130-fold Enhancement of TiO2Photocatalytic Activities by Ball Milling
Ken-ichi Saitow1,2,a) and Tomoji Wakamiya2
1Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University, 1-3-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan
2Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan
Supplemental materials
Experimental details
The planetary ball milling apparatus used is commercially available (Premium line P-7, Fritsch Japan Co., Ltd.). A ZrO2 milling vessel and ZrO2 milling ballswere used to grind the TiO2 particles. Milling balls, TiO2, and methanol were placed in the milling vessel. Milling was performed by changing two parameters: milling time from 0 to 12 h and revolution speed from 0 to 1,000 rpm (revolution speed/rotation speed = -1:2). Milling was conducted at a fixed revolution of 600 rpm for 1 h, and this step was followed by a pausing time for cooling. This cycle was repeated to establish a net milling time in the range 1– 12 h. The milling was conducted at a constant milling time of 1 h as a function of revolution speed. To reduce the increase in temperature and pressure in the vessel caused by milling at higher revolution speeds, milling was conducted for 1 h in three cycles, each of 20-min milling duration, with a pausing time for cooling. For all procedures, no solvent was added during milling. Anatase-type TiO2 (Kanto Chemical Co., Inc.) and P25 (Degussa) TiO2 were used as samples.
The MB (Sigma-Aldrich) was used to evaluate the photocatalytic performance of TiO2. MB aqueous solution (concentration2.94 × 10−5 M, volume 3.0 ml), TiO2methanol solution (concentration~25 mg/ml, volume 80 l), and a stirring bar were placed in a quartz cuvette. Photoreduction was performed by irradiating the UV light of a He/Cd laser ( = 325 nm, intensity = 8 mW, CVI MellesGriot) onto the cuvette while stirring the solution at room temperature. The laser irradiation time ranged from 0 to 120 min. After laser irradiation, the absorption spectra were recorded, using a UV–Vis spectrophotometer (V-660, Jasco). The time profiles of the absorption spectra, shown in Fig. 2(c), were evaluated using the absorbance at the wavelength of 662 nm. These profileswere well fittedby a single exponential function, i.e. Abs(t) = a + bexp(- kt), where a, b, and k are baseline, amplitude, and rate constant, respectively. For the clearness, their absorbances at t = 0 were normalized to Abs=2.9by subtle changes of the base lines,a, becausethe rate constantkin the exponential function should be independent of thevalue of base line.
A DLS apparatus (Zetasizer Nano, Malvern Instruments Ltd.) was used to determine the grain sizes of TiO2 particles dispersed in methanol. The morphology and surface roughness of TiO2 before and after milling was observed, using FE-SEM (S-5200, Hitachi Technologies Co.). The FE-SEM samples were prepared by dropping a methanol dispersion of TiO2 particles on TEM grids. After drying, the grid was coated by osmium film with a coater machine (Neoc-STB, Meiwafosis Co., Ltd.). The specific surface area of the TiO2 particles in the dispersion was derived by measuring the relaxation time of solvent molecules with a commercial wet-process specific surface area analyzer based on the NMR apparatus (Acorn Area, XigoNanotools). In XRD (Rint 2500, Rigaku) analysis (Cu Kα, 40 kV, 200 mA), a sample-holder plate of nonreflective silicon specialized for producing a background-free signal was used. EPMA (JXA-8200, JEOL) was performed to assay the prepared samples for Zr contamination.
Figure S1. Typical XRD patterns (red) and fitting curves (green) a) before and b) after milling of anatase TiO2 at 600 rpm for 6 h.
Figure S2. MB absorbance measured at 662 nm as a function of laser irradiation time. a) Milled TiO2 is used as a photocatalyst. The time and revolution speed for milling of TiO2 with the weight of 1g are 6 h and 600 rpm, respectively. b) TiO2 (1g) (0.05g) is milled with ZrO2, and the product is tested as a photocatalyst. The time and revolution speed for milling are 6 h and 600 rpm, respectively.
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