Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Phenol-photodegradation on ZrO2. Enhancement by semiconductors
Graphical abstract
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, O2–. In aqueous medium, O2–in turn generates reactive species such as HO, HO2 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 HO 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)
- et al.
Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems
J. Photochem. Photobiol. C
(2008) - et al.
Photocatalytic activity of ZrO2 nanoparticles prepared by electrical arc discharge method in water
Polyhedron
(2010) - et al.
Photoconductive and photocatalytic properties of ZrTiO4. Comparison with the parent oxides TiO2 and ZrO2
J. Photochem. Photobiol. A
(1997) - et al.
Photocatalytic properties of ZrO2 and Fe/ZrO2 semiconductors prepared by a sol–gel technique
J. Photochem. Photobiol. A
(1999) - et al.
Photocatalysis with ZrO2: oxidation of aniline
J. Mol. Catal. A
(2005) - et al.
Effect of carbonate addition on the photocatalytic decomposition of liquid water over a ZrO2 catalyst
J. Photochem. Photobiol. A
(1996) - et al.
Photocatalytic generation of hydrogen on Zr-MCM-41
Int. J. Hydrogen Energy
(2002) - et al.
Studies on the heterogeneous photocatalytic oxidation of 2,6-dinitrophenol in aqueous TiO2 suspension
J. Mol. Catal. A
(2004) - et al.
Semiconductor-catalyzed degradation of phenols with sunlight
Sol. Energy Mater. Sol. Cells
(2008) Phthalocyanine-modified titania-catalyst for photooxidation of phenols by irradiation with visible light
J. Photochem. Photobiol. A
(2002)
Semiconductor-sensitized photodegradation of 4-chlorophenol in water
J. Photochem. Photobiol. A
Photocatalytic behavior of mixed WO3/WS2 powders
Catal. Today
Degradation of chlorophenols by means of advanced oxidation processes: a general review
Appl. Catal. B
Capability of coupled CdSe/TiO2 for photocatalytic degradation of 4-chlorophenol
J. Hazard. Mater.
Enhanced phenol-photodegradation by particulate semiconductor mixtures: interparticle electron-jump
J. Hazard. Mater.
Solar-powered potentially induced TiO2, ZnO and SnO2-catalyzed iodine generation
Sol. Energy Mater. Sol. Cells
Degradation of phenol by nanomaterial TiO2 in wastewater
Chem. Eng. J.
TiO2-photocatalyzed oxidation of aniline
J. Photochem. Photobiol. A
Photooxidation of iodide ion on some semiconductor and non-semiconductor surfaces
Catal. Commun.
Cited by (30)
Vapor-solid growth ZnO:ZrO<inf>2</inf> micro and nanocomposites
2021, Journal of Alloys and CompoundsCitation Excerpt :al [10]. However, as its bandgap (5 eV) belongs to the UV-C region of the electromagnetic spectrum, the efficiency of electron-hole pairs generation by solar light is limited [11]. Vapor Solid (VS) growth has shown to be a suitable method to obtain a large amount of micro- and nanostructures of both pure ZnO and doped with several transition metal [12,13] and rare earth ions [14,15].
Immobilization of TiO<inf>2</inf>and TiO<inf>2</inf>-GO hybrids onto the surface of acrylic acid-grafted polymeric membranes for pollutant removal: Analysis of photocatalytic activity
2020, Journal of Environmental Chemical EngineeringCitation Excerpt :Heterogeneous photocatalysis has been recognized as a promising method for the removal of organics in water. Various metal oxide photocatalysts have been commonly applied including TiO2, ZnO [1,2], Al2O3 [3], SnO2 [4], Fe3O4 [5], and ZrO2 [6]. Among these, TiO2 has received much attention particularly for practical applications due to its high photocatalytic activity, high chemical stability, and cost-effectiveness [7].
Mitigation of pollutants by chitosan/metallic oxide photocatalyst: A review
2020, Journal of Cleaner ProductionEnhanced UV–Visible triggered photocatalytic degradation of Brilliant green by reduced graphene oxide based NiO and CuO ternary nanocomposite and their antimicrobial activity
2020, Arabian Journal of ChemistryCitation Excerpt :In this connection, Nickel oxide (NiO), a p-type oxide semiconductor with cubic lattice structure, is used in versatile applications like catalysis, battery cathodes, gas sensors, magnetic materials and electrochromic films depending on the effect of quantum size, volume, surface area which implies anodic electrochromism, terrific durability, electrochemical stability, and huge spin optical density (El-Kemaryn et al., 2013). The former reports state that NiO has a wide bandgap energy range of 3.6–4.0 eV, translucent to UV, visible and near infrared radiation (El-Kemaryn et al., 2013; Karunakaran et al., 2012). Due to the wider bandgap, NiO requires a UV-C light (280 nm) for excitation and create electron-hole pairs (Karunakaran et al., 2012).