石墨烯氧化物薄膜電極的光電化學特性

張曉艷 孫明軒 孫鈺珺 李 靖 宋 鵬 孫 通 崔曉莉

(復旦大學材料科學系,上海200433)

石墨烯氧化物薄膜電極的光電化學特性

張曉艷 孫明軒 孫鈺珺 李 靖 宋 鵬 孫 通 崔曉莉*

(復旦大學材料科學系,上海200433)

采用浸漬-提拉法制備了一系列石墨烯氧化物(GO)薄膜,并通過X射線衍射(XRD),掃描電鏡(SEM),傅里葉變換紅外光譜,紫外-可見吸收光譜和光電化學測量等技術對樣品進行了表征.在GO電極上觀察到陰極光電流,且光電流密度受薄膜的厚度影響.GO薄膜電極厚度為27 nm時,光電流密度為0.25μA·cm-2.此外,GO電極的光電響應還受紫外光照影響,隨著紫外光照時間的延長,陰極光電流逐漸減小.該工作提供了簡便的通過控制薄膜厚度或紫外光照時間來控制GO薄膜半導體光電化學性能的方法.

石墨烯氧化物;光電化學性能;陰極光電流;膜厚;紫外光照

1 Introduction

Graphene has brought about a hot research field due to its unique transport properties,high conductivity,and good optical transparency.1-5It has been widely investigated in many areas such as sensors,6capacitors,7,8and catalysts.9Graphene oxide(GO),viewed as graphene with oxygen functional groups, has also triggered great research interests due to its promising application in large areas.10-15Previous reports have confirmed that GO has semiconductor properties.16-20Yeh et al.16studied the photocatalytic activity of GO,which exhibited photocata-lytic activity for hydrogen evolution in methanol aqueous solution.

It is fundamentally interesting to study the photoelectrochemical properties of semiconductor oxides for applications in photovoltaics and photocatalysis.Recently,the photoelectrochemical property of reduced GO based composites such as GO-TiO2has been investigated intensively.20-25However,there is little work focusing on studying the photoelectrochemical property of GO film itself as well as the influencing factors.In this work,we provide a complete picture of the photoelectrochemical performance of GO electrodes.A series of GO film electrodes were prepared by a dip-coating method,both the effects of film thickness and UV irradiation on the photoelectrochemical performance have been investigated.

2 Experimental

2.1 Preparation of GO thin film electrodes

GO was obtained from natural graphite(CP,99.85%,d≤30 μm,Sinopharm Group Chemical Reagent Co.,Ltd.)with Hummers method.26,27The resulting oxidized material was dispersed in water by ultrasonication.GO thin films were produced by a dip-coating process on freshly washed fluorine-doped tin oxide(FTO)substrates.In a typical process,FTO substrate was dip-coated in 1 mg·mL-1GO aqueous solution with a rate of 11 cm·min-1followed by drying under infrared lamp for 5 min,and then repeated for 5 times.The obtained GO film was noted as GO-5.Similarly,GO films dip-coated with 7 and 9 times were also prepared,noted as GO-7 and GO-9,respectively.

2.2 Characterizations

The X-ray diffraction(XRD)was recorded with Cu Kαradiation using a D8-Advance X-ray diffractometer(Bruker,Germany).The Fourier-transform infrared(FTIR)spectra experiments were performed by FTIR spectrometer(Nicolet Nexus 470).The light absorption properties were measured by UV-Vis absorption spectrometer(UV-2300,Tianmei,China). Field emission scanning electron microscopy(FE-SEM)was used to measure the film thickness in a vertical angle mode (S-4800,Hitachi,Japan).

2.3 Photoelectrochemical characterizations

The photoelectrochemical measurements were performed in a conventional three-electrode system on a CHI 660 electrochemical workstation(Chenhua Instruments,China)in 0.5 mol·L-1Na2SO4electrolyte in a cell with a quartz window with Pt sheet as counter electrode and saturated calomel electrode (SCE)as reference electrode.The specimens were sealed with epoxy to expose an active area of 0.5 cm×0.5 cm.A 500 W Xe-lamp was used to provide the UV-Vis light with the intensity as 200 mW·cm-2.All the potentials were reported versus SCE.

3 Results and discussion

Fig.1 (a)Typical XRD patterns and(b)FTIR spectra of the as-prepared GO before and after UV irradiation, (c)absorption spectra of GO films,and(d)the corresponding plots of(αE)2against the photon energy(E)

Fig.1(a)shows the typical XRD patterns of GO powders. The strong peak at 11.8°is ascribed to(001)facet of GO.A very weak and broad peak at around 23°is observed which is attributed to graphite,suggesting an incomplete oxidation of graphite into GO.Fig.1(b)shows the FTIR spectra of GO powders,which were obtained from GO aqueous solution and that after 6 h of UV irradiation,respectively.As shown in Fig.1(b), GO exhibits the following characteristic features:a weak band at 1721 cm-1ascribed to C＝O(carboxylic acid)stretching vibration,a strong peak at 1622 cm-1for water H―O―H bending,a small peak at 1220 cm-1attributed to phenolic C―OH stretching,a strong band at 1070 cm-1corresponding to C―O or phenyl hydroxyl stretching vibration,and a weak shoulder band at 980 cm-1attributed to epoxy stretching.These results confirm the presence of a large amount of oxygen-containing groups on GO planes.17,28For GO after UV irradiation,the intensity of peaks ascribed to C＝O and alkyl C―OH stretching vibrations and H―O―H bending decreases obviously,suggesting that UV irradiation of GO can induce the elimination of oxygen-containing groups on GO.It has been realized that GO can be reduced under UV light irradiation.17,29

Fig.2 Typical FE-SEM images of the cross sections of the GO films

Fig.1(c)displays the UV-Vis light absorption of the obtained GO films.A broad absorption peak in the visible light region of 450-650 nm is observed for GO films with different film thicknesses.The intensity of the absorption peak in the visible light region increases from GO-5 to GO-9,suggesting that the light absorption ability can be enhanced by increasing the film thickness.The optical band gap of GO films is evaluated to be about 3.06 eV from the Tauc plot30as shown in Fig.1(d)using the relation(E-Eg)∝(αE)2,where α is the absorption coefficient,E is the photon energy,and Egis the optical band gap. The results demonstrate that the film thickness has little effect on the optical band gap of GO films.

The morphology and film thickness of the obtained GO films were investigated by field emission scanning electron microscopy(FE-SEM).As shown in Fig.2,GO nano-films can be observed for all the three samples.The average thickness of the films is measured to be about 9 nm for GO-5 film and 27 nm for GO-9 film.The changes of the film thickness are in consistency with the UV-Vis light absorption in Fig.1(c),which increases in the whole spectrum with the film thickness.

Fig.3(A)depicts the photoelectrochemical performance of the as-prepared GO electrodes with different film thicknesses at 0 V versus SCE.A cathodic photocurrent is observed and the value increases with increasing the film thickness.GO-5 film electrode in Fig.3(A-a)with an average film thickness of 9 nm exhibits a photocurrent density of 0.10μA·cm-2.The photocurrent density increases to 0.16 and 0.25μA·cm-2for GO-7 and GO-9 in Fig.3(A-b)and(A-c),respectively,which is in line with their UV-Vis light absorption property.This fact suggests that the photocurrent density increases with the film thickness because more light can be absorbed with thicker film.Further experimental results demonstrate that the photoelectrochemical performance of GO electrode will decrease with thicker film thickness.It should be reasonable to observe cathodic photocurrent for GO film electrode since GO is p-type semiconductor.When GO was under light illumination,the photo generated holes tended to be swept into GO layer and the photogenerated electrons tended to be driven towards the interface with electrolyte,31generating cathodic photocurrent.The photogenerated electrons were captured by adsorbed H2O on electrode surface,producing hydrogen.17

The photoelectrochemical performance of GO electrodes was also investigated at different applied potentials.As shown in Fig.3(B),the photocurrent density increases when cathodic bias was applied,which is also a characteristic of p-type semiconductors.32When the electrode was applied anodic bias,the cathodic photocurrent is converted into anodic photocurrent.It can be observed that the cathodic photocurrent density first decreases to zero and then the newly generated anodic photocurrent increases along with the applied anodic bias increasing from 0.02 to 0.10 V due to the existence of an intrinsic filed.

Fig.4 shows the influence of UV irradiation on the photocurrent density for GO films under chopped UV-Vis light illumination.Obviously,the cathodic photocurrent decreases with extending the UV irradiation time of GO films as shown in Fig.4 (A,B).It can be observed that the photoresponse decreases from 0.16 and 0.25μA·cm-2to 0.04 and 0.05μA·cm-2for GO-7 and GO-9 film electrodes,respectively,as shown in Fig.4 (C).Photocurrent density response dependence on UV irradiation is observed for the GO film electrodes.Combined with FTIR results,the influence of UV irradiation on the photoelectrochemical property of GO film electrodes can be attributed to the change of amounts of oxygen-containing groups on GO. The results confirm that UV irradiation takes a great effect on the photoelectrochemical performance of GO film electrodes and the photoresponse of GO films can be controlled by UV irradiation.

Interestingly,anodic and cathodic sharp spikes appeared as light was switched on and off for GO film electrodes after UV irradiation,especially when with long time of UV irradiation. The generation of the spikes is still unclear and further investigations are underway.

4 Conclusions

In this work,we investigated the photoelectrochemical performance of GO film electrodes with different thicknesses.The results show that both the film thickness and UV irradiation can significantly affect the photoelectrochemical performance of GO film electrodes.The results demonstrate that the photoresponse of GO film electrodes can be increased by increasing the film thickness and decreased by UV irradiation.A possible way to fabricate graphene oxide film with tunable photoelectrochemical performance has been demonstrated through only changing film thickness or UV irradiation time.

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Photoelectrochemical Properties of Graphene Oxide Thin Film Electrodes

ZHANG Xiao-Yan SUN Ming-Xuan SUN Yu-Jun LI Jing SONG Peng SUN Tong CUI Xiao-Li*
(Department of Materials Science,Fudan University,Shanghai 200433,P.R.China)

A series of graphene oxide(GO)thin films were prepared by a dip-coating method and characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),Fourier-transform infrared (FTIR)spectroscopy,ultraviolet-visible(UV-Vis)light absorption,and photoelectrochemical measurements. A cathodic photocurrent was observed for the GO electrodes and the photocurrent density was influenced by the thickness of the films.The GO film electrode with an average thickness of 27 nm gave a photocurrent density of 0.25μA·cm-2.The photoresponse of the GO electrodes was found to be influenced by UV irradiation and the cathodic photocurrent decreased gradually with UV irradiation time.This work provides a simple method to change the photoelectrochemical property of GO films by controlling the film thickness or UV irradiation time.

Graphene oxide;Photoelectrochemical property;Cathodic photocurrent; Film thickness;UV irradiation

10.3866/PKU.WHXB20112831

?Corresponding author.Email:xiaolicui@fudan.edu.cn;Tel:+86-21-65642397;Fax:+86-21-65642682.

The project was supported by the National Key Basic Research Program of China(973)(2011CB933302,2010CB933703),Shanghai Science and Technology Commission,China(1052nm01800),and Key Disciplines Innovative Personnel Training Plan of Fudan University,China.

國家重點基礎研究發展規劃項目(973)(2011CB933302,2010CB933703),上海市科學技術委員會納米項目(1052nm01800)及復旦大學研究生創新基金資助

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