Stability of TiO2-coated ZnO photocatalytic thin films for photodegradation of methylene blue

Abstract

Investigations on the stability of titanium dioxide (TiO2)-coated zinc oxide (ZnO) thin films upon repeated uses for methylene blue (MB) degradation were conducted. Photocorrosion of ZnO, upon exposure to light in aqueous media, can affect the photocatalytic performance due to loss of material. Hence, coating with a more stable metal oxide was seen as a way to suppress the effects of photocorrosion. In this study, homogeneous wurtzite ZnO nanostructured thin films were obtained from thermal oxidation of sputter-deposited Zn films on glass substrates. TiO2 was subsequently deposited onto the ZnO nanostructured thin films using a reactive magnetron sputtering system in an admixture of argon and oxygen gases. After deposition, the thin films were annealed at 500 °C for 1 h. The photocatalytic efficiency and stability of the thin films were investigated after multiple degradation cycles. The addition of a TiO2 film increased the surface roughness and blueshifted the absorption edge of the ZnO thin films. The coated films obtained up to 94.3% degradation efficiency of MB after a 180-min exposure cycle using a solar light simulator. After three cycles, degradation efficiency decreased for the uncoated ZnO photocatalysts. Analysis of the MB solution after one degradation cycle revealed the presence of Zn2+ ions attributed to the effects of photocorrosion. Higher Zn2+ concentrations were observed when the ZnO surface is uncoated. This study showed that the addition of a thin, antiphotocorrosion material such as TiO2 layer decreased the dissolution of ZnO caused by photocorrosion without a significant reduction in the photodegradation efficiency.

Characterization and Growth of TiO2/ZnO on PTFE Substrates at Different Volumetric Ratios Using Chemical Bath Deposition

“…Rough surfaces are desirable because they enhance multiple light reflections from the samples. Moreover, the absorptions and tandem reflections of radiation increase based on the effect of the wavelength accompanying the measurement, whereas vertical or patterned surfaces are necessary for the entire retention of incident light [46][47][48].…”

Shaping the Structure and Properties of TiO2-ZnO Oxide Coatings Produced by Plasma Electrolytic Oxidation on Titanium Substrate

“…It is also worth comparing the energy gap values of the obtained oxide coatings with those for pure semiconductor oxides that can be found in the literature. The hybrid material we obtained has an energy gap with a value that falls between the energy gap value of pure TiO2, i.e., Eg = about 3.0 eV [11], which provides activity in the UV region, and ZnO where Eg = 3.37 eV (VIS/UVnear) [24], which is a good sign for the aspiration in obtaining a material with photocatalytic activity in visible light. In addition, they are comparable to the energy gap of other ZnO-based materials that have been studied in recent years such as Al-doped ZnO, where Eg ranges from 3.18 to 3.27 eV depending on the synthesis method used [34], or PEO-produced ZnO thin film on zinc substrate with Eg = 3.17, respectively [35].…”

“…On the other hand, despite prospects, pure ZnO-based films have a high tendency to corrode or are subject to photodegradation [8,9], which means that these materials have limited 2 of 14 durability and therefore limited usefulness. The combination of ZnO with TiO 2 results in the increased chemical durability of the layers [10,11]. Some researchers have proven that TiO 2 -ZnO semiconductor systems with various band gap widths can enhance the charge separation rate and visible-light photocatalytic activity compared with a single-component catalyst [12,13].…”

Promising Cr-Doped ZnO Nanorods for Photocatalytic Degradation Facing Pollution

“…A recycle experiment was used to inspect the photostability of the photocatalyst, as reported in several studies [57][58][59]. The used ZnO-Cr 1 nanoparticles were centrifuged, washed and then dried for the next experiment.…”