Size dependency of nanocrystalline TiO2 on its optical property and photocatalytic reactivity exemplified by 2-chlorophenol
Introduction
Since the discovery of photovoltaic property of titanium dioxide TiO2 by Fujishima and Honda [1], great efforts have been focused on elucidating the electronic structure [2], [3], [4], [5], catalytic reactivity [6], [7], [8] and surface property [9] of TiO2. Inexpensive and thermal-dynamically stable at room temperature, this semiconductor material has been widely used in heterogeneous photocatalysis and proven to be capable of decomposing a host of organic pollutants such as phenolic compounds [10], [11], metal ethylene diamine tetra acetate (EDTA) complexes [12], [13], airborne microbes [14] and odorous chemicals [15]. Most of these studies involved ultraviolet (UV) photons as the major exciting light sources. Considering that there is only 5% of solar irradiation within the UV range, intuitively it is desirable to enhance the photocatalytic performance of TiO2 by enabling it to utilize photons from the near-visible to visible region. It has been suggested that this can be achieved by manipulating the particle size of photocatalyst [3], [6], [16] or doping the TiO2 with foreign ions [17], [18], [19].
The initiation of a photocatalytic reaction requires a minimum photon energy that exceeds the band gap of the material in order to trigger the interband transition of electrons between the lowest unoccupied molecule orbital (LUMO) and the highest occupied molecule orbital (HOMO); that is, the incident wavelength needs to be smaller than the wavelength of the band gap threshold, λbg. Thus, it is speculated that reducing the band gap of TiO2 can enhance its photocatalytic performance through more efficient utilization of lower energy photons. TiO2 has three distinct crystalline structures: rutile, anatase, and brookite. Most studies on the photocatalytic reactivity were conducted with either rutile or anatase, which reported that the indirect band gap was 3.0 eV (or λbg 413 nm) and 3.2 eV (or λbg 387 nm), respectively. Although rutile has a lower band gap than anatase, it has been demonstrated that anatase-structure TiO2 exhibits a better photocatalytic performance than that of rutile [20], [21], [22]. This is attributed in part to a wider optical absorption band and smaller electron effective mass of rutile than those of anatase, which leads to higher mobility of charge carriers in rutile than anatase [5]. The band gap of TiO2 is commonly believed to be indirect. In contrast to direct interband transition, indirect transition requires phonons (lattice vibration) to compensate for the change in wave vector during electron transition.
It has been reported that the band gap of semiconductor crystalline is a function of the particle size [3], [4], [23]. Below a certain threshold, the density of point/surface defects of semiconductor crystalline increases with decrease in particle size. Due to mild delocalization of molecular orbitals on the surface, defects in the bulk semiconductor create deep and shallow traps near the band edge of its electronic state, which brings about reduction in band gap, that is, red-shift in absorption spectrum [3], [4]. When the size of semiconductor particle decreases from its bulk to that of Bohr radius, e.g., the first excitation state, the size quantization (Q-size) effect arises due to the spatial confinement of charge carriers. Consequently, electrons and holes in the quantum sized semiconductor are confined in a potential well and do not experience the delocalization that occurs in the bulk phase. Therefore, the band gap of ultra-fine semiconductor particle increases with the decrease in particle size when it is smaller than the band gap minimum [3], [4]. This phenomenon has been described by the Brus’ effective-mass model (EMM) [3]. Size quantization effect has been studied using various semiconductors including CdS [24], HgSe, PbSe, CdSe [25], ZnO [26], Cd3S2 [27], and TiO2 [6], [16], [28]. The reported Q-size effect of semiconductor clusters appears to be between 1 and 12 nm. Results of these studies were mostly obtained from liquid phase UV–vis absorption spectrum, and few had focused on the photodegradation of organic compound.
A good photocatalyst must have high photon conversion efficiency in addition to high specific surface area. In fact, the primary particle size of photocatalysts determines both the specific surface area and the photon conversion efficiency. Although the size dependency of band gap has been studied theoretically, only a few investigations have been conducted on the change of band gap as a function of particle size using TiO2 [6], [16], [29]. Anpo et al. [6] studied the change of band gap of Ti02over a wide range of particle sizes (e.g., 3.8–200 nm) and found significant blue shifts of the absorption edge by 0.093 and 0.156 eV for rutile and anatase crystalline, respectively, when particle size was less than 12 nm. Kormann et al. [16] observed the quantum confinement effect upon illumination of TiO2 colloids (e.g., <3 nm) and reported a blue-shift by 0.15–0.17 eV in absorption spectrum. However, Serpone et al. [29] did not observe a quantum size effect in the particles size range between 2.1 and 26.7 nm of TiO2. It must be mentioned that these authors depended on UV–vis absorbance measurements to estimate the band gap energy. Obviously, this method cannot exclude the light scattering effect during absorbance measurements, which would lead to an over-estimation of the absorbance, especially for large aggregates. It is known that the scattering efficiency is proportional to the fourth-power of particle radius (e.g., r4) according to Rayleigh scattering theory [30]. During UV–vis absorbance measurements, the size of aggregates will increase with time in suspensions. Furthermore, most of these UV–vis absorbance measurements were conducted during the agglomeration/reflux stage of particle preparation using the sol–gel process [6], [16], [29]. The presence of precursor intermediates can contribute light absorbance also, which, in turn, might affect the reliability and the interpretation of results observed.
As mentioned above, the particle size can affect the photocatalytic reactivity. A few studies have been conducted to assess the relationship between particle size and photocatalytic reactivity [6], [22], [30], [31], [32], [33], [34], [35], [36], [37], [38]. Anpo et al. [6] observed an increase in quantum yield in the hydrogenation of CH3CCH when the particle size of anatase TiO2 decreased from 11 to 3.8 nm and concluded that it was caused by the quantum size effect. Maira et al. [30] studied the photocatalytic degradation of trichloroethylene (TCE) in gas phase upon TiO2 in the size range of 2.3 and 27 nm and reported an optimum particle size of 7 nm. They further concluded that the lower reactivity at particle size less than 7 nm was due to changes in structure and electronic properties of the ultra-fine particles. Almquist and Biswas [31] compared the effect of particle size using both flame synthesized and commercial TiO2 particles, on the photodegradation of phenol. They reported an optimal particle size in the range of 25–40 nm with respect to photo-degradation rate and optical responses. However, the TiO2 samples consisted of a mixture of anatase and rutile crystallines at various proportions that results may be affected by the anatase/rutile ratio in addition to particle size. Hao et al. [39] and Jang et al. [33] studied the effect of anatase particle size on the photodegradation of rhodamine B and methylenen blue dye. They observed that the photocatalytic reactivity monotonically increased with decrease in particle size in the range of 8–15 and 15–30 nm. However, the effect of size-dependent band gap change and its corresponding excitation source spectrum was not considered in their work. Nam et al. [34] studied the photocatalytic degradation of TCE (tetrachloroethene) using TiO2 (anatase) thin film and found an increase in the photocatalytic degradation rate as the primary particle size decreased from 37 to 25 nm. Zhang et al. [35] examined the effect of anatase particle size on the oxidation of trichloromethene, CHCl3, and reported that TiO2 particles at 11 nm yielded the highest photonic efficiency on the oxidation of CHCl3. They reported that photocatalytic efficiency did not increase monotonically with decrease in particle size; rather it increased with an increase in surface combination of the electron–hole pairs. Xu et al. [36] and Gerischer [37] have experimentally and theoretically studied the effect of particle size of on the photocatalytic efficiency of TiO2. Both researchers concluded that the photocatalytic reactivity of TiO2 increases with decrease in particle size. Unfortunately, the TiO2 used in the studies conducted by Xu et al. [36] and Gerischer [37] had a particle size mainly in the range of micronmeters rather than nanometers. As indicated above, there appears to be no agreement on the effect of particle size on the photocatalytic activities of TiO2; the optimal particles size reported has covered a rather wide range, e.g., between 3.8 and 40 nm. Another issue related to the effect of particle size on the photocatalytic reaction is the primary versus the secondary particle size. This is of particular importance when dealing with aqueous systems, as particle aggregation is inevitable in the water environment [38]. Will particle aggregation affect the photocatalytic reactivity during the course of water treatment process? Maira et al. [30] reported that the degradation of TCE in the gas phase was affected by both the primary and the secondary particle sizes. Little information with regard to the importance of secondary particle size on photocatalytic reactivity in aqueous solution is available. There is a need for more systematic studies on the effect of particle size, both primary and secondary, on photocatalytic reactions in aqueous solutions.
The objectives of this study were (1) to elucidate the effect of primary particle size on the electro-optical property of TiO2 in terms of band gap changes, (2) to assess the effect of primary and secondary particle size on the photocatalytic reactivity of TiO2 exemplified by 2-chlorophenol (2-CP), and (3) to determine whether any relationship exists between the optical property and photocatalytic reactivity. In order to better evaluate the effect of particle size on the optical property of TiO2 with minimum optical interference (e.g., aggregation in the aqueous phase), band gap measurements were made using the transformed diffuse reflectance technique according to the Kubelka–Munk theory [40], [41], [42]. It is expected that this technique will enable better correction of interferences in absorbance measurements caused by particle light scattering and other factors such as the presence of precursor intermediates. Results of band gap measurements were then compared with those predicted by the Brus’ EMM model. The effect of particle size on the photocatalytic reactivity of TiO2was assessed using a 2-chlorophenol (2-CP) probe over a wide range of primary particle sizes, e.g., from 3.8 nm (reported known quantization effect) to 29 nm (approximated bulk size). The next step was to compare the effect of primary and secondary particle size on the photocatalytic degradation of hazardous organic compounds exemplified by 2-CP. Finally, the band gap change was compared with the photocatalytic degradation rate of 2-CP for assessing the overall effect of particle size.
Section snippets
TiO2 particle preparation
Pure anatase TiO2 particles with diameter ranging from 12 to 33 nm were prepared by metal–organic chemical vapor deposition (MOCVD). Details of the system configuration and operation have been reported previously [43]. Titanium tetraisopropoxide—Ti[OCH(CH3)2]4 (TTIP), 97% purity, purchased from Aldrich, was selected as the precursor. The liquid TTIP was placed in a Pyrex thermal well. Argon (99.999% purity) was used as the carrier gas. The precursor flow rate was adjusted by controlling the
Optical properties
From the diffuse reflectance plot (Fig. 5a), we can see that A20, A17 and A23 have better light absorbance than A3.8, A4.9, A5.7, and A29 in the range of wavelength (λ) between 400 and 350 nm. The damping of the F(R∞) for A3.8 and A4.9 was observed when the wavelength was below 360 nm. We suspect that is due to the size quantization effect and defects induced deep energy traps that are more discretely created (compared to A5.7). The damping of the diffuse reflectance may be caused by attenuation
Conclusion
TiO2 crystallines of different particle size were successfully synthesized by the MOCVD method (particle size ranged between 12 and 29 nm). The particle size of TiO2 can be simply manipulated by moderate control of the O2/precursor flow rate and system pressure. Results from the transformed Kubelka–Munk equation showed that the band gap of the TiO2 nanoparticles was a function of the primary particle size. When the TiO2 particle size decreased from its bulk (i.e., =29 nm) to 17 nm, the band gap
Acknowledgement
The authors wish to acknowledge our two anonymous reviewers for their excellent comments. This work was supported by a NSF grant NIRT #0210284.
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