New visible-light photocatalysts were prepared by doping In2O3 with nitrogen from ethylenediamine
(NH2(CH2)2NH2) or ammonium chloride (NH4Cl) as the nitrogen source. Nitrogen-doped In2O3 powder prepared
from NH2(CH2)2NH2 shows a rhombic structure and a substitutional N-doping, while the powder prepared
from NH4Cl shows a cubic structure and NH
x
in interstitial sites. N-doping extended the light absorption of
In2O3 to the visible region (λ < ∼650 nm), narrowing the band gap from 3.5 eV to approximately 2.0 eV.
The photocurrent densities of N-doped In2O3 electrodes are at least double those of undoped In2O3 and
approximately 50 times better than N-doped TiO2 electrodes in the visible region, although optimization will
be needed to deliver high photocurrents. This present work shows that In2O3 can be suitably doped to produce
a promising photocatalyst with improved photoelectrochemical properties for solar hydrogen conversion
applications.
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 (NO
x
−) 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.
Composite WO3/TiO2 nanostructures with optimal properties that enhance solar photoconversion reactions were developed, characterized, and tested. The TiO2 nanotubes were prepared by anodization of Ti foil and used as substrates for WO3 electrodeposition. The WO3 electrodeposition parameters were controlled to develop unique WO3 nanostructures with enhanced photoelectrochemical properties. Scanning electron microscopy (SEM) images showed that the nanomaterials with optimal photocurrent density have the same ordered structure as TiO2 nanotubes, with an external tubular nanostructured WO3 layer. Diffuse reflectance spectra showed an increase in the visible absorption relative to bare TiO2 nanotubes and in the UV absorption relative to bare WO3 films. Incident simulated solar photon-to-current efficiency (IPCE) increased from 30% (for bare WO3) to 50% (for tubular WO3/TiO2 composites). With the addition of diverse organic pollutants, the photocurrent densities exhibited more than a 5-fold increase. Chemical oxygen demand measurements showed the simultaneous photodegradation of organic pollutants. The results of this work showed that the unique structure and composition of these composite WO3/TiO2 materials enhance the IPCE efficiencies, optical properties, and photodegradation performance compared with the parent materials.
A composite material consisting of TiO 2 nanotubes (NTs) with WO 3 electrodeposited homogeneously on its surface has been fabricated, detached from its substrate, and attached to a fluorine-doped tin oxide film on glass for application to electrochromic (EC) reactions. A paste of TiO 2 made from commercially available TiO 2 nanoparticles creates an interface for the TiO 2 NT film to attach to the FTO glass, which is conductive and does not cause solution-phase ions in an electrolyte to bind irreversibly with the material. The effect of NT length on the current density and the EC contrast of the material were studied. The EC redox reaction seen in this material is diffusion-limited, having relatively fast reaction rates at the electrode surface. The composite WO 3 /TiO 2 nanostructures showed higher ion storage capacity, better stability, enhanced EC contrast and longer memory time compared with the pure WO 3 and TiO 2 . 4 5
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