Impact of the absolute rutile fraction on TiO₂ visible-light absorption and visible-light-promoted photocatalytic activity

Highlights

• Phase fractions & microstructure solved via XRD & HR-EELS, uncommon in catalysis.

• A 2 wt% rutile fraction is able to red-shift the optical E_g of the mixture.

• PC activity tested in gas- and liquid-solid phase using purely visible-light.

• A rutile fraction ≥10 wt% granted vis-light absorption & less charge recombination.

• A rutile fraction ≥10 wt% gave real vis-light PC activity.

Abstract

Titanium dioxide is by far the most used semiconductor material for photocatalytic applications. Still, it is transparent to visible-light. Recently, it has been proved that a type-II band alignment for the rutile − anatase mixture would improve visible-light absorption. In this research paper we thoroughly characterised the real crystalline and amorphous phases of synthesised titanias – thermally treated at different temperatures to get distinct ratios of anatase-rutile-amorphous fraction – as well as that of three commercially available photocatalytic nano-TiO₂.

Optical spectroscopy showed that even a small fraction of rutile (2 wt%) is able to shift to lower energies the apparent optical band gap of an anatase-rutile mixed phase. But is this enough to attain a real photocatalytic activity promoted by merely visible-light? We tried to give an answer to that question.

Photocatalytic activity was assessed in the liquid- and gas-solid phase (employing rhodamine B and 4-chlorophenol, and isopropanol, respectively, as the organic substances to degrade) using a light source irradiating exclusively in the visible-range.

Photocatalytic activity results in the liquid-solid phase showed that a high surface hydroxylation led to specimen with superior visible light-promoted catalytic activity – i.e. dye and ligand-to-metal charge transfer complexes sensitisation effects, not photocatalysis sensu-strictu.

On the other hand, the gas-solid phase results showed that a higher amount of the absolute rutile fraction (around 10 wt%), together with less recombination of the charge carriers, were more effective for both visible-light absorption and a “real” visible-light promoted photocatalytic oxidation of isopropanol.

Introduction

1972 was the year when Honda and Fujishima firstly demonstrated electrochemical photolysis of water using a titanium dioxide (TiO₂, titania) electrode (and a Pt counter-electrode) [1]. Since then, the research about TiO₂ and its photocatalytic (PC) application has been growing exponentially [2]. Indeed, TiO₂ still remains the most widely used oxide for PC applications, owing to its low cost, high activity, and stability in both basic and acidic media [3].

TiO₂ crystallises in a large number of polymorphs [4], though anatase and rutile are the most widely used for PC applications, with anatase believed to have better PC performances than rutile [5,6]. [This is the reason why the vast majority of the research to improve the PC performances and extend its light absorption to the visible (see below), is headed to the anatase TiO₂ polymorph.] TiO₂ electronic configuration retains zero 3d electrons, and the optical band gap (E_g) of anatase and rutile is accepted to be around 3.2 and 3.0 eV [7], respectively, thus being transparent for most of the visible radiation region. This means that the PC reaction is exploited by 3–5% of the solar spectrum [8], this is the reason why the scientific community is challenging in extending its light response to the visible-region [9].

First attempts to modify TiO₂ electronic structure were made by doping TiO₂ with transition metals [10]. However, this strategy proved itself to be counterproductive, because transition metals behaved as recombination centres for the photo-generated couple e–/h⁺ [11]. Then, researcher successfully synthesised TiO₂ with Ti³⁺ dopants. This so-called “Ti³⁺ self-doping” is a technique to harvest visible-light without introducing any foreign element into titania structure [12]. However, to be successful, this method involves toxic precursors and solvents, i.e. hydrofluoric acid (HF), TiF₄ and TiCl₃ [12,13], thus being contrary to most of the principles of green chemistry – photocatalysis can be certainly considered as one of the most innovative approaches in green chemistry [14].

Asahi and co-workers succeeded in doping TiO₂ with nitrogen, thus extending titania’s light response to the visible [15]. However, also this approach proved itself to be somehow unsuccessful. Indeed, if from one hand anionic doping extends TiO₂ light absorption to the visible, from the other it is reasonable that, during the oxidising working conditions of a PC reaction, titania might have a “self-cleaning” disposition, ejecting N^3– anions from the structure [16] – provided that nitrogen is present in the nitride state. The photocatalyst is thus not stable and cannot be recycled after repeated PC runs [17].

Other strategies adopted to extend TiO₂ light response to the visible region were to graft / decorate its surface with noble-metals atoms [[18], [19], [20], [21]] – although effective, these methods proved to be quite expensive. It was even found that high-pressure cubic TiO₂ would absorb visible-light, but those phases are not stable at atmospheric pressure, thus unsuitable for real-world PC applications [22,23].

The investigations reported so far mainly regarded the doping and/or modifications of the anatase TiO₂ polymorph. However, Degussa P25 is known to be one of the most effective commercial TiO₂-based photocatalysts. This is a mixture of nano-anatase and nano-rutile, as well as some amount of amorphous fraction [24], and it possesses some weak absorption into the visible region [25]. From this starting point, very recent literature has been paid to investigate the electronic structure of anatase/rutile mixed-phase TiO₂ nano-powders [26], and its possible synergy in PC reactions [27]. As a matter of fact, it is believed that the phase junction of nanocomposite materials is key to obtain improved PC performances [28]. Scanlon et al. showed that a type-II (staggered) band alignment of ∼0.4 eV exists between anatase and rutile TiO₂ polymorphs when they form a hetero-junction, with anatase possessing the higher electron affinity [29]. This was further confirmed by a very recent work of Nosaka and Nosaka, who suggested that, in an anatase-rutile hetero-junction, the conduction band bottom (E_CB), for the indirect E_g of 3.2 eV, should be 0.4 V lower than the E_CB of rutile TiO₂ [30]. Indeed, such band alignment has a twofold advantage [31]: (i) on excitation, it is beneficial for electrons to flow from rutile to anatase, and for holes to flow in the opposite direction, this leading to efficient separation of the charge carriers. (ii) The effective E_g of the mixture is lower than that of the constituent polymorphs, this leading to enhanced visible light absorption.

Aware of these findings, the quest for attaining a visible-light activated PC material showing good performances might be closed by investigating anatase-rutile hetero-junctions composed of different fraction of anatase and rutile (and amorphous phase) [32]. Therefore, in this work, the real crystalline/amorphous composition of several synthesised and commercial TiO₂-based PC nanoparticles (NPs) was thoroughly explored via X-ray powder diffraction (XRPD), and the materials fully characterised. This is a very innovative approach within the catalysis community, as TiO₂ absolute crystallinity has been rarely determined [[33], [34], [35], [36], [37], [38]]. Furthermore, it has been shown that the amorphous fraction of TiO₂ has no or little PC activity [39], this also influences the interpretation of the results.

The PC activity was evaluated using exclusively visible-light irradiation from a white LED lamp source, in the liquid-solid as well as gas-solid phase. In the former, the PC activity was assessed monitoring the decolouration of an organic dye, rhodamine B (RhB) and the degradation of a phenolic compound (highly toxic and poorly biodegradable compounds) [40], 4-chlorophenol (4-CP). In the latter, isopropanol was used as the organic substance to degrade.