Yogesh A. Sethi

Self-assembled hierarchical nanostructures of Bi2WO6 for hydrogen production and dye degradation under solar light

Three dimensional (3D) hierarchical nanostructures of orthorhombic Bi2WO6 with unique morphologies were successfully synthesized by a solvothermal method. The precursor concentration plays a key role in the architecture of the hierarchical nanostructures. A peony flower-like morphology was obtained at higher precursor concentrations, and a red blood cell (RBC)-like morphology with average diameter of 1.5 μm was obtained at lower concentrations. These hierarchical nanostructures were assembled by self-alignment of 20 nm nanoplates. As their band gap is in the visible region, the photocatalytic activity of the Bi2WO6 hierarchical nanostructures for the production of hydrogen from glycerol, and the degradation of rhodamine B (RhB) and methylene blue (MB) under ambient conditions in the presence of solar light was investigated. The Bi2WO6 with peony flower morphology was observed to be the most efficient photocatalyst (H2: 7.40 mmol h−1 g−1, kRhB: 0.240 and kMB: 0.100) of the reported nanostructures. The higher activity of the peony flowers was due to their porous nature, high surface area and lower band gap. Such unique 3D nanostructures of Bi2WO6 have been fabricated for the first time, and their use as photocatalysts in the production of hydrogen from glycerol has hitherto not been attempted. These nanostructures may have potential in ferroelectric, piezoelectric, pyroelectric and nonlinear dielectric applications.

Environmentally benign enhanced hydrogen production via lethal H2S under natural sunlight using hierarchical nanostructured bismuth sulfide

Nanorods and hierarchical nanostructures (dandelion flowers) of bismuth sulfide (Bi2S3) were synthesized using a solvothermal method. The effects of solvents such as water and ethylene glycol on the morphology and size of the Bi2S3 nanostructures were studied. A structural study showed an orthorhombic phase of Bi2S3. We observed nanorods 30–50 nm in diameter and dandelion flowers assembled with these nanorods. A formation mechanism for the hierarchical nanostructures of Bi2S3 is proposed. Based on the tuneable band gap of these nanostructures in the visible and near-IR regions, we demonstrated the photocatalytic production of hydrogen from H2S under normal sunlight. Abundantly available toxic H2S was used to produce hydrogen under normal sunlight conditions. We observed an excellent hydrogen production of 8.88 mmol g−1 h−1 under sunlight (on a sunny day between 11.30 am and 2.30 pm) for the Bi2S3 flowers and 7.08 mmol g−1 h−1 for the nanorods. The hierarchical nanostructures suppress charge carrier recombination as a result of defects, which is ultimately responsible for the higher activity. The evolution of the hydrogen obtained is fairly stable when the catalyst is used repeatedly. The evolution of hydrogen via water splitting was observed to be lower than that via H2S splitting. Bi2S3 was observed to be a good eco-friendly photocatalyst active under natural sunlight. The photo-response study showed that the Bi2S3 microstructures are good candidates for applications in highly sensitive photo-detectors and photo-electronic switches.

Nanostructured CdS sensitized CdWO4 nanorods for hydrogen generation from hydrogen sulfide and dye degradation under sunlight.

In this report, CdS nanoparticles have been grown on the surface of CdWO4 nanorods via an in-situ approach and their high photocatalytic ability toward dye degradation and H2 evolution from H2S splitting under visible light has been demonstrated. The structural and optical properties as well as morphologies with varying amount of CdS to form CdS@CdWO4 have been investigated. Elemental mapping and high resolution transmission electron microscopy (HRTEM) analysis proved the sensitization of CdWO4 nanorods by CdS nanoparticles. A decrease in the PL emission of CdWO4 was observed with increasing amount of CdS nanoparticles loading possibly due to the formation of trap states. Considering the band gap in visible region, the photocatalytic study has been performed for H2 production from H2S and dye degradation under natural sunlight. The steady evolution of H2 was observed from an aqueous H2S solution even without noble metal. Moreover, the rate of photocatalytic H2 evolution over CdS modified CdWO4 is ca. 5.6 times higher than that of sole CdWO4 under visible light. CdS modified CdWO4 showed a good ability toward the photo-degradation of methylene Blue. The rate of dye degradation over CdS modified CdWO4 is ca. 7.4 times higher than that of pristine CdWO4 under natural sunlight. With increase in amount of CdS nanoparticle loading on CdWO4 nanorods the hydrogen generation was observed to be decreased where as dye degradation rate is increased. Such nano-heterostructures may have potential in other photocatalytic reactions.

In situ fabrication of highly crystalline CdS decorated Bi2S3 nanowires (nano-heterostructure) for visible light photocatalyst application

In situ synthesis of the orthorhombic Bi2S3 nanowires decorated with hexagonal CdS nanoparticles (nano-heterostructure) has been demonstrated by a facile solvothermal method. The tiny 5–7 nm CdS spherical nanoparticles are decorated on the surfaces of 30–40 nm Bi2S3 nanowires, successfully. Structural, morphological and optical studies clearly show the existence of CdS on the nanowires. A possible sequential deposition growth mechanism is proposed on the basis of experimental results to reveal the formation of the nano heterostructure. The heterostructures have been used as a photocatalyst for hydrogen production as well as degradation of methylene blue under solar light. The maximum hydrogen evolution i.e. 4560 and 2340 μmol h−1 0.5 g was obtained from H2S splitting and glycerol degradation for Bi2S3 NWs decorated with CdS nanoparticles (nano-heterostructure) which is higher than that of the Bi2S3 NWs (3000 and 1170 μmol h−1 0.5 g, respectively). The enhanced photocatalytical hydrogen evolution efficiency of the heterostructures is mainly attributed to its nanostructure. In the nano heterostructure, the CdS nanoparticles control the charge carrier transition, recombination, and separation, while the Bi2S3 nanowire serves as a support for the CdS nanoparticles. The photogenerated electrons migration is faster than the holes from the inside of a CdS nanoparticle to its surface or to the phase interface, resulting in a relatively higher hole density inside the CdS nanoparticle leaving electron density at surface of the Bi2S3 NWs. This influences the photocatalytic activity under solar light. Such nano-heterostructures may have potential in other photocatalytic reactions.

Nanostructured layered Sn3O4 for hydrogen production and dye degradation under sunlight

Nanostructured layered Sn3O4 nanoplatelets were synthesized via a facile hydrothermal method. At low pH, the formation of SnO2 was observed due to the existance of Sn(II). By tuning the pH of the hydrothermal reaction, Sn3O4 was conferred. The broadening of XRD peaks reveals the existance of nanocrystalline triclinic Sn3O4, which was also confirmed by Raman spectroscopy. Morphological examination displays the nanostructured layered plate-like structure with diameter of 20 nm and length of 100–150 nm. The optical band gap was observed to be 2.76 eV for Sn3O4. Considering the band gap was in the visible light region, investigations on the photocatalytic production of hydrogen and dye degradation were performed under sunlight. A maximum hydrogen evolution (3916 μmol h−1/0.5 g) was obtained using the Sn3O4 nanoparticles, which is much higher than that found with SnO2 (729 μmol h−1/0.5 g) due to narrowing of the band gap. Hydrogen production from H2S using Sn3O4 was reported for the first time. Sn3O4 also demonstrates excellent dye degradation activity. The high photocatalytic activity of Sn3O4 was attributed to its nanocrystalline nature and narrowing of the band gap.

3 D Hierarchical heterostructures of Bi2W1-XMoXO6 with enhanced oxygen evolution reaction from water under natural sunlight

Self-assembled 3D hierarchical Bi2W1−xMoxO6 heterostructures with varying x (x = 0, 0.2, 0.4, 0.6, 0.8 or 1.0) with different morphologies were synthesised via a facile one-pot solvothermal method and their photocatalytic activity towards the oxygen evolution reaction (OER) from water under natural sunlight was tested. The structural properties of Bi2W1−xMoxO6 were studied by the X-ray diffraction (XRD) technique, which showed an orthorhombic Aurivillius layered crystal structure. The microstructural features were examined by FE-SEM and FE-TEM techniques which showed that the morphology of Bi2WO6 varies with substitution of Mo and each morphological structure grows via the assembly of tiny nanoparticles of size 50 nm. The effective substitution of Mo in Bi2WO6 extends the optical absorption towards the visible region. The substitution of Mo in place of W was confirmed by X-ray photoelectron spectroscopy. The photocatalytic activities were evaluated by OER under solar light irradiation. The sample Bi2W0.6Mo0.4O6 (S3) shows enhanced photocatalytic activity for OER from aqueous AgNO3 solution (652 μmol h−1 g−1) which is higher than for pristine Bi2MoO6 or Bi2WO6 photocatalysts. Enhanced photocatalytic activity can be attributed to the extended absorption in the visible light region, which enhances the photocatalytic efficiency of the photocatalysts. More significantly, the 3D intrinsically layered nanosheet structure based morphology, and the unique band structure are beneficial for efficient charge transfer, which enhances the photocatalytic activity. This work demonstrates an effective strategy for developing an active photocatalyst with greater utilization of solar light.

Perforated N-doped monoclinic ZnWO4 nanorods for efficient photocatalytic hydrogen generation and RhB degradation under natural sunlight

The synthesis of novel nitrogen-doped zinc tungstate (N-doped ZnWO4) perforated nanostructures and their photocatalytic activity for hydrogen production from water and rhodamine B degradation under direct sunlight have been demonstrated for the first time. ZnWO4 was synthesized by a simple hydrothermal method and doped with nitrogen by precise thermal treatment in the presence of thiourea to obtain perforated nanorods. The structural analysis carried out by X-ray diffractometry (XRD) and first principles density functional theory (DFT) based calculations shows a monoclinic structure. The microstructural and morphological studies show unique perforated nanorods with diameters of 25–20 nm of N-doped ZnWO4. The substitution of nitrogen in place of oxygen atoms was confirmed by X-ray photoelectron spectroscopy (XPS). The effective substitution of nitrogen in ZnWO4 extends the absorption bands into the visible region. Hence, a computational study of N-doped ZnWO4 was also performed for the investigation and confirmation of its crystal and electronic structures. UV-DRS and analysis of the density of states (DOS) indicate a band gap of ∼2.4 experimentally and 2.9 eV theoretically. Considering the band structure, its functionality as a sunlight driven photocatalyst for water splitting and dye degradation has been investigated. N-Doped ZnWO4 exhibits enhanced photocatalytic activity towards hydrogen evolution (5862.1 μmol h−1 g−1) for water splitting as well as RhB degradation under natural sunlight. The enhanced photocatalytic activity of N-doped ZnWO4 is attributed to extended absorbance in the visible region, which in turn generates more electron–hole pairs responsible for higher H2 generation. DFT calculations suggest that the hybridization between O-2p and N-2p at the valence band edge is the reason for the narrowing band gap, and the degree of hybridization is likely to be increased with an increase in N doping which is responsible for the higher activity. The present investigation demonstrates a novel approach for the synthesis of perforated N-doped ZnWO4 with great prospects of scaling up and high yields.