Elsevier

Journal of Alloys and Compounds

Volume 629, 25 April 2015, Pages 105-112
Journal of Alloys and Compounds

Correlating oxygen vacancies and phase ratio/interface with efficient photocatalytic activity in mixed phase TiO2

https://doi.org/10.1016/j.jallcom.2014.12.218Get rights and content

Highlights

  • Pristine and mixed phases (A/R ratio) TiO2 synthesized by sol gel route.

  • Photoactivity variation has been correlated with the changes in the phase ratio.

  • Enhanced UV and visible activity attributable to oxygen vacancy present at the interface.

  • Role of A/R ratio and oxygen vacancy in the photoactivity of mixed TiO2 depicted through a model.

Abstract

The photocatalytic activity is a result of the synergy of a succession of phenomena-photogeneration, separation, and participation of the charge carriers in redox reaction at the catalyst surface. While the extent of photogeneration is assessable in terms of absorption spectrum (band gap), the redox reaction can be correlated to specific surface area. However the respective change in the photocatalytic activity has not been rationally and consistently correlated with the above mentioned parameters. A satisfactory explanation of suppression of recombination based on separation of carriers due to differential mobility/diffusivity in the material phase(s) and/or intrinsic potential barrier exists but its correlation with common identifiable parameter/characteristics is still elusive.

This paper attempts to address this issue by correlating the carrier separation with the phase ratio (phase interface) in mixed phase titania and generalizing it with the presence of oxygen vacancy at the phase interface. It essentially appears to complete the quest for identifiable parameters in the sequence of phenomena, which endow a photocatalyst with an efficient activity level. It has been done basically using photoluminescence; X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy in addition to other characterization tools such as X-ray diffraction (XRD), transmission electron microscope (TEM), UV–Vis absorption (UV–Vis DRS), and Fourier transform infrared (FTIR) studies.

Graphical abstract

The correlation of interfacial behavior and oxygen vacancies in mixed phase titania nanoparticles on their performance as photocatalyst has been investigated to explain the impact of photoactivity under UV and visible irradiation compared to pristine counterparts. The defects at the junction effectively reduce the band gap as well decrease the carrier recombination to enhance the photocatalytic activity.

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Introduction

The efficacy of heterogenous photocatalysis has come of age since its discovery by Fujishima and Honda [1]. Subsequent development of a number of photocatalysts with each variant reported to be better than the original; based mainly on the kinetics of the photocatalytic process has raised a number of issues which have attracted the attention of researchers at regular intervals. One of the issues is of correlating the performance with the precisely measurable physico-chemical characteristics of the catalyst material. It is well known that only some of the metal oxide semiconductors with precisely located band edges are suitable for the intended photocatalytic activity. Out of these titanium dioxide (TiO2) takes the lead because it has some other superior characteristics such as high chemical stability, non-toxicity and low cost in addition to photocatalytic activity as well [2], [3], [4], [5]. But the photocatalytic activity has invariably been correlated with absorption spectrum (band gap or surface plasmon resonance based), specific surface area and in some specific cases of mixed phase materials with phase ratio. It is reasonable to take titania to understand it further. Anatase titania as a photocatalyst is blind to visible light essentially because of the band gap (3.2 eV) limitation with its rutile (3.0 eV) variant only slightly better making both of them inadequate for exploitation of solar radiation. The enhancement and extension of absorption in the solar spectrum range through doping, sensitization, and development of hierarchical morphology was the natural course taken [6], [7], [8], [9], [10], [11]. The requirement of more redox reaction sites through increase in the specific surface area of a photocatalyst and the subsequent development of photocatalyst at nanoscale from three to single digit has resulted in only a marginal gain. But neither band gap reduction nor specific area enhancement has provided the gains on a consistent basis which may be directly correlated with its performance. For example, the band gap reduction using cations like V, Cr, Fe, Ni, W, Mo, Sn, Zn and anions like C, B, N, S [12], [13], etc. may result in identical band gap reduction in titania. But their respective performance is so different that they cannot be equated even after rationalizing with the small difference in their band gap, crystallite size, specific surface area, etc. One of the recent reports on mixed phase titania attributed the activity to the phase ratio of the anatase and rutile phases ignoring the respective crystallite size [14]. The phase interface in a crystallite plays an important role in separation of photogenerated carriers and, consequently, in suppression of their recombination. In the following related work on mixed phase titania with different phase ratio but identical crystallite sizes of each phase conclusively showed that the phase ratio is a more dominant factor in determining the activity [15]. This has been further supported in a related work wherein the mixed phase (wurtzite and zinc blende) of ZnO has been shown to have higher activity than its mono-phasic counterpart support [16]. It clearly shows that synergy of the radiation absorption, photo-generated charge carrier separation, and their utilization through redox reaction at the large specific surface area is responsible for overall enhancement in the photocatalytic performance. Out of these three aspects the absorption (band gap) and specific surface area are quantifiable but the charge carrier separation has neither been quantified nor correlated with any material parameter. The present work attempts to do the same by finding a correlation between the phase ratio and oxygen vacancy in the case of mixed phase titania. Mixed phase titania is uniquely poised here because of dependence of its activity on phase-ratio. It may pave the way for quantifying the performance of heterogenous catalysis with the material physico-chemical characteristics. It may be noted that a number of recent studies attribute the photoactivity to parameters such as oxygen vacancies and lattice disorder. A recent work on black titania attributed its high activity to enhancement/extension of absorption spectrum in the visible range by oxygen vacancy/hydrogen doping induced localized surface plasmon resonance as it happens in Pt-loaded titania (which does not have any secondary band gap) [17]. In the earlier work on black titania the high activity was attributed to lattice disorder induced and hydrogen stabilized mid-gap states in the surface region which account for a secondary band gap and the resultant band gap reduction [18]. The band gap reduction enhances the creation of charge carriers but their separation can only be ensured through mechanisms/lattice-feature wherein (i) carriers are separated under the influence of a barrier field at an interface of two different regions, (ii) carriers move in two different directions with preferred conductivity/diffusivity, respectively, or (iii) carriers are generated at the surface of the material which provides suitable conditions/trap centers facilitating one of them to take part in the desired redox reaction. The mixed phase system is dominated by the first mechanism with the second mechanism acting as a facilitator. The sensitized system is dominated by the second mechanism. The third option is the one which dominates the systems with structural/lattice disorder at the surface. These systems, as stated earlier with reference to black titania, have been shown to possess sub-band gap and/or show surface plasmon resonance by virtue of having lattice disorder induced oxygen vacancies. It appears that these oxygen vacancies also facilitate separation because of preferential conductivity for electrons [19].

Although it is quite involved to find commonality in the three mechanisms/lattice-feature our quest to find a correlation between the phase ratio and oxygen vacancy may throw some light on it. This has been done by analyzing their photoactivity in UV as well as visible radiation flux.

Section snippets

Synthesis of TiO2 nanoparticles

The chemical reagents used for the synthesis of TiO2 were titanium iso-propoxide [Ti(OCH(CH3)2)4] purchased from Sigma–Aldrich and isopropyl alcohol [(CH3)2CHOH] purchased from Merck (India). All the reagents were of analytical grade and double-distilled water was used throughout the experiments. TiO2 nanoparticles were synthesized via sol gel method. For this titanium iso-propoxide was mixed with 2-propanol in 1:10 ratio. After 15 min, 0.5 mL of water was added drop wise under constant stirring

X-ray diffraction study

Fig. 1(a)–(d) shows the XRD spectra of TiO2 nanoparticles at four different temperatures. From XRD pattern it is revealed that the T60 corresponds to pure anatase phase (JCPDS File No. 894203).The samples T65 and T70 correspond to anatase–rutile mixed phases, and T75 corresponds to pure rutile phase (JCPDS File No. 894920). The crystallite sizes of the particles were calculated using Scherrer’s formula, d = /β cos θ; where d is the average crystallite size obtained in angstroms (Å) and k is the

Conclusions

It may be concluded on the basis of this study on mixed phase titania that phase ratio dependent oxygen vacancy has important role to play in determining the band gap of the material and hence photocatalytic activity of the material. But the overall functional behavior of the material cannot be attributed to this aspect only. The role of the interface of the two materials with different band gap/oxygen vacancy level has an important role to play. The most important role is in separation of the

Acknowledgements

Authors thankfully acknowledge the support provided by DST, New Delhi and AICTE, New Delhi (India). The help provided by Dr. B. Choudhury, Department of Physics, Tezpur University, Tezpur, India is also thankfully acknowledged.

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