Vaidyanathan (Ravi) Subramanian

Encapsulating Bi2Ti2O7 (BTO) with Reduced Graphene Oxide (RGO): An Effective Strategy to Enhance Photocatalytic and Photoelectrocatalytic activity of BTO

Multimetal oxides (AxByOz) offer a higher degree of freedom compared to single metal oxides (AOx) in that these oxides facilitate (i) designing nanomaterials with greater stability, (ii) tuning of the optical bandgap, and (iii) promoting visible light absorption. However, all AxByOz materials such as pyrochlores (A2B2O7)--referred to here as band-gap engineered composite oxide nanomaterials or BECONs--are traditionally prone to severe charge recombination at their surface. To alleviate the charge recombination, an effective strategy is to employ reduced graphene oxide (RGO) as a charge separator. The BECON and the RGO with oppositely charged functional groups attached to them can be integrated at the interface by employing a simple electrostatic self-assembly approach. As a case study, the approach is demonstrated using the Pt-free pyrochlore bismuth titanate (BTO) with RGO, and the application of the composite is investigated for the first time. When tested as a photocatalyst toward hydrogen production, an increase of ∼ 250% using BTO in the presence of RGO was observed. Further, photoelectrochemical measurements indicate an enhancement of ∼ 130% in the photocurrent with RGO inclusion. These two results firmly establish the viability of the electrostatic approach and the inclusion of RGO. The merits of the RGO addition is identified as (i) the RGO-assisted improvement in the separation of the photogenerated charges of BTO, (ii) the enhanced utilization of the charges in a photocatalytic process, and (iii) the maintenance of the BTO/RGO structural integrity after repeated use (established through reusability analysis). The success of the self-assembly strategy presented here lays the foundation for developing other forms of BECONs, belonging to perovskites (ABO3), sillenite (A12BO20), or delafossite (ABO2) groups, hitherto written off due to limited or no photoelectrochemicalactivity.

Photoinduced transformations at semiconductor/metal interfaces: X- ray absorption studies of titania/gold films

We have measured x-ray absorption fine structure of pre- and post-ultraviolet(UV)-irradiated gold (Au) deposited- titania (TiO2) nanocomposites in order to study the effect of UV irradiation on the charge state and local structures around Au in TiO2. Our results indicate a positive oxidation state of Au in TiO2 following UV irradiation and, in addition, a remarkable change is observed in the local environment between these samples as an effect of UV irradiation. The environment around Au, which is comprised mostly of Ti and O atoms prior to UV illumination, is seen to form Au clusters following UV illumination. The photoinduced chemical transformation established in this study demonstrates the nature of semiconductor/metal interface under UV irradiation, and its role in dictating long-term photoelectrochemical performance of nanocomposite photocatalysts.

Photoassisted Enhancement of the Electrocatalytic Oxidation of Formic Acid on Platinized TiO2 Nanotubes

A solvothermal method is used to deposit Pt nanoparticles on anodized TiO2 nanotubes (T_NT). Surface characterization using SEM, EDX, and XRD indicates the formation of polycrystalline TiO2 nanotubes of 110 ± 10 nm diameter with Pt nanoparticle islands. The application of the T_NT/Pt photoanode has been examined toward simultaneous electrooxidation and photo(electro)oxidation of formic acid (HCOOH). Upon UV-vis photoillumination, the T_NT/Pt photoelectrode generates a current density of 72 mA/cm(2), which is significantly higher (∼39-fold) than that of the T_NT electrode (1.85 mA/cm(2)). This boosting in the overall current is attributable to the enhanced oxidation of formic acid at the T_NT/Pt-electrolyte interface. Further, a series of cyclic voltammetric (CV) responses, of which each anodic scan is switched to photoillumination at a certain applied bias (i.e., 0.2 V, 0.4 V, etc.), is used to identify the role of T_NT/Pt as a promoter for the photoelectrooxidation of formic acid and understand a carbon monoxide (CO)-free pathway. Chronoamperometric (j/t) measurements demonstrate the evidence of an external bias dependent variation in the time lag during the current stabilization. An analysis of the CV plots and j/t profiles suggests the existence of both the charge-transfer controlled process and the diffusion-controlled process during formic acid photoelectrooxidation.

Probing photochemical transformations at TiO2∕Pt and TiO2∕Ir interfaces using x-ray absorption spectroscopy

Structural transformations at the TiO2Pt and TiO2Ir interfaces during UV-irradiation have been probed by X-ray absorption spectroscopy. Oxidation by the photogenerated holes results in the intercalation of Pt and Ir into the Titania matrix. The structural transformations observed with Pt and Ir nanoparticles anchored on TiO2 is different than the clustering of gold atoms observed in the TiO2/Au system. Implications of such structural transformations on the photocatalytic activity of semiconductor photocatalyts are discussed.

A Unique Architecture Based on 1 D Semiconductor, Reduced Graphene Oxide, and Chalcogenide with Multifunctional Properties

A unique heterostructured optoelectronic material (HOM), consisting of a reduced graphene oxide (RGO) layer with spatially distributed CdS, suspended by zinc oxide (ZnO) nanorods, is presented. The key features of this HOM are the assembly of the components in a manner so as to realize an effective integration between the constituents and the ability to modify the electronic properties of the RGO. For the first time, the location of RGO (as a suspended layer) along with the tuning of its charge-transport properties (n-/p-type) and its influence on the photo(electro)chemical processes has been examined systematically by using this ZnO/RGO/CdS HOM as a case study. The n-type RGO interlayer facilitates >100 % increase in the photocurrent density and 25 % increase in the photodegradation of a dye, compared to ZnO/CdS, thus demonstrating its multifunctionality. At 3.2 mA cm(-2) , this HOM architecture helps to achieve the highest photocurrent density utilizing ZnO, RGO, and CdS as building blocks in any form. The work is significant for the following reasons: i) other one dimensional (1D) oxides/chalcogenides or 1D oxides/dyes may be designed with similar architectures; ii) HOMs with tunable optical absorbance and charge-transport properties could be realized; iii) related application areas (e.g., sensing or solar fuel generation) should be greatly benefited.

Role of reduced graphene oxide in the critical components of a CdS-sensitized TiO2 -based photoelectrochemical cell.

Nitrogen (N)-doped reduced graphene oxide (nRGO) is systematically incorporated into a TiO(2) -CdS photoelectrochemical (PEC) cell and its role is examined in the three main components of the cell: 1) the CdS-sensitized TiO(2) photoanode, 2) the cathode, and 3) the S(2-)/S(.-) aqueous redox electrolyte. The nRGO layer is sandwiched between TiO(2) nanorods (deposited by using a solvothermal method) and CdS (deposited by using the successive ionic-layer-adsorption and -reaction method). Scanning electron microscopy with energy dispersive X-ray analysis (EDS) reveals the spatial distribution of CdS and nRGO, whereas nRGO formation is evident from Mott Schottky analysis. Chronoamperometry and PEC analysis indicate that upon incorporation of nRGO, a photocurrent density that is at least 27 times higher than that of pristine TiO(2) is achieved; this increase is attributable to the ability of the nRGO to efficiently separate and transport charges. Stability analysis performed by continuous photoillumination over ∼3 h indicates a 26% and 42 % reduction in the photocurrent in the presence and absence of the nRGO respectively. Formation of SO(4)(2-) is identified as the cause for this photocurrent reduction by using X-ray photoelectron spectroscopy. It is also shown that nRGO-coated glass is as effective as a Pt counter electrode in the PEC cell. Unlike the benefits offered by nRGO at the anode and cathode, introducing it in the redox electrolyte is detrimental. Systematic and complementary electrolyte and film-based studies on this aspect reveal evidence of the capacitive behavior of nRGO. Competition between the nRGO and the oxidized electrolyte is identified, based on linear-sweep voltammetry analysis, as the limiting step to efficient charge transport in the electrolyte.

Effects of Carbon Allotrope Interface on the Photoactivity of Rutile One-Dimensional (1D) TiO2 Coated with Anatase TiO2 and Sensitized with CdS Nanocrystals.

The assembly of a large-bandgap one-dimensional (1D) oxide-conductive carbon-chalcogenide nanocomposite and its surface, optical, and photoelectrochemical properties are presented. Microscopy, surface analysis, and optical spectroscopy results are reported to provide insights into the assembly of the nanostructure. We have investigated (i) how the various carbon allotropes (C60), reduced graphene oxide (RGO), carbon nanotubes (CNTs), and graphene quantum dots (GQDs) can be integrated at the interface of the 1D TiO2 and zero-dimensional (0D) CdS nanocrystals; (ii) the carbon allotrope and CdS loading effects; (iii) the impact of the carbon allotrope presence on 0D CdS nanocrystals; and (iv) how they promote light absorbance. Subsequently, the functioning of the integrated nanostructured assembly in a photoelectrochemical cell has been systematically investigated. These studies include (i) chronoamperometry, (ii) impedance measurements or EIS, and (iii) linear sweep voltammetry. The results indicate that the presence of a GQD interface shows the most enhancement in the photoelectrochemical properties. The optimized photocurrent values were respectively noted to be 2.8, 2.2, 1.9, and 1.6 mA/cm(2), indicating JGQD > JRGO > JCNT > Jfullerene. Furthermore, the annealing conditions have indicated that ammonia treatment leads to an increase in the photoelectrochemical responses when using any form of the carbon allotropes.

Development of a highly efficient 1D/0D TiO2 nanotube/n-CdTe photoanode: single-step attachment, coverage, and size control by a solvothermal approach

The creation of an n–n heterojunction between TiO2 nanotubes (T_NT) and CdTe nanocrystals (which mostly exist as p-type) is crucial for realizing the benefits of efficient directional charge transport in a photoanode of 1D/0D architecture. The presented one-pot solvothermal approach leverages a temperature control to achieve a linker-free spatial distribution of CdTe on T_NT in a non-coordinating solvent resulting in highly efficient optical and photoelectrochemical responses. Quantum confinement, broad spectral absorbance, and multiple excitonic generations – areas that are critical to transforming solar energy utilization, will benefit from the presented strategy.

Free energy dependence of pure phase iron doped bismuth titanate from first principles calculations

A density functional theory study of Fe substitutions in Bi2Ti2O7 photocatalyst (Fe-BTO) is presented. It models an experiment where H2 production of Fe-BTO peaked for samples loaded with 1% Fe concentration then decreased for samples with heavier loadings. The total energy calculations were used to determine defect formation energies and the chemical potential landscape that suggests the observed formation of Fe2O3 (in samples at 2% Fe concentration) was detrimental to H2 production. Doping configurations as a function of oxygen chemical potential are discussed, and the chemical potential ranges that avoid formation of the Fe2O3 phase in Fe-BTO are predicted.

Biobased aerogels with different surface charge as electrolyte carrier membranes in quantum dot-sensitized solar cell

Biobased aerogels were used as environmentally friendly replacement for synthetic polymers as electrolyte carrier membranes in quantum dot-sensitized solar cell (QDSC). Integration of polymeric components in solar cells has received increased attention for sustainable energy generation. In this context, biobased aerogels were fabricated to apply as freestanding, porous and eco-friendly electrolyte holding membranes in QDSC. Bacterial cellulose (BC), cellulose nanofibers (CNF), chitin nanofibers (ChNF) and TEMPO-oxidized CNF (TOCNF) were selected because of their fibrilar structures and water-holding capability to investigate their inherent differences in terms of surface groups and electrostatic charge on the electrolyte redox reaction and the photocell function. BC, CNF, ChNF and TOCNF were selected due to different surface functional groups (hydroxyl, N-acetylglucosamine and carboxyl units) and fibrilar structures that can form highly interconnected and robust network. These aerogels enabled easy handling, effective electrolyte filling and efficient redox reactions, while keeping the solar cell performance on par to that of traditional reference cells without membranes. The aerogel membranes maintained the photocell performance since they took only a very small space of the electrolyte volume, which allowed efficient charge transfer. The results indicated that aerogels did not interfere with the cell operation, as confirmed by quartz crystal microgravimetry with bio-interphases in contact with the polysulfide-based electrolyte. The electrochemical measurements also suggested that the respective functional groups (hydroxyl, N-acetylglucosamine and carboxyl units) did not interfere with the redox reaction of the polysulfide electrolyte.