Debajeet K. Bora

“In rust we trust”. Hematite – the prospective inorganic backbone for artificial photosynthesis

The search for affordable high performance electrode materials in photoelectrochemical hydrogen production by solar water splitting is an ongoing quest. Hematite is a photoanode material with an electronic band gap suitable for efficient absorption of visible light in a photoelectrochemical cell (PEC). Although its poor electronic structure makes hematite a controversial candidate for PEC, it remains promising because it is an earth abundant, chemically stable and low cost material – necessary prerequisites for PEC to become a competitive cost-efficient solar fuel economy. In addition to reviewing some recent PEC research on hematite and its relevant physical and chemical characteristics, we show how hematite obtained by a low cost synthesis can be refined by hydrothermal treatment and further functionalized by coating with phycocyanin, a light harvesting protein known for photosynthesis in blue-green algae.

Formation of an electron hole doped film in the α-Fe2O3 photoanode upon electrochemical oxidation

Solar hydrogen generation by water splitting in photoelectrochemical cells (PEC) is an appealing technology for a future hydrogen economy. Hematite is a prospective photoanode material in this respect because of its visible light conjugated band gap, its corrosion stability, its environmentally benign nature and its low cost. Its bulk and surface electronic structure has been under scrutiny for many decades and is considered critical for improvement of efficiency. In the present study, hematite films of nominally 500 nm thickness were obtained by dip-coating on fluorine doped tin oxide (FTO) glass slides and then anodised in 1 molar KOH at 500, 600, and 700 mV for 1, 10, 120 and 1440 minutes under dark conditions. X-ray photoelectron spectra recorded at the Fe 3p resonant absorption threshold show that the e(g) transition before the Fermi energy, which is well developed in the pristine hematite film, becomes depleted upon anodisation. The spectral weight of the e(g) peak decreases with the square-root of the anodisation time, pointing to a diffusion controlled process. The speed of this process increases with the anodisation potential, pointing to Arrhenius behaviour. Concomitantly, the weakly developed t(2g) peak intensity becomes enhanced in the same manner. This suggests that the surface of the photoanode contains Fe(2+) species which become oxidized toward Fe(3+) during anodisation. The kinetic behaviour derived from the experimental data suggests that the anodisation forms an electron hole doped film on and below the hematite surface.

A dip coating process for large area silicon-doped high performance hematite photoanodes

A facile and low-cost dip-coating process for the deposition of silicon doped hematite films (Si:α-Fe2O3) for hydrogen production by solar water splitting in photo-electrochemical cells (PEC) is presented. The precursors include iron nitrate, oleic acid, tetraethyl orthosilicate (TEOS) and tetrahydrofuran as dispersion agent. Sequential dip coating on transparent conducting oxides glass substrates with heat treatment steps at 500 °C and 760 °C yields mesoporous Si:α-Fe2O3 with a roughness factor of 17 and photocurrent densities >1 mA/cm2 at 1.23 V vs. reversible hydrogen electrode with SiOx underlayer and surface modification. A PEC demonstrator with 80 cm2 active area in 1 M potassium hydroxide yields a photocurrent of 35 mA at 1.5 AM irradiation with the corresponding hydrogen evolution at a Pt wire counter electrode.

Biological Components and Bioelectronic Interfaces of Water Splitting Photoelectrodes for Solar Hydrogen Production

Artificial photosynthesis (AP) is inspired by photosynthesis in nature. In AP, solar hydrogen can be produced by water splitting in photoelectrochemical cells (PEC). The necessary photoelectrodes are inorganic semiconductors. Light-harvesting proteins and biocatalysts can be coupled with these photoelectrodes and thus form bioelectronic interfaces. We expand this concept toward PEC devices with vital bio-organic components and interfaces, and their integration into the built environment.

Solution processed transparent nanoparticulate ZnO thin film electrode for photoelectrochemical water oxidation

A non-aqueous solution processing route followed by dip-coating has been developed to deposit a ZnO thin film with average nanoparticle size of around 21 nm. The optical, morphological and structural characterization of the film is studied in order to confirm its band gap, particle size distribution and crystallographic properties. Finally the photoelectrochemical studies of different layers of deposited ZnO films has shown that the optimized film for 2 layer deposition at 500 °C exhibits an anodic photocurrent density of 1 mA cm−2 without any modification. Surprisingly the photocurrent density at a water splitting potential of 1.23 V is quite high with a magnitude of 0.817 mA cm−2. The photocurrent experiment is followed by chopped light investigation in order to have information about the charge transfer characteristics and here the absence of a cathodic spike rules out any recombination effect in the bulk of the ZnO electrode. In this study, we also propose that this kind of transparent electrode having photoelectrochemical functionality can be used as a large window panel in a modern house with the aim of performing dual purpose work: UV filtering and photoelectrochemical water splitting. The evolved oxygen by the electrode is further quantified with an in situ gas chromatographic study.

Affinity-Driven Immobilization of Proteins to Hematite Nanoparticles

Functional nanoparticles are valuable materials for energy production, bioelectronics, and diagnostic devices. The combination of biomolecules with nanosized material produces a new hybrid material with properties that can exceed the ones of the single components. Hematite is a widely available material that has found application in various sectors such as in sensing and solar energy production. We report a single-step immobilization process based on affinity and achieved by genetically engineering the protein of interest to carry a hematite-binding peptide. Fabricated hematite nanoparticles were then investigated for the immobilization of the two biomolecules C-phycocyanin (CPC) and laccase from Bacillus pumilus (LACC) under mild conditions. Genetic engineering of biomolecules with a hematite-affinity peptide led to a higher extent of protein immobilization and enhanced the catalytic activity of the enzyme.

Charge transfer between photosynthetic proteins and hematite in bio-hybrid photoelectrodes for solar water splitting cells

Functionalization of the hematite photoanode with the photosynthetic light antenna protein C-phycocyanin (PC) can yield substantial enhancement of the photocurrent density. Photoelectrochemical cells with bio-hybrid electrodes from photosynthetic proteins and inorganic semiconductors have thus potential for the use in artificial photosynthesis. We investigate here processing routes for the functionalization of hematite photoanodes with PC, including in situ co-polymerization of PC with enzymatically-produced melanin, and using a recombinant PC genetically engineered to carry a hexa-histidine tag (αHisPC). First, the effect of the immobilisation of PC on the electrode morphology and photocurrent production is evaluated. Then, the electronic charge transfer in dark and light conditions is assessed with electrochemical impedance spectroscopy and valence band (VB) X-ray photoemission spectroscopy. The relative shift of the VB spectrum towards the Fermi energy EF upon illumination is smaller for the more complex processed coating and virtually disappears for αHisPC immobilised with a melanin film. Optimal conditions for protein immobilisation are determined and the dark currents benefit most from the most advanced protein coating processes.

Solar photoelectrochemical water splitting with bioconjugate and bio-hybrid electrodes

Biohybrid electrodes of different types have been described in the current chapter. The biohybrid photoanodes and photocathodes are new class of materials for solar energy application by utilizing the mother nature’s photosynthetic units and its biomimetic counterparts. The physical characteristics and the performance of the biohybrid electrodes have been described in terms of charge transfer, hydrogen evolution, and stability in harsh electrochemical environment.

Assessment of the Electronic Structure of Photo-electrodes with X-Ray and Electron Spectroscopy

The photoelectrodes are used in solar ware splitting reaction in order to generate future green fuel in the form of hydrogen. However the detailed assessment of the electronic structure of these materials is necessary in order to increase the performance of the electrode by tuning at the molecular level. The assessment of the photoelectrodes electronic structure with synchrotron spectroscopy is described within this chapter. The scope of the chapter is to give the reader a concise overview of the X- Ray and electron based spectrosocpic characterization techniques to shed light upon the electronic structure of solar water splitting photoelectrodes.