Somnath Chanda Roy

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.

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.

Irradiation of materials by high energy (∼MeV) ions causes intense electronic excitations through inelastic transfer of energy that significantly modifies physicochemical properties. We report the effect of 100 MeV Ag ion irradiation and resultant localized (∼few nm) thermal spike on vertically oriented TiO2nanorods (∼100 nm width) towards tailoring their structural and electronic properties. Rapid quenching of the thermal spike induced molten state within ∼0.5 picosecond results in a distortion in the crystalline structure that increases with increasing fluences (ions per cm2). Microstructural investigations reveal ion track formation along with a corrugated surface of the nanorods. The thermal spike simulation validates the experimental observation of the ion track dimension (∼10 nm diameter) and melting of the nanorods. The optical absorption study shows direct bandgap values of 3.11 eV (pristine) and 3.23 eV (5 × 1012ions per cm2) and an indirect bandgap value of 3.10 eV for the highest fluence (5 × 1013ions per cm2). First principles electronic structure calculations corroborate the direct-to-indirect transition that is attributed to the structural distortion at the highest fluence. This work presents a unique technique to selectively tune the properties of nanorods for versatile applications.

Synthesis conditions and processing parameters profoundly affect the growth and morphology of nanostructures. In particular, when nanostructures are fabricated through a chemical technique such as hydrothermal, the process parameters such as reaction time, temperature, precursor concentration, and substrate orientation play a crucial role in determining the structure-property relationships. In this work, we report the hydrothermal growth of Titanium dioxide (TiO2) nanostructures as a function of these parameters and show that specific morphologies can be obtained by a variation of these parameters. A systematic study is carried out to understand the influence of reaction time (from 0.5 h to 3.0 h), reaction temperature (180 °C-200 °C), titanium precursor concentration (0.25 ml and 0.50 ml in 20 ml solution of HCl and deionized water) and substrate orientation (horizontal and tilted at an angle), and we show that significant variation in morphology- from nanowires to nanorods and then dandelions can be achieved. In particular, we demonstrate that high surface area multidirectional growth of nanorods leading to flower-like nanostructures or dandelions resulting from precipitation during the hydrothermal process. This is in contrast with previous reports on similar structures, where the role of precipitations was not analyzed. The work shows a possibility to control such growth by manipulating substrate position inside the autoclave during the hydrothermal process and will be useful for surface-dependent applications.

Doping of rare earth elements into TiO2 nanorods allows tailoring of the electrical and optical properties, which in turn, modifies the photoelectrochemical behavior. This study presents hydrothermal synthesis of Ce doped TiO2 nanorod arrays on FTO coated glass substrate. XRD pattern confirms the rutile phase in both pristine and Ce doped samples; however, Ce doping results in an enhancement of peak intensity compared to that of pristine TiO2 nanorods. FESEM shows a significant change in the growth rate and the surface morphology of the nanorods with an apparent clustering of grains caused by Ce doping. HRTEM of the Ce doped sample shows regions of different crystallographic orientations. EDX data also confirms a uniform presence of Ce in the TiO2 nanorods. The optical absorption spectra show a slight redshift in the band gap from 3.08 eV for the pristine sample to 3.02 eV for the Ce doped nanorods that is attributed to the presence of shallow energy states within the band gap. Photoelectrochemical measurements indicate a negative shift in the flat-band potential from −0.80 V (for pristine) to −0.94 V (for Ce doped nanorods), resulting in a lower charge transfer resistance at the semiconductor/electrolyte interface, which is also corroborated by impedance spectroscopy. Although there is a reduction in the charge density, Ce doping facilitates better interfacial charge transport at the semiconductor/electrolyte interface.

Ultrathin g-C3N4 (g-CN) nanosheets are prepared by a two-step thermal exfoliation process in an air atmosphere. X-ray diffraction pattern (XRD) shows that ultrathin g-CN nanosheets have the same crystal structure as that of the bulk g-CN with a slight shift in the peak positions due to the reduction of sheet thickness. Field emission scanning electron microscope (FESEM) images show that the bulk g-CN is converted into ultrathin g-CN nanosheets due to thermal oxidation in an atmosphere, which increases the number of reactive active sites for water splitting. Due to a reduction in sheet thickness of g-CN, quantum confinement happened and thereby increase the bandgap from 2.88 (bulk g-CN) eV to 3.10 eV (ultrathin g-CN). Steady-state photoluminescence (PL) shows a blue shift, and time-resolve photoluminescence (TRPL) shows that the average lifetime of photogenerated charge carrier increases when bulk g-CN is converted into ultrathin g-CN nanosheets. Enhancement in photoelectrochemical performance is observed in ultrathin g-CN nanosheets compared to bulk g-CN due to the increased average lifetime of photogenerated charge carriers and a large number of reactive active sites.

Due to a unique planar structure, hydrophilic functional groups, and excellent metallic conductivity, recently developed two-dimensional (2D) MXene offers great potential in applications such as energy storage, photothermal conversion, gas sensing, etc. Herein, we have fabricated 2D Ti3C2Tx MXene functionalized TiO2 nanotube arrays (TNTs) to achieve enhanced photoelectrochemical (PEC) water splitting. We have observed ∼30% increase in photocurrent density for MXene functionalized TNTs over that of bare TNTs. Impedance spectroscopy analysis suggests a decrease in charge transfer resistance from ∼11.38 kΩ to ∼2.94 kΩ at an optimal amount of MXene functionalization. Analysis of Mott-Schottky measurement shows that donor density increases in MXene functionalized TNT sample. Under light irradiation, Ti3C2Tx MXene shows a localized surface plasmon resonance (LSPR) effect and injects electrons to the conduction band of TNT thereby increasing the photogenerated charge carriers, which in turn, enhances the PEC performance of bare TNTs. The work provides a pathway to design new plasmonic heterostructure materials based on monolayers MXene flakes for overall water splitting application.

The photoelectrochemical performance of Ag coated CeO2 functionalized TiO2 nanotube arrays (AgCeTNT) is investigated here. The sample is prepared via three steps, such as synthesis of TiO2 nanotube by electrochemical anodization, CeO2 functionalization by hydrothermal and Ag coating by Successive Ionic Layer Adsorption and Reaction (SILAR) method. X-ray diffraction (XRD) analysis showed the anatase phase formation after annealing of TiO2 nanotubes. Field Emission Scanning Electron Microscopic (FESEM) images showed the conformal coverage of CeO2 and Ag nanoparticles on the surface of TiO2 nanotubes. Energy Dispersive X-Ray (EDX) analysis confirmed the presence of Ce, Ag, Ti and O in the AgCeTNT sample. Diffuse Reflectance Spectroscopy (DRS) study revealed the enhancement in visible light sensitivity with CeO2 functionalization and further with Ag coating. The photoelectrochemical measurement showed that the AgCeTNT sample exhibits improved photoelectrochemical performance compared to bare TiO2 nanotubes (TNT), CeO2 functionalized TiO2 nanotubes (CeTNT) and Ag coated TiO2 nanotubes (AgTNT) under the illumination of sunlight. This improvement in photoelectrochemical performance happeneddue to the combined effect of favorable band alignment of CeO2 and TiO2 and Surface Plasmon Resonance (SPR) effect of Ag nanoparticles, which lead to superior charge separation, efficient charge transport and enhanced visible light response.