AVS2010 Session EN+SS-FrM: Photocatalysis and Solar Fuels

For the full utilization of intermittent alternative energy sources such as wind and solar energy, issues regarding energy storage must be addressed. An attractive solution is the production of a chemical fuel, allowing for both storage and transportation from so called “stranded” production sites. The electrochemical splitting of water to generate hydrogen allows for the production of such fuel, be it in a denser chemical form or H₂. Electrode materials for such an electrocatalytic process must have a high surface area, and be catalytically efficient and robust in order to be viable for such a process.

The research presented addresses the aforementioned challenges using two elegant, Los Alamos National Lab exclusive nanotechnologies: 1) combustion synthesis of conducting noble metal nanofoam scaffolding to serve as the cathode and 2) Polymer Assisted Deposition (PAD) of catalytically active films onto a conductive metal scaffold to functionalize the anode. The resulting foams have ultralow densities, controlled nanopore diameters, and rank with the highest surface area metals ever produced, making them ideal candidates for various catalytic applications. The deposition mechanism of PAD, as well as the high conductivity of the metal foam, enables the utilization of the entire interior surface of the foam for electrochemical reactions, exploiting the advantages of the thin film form of the coating while retaining the electrical advantages of a bulk metal electrode. Metallic copper foams have been coated with CIS-based thin-film absorber layers. These materials show photocurrent under illumination with 1.5 AM solar simulated light. Electrochemical water-splitting data will also be presented.

We used a complement of X-ray photoemission spectroscopy (XPS), depth-resolved cathodoluminescence spectroscopy (DRCLS), Kelvin Probe Force Microscopy (KPFM), and Atomic Force Microscopy (AFM) to measure the surface electronic properties of the tantalum oxynitride series ATaO₂N (A = Ca, Sr, Ba) and RTaON₂ (R = La, Pr), promising candidates for photocatalytic splitting of water under illumination by visible light. Besides creating perovskites with band gaps that straddle the redox potentials of water closely, a major challenge to conversion efficiency is the recombination of free carriers by trap states formed by lattice defects. We used DRCLS to measure the energies and densities of these recombination centers with respect to the bulk oxynitride energy bands and Fermi levels. Previously reported UV-VIS diffuse reflectance measurements indicate band gap absorption onsets at 2.4, 2.1, 1.8 [1], 2.0, and 2.0 eV [2] for CaTaO₂N, SrTaO₂N, BaTaO₂N, LaTaON₂ and PrTaON₂, respectively. DRCL spectra reveal both broad band-to-band transitions in the 2-5 eV range as well as intense and narrow sub-band gap peak features at 1.95, 1.70, 1.70, 1.70 and 1.79 ± 0.01 eV for the same oxynitride sequence. The relatively constant DRCLS gap state energies indicate similar defects derived from the oxygen and nitrogen 2p orbitals comprising the valence band for all five compounds. By varying the incident beam energy, we probed the surface to the bulk with DRCLS, showing these peak energies nearly unchanged as a function of depth. However 0.1-0.2 eV shifts within the outer 10 nm suggest surface interactions that modify these localized states. The higher energy CL features reflect the slowly rising conduction band densities of states plus more pronounced O 2p-Ta 5d transitions calculated by density functional theory [1]. XPS valence band spectra show Fermi levels 0.5-2 eV above the valence bands, while KPFM work functions vary in the range 4.7 – 4.9 eV, indicating valence band maxima comparable to the 5.67 eV oxidation potential of water. Charging and potentials that vary laterally with nanoscale thickness can affect the XPS and KPFM values significantly but not DRCLS. The appearance of strong defect emissions at energies well within the band gap is indicative of strong recombination that can limit optical conversion efficiencies. Hence, these studies reveal the importance of O- and N-derived native point defects in limiting the efficiency of oxynitride photocatalysts.

[1] Y-I. Kim, P. M. Woodward, K.Z. Baba-Kishi, and C.W. Tai Chem. Mater. 2004, 16, 1267.

Solid solutions of GaN and ZnO have been shown to be a promising class of photocatalysts, capable of splitting water under visible-light irradiation. The structural and electronic properties of Ga_1-xZn_xN_1-xO_x have been studied using density-functional theory with the Linear Augmented Plane Wave (LAPW) method at varying values of x. A GGA+U approach is used to better describe the semicore 3d states of Ga and Zn. These calculations show that there exists a p-d coupling between the N 2p and Zn 3d states, leading to a decreased band gap. The band gaps in the mixed metal oxynitrides are lower than either ZnO or GaN, thus allowing excitation by visible light. The trend in band gaps over the range of Zn concentrations (x) is consistent with experimental results. The expected band gap minimum is at a composition that is difficult to synthesize. Formation energies have been calculated to understand the limitations on synthesis of these materials. Several starting materials and synthesis environments have been studied in the formation energy calculations to determine how the thermodynamically preferred products depend on experimental conditions and whether high concentrations of zinc can be obtained in these materials.

A sol-gel precursor was used to synthesize zinc gallium oxy-nitrides with visible light band gaps. At low temperatures, novel spinel oxynitrides were produced with band gaps of 2.5 to 2.7eV, surface areas of 16 to 36 m²/g, and nitrogen content less than 1.5%. As the temperature was raised, these spinels get consumed to form wurzitic oxy-nitrides also with band gaps less than 3 eV but with surface areas of 4 to 6 m²/g. The reduction in the band gap for the spinel oxy-nitrides is associated with the incorporation of N2p orbitals in the valence band with corresponding changes in the anion position parameter. We established that the presence of a small fraction of gallium tetrahedral centers and anion vacancies might affect its unique electronic properties. The changes associated with the gallium coordination environment as the spinel zinc gallate precursor transforms to a spinel oxynitride at 550^oC and further changes into a wurzite oxynitride at 850^oC are studied through x-ray diffraction, ultraviolet-visible diffuse reflectance spectroscopy, neutron powder diffraction, x-ray absorption spectroscopy and other techniques. Electronic structure and formation energies of the spinel and wurzite oxy-nitrides were studied using density-functional theory (DFT) with the Linear Augmented Plane Wave (LAPW) method at varying dopant concentrations. Furthermore, these novel spinel photocatalysts were found to be active in degrading methylene blue in visible light and oxygen production from silver nitrate. The protocol developed opens a different avenue for the synthesis of semiconductors possessing the spinel crystal structure and with band gaps engineered to the visible region with potential applications for both opto-electronics and photocatalytic applications.