Elsevier

Applied Surface Science

Volume 319, 15 November 2014, Pages 151-157
Applied Surface Science

Singular effect of crystallite size on the charge carrier generation and photocatalytic activity of nano-TiO2

https://doi.org/10.1016/j.apsusc.2014.06.071Get rights and content

Highlights

Abstract

Photocatalytic processes on nanometric titanium oxide have been extensively studied from the standpoint of solar energy utilization and remediation of water and gas streams polluted with organic molecules. It is accepted that TiO2 crystallite size and crystalline phase are among the parameters involved in the control of the photocatalytic activity. However, while changing the catalyst nanoparticle size, other attributes important to assess photocatalytic activity may be modified, making thus difficult to define to which extension the photoactivity changes are related only to size differences. Therefore, aiming at studying exclusively the size effect and the parameters directly related to size on the photocatalytic activity of nanometric TiO2, in this work it was explored a method to synthesize TiO2 nanoparticles with controlled size, highly similar morphology and comparable phase and degree of crystallinity. A set composed of four samples of nano-TiO2 loaded porous Vycor glass, each sample having a specific TiO2 nanoparticle size, was tested on the photoactivated process of depollution of solutions of salicylic acid and methylene blue. The photocatalytic activity observed for the organic compounds tested was inversely proportional to the TiO2 nanoparticle size. An opposite tendency was observed for the generation of OH radicals during photocatalyst illumination, as more radicals are formed on the material containing the larger TiO2 nanoparticles. Results of this study suggest that photocatalytic activity of nano-TiO2 is less favored by the enhanced light absorption response and the higher generation of oxidative species observed for the larger nanoparticles. Better catalysts were obtained when nano-TiO2 exhibited high surface-to-volume ratio and had small recombination volumes, which respectively favors pollutant adsorption-desorption on catalyst surface and reduce the number of recombined charge carriers.

Introduction

Photocatalytic processes employing TiO2 as catalyst have been widely explored in the literature from the viewpoint of reaction mechanisms; limiting parameters of reaction; and electronic processes [1], [2], [3], [4]. In addition, there is a broad literature exploring to which extension the photocatalytic activity of TiO2 is affected by titania crystalline phase and by the presence of heterojunction of anatase-rutile [5], as well as by the surface area of the catalyst, considerably large in mesoporous titania [6], [7], [8]; by the specific titania crystal facets exposed to the reactants [9], [10]; and by the type and number of surface and bulk defects [11]. Although these topics have already been widely examined, the development of nanometric titania has created new challenges. When TiO2 is under a nanometric regime, especially when it has size smaller than the Bohr radius, modification of particles size results not only in change in the surface area-to-volume ratio of the particle, but also in other parameters related to the photocatalytic activity of TiO2, such as band gap value, light absorption response and mean free path of the charge carriers (recombination volume) [12], [13]. Changes in nanoparticle size can also result in differences in crystallinity and morphology of TiO2 particle, thus further influencing the photoactivity of the material [14], [15].

To understand size-related properties in nano-TiO2, an even more complex scenario is set when titania is supported in matrices, as there can be a synergistic interaction between the nanoparticles and the matrix, thus changing the final photocatalytic activity of the material. Therefore, there is a list of concurring changes that can happen when varying the size of supported-titania nanoparticle, thus making it difficult to assess the effects on TiO2 photocatalytic activity caused solely by modifications in TiO2 nanoparticle size. However, the understanding of how the single size change can affect the photocatalytic activity should give advantages to material scientists in their search for novel titania-based photocatalysts. In this manner, a synthetic approach was explored in this work to prepare spherical TiO2 nanoparticles with controlled sizes, similar morphology and comparable crystallinity supported in porous Vycor glass (PVG). Photoactive TiO2 nanoparticles were immobilized in an inert support as a strategy to help to reduce costs related to the catalyst separation process [16]. Nanoparticle size effect on the electronic and optical properties was studied. Salicylic acid and methylene blue were the molecules chosen to test the catalysts’ ability to photodegrade organic pollutants, while the ability to photogenerate oxidizing species (OH radicals) was evaluated using a model reaction with terephthalic acid. Correlations between the photocatalytic activity due to size dependent properties like surface area, recombination volume and light absorption response are then proposed.

Section snippets

Catalyst preparation

PVG monoliths of 0.5 cm diameter and 0.1 cm of thickness purchased from Corning Glass Company were submitted to ultrasound during 5 min in HCl (1.0 mol L−1) and 5 min in acetone and thereafter were thermally treated at 393 K for 2 h followed by 1023 K for 72 h to remove organic impurities and to activate the silanol groups on the PVG surface. For the preparation of TiO2 inside PVG pores, the clean PVG monoliths were immersed in a 0.75 mol L−1 solution of titanium di-(n-propoxy)-di-(2-ethylhexanoate),

PVG/xTiO2 characterization

The method described in this work for the synthesis of PVG/xTiO2 involves sequential cycles of impregnation of PVG pieces with Ti(nPr)2(hex)2 followed by thermal treatment for the decomposition of chemical. XRF analyses were performed to determine TiO2 wt.% in each sample. As expected, increasing numbers of IDC performed result in increasing TiO2 wt.% content (Table 1).

TEM images of PVG/xTiO2 samples were obtained in order to observe the morphology of TiO2 produced by the IDC method. All PVG/xTiO2

Conclusions

TiO2 nanoparticles supported in PVG pores were synthesized by successive impregnation–decomposition cycles using Ti(nPr)2(hex)2 and by this method, it was possible to prepare a set of four different materials having TiO2 nanoparticles with similar morphology and comparable crystallinity, while showing different particle sizes. This set of materials was used to study size effects in TiO2 photoactivity probing the liquid-phase degradation of salicylic acid and methylene blue. Catalytic activity

Acknowledgement

Financial support from FAPESP, CNPq and CAPES are gratefully acknowledged. The authors would like to thank Prof. C. H. Collins (IQ-UNICAMP, Campinas, Brazil) for English revision, the LME/LNNano/CNPEM for technical support during electron microscopy work and the Multiuser Laboratory of Advanced Optical Spectroscopy (LMEOA/IQ-UNICAMP) for use of Raman. This work is a contribution of the National Institute of Science and Technology in Complex Functional Materials (CNPq-MCT/Fapesp).

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