WO3TiO2 composite with morphology change via hydrothermal template-free route as an efﬁcient...

Chemical Engineering Journal 166 (2011) 906–915 Contents lists available at ScienceDirect Chemical Eng ineeri ng Jour nal j o u r nal home p a g e : www.elsevier.com/locate/cej WO 3 /TiO 2 composite with morphology change via hydrothermal template-free route as an efficient visible light photocatalyst Sajjad Ahmed Khan Leghari, Shamaila Sajjad, Feng Chen, Jinlong Zhang ∗ Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, PR China a r t i c l e i n f o Article history: Received 13 October 2010 Received in revised form 17 November 2010 Accepted 17 November 2010 Keywords: Template-fre e route Hydrothermal method WO3/TiO2 composite Ammonium sulfate Visible photocatalyst a b s t r a c t The WO3/TiO2 composites are prepared by a template-free synthetic approach based on hydrothermal react ion thatleads to theformation of nano parti cles andmicrosphe res withthe chan ge inconcentrations of ammonium tungstate used as a dopant precursor. 5.0% composite generally exists in loose aggregate nanoparticles. These particles are aggregated to form the peculiar morphology of microspheres at 10.0% WO3/TiO2 due to the in situ formation of ammonium sulfate in supersaturated state. 5.0% composite exhibits the best photoactivity as compared to pure TiO 2 , P-25 and pure WO 3 in the degradation of met hylorang e and 2, 4-d ichlor oph eno l in UV andvisib le light.10.0%WO3/TiO2 composi te conta ins more dopant contents but exhibits comparable higher activity due to its specific morphology of spheres. Mor- phological variations of a photocatalyst also influence the photocatalytic efficiencies. Catalysts exhibit high activity owing to the combined effects of both the unique structural characteristics and the tungsten doping. The doped tungsten (W) inhibits the electron–hole recombination rate. The kinetics of the organics degradation is found to follow the Langmuir–Hinshelwood model. © 2010 Elsevier B.V. All rights reserved. 1. Intro ducti on The spont aneous assembly and trans formation of inorga nic matter into complex higher-order architectures is a key challenge in materials chemistry. In general, template-directed approaches have been used extensively to control the spatial patterning of inorganic materials. For example, inorganic spheres which have numerous potential applications as catalysts, lightweight fillers, low-dielectric materials, drug-release vectors and photonic crys- tals [1] have been fabr icate d using confin ed react ion enviro nments suc h as emulsions [2] and spra y-dri ed dropl ets [3] or pol y- mer/surfactant micellar templates [4] or by surface depo sitio n onto monodisperse latex particles followed by template removal [5]. Alte rnati vely, higher -ord er architectu res can be spont aneou sly assembled by mesos cale trans formation processes that invol ve metastabl e inorga nic-based colloidal aggre gates [6]. Typically nucleation and growth occur within the aggregates of amorphous inorga nic parti cles that are part ially stabilized by surfa ce-ad sorbed polymers/block copolymers [7] or surfactant molecules [8]. Under such conditions, crystallization of the precursor phase results in complex forms that bear little resemblance to the initial mor- pholo gy of the colloida l aggreg ates due to emergent processe s that radically transform the systems [9]. I n some cases, the aggregated precursor particles undergo self-transformation such that ∗ Correspondin g author. Tel.: +86 21 64252062; fax: +86 21 64252062. E-mail address: jlzhang@e cust.edu.cn (J. Zhang). new forms are produced by a redistribution of matter essentially within the same morphological boundary. In particular, interior structures canbe obt ain ed by the loc ali zedOstwald rip eni ng or dif - ferential diffusion (the Kirkendall effect) within metastable solid microspheres [10]. However, all of the aforementioned methods generally require the use of surfactants or polymers that must be remov ed. Moreo ver, thesematerial s are usual ly unsta ble and hence are limited in their potential applications. Coupling titania (TiO 2 ) with metal oxides and non metals is an approach which has received much attention for improving the photo catal ytic prop ertie s of TiO 2 [11–18]. Mic ro arc oxi dat ion pro - cesses have also been employed to grow WO 3 –TiO 2 composites [19,20]. In case of tungsten, this cation tends to migrate toward the surface in comparison to pristine TiO 2 by shifting the isoelec- tric poi nt to lower val ues [21,22]. Bul k pro per ties are als o mod ifie d because of the iso mor phi c sub stitution wit hin the lat tice; theelec- tronic properties of the doped material are notably different when compared with the pristine oxide [23]. The tungsten surface clus- ters work as electron traps which lead to photochromic materials with energy storage capacity [24,25]. In principle, the presence of tungs ten couldimprovethe phot odegr adat ion abili ty of TiO 2 insev- eral ways [26]. I t prevents the recombination of the electron–hole pairs [27] as well as expands the range of useful excitation light to thevisibl e spectra. In add iti on, the chemic al changes of thesurface result in enhanced acidity [28]. Taking into account the aforementioned factors, the photodegradation mechanism can be affected when tungsten is incorporated into the catalyst [29]. The addition of tungsten oxide (WO 3 ) to TiO 2 has been observed to improve the 1385-8947/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2010.11.065

WO3TiO2 composite with morphology change via hydrothermal template-free route as an efﬁcient...

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