Somnath Chanda Roy

Semiconductor heterostructures such as TiO2/Sb2S3 and ZnO/Sb2S3 are promising for photoelectrochemical applications. In this work, we demonstrate the photoelectrochemical performance with enhanced current densities of Sb2S3 sensitized nanorods of TiO2 (TNR) and ZnO (ZNR) photoanodes. ZNR and TNR are synthesized by the hydrothermal method and annealed in the air (AA) and hydrogen (HA) ambient. Stoichiometric Sb2S3 are coated on ZNR and TNR by a facile one-step method using thermolysis of Sb-MPA precursor in air ambient. Structural, optical and microstructural studies are carried out to confirm the formation of phase pure Sb2S3 on ZnO and TiO2 oxide nanorods in the TiO2/Sb2S3 and ZnO/Sb2S3 heterojunction photoanodes. Photoelectrochemical studies show enhanced performance from the Sb2S3 sensitized photoanodes in comparison to the bare TNR or ZNR photoanodes. TNR-AA/Sb2S3(MPA) and TNR-HA/Sb2S3(MPA) heterostructures exhibit current densities of 1.79 mA/cm2 and 1.58 mA/cm2, respectively, which is six times higher than the uncoated photoanodes (0.38 mA/cm2 for TNR-AA and 0.20 mA/cm2 for TNR-HA). In the case of ZNR, we see a 10 to 15 fold increase in current density (~3.3 to 4 mA/cm2) upon sensitizing with Sb2S3 on photoanodes. Further hydrogen annealed ZNR sensitized with Sb2S3 (ZNR-HA/Sb2S3(MPA)) shows a slightly higher current density (3.9 mA/cm2) than the air annealed ZNR-AA/Sb2S3(MPA) (3.32 mA/cm2). Hydrogen annealing is beneficial for ZNR, whereas air-annealing is favored for TNR. Also, stability studies and photocurrent measurements are discussed for these photoanodes.

Recombination of photo-generated charges is one of the most significant challenges in designing efficient photo-anode for photo electrochemical water oxidation. In the case of TiO2, mixed phase (anatase-rutile) junctions often shown to be more effective in suppressing electron-hole recombination compared to a single (anatase or rutile) phase. Here, we report the study of bulk and surface recombination process in TiO2 multi-leg nanotube (MLNTs) anatase-rutile (A-R) junctions decorated with reduced graphene oxide (rGO) layers, through an analysis of the photo-current and impedance characteristics. To quantify the charge transport/transfer process involved in these junctions, holes arriving at the interface of semiconductor/electrolyte were collected by adding H2O2 to the electrolyte. This enabled us to interpret the bulk and surface recombination process involved in anatase/rutile/rGO junctions for photo-electrochemical water oxidation. We correlated this quantification to the electrochemical impedance spectroscopy (EIS) measurements, and showed that in anatase/rutile junction the increase in PEC performance was due to suppression in electron-hole recombination rate at the surface states that effectively enhances the hole transfer rate to the electrolyte. On the other hand, in rGO wrapped A-R MLNTs junction it was due to both phenomenon i.e decrease in bulk recombination rate as well as increase in hole transfer rate to the electrolyte at the semiconductor/electrolyte interface.

Current manuscript describes porous alumina (PA) template assisted electrodeposition of high aspect ratio nanowires and dense hierarchical structures of CdTe. We demonstrate here for the first time that simple structural engineering of a PA template can lead to electrochemical growth of diverse shapes of CdTe nanostructures. Facile and cost-effective modifications have been implemented for the fabrication of self-organized through-hole PA membrane and its transfer onto any rough substrate. These modifications have facilitated extended duration (30 minute to 1 hour) electrodeposition of CdTe nanostructures at high bath temperature of 60◦C without delaminating the PA membrane. High aspect ratio nanowires of 60 nm diameter and 2.8 μm length were growth through the self-ordered PA membrane without any underlying metal coating i.e. without altering its optical properties. An average of 56% optical absorption (within 350 nm - 1400 nm wavelength) and a moderate photoluminescence was observed for the CdTe nanowires. Minor variation in the anodization process resulted into a non-uniform/branched PA template that enabled the formation of dense 3D hierarchical structures of CdTe using similar electrodeposition conditions as that used for CdTe nanowires. The hierarchical CdTe nanostructures exhibited very high total optical absorption of ∼90% within 350 nm - 1400 nm wavelength and a strong photoluminescence was also demonstrated that was almost 10 fold more intense than the CdTe nanowires.

The search for a suitable photoelectrode material remains the most challenging problem in photoelectrochemical applications. Metal oxides are favorable for their chemical stability under an aqueous environment. This work presents size and distribution controlled CeO2 nanoparticles on TiO2 nanorod arrays achieved through a systematic variation of the hydrothermal process temperature. In a two-step hydrothermal process, single crystalline TiO2 nanorods are first grown on fluorine doped tin oxide (FTO) coated glass substrate using titanium (IV) butoxide precursor followed by a treatment with cerium nitrate to obtain CeO2 nanoparticles over TiO2. Variation of the hydrothermal process temperature in the second step from 80 °C to 150 °C results in CeO2 nanoparticles with a systematic variation of size and distribution over TiO2 nanorods. We demonstrate that an effective heterojunction between the CeO2 nanoparticles and TiO2 nanorod forms at a process temperature of 120 °C, which is manifested by improved photoelectrochemical performance. The CeO2–TiO2 heterojunction photoanode shows a photocurrent density of 3.77 mA/cm2 (at 1.23 V vs. RHE) in 1 M KOH solution under one Sun (100 mW/cm2) illumination, which is approximately three times higher than that of bare TiO2 nanorod arrays. Further, Applied Bias Photon-to-current Efficiency (ABPE) is estimated to be 2.01%. Diffuse Reflectance Spectra (DRS) shows a redshift of ~0.1 eV in CeO2–TiO2 heterojunction that signifies the contribution of CeO2 towards visible light absorption. The Electrochemical Impedance Spectroscopy (EIS) shows a lower value for charge transfer resistance in samples processed at 120 °C. The superior photoelectrochemical performance of CeO2–TiO2 heterojunction is attributed to the collective contributions of visible light absorption and efficient charge transfer at the CeO2–TiO2 interface.

The present review concerns thin film field effect transistor (TFT)-based gas sensors with special emphasis on material synthesis, electrical characterizations and sensing properties. A comparative analysis of TFT sensors with that of resistive and capacitive sensors is carried out. Comparison reveals that resistive and capacitive sensors have higher sensitivity, whereas TFT sensors offer improved selectivity and higher reliability. This is attributed to gate voltage-induced carrier transport through the sensing channel that reduces the need for higher operating temperature unlike in the case of resistive and capacitive devices. The gas-sensing mechanism is co-related considering adsorption-desorption isotherms and work function modulation at the gas-solid interface. Type of electrode and density of surface states play a major role for sensitivity and reliability enhancement. Factors influencing stability, repeatability and selectivity improvement are also discussed. Graphical Abstract: [Figure not available: see fulltext.]

Harnessing the solar energy and producing clean fuel hydrogen through efficient photoelectrochemical water splitting has remained one of the most challenging endeavors in materials science. The core problem is to develop a suitable photo-catalyst material that absorbs a significant part of the solar spectrum and produces electron-hole pairs that can be easily separated without recombination. In the recent times, the composite of Titanium dioxide with graphene have been investigated to explore the advantages of both class of materials. Here we report on the photo-electrochemical properties of reduced graphene oxide functionalised TiO2 whiskers. The TiO2 whiskers are obtained from potassium titanium oxide (KTi8O16) synthesized through hydrothermal technique followed by ion exchange method and heat treatment. Graphene oxide was deposited on the as repared TiO2 whiskers using hydrothermal method. As formed samples were characterized by Raman pectroscopy to confirm the presence of reduced graphene oxide (RGO) attached to TiO2 whiskers. Comparative photo electrochemical studies were carried out for TiO2 and reduced graphene oxide modified TiO2 whiskers. Among these, RGO modified TiO2 whiskers show significantly higher photo current density possibly due to enhancement in charge separation ability and longer electron life times.

Highly aligned, vertically oriented TiO2 nanotube arrays formed by electrochemical anodization have been extensively investigated in the recent times because of various potential applications such as in solar cells, hydrogen generation, CO2 reduction, gas sensors and as biocompatible material. The as prepared nanotubes, however, are amorphous and require a thermal annealing process to achieve the desired crystallographic phase. Here we report a simple solvothermal technique by which the crystallinity of the nanotube arrays can be tailored at temperatures around 200 C to achieve the anatase phase without destroying the tubular morphology. In this alcohol-based solvothermal process, the crystallinity can be enhanced by changing the solvent from methanol to isobutanol and the sample treated with isobutanol for 2 h shows crystallinity and strain comparable to that of a sample annealed in a conventional furnace at 550 C. A mechanism for the solvothermal crystallization and the enhancement of crystallinity has been proposed. Furthermore, the gas sensing measurements on solvothermally treated samples have been performed with 1000 ppm methane which showed higher room temperature sensitivity compared to that of conventionally annealed sample. © 2013 American Chemical Society.

Development of photocatalysts for hydrogen generation is highly imperative in the current scenario for resolving the worldwide energy crisis. Continuous efforts are being made to find low-cost and durable photocatalysts with better light absorption capacity to mitigate energy issues. Herein, a new p-n heterojunction photocatalyst has been synthesized successfully using a polyoxometalate (POM), phosphomolybdic acid (PMo12), and zeolitic imidazolate framework (ZIF-9). Photoelectrochemical studies under visible-light irradiation revealed that ZIF-9–PMo12 exhibits a higher photocurrent density than pure ZIF-9. The support of ZIF-9 prevented the instability of PMo12 in aqueous solutions and improved the photoresponse ability of ZIF-9. The p-n junction formation impedes the recombination of electrons and holes, resulting in the improved photocatalytic property. Photoelectrochemical experiments confirmed the photocatalytic features, and thus this work paves the way for the development of an efficient, stable, and low-cost photocatalyst for green H2 generation.

Designing a photocatalyst material with reduced recombination of photogenerated charges is one of the most important aspects of hydrogen generation through solar water splitting. Here, we report hydrogen generation using the TiO2/ultrathin g-C3N4 (U-g-CN) heterostructure fabricated using a unique in situ thermal exfoliation process. Multilayer g-CN is converted into U-g-CN having a high surface (∼190 m2/g) area by calcination at ∼550 °C through oxygen-induced exfoliation, which also forms a robust heterostructure with TiO2. In addition, the presence of g-CN also inhibits further growth of TiO2 nanoparticles, thereby retaining a high specific surface area. The presence of U-g-CN causes a redshift (∼0.13 eV) in the absorption edge of heterostructure compared to that of bare TiO2, which extends the light absorption capability. Addition of 40 wt. % of multilayer g-CN to TiO2 shows an enhanced H2 evolution rate, which is ∼15 times and ∼4 times higher compared to that of bare TiO2 and U-g-CN, respectively. Photoluminescence (PL) and time-resolved PL (TRPL) studies indicate a reduced recombination rate of photogenerated charge carriers with an increase in the average lifetime from 10.53 (TiO2) to 13.32 ns (TiO2/U-g-CN40). The interfacial charge transport characteristics studied through impedance spectroscopy reveal a reduced charge transfer resistance at the semiconductor-electrolyte interface, which facilitates faster charge separation due to the heterostructure formation. The band edge positions are estimated through flatband potential from the Mott-Schottky measurements and optical absorption data, indicating a type-II heterojunction. More light absorption and enhanced separation of photogenerated charges at the heterojunction interface lead to better photocatalytic H2 generation.

In this report we study the effect of annealing temperature on the structural stability, phase transformation and photo-electrochemical performance of TiO2 multi-leg nanotubes. Multi-leg nanotubes were synthesized using electrochemical anodization method, and as synthesized nanotubes were annealed in air at the temperatures ranging from 500 to 900 °C.We observed that multi-leg morphology is preserved upto 900 °C and a dominant rutile phase is observed at this temperature. X-ray diffraction and Raman spectra measurements were carried out to study the crystallite size, phase transformation and phonon confinement effects. The multi-leg nanotubes annealed at 900 °C shows enhanced photo-electrochemical performance; this is attributed to mixed phase, increase in crystallite size and decrease in defects.