Visible photoactivity and antiphotocorrosion performance of PdS e CdS photocatalysts modi?ed by polyaniline
Shu Zhang,Qingyun Chen *,Dengwei Jing,Yunhai Wang,Liejin Guo
State Key Laboratory of Multiphase Flow in Power Engineering,Xi’an Jiaotong University,28West Xianning Road,Xi’an City,Shanxi Province 710049,China
a r t i c l e i n f o
Article history:
Received 17November 2010Received in revised form 7April 2011
Accepted 8April 2011Available online 8May 2011Keywords:Photocatalysts Visible photoactivity Antiphotocorrosion PANI/PdS e CdS
a b s t r a c t
PdS e CdS modi?ed by polyaniline (PANI)as a novel photocatalyst was synthesized via a sonochemical approach for the ?rst time.The as-prepared photocatalysts were charac-terized by transmission electron microscopy (TEM),scanning electron microscopy (SEM),X-ray diffraction (XRD),UV e vis absorption spectra,FT-IR spectra,and ?uorescence spectra (PL).Under visible light,the photocatalysts showed excellent photocatalytic activity for hydrogen evolution in the absence of noble metal Pt and the antiphotocorrosion perfor-mance of the composite photocatalyst was also found to be signi?cantly improved simultaneously compared to PdS e CdS without PANI modi?cation.The highest rate for H 2evolution was 3.32mmol/h with the concentration of 1wt.%PANI and 1wt.%PdS.Copyright a2011,Hydrogen Energy Publications,LLC.Published by Elsevier Ltd.All rights
reserved.
1.Introduction
Energy is the material basis of human survival,developing new energy sources to replace traditional ones becomes increasingly important with the energy depletion.Hydrogen,as a kind of renewable energy,has attracted more and more attention due to its pollution-free and ef?ciency and other advantages [1e 3].Therefore,how to make full use of solar energy to produce hydrogen becomes a hot and focal point for researchers and it is regarded as one of the “dream”techniques [4].Among all the preparation methods of hydrogen,photocatalytic water split-ting using semiconductors and solar energy is the most ideal way from the energy and environment point of view [5].
In 1972,Fujishima and Honda et al.[6]successfully split water into hydrogen by photoelectrolysis using TiO 2as elec-trode.Since then,semiconductors are widely used as catalyst for photocatalytic hydrogen production.So far,TiO 2is the most studied photocatalyst,but it can split water only under
UV light irradiation due to its large band gap (w 3.2eV)[7].In order to utilize the visible light (43%of solar spectrum),many other semiconductor materials,including oxides [8e 11],oxy-nitrides [12e 16]and oxysul?des [17e 19]have been developed.It’s reported that the most suitable semiconductor band gap should be around 2.0e 2.2eV [20].CdS is one of the most ef?-cient visible-light-driven photocatalysts because its band gap is narrow (2.4eV)and its conduction band edge is more negative than the H 2O/H 2electrode potential [21].But in the photocatalytic process of CdS,the phenomenon of photo-corrosion is prone to occur,which can badly destroy its pho-toactivity.Enormous efforts have been devoted to increase photoactivity and suppress photocorrosion of CdS,such as combining CdS with other semiconductors,or embedding CdS particles in mesoporous materials [22]or polymer matrix to form hybrid photocatalysts,or exploiting alternative prepa-ration approaches [23]as has also been successfully employed in our previous report [24,25].
*Corresponding author .Tel.:t862982663895;fax:t862982669033.
E-mail addresses:qychen@https://www.doczj.com/doc/fc8686464.html, (Q.Chen),lj-guo@https://www.doczj.com/doc/fc8686464.html, (L.
Guo).
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
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 37(2012)791e 796
0360-3199/$e see front matter Copyright a2011,Hydrogen Energy Publications,LLC.Published by Elsevier Ltd.All rights reserved.doi:10.1016/j.ijhydene.2011.04.060
Recently,conducting polymers arouse a great interest among researchers because of their curious electronic,magnetic and optical properties [26].As a typical conducting polymer,polyaniline (PANI)has good processibility,environ-mental stability and photoelectric property,and it is cheaper than other conducting polymers [27].Considering these good properties of PANI,it can be used as photosensitizer to modify photocatalyst to enhance the photoactivity [28].There have been several photocatalysts modi?ed by PANI reported,such as PANI/TiO 2[27],PANI/BiVO 4[28],PANI/SnO 2[29],and PANI/MoO 3[30].Zhang et al.[23]hybridized CdS photocatalysts by monolayer polyaniline and found that the photocatalytic activity and antiphotocorrosion performance were enhanced under visible-light irradiation.In view of practical application,reducing the cost and enhancing the stability of the photo-catalyst are the key issues for the technique to be commer-cially available.Stable photocatalyst free of noble metal cocatalyst or with less amount of noble metal is therefore highly desired.So it is of great value to use PANI to modify photocatalyst to improve the photocatalytic activity and suppress antiphotocorrosion performance.
In this work,nanostructured CdS were synthesized by hydro-thermal method,then PdS were successfully doped into CdS by coprecipitation method.At last the PdS e CdS photocatalysts were hybridized for the ?rst time by polyani-line.The as-prepared photocatalyst was found to be both ef?cient and stable for photocatalytic hydrogen production under visible light in the absence of Pt.The effect of PdS and polyaniline on the photoactivity and antiphotocorrosion of
CdS were studied for hydrogen evolution under visible light from water with Na 2S and Na 2SO 3as sacri?cial reagents.
2.
Experimental
2.1.
Preparation of the catalysts
All chemicals were of analytical grade and were used without further puri?cation.PdS e CdS nanocrystals were prepared as described in Ref.[31].PdS e CdS photocatalysts were hybrid-ized by polyaniline as follows:PANI were dissolved in THF to obtain a concentration of 0.02g/L solution,then a certain amount of PdS e CdS powder was added into 100ml of the above solution.The suspension was ultrasonicated for 30min,stirred for 10h,and then ?ltered.The as-produced precipitate was washed with deionized water for three times,then transferred to an oven to dry at 70 C for 12h.Thus,a series of PANI/PdS e CdS photocatalysts from 0.05to 2.6wt.%were synthesized.
2.2.Characterization
The X-ray diffraction (XRD)patterns of catalysts were obtained from a Panalytic X’pert Pro X-ray diffractometer
a
R a t e o f H 2 e v o l u t i o n (m m o l /h )
Content of PdS on CdS (wt%)R a t e o f H 2 e v o l u t i o n (m m o l /h )
Content of PANI on 1 wt% PdS-CdS(wt%)
b Fig.1e Rate of hydrogen evolution
for:(a)CdS Photocatalysts with different content of PdS;(b)PdS e CdS Photocatalysts with different content of PANI.
0A m o u n t o f H 2 (m m o l )
Time (h)
Fig.2e Reaction time courses of H 2evolution under visible irradiation.
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equipped with CuKa irradiation.The UV e vis absorption spectra were measured by a HITACHI UV4100instrument,with the scanning range from 300to 800nm.The FT-IR spectra were carried out on a BRUKER VERTEX70instrument with KBr as the reference sample.The analysis of photoluminescence spectra (PL)was measured at room temperature using a PTI QM-4?uorescence spectrophotometer.The crystallite morphologic micrograph was observed on a high resolution transmission electron microscopy (HRTEM)JEOL JEM-3010instrument and a ?eld emission scanning electron micros-copy (SEM)JSM-6700F (Japan).
2.3.Photocatalytic experiments
Photocatalytic reaction was carried out in a side-irradiation Pyrex cell.The effective irradiation area for the cell is 12.56cm 2.The powder of photocatalyst (0.2g)was dispersed by a magnetic stirrer in an aqueous solution (200ml)con-sisting of Na 2SO 3(0.5M)and Na 2S (0.5M)as electron donors in the cell.The photocatalysts were irradiated with visible light through a cutoff ?lter (l >430nm,T ?65%)from a 300W Xe-lamp.The amount of H 2gas was determined by an online thermal conductivity detector (TCD)gas chromatograph (NaX zeolite column,nitrogen as a carrier gas).
The solar-hydrogen energy conversion ef?ciency was calculated by the following equation.
h ?
Chemical energy stored in evolved H 2
Energy of incident solar light
(1)
3.Results and discussion
Fig.1shows the hydrogen evolution rates of various PdS e CdS and PANI/PdS e CdS photocatalysts under visible-light irradi-ation.It can be learned that PdS can signi?cantly improve the photoactivity of CdS.The content of PdS had important in?uence on the photoactivity of PdS e CdS photocatalyst,and 1wt.%PdS e CdS showed the highest activity of hydrogen production.The rate of H 2evolution of 1%PdS e CdS photo-catalyst can reach 2.98mmol/h,which is 124.3times of pure CdS (0.02mmol/h).Besides,we can ?nd that PANI can only slightly improve the photoactivity of PdS e CdS.And 1wt.%PANI/1%PdS e CdS showed the best activity of hydrogen production,its rate of H 2evolution can reach 3.32mmol/h (138.4times of pure CdS,and 1.1times of PdS e CdS).The energy conversion ef?ciency of the as-prepared photo-catalysts under visible light was calculated by Eq.(1);the results were shown in Table 1.
Fig.2reveals the antiphotocorrosion performance of as-prepared composite photocatalysts.CdS alone can act stably as photocatalyst for less than 10h;with the addition of 1wt.%PdS,the PdS/CdS composite showed much improved stability.
A b s (a .u .)
Wavelength (nm)
400
500
600
700
800
0.0
0.20.40.60.8
1.06A b s (a .u .)
Wavelength (nm)
1
4
a
b
Fig.3e UV e visible
absorption spectra of photocatalysts.
P L I n t e n s i t y (a .u .)
Wavelength (nm)
Fig.4e PL spectra of as-prepared photocatalysts excitation at 420nm.
0102030405060708090
I n t e n s i t y (a .u .)
2Theta (Degree)
111
220
311
CdS
1%PdS-CdS
1%PANI-1%PdS-CdS
5001000
150020002500300035004000
Fig.5e XRD patterns of CdS,1%PdS e CdS,and 1%PANI e 1%PdS e CdS composite powders.
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However the activity of the photocatalyst obviously decreased after reaction for 30h.It is noted that PANI/PdS e CdS can exhibit signi?cantly improved stability,showing steady hydrogen production more than 40h.Our result indicated that although PANI can only exert slight effect on the activity of the photocatalyst,it is indeed bene?cial for enhancing the stability of the photocatalyst.The mechanism for this modi-?cation effects is worth study in depth.As shown in our following result,no notable changes were found for the XRD image and FT-IR spectra of PANI/PdS e CdS composite after reaction (support information).So PANI/PdS e CdS is a kind of stable and ef?cient photocatalyst.
To analyze the reason,Fig.3shows the UV e vis absorption spectra for pure CdS,PdS e CdS and PANI/PdS e CdS samples.It can be seen that the curve shapes of the composite powders were alike to CdS curve shape.With the increase of PdS concentration,the absorption edges of these PdS e CdS powders were slightly shifted to longer wavelength while the visible absorption of which was enhanced.It’s reported that PdS could not absorb visible light [32],so we can conclude that the PdS phase can enhance the absorption of CdS phase.For 1%PdS e CdS photocatalysts,the spectra of the PANI/PdS e CdS samples showed an enhanced visible absorption but the absorption edge did not change.Also,the absorption band at wavelength about 625nm was comparable with neat PANI [33].As known,PANI as a conducting polymer with an extended e conjugated electron system has high absorption coef?cients in the visible-light range and high mobility of charge carriers.The UV e vis result suggested the PdS e CdS was hybridized by PANI but its band gap energy was not in?uenced by PANI.
PL emission has been widely used to investigate the ef?-ciency of charge carrier trapping,migration and transfer and to understand the recombination rate of electron/hole pairs in semiconductor particles [34].PL spectra of the as-prepared composite powders were recorded and shown in Fig.4,the pure CdS composite powders exhibited broad ?uorescence peaks related to Stoke’s shift around 550nm and 750nm [35].The band around 550nm is due to intrinsic emission,whereas that at 750nm should originate from transition of electrons trapped at surface states to the valence band of CdS [22].It can be seen from the ?gure that the PL peak intensity around 750nm of CdS is weakened than that of PdS e CdS and PANI/PdS e CdS powders.This change suggested that the recombi-nation of photoelectrons and holes was ef?ciently sup-pressed,resulting in the decrease of ?uorescence intensity [35].Besides,the intensity of the peak around 550nm for the 1%PANI/1%PdS e CdS is the weakest.It can be concluded that loaded with PdS and PANI,the PL peak intensity of CdS would decrease a lot.The weaker the PL peak intensity of the pho-tocatalyst is,the higher the photoactivity is.Similar condition has also been revealed by Zhang Kai et al.[35]this phenom-enon might be bene?cial for the stability of composite.So it suggested that the stable and ef?cient photocatalysts of PdS e CdS modi?ed by PANI is due to the high separation and transferring ef?ciency of electron and hole.
Fig.5shows the XRD patterns of CdS,PdS e CdS,and PANI/PdS e CdS powders.It can be seen from the ?gures that the photocatalysts were well crystallized,and the diffraction peaks at 2q values of 27.0 ,44.0 and 52.5 match perfectly with the (111),(220)and (311)lattice planes of cubic (b )phase CdS (JCPDS ?le No.10-454).No peaks assigned to PdS and PANI were observed because the amount of PdS was too little and the PANI layer was too thin.The XRD patterns also show that no obvious changes occurred with adding PdS and PANI,which in turn suggest that PdS and PANI form the photo-catalysts but don’t change the phase of CdS.
Morphology of as-prepared photocatalysts is shown in Fig.6.From Fig.6(a),we con?rmed that the photocatalysts were nanostructured.As seen from Fig.6(b),the nanoparticles were well crystallized.But in this case,the PANI layer was very thin to be observed clearly.According to Zhang et al.[36]who once reported that the PANI molecule was dispersed on the surface of CdS with a monolayer structure.As discussed above,PANI has effect on the photophysical properties.It may conclude that PANI was adsorbed evenly and stably on the surface of CdS and PdS.
PdS by itself displays no photoactivity for H 2production,but PdS can act as an oxidation cocatalyst together with CdS.PdS can not only enhance the photoactivity of CdS but also protect CdS from photocorrosion [31].PANI possesses p elec-tron conjugated system,which is helpful to the separation of photogenerated charge,so it can improve the photoactivity of CdS [28].PANI can form a layer of Conductive network on the surface of PdS e CdS,which can promote the generation of photogenerated electron (e à)and holes (h t),accelerate
the
Fig.6e (a)High-magni?cation SEM image of PANI/PdS e CdS;(b)HRTEM image of PANI/PdS e CdS.
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migration of photogenerated hole from VB of CdS to PdS,and prevent the recombination of them.Thus PANI is favorable to suppressing the photocorrosion of CdS and the pollution from other heavy metallic salt to CdS as well.So PANI has impor-tant meaning for the construction of the development of new, stable and nontoxic sulphide photocatalysts.
4.Conclusions
In conclusion,the PANI/PdS e CdS composite photocatalyst was synthesized for the?rst time to produce H2.When the concentration of PdS and PANI was1%by weight,the photo-catalyst displayed the highest photoactivity for hydrogen production under visible-light irradiation.The reaction time and stability of the composite photocatalyst was enhanced a lot.Through this research we found that PdS and PANI both have great in?uence on the photocatalytic activity of CdS. Furthermore,we concluded that PANI was very ideal to be used to modify sulphide photocatalysts,and may be also useful to modify other kinds of photocatalysts.
Acknowledgments
This work was?nancially supported by the National Basic Research Program of China(No.2009CB220000),the National Natural Science foundation of China(Contracted No. 50821064)and the Fundamental Research Funds for the central Universities.
Appendix.Supplementary material
Supplementary data related to this article can be found online at doi:10.1016/j.ijhydene.2011.04.060
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