Mohammad Mansoob Khan

Band gap engineered TiO2 nanoparticles for visible light induced photoelectrochemical and photocatalytic studies

Visible light-active TiO2 (m-TiO2) nanoparticles were obtained by an electron beam treatment of commercial TiO2 (p-TiO2) nanoparticles. The m-TiO2 nanoparticles exhibited a distinct red-shift in the UV-visible absorption spectrum and a much narrower band gap (2.85 eV) due to defects as confirmed by diffuse reflectance spectroscopy (DRS), photoluminescence (PL), X-ray diffraction, Raman spectroscopy, electron paramagnetic resonance, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and linear scan voltammetry (LSV). The XPS revealed changes in the surface states, composition, Ti4+ to Ti3+ ratio, and oxygen deficiencies in the m-TiO2. The valence band XPS, DRS and PL results were carefully examined to understand the band gap reduction of m-TiO2. The visible light-responsive enhanced photocatalytic activity of m-TiO2 was demonstrated by degrading methylene blue and brilliant blue G. The EIS and LSV in the dark and under visible light irradiation further support the visible light-induced photocatalytic activities of the m-TiO2 due to a decrease in electron transfer resistance and an increase in photocurrent. This study confirms that m-TiO2 can be used effectively as a photocatalyst and photoelectrode material owing to its enhanced visible light-induced photocatalytic activity.

Oxygen vacancy induced band gap narrowing of ZnO nanostructures by an electrochemically active biofilm

Band gap narrowing is important and advantageous for potential visible light photocatalytic applications involving metal oxide nanostructures. This paper reports a simple biogenic approach for the promotion of oxygen vacancies in pure zinc oxide (p-ZnO) nanostructures using an electrochemically active biofilm (EAB), which is different from traditional techniques for narrowing the band gap of nanomaterials. The novel protocol improved the visible photocatalytic activity of modified ZnO (m-ZnO) nanostructures through the promotion of oxygen vacancies, which resulted in band gap narrowing of the ZnO nanostructure (Eg = 3.05 eV) without dopants. X-ray diffraction, UV-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy, Raman spectroscopy, photoluminescence spectroscopy and high resolution transmission electron microscopy confirmed the oxygen vacancy and band gap narrowing of m-ZnO. m-ZnO enhanced the visible light catalytic activity for the degradation of different classes of dyes and 4-nitrophenol compared to p-ZnO, which confirmed the band gap narrowing because of oxygen defects. This study shed light on the modification of metal oxide nanostructures by EAB with a controlled band structure.

ZnO/Ag/CdO nanocomposite for visible light-induced photocatalytic degradation of industrial textile effluents.

A ternary ZnO/Ag/CdO nanocomposite was synthesized using thermal decomposition method. The resulting nanocomposite was characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, UV-Vis spectroscopy, and X-ray photoelectron spectroscopy. The ZnO/Ag/CdO nanocomposite exhibited enhanced photocatalytic activity under visible light irradiation for the degradation of methyl orange and methylene blue compared with binary ZnO/Ag and ZnO/CdO nanocomposites. The ZnO/Ag/CdO nanocomposite was also used for the degradation of the industrial textile effluent (real sample analysis) and degraded more than 90% in 210 min under visible light irradiation. The small size, high surface area and synergistic effect in the ZnO/Ag/CdO nanocomposite is responsible for high photocatalytic activity. These results also showed that the Ag nanoparticles induced visible light activity and facilitated efficient charge separation in the ZnO/Ag/CdO nanocomposite, thereby improving the photocatalytic performance.

ZnO/Ag/Mn2O3 nanocomposite for visible light-induced industrial textile effluent degradation, uric acid and ascorbic acid sensing and antimicrobial activity

A facile and inexpensive route has been developed to synthesize a ternary ZnO/Ag/Mn2O3 nanocomposite having nanorod structures based on the thermal decomposition method. The as-synthesized ternary ZnO/Ag/Mn2O3 nanocomposite was characterized and used for visible light-induced photocatalytic, sensing and antimicrobial studies. The ternary ZnO/Ag/Mn2O3 nanocomposite exhibited excellent and enhanced visible light-induced photocatalytic degradation of industrial textile effluent (real sample analysis) compared to pure ZnO. Sensing studies showed that the ternary ZnO/Ag/Mn2O3 nanocomposite exhibited outstanding and improved detection of uric acid (UA) and ascorbic acid (AA). It also showed effective and efficient bactericidal activities against Staphylococcus aureus and Escherichia coli. These results suggest that the small size, high surface area and synergistic effect among ZnO, AgNPs and Mn2O3 induced visible light photocatalytic activity by decreasing the recombination of photogenerated electrons and holes, and extending the response of pure ZnO to visible light, enhanced sensing of UA and AA and antimicrobial activity. Overall, the ternary ZnO/Ag/Mn2O3 nanocomposite is a valuable material that can be used for a range of applications, such as visible light-induced photocatalysis, sensing and antimicrobial activity. Therefore, ternary nanocomposites could have important applications in environmental science, sensing, and biological fields.

Ce3+-ion-induced visible-light photocatalytic degradation and electrochemical activity of ZnO/CeO2 nanocomposite

In this study, pure ZnO, CeO2 and ZnO/CeO2 nanocomposites were synthesized using a thermal decomposition method and subsequently characterized using different standard techniques. High-resolution X-ray photoelectron spectroscopy measurements confirmed the oxidation states and presence of Zn2+, Ce4+, Ce3+ and different bonded oxygen species in the nanocomposites. The prepared pure ZnO and CeO2 as well as the ZnO/CeO2 nanocomposites with various proportions of ZnO and CeO2 were tested for photocatalytic degradation of methyl orange, methylene blue and phenol under visible-light irradiation. The optimized and highly efficient ZnO/CeO2 (90:10) nanocomposite exhibited enhanced photocatalytic degradation performance for the degradation of methyl orange, methylene blue, and phenol as well as industrial textile effluent compared to ZnO, CeO2 and the other investigated nanocomposites. Moreover, the recycling results demonstrate that the ZnO/CeO2 (90:10) nanocomposite exhibited good stability and long-term durability. Furthermore, the prepared ZnO/CeO2 nanocomposites were used for the electrochemical detection of uric acid and ascorbic acid. The ZnO/CeO2 (90:10) nanocomposite also demonstrated the best detection, sensitivity and performance among the investigated materials in this application. These findings suggest that the synthesized ZnO/CeO2 (90:10) nanocomposite could be effectively used in various applications.

Nitrogen-doped titanium dioxide (N-doped TiO2) for visible light photocatalysis

TiO2 is an effective and well-known photocatalyst for water and air purification, but its practical applications in visible light-assisted chemical reactions are hindered mainly by its poor visible light absorption capacity. Nitrogen-doped TiO2 (N-doped TiO2) has attracted considerable attention as a photocatalyst, and rapid progress has been made in enhancing the photocatalytic efficiency of TiO2 under visible light irradiation. N-doped TiO2 exhibits broad absorption in the visible region, which can allow the utilization of a large part of the solar spectrum. This might be useful for environmental and energy applications, such as the photocatalytic degradation of organic pollutants, solar cells, sensors, and water splitting reactions. This review focuses on the major developments in the synthesis of N-doped TiO2 and its possible applications in the photocatalytic degradation of organic pollutants and environmental remediation under visible light irradiation.

Band gap engineering of CeO2 nanostructure using an electrochemically active biofilm for visible light applications

Narrowing the optical band gap of cerium oxide (CeO2) nanostructures is essential for visible light applications. This paper reports a green approach to enhance the visible light photocatalytic activity of pure CeO2 nanostructures (p-CeO2) through defect-induced band gap narrowing using an electrochemically active biofilm (EAB). X-ray diffraction, UV-visible diffuse reflectance/absorption spectroscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy, Raman spectroscopy, photoluminescence spectroscopy and high resolution transmission electron microscopy confirmed the defect-induced band gap narrowing of the CeO2 nanostructure (m-CeO2). The structural, optical, photocatalytic and photoelectrochemical properties also revealed the presence of structural defects caused by the reduction of Ce4+ to Ce3+ as well as an increase in the number of oxygen vacancies. The as-modified CeO2 (m-CeO2) nanostructure exhibited substantially enhanced, visible light-driven photoactivity for the degradation of 4-nitrophenol (4-NP) and methylene blue (MB) compared to the p-CeO2 nanostructure. The enhancement in visible light performance was attributed to defects (Ce3+ and oxygen vacancy), resulting in band gap narrowing and a high separation efficiency of photogenerated electron–hole pairs. Photoelectrochemical investigations also showed a significantly-enhanced separation efficiency of the photogenerated electron–hole charge carriers in the m-CeO2 nanostructure under visible light irradiation. The DC electrical conductivity of m-CeO2 showed higher electrical conductivity than p-CeO2 under ambient conditions. This study provides a new biogenic method for developing narrow band gap semiconductor nanostructures for efficient visible light driven photocatalysis and photoelectrode applications.

Fabrication of WO3 nanorods on graphene nanosheets for improved visible light-induced photocapacitive and photocatalytic performance

Tungsten oxide (WO3) nanorods were grown on pure-graphene (P-graphene) nanosheets using a template-free and surfactant-less hydrothermal process at 200 °C. The synthesis and purity of the synthesized WO3 nanorods-graphene nanostructure was confirmed by UV-vis diffuse reflectance measurements, photoluminescence spectroscopy, X-ray diffraction, Raman spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The results showed that WO3 nanorods were well distributed over the graphene nanosheets. The photocatalytic activity of the WO3 nanorods–graphene nanostructure was tested for the photocatalytic degradation of the organic model pollutant dye under visible light irradiation. The photocapacitance performance of the as-prepared nanostructure was examined by cyclic voltammetry. The superior photocapacitive and photocatalytic performances of the WO3 nanorods–graphene nanostructure were observed which was mainly attributed to the combination of WO3 nanorods with graphene nanosheets. WO3 nanorods themselves have photocatalytic properties but the overall performance of the WO3 nanorods–graphene nanostructure was significantly improved when WO3 nanorods were combined with the graphene nanosheets because of the fascinating properties such as high mobility of charge carriers and unique transport performance of graphene nanosheets. The robust nanocomposite structure, better conductivity, large surface area, and good flexibility of the WO3 nanorods–graphene nanostructure appears to be responsible for the enhanced performances. This methodology and the highlighted results open up new ways of obtaining photoactive WO3 nanorods–graphene nanostructure for potential practical applications such as visible light-induced photocatalysis and photocapacitive studies.

Visible light-induced enhanced photoelectrochemical and photocatalytic studies of gold decorated SnO2 nanostructures

This paper reports a novel one-pot biogenic synthesis of Au–SnO2 nanocomposite using electrochemically active biofilm. The synthesis, morphology and structure of the as-synthesized Au–SnO2 nanocomposite were in-depth studied and confirmed by UV-vis spectroscopy, photoluminescence spectroscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. It was observed that the SnO2 surface was decorated homogeneously with Au nanoparticles. The photoelectrochemical behavior of the Au–SnO2 nanocomposite was examined by cyclic voltammetry, differential pulse voltammetry, electrochemical impedance spectroscopy, and linear sweep voltammetry in the dark and under visible light irradiation. Visible light-induced photoelectrochemical studies confirmed that the Au–SnO2 nanocomposite had enhanced activities compared to the P–SnO2 nanoparticles. The Au–SnO2 nanocomposite was also tested for the visible light-induced photocatalytic degradation of Congo red and methylene blue, and showed approximately 10 and 6-fold higher photocatalytic degradation activity, respectively, compared to P–SnO2. These results showed that the Au–SnO2 nanocomposite exhibits excellent and higher visible light-induced photoelectrochemical and photocatalytic activities than the P–SnO2 nanoparticles, and can be used for a wide range of applications.

Gold nanoparticles-sensitized wide and narrow band gap TiO2 for visible light applications: a comparative study†

Gold nanoparticles (AuNPs)-sensitized wide band gap TiO2 (Au/P-TiO2) and narrow band gap TiO2 (Au/M-TiO2) nanocomposites were prepared using an electrochemically active biofilm. The optical and structural properties of the Au/P-TiO2 and Au/M-TiO2 nanocomposites were characterized using standard techniques. The surface plasmon resonance (SPR) absorption characteristics of the AuNPs on the TiO2 surface extended the absorption edge of P-TiO2 and M-TiO2 to the visible light region. The photocatalytic activity of the Au/P-TiO2 and Au/M-TiO2 nanocomposites was evaluated by the photodegradation of methylene blue and methyl orange, and 2-chlorophenol under visible light irradiation, where Au/M-TiO2 nanocomposite exhibited enhanced photocatalytic activity compared to the Au/P-TiO2 nanocomposite and P-TiO2 and M-TiO2 nanoparticles. Furthermore, the higher photoelectrochemical performance of the Au/M-TiO2 nanocomposite compared to the Au/P-TiO2 nanocomposite and P-TiO2 and M-TiO2 nanoparticles further support its higher visible light active behavior under visible light irradiation. The pronounced photoactivities of the Au/M-TiO2 nanocomposite in the visible region were attributed to the interfacial synergistic effects of the two phenomena, i.e. the SPR effect of AuNPs and the defect-induced band gap reduction of M-TiO2 nanoparticles. The present work provides a newer insight into the development of nanocomposites of noble metals and defective metal oxides with high efficiency in the field of visible light-induced photoactivity.