Optical excitation of Ru II (2,2′-bipyridyl-4,4′dicarboxylate) 2 (NCS) 2 -sensitized nanocrystalline TiO 2 films results in injection of an electron into the semiconductor. This paper addresses the kinetics of charge recombination which follows this charge separation reaction. These charge recombination kinetics were found to be strongly dependent upon excitation intensity, electrolyte composition, and the application of an electrical bias to the TiO 2 film. For excitation intensities resulting in less than one excited dye molecule/TiO 2 particle, the recombination kinetics were independent of excitation intensity. Increasing the excitation intensity above this level resulted in a rapid acceleration in the charge recombination kinetics. Similarly, for positive electrical potentials applied to the TiO 2 electrode, the recombination kinetics were independent of applied potential. If the applied potential was more negative than a threshold potential V kin , a rapid acceleration of the charge recombination kinetics was again observed, for example from ∼1 ms at +0.1 V vs Ag/AgCl to ∼3 ps at -0.8 V (∼10 8 fold increase in the rate). Moreover, at a constant applied potential the charge recombination kinetics were found to be strongly dependent upon electrolyte composition (up to 10 6 -fold change in rate). This strong dependence upon the electrolyte composition was found to be associated with shifts in the threshold potential V kin . Spectroelectrochemical measurements were used to monitor the shift in the trap/conduction band density of states induced by the electrolyte composition. A direct correlation was observed between the threshold voltage V kin observed from kinetic measurements, and the threshold voltage for electron occupation of conduction band/trap states of the TiO 2 observed from spectroelectrochemical measurements. This direct correlation was observed for a wide range of electrolyte compositions including protic and aprotic solvents and the addition of Li + ions and 4-tert-butylpyridine. We conclude that the charge recombination kinetics in such dye-sensitized films are strongly dependent upon the electron occupation in trap/conduction band states of the TiO 2 film. This occupation may be modulated by variations in light intensity, applied electrical potential, and electrolyte composition. These results are discussed with relevance to the function of dye-sensitized photoelectrochemical devices.

Potential step chronoamperometry is employed to compare the capacitances of nanocrystalline ZnO and TiO 2 electrodes. These capacitance data are complemented by transient optical absorption studies of charge recombination following adsorption of molecular sensitizer dyes to these metal oxide electrodes. Both measurements are conducted as a function of electrochemical bias applied to the metal oxide film in a threeelectrode photoelectrochemical cell. For both metal oxides, a power law dependence was observed between the half times for charge recombination (t 50% ) and the metal oxide electron density n determined from integration of the capacitance data, t 50% ∝ n -1/R , where R ) 0.27 and 0.30 ( 0.05 for ZnO and TiO 2 , respectively. A numerical model for the recombination dynamics based upon a random walk of electrons between localized sub-bandgap states is found to be in good agreement with experimental observations for both metal oxides. At negative applied potentials, the film capacitance, and therefore electron density, is observed to increase more rapidly with increasingly negative applied potential for the ZnO film compared to the TiO 2 film. This observation is quantitatively correlated with a more rapid acceleration of the recombination dynamics observed for dye sensitized ZnO films under negative biases. It is suggested that the faster recombination dynamics observed under negative bias may be the origin of the lower open circuit voltages reported previously for dye sensitized photoelectrochemical cells employing ZnO electrodes relative to comparable devices employing TiO 2 .

The preparation and characterization of thick (9 microns), clear, mechanically robust and photocatalytically active films of nanocrystalline anatase titania are described. XRD and SEM analysis show the films comprise 13 nm particles of anatase TiO2. Thin (54 nm) films of the 'paste' TiO2, along with sol-gel titania films made by a more traditional route are also prepared and characterised. All titania films mediate the photocatalytic destruction of stearic acid with a quantum yield of 0.0016 +/- 0.0003, using either 365 nm (i.e. BLB) or 254 nm (germicidal) light. P25 TiO2 films also appear to mediate the same process with a similar formal quantum efficiency. Of all the films tested, the thick paste TiO2 films are the most ideally suited for use with near UV light, for reasons which are discussed. All the titania films tested exhibit photoinduced superhydrophilicity.

We have investigated the use of nanoporous Ðlms as substrates for protein TiO 2 immobilisation. Such Ðlms are of interest due to their high surface area, optical transparency, electrochemical activity and ease of fabrication. These Ðlms moreover allow detailed spectroscopic study of protein/electrode electron transfer processes. We Ðnd that protein immobilisation on such Ðlms may be readily achieved from aqueous solutions at 4 ¡C with a high binding stability and no detectable protein denaturation. The nanoporous structure of the Ðlm greatly enhances the active surface area available for protein binding (by a factor of up to 850 for an 8 lm thick Ðlm). We demonstrate that the redox state of proteins such as immobilised cytochrome-c (Cyt-c) and haemoglobin (Hb) may be modulated by the application of an electrical bias potential to the Ðlm, without the TiO 2 addition of electron transfer mediators. The binding of Cyt-c on the Ðlms is TiO 2 investigated as a function of Ðlm thickness, protein concentration, protein surface charge and ionic strength. We demonstrate the potential use of immobilised Hb on such TiO 2 Ðlms for the detection of dissolved CO in aqueous solutions. We further show that protein/electrode electron transfer may be initiated by UV bandgap excitation of the TiO 2 electrode. Both photooxidation and photoreduction of the immobilised proteins can be achieved. By employing pulsed UV laser excitation, the interfacial electron transfer kinetics can be monitored by transient optical spectroscopy, providing a novel probe of protein/electrode electron transfer kinetics. We conclude that nanoporous Ðlms may TiO 2 be useful both for basic studies of protein/electrode interactions and for the development of novel bioanalytical devices such as biosensors.

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Health and safety considerations of room occupants in enclosed spaces is crucial for building management which entails control and stringent monitoring of co 2 levels to maintain acceptable air quality standards and improve energy efficiency. Smart building management systems equipped with portable, low-power, non-invasive co 2 sensing techniques can predict room occupancy detection based on co 2 levels exhaled by humans. In this work, we have demonstrated the development and proof-offeasibility working of an electrochemical RtiL-based sensor prototype for co 2 detection in exhaled human breath. The portability, small form factor, embedded RTIL sensing element, integrability with low-power microelectronic and iot interfaces makes this co 2 sensor prototype a potential application for passive room occupancy monitoring. This prototype exhibits a wide dynamic range of 400-8000 ppm, a short response time of ~10 secs, and a reset time of ~6 secs in comparison to commercial standards. The calibration response of the prototype exhibits an R 2 of 0.956. With RTIL as the sensing element, we have achieved a sensitivity of 29 pF/ppm towards CO 2 at ambient environmental conditions and a three times greater selectivity towards co 2 in the presence of n 2 and o 2. CO 2 detection is accomplished by quantifying the capacitance modulations arising within the electrical double layer from the RtiL-co 2 interactions through Ac-based electrochemical impedance spectroscopy and Dcbased chronoamperometry. Monitoring of CO 2 levels has been a crucial subject of research interest worldwide in regard to efficient building occupancy management for indoor occupancy comfort and energy-savings 1. In the US, indoor air quality monitoring and occupancy comforts account for 40% of the total energy usage 2. Intelligent buildings have adopted system controls that communicate with the deployed sensor network within the building to optimize occupancy comforts and energy consumption. IOT based sensor technology is gaining attraction and has made its way into building management systems to monitor vital indoor environmental parameters such as acoustics, CO, VOC, small particulate matter, CO 2 , temperature, and humidity. Information collected from all these sensors can be utilized to predict patterns for preventing mishaps and take corrective actions in advance for effective building maintenance 3. The smart sensor network should be capable of automatically modulating its air ventilation to avoid excessive ventilation for energy savings in areas with highly variable and dense occupancy 4. Exhaled human breath is the main source of CO 2 production in indoor spaces and is a widely used indicator of room occupancy. CO 2 is regarded as a toxic contaminant with acceptable exposure limit of 5000 ppm over an 8-hour window or a short exposure limit of 15,000-30,000 ppm for 15-minutes according to OSHA and ASHREA standards. The CO 2 levels produced by humans are much higher than the CO 2 present in outdoor environment. Studies show that the indoor CO 2 concent...

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Rising environmental concerns have led to the development of sensors to monitor environmental conditions such as CO 2 and relative humidity (RH). Requirement of sensor performance metrics such as low power, high stability, increased sensitivity has led to the investigation of RTIL as a suitable candidate for environmental sensing. A versatile, robust electrochemical duplex sensor for the detection of CO 2 and humidity using a novel sensing material -Room temperature ionic liquid (RTIL) is presented. RTILs interfaced with a sensing electrode platform results in the formation of multiple electrochemical double layers (EDL). AC perturbation of the system causes charge redistribution in the EDL because of CO 2 and water adsorption which is studied through an AC based technique-Electrochemical impedance spectroscopy. The frequency response of the EDL provides an insight of the impedance changes of the EDL as the environment changes around them. The effects of CO 2 concentrations and RH levels across temperatures on three RTILs -MMIM [MeSO 4 ], EMIM[TF 2 N] and EMIM [FAP] have been investigated. Furthermore, the adsorption/desorption dynamics of the best performing RTILs were evaluated to understand the repeatability in CO 2 sensing behavior. RTILs have the potential to be integrated with semiconductor technology for the sensitive detection of CO 2 concentrations and humidity ranges across varying temperatures. Hence, it has the potential of a new strategy for achieving low power environmental sensors.