Recent developments of zinc oxide based photocatalyst in water treatment technology: A review

The degradation of acid red 14 (AR14), commonly used as a textile dye, can be photocatalysed by ZnO. Using advanced oxidation processes (AOPs), zinc oxide appears to be a suitable alternative to TiO2 for water treatment. In this study, a detailed investigation of photocatalytic degradation of acid red 14 is presented. Photodegradation efficiency was small when the photolysis was carried out in the absence of ZnO and it was also negligible in the absence of UV light. The semi-log plot of dye concentration versus time was linear, suggesting first order reaction (K=0.0548 min−1). The effects of some parameters such as pH, amount of photocatalyst, hydrogen peroxide and ethanol concentration were also examined. The addition of proper amount of hydrogen peroxide improved the decolorization, while the excess hydrogen peroxide could quenched the formation of hydroxyl radicals ( OH). As our results indicated that ethanol inhibited the photodegradation of dye, we concluded from the inhibitive effect of ethanol that hydroxyl radicals played a significant role in the photodegradation of dye. This should not undermine direct oxidation caused by positive holes.

Photocatalytic degradation of azo dye acid red 14 in water on ZnO as an alternative catalyst to TiO2

A ZnO photocatalyst was hybridized with graphite-like C3N4via a monolayer-dispersed method. After hybridization with C3N4, the photocurrent of ZnO was enhanced by 5 times under UV irradiation and a photocurrent under visible light irradiation was observed. The photocatalytic activity of C3N4/ZnO under UV irradiation was increased by 3.5 times, the visible light photocatalytic activity was generated and the photocorrosion of ZnO was suppressed completely after ZnO was hybridized with C3N4. The enhancement in performance and photocorrosion inhibition under UV irradiation was induced by the high separation efficiency of photoinduced holes from ZnO to the HOMO of C3N4. Under visible light irradiation, the electron excited from the HOMO to the LUMO of C3N4 could directly inject into the CB of ZnO, making C3N4/ZnO present visible light photocatalytic activity. The optimum synergetic effect of C3N4/ZnO was found at a weight ratio of 3%, which corresponded to a monolayer dispersion of C3N4 on the surface of ZnO.

Enhancement of photocurrent and photocatalytic activity of ZnO hybridized with graphite-like C3N4

Considerable efforts have been devoted to enhancing the photocatalytic activity and solar energy utilization of photocatalysts. The fabrication of type II heterostructures plays an important role in photocatalysts modification and has been extensively studied. In this review, we briefly trace the application of type II heterostructured semiconductors in the area of environmental remediation and water splitting, summarize major fabrication methods, describe some of the progress and resulting achievements, and discuss the future prospects. The scope of this review covers a variety of type II heterostructures, focusing particularly on TiO2 and ZnO based visible light driven type II 0D and 1D heterostructured photocatalysts. Some other low dimensional nanomaterials which have shown high-performance photocatalysis are also presented. We expect this review to provide a guideline for readers to gain a clear picture of fabrication and application of type II heterostructures.

Visible light driven type II heterostructures and their enhanced photocatalysis properties: a review

Enhanced photocatalytic activity of ZnO/CuO nanocomposite for the degradation of textile dye on visible light illumination.

The degradation of 2-phenylphenol (OPP), commonly used as fungicide, can be photocatalysed by TiO 2 and ZnO. The latter is a little more efficient but cannot be used in acidic solution. In both cases, the main identified photoproducts are hydroquinone (HQ), p -benzoquinone (BQ), phenylhydroquinone (PHQ), phenylbenzoquinone (PBQ), 2,2′- and 2,3-dihydroxybiphenyls (2,2′- and 2,3-BPh(OH) 2 ). The observed minor formation of 2-hydroxydibenzofuran is due to the photocyclisation of PBQ. The influence of various parameters is considered. The phototransformation of OPP is slightly favoured by increasing pH. It is enhanced by the addition of small amounts of H 2 O 2 , but it is inhibited by high concentrations of chloride, sulphate or nitrate. From the inhibiting effect of alcohols, it is deduced that the main degradation pathway involves hydroxyl radicals, but a direct oxidation by positive holes h + is probably not negligible. The observed slow photodegradation of ZnO in unbuffered solution is attributed to an electrochemical process. A mechanism is proposed for the photocatalytic transformation of OPP.

Photocatalytic degradation of 2-phenylphenol on TiO2 and ZnO in aqueous suspensions

The heterogeneous photocatalytic degradation of carbamazepine (CBZ) was investigated in the presence of BiOCl/AgCl composite photocatalyst under simulated sunlight irradiation in water. BiOCl/AgCl composite showed higher photocatalytic activity than pure BiOCl for CBZ degradation. The photocatalytic mechanism analysis was based on byproducts identification by LC–MS-QTof and active species trapping or inhibiting experiments. The results revealed that the first step of the transformation mainly results in an electron transfer implying positive holes and to a lesser extent in hydroxyl radical’s involvement. The enhanced photocatalytic performance of BiOCl/AgCl was proved to be related to the suitable conduction and valence band interaction that extends optical response range but also improves the efficient separation of photoinduced electron-hole pairs. BiOCl/AgCl composite totally removed CBZ from natural surface water after 30 min irradiation, suggesting its potential application to wastewater treatments. Eight intermediate products were identified demonstrating that CBZ transformation occurs through two main routes from CBZ radical cation, hydroxylation of ring (aromatic or seven membered rings), followed by further oxidation, rearrangement ring and hydroxylation.

Photocatalytic degradation of antiepileptic drug carbamazepine with bismuth oxychlorides (BiOCl and BiOCl/AgCl composite) in water: Efficiency evaluation and elucidation degradation pathways

The photocatalytic degradation of metoxuron [3-(3-chloro-4-methoxyphenyl)-1,1-dimethylurea] in aqueous suspensions of TiO2 was investigated. Several intermediate photoproducts were identified using NMR and MS techniques. Oxidation or removal of the methyl of terminal nitrogen and ring hydroxylation were found to occur. 2-Propanol was shown to decrease the rate of photocatalytic degradation, inhibiting partly ring hydroxylation and completely reactions on terminal nitrogen. In contrast, basification of the suspensions accelerated the degradation significantly. Degussa P25 TiO2 was found to exhibit a higher photocatalytic activity than Millennium TiO2 in spite of a generally smaller specific surface area. Within the Millennium TiO2 series, the photocatalytic efficiency increased with the specific surface area.

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Amina Amine Khodja

Papers

Photocatalytic degradation of 2-phenylphenol on TiO2 and ZnO in aqueous suspensions

Photocatalytic degradation of antiepileptic drug carbamazepine with bismuth oxychlorides (BiOCl and BiOCl/AgCl composite) in water: Efficiency evaluation and elucidation degradation pathways

Photocatalytic degradation of metoxuron in aqueous suspensions of TiO2. Analytical and kinetic studies