R.V. Kumar

Efficient visible-light-driven photocatalytic hydrogen production using CdS@TaON core–shell composites coupled with graphene oxide nanosheets

Large-scale hydrogen production through water splitting using photocatalysts with solar energy can potentially produce clean fuel from renewable resources. In this work, photocatalytic evolution of H2 with a high efficiency was achieved using graphene oxide (GO) nanosheets decorated with CdS sensitized TaON core–shell composites (GO–CdS@TaON). The CdS@TaON core–shell nanocomposites were prepared by an ion-exchange route with assistance from a hydrothermal process on GO as the support. The TaON core–shell composites containing 1 wt% CdS nanocrystals showed a high rate of H2-production at 306 μmol h−1 with an apparent quantum efficiency (QE) of 15% under 420 nm monochromatic light. The rate of hydrogen formation was 68 times faster in comparison with the rate observed on pure TaON. The rate was further increased to 633 μmol h−1 with a high quantum efficiency of 31% when the GO–CdS@TaON hybrid composite was coupled with 1 wt% of graphene oxide and 0.4 wt% of Pt (about 141 times higher than that of the pristine TaON). This high photocatalytic H2-production activity is ascribed firstly to the presence of CdS nanocrystals that alter the energy levels of the conduction and valence bands in the coupled semiconductor system; secondly to the involvement of graphene oxide that serves as an electron collector and transporter to efficiently lengthen the lifetime of the photogenerated charge carriers from CdS@TaON composites. This investigation can open up new possibilities for the development of highly efficient TaON-based photocatalysts that utilize visible light as an energy source.

Hierarchical assemblies of bismuth titanate complex architectures and their visible-light photocatalytic activities

The shape-controlled synthesis of hierarchically micro- and nanostructured materials has provided new possibilities to improve their physical and chemical properties. In this work, Bi12TiO20 complex architectures, such as nanospheres, nanowires, microflowers and microspheres were prepared through a facile solution-phase hydrothermal process, by controlling the reaction parameters, such as temperature, reagent concentration, and reaction time. Within the hydrothermal temperature range from 150 to 180 °C, the morphology transformed progressively from nanosphere agglomerates to microflowers, nanowires and then to microspheres with increasing time. A possible growth mechanism is proposed to explain the transformation of nanoparticles to microflowers via an Ostwald ripening mechanism followed by self-assembly and then break-up of the microflowers to nanowires and final consolidation into microspheres. Most importantly, these Bi12TiO20 samples dependent on their shape, size, and crystallinity, displayed high photocatalytic activities in comparison with bulk bismuth titanate powders, as demonstrated by the degradation of Rhodamine B under visible-light irradiation. The reasons for the difference in the photocatalytic properties of these Bi12TiO20 architectures are further investigated.

Thick film CO2 sensors based on Nasicon solid electrolyte

Abstract A planar miniaturised CO 2 sensor based on thick films of Nasicon (Na + conductor, Na 3 Zr 2 Si 2 PO 12 ) electrolyte has been developed. The thick film was fabricated by screen printing Nasicon paste on an alumina substrate and then firing at 1543 K. scanning electron microscopy (SEM) analysis shows that the films achieved good densification, although reactions occur between Nasicon and alumina substrate, so conductivity of the thick film is lower than that of bulk Nasicon. The sensors interfaced with a binary carbonate auxiliary phase (Na 2 CO 3 –BaCO 3 ) applied as a slurry at 373 K showed good CO 2 sensing properties at 673–873 K. The electromotive force (emf) values obtained were linearly dependent upon the logarithm of CO 2 concentration over a wide range (6 ppm–100%). The slopes of emf versus log P CO 2 agreed well with the Nernstian slopes based on two electrons electrode reactions.

Application of Nafion electrolytes for the detection of humidity in a corrosive atmosphere

Abstract Using platinum electrodes and Nafion as the solid electrolyte, a device for measuring humidity in a chlorine–air atmosphere has been assembled. Deposition of platinum on the polymer is achieved by electroless plating from tetraamine platinum chloride, Pt(NH 4 ) 2 Cl 2 ·H 2 O, and reduction by sodium borohydride (NaBH 4 ). The sensor was calibrated in terms of impedance as a function of relative humidity, using a Frequency Response Analyser. The frequency range was 10 −2 –10 7 Hz and an amplitude of either 70 or 100 mV was applied. At very low relative humidity, only a bulk process is observed but when the moisture content is increased, an additional process takes place at the electrode/electrolyte interface. This is reflected in the form of a spike at 45° to the real axis and represents diffusion of moisture into and out of the platinum/Nafion composite. Maximum conductivity is observed at 433 K. The maximum operating temperature is about 473 K as ascertained by thermal analysis of Nafion. In industrial practice, the sensor can be operated with a simpler frequency controller, which would allow the sensor to be operated at a fixed frequency. The sensor retains its physical and chemical integrity after use in a chlorine–air atmosphere.

Novel sensors for monitoring high oxygen concentrations

Using a novel approach, an oxygen sensor for monitoring high concentrations has been developed. The technique consists of applying a potential to an oxygen conducting solid electrolyte in one direction and the current measured, the potential reversed and the current re-measured, to obtain a current ratio which strongly and linearly depends on % oxygen in the high concentrations. Complications such as pinhole or porous diffusion barrier for limiting current are not required in this approach. Furthermore, by choosing appropriate values of the applied potential, it is possible to obtain extremely high sensitivity at any predetermined oxygen concentrations. Using this concept, it is possible to use a standard solid electrolyte oxygen sensor for application in automotive engines and other combustion systems.

Preparation and impedance spectroscopic studies of Ba β-Al2O3☆

Abstract Samples of Ba β-Al 2 O 3 with the stoichiometries BaAl 9 O 15 to BaAl 14 O 22 were prepared by a solid state route from Al 2 O 3 and BaCO 3 . Samples of MgO-stabilised and ZrO 2 -added material were also prepared. After rationalising the calcining and sintering procedure to produce dense samples (99% of theoretical) such samples were then analysed by Impedance Spectroscopy. Differences in the Arrhenius plots e.g. activation energies for the different materials are shown. XRD patterns showed BaAl 9 O 15 and BaAl 14 O 22 to be single phase with the BaAl 12 O 19 material being a mixture of BaAl 9 O 15 and BaAl 14 O 22 .

Thermodynamic activity of Na2O in Na β-alumina

Abstract β-alumina samples prepared by sintering Na 2 O–xAl 2 O 3 (5.5 1900 K were subjected to XRD at room temperature. For all compositions from 5.5 8.5. The proportion of β″-alumina is observed to reach a maximum value at x = 8.5, which is near the β/α phase boundary and decreases to lower values for both x 8.5. The c parameter for β-alumina is observed to increase from 22.45 A at x = 5.5 to 22.75 A at x = 8.5 and then is almost constant for x > 8.5. Emf measurements were made at higher temperature (973 K) using the following galvanic cell: (−) Na (reference) | Pt | Na β-alumina | Pt | Na 2 O–xAl 2 O 3 (5.5 2 O in the Na 2 O–xAl 2 O 3 binary system, decreases with increasing value of x for 5.5 8.5. These results suggest that β″-alumina is not a separate phase and provide new edvidence that β″-alumina can be considered as a region of stacking disorder within the β-alumina phase, as reported by D. Gratias and coworkers. Although XRD can identify the presence of both structures, thermodynamic evidence is provided to suggest that the microdomains of β″-alumina and β-alumina structures are part of one single β-alumina phase, formed syntaxially as long-range stacking disorder.

Solid-state hydrogen sensors based on SrCl2 electrolyte

Abstract A solid-state sensor for measuring the partial pressure of hydrogen has been developed using a SrCl 2 electrolyte, with either a Ag/AgCl or a Ni/NiCl 2 reference electrode for application in the temperature ranges of 600–700 K and 700–900 K, respectively. Although it is a chloride ion conductor, SrCl 2 can be applied in the detection of hydrogen by using a thin layer of SrHCl/SrCl 2 as an auxiliary phase on the working surface of the electrolyte.

Important materials aspects for sensors based on Na-β-alumina

Abstract A critical view is presented of the feasibility and reliability of electrochemical sensors combining Na-β-alumina (i.e. Na 2 O· x Al 2 O 3 ) solid electrolyte material with an in-situ created auxiliary phase and electrochemical sensors using a Na-β-alumina based reference. Thermodynamic information on the stability of Na-β-alumina and the activity of Na 2 O in the Na 2 O– x Al 2 O 3 binary system and in the Na-β-alumina phase in particular, is critical in applications for sensors, and especially in situations where the operating temperatures are high and the oxygen pressures are low and where the Na-β-alumina phase is used as a part of the electrode providing reference potentials in galvanic cells.