Chem Letter-2008-SnO2 and ZnO

SnO 2and ZnO Nanostructured Spheres Self-assembled by Nanocrystals:Microwave-assisted Preparation and Enhancement of Photocatalytic Activity

Ling Zhang,Ying-Jie Zhu,?and Shao-Wen Cao

State Key Laboratory of High Performance Ceramics and Super?ne Microstructure,Shanghai Institute of Ceramics,

Chinese Academy of Sciences,Shanghai 200050,P.R.China

(Received June 20,2008;CL-080624;E-mail:y.j.zhu@https://www.wendangku.net/doc/9a10673312.html,)

We have developed a simple microwave-assisted solvother-mal method for the preparation of SnO 2and ZnO hierarchically nanostructured spheres self-assembled by nanocrystals.The photocatalytic activity is signi?cantly enhanced by simply mix-ing SnO 2and ZnO nanostructured spheres without any treatment or processing.

In the past three decades,the photocatalytic properties of semiconductor-based catalysts for the degradation of organic pollutants and production of clean hydrogen fuel from water have received considerable attention owing to energy and envi-ronmental concerns.In order to enhance the photocatalytic prop-erties,a number of strategies have been developed,for example,doping,surface modi?cation,and using composites.Doping with anions 1or cations 2–4in the semiconductor can change the energy states and narrow the band gap which enhances the absorption in the visible light region or change the crystalline structure and retard the recombination of the photoexcited charge carriers.Surface modi?cation with inorganic or organic species,using composites,core/shell structures and heterostruc-tures can also increase the surface active sites or trap the elec-trons and holes to prevent their recombination by the interaction of di?erent components in the composites,leading to the improvement of photocatalytic properties.5,6Another way to in-crease the photocatalytic property is to control the crystal size,crystallinity,crystalline phase,and morphology of the semicon-ductors.Controlling the size of the semiconductor allows for the tuning of the band gap,which determines the absorption edge of the semiconductor.In addition,the semiconductor nanocrystals have a higher speci?c surface area and provide more surface sites where free radicals can be generated by the reaction between the photogenerated charge carriers and adsorbed molecules,leading to a superior photocatalytic activity.

The complex hierarchical nanostructures with large surface areas are promising for new kinds of photocatalysts.7–9The hierarchical nanostructures are constructed by the assembly of nanocrystal building blocks (such as nanosheets,nanorods,and nanoparticles).Recently,we reported the preparation of homo-geneous nanocomposite ZnO–SnO 2hollow spheres and hier-archical nanosheets using ZnO rods,SnCl 4.

5H 2O,NaOH,and CTAB in aqueous solution by the hydrothermal method standing for 12h,and the ZnO–SnO 2nanocomposites showed a high pho-tocatalytic activity.5Herein,we report the preparation of single-phase SnO 2or ZnO hierarchically nanostructured spheres self-assembled by nanoparticles or nanorods by the microwave-assisted solvothermal method using mixed solvents of water and methanol at 160 C for a short period of time (30min).By simply mixing ZnO and SnO 2hierarchically nanostructured spheres without any further treatment,we obtained the photoca-

talyst with much higher photocatalytic activity compared with singe-phase ZnO or SnO 2.

In a typical procedure,0.351g of SnCl 4.

5H 2O or 0.219g of Zn(CH 3COO)2.

2H 2O,0.5g of cetyltrimethylammonium bro-mide (CTAB),and 1.2g of NaOH were dissolved in a mixture of 20mL of deionized water and 10mL of methanol.Then,2.75mL of ethyl acetate was added into the mixture under mag-netic stirring for 10min.The resultant mixture was heated at 160 C by microwave–solvothermal process (MDS-6,Sineo,Shanghai)for 30min.For the photocatalytic activity measure-ment,20mg of the catalyst (for the mixture,the total amount was still 20mg,and ZnO and SnO 2were added into the solution,respectively)was suspended in 40mL of aqueous Methyl Or-ange (MO)solution (20mg/L),and then the suspension was shaken for 30min in the absence of light.UV irradiation was car-ried out using a 300-W high-pressure Hg lamp (maximum 365nm).MO concentrations were analyzed using a UV–vis spectrophotometer (UV-2300,Techcomp)at its maximum adsorption wavelength of 464nm.

The XRD patterns show that the product was a single phase of SnO 2or a single phase of ZnO.Figure 1shows the TEM (JEOL JEM 2100F)micrographs of the SnO 2and ZnO samples.From Figure 1a,one can see that the SnO 2sample consisted of spheres with diameters of about 200–400nm.The high-magni?-cation TEM micrograph (inset of Figure 1a)shows that the SnO 2spheres were constructed by the self-assembly of small nanopar-ticles with diameters of several nanometers.SEM (JSM-6700F)micrograph (Figure 1b)shows that the ZnO sample consisted of spheres with diameters of about 1–2m m which were constructed by self-assembled nanorods.

The microwave–solvothermal method has been rapidly developed owing to rapid heating,faster kinetics,homogeneity,higher yield,better reproducibility,and energy saving compared with conventional solvothermal method.Moreover,the micro-wave–solvothermal method also enables chemical reactions to occur in an elevated-temperature and pressurized closed system similar to a solvothermal condition in a short preparation time

of

Figure 1.(a)TEM micrograph of as-prepared SnO 2nanostruc-tured spheres self-assembled by nanocrystals,(b)SEM micro-graph of as-prepared ZnO nanostructured spheres self-assembled by nanorods.

Copyright ó2008The Chemical Society of Japan

minutes rather than days.Here,we prepared single-phase SnO 2or ZnO hierarchically nanostructured spheres self-assembled by nanoparticles or nanorods by the microwave-assisted solvother-mal method at 160 C for a short period of time (30min).

The photocatalytic degradation of MO was investigated.Figure 2a shows the degradation of MO in the presence of SnO 2,ZnO nanostructured spheres and the mixture of both at a mass ratio of 15:5.About 56%of MO was degraded in 60min in the presence of ZnO or SnO 2nanostructured spheres.ZnO nanostructured spheres showed a better photocatalytic activity than SnO 2,while the di?erence in the photocatalytic activity narrowed with increasing time;the degradation rate of MO was almost the same over both samples when the time reached 60min.

A signi?cant enhancement in the degradation rate of MO was observed by just simple mixing of ZnO and SnO 2nanostruc-tured spheres (15mg SnO 2/5mg ZnO)at room temperature without any processing or treatment.Figure 2a shows the photo-catalytic activity for the photodegradation of MO over the mixture of SnO 2and ZnO nanostructured spheres,from which one can see that about 50%MO was degraded in 20min,90%in 30min and almost 100%in 40min,implying that the degrada-tion rate of MO over the mixture of ZnO and SnO 2nanostruc-tured spheres was much higher than that of single-phase ZnO or SnO 2nanostructured spheres.The photocatalytic activity can be greatly enhanced by simple mixing of the two compo-nents of ZnO and SnO 2nanostructured spheres at room temper-ature without any processing.Our experiments showed that the photocatalytic activity of the mixture increased with increasing quantity of SnO 2and that the optimal mass ratio of SnO 2to ZnO was 15:5.

The photocatalytic activities of the commercial ZnO,SnO 2,and their mixture were also investigated for comparison.The commercial ZnO sample consisted of irregularly shaped crystals with sizes of about 0.5–2m m ,and the commercial SnO 2sample consisted of irregularly shaped nanocrystals with an average diameter of about 100nm.As shown in Figure 2b,the two commercial samples showed lower photocatalytic activity in the degradation of MO compared with the nanostructured samples.In the presence of the commercial SnO 2,there was only 18%of MO degraded after 60min UV irradiation,while 56%of MO degraded in the presence of SnO 2nanostructured spheres.The commercial ZnO also showed lower photocatalytic activity compared with ZnO nanostructured spheres.The photocatalytic activity of the mixture of the commercial SnO 2and ZnO was slightly higher than single-phase commercial ZnO and much

higher than single-phase commercial SnO 2.The enhancement e?ect for the photocatalytic activity of the mixture of the com-mercial SnO 2and ZnO was much weaker than that for the mix-ture of the ZnO and SnO 2nanostructured spheres.

Previous work reported that SnO 2/ZnO composite exhibits a superior photocatalytic activity compared with single-phase SnO 2or ZnO.5,10,11The reason for the increase in the photocata-lytic activity was ascribed to the enhanced charge separation de-rived from the coupling of ZnO with SnO 2.5,10,11It is known that three factors contribute to the enhancement of the photocatalytic activity:modifying the band of the semiconductor,increasing the surface active sites and hindering the recombination of pho-togenerated electrons and holes.The room-temperature UV–vis di?use re?ectance spectra showed that the band gap of the mix-ture was very similar to that of the single-phase ZnO.Moreover,the surface active sites of the catalyst were not changed by the simple mixing of the two components.So the enhancement of photocatalytic activity of the mixture may mainly be attributed to the e?ective separation of the photoinduced electrons and holes.It has been reported that SnO 2is a good electron-trapping agent.9We propose that the photogenarated electrons on the con-duction band of ZnO can be transferred to the conduction band of SnO 2when they are in contact with each other in the solution,hindering the recombination of photogenerated electrons and holes.In addition,SnO 2and ZnO nanostructured spheres are fa-vorable for the transfer of electrons or holes generated inside of the crystal to the surface and facilitate the degradation of MO.

In summary,we have developed a simple microwave-assist-ed solvothermal method for the preparation of SnO 2or ZnO nanostructured spheres self-assembled by nanocrystals.The photocatalytic activity is signi?cantly enhanced by simply mixing SnO 2and ZnO nanostructured spheres self-assembled by nanocrystals compared with the single-phase ZnO or SnO 2.This result provides a new promising route for the design and preparation of new kinds of photocatalysts with high photocata-lytic activities,thus expanding their applications in the ?eld of photocatalysis.

Financial support from the National Natural Science Foundation of China (No.50772124,the Fund for Innovative Research Groups),the Program of Shanghai Subject Chief Scientist (No.07XD14031),CAS International Partnership Program for Innovative Research Team,and the fund of State Key Laboratory of High Performance Ceramics and Super?ne Microstructure is gratefully acknowledged.References

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Time / min

C /C 0

Time / min

Figure 2.The degradation of MO over di?erent photocatalysts:(a)as-prepared SnO 2,ZnO nanostructured spheres,and the mixture of both at a mass ratio of 15:5,(b)commercial SnO 2and ZnO,and the mixture of both at a mass ratio of 15:5.

Published on the web (Advance View)September 5,2008;doi:10.1246/cl.2008.1002