Mohd Omaish Ansari

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.

Simultaneous sulfur doping and exfoliation of graphene from graphite using an electrochemical method for supercapacitor electrode materials

Doping with heteroatoms has become a significant strategy for modifying the electronic properties and enhancing the electrochemical properties of graphene (GN). In this study, an environmental friendly, economical and facile one pot electrochemical method was developed to synthesize sulfur-doped graphene (S-GN). Sodium thiosulphate (Na2S2O3), in addition to acting as a sulfur source, also catalyzed the exfoliation process, resulting in sulfur-doped GN structures. The exfoliation of graphite to GN and sulfur (S) doping occurred simultaneously resulting in well dispersed S-GN frameworks. Transmission electron microscopy and high-resolution transmission electron microscopy revealed the presence of the heteroatom in S-GN, and X-ray photoelectron spectroscopy confirmed the high S content (3.47%), as well as the existence of high-quality sulphureted species (mainly as C–S–C–). The incorporation of S species in GN during the exfoliation process modified the surface chemistry of carbon in the GN. The electrochemical performance of the as-prepared S-GN electrode exhibited a high specific capacitance of 320 F g−1 at a current density of 3 A g−1 and excellent cycling stability up to 1500 cycles as well as high energy density of 160 W h kg−1 at a power density of 5161 W kg−1 in an aqueous electrolyte.

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.

Visible light-driven photocatalytic and photoelectrochemical studies of Ag–SnO2 nanocomposites synthesized using an electrochemically active biofilm

Ag–SnO2 nanocomposites (1 mM and 3 mM) were synthesized in water at room temperature using an electrochemically active biofilm. The resulting nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, diffuse reflectance spectroscopy, photoluminescence spectroscopy and X-ray photoelectron spectroscopy. The Ag–SnO2 nanocomposites exhibited enhanced photocatalytic activity under visible light irradiation for the degradation of methyl orange, methylene blue, 4-nitrophenol and 2-chlorophenol compared with pure SnO2 nanostructures. Photoelectrochemical measurements, such as electrochemical impedance spectroscopy, linear scan voltammetry and differential pulse voltammetry in the dark and under visible light irradiation, further supported the visible light activity of the Ag–SnO2 nanocomposites. These results showed that the Ag nanoparticles induced visible light activity and facilitated efficient charge separation in the Ag–SnO2 nanocomposites, thereby improving the photocatalytic and photoelectrochemical performance.

Highly photoactive SnO2 nanostructures engineered by electrochemically active biofilm

This paper reports the defect-induced band gap narrowing of pure SnO2 nanostructures (p-SnO2) using an electrochemically active biofilm (EAB). The proposed approach is biogenic, simple and green. The systematic characterization of the modified SnO2 nanostructures (m-SnO2) revealed EAB-mediated defects in the pure SnO2 nanostructures (p-SnO2). The modified SnO2 (m-SnO2) nanostructures in visible light showed the enhanced photocatalytic degradation of p-nitrophenol and methylene blue compared to the p-SnO2 nanostructures. The photoelectrochemical studies, such as the electrochemical impedance spectroscopy and linear scan voltammetry, also revealed a significant increase in the visible light response of the m-SnO2 compared to the p-SnO2 nanostructures. The enhanced activities of the m-SnO2 in visible light was attributed to the high separation efficiency of the photoinduced electron–hole pairs due to surface defects mediated by an EAB, resulting in a band gap narrowing of the m-SnO2 nanostructures. The tuned band gap of the m-SnO2 nanostructures enables the harvesting of visible light to exploit the properties of the metal oxide towards photodegradation, which can in turn be used for environmental remediation applications.

Enhanced thermoelectric behaviour and visible light activity of Ag@TiO2/polyaniline nanocomposite synthesized by biogenic-chemical route

This paper reports the synthesis of a visible light-active Ag@TiO2/Pani nanocomposite through a facile biogenic-chemical route. The Ag@TiO2/Pani nanocomposite was prepared by the in situ oxidative polymerization of aniline in the presence of an Ag@TiO2 nanocomposite, which was obtained via a biogenic route. The synthesized Ag@TiO2/Pani nanocomposite was confirmed by UV-visible spectroscopy, photoluminescence spectroscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and thermogravimetric analysis. The Ag@TiO2/Pani nanocomposite was doped with HCl or para-toluene sulfonic acid to render it conducting. An analysis of the thermoelectrical behavior using a cyclic aging technique showed that the electrical conductivity and thermal stability of Pani was improved after incorporation of Ag@TiO2 to form an Ag@TiO2/Pani nanocomposite system. Photocatalytic studies of the Ag@TiO2/Pani nanocomposite revealed superior photodegradation properties in comparison to Pani towards the degradation of methylene blue and brilliant blue under visible light, even after repeated use. Electrochemical impedance spectroscopy and linear sweep voltammetry in the dark and under visible light irradiation also supported the visible light photocatalytic activity of Ag@TiO2/Pani due to a decrease in the electron transfer resistance resulting in an increase in photocurrent. Therefore, the enhanced thermoelectric, photoelectrochemical and photodegradation properties of these materials suggest them to be a suitable replacement for Pani in the near future.

Enhanced Thermal Stability under DC Electrical Conductivity Retention and Visible Light Activity of Ag/TiO2@Polyaniline Nanocomposite Film

The development of organic-inorganic photoactive materials has resulted in significant advancements in heterogeneous visible light photocatalysis. This paper reports the synthesis of visible light-active Ag/TiO2@Pani nanocomposite film via a simple biogenic-chemical route. Electrically conducting Ag/TiO2@Pani nanocomposites were prepared by incorporating Ag/TiO2 in N-methyl-2-pyrrolidone solution of polyaniline (Pani), followed by the preparation of Ag/TiO2@Pani nanocomposite film using solution casting technique. The synthesized Ag/TiO2@Pani nanocomposite was confirmed by UV-visible spectroscopy, photoluminescence spectroscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and thermogravimetric analysis. The Ag/TiO2@Pani nanocomposite film showed superior activity towards the photodegradation of methylene blue under visible light compared to Pani film, even after repeated use. Studies on the thermoelectrical behavior by DC electrical conductivity retention under cyclic aging techniques showed that the Ag/TiO2@Pani nanocomposite film possessed a high combination of electrical conductivity and thermal stability. Because of its better thermoelectric performance and photodegradation properties, such materials might be a suitable advancement in the field of smart materials in near future.

Electrically conductive polyaniline sensitized defective-TiO2 for improved visible light photocatalytic and photoelectrochemical performance: A synergistic effect

Sulfonated polyaniline@pure-TiO2 (s-Pani@p-TiO2) and polyaniline@defective-TiO2 (s-Pani@m-TiO2) nanocomposites were prepared by the in situ oxidative polymerization of aniline in the presence of TiO2 (p-TiO2 and m-TiO2) nanoparticles followed by sulfonation with fuming sulfuric acid. Defect-induced TiO2 (m-TiO2) nanoparticles were obtained by an electron beam (EB) treatment of commercial TiO2 (p-TiO2) nanoparticles. The resulting s-Pani@p-TiO2 and s-Pani@m-TiO2 nanocomposites were characterized by UV-visible diffuse absorbance spectroscopy, photoluminescence spectroscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Polyaniline (Pani) was dispersed uniformly over the defective m-TiO2 surface with intimate contact on the interface to act cooperatively with the deliberately induced defects to achieve remarkably enhanced properties. The s-Pani@m-TiO2 nanocomposite showed better photocatalytic activity and photoelectrochemical performance than s-Pani@p-TiO2 under visible light irradiation, which was attributed partly to the sensitizing effect of Pani, the narrowed band gap of m-TiO2 and the effective interfacial interaction between Pani and m-TiO2. The electrical conductivity measured using a four-point probe revealed s-Pani@m-TiO2 to have much higher conductivity than s-Pani@p-TiO2. Therefore, s-Pani@m-TiO2 may be used for a wide range of applications owing to its higher charge mobility and high photocatalytic activity. The proposed methodology can also be a potential route for the development of nanocomposites via EB treatment and can be commercialized.