Characterization of Photoactive Centers in N-Doped In2O3 Visible Photocatalysts for Water Oxidation

N-doped In2O3 films and powders were synthesized, characterized, and evaluated for photoelectrochemical water splitting. The synthetic process was followed in detail by FTIR and UV−vis spectroscopy and the In complex was characterized by X-ray crystallography. NMR, XPS, and EPR were combined in an effort to track the N speciation at each step of the synthesis. The structural, optical and photoelectrochemical properties of the final products (films and powders) were analyzed. Compared to undoped In2O3, N-doped In2O3 showed an increased absorption in the 350−500 nm range with a red shift in the band gap transition. Electrodes prepared from NH4Cl exhibit higher photoactivity compared to the electrodes prepared from urea. NMR, XPS, and EPR results showed that inert amino- and nitrate-type species adsorbed on the surface were produced from urea and NH4Cl, which count toward the N atomic percent but do not increase the activity of In2O3. However, a nitrate-type species in interstitial sites and a paramagnetic species attributed to an F-center play an important role in the photoelectrochemical improvement of N-doped In2O3 prepared using NH4Cl as the dopant source. A mechanism for the formation of the F-centers is proposed based on the electron donation of the nitrate-type species (NOx−) at oxygen vacancies. N-doped In2O3 prepared using NH4Cl at an optimal N content of 1 to 2% (initial N/In = 0.50) produced 5 fold better photocurrent density than undoped In2O3, reaching close to 1 mA/cm2 with a film thickness of 15 μm and applied voltage of 0.7 V. This paper also illustrates how the combination of NMR, XPS, and EPR has excellent potential for characterizing dopant species and for determining the origin of visible photoelectrochemical activity of doped metal oxides.