Phenol-photodegradation on ZrO2. Enhancement by semiconductors

https://doi.org/10.1016/j.saa.2012.02.040Get rights and content

Abstract

On illumination with light of wavelength 365 nm phenol undergoes degradation on the surface of ZrO2. The rate of degradation enhances linearly with the concentration of phenol and also the light intensity but decreases with increase of pH. The photonic efficiency of degradation is higher with illumination at 254 nm than with 365 nm. The diffuse reflectance spectral study suggests phenol-sensitized activation of ZrO2 with 365 nm light. TiO2, Fe2O3, CuO, ZnO, ZnS, Nb2O5 and CdO particles enhance the photodegradation on ZrO2, indicating inter-particle charge-transfer. Determination of size of the particles under suspension, by light scattering technique, shows agglomeration of particles supporting the proposition of charge-transfer between particles.

Highlights

► Phenol-adsorbed ZrO2 absorbs UV-A light. ► Phenol degradation on ZrO2 is larger with UV-C light than with UV-A light. ► Particulate semiconductors enhance phenol photodegradation on ZrO2 particles. ► Particles agglomerate.

Introduction

Semiconductors on illumination of light with energy not less than the band gap produce electron–hole pairs, holes in the valence band and electrons in the conduction band. Some of these charge carriers reach the crystal surface and react with the adsorbed substrates resulting in photocatalysis and the rest recombine lowering the photocatalytic efficiency [1], [2], [3]. The hole reacts with the adsorbed organics generating intermediates, which through fast reactions finally yield carbon dioxide and water. The adsorbed oxygen molecule takes up the conductance band electron transforming into highly active superoxide radical, O2radical dot. In aqueous medium, O2radical dotin turn generates reactive species such as HOradical dot, HO2radical dot and H2O2, which oxidize the organics. Water is adsorbed on the semiconductor, molecularly as well as dissociatively. Hole-trapping by either the surface hydroxyl groups or the adsorbed water molecules yields short-lived HOradical dot radicals, which are the primary oxidizing agents. ZrO2 is resistant to photocorrosion and is used as photocatalyst. But its band gap is very large (∼5.0 eV) [4] and requires UV-C light (<280 nm) for generation of electron–hole pairs. Illumination with light of wavelength 254 nm enables band gap excitation and photocatalytic reduction of CO2 [5] and degradation of rhodamine B [6] by ZrO2 have been reported. Surprisingly, illumination at 365 nm also brings in photoreaction on the surface of ZrO2 and the reported oxidation of isopropanol [7], [8], nitrite [9], aniline [10] and ethylene diamine tetraacetate (EDTA) [9], reduction of chromium(VI) [9] and decomposition of water [11] on ZrO2, photocatalytic oxidation of 4-nitrophenol on ZrO2-SiO2 [12] and photocatalytic generation of hydrogen on Zr-MCM-41 [13] are with 365 nm-light. But, there is no study on the mechanism of ZrO2-photocatalysis under 365 nm-illumination and here we address the same taking phenol as the model substrate. The present results indicate substrate sensitized photoactivation of ZrO2. Reports on the photodegradation of phenol are numerous and the semiconductors used as catalysts are TiO2 in different modifications [14], [15], [16], [17], [18] including metal-doped [15] and dye-sensitized TiO2 [19], MoO3 [20], MoS2 [21], WO3 [16], WS2 [22], PW12O403− [23], α-Fe2O3 [17], [18], [23], CuO [17], [18], GdCoO3 [24], ZnO [16], [17], [18], ZnS [17], [18], CdO [18], CdS [16], [18], CdSe/TiO2 [25], In2O3/TiO2 [26], SnO2 [21], Bi2O3 [18], Sb2O3 [18] and CeO2 [18]; all these semiconductors undergo band gap excitation with UV-A light.

Mixture of particulate semiconductors show enhanced photocatalytic activity. On illumination, both the semiconductors are simultaneously excited and electrons slip to the low-lying conduction band of one semiconductor while holes move to the less anodic valence band. Improved electron–hole pair separation enhances the photocatalytic efficiency [27], [28]. Enhanced photocatalysis by particulate semiconductor mixtures under potential bias has also been observed recently [29], [30]. But, here we report enhancement of phenol-photodegradation on ZrO2, illuminated with 365 nm-light, by particulate semiconductors – an unusual synergism. The present results are unique; the energy of illumination is sufficient to excite the added semiconductor particles but not ZrO2 particles.

Section snippets

Materials

TiO2 (Merck), ZrO2 (Chemco), Fe2O3 (Fischer), CuO (Sd fine), ZnO (Merck), ZnS (Sd fine), Nb2O5 (Sd fine) and CdO (Chemco) of analytical grade were used as received. Analytical grade phenol was distillation before used. Other chemicals used were also of analytical or laboratory grade.

Characterization

The powder XRDs of the oxides were obtained using a Bruker D8 system with Cu Kα radiation of 1.5406 Å in a 2θ range of 5–60° at a scan speed of 0.050° s−1or a Siemens D-5000 XRD with Cu Kα X-rays of wavelength 1.5406 Å

Catalysts characterization

The TiO2 used is of anatase phase. The XRD of the sample totally matches with the standard pattern of anatase (JCPDS 00-021-1272*) and the rutile lines (00-034-0180 D) are absent. The crystal parameters are as follows: tetragonal, a 3.7845 Å, c 9.5143 Å, body-centered. The XRD of ZnO is identical with the JCPDS pattern of zincite (00-005-0664 D) and reveals the crystal structure as hexagonal (primitive) with the cell constants a and b as 3.2490 Å and c as 5.2050 Å. The BET surface areas (S),

Conclusions

Phenol degrades on ZrO2 under UV-A light and the degradation is larger with UV-C light. The degradation follows first-order kinetics with respect to phenol and photon flux and slows down with increase of pH. Phenol-sensitized excitation of ZrO2 is the likely mechanism of the degradation with UV-A light. TiO2, Fe2O3, CuO, ZnO, ZnS, Nb2O5 and CdO mixed with ZrO2 show enhanced photodegradation. The semiconductor particles agglomerate under suspension and electron-transfer from the conduction band

Acknowledgements

The authors thank the Council of Scientific and Industrial Research (CSIR), New Delhi, for the financial support through research grant no. 01(2031)/06/EMR-II. R.D. and P.G are grateful to Annamalai University and CSIR for UF and JRF, respectively.

References (38)

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