,the pre-treated CC was immersed into the obtained solution,heated at 150 °C for 4 h.Then the obtained TiO2@CC was thoroughly washed with ultrapure water and dried under vacuum at 60 °C.Finally,the TiO2@CC was annealed at 400 °C in air for 2 h.

2.3.Synthesis of the Ag NPs

Ag NPs was synthesized according to the reported literature[50].Firstly,0.38 g AgNO3was dissolved into the mixture of solution of OAm and OA(volume ratio=50:1),heated to 120°C under magnetic stirring for 24 h.Upon cooled to room temperature naturally,30 mL of acetone was added to the above solution to generate a precipitate.After,centrifuge at 10,000 r·min-1for 10 min,the supernatant was saved and stored in the refrigerator for later use.

2.4.Synthesis of the N,S-C NPs

2.5.Synthesis of the broccoli-like Ag-N,S-C/TiO2@CC nanocomposites

Firstly,a certain amount of the as-obtained N,S-C NPs and Ag NPs solution was added to 10 ml ethanol,transfer the solution to a 25 ml Teflon-lined stainless-steel autoclave,and heated at 200 °C for 6 h.After that,the broccoli-like Ag-N,S-C/TiO2@CC nanocomposites were obtained and washed thoroughly with ethanol and dried under vacuum at 60 °C.

2.6.Characterization

The morphologies of the broccoli-like Ag-N,S-C/TiO2@CC nanocomposites were observed under a Hitachi S-4800 field emission scanning electron microscope(FESEM).Transmission electron microscopy (TEM) images of Ag NPs and N,S-C NPs were taken by FEI Tecnai-G2-F20 S-TWIN with an accelerating voltage of 200 kV.X-ray diffraction (XRD) patterns were obtained by employing X’Pert3 Powder diffractometer with Cu Kα radiation that operated at 40 kV and 40 mA.X-ray photoelectron spectroscopy(XPS)spectrums were provided by a K-Alpha+instrument (Thermo Fisher Scientific),applying a micro-focused,monochromatic Al-Kα X-ray source with 30–500 μm spot size.Optical properties of photocatalyst were revealed by Agilent Cary 7000 UV–Vis NIR spectrophotometer in the range of 200–800 nm,and equipped with an integrating sphere.

2.7.PEC measurements

The light source was a Xe 500 W lamp (AuLight Beijing,CELS500,China) with an AM 1.5G filter (100 mW·cm-2).All the PEC measurements were measured by a ModuLab XM electrochemical system (Solartron Analytical,Ametek Inc.,Berwyn,USA) with a typical three electrode configuration.Broccoli-like Ag-N,S-C/TiO2@CC nanocomposites,Pt plate(2 cm×2 cm)and Ag/AgCl electrode (3 mol·L-1KCl) are used as working electrode,counter electrode and reference electrode,respectively.The 0.5 mol·L-1phosphate buffer solution (PBS,pH=7) was used as electrolyte.The linear sweep voltammetry (LSV) curve test range is -0.2 to 1.0 V (vs.Ag/AgCl).Using the Nernst equation to convert potential vs.Ag/AgCl electrode to potential vs.reversible hydrogen electrode(RHE):

where the ERHEis the potential vs.RHE and EAg/AgClis the potential vs.Ag/AgCl electrode [5].

3.Results and Discussion

The Ag-N,S-C/TiO2@CC nanocomposites was prepared by multistep hydrothermal method (Scheme 1).The morphology and element distribution of TiO2@CC and Ag-N,S-C/TiO2@CC nanocomposites were characterized by SEM and EDS-mapping(Fig.1).It can be seen that the TiO2nanorods,like grass,with a width about 50 nm are grown on CC uniformly(Fig.1(a)-(c)).Then,the Ag NPs and N,S-C NPs obtained in advance are modified on the TiO2nanorods with hydrothermal process to obtain Ag-N,S-C/TiO2@CC nanocomposites(Fig.1(d)-(f)).The Ag-N,S-C/TiO2@CC nanocomposites contain Ti,Ag,N,C,S and O elements,as shown in the EDS-mapping images (Fig.1(g)).It can be clearly found that Ag NPs and N,S-C NPs are uniformly modified on the surface of TiO2nanorods.The average size of Ag NPs is 3.1 nm,the average size of N,S-C NPs is 1.2 nm (Figs.S3 and S4).

Scheme 1.Synthesis process of broccoli-like Ag-N,S-C/TiO2@CC using hydrothermal method.

XRD patterns were measured to analyze the crystal structure and phase composition of the obtained samples.By comparison with the rutile TiO2PDF card (JPDF #87-0710),the TiO2prepared by the hydrothermal reaction is rutile(Fig.2(a)).The sharp diffraction peak appears at about 25.0°,belongs to the carbon cloth [51].After annealing at 400 °C,the carbon diffraction peaks near 25.0°was disappeared,while the TiO2related peaks did not change significantly,indicating that the annealing treatment only removed the unstable carbon on the carbon cloth and did not change the TiO2crystal structure.The XRD results (Fig.2(b)) revealed clear diffraction peaks at 27.68°,36.35°,39.55°,41.51°,44.46°,54.57°and 56.91°,corresponding to the (110),(101),(200),(111),(210),(211) and (220) planes of rutile TiO2(JPDF #87-0710) [52,53],respectively.The main peak centered at 27.68° corresponds to the (110) crystal plane,indicating the growth direction of TiO2nanorods.Surprisingly,the peak of carbon cloth was not detected in the XRD pattern of the TiO2@CC,indicating the TiO2film layer is thick and cover the carbon cloth completely.Additionally,a graphitized carbon peak was detected at 26.1° in Ag-N,S-C/TiO2@CC,which was attributed to the biomass carbon N,S-C NPs.Meanwhile,the diffraction peaks at 38.1°,44.3° and 64.2° were correspond to the (111),(200) and (220) plates of crystalline Ag(JPDF#87-0589)[54,55],demonstrating the successful preparation of Ag-N,S-C/TiO2@CC hybrid.After the test of PEC performance,the diffraction peaks of TiO2and Ag0shows a negligible change while the graphitized carbon peak at 26.1° almost disappeared.

Fig.3 shows the XPS spectrums of TiO2@CC and Ag-N,S-C/TiO2@CC nanocomposites.Ti 2p (458.5 eV),O 1s (532.0 eV) and C 1s (284.8 eV) appeared in both TiO2@CC and Ag-N,S-C/TiO2@CC nanocomposites(Fig.3(a)).Moreover,compared with the TiO2@CC,the slight peak intensity of Ag 3d,N 3d and S 2p can be observed in Ag-N,S-C/TiO2@CC,demonstrated the successfully deposited of Ag NPs and N,S-C NPs,which is consistent with the result of SEM and XRD patterns.The high-resolution XPS spectrum of Ag 3d shows two obvious peaks at 368.4 and 374.4 eV (Fig.3(e)),corresponding to Ag 3d5/2and Ag 3d3/2,and the distance of approximately 6.0 eV reveals that Ag is in the form of metallic Ag0[55,56].The peaks at 161.3 eV,163.0 eV,and 396.9,399.3 and 401.2 eV (Fig.3(h)-(i)) are corresponded to S 2p3/2,S 2p1/2,and Ti-N,C-N and C-N-H bonds,respectively.Indicated that N,S-C NPs are successfully modified to the surface of TiO2nanorods by chemical bonds.

Little Thumb, who had observed that the Ogre s daughters had crowns of gold upon their heads, and was afraid lest the Ogre should repent his not killing41 them, got up about midnight, and, taking his brothers bonnets42 and his own,26 went very softly and put them upon the heads of the seven little ogresses, after having taken off their crowns of gold, which he put upon his own head and his brothers , that the Ogre might take them for his daughters, and his daughters for the little boys whom he wanted to kill.

The effect of Ag NPs prepared under different conditions on broadening the range of light absorption of the Ag-N,S-C/TiO2@CC were studied by UV–vis absorption spectra (Fig.S5).It can be found that the TiO2@CC film mainly absorbs ultraviolet light(λ ＜400 nm),and the light absorption in the visible light region(λ ＞400 nm)is negligible.While TiO2@CC was modified by Ag NPs and N,S-C NPs,the intensity of absorption peak in visible light region (λ=400–800 nm) has been significantly improved.This result may be caused by the strong surface plasmon resonance effect of Ag NPs,which is consistent with previous reports.[57].Simultaneously,the influence of TiO2nanorods modified by Ag

NPs prepared under different conditions on the visible light absorption ability was also explored.The result demonstrated that Ag NPs prepared at 18 h exhibit the strongest absorption capacity for visible light,indicating that a proper extension of the reaction time is beneficial to enhance the Ag NPs of absorption on visible light region.

Fig.1.SEM images of(a)-(c)TiO2@CC and(d)-(f)broccoli-like Ag-N,S-C/TiO2@CC.(g)Corresponding EDS elemental mapping images of the Ag-N,S-C/TiO2@CC containing C,N,O,S,Ti and Ag elements.

Fig.2.XRD patterns of (a) TiO2@CC annealed and TiO2@CC unannealed.(b) TiO2@CC and broccoli-like Ag-N,S-C/TiO2@CC before/after PEC.

Fig.3.XPS survey spectra of(a)TiO2@CC and Ag-N,S-C/TiO2@CC.High-resolution XPS spectrums of(b)Ti 2p and(c)O 1s of TiO2@CC,(d)Ti 2p,(e)Ag 3d,(f)O 1s,(g)C 1s,(h)S 2p and (i) N 1s of Ag-N,S-C/TiO2@CC.

To explore an optimized condition for preparing photoanode semiconductors,the effects of the amount of titanium butoxide and different calcination temperatures for the intensity and stability of the PEC photocurrent of TiO2@CC were explored (Figs.S6–S8).The PEC photocurrent intensity increased linearly with the amount of precursor titanium butoxide increased.Moreover,enhancing the calcination temperature will improves the stability of PEC photocurrent signal,which may be related to the fact that high temperature calcination is beneficial to the removal of unstable substances.

The I-t results were studied to reveal the influence of the deposition amount of Ag NPs and N,S-C NPs on TiO2@CC for PEC water oxidation activities.The Ag-N,S-C/TiO2@CC displays the optimal PEC water oxidation ability as the volumes of the prepared dispersion for N,S-C NPs and Ag NPs are 4 μl and 2 μl,respectively(Fig.4).Both N,S-C/TiO2@CC and TiO2@CC photoelectrodes show negligible photocurrent in the dark,indicating that there was no obvious electrocatalytic water oxidation reaction.Interestingly,the LSV curves of Ag-N,S-C/TiO2@CC and Ag/TiO2@CC has a sharp peak near 0.8 V (Fig.S9a).This is the characteristic peak of Ag NPs,corresponding to XRD and XPS results,proving that Ag exists in the form of 0 valence.Obvious photocurrent signals were observed under illumination,which was caused by the effective photo-induced charge separation/transport on the photoanode.The photocurrent sequences of photoanodes are as followed:Ag-N,S-C/TiO2@CC ＞Ag/TiO2@CC ＞N,S-C/TiO2@CC ＞TiO2@CC,demonstrated that Ag NPs is the main influencing factor in improving PEC water splitting performance,and the N,S-C NPs plays a synergistic promotion effect.

Applied-bias photon-to-current efficiency (ABPE) was calculated using the equation:

Fig.4.Transient photocurrent responses of modification amount of(a)Ag NPs and(b)N,S-C NPs of Ag-N,S-C/TiO2@CC,(c)and(d)the corresponding line graphs,at 1.23 V vs.RHE under AM 1.5G simulated sunlight irradiation in 0.5 mol·L-1 PBS aqueous solution (pH=7),respectively.

where j is the photocurrent density(mA·cm-2),Vbis the bias potential(V)vs.RHE,and P is the incident light intensity(mW·cm-2)[14].The maximum ABPE for Ag-N,S-C/TiO2@CC is 0.059%,at 1.23 V vs.RHE,which is higher than Ag/TiO2@CC,N,S-C/TiO2@CC and TiO2@CC(Fig.S9b).The photocurrent density sequences of different photoanodes is Ag-N,S-C/TiO2@CC (89.8 μA·cm-2) ＞ Ag/TiO2@CC (75.3 μA·cm-2)＞N,S-C/TiO2@CC(14.5 μA·cm-2)＞TiO2@CC(7.6 μA·cm-2)(Fig.5(a)).And,the four photoanodes have good chemical stability during the switching cycle,indicating the prepared photoanodes with good photo-corrosion resistance.More interestingly,once N,S-C/TiO2@CC was modified by Ag NPs,the photocurrent density of Ag-N,S-C/TiO2@CC shows a “increased” trend.This phenomenon shows that the SPR of Ag can effectively inhibit interfacial recombination through the “fast” hole transfer kinetics on the Ag-N,S-C/TiO2@CC interface,which can be confirmed by the ABPE and OCP results.

The lifetime of hot carriers in the conduction band of semiconductors(i.e.,TiO2)is a key factor in achieving high-efficiency catalysis.The carrier lifetime can be calculated by measuring the open circuit potential decay.The plasma electrode absorbs photons and reduces its open circuit potential (OCP) under the irradiation of simulated solar light,OCP gradually decays to the initial value upon turn off the light source (Fig.5(b)).Decay rate of photovoltage (Voc) is associated with the lifetime of photocarriers (τn),and there can be caculated according to the formula:

Fig.5.(a)transient photocurrent responses and(b)OCP decay of TiO2@CC,Ag/TiO2@CC,N,S-C/TiO2@CC and Ag-N,S-C/TiO2@CC under AM 1.5G simulated sunlight irradiation in 0.5 mol·L-1 PBS aqueous solution (pH=7) at 1.23 V vs.RHE,respectively.

where τnrepresents the lifetime of free electron,κBis Boltzmann’s constant(1.38×10-23J·K-1),T=298 K,e=1.6×10-19C,and Vocis the voltage[58].The photocarriers lifetime of the photoanode is Ag-N,S-C/TiO2@CC ＞N,S-C/TiO2@CC ＞Ag/TiO2@CC ＞TiO2@CC in order.It is worth noting that although the surface plasmon resonance effect of Ag NPs can broaden the light absorption region,no obvious improvement on the lifetime of photocarriers was founded,the lifetime of N,S-C NPs/TiO2@CC is the second only to Ag-N,S-C/TiO2@CC.

In order to explore the utilization of the photocatalyst to visible light,a light source with a filter (λ ＞420 nm) was used to illuminate the photoanode.It can be seen from Fig.6(a),the photocurrent density of TiO2@CC under visible light is almost negligible(0.6 μA·cm-2).When the TiO2nanorods was modified by N,S-C NPs and Ag NPs,the photocurrent density was increased rapidly by 12.3 times(7.4 μA·cm-2)and 14.7 times(8.8 μA·cm-2),respectively.At the same time,the N,S-C NPs and Ag NPs prepared for 24 h co-modified TiO2nanorods are the best,and the photocurrent density is increased by 21.0 times (12.6 μA·cm-2).As the acidity and alkalinity of the electrolyte are important factors to assess the photocatalyst.The Ag-N,S-C/TiO2@CC displays a photocurrent responses in a full pH range(Fig.6(c)).The transient photocurrent density is the largest under alkaline conditions,but the stability is poor.The photocurrent density under neutral conditions(pH=7)is the most stable,and there is almost no attenuation during the entire operation.

Based on the above analysis,a possible charge transfer mechanism for broccoli-like Ag-N,S-C/TiO2@CC has been proposed(Scheme 2).Under AM 1.5G simulated sunlight,Ag-N,S-C/TiO2@CC absorb sunlight and are excited to generate electron/hole pairs due to the surface plasmon resonance effect of Ag NPs.At the same time,holes migrate to the surface of TiO2nanorods to contact the electrolyte,and water oxidation reaction occurs.Photogenerated electrons migrate along the external circuit to the surface of the counter electrode (Pt plate),and water reduction reaction occurs.Subsequently,N,S-C NPs further extract holes from the Ag-N,S-C/TiO2@CC surface through the Ti-N bond,which can reduce the charge depletion layer and promote the separation of photogenerated carriers,extending the lifetime of photogenerated carriers.Therefore,N,S-C NPs and Ag NPs play their respective roles and coordinate to promote charge separation and transfer.

Scheme 2.Schematic illustration of broccoli-like Ag-N,S-C/TiO2@CC for PEC water splitting under AM 1.5G simulated solar light.

4.Conclusions

To summarize,we have developed a new strategy for constructing broccoli-like Ag-N,S-C/TiO2@CC by multi-step hydrothermal method.The Ag-N,S-C/TiO2@CC not only shows a good water oxidation ability for PEC water splitting,but also displays outstanding chemical stability.Both in the range of visible light and solar light,as-prepared materials express better photocurrent than unmodified TiO2.The excellent performance of Ag-N,S-C/TiO2@CC ascribe the SPR of Ag NPs,which can improve the utilization of TiO2for visible light,and the N,S-C NPs prolong the lifetime of photocarriers.Under AM 1.5G solar light irradiation,the photocurrent density and ABPE of Ag-N,S-C/TiO2@CC is 89.8 μA·cm-2and 0.059%,which is higher than TiO2@CC.Under visible light irradiation,the photocurrent density of Ag-N,S-C/TiO2@CC reaches to 12.6 μA·cm-2,which is 21.0 times that of TiO2@CC.Moreover,Ag-N,S-C/TiO2@CC exhibits a photocurrent responses in full pH range.This research provides a new strategy for exploration surface plasma metal coupled with biomass carbon materials in the field of PEC water splitting.

Fig.6.(a) Transient photocurrent responses of TiO2@CC,N,S-C/TiO2@CC,Ag/TiO2@CC,6 h-Ag-N,S-C/TiO2@CC,18 h-Ag-N,S-C/TiO2@CC and 24 h-Ag-N,S-C/TiO2@CC in 0.5 mol·L-1 PBS aqueous solution (pH=7).(b) Transient photocurrent responses of broccoli-like Ag-N,S-C/TiO2@CC in electrolytes with different pH values.(c) and (d) The corresponding histogram of photocurrent density,under visible light (λ ＞420 nm) irradiation,at 1.23 V vs.RHE,respectively.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors thank the National Natural Science Foundation of China (22075046,51972063,21501127 and 51502185),National Key Research and Development Program of China(2019YFE0111200),Natural Science Funds for Distinguished Young Scholar of Fujian Province (2020J06038),Natural Science Foundation of Fujian Province(2019J01256),and Overseas Expertise Introduction Project for Discipline Innovation (111 Project) (No.D17005).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.10.022.