The influence of crystallite size and crystallinity of anatase nanoparticles on the photo-degradation of phenol
Graphical abstract
The effect of crystallite size and crystallite on the photoreactivity of anatase was studied by phenol photodecomposition. It was found that the decomposition process was strongly influenced by the crystallite size but not by the crystallinity. Increasing the crystallite size led to a significant suppression of undesired hydroquinone-benzoquinone redox reactions thus enhanced the photoreactivity of anatase.
Introduction
Titanium dioxide (TiO2) has attracted great attention in heterogeneous photocatalysis applications due to its high reactivity, strong oxidizing power, long-term stability, and non-toxicity [1], [2]. It has been found that the recombination kinetics, charge separation, charge trapping efficiency, and optical properties of TiO2 nanoparticles are governed by their size, shape, polymorph composition, crystallinity, and impurity concentration [3], [4], [5], [6]. Fine tuning of these parameters is expected to impact the photo-reactivity strongly [7], [8], [9].
Anatase is considered to be the most active polymorph of TiO2 due to its high adsorption affinity for organic molecules [10] and low recombination rate [5]. The crystallite size has been recognized as an important factor that influences the photo-reactivity because it dictates many physical properties of the pristine anatase nanoparticles (i.e., surface area, surface energy, light absorptivity, and lattice distortion) [11], [12]. Anpo et al. first reported the size effect of anatase on the photocatalytic hydrogenation of propyne [13]. It has been pointed out that the energy of photo-generated radicals increases by reducing the crystallite size due to the quantum confinement effect, which improves the performance of the photocatalyst. This effect has been further supported by other authors who have studied various other photo-reactions [2], [14], [15]. Additionally, a reduction in the crystallite size leads to larger surface areas thus improves the adsorption of reactants and subsequently enhances the photo-reactivity [16], [17]. However, contradictive reports also exist [18], [19], [20], which claims that the quantization effect alters the electric field gradients and reduces the separation of electron–hole pairs [21]. Almquist and co-workers reported that the photocatalytic performance of anatase nanoparticles increases upon an increase in the crystallite size due to the optimization of the optical property and charge carrier dynamics [22]. Interestingly, there are also reports suggesting that the optimum crystallite size is in the range of 7–15 nm for various photocatalytic reactions [23], [24], [25], [26].
Crystallinity is believed to be another critical parameter that affecting the photocatalytic performance of anatase. Ohtani et al. showed that amorphous titania plays a role as a recombination center, and it exhibits negligible reactivity in several photocatalytic reactions [27]. Even though reports suggesting that hydrated amorphous titania can be photo-reactive exist [28], it is generally accepted that the photo-reactivity can be enhanced by improving the crystallinity [29], [30], [31], [32], [33], [34], [35], [36]. This may be attributed to the enhanced electron transport from the conduction band of TiO2 to the surface adsorbed molecules [37]. However, it is very difficult to interpret these data as the degrees of crystallinity have been estimated indirectly from the annealing temperatures, and the absolute values are missing in most studies.
The general approach of tuning the crystallite size and crystallinity is by heat treatment of either the amorphous phase or of small anatase nanoparticles in previous studies [12], [13], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [28], [29], [30], [31], [32], [33], [35], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47]. Unfortunately, anatase nanoparticles prepared via conventional synthesis routines normally exhibit a broad distribution of crystallite sizes [48]. Moreover, the calcination process produces variations in crystallite size and crystallinity simultaneously. These drawbacks render any clear conclusion about the individual impact of crystallite size and crystallinity impossible. Despite the large interest in this topic, no previous studies where pure anatase nanoparticles with tuneable crystallite size and crystallinity but otherwise identical properties have been evaluated for their photocatalytic activities exist.
Here, we have prepared a series of crystallite size and crystallinity controlled anatase nanoparticles by supercritical flow synthesis. The crystallite size series ranged from 6 nm to 27 nm with a fixed crystallinity of ∼84%, whereas the crystallinity series varied the absolute crystallinity from 12% to 82% with a fixed crystallite size of ∼9 nm. Thus, it was possible to directly correlate the photocatalytic performance of anatase nanoparticles to the crystallite size and crystallinity. The photo-reactivity of these catalysts was systematically evaluated using phenol as the probing molecule. The influence of crystallite size and crystallinity of the anatase nanoparticles on the degradation of phenol was studied by tracking the evolution of the phenolic intermediates and the reaction kinetics.
Section snippets
Sample preparation
TiO2 nanoparticles were prepared by supercritical synthesis using a continuous flow reactor [48], [49], [50]. The supercritical synthesis method is featured by a rapid mixing of the cold reactant solution with the superheated solvent stream in a continuous process, which results in a high degree of super-saturation and thus a rapid nucleation that facilitates the formation of primary crystallites in large quantities. The crystallite size and crystallinity depend on temperature, pressure, and
Physical properties
All samples were chemically pure TiO2 in the polymorph of anatase according to XPS and XRD analysis [55]. Rietveld refinement of the XRD patterns suggests that all samples were randomly oriented with unit cell parameters of a = b = 3.8 Å and c = 9.5 Å. Representative TEM images of the crystallite size series, S1–S6, are depicted in Fig. 2a–e. The anatase nanoparticles showed a clear change of the particle size from ∼8.9 nm (Fig. 2a) to ∼25 nm (Fig. 2e), but otherwise a typical plate-like (Fig. S4)
Conclusions
Two series of crystallite size and crystallinity controlled pristine anatase nanoparticles have been synthesized under supercritical water–isopropanol conditions. Photocatalytic degradation of phenol was carried out to evaluate the influence of crystallite size and crystallinity on the performance of anatase. We discovered that the reactivity of anatase nanoparticles is independent of the crystallinity. In contrast, increasing the crystallite size from 6.6 nm to 26.6 nm resulted in an enhancement
Acknowledgments
We would like to thank Dr. Helene Zeuthen at iNANO for fruitful discussions on the reaction kinetics analysis. Financial support by the Center of Energy Materials (Danish Strategic Research Council), the Center for Materials Crystallography (Denmark National Research Foundation, DNRF93), the iNANO Center through the Danish Strategic Research Council, and the Chinese Scholarship Council (CSC) is acknowledged.
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