Heterostructured Metaloxide Photocatalytic (Copper Oxide and Titanium Oxide) for Degradation and Removal of Water Organic Pollutants

Metal oxides with favourable light absorption properties and charge transport characteristics play wide application as a photocatalyst. Titanium oxide and Copper oxide are the two major earth abundant metal oxides, and this dissertation is aimed to synthesize the heterostructured copper oxide and titanium oxide for the application of water treatment by enhancing their photocatalytic activity. Metal oxides are treated with atmospheric pressure plasma or by altering the structural level in nano size region to enhance the photocatalytic activity. Cupric oxide (CuO) thin film has found widespread application as a low-cost material for photo catalytic applications. High surface wettability is a key factor to achieve enhanced efficiency in these catalytic applications. Here, we report a fast and environment friendly route to fabricate super hydrophilic CuO thin films using a low power (5–10 W) atmospheric pressure plasma jet (APPJ). With APPJ treatment for 5 min, the CuO surface transforms from hydrophobic to super-hydrophilic with increase in catalytic activity. APPJ introduces anisotropy in the crystal structure and creates unique three-dimensional surface morphology with distinct surface chemical and electronic features. The treated films exhibited a higher rate of photo degradation of Methylene Blue and phenol indicating efficient charge separation. An environment friendly alternative to epitaxially grown process of copper oxide nanowires (NWs) on copper substrates using single step atmospheric pressure plasma jet assisted oxidation is used. NWs of average length 300 nm are grown rapidly in 5 minutes along with transforming the surface to superhydrophilic. This method introduces defects in the nanowire structure which is otherwise difficult to achieve due to the highly isotropic nature of nanowire growth. High resolution transmission electron microscopy reveals vacancies and structural defects such as lattice twinning and kinks. Copper oxide NWs have an excellent degradation activity towards organic pollutants Methylene Blue. Two-dimensional (2D) Molybdenum disulfide (MoS2) has become one of the most exciting areas of research for adsorbents due to its high surface area and abundant active sites. Mainly, 2D MoS2 show promising removal of textile dye pollutants by adsorption process, but it shows high affinity for anionic type of dyes, that limits its performance in mixed dye pollutants treatment. Herein, we demonstrate an integrated approach to remove mixed dye pollutants (anionic and cationic) concurrently by combining adsorption and photocatalysis process. We synthesize MoS2/TiO2 nanocomposites for different weight percentages 2.5, 5, 10, 20, 30 and 50 wt% of pre-synthesized flower-like MoS2 nanoparticle by a two-step hydrothermal method. We demonstrate a new process of two-stage adsorption / photocatalysis using high wt% of MoS2 (Stage-I) and low wt% of MoS2 (Stage-II) nanocomposites. The proposed two-stage integrated adsorption and photocatalysis process using 50% and 2.5% of MoS2 coated TiO2, respectively showed complete removal of methylene blue dye ~5 times faster than conventional single-stage (adsorption or photocatalysis) water treatment process. Furthermore, the feasibility of the proposed two stage method in mixed dye pollutants removal (anionic and cationic) showed excellent performance even in doubling the dye pollutant concentration. This work brings a deeper insight into understanding the morphology and concentration of 2-D MoS2 in MoS2/TiO2 nanocomposite in tackling mixed dye pollutants and the possibilities of applying in textile dyeing industries wastewater treatment plants

Microwave- and Formaldehyde-Assisted Synthesis of Ag–Ag₃PO₄ with Enhanced Photocatalytic Activity for the Degradation of Rhodamine B Dye and Crude Oil Fractions

The release of crude oil and water-soluble dyes into our marine environment is a major global problem. An efficient semiconductor Ag–Ag3PO4 photocatalyst was synthesized using formaldehyde as a reducing agent to form surface active Ag on Ag3PO4 under microwave radiation for heating, and its potential in destroying environmental pollutants has been examined. The diffuse reflectance spectroscopy of Ag–Ag3PO4 revealed an enhanced absorption in the visible light region. The rate of photocatalytic degradation of rhodamine B by Ag–Ag3PO4 was over 4-fold compared to Ag3PO4. The potential application of Ag–Ag3PO4 in oil spill remediation was also examined through photocatalytic degradation of benzene, n-hexane, and 1:1 v/v benzene/methanol crude oil-soluble fractions. UV–vis and gas chromatography–mass spectrometry analysis of the crude oil components after visible light irradiation showed excellent degradation. The photocatalytic efficiency enhancement of Ag–Ag3PO4 is attributed to the excellent electron trapping of silver nanoparticles deposited on the surface of Ag3PO4. This work will motivate future studies to develop recyclable visible light photocatalysts for many applications

Ultrafast epitaxial growth of CuO nanowires using atmospheric pressure plasma with enhanced electrocatalytic and photocatalytic activities

This work reports an environment friendly alternative to epitaxially grow copper oxide nanowires (NWs) on copper substrates using single step atmospheric pressure plasma jet assisted oxidation. NWs of average length 300 nm are grown rapidly in 5 minutes along with transforming the surface to superhydrophilic. This method introduces defects in the nanowire structure which is otherwise difficult to achieve due to the highly isotropic nature of nanowire growth. High resolution transmission electron microscopy reveals vacancies and structural defects such as lattice twinning and kinks. Theoretical investigations using density functional theory calculations indicated that oxygen vacancies reduces the adsorption energy of methanol molecules onto the CuO (111) surface and shifts the Fermi level towards conduction band. During electrocatalysis, these defect-rich nanowires exhibit twice the catalytic activity toward oxygen evolution reaction (OER) and methanol oxidation reaction (MOR) in comparison to the traditionally thermally grown nanowires. Moreover, retreating the electrodes after each stability test drops the contact resistance similar to the prisitine sample. Additionally, these NW photocathodes demonstrate an exceptional photocurrent of 2.2 mAcm–2 and have an excellent degradation activity towards organic pollutants namely phenol and paracetamol. This facile growth method can be used to engineer nanowires of other transition metals with enhanced activities

Tailoring the properties of Cu2ZnSnS4 thin films grown by ultrasonic spray pyrolysis

Cu2ZnSnS4 (CZTS) thin films are grown using an automated ultrasonic spray pyrolysis setup. Sn concentration is varied to attain properties suitable for solar cell applications. Kesterite tetragonal structured CZTS phase is dominant in all films. When Cu/(Zn + Sn) > 0.83, films showed Cu2S phase in addition to CZTS phase. Film is nearly stoichiometric when Cu/(Zn + Sn) concentration ratio in the precursor solution is 1.07. When Sn concentration is reduced, Cu concentration increases proportionally and a significant change in morphology occurred for the Cu-rich films. All the samples are p-type in nature and their resistivity varies between 255 and 0.9 x 10(-3) ohm cm. Sample with Cu/(Zn + Sn) ratio of 0.83 is found to be suitable for solar cell applications. (C) 2016 Elsevier B.V. All rights reserved

Improved photoelectrochemical properties of TiO₂-graphene nanocomposites: Effect of defect induced visible light absorption and graphene conducting channel for carrier transport

Use of heterojunctions between two materials having favorable optical and electronic properties can lead to increased photon absorption and charge separation resulting in enhanced photo-electro-chemical energy conversion. In the present study, graphene monolayer nano-flakes are mixed with TiO2 nanoparticles to form nanocomposites having different weight percentages of graphene. The microstructural, morphological and structural properties of the composite samples are investigated using X-ray diffraction, Raman spectroscopy and transmission electron microscopy techniques. Raman studies carried out on samples annealed at different temperatures show the interfacial interaction between TiO2 and graphene, although Anatase TiO2 and graphene maintain their phase integrity. PEC measurements show higher photo-electro-chemical activity in TiO2-graphene nanocomposite at an optimized concentration (2.0 weight percent) due to increased surface area, higher optical absorption in the visible part of the solar spectrum and favorable carrier transport due to increased concentration of defect states and graphene acting as a charge carrier medium

Cu₂O/CuO heterojunction catalysts through atmospheric pressure plasma induced defect passivation

A novel route to fabricate Cu2O/CuO heterojunction electrodes using an atmospheric pressure plasma jet (APPJ) is demonstrated. This process promotes favourable band alignment and produces nanoscale CuO surface features from Cu2O with low density of interfacial defects. This electrode can operate without any transparent current collector, showing remarkable currents and stability towards oxygen evolution reaction (OER) (6 mA cm−2 for 2 h at pH14) as well as photocatalytic hydrogen evolution reaction (HER) activity (−1.9 mA cm−2 for 800 s at pH7). When the electrocatalytic oxygen evolution (OER) activity was measured for Cu2O/CuO electrode deposited on FTO substrate the currents increased to ~40 mA cm−2 at 0.8 V vs SCE in 1 M KOH without compensating for the electrode electrolyte surface resistance (iR correction). The composite films also exhibited a high rate towards photo degradation of Methylene Blue (MB) and phenol in the visible spectra, indicating efficient charge separation. We modelled the electronic structure of this epitaxially grown Cu2O/CuO heterojunction using density functional theory. The calculations revealed the distinctive shifts towards Fermi level of the p-band centre of O atom in Cu2O and d-band centre of Cu atom in CuO at the interface contribute towards the increased catalytic activity of the heterostructure. Another factor influencing the activity stems from the high density of excited species in the plasma introducing polar radicals at the electrode surface increasing the electrolyte coverage. This work presents the potential of APPJ functionalization to tune the surface electronic properties of copper oxide based catalysts for enhanced efficiency in OER and HER water splitting

Atmospheric pressure plasma engineered superhydrophilic CuO surfaces with enhanced catalytic activities

Cupric oxide (CuO) thin film has found widespread application as a low-cost, earth-abundant material for electro and photo catalytic applications. High surface wettability is a key factor to achieve enhanced efficiency in these catalytic applications. Here, we report a fast and environment friendly route to fabricate super hydrophilic CuO thin films using a low power (5 to 10 Watts) atmospheric pressure plasma jet (APPJ). With APPJ treatment for 5 minutes, the CuO surface transforms from hydrophobic to super-hydrophilic with threefold increase in catalytic activity. The electrodes were extensively characterized using various bulk and surface-sensitive techniques. APPJ introduces anisotropy in the crystal structure and creates unique three-dimensional surface morphology with distinct surface chemical and electronic features. Interestingly, presence of oxygen in the plasma was found to be critical for the enhanced activities and the activity decreased when the functionalised with nitrogen plasma. Oxygen plasma functionalisation of CuO electrodes resulted in a 130 mV reduction in the onset potential for oxygen evolution reaction along with enhanced current density ,10 mA cm-2 against 3 mA cm-2 at 1 V vs Saturated Calomel Electrode in 0.1M KOH without iR compensation. Importantly, without introducing any external dopants the work function could be decreased by 80mV. Moreover, the treated films exhibited a higher rate of photo degradation (0.0283 min-1 compared to 0.0139 min-1) of Methylene Blue and phenol indicating efficient charge separation. This work presents the potential of APPJ functionalisation of CuO surface to boost the activity of other thin film catalyst materials and solutions processed systems