Novel Carboxy-Functionalized PVP-CdS Nanopopcorns with Homojunctions for Enhanced Photocatalytic Hydrogen Evolution

Na Zhao,Jing Peng,Jianping Wang,Maolin Zhai,＊

1 The National Museum of China,Beijing 100006,China.

2 Beijing National Laboratory for Molecular Sciences,Radiochemistry and Radiation Chemistry Key Laboratory of Fundamental Science,College of Chemistry and Molecular Engineering,Peking University,Beijing 100871,China.

Abstract:Photocatalytic hydrogen evolution is a scalable pathway to generate hydrogen fuels while mitigating environmental crisis.Strategies based on modification of the host photocatalyst surface are key to improve the adsorption/activation ability of the reaction molecules and the efficiency of charge transport,so that high-efficiency photocatalytic systems can be realized.Cadmium sulfide(CdS),a visible light-responsive semiconductor material,is widely used in photocatalysis because of its simple synthesis,low cost,abundant raw materials,and appropriate bandgap structure.Many researchers have focused on improving the photocatalytic efficiency of CdS because the rapid charge recombination in this material limits its applications.Among the various strategies proposed in this regard,surface modification is an effective and simple method used to improve the photocatalytic performance of materials.In this work,polyvinyl pyrrolidone(PVP)-capped CdS(denoted as P-CdS)nanopopcorns with hexagonal wurtzite(WZ)-cubic zinc blende(ZB)homojunctions were fabricated via one-step gamma-ray radiation-induced reduction under ambient conditions.Subsequent alkaline treatment under ambient conditions led to a dramatic improvement in the activity of the alkalized PVP-capped CdS(MP-CdS)photocatalyst.The structure and properties of the photocatalyst were determined by X-ray diffraction(XRD)analysis,field-emission scanning electron microscopy(FE-SEM),transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS),Fourier transform infrared(FTIR)analysis,Brunauer-Emmett-Teller(BET)specific surface area measurements,and photoelectric tests.The photocatalytic performance was evaluated based on the photocatalytic H2 evolution under visible-light irradiation.The mechanism underlying the enhancement of the photocatalytic activity is also discussed.The results showed that after the alkaline treatment,the crystal structure of CdS with WZ-ZB homojunctions was preserved,but PVP on the surface of CdS hydrolyzed to form PVP hydrolysis product(MPVP)with carboxyl and amino groups.Owing to the increased alkaline solubility,a portion of MPVP dissolved into the solution and was removed from the surface of MP-CdS,exposing a greater number of active sites of the WZ-ZB phase junctions with a larger specific surface area.On the other hand,the carboxyl groups in MPVP coordinated with CdS could affect the bandgap and valence band position of CdS to facilitate the photocatalysis.Because of the synergistic effects of the exposure of WZ-ZB phase junctions and band structure engineering,the alkalized samples at a 1 mol·L-1 concentration of NaOH showed a H2 evolution rate of 477 μmol·g-1·h-1 under visible-light illumination,which was twice that obtained for the pristine P-CdS photocatalysts.This simple and low-cost post-synthesis strategy can be extended to the preparation of diverse functional photocatalysts.The present work is expected to contribute to the practical application of sulfide photocatalysts.

Key Words:CdS;Homojunction;Alkaline treatment;Photocatalytic hydrogen evolution

1 Introduction

Converting solar energy into clean hydrogen via photocatalysis is a promising method to fulfill the increasing worldwide demands of energy,as well as the environmental issues aroused by the burning of fossil fuels1-4.In the photocatalytic process,three critical steps,i.e.light harvesting,charge transfer and surface reaction,occur in a huge range of time scale(from 10-12s to 100s)5.Thus,to achieve efficient photocatalytic activities,managing an overall cooperation between each step is essentially important.So,an optimization of visible light active photocatalysts is based on these considerations:a narrow band gap(1.9-3.0 eV)and a wide spectrum response,a suitable band alignment,that is,a conduction band/valence band satisfying the water oxidation and reduction potentials,faster charge separation and transportation,photostability and so on.

As one of the most promising photocatalytic materials,cadmium sulfide(CdS)presents a suitable bandgap(Eg=2.42 eV)and redox potential6-8,which enables it to efficiently absorb visible-light to split water for hydrogen generation.Unfortunately,CdS suffers from the rapid charges recombination that limit its photocatalytic H2-evolution activity.To overcome these drawbacks,it is necessary to stimulate the surface reaction to match with the ultrafast light absorption process through charge transfer.Surface phase junctions,including the cocatalyst modification9,10,hetero-11-13and homo-junction14-16structure,and surface functionalization of semiconductors17-19are considered as the effective strategies.As we all know,the surface phase of CdS,which is directly exposed to the reactants,contributes to photocatalytic H2evolution because the photocatalytic reaction takes place only when photo-induced charges are available on the surface.Zhong et al.20synthesized a CdS homojunction photocatalysts with a massive S2-adsorbed surface phase,and the numerous adsorbed S2-ions on the cubic phase CdS nanocrystal surface can effectively function as H2-generation active centers to boost the H2formation.Meng et al.21utilized the surface modification of TiO2with NaOH to promote the chemisorption,activation and photocatalytic CO2reduction.The above results obviously illustrate that the surface modification to construct or expose more functional sites are thus required to boost the H2generation.While,the design of a facile and scalable method to synthesize solar light driven photocatalyst is still highly challenging.In the recent study in our laboratory16,a series of PVP-capped CdS(P-CdS)nanopopcorns with ZB-WZ type-II homojunctions are successfully synthesized by a one-step gamma-ray radiationinduced reduction approach.And after the Pt cocatalysts were photo-deposited on the P-CdS nanopopcorns,the photocatalysts exhibit an excellent photocatalytic activity.While,the scarcity and high-cost of noble metals may discourage its large-scale application in practice.

What’s more,the band structure engineering of the photocatalysts,i.e.narrowing the bandgap to increase the visible light harvesting,the more positive shift of valence band(VB)energy to promotes easy oxidation of H2O to O2and more negative conduction band(CB)energy to facilitate the photoreduction ability,is another key indicator for evaluating photocatalysts22,23.In this work,a novel carboxy-functionalized PVP-CdS nanopopcorns with hexagonal wurtzite(WZ)-cubic zinc blende(ZB)homojunction(named as MP-CdS)were synthesized via gamma-ray radiation-induced reduction and subsequent alkaline treatment under ambient conditions.It has been found that the alkaline treatment leads to the hydrolysis of the PVP on the surface of MP-CdS,and the hydrolysis degree depends on the alkali concentrations.What’s more,upon the alkaline treatment,the synergistic effects of the exposure of active sites of homojunctions and the band structure engineering on the hydrogen evolution were systematically investigated.The possible mechanisms for the formation of MP-CdS nanopopcorns and their photoactivity were proposed.

2 Experimental section

2.1 Materials

Cadmium chloride hemipentahydrate(CdCl2·2.5H2O,≥99.0%),sodium thiosulfate pentahydrate(Na2S2O3·5H2O,≥99.0%),polyvinylpyrrolidone(PVP,MW≈ 40000),ethylene glycol(EG,≥ 99.0%),lactic acid(85.5%-90%),and sodium hydroxide(NaOH,≥ 99.7%)were used without any further purification.

2.2 Synthesis of P-CdS nanocomposites

P-CdS was synthesized by using a one-step gamma-rayinduced reduction method16.In brief,0.1 mol·L-1CdCl2·2.5H2O was injected into the 5%(w,mass fraction)PVP solution and sonicated for 30 min.Subsequently,0.1 mol·L-1Na2S2O3·5H2O was added and sonicated for 30 min to obtain a clear mixture.The solution was bubbled with nitrogen for 20 min,followed by gamma-ray irradiation for ca.300 kGy at dose rate of 200 Gy·min-1via a60Co source at the Department of Applied Chemistry of Peking University.The actual absorbed doses were calibrated by a Fricke dosimeter.Upon irradiation,the CdS colloidal solution was obtained and the precipitates were collected by centrifugation.The collected precipitates were washed with respective Milli-Q water(18 MΩ·cm,Millipore)and ethanol for 3 times,and then dried at 40 °C for 24 h.

2.3 Alkaline treatment

A simple post-synthesis approach was developed to prepare the product.Typically,0.2 g of the as-synthesized P-CdS was added into 20 mL of Milli-Q water and sonicated for 30 min.Then,a certain amount of NaOH was added and stirred for 4 h.The resultant product was washed several times with Milli-Q water and ethanol,respectively;collected by centrifugation,and then dried at 60 °C for 24 h.The abbreviations of the samples prepared under different alkali concentrations are listed in Table 1.

Table 1 Sample abbreviations and preparation conditions.

2.4 Materials characterization

X-ray diffraction(XRD)measurements were carried out on Philips X'Pert PRO Super diffractometer(Philips X'Pert-PRO,Philips Japan,Ltd.,Japan)using Cu Kαradiation(λKα1=0.154178 nm)at a scanning rate of 4(°)·min-1.Transmission electron microscopy(TEM)images were acquired on a FEI TECNAI F30 field emission electron microscope working at an acceleration voltage of 300 kV.Field emission scanning electron microscopy(FE-SEM)images were obtained on a Hitachi S-4800 microscope(S-4800,Hitachi,Japan).X-ray photoelectron spectroscopy(XPS)spectra were obtained on an AXIS Ultra X-ray photoelectron spectrometer(AXIS Supra,Kratos Analytical Ltd.,UK)with an exciting source of Al Kα(1486.7 eV).The binding energies were calibrated by using adventitious carbon(C 1s=284.8 eV)as a reference.Fourier transform infrared(FTIR)spectra were recorded on a Nicolet spectrometer(Nicolet iS50,Thermo Fisher Scientific,USA)with the samples dispersed in KBr.The Brunauer-Emmett-Teller(BET)specific surface area was measured using an ASAP 2020 nitrogen adsorption apparatus(ASAP 2020,Micromeritics Instruments,Norcross,Georgia,USA).

2.5 Photocatalytic activity of H2 evolution

The photocatalytic hydrogen evolution tests were conducted in a Pyrex top-irradiation reaction vessel connected to a closed glass-gas system(Labsolar-6A,Beijing Perfect Light Science & Technology Co.,Ltd.).A 300 W Xe lamp with a cut-off filter(λ ≥ 420 nm)was used as visible-light source.In a typical photocatalytic measurement,50 mg of photocatalysts was dispersed into an aqueous solution(100 mL)containing 20 mL of lactic acid.The above mixture was sonicated and stirred for 30 min and then transferred into the reactor.During the photocatalytic reaction,the reactor system was maintained at room temperature by a closed cycle cooling water system.The hydrogen gas was analyzed by an online gas chromatograph(GC7900 Tianmei Shanghai China)equipped with a TCD detector,using Ar as the carrier gas.

2.6 Photoelectrochemical measurement

The photoelectrochemical tests were performed on the electrochemical workstation(PGSTAT302NM,etrohm Autolab,Switzerland)in a typical three-electrode setup with an electrolyte solution of 0.5 mol·L-1Na2SO4,using Pt foil as the counter electrode,Ag/AgCl as a reference electrode and the modified F-doped tin oxide(FTO)glass as the working electrode.The working electrode was prepared as follows:sample(4 mg)was dispersed into a 0.2 mL of N,N-dimethylformamide(DMF)to get a slurry by ultrasonication.Then,5 μL of the suspension was spin-coated onto an FTO glass electrode pretreated by plasma.The coated area of the working electrode was 0.25 cm2.The photocurrent measurements were performed at a 5 mV potential bias under a 300 W Xe lamp(light on/off at a regular interval of 20 s)equipped with a cut-off filter(λ ≥ 420 nm).Electrochemical impedance spectroscopy(EIS)curve was measured under open circuit potential conditions.

3 Results and discussion

3.1 Sample characterization

P-CdS nanostructures were synthesized through a one-step gamma-ray radiation induced reduction process according to a previously reported method16.Then,the MP-CdS were prepared via a simple alkaline treatment approach.XRD patterns are used to analyze the crystallinity and crystalline phases of the P-CdS and MP-CdS samples.As shown in Fig.1,the as-prepared samples present both ZB(JCPDS 10-0454)and WZ(JCPDS 77-2306)phases of CdS,which is similar to those reported in our previous work16.

Fig.1 XRD patterns of P-CdS,MP-CdS-1,MP-CdS-2,MP-CdS-3,MP-CdS-4,MP-CdS-5.

SEM measurements were applied to directly observe the morphologies of the P-CdS and MP-CdS(Fig.2).As treated with different concentrations of NaOH,the morphology of MPCdS is similar to that of P-CdS,but the MP-CdS’s surface become rougher than the P-CdS.In order to explore the microstructure of P-CdS and MP-CdS,the TEM and SEAD were measured in Fig.3.Both P-CdS and MP-CdS-3 are composed of some small grains,and these grains aggregate together to form the popcorn-shaped CdS nanostructures(Fig.3b,f).Besides,the proximity between the WZ grains and ZB grains shows that the intimate contact between these two phases evidences the formation of a homojunction in good agreement with our previous report(Fig.3c,g)16.And in the corresponding selected area electron diffraction(SAED)patterns(Fig.3d,h),the diffraction rings suggest that both the P-CdS and MP-CdS consist of the randomly oriented polycrystallites.The three major peaks labeled as(111),(220)and(311)are assigned as the ZB phase,which agrees with the results of XRD patterns.It’s worth noting that,compared with the P-CdS(Fig.3a-c),the surface of MP-CdS-3(Fig.3e-g)become rougher,and the MPCdS-3(Fig.3h)shows some clear diffraction spots,but only diffraction rings are shown in P-CdS(Fig.3d).All these phenomena suggest that the surface of P-CdS has changed,but the crystal structures of CdS are remained after the alkaline treatment.

Fig.2 SEM image of(a)P-CdS,(b)MP-CdS-1,(c)MP-CdS-2,(d)MP-CdS-3,(e)MP-CdS-4,(f)MP-CdS-5.

Fig.3 TEM images with different magnifications and corresponding SAED patterns of(a-d)P-CdS and(e-h)MP-CdS-3.

In order to investigate the structure changes of P-CdS after alkaline treatment,the FTIR of P-CdS and MP-CdS-3 is performed.As shown in Fig.4a,two intense bands appear at 640 and 1002 cm-1resulting from the respective Cd―S and S＝O stretching vibration24.And the characteristic peaks at 2927,1451,1658,1289 cm-1for P-CdS and MP-CdS-3 are assigned to C―H stretching,C―H bending,C＝O stretching and C―N stretching vibration modes of PVP,respectively16,25,26.It’s worth noting that the signal at 1372 cm-1in the alkalized MPCdS-3 enlarges significantly,which belongs to the COO-asymmetric stretching vibration27.Besides,the peak at 1568 cm-1occurred in the MP-CdS-3 corresponds to N―H bending vibration28,29.What’s more,the C 1s high-resolution XPS spectra in Fig.4b also shows the presence of COO-in MP-CdS-3 with the binding energy of 289.1 eV,while for the P-CdS samples,no peaks appear at this binding energy.And the peak of MP-CdS-3 centered at 286.0 eV could be assigned to C―N bonds,a positive shift(ca.0.2 eV)with respect to that of P-CdS is ascribed to the changes of amide bond of PVP.All those phenomena indicate that the amide group in PVP on the P-CdS’s surface has been hydrolyzed to COO-and ―NH― groups upon the alkaline treatment.

What’s more,the changes of N element contents on the surface of P-CdS after alkaline treatment was investigated by the XPS semi-quantitative analysis(in Table 2).From the C 1s spectrum,with an increase in the treated alkali concentrations,the relative percentage of C―N bonds decreases first and then increases slightly,which indicate the removal of PVP after the alkaline treatment.Besides,in the N 1s and Cd 3d spectrum,the atomic ratios of N/Cd exhibit the similar behaviours.It’s worth noting that the MP-CdS-3 shows the highest COO-contents and the highest hydrolysis degree compared with P-CdS and MPCdS treated in other alkali-concentrations.These above reports clearly validate that via the alkalization induced-hydrolysis,the amide group of PVP has been changed to COO-and ―NH―groups,and the formation of carboxy-functionalized PVP could increase the solubility of MP-CdS in the alkalized solution,leading to a portion of carboxy-functionalized PVP dissolving into the solution and removing from the CdS to expose more active sites.What’s more,by properly controlling the alkali concentrations,MP-CdS-3 photocatalyst displays the highest COO-contents and the capping of lowest carboxyfunctionalized PVP with the highest hydrolysis degree as shown in Table 2,which may influence the photocatalytic process.

Then,the specific surface area of the P-CdS and MP-CdS was obtained by the Brunauer-Emmett-Teller(BET)method(Fig.5a).After the P-CdS was treated by NaOH,the BET specific area of MP-CdS become larger,and the MP-CdS-3 shows the highest surface area(87.10 m2·g-1).This observed increase in surface area is attributed to the removing of carboxy-functionalized PVP on the MP-CdS surface as supported by the above XPS semiquantitative of C 1s and N 1s.Besides,the N2adsorptiondesorption isotherm curves of P-CdS and MP-CdS(Fig.5b)exhibit typical type IV curves with a hysteresis loop,which is related to the capillary condensation of N2in the mesopores of the nanopopcorns.

Further,the elemental valence states of Cd and S were investigated by XPS(Fig.6a,b).For the P-CdS samples,the Cd 3d5/2and 3d3/2peaks were located at ～404.8 and 411.6 eV,respectively,confirming the Cd2+in CdS30,31(Fig.6a).In the high-resolution S 2p spectrum(Fig.6b),two peaks located at 162.4 and 161.1 eV are observed,revealing the existence of S2-32.These results adequately support the existence of CdS in P-CdS.After the alkaline treatment,Cd 3d5/2of MP-CdS exhibits a positive shift with respect to that of P-CdS,and the shift(ca.0.4 eV)of Cd 3d5/2is the highest for the MP-CdS-3.Combined with S 2p(Fig.6b)core level peak,the S 2p3/2of MP-CdS-3 shows a positive shift(ca.0.3 eV)compared with the P-CdS,too.All of these results are ascribed to the bonding between the electron withdrawing carboxyl groups of carboxy-functionalized PVP with CdS,instead of the bonding between the amido bond of PVP with CdS33.

Fig.5 (a)The N2 adsorption-desorption isotherm curves and(b)the BET specific area of P-CdS and MP-CdS treated in different alkali concentrations.

Fig.6 (a)The high-resolution Cd 3d XPS spectra,(b)the high-resolution S 2p XPS spectra,(c)the valence-band XPS spectra and(d)the Tauc plots of(αhν)2 versus photon energy(hν)for P-CdS and its alkalized products in different alkali concentrations.

Then,the energy band structure was tested by characterization method.Firstly,the XPS valence band spectra of P-CdS and MPCdS were measured in Fig.6c.After the alkaline treatment,the valence band level of MP-CdS shifts higher compared with that of P-CdS.And the position of valence band maximum(VBM)of MP-CdS-3 with the most COO-groups is the highest to increase the oxidizing ability34.What’s more,the band gap energies of the samples are estimated from the extrapolation of the plot of(αhν)2versus photon energy(hν)35.As shown in Fig.6d,the band gap energy of MP-CdS is lower than that of P-CdS to increase absorption in the visible light region.The testing results prove that the formation of COO-and ―NH― group after the alkaline treatment could regulate the band structures due to the coordination of COO-groups and CdS,further affecting the photocatalysis.

3.2 Formation mechanism of MP-CdS nanopopcorns

Fig.7 Schematic illustration for the formation of MP-CdS nanocomposites.

3.3 Photocatalytic studies

To further investigate the significant effect of the alkaline treatment on the photocatalytic performance,we evaluate P-CdS and alkalized MP-CdS towards photocatalytic H2evolution under visible-light irradiation(λ ≥ 420 nm,Fig.8).Compared with pristine P-CdS,MP-CdS photocatalysts exhibit dramatically enhanced photocatalytic efficiency and the alkaliconcentration plays an essential role in controlling the hydrolysis degrees for improvement of the photocatalytic activity.The MPCdS-3 sample performed the highest H2production rate(477 μmol·h-1·g-1)when the treated NaOH concentration is 1 mol·L-1,which was about 2 times higher than that of P-CdS.As the MP-CdS-3 exhibit the highest BET specific area and highest COO-contents,the highest catalytic activity of MP-CdS-3 could be ascribed to the removal of PVP to expose more active sites of WZ-ZB phase junctions and the modulation of band structure due to the coordination of COO-groups and CdS as shown in Table 3.The photocatalytic activity of MP-CdS-3 is compared with reported typical CdS-based photocatalysts in terms of photoactivity(Table 4)and we found that the carboxyl functionalized MP-CdS-3 nanocomposites show relatively higher hydrogen production activity.

Fig.8 Comparison of the photocatalytic H2 production rates of CdS-based photocatalysts treated with different alkali-concentration.

Table 3 Physicochemical data of CdS-based photocatalysts.

Table 4 Comparison of photocatalytic H2 production rate of representative CdS-based photocatalyst.

Fig.9 (a)Photocurrent responses and(b)EIS spectra of MP-CdS-3 and P-CdS.

The enhanced charge separation and the rapid charge transfer of the homojunction CdS photocatalyst can be further demonstrated via photoelectrochemical measurements20.The photoelectrochemical measurements were performed in an electrolyte solution of 0.5 mol·L-1Na2SO4,as shown in Fig.9.Clearly,the photocurrent response of MP-CdS-3 is much larger than that of P-CdS(Fig.9a).Moreover,compared with P-CdS,MP-CdS-3 shows the smaller hemicycles in the Nyquist plot(Fig.9b),revealing that MP-CdS-3 is more favorable for charge separation with high charge mobility43-45.Overall,due to the synergistic effects of the exposure of active sites of WZ-ZB phase junctions and the modulation of energy band structures via alkaline treatment,the separation and transfer efficiency of electrons and holes could increase,thereby enhancing photoactivity under visible-light excitation.

3.4 Photocatalytic mechanisms

In light of the aforementioned discussion,a possible mechanism for the photocatalytic H2production over MP-CdS samples is proposed.As illustrated in Fig.10,under visible light irradiation,CdS homojunctions with WZ CdS grains and ZB CdS grains are photoexcited to generate electrons and holes.Clearly,the photoinduced electrons transfer from the conduction band of WZ-CdS to that of the ZB-CdS,and then react with the H+ions adsorbed on the surface of MP-CdS to form H2.Simultaneously,the photogenerated holes transfer from the valence band of the ZB-CdS to that of WZ-CdS,and then react with lactic acid to form the oxidic products15,16.By the alkali modification,a portion of carboxy-functionalized PVP removed from the MP-CdS,exposing more active sites of WZ-ZB phase junctions.Additionally,it is believed that the narrower band gap and a suitable band alignment can improve the light absorption capacity of the semiconductor materials and the H2-generation reduction.In a word,during the photocatalysis process,the H+ions can be quickly reduced by the photoexcited electrons migrated from WZ-CdS to ZB-CdS to form H2because of the type II homojunction and proper band structure of MP-CdS,thereby greatly boosting the photocatalytic H2-evolution rates.

Fig.10 Schematic diagrams of visible-light-induced photocatalytic H2 evolution of MP-CdS.

4 Conclusions

P-CdS nanopopcorns with ZB-WZ homojunctions are synthesized by a gamma-ray radiation-induced reduction approach.Then,via a simple alkaline treatment,the photocatalytic activity of MP-CdS photocatalyst is dramatically promoted.The alkali concentration plays an essential role in controlling the hydrolysis of PVP on the surface of P-CdS.The MP-CdS-3 photocatalyst prepared under 1 mol·L-1NaOH exhibits the highest photocatalytic efficiency of H2-generation under visible-light irradiation.Herein,the promoted H2-generation rate of MP-CdS samples can be well explained by the synergistic action mechanism of the homojunction structure and modulation of band structure via the alkaline treatment;namely,the homojunction between WZ-CdS and ZB-CdS nanocrystals can achieve effective separation and transportation of photoexcited charges,while the hydrolysis of PVP leads to the removal of PVP and the formation of carboxy-functionalized PVP,which expose more active sites of WZ-ZB phase junctions and modulate the band structure to boost the surface hydrogen reduction.The present research provides a facile strategy to the practical application of a range of functional photocatalysts.