Evaluation of nitrogen doping of tungsten oxide for photoelectrochemical water splitting

Thin films of tungsten oxide were fabricated by reactive RF magnetron sputtering for applications related to the direct solar splitting of water. To investigate band gap reduction by nitrogen doping, films were deposited with nitrogen introduced into the sputtering ambient at partial pressures in the range of 0-6 mTorr N-2. For dilute doping (pN(2) < 2 mTorr), the films exhibit a small increase in band gap, and show a degradation of the carrier collection efficiency. Structurally, X-ray diffraction indicates a decrease in crystallinity with nitrogen incorporation. For higher levels of nitrogen doping (pN(2) > 3 mTorr), the diffraction pattern shows the evolution of a new phase that shows an increase in scattering power relative to the pure WO3. For these samples, a reduction of the optical band gap to < 2.0 eV is measured. However, the photocurrent density under AM 1.5G illumination showed a degradation from 2.68 mA/cm(2) for pure WO3 to 0.67 mA/cm(2) for the nitrogen doped sample (6 mTorr). The poor photocurrent for the nitrogen doped sample is attributed a degradation of the electron transport properties as a result of a highly defective lattice.