UV-absorption--the primary process in photocatalysis and some practical consequences

Abstract

TiO2 photochemistry studies generally address reactions of photogenerated charge-carriers at the oxide surface or the recombination reactions which control the proportion of charge carriers that reach the surface. By contrast, this review focuses on UV absorption, the first photochemical step in semiconductor photocatalysis. The influence of particle size on absorption and scattering of light by small TiO2 particles is summarized and the importance of considering, the particle size in the application, not the BET or X-ray line broadening size, is emphasized. Three different consequences of UV absorption are then considered. First, two commercially important systems, pigmented polymer films and paints, are used to show that TiO2 can protect from direct photochemical degradation. Then the effect of UV absorption on the measured photocatalytic degradation of aqueous solutions of organics is considered for two separate cases. Firstly, the consequences of UV absorption by TiO2 on the generation of hydroxyl radicals from H2O2 are considered in the context of the claimed synergy between H2O2 and TiO2. Secondly, the effect of altered UV absorption, caused by changed effective particle size of the catalyst, is demonstrated for photocatalysis of propan-2-ol oxidation and salicylic acid degradation.

Figure 1

Rayleigh (▲, dashed line) and Mie scattering (◊ and ■, full lines) of 555 nm radiation by isolated rutile spheres as a function of particle diameter. The curves plotted through ▲ and ◊, points assume that all particles are of identical. The curve plotted through the ■ points assumes a log-normal distribution of particle size.

Figure 2

Variation of the real, ◊, and imaginary, ●, components of the refractive index of rutile between 275 and 400 nm based on results of Vos and Krusmeyer [22]. Above 400 nm the imaginary component, which controls light absorption, is negligible even though the real component is not. Pure rutile crystals are transparent in the visible region of the spectrum but can scatter visible radiation.

Figure 3

Mie theory calculations of (a) q_sca, scattering and (b) q_abs, absorption per unit volume for 20 nm, ×; 50 nm, Δ; 100 nm ■; and 220 nm ♦ rutile particles dispersed in an organic medium, plotted as a function of mean size for a log normal particle distribution with σ = 1.33. Reproduced with permission from Egerton & Tooley, International Journal of Cosmetic Science 2012, 34, 117–122 [23] published by Society of Cosmetic Sciences; Société Française de Cosmétologie and Blackwell Publishing.

Figure 4

Experimental extinction coefficients for 35 ■, 50 ▲ and 145 nm ● rutile particles and the calculated coefficients for 50 nm ∆, derived from Figure 3a,b.

Figure 5

(a) The reflectance spectra (measured on a Jasco 670 spectrometer fitted with an integrating sphere) of pressed discs made from the three different rutile samples of mean size 35 ─; 50 ─ and 145 ─ whose suspension spectra are shown in Figure 4 and whose size distributions are shown in Figure 5b; (b) The particle-size distributions measured by X-ray size sedimentation (Brookhaven X-ray disc sedimentometer) of three rutile samples (reprinted with permission from Egerton & Tooley, International Journal of Cosmetic Science 2012, 34, 117–122 [23] published by Society of Cosmetic Sciences; Société Française de Cosmétologie and Blackwell Publishing).

Figure 6

A comparison of published XRD sizes with the BET-derived sizes of (mainly) anatase samples identified from the publications listed in the caption to figure 15 of Egerton, T.A.; Tooley, I.R., Intl. J. Cosmetic Sci. 2014, 36, 195–206 [35]. The dashed line corresponds to S = 6D/ρ. where S is the surface area, D the particle diameter and ρ the particle density.

Figure 7

Schematic representation of the aggregation and agglomeration of titanium dioxide nanoparticles. Primary particles may flocculate to form weakly bound agglomeratesor sinter to form much stronger aggregates. Agglomerates may sinter to form more strongly bound aggregates. Agglomerates break down more easily than aggregates.

Figure 8

The development of the infrared absorption characteristic of carbonyl oxidation products in an unpigmented polyethylene film as the UV exposure in QUV accelerated weathering equipment, fitted with UVA-340 tubes and operated at 40 °C, increases from 125 to 1348 h. (reprinted from Polymer Degradation and Stability. 2007, 92, 2163–2172 [41] with permission).

Figure 9

Transmission electron micrographs showing (a) the uncoated TiO₂ crystals and (b) surface treated (coated) TiO₂ rutile pigment. The ZrO2/Al2O3 coating, with a thickness of 3–10 nm, shows as a less dense outline to the images (reprinted from J. Mater. Sci. 2002, 37, 4901–4909 [47] with permission).

Figure 10

(a) The development of carbonyl absorption in unpigmented and pigmented polythene films (PE A-1, ■; PE R1-1, ▲, PE U-1, ●; PE R2-1, ♦; and PE R3-1, ▼ and PE R4-1, ┼. as a function of exposure in QUV accelerated weathering equipment. After recording each IR spectrum the disc was returned to the exposure unit for further UV exposure); (b) CO₂ evolution from the photo-oxidation of the same films as used for the carbonyl development measurements in Figure 10a but exposed to irradiation from a xenon lamp [46].

Figure 11

(a) UV absorption of a 90 μm unpigmented alkyd film compared with the spectral distribution of the carbon arc lamps used in an “accelerated weathering test”. LH scale, arc lamp distribution; RH scale, Film Absorbance (reprinted from J. Mater. Sci. 2002, 37, 4901–4909 [47] with permission); (b) The degradation (weight loss as a function of time exposed to radiation from carbon arcs) of paint films of long-oil soya alkyd opacified with either uncoated rutile (solid points) or coated rutile (empty points) by 40 (■,□), 25 (●○) and 5 (♦ ◊) volume % pigment.

Figure 12

UV-C decolouration of 0.05 mM RO16 in the presence of TiO₂ (● UV-C only: ▲ 2 g·dm⁻³ TiO₂: ♦ 2 g·dm⁻³ TiO₂, 20 mM H₂O₂: ◊ No TiO₂, 20 mM H₂O₂). Reprinted from Egerton, T.A.; Purnama, H. Dyes Pigments 2014, 101, 280–285 [53] with permission.

Figure 13

A schematic depiction of the effects of increased UV absorption associated with improved dispersion of TiO₂ particles. The second and third rows represent changes in the transmission spectrum and the attenuation of the incident UV beam as the particle dispersion is altered in the way depicted in the top row.

Figure 14

(a)Optical transmission curves measured on (A) unmilled 140 m²·g⁻¹ rutile, and the same rutile milled for (B) 7.5 minute, (C) 15 minutes and (E) 30 minutes. Curve D was measured on a 15-minute milled and diluted sample that had been left to stand for the duration of a typical oxidation experiment prior to making the measurement; (b) The time dependence of propanone formation during the photocatalytic oxidation of propan-2-ol by the same 140 m²·g⁻¹ rutile before sand-milling ●, and after milling for 7.5 ■, 15 ▲ and 30 ×, minutes. Reprinted from Egerton & Tooley, J. Phys. Chem. B 2004, 108, 5066–5072 [36] with permission.

Figure 14

(a)Optical transmission curves measured on (A) unmilled 140 m²·g⁻¹ rutile, and the same rutile milled for (B) 7.5 minute, (C) 15 minutes and (E) 30 minutes. Curve D was measured on a 15-minute milled and diluted sample that had been left to stand for the duration of a typical oxidation experiment prior to making the measurement; (b) The time dependence of propanone formation during the photocatalytic oxidation of propan-2-ol by the same 140 m²·g⁻¹ rutile before sand-milling ●, and after milling for 7.5 ■, 15 ▲ and 30 ×, minutes. Reprinted from Egerton & Tooley, J. Phys. Chem. B 2004, 108, 5066–5072 [36] with permission.

Figure 15

The effect increasing milling times on the first order rate constant for the degradation by high area rutile of 0.36 mM salicylic acid at pH 4. Reprinted from J. Photochem. Photobiol. A 2010, 216, 268–274 [40] with permission.

Figure 16

Effect of milling time on the DCA oxidation rate by a high area rutile ♦ and by P25 ■. Reprinted from J. Photochem. Photobiol. A 2010, 216, 268–274 [40] with permission.