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Solid State Sciences 32 (2014) 8e12

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Solid State Sciences

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Synthesis of 3D hierarchical porous TiO2/InVO4 nanocompositeswith enhanced visible-light photocatalytic properties

Jianchao Shen a, Hui Yang a,b, Yu Feng a, Qifeng Cai a, Qianhong Shen a,b,*

a State Key Laboratory of Silicon Materials, Department of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, PR Chinab Zhejiang California International NanoSystems Institute, Zhejiang University, Hangzhou 310027, PR China

a r t i c l e i n f o

Article history:Received 13 February 2014Received in revised form15 March 2014Accepted 17 March 2014Available online 26 March 2014

Keywords:InVO4

TiO2

3D hierarchical structurePorosityNanojunctionVisible-light photocatalysis

* Corresponding author. State Key Laboratory of SiliMaterials Science & Engineering, Zhejiang University,Tel./fax: þ86 571 87953313.

E-mail address: s_qianhong@163.com (Q. Shen).

http://dx.doi.org/10.1016/j.solidstatesciences.2014.03.01293-2558/� 2014 Elsevier Masson SAS. All rights re

a b s t r a c t

Three-dimensional (3D) hierarchical porous TiO2/InVO4 nanocomposites were fabricated by loading TiO2

nanoparticles on the surface of porous InVO4 microspheres. X-ray diffractometer (XRD), scanning elec-tron microscopy (SEM), transmission electron microscopy (TEM), UVevis spectroscopy and photo-luminescence spectroscopy (PL) were adopted to analyze the structureeproperty relationship of samples.The results show that the surface of as-prepared TiO2/InVO4 nanocomposites are composed of uniformlyinterconnected bi-phase nanocrystals, forming a close interface between these two components, whichis favorable for the highly efficient interparticle electron transfer to achieve enhanced photocatalyticproperties. However, the adsorption ability is decreased due to the loading of TiO2 nanoparticles on thesurface of InVO4. Therefore, under the joint action of these factors, the TiO2/InVO4 nanocompositesachieve the best photocatalytic activity when the mole ratio of In:Ti reaches 4:1, and the visible-lightphotocatalytic activity is about as 3.3 times high as that of pure InVO4 without modification.

� 2014 Elsevier Masson SAS. All rights reserved.

1. Introduction

Semiconductor photocatalysis has attracted a wide attention forits outstanding performance on environmental purification andenergy production [1e3]. In recent decades, extensive studies havefocused on the TiO2 based systems in photocatalysis field since thediscovery of photocatalytic water splitting into H2 and O2 in 1972[2]. However, TiO2 has a band gap of about 3.2 eV (for anatasephase), meaning that only the UV light with a wavelength less than388 nm, which is only a small part (3e5%) of solar irradiation, canexcite electrons in the valence band of titanium dioxide. To extendthe wavelength response range to the visible region and therebyincreasing the quantum yield of TiO2, a lot of methods have beendeveloped, such as doping with foreign elements [4], and couplingwith organic photosensitizer dyes [5], noble metal [6] or othersemiconductors [7]. Among them, coupling TiO2 with other narrowgap semiconductors of matching band potentials can not onlyextend the absorption wavelength range to visible region, but alsoinhibit the recombination of photo-generated electronehole pairs,

con Materials, Department ofHangzhou 310027, PR China.

09served.

and thus has become a promising strategy for improving thevisible-light photocatalytic activity of TiO2 [8]. To date, many novelvisible-light-response photocatalysts have been developed andapplied to couple with TiO2.

InVO4, with band gap of about 2.0 eV, was found to be animportant fundamental semiconductor material [9,10], and hasreceived considerable attention due to its unique optical andelectrical properties in various fields, such as visible-light photo-catalyst [11,12], gas sensor [13], biomolecules detection [14] andlithium electrode [15], etc. Considering the matched band potentialbetween InVO4 and TiO2, InVO4 has been employed to combinewith TiO2, thereby achieving enhanced visible-light photocatalyticactivity by promoting the high effective migration of photo-generated electrons from the conduction band of InVO4 to that ofTiO2 [16e18]. However, the current most studied TiO2/InVO4

composite nanoparticles are difficult to be separated and recycledin actual application, although they exhibit better photocatalyticactivity than bulk composites due to the strong adsorption capacityand high quantum efficiency [17,18]. Furthermore, the compositeuniformity, which has an important influence on the photocatalyticperformance, is still a challenging problem for the nano-scalecombination of InVO4 and TiO2 [17]. Therefore, it is quite mean-ingful to develop an effective and robust method to prepare theuniform TiO2/InVO4 composites with high photocatalytic activityand easy recycling characteristics.

Fig. 1. XRD patterns of as-prepared samples.

J. Shen et al. / Solid State Sciences 32 (2014) 8e12 9

In our previous work [19], we have successfully synthesizedthree-dimensional (3D) hierarchical InVO4 porous microspheres,which exhibits well photocatalytic activity because of its highspecific surface area, more importantly, can be easily separatedand reused due to its relative large size. In this article, we report afacile hydrothermal method to fabricate TiO2/InVO4 nano-composites based on the in-situ growth of TiO2 nanoparticles on3D hierarchical InVO4 porous microspheres. Such structural ma-terial offers two advantages: i) TiO2 nanoparticles can be uni-formly loaded on the surface of InVO4, forming interconnected bi-phase TiO2/InVO4 nanocomposites with the close interface, whichleads to the highly efficient interparticle electron transfer and thusachieving improved photocatalytic properties [20]; ii) The 3D hi-erarchical porous microspheres can combine the advantages ofhigh surface area materials and large-scale materials, achievingboth high photocatalytic activity and easy recycling characteristics,thereby showing a promising potential in environmental protec-tion. To the best of our knowledge, this work is still lack of study,and such convenient strategy also provides a new idea for syn-thesizing nanocomposite materials with high compositeuniformity.

2. Experimental

2.1. Synthesis

All of the chemicals were analytical grade and used as receivedwithout further purification. 3D hierarchical structure InVO4porous microspheres were synthesized according to our previousreport with some modifications [19]. In a typical synthesis, 8 mmolof NH4VO3 was dissolved in 60 mL of deionized water at 60 �C, andthen the NH4VO3 solution was slowly added to 40 mL of InCl3aqueous solution with concentration about 0.1 M under vigorousstirring. The pH of the mixture was adjusted to 4 with 1 M aqueousNH3$H2O. After stirred for another 1 h, the yellowish precipitatewas collected by centrifugation and washed with deionized waterthree times. Finally, the as-obtained precipitate was mixed with1 wt% CTAB aqueous solution under constant magnetic stirring for1 h. The obtained solution was transferred to a Teflon-linedstainless steel autoclave of 50 mL capacity and maintained at180 �C for 24 h. After cooling to room temperature naturally, theprecipitate was collected, washed with deionized water and ab-solute ethanol several times, and then dried in air at 60 �C for 4 h.Finally, the product was heat treated at 350 �C for 2 h in air, andthen milled into powder, obtaining InVO4 porous microspheres forfurther use.

TiO2/InVO4 nanocomposites were obtained via the in-situgrowth of TiO2 nanoparticles on the surface of InVO4 porous mi-crospheres. First, 0.2 g of as-prepared InVO4 porous microsphereswere dispersed in 20 mL of ethanol and then 10 mL of ethanolsolution of tetrabutyl titanate (TBT) was slowly dropped into thedispersion. After stirred magnetically for 18 h, 10 mL of deionizedwater was added into the solution, followed by water bath treatedat 80 �C for another 4 h under stirring. The obtained solution wasthen transferred to a Teflon-lined stainless steel autoclave of 50 mLcapacity and maintained at 150 �C for 20 h. After cooling to roomtemperature naturally, the precipitate was collected, washed withdeionized water and absolute ethanol several times, and then driedin air at 60 �C for 12 h. According to this method, a series of TiO2/InVO4 nanocomposites were preparedwith differentmolar ratios ofIn:Ti. For clear presentation, the as-obtained samples were denotedas IT-2, IT-4, IT-8 and IT-16 when the molar ratio of In:Ti was 2:1,4:1, 8:1 and 16:1, respectively. In addition, the pure InVO4 (denotedas IneH) was also prepared under the same conditions withoutadding TBT for reference.

2.2. Characterization

The phase compositions and crystal structures of as-synthesizedsamples were determined by an X-ray powder diffractometer (XRD,APEXII, Bruker, Germany) using Ni-filtered Cu Ka radiation(l ¼ 1.542 �A) at 40 kV and 40 mA, and a scan rate of 3� min�1 wasapplied to record the patterns in the range of 2q¼ 10�e80� at a stepof 0.02�. Morphology and structure were investigated using atransmission electron microscopy (TEM, TECNAI-10, Hitachi, Japan)and field-emission scanning electron microscope (FE-SEM, SU-70,Hitachi, Japan). Photoluminescence (PL) spectra were recorded ona fluorescence lifetime and steady state spectrometer (FLS920,Edinburgh, UK) with excitation wavelength of 325 nm at roomtemperature.

2.3. Photocatalytic activity

Rhodamine B (RhB) was used as target substrate for evaluatingthe photocatalytic activity of as-synthesized samples under visible-light irradiation. The visible-light was obtained by a 35 W CDM-TPhilips metal halide lamp with a 420 nm UV-cutoff filter. In eachexperiment, 50 mg of photocatalyst powder was suspended into100 mL RhB solution with a concentration of 10 mg/L. Prior toirradiation, the suspension was magnetically stirred in the dark for2 h to reach adsorptionedesorption equilibrium between the RhBand the photocatalyst. During the photocatalytic reaction, 4 mL ofmixture was collected at irradiation time intervals of every 30 minand centrifuged (9000 rpm, 5 min) to remove solid particles. Theresulting supernatants were analyzed by recording variations in theabsorption band at 554 nm in the UVevis spectra of RhB using aUVevis spectrophotometer (Lambda20, PerkinElmer, USA).

3. Results and discussion

Fig. 1 presents the XRD patterns of as-prepared samples. Thediffraction peaks of pure InVO4 microspheres (IneH) can beindexed to the pure orthorhombic phase of InVO4 according to theJCPDS card no. 71-1689. While for the TiO2/InVO4 nanocomposites,the intensity of characteristic diffraction peaks of TiO2 enhanceswith the increase of TBT dosage. IT-2 sample shows a typical XRDpattern, of which the diffraction peaks correspond to both InVO4and anatase TiO2, and no other impurities can be observed, sug-gesting the bi-phase composition of InVO4 and TiO2 in thesenanocomposites.

J. Shen et al. / Solid State Sciences 32 (2014) 8e1210

Fig. 2 shows the typical field-emission scanning electron mi-croscopy (FE-SEM) images of as-prepared samples. It can be seenthat the IneH sample with diameter between 2 and 5 mm displays3D hierarchical structure composed of interconnected nano-particles and pores (Fig. 2a).When introducing TBTas the precursorof TiO2, TBT may be firstly adsorbed on the InVO4 porous micro-spheres via capillary action, thereby inducing the in-situ formationof nuclei on the InVO4 porous microspheres, and subsequentlygrowing into nanocrystals during the hydrothermal process. Withthe increase of TBT dosage, the surface of TiO2/InVO4 compositesbecomes more compact, and the porosity gradually decreases(Fig. 2bee). In addition, owing to the in-situ growth of TiO2, it canbe seen from the insets of Fig. 2bed that the TiO2 nanoparticles aremainly loaded on the InVO4 porous microspheres, and there are

Fig. 2. FE-SEM images of as-prepared samples: IneH (a), IT-16 (b), IT-8 (c), IT-4 (d), IT-2 (e).EDS mapping data obtained from the IT-4 sample (f).

hardly any TiO2 nanoparticle aggregates around InVO4 porous mi-crospheres. The SEMeEDSmapping of IT-4 sample further confirmsthat the TiO2 nanoparticles are uniformly loaded on the surface ofInVO4 porousmicrospheres via in-situ growth process (As shown inFig. 2f). However, the excessive TBTwill lead to the fast formation ofTiO2 crystals by homogeneous nucleation and growth in the liquidphase, thus some TiO2 aggregates can be obviously observed in theinset of Fig. 2e.

The morphology and microstructure of TiO2/InVO4 compositeswere further investigated by TEM. Fig. 3a presents a typical TEMimage of several microspheres of the IT-4 sample, and Fig. 3b is themagnified TEM image taken from an individual microsphere(marked in Fig. 3a). Fig. 3c shows the high-resolution TEM image,which is taken from the fringe of a microsphere marked by a small

The insets are the corresponding images of low magnification. FE-SEM image and SEM-

Fig. 3. TEM (a, b) and HRTEM (c) images of IT-4 sample.

Fig. 4. PL emission spectra of as-prepared samples.

Fig. 5. Photocatalysis process analysis of as-prepared samples. (a) Photocatalytic degradatioirradiation.

Fig. 6. UVevis spectra of RhB after absorption by the IneH, IT-4 and IT-2 sample in thedark for 2 h.

J. Shen et al. / Solid State Sciences 32 (2014) 8e12 11

blank rectangle in Fig. 3b. It can be clearly found that there existsdifferent kinds of lattice fringes, of which the lattice interplanarspacing of d ¼ 0.353 nm matches the (101) crystallographic planesof anatase TiO2 [21], while the fringes of d ¼ 0.389 nm correspondto the (111) plane of orthorhombic InVO4 [22], respectively. Basedon the above results, it is suggested that these two componentshave formed a close interface by the means of in-situ growth ofTiO2, which is favorable for the highly efficient interparticle elec-tron transfer [16,20].

Fig. 4 illustrates PL emission spectra of as-prepared samplesmonitored at an excitation wavelength of 325 nm. The observed PLpeak is attributed to the radiative recombination process of eitherself-trapped excitons, and thus the decrease of PL intensity in-dicates a better charge separation [23]. After combining InVO4 withTiO2, the fluorescence intensity of as-prepared samples are signif-icantly reduced, especially when the molar ratio of In:Ti reaches8:1, meaning that the recombination of photo-generated chargecarriers is greatly inhibited due to the efficient electron transferbetween InVO4 and TiO2. This result shows good agreement withother heterojunction semiconductors [24].

The photocatalytic performance of as-prepared samples areevaluated by Rhodamine B (RhB) photocatalytic degradation undervisible-light (l > 420 nm) irradiation, as shown in Fig. 5. It is wellknown that, as a typical organic contaminant, RhB is stable undervisible-light irradiation if there is no photocatalyst involved, whichis also observed in our experiment. After 3 h of visible-light irra-diation, the RhB degradation rate over IneH sample is merely20.3%. The relatively poor photocatalytic performance of pure

n and (b) corresponding kinetics of RhB over as-prepared samples under visible-light

Fig. 7. Illustration of electronehole separation process in the TiO2/InVO4 nano-composite under visible-light irradiation.

J. Shen et al. / Solid State Sciences 32 (2014) 8e1212

InVO4 microspheres may due to the rapid and easy recombinationof photo-generated charge carriers, although they exhibit goodporosity. Interestingly, the coupling of InVO4 with TiO2 results in anoteworthy increase of RhB degradation, especially when themolarratio of In:Ti is 4:1, which leads to the highest degradation rate of55.3%. It should also be noted that the IT-2 sample shows aremarkable decrease in photocatalytic performance. The reasonmay lie in the large area of pore blocking caused by excessiveamount of TiO2 particles on the surface of InVO4 microspheres,which significantly affects the adsorption of RhB molecule to themicrospheres, and thus decreasing photocatalytic activity (seeFig. 6).

To quantitatively understand the reaction kinetics of the RhBdegradation in our experiments, we analyzed the degradation datawith the pseudo-first-order model as expressed by the equation ofln(C0/C)¼ kt, where C0 and C are the RhB concentrations in solutionat times 0 and t, respectively, and k is the apparent first-order rateconstant. The results of Fig. 5b show that the IT-4 sample exhibitsan excellent photocatalytic degradation rate of 0.271 h�1, which isabout as 3.3 times high as that of IneH sample, indicating that thevisible-light photocatalytic activities of InVO4 are significantlyenhanced after the formation of the high efficient 3D hierarchicalporous TiO2/InVO4 nanocomposites.

Based on the characterization of structure and properties, theefficient photocatalytic activity should be mainly ascribed to theenhanced charge separation efficiency of TiO2/InVO4 nano-composites, as they have a highly close combination of interfacewith the well-matched band potentials [25,26] (see Fig. 7). The CBedge potential of InVO4 is more negative than that of TiO2 [16,27],hence, photo-generated electrons on the InVO4 surface wouldeasily transfer to TiO2, leaving holes on the InVO4 valence band. Inthis way, the photo-generated electronehole pairs could be effec-tively separated and a highly efficient visible-light photocatalystwith enhanced quantum efficiency can be obtained.

4. Conclusions

In summary, 3D hierarchical porous InVO4 microspheres withTiO2 nanoparticles uniformly loaded on the surface were success-fully synthesized by a facile hydrothermal method. The resultingTiO2/InVO4 nanocomposites show high efficiency of degradingRhodamine B dye under visible-light irradiation due to the efficientcharge separation between InVO4 and TiO2 nanoparticle. The molarratio of In:Ti is quite important to affect the photocatalytic activityof TiO2/InVO4 photocatalyst. When the molar ratio of In:Ti is 4:1,the TiO2/InVO4 sample exhibits an excellent photocatalytic degra-dation rate of 0.271 h�1, which is about as 3.3 times high as that ofpure InVO4 microspheres. Furthermore, these micron size powderscan be easily separated and reused, thereby showing a promisingpotential in environmental protection.

Acknowledgments

This work was supported by Zhejiang Provincial Natural ScienceFoundation of China under Grant No. LQ12E02008, Key Technolo-gies R&D Program of China under Grant No. 2013BAJ10B05 andFundamental Research Funds for the Central Universities underGrant No. ZJUR011104.

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