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Photoelectrochemical evaluation of undoped and C-doped CdIn 2O 4thin ?lm electrodes

Yanping Sun,Jason M.Thornton,Nathan A.Morris,Rina Rajpura,Sarah Henkes,Daniel Raftery *

Department of Chemistry,Purdue University,560Oval Drive,West Lafayette,IN 47907,USA

a r t i c l e i n f o

Article history:

Received 24August 2010Received in revised form 23November 2010

Accepted 2December 2010Available online 19January 2011Keywords:CdIn 2O 4

Photocatalysis

Solar hydrogen production Water splitting C-doping XPS

a b s t r a c t

Undoped and C-doped cadmium indate (CdIn 2O 4)thin ?lms and powders were synthe-sized,characterized,and evaluated for photoelectrochemical water splitting.Both undoped and C-doped CdIn 2O 4samples have cubic lattices,and the presence of carbonate-type species was con?rmed in the C-doped sample by XPS.Doping C into CdIn 2O 4leads to a red shift (but no separate peak)in light absorption and band gap narrowing.The photocurrent densities of CdIn 2O 4electrodes are at least three-fold greater than either CdO or In 2O 3electrodes with equivalent ?lm thickness.Carbon doping further improved the photocur-rent densities by 33%.The photoelectrochemical performance of C-doped CdIn 2O 4was optimized with respect to several synthetic parameters,including the C:In molar ratio and glucose concentration in the spray precursor solution,the calcination temperature,and the ?lm thickness.The present work shows that CdIn 2O 4is a promising photocatalyst and can be suitably doped to improve the electrochemical properties for solar conversion applications.

1.Introduction

Photocatalytic water splitting to produce H 2and O 2over a semiconductor catalyst has recently received a great deal of attention due to the importance of using hydrogen as a clean and renewable fuel source [1,2].Since Fujishima and Honda ?rst demonstrated photocatalytic water splitting using UV irradated TiO 2in the early 1970s [3],many other semi-conductors have been investigated as suitable photocatalysts for water splitting,

including Fe 2O 3,WO 3,ZnO,SrTiO 3and In 2O 3[4e 9].Of these materials,TiO 2is the most popular material due to its low cost,chemical stability over a wide pH range,and excellent functionality.However,a major disad-vantage of TiO 2and most other potentially viable metal oxide semiconductors is that these oxides have relatively large band gaps which correspond to the absorption of UV light only.

Semiconductors with smaller band gaps are normally unstable photochemically in aqueous solution,and thus they cannot offer a practical solution for water splitting [10].

Doping with transition metals is a useful approach to narrow these large band gaps and shift the absorption to the visible region.However,the photocurrent generated from these transition metal doped materials has not improved much due to rapid electron-hole recombination and/or thermal instability [11e 13].Doping anions into photoactive metal oxides for the purpose of improving the photoresponse was proposed in 2001[14],and in particular,anion-doped TiO 2materials have been widely studied in recent years [5e 21].Among different doping anions,nitrogen and carbon have been proven to be the most promising dopants for TiO 2materials.Recently,this approach has been extended to ZnO [22,23],Fe 2O 3[24],In 2O 3[8,9,25]and WO 3[5,26,27].

*Corresponding author .Tel.:t1(765)4946070;fax:t1(765)4940239.E-mail address:raftery@https://www.360docs.net/doc/5419006576.html, (D.Raftery).

A v a i l a b l e a t w w w.s c i e n c e d i r e c t.c o m

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /h e

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Although many different semiconductors have been studied in the search for suitable water splitting photo-catalysts,no material which is ef?cient enough to produce suf?cient H2for practical use has been found thus far. Therefore,it is necessary to explore new photocatalysts for water splitting.

Some semiconductors may have the correct band positions to split water,but have large band gaps;while other semi-conductors may have smaller band gaps,but do not have right band positions.Our goal is to identify materials that may combine the bene?cial characteristics of both these types of materials.One such interesting material is CdIn2O4.In2O3has the right band positions to split water;however,it is trans-parent to visible light(band gap w3.5eV).On the other hand, CdO has relatively a small band gap(2.6eV);however,the conduction band of CdO is more positive than the H2/H2O potential.CdIn2O4,as a new photocatalyst,may combine the advantages of both In2O3and CdO and result in good photoactivity.

CdIn2O4is known to have excellent electrical conductivity and good transparency in the visible light range,and this material has applications in gas sensors and device tech-nology[28e31],such as solar cells,?at panel displays,invisible security circuits and windshield defrosters[28e31].Though CdIn2O4has not been studied to date for water splitting,this material possesses a number of important and suitable characteristics for photocatalysis,including good conductivity and high stability.

As a wide band gap material(w3.1eV),CdIn2O4does not absorb visible light,which gives rise to a low absorption ef?-ciency.However,a very recent report indicates that the CdIn2O4band gap energy may actually be as low as2.7eV when small amounts of In2O3and CdO coexist[32].Carbon doping may provide a good opportunity to modify the band gap of CdIn2O4and enhance its photoresponse under visible light irradiation.

In this study,we describe the photoelectrochemical performance of undoped and C-doped CdIn2O4thin?lms prepared by spray pyrolysis.Spray pyrolysis offers a repro-ducible,simple and inexpensive method to synthesize CdIn2O4?lms.The deposition rate and the thickness of the ?lms can be easily controlled over a wide range by changing the spray parameters,thus eliminating a major drawback of chemical methods such as sol e gel which produces?lms of limited thickness.In this paper,the structural,optical, morphologic and photoelectrochemical properties of undo-ped and C-doped CdIn2O4materials were evaluated and show that these materials have promise for water splitting applications.

2.Experimental methods

2.1.Preparation of undoped and C-doped CdIn2O4 electrodes

Undoped and C-doped CdIn2O4thin?lms were synthesized by spray pyrolysis.The spray precursor solution for undoped CdIn2O4was prepared by mixing1.0g of indium(III)nitrate hydrate(In(NO3)3 5H2O)(Aldrich)and0.34g of cadmium nitrate tetrahydrate Cd(NO3)2$4H2O(Mallinckrodt)with0.8mL acetylacetone(Aldrich)in50mL methanol(Mallinckrodt)for 30min.Ammonium hydroxide(Mallinckrodt)solution, 0.35mL,was then added,and the solution was stirred for2h before spraying.The Cd:In molar ratio was kept at0.5:1,except for the cadmium concentration optimization study.The spray precursor solution for C-doped CdIn2O4?lms was prepared by adding0.090g glucose into the above solution(?nal concen-tration,0.010M).

Glass slides and conductive?uorine-tin oxide(FTO)sheets were used separately as the substrates for the spray-pyrolysis deposition.Thin?lms synthesized on glass slides were scra-ped and collected after calcination for X-ray photoelectron spectroscopy(XPS)measurements,while?lms made on FTO substrates were used for X-ray diffraction(XRD),UV e vis, surface scanning electron microscopy(SEM)and photo-electrochemical analysis.A portion of the FTO was covered with aluminum foil to avoid spray deposition and allow for an electrical connection.

The spray pyrolysis method was carried out by placing the FTO sheets on a hotplate set to200 C and spraying the precursor solution for15min.For the?lm thickness opti-mization study,this time was varied.The resulting?lms were then calcinated in air at550 C for2h.Undoped In2O3 and CdO?lms were used as reference electrodes and were synthesized in exactly the same manner described above, except in the absence of Cd(NO3)2$4H2O and(In(NO3)3$5H2O), respectively.The electrical contact for all synthesized?lms was made by connecting the uncoated area of FTO with a copper wire using silver epoxy;the metallic contact was then covered by epoxy resin to isolate it from the basic electrolyte solution.

2.2.Characterization

XRD analysis was performed on a Siemens D500diffractom-eter employing Cu K a radiation(0.1540nm)and the data were recorded in the range of20 2q80 with a step width of

0.02 2q sà1.UV e vis absorption spectra were taken using

a Cary Bio300Varian spectrophotometer.The?lm thickness and morphology were characterized using cross-sectional and surface SEM images,respectively.XPS high-resolution scans were collected using a Kratos Axis Ultra X-ray photo-electron spectrometer employing monochromatic Al K a excitation.Spectral processing was conducted with the CASA XPS software to analyze the XPS data[33],and binding energies were calibrated with respect to the residual C(1s) peak at284.6eV.

Photoelectrochemical measurements were carried out using a Gamry reference600potentiostat in a three-electrode con?guration.A1M KOH solution was used as the electrolyte, platinum foil was used as the counter electrode,and Ag/AgCl/ KCl(sat.)electrode was utilized as the reference electrode. A300W xenon arc lamp was employed as the light source with an intensity of0.13W/cm2at the electrode surface to simulate the reported total solar irradiance of0.1366W/cm2 [34].A400nm cutoff?lter was placed into the light path to remove the UV irradiation,and a water?lter was used to remove the IR energy and avoid overheating.

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3.Results and discussion

3.1.XRD

Fig.1a shows the XRD patterns of undoped and C-doped CdIn2O4?lms.The XRD patterns of CdO and In2O3?lms are also shown in Fig.1a for comparison.The peaks correspond-ing to a SnO2phase in all the patterns are attributed to the FTO substrates.It can be seen that the synthesized undoped and C-doped CdIn2O4?lms mostly contain the cubic CdIn2O4 structure,while a small concentration of cubic In2O3phase (peaks at30.7 and35.6 )still exists.No additional peaks were detected at the sensitivity limit of the instrument,indicating that the CdO phase was not found in undoped or C-doped CdIn2O4samples and C-doping did not change the crystalline structure of the CdIn2O4?lms.The average particle size calculated from the Scherrer equation based on the32.4 peak was found to be41nm for the CdO sample,11nm for the In2O3sample,21nm for the undoped CdIn2O4sample,and23nm for the C-doped CdIn2O4sample.

Fig.1b shows the XRD patterns for the C-doped CdIn2O4?lms with different Cd(NO3)2to In(NO3)3M ratios(2:1,1:1, 0.5:1,and0.25:1)in the spray precursor solutions.Based on peak intensities,when Cd:In?2:1the majority phase in the sample after calcination was CdO,although the CdIn2O4and In2O3phases were present.When Cd:In?1:1,CdO and CdIn2O4were the main products;when Cd:In?0.5:1,the majority phase changed to CdIn2O4;and when Cd:In?0.25:1, only the In2O3phase was detected.

3.2.UV e vis spectroscopy

Fig.2a shows the UV e vis absorption spectra of CdO,In2O3, undoped CdIn2O4and C-doped CdIn2O4?https://www.360docs.net/doc/5419006576.html,pared with CdO or In2O3,undoped CdIn2O4shows increased absorption not only in the ultraviolet range but also in the visible

range.

Fig.1e(a)XRD patterns for(I)CdO,(II)In2O3,(III)undoped CdIn2O4,and(IV)C-doped CdIn2O4?lms.(b)XRD patterns for C-doped CdIn2O4?lms with different Cd:In molar ratios:(I)Cd:In[2:1,(II)Cd:In[1:1,(III)Cd:In[0.5:1,and(IV)

Cd:In[0.25:1.(*)SnO2phase from the FTO substrate,()In2O3phase,()CdO phase,and(C)CdIn2O4phase.

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After doping with carbon,CdIn 2O 4absorptions are red-shifted in the optical response and show no separate absorption peaks,which indicates a reduction of the band gap.Assuming the materials are indirect semiconductors,a plot of the modi?ed Kubelka e Munk function versus the excitation energy [35,36]can be used to determine the band gap.Kubel-ka e Munk plots for CdO,In 2O 3,undoped,and C-doped CdIn 2O 4are shown in Fig.2b.The estimated band gap energies are 2.5eV for CdO,3.2eV for In 2O 3,and 2.6eV for undoped CdIn 2O 4.These values are in agreement with those known for CdO,In 2O 3,and CdIn 2O 4[32,37,38].The estimated band gap energy for CdIn 2O 4is decreased to 2.4eV after carbon doping,and this band gap reduction can be attributed to the doping of C into the CdIn 2O 4lattice.

3.3.XPS

Fig.3shows XPS high-resolution spectra of C 1s ,Cd 3d ,In 3d ,and O 1s core levels for the undoped and C-doped CdIn 2O 4powders.Table 1summarizes corresponding binding energies.In the C 1s spectra,binding energies of 284.6eV and 286.1eV were observed in both undoped and C-doped CdIn 2O 4samples.These peaks are due to the presence of extrinsic carbon which is an unavoidable presence on all air-exposed materials.The peak at 289.1eV was also found in both samples corresponding to a carbonate species.The C atomic ratio based on 289.1eV peak was 0.9and 5.2%for undoped and C-doped CdIn 2O 4samples,respectively.The low C concentration (0.9%)in the undoped powder is due to super?cial environmental contam-ination,as it has been reported previously for undoped In 2O 3[8,25]and WO 3[5].The C excess of the doped material comes from glucose in the spray precursor solution.This 289.1peak is in good agreement with C-doped TiO 2[39],C-doped In 2O 3[8],C-doped SiO 2[40],C-doped WO 3[5]and C-doped SrTiO 3

[41]

Fig.2e (a)UV e Vis absorption spectra and (b)transformed diffuse re?ectance spectra of (I)CdO,(II)In 2O 3,(III)undoped CdIn 2O 4,and (IV)C-doped CdIn 2O 4?lms.The insert plot shows the estimated band gap for

CdO.

Fig.3e XPS high-resolution spectra for the (a)C 1s ,(b)Cd 3d ,(c)In 3d ,and (d)O 1s core levels.The individual spectra correspond to (I)undoped CdIn 2O 4and (II)C-doped CdIn 2O 4.

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materials that contain carbonate species.No C peak appears Array around281eV,suggesting carbide species were not found in

the C-doped sample[42,43].Since XPS only probes the top5nm

of the?lm,it is possible the carbon signals are due to surface

carbonates.This is unlikely since the results obtained are in

good agreement with previously published work,and surface

carbonates typically block catalytic sites.However surface

etching experiments,where the top layers of the?lms

are removed and XPS measurements are repeated on the bulk,

are possible to con?rm the carbon signals are from intrinsic

doping throughout the sample.

Fig.4e SEM surface images of(a)CdO,(b)In2O3,(c)undoped CdIn2O4,and(d)C-doped CdIn2O4?lms;SEM cross-sectional images of(e)C-doped CdIn2O4?lm with a15min spray time,and(f)C-doped CdIn2O4?lm with a40min spray time.

As shown in Fig.3b,the binding energies of Cd3d3/2and Cd 3d5/2are found to be411.0eV and404.3eV for undoped CdIn2O4,which are in good agreement with the literature values for CdIn2O4[44,45].However,the C-doped CdIn2O4 shows that the Cd3d peaks shift to higher binding energies (412.3eV and405.5eV).The binding energies of In3d3/2and In 3d5/2are also found to have a higher shift in C-doped CdIn2O4 compared with undoped CdIn2O4(from451.2to451.6eV for In 3d3/2and from443.6eV to444.0eV for In3d5/2),as shown in Fig.3c.These higher binding energy shifts are attributed to C substitutional doping into cation sites(Cd and/or In site)in the CdIn2O4lattice.As C is more electronegative than Cd and In,the electron density around the cations increases.It is dif?cult to estimate whether Cd or In sites are substituted due to the similar ionic properties and ionic radii for Cd2tand In3tions.More detailed experiments are required to understand the structural properties of C-doped CdIn2O4.

Fig.3d shows that the O1s core level is composed of two components.The O1s signal at529.1eV is assigned to CdIn2O4 according to the literature[45],while the signal at531.1eV is probably due to oxidized carbon(C]O)at the surface[46].The C-doped CdIn2O4sample shows a higher energy shift (529.5eV)for the O1s peak that is assigned to CdIn2O4.

3.4.SEM

Fig.4a shows the SEM image of CdO particles on the FTO substrate.The sample exhibits aggregated small crystals in a fairly con?uent coating,which was also observed in previous studies for CdO?lms deposited by sol e gel dip coating[47],electrodeposition[37]and the sonochemical method[48].The surface micrographs of the In2O3,CdIn2O4, and C-doped CdIn2O4?lms(Fig.4b e d)all show cracked

morphologies that are likely due to the combustion and shrinkage of the?lms during the calcination process.The cracked morphology in C-doped CdIn2O4is worse than that for the undoped?lm most likely due to the pyrolysis of excess glucose during the calcination process.Similar cracked SEM images were observed in C-doped TiO2[49],C-doped In2O3[8], and C-doped WO3?lms[5]by spray pyrolysis using glucose as the dopant source.Cross-sectional analysis(Fig.4e and f) shows that the?lms are approximately4m m thick for a15min spray time and8m m for a40min spray time.

3.5.Photoelectrochemical analysis

Fig.5shows a plot of photocurrent density versus the applied potential for undoped and C-doped CdIn2O4electrodes.The photoelectrochemical performance of CdO and In2O3is also shown in Fig.5as a comparison.The samples were evaluated under near UV e visible light and visible light only(l>400nm) irradiation conditions.Each reported photocurrent density has been background subtracted to correct for any dark current.As shown in Fig.5,with the experimental conditions kept constant the photocurrent densities for undoped CdIn2O4 electrodes were at least triple those of either the CdO or In2O3 electrodes with equivalent?lm thicknesses.The improved photoresponse of CdIn2O4over CdO and In2O3is consistent with the UV e vis absorption spectra.Under near UV e vis irra-diation and without any sacri?cial reagents,the photocurrent density increased from1.10mA/cm2for undoped CdIn2O4to 1.47mA/cm2for C-doped CdIn2O4at1.0V,and the photocur-rent density under visible light irradiation increased from 0.23mA/cm2for the undoped electrode to0.51mA/cm2for the C-doped electrode at1.0V.This enhanced photoactivity can be attributed to carbon doping.Visible light photocurrent contributes21%of the total response for the undoped elec-trode and35%for the C-doped electrode.This improved visible light photocurrent percentage is highly encouraging. The cell ef?ciency,h was calculated using the following equation[50],

h?j p

V wsàV app

?100%(1) where j p is the photocurrent produced per unit area,V WS is the potential corresponding to the Gibbs free energy change per photon required to split water,V app is the applied voltage between the working the counter electrode and I is the irra-diation power per unit area.The photocurrents measured under UV e Vis irradiation correspond to an overall photo-conversion ef?ciency of0.42%for the C-doped and0.27%for the undoped.

The photoresponse was studied as a function of several synthetic variables.The dependence of the C-doped CdIn2O4?lm photoresponse on Cd:In molar ratio in the precursor spray solution is shown in Fig.6a.Under both near UV e visible light and visible light only illumination,the photocurrent

density b

Fig.5e Photocurrent density for the(I)CdO,(II)In2O3,(III) CdIn2O4,and(IV)C-doped CdIn2O4electrodes.All the electrodes were evaluated in1M KOH electrolyte and illuminated with130mW/cm2illumination using(a)

UV e visible light and(b)visible light only(l>400nm).

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increased with an increasing Cd:In molar ratio from 0.25:1to 0.5:1,and then decreased with further increases in the molar ratios.Since the XRD patterns (Fig.1b)con?rmed that CdIn 2O 4was the majority phase in the sample with a 0.5:1ratio,while In 2O 3was the majority phase with 0.25:1and CdO was the majority phase with 3:1,it can be concluded that CdIn 2O 4performs much better photoelectrochemically than In 2O 3or CdO.

The dependence of the C-doped CdIn 2O 4?lm photo-response on the precursor spray glucose concentration is shown in Fig.6b.Under near UV e visible light and visible light illumination,photocurrent densities of undoped CdIn 2O 4increased when glucose was added to the precursor solution (0.05M).The photocurrent density increased with increasing glucose concentration up to 0.07M,and then decreased with further increases in the glucose concentration.

The dependence of the photocurrent density on calcina-tion temperature for the C-doped CdIn 2O 4electrodes is shown in Fig.6c.Four different calcination temperatures were applied to optimize the heating conditions.Under near UV e vis irradiation,the photocurrent density ?rst increased from 0.25mA/cm 2at 450 C to 0.92mA/cm 2at 500 C,and then to 1.47mA/cm 2at 550 C,and ?nally dropped to 0.78mA/cm 2at 600 C.The photocurrent density under visible light irradi-ation displayed a similar trend.Therefore,the photocurrent density depends on the calcination temperature and 550 C was found to be the best calcination temperature for C-doped CdIn 2O 4electrodes.

To determine the optimized synthetic conditions for achieving the highest photoactivity,C-doped CdIn 2O 4elec-trodes of various thicknesses were prepared by increasing the

spray time.A spray time of 15min produced a ?lm with a thickness of 4m m and a 40min spray time produced a ?lm with a thickness of 8m m,which were determined by SEM cross-sectional images.Fig.6d shows the photocurrent densities as a function of the spray time used in the electrode preparation.It was found that the photocurrent density ?rst increased with an increasing spray time,reaching maxima at 30min (1.87mA/cm 2under near UV e vis irradiation and 0.82mA/cm 2under visible light only irradiation),and then decreased with further increases in spray time.Similar results have been reported for many other semiconducting materials,such as TiO 2[51],Fe 2O 3[52],and CdSe [53]thin ?lms.The photoresponse of a ?lm depends on the light absorption and the decay of electron current to the back electrode due to diffusion-related losses.When the ?lm is thin,not enough light is absorbed,and therefore the performance improves with longer spray times.However,when the ?lm thickness is considerably larger than the penetration depth of the light,the photocurrent density starts to decrease,which can be attrib-uted to the combination of increased resistance and a higher recombination rate of photogenerated carriers due to reduc-tion of the electric ?eld gradient in a thicker ?lm [53].

4.Conclusions

Undoped and C-doped CdIn 2O 4electrodes and powders have been synthesized by spray https://www.360docs.net/doc/5419006576.html,pared with CdO and In 2O 3,CdIn 2O 4shows more light absorption in both the ultraviolet and the visible ranges.C-doped CdIn 2O 4is red shifted (but without a de?ned peak)in its absorption,

and

Fig.6e Photocurrent density for the C-doped CdIn 2O 4electrodes dependence on (a)Cd:In molar ratio in the spray precursor solution,(b)glucose concentration on the spray precursor solution,(c)the calcination temperature and (d)the spray time,and (I)illuminated under near UV e visible light,or (II)illuminated under visible light (l >400nm).The photocurrent densities were measured at 1.0V.

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therefore carbon doping results in band gap narrowing.XPS indicates the existence of a carbonate species in the substi-tutional C-doped CdIn2O4samples.CdIn2O4electrodes show much better photoactivity compared to CdO and In2O3elec-trodes,and the photoelectrochemical performance of the C-doped CdIn2O4electrode prepared from glucose is further improved.The observed photocurrent densities are as high as 1.87mA/cm2without any sacri?cial agents.The visible light photocurrent contribution to the total response for undoped electrode is increased from21to33%after C-doping.The photocurrents are strongly related to the Cd:In molar ratio in the precursor solution,calcination temperature and the?lm thickness.A0.5:1Cd:In molar ratio,0.02M glucose concen-tration,550 C calcination temperature,and30min spray time gave the highest photoresponse.

Acknowledgments

Support for this work from the National Science Foundation (CHE-0616748and DMR-0805096),and the Purdue Research Foundation is gratefully acknowledged.The authors also thank Dr.D.Zemlyanov of the Surface Analysis Laboratory, Birck Nanotechnology Center,Purdue University,for helpful comments on and acquisition of the XPS spectra,and to Professor K.Choi for the diffuse re?ectance measurements.

D.R.is a member of the Purdue Energy Center.

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