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TiO2 nanotube (TNT) arrays were fabricated in NaOH/fluoride/ethylene glycol (EG) electrolyte at 30 V and 60 V for 60 min via anodization method. Three different concentrations of NaOH were studied: 1 M, 1.5 M and 2 M. Self-organized, vertically aligned and uniform nanotubes were produced for all samples but longer nanotubes were formed when 60 V was used instead of 30 V. The fabricated TNTs at 60 V were used as photoanode in a photoelectrochemical (PEC) cell. PEC cell with photoanode fabricated in 1 M NaOH/fluoride/EG electrolyte displayed photocurrent of 3.5 mA/cm2 under light illumination.

Anodisation of zirconium, Zr at 40 V for 60 min in 0.1 wt.% NH4F/ethylene glycol (EG) added to it 1 ml, 2 ml or 3 ml of water resulted in the formation of self-aligned ZrO2 nanotube (ZNT) arrays. Water content did not affect the length and diameter of the ZNTs but was influencing surface etching whereby better surface etching was observed for sample anodised in 3 ml-water/NH4F/EG. From transmission electron microcopy (TEM) image, the ZNTs were found to have an outer diameter of ~ 40 nm and wall thickness of 10 nm. Annealing at 400 °C was resulted in monoclinic (M-ZrO2) and tetragonal (T-ZrO2) formation, but at 600 °C M-ZrO2 dominates. Cr(VI) reduction is higher for ZNTs annealed at 400 °C compared to 600 °C sample.

Even though heavy metals are naturally occurring compounds, in recent years, anthropogenic activities have introduced them in excessive quantities to different environmental matrices, imposing severe threats to the ecosystem. This chapter is focused on the occurrence of heavy metals in Malaysia, reviewing their potential sources, their existence in water, sediment, and aquatic life. However, despite the contents of heavy metals found in water, sediment, and aquatic life, they remain within the permissible limit. Heavy metals are known to be nondegradable and hence can lead to bioaccumulation in humans or animals. Avoiding heavy metals from entering the environment is, therefore, critical. From the literature survey undertaken in this chapter, it appears that the leading cause of heavy metal pollution in rivers is from industrial establishments and perhaps from the inadequate treatment of heavy metal–polluted wastewater discharging into rivers. Therefore treating wastewater is essential to ensure that no heavy metal ions are released into the environment. In undertaking this initiative the initial step is by having a proper detection device such as having sensors that can monitor the concentration of different types of heavy metal ions found in wastewater. Among the various technologies that are available, sensors utilizing nanostructured transition metal oxide are seen as an effective technique for the detection of various metal ions. The removal of heavy metal ions from wastewater can be accomplished in several ways of which metal oxide has been used for removal purposes. Clay and traditional oxides have also been used as an effective absorbent for highly concentrated heavy metal ions in wastewater, whereas nanostructured metal oxide semiconductors can be used to absorb and reduce the minute amount of heavy metal ions via photocatalysis process.

Visible-light-active freestanding zirconia (ZrO

In this work, iron oxide nanosheets (NSs) and nanowires (NWs) were fabricated by thermal oxidation of iron foils at 500 and 800 °C in the presence of water vapour. Oxidation resulted in multi-scale oxide with the surface oxide comprising of α-Fe2O3 and inner oxide comprising of mostly Fe3O4. The mechanism of formation is discussed to be based on stress-driven mechanism and diffusion paths of iron ions. It was found that the NSs formed could adsorb 112 mg/L aqueous Cr(VI) and the NWs could adsorb 200 mg/L of Cr(VI). It is suggested that the Cr(VI) removal by the iron oxide NSs/NWs due to the electrostatic reaction between the adsorbent and adsorbate.

Self-organized TiO2 nanotube (TNT) arrays were fabricated by anodic oxidation of titanium foil in three different fluoride electrolytes: ethylene glycol (EG), glycerol, and sodium sulfate (Na2SO4) at different voltage and anodization time. In these electrolytes, the TNTs were produced in ∼ 2 µm long, but at the different diameter. The size of inner diameter can be arranged from the largest to the smallest value in the order of glycerol > EG > Na2SO4. Crystallization of the TNTs by annealing was led to the formation of anatase-TiO2. The sample can be activated under natural sunlight for reduction of hexavalent chromium, Cr(VI) into trivalent chromium, Cr(III). The reduction was thought to occur via synergistic reactions between adsorption and photoreduction. The result demonstrates 100 % of Cr(VI) removal efficiency over the TNTs grown in EG after 120 min. Whereas, only 48 % and 45 % of Cr(VI) can be removed over the TNTs fabricated in glycerol and Na2SO4, respectively. High photocatalytic activity of the ...

ZrO

Visible-light-active freestanding zirconia (ZrO2) nanotube (FSZNT) arrays were fabricated by a facile electrochemical anodization method in fluoride containing ethylene glycol electrolyte added to it was 1 vol% of potassium carbonate (K2CO3) at 60 V for 1 h. Poor adhesion at the metal∣oxide interface was induced by K2CO3 leading to the formation of FSZNT flakes. The effect of the crystal structures of the FSZNTs e.g., amorphous, amorphous/tetragonal, and tetragonal/monoclinic was investigated towards the photocatalytic reduction of 10 ppm hexavalent chromium, Cr(VI) at pH 2 under sunlight. The results demonstrate the amorphous FSZNTs exhibited the highest Cr(VI) removal efficiency than the crystalline FSZNTs (95% versus 33% after 5 h). The high photocatalytic activity of the amorphous FSZNTs can be attributed to enhanced Cr(VI) adsorption, high visible light absorption, and better charge carrier separation. The low photocatalytic activity of the crystalline FSZNTs annealed at 500 °C was mainly attributed to poor Cr(VI) adsorption, low visible light absorption, and less photoactive monoclinic-ZrO2.

In this work, controllable growth of ZnO nanorods by hydrothermal process was achieved on a seeded substrate prepared by thermal oxidation of Zn foil. The Zn foil was oxidised so that the surface grain would consists of only uniform circular nanosized grains and hence, suitable as a template for the formation of ZnO nanorods. The formation of ZnO nanorods occurred via the reaction of hydroxyl ions released during the thermal degradation of hexamethylamine (HMT) with Zn ions in the precursor solution. By keeping the ratio of Zn ions to HMT constant at 1:1, the dimensions of the nanorods are found to be a function on the properties of HMT which are time and temperature dependent. The optimum hydrothermal reaction parameters of 4–6 h at 80 °C are found suitable to grow needle-like nanorods with length of ∼700 nm and base diameter of ∼200 nm and top diameter of less than 30 nm. The results prove that ZnO nanorods formation via hydrothermal process is highly dependent on the rate of HMT decomposition. Suitable temperature and hydrothermal reaction time are essential to allow decomposition of HMT to produce the hydroxyl ions as oxygen source for ZnO formation. The success in the formation of ZnO nanorods of controllable dimensions on seeded substrate formed by the oxidized Zn foil is noteworthy as an alternative approach in the formation of self-aligned ZnO nanostructures.