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

Abstract

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.

In2O3 microbundles constructed with well-aligned single-crystalline nanorods: F127-directed self-assembly and enhanced gas sensing performance

“…Examples include MnWO 4 [14], PbTiO 3 [16], LiFePO 4 [17], In 2 O 3 [13,[18][19][20], PbWO 4 [21], LaPO 4 [5], and so on. In 2 O 3 , an important n-type semiconductor with a direct band gap of 3.55-3.75 eV, has been extensively studied and applied in the field of solar cells [22], transistor [23], photocatalysis [24] and gas sensors for detection of ethanol [13], 2-chlorophenol [20], NO 2 [25], NH 3 [26], H 2 S [27], and CO [28]. Up to now, various wet-chemical routes, such as hydrothermal/solvothermal reaction [29,30], sonohydrolysis [31], nanocasting procedure [32,33], and sol-gel technique [34], have been employed to generate diverse In 2 O 3 nanomaterials with controllable structure and morphology including nanotubes [35], nanocubes [36], nanofibers [37], nanorods [36], hollow spheres [13], and mesoporous In 2 O 3 [32,33,[38][39][40].…”

Photoelectrochemical and structural characterization of carbon-doped In 2 O 3 and carbon-doped WO 3 films prepared via spray pyrolysis

“…Recently, our group synthesized N-doped and C-doped In 2 O 3 and showed that In 2 O 3 can be suitably doped with anions to produce a promising photocatalyst with smaller band gap and improved photoelectrochemical properties [16][17][18]. Previous work on N-doped WO 3 shows that nitrogen doping is an effective way to reduce the band gap of WO 3 [19,20], however, the photoelectrochemical performance of N-doped WO 3 films has not been promising.…”

Cubic In2O3 Microparticles for Efficient Photoelectrochemical Oxygen Evolution

“…5,15,[19][20][21][22] Unique to In 2 O 3 facets is that polar {001} facets have the capability to dissociate adsorbed H 2 O molecules into H + and OH − , whereas nonpolar {111} facets cannot do so. 5,15 This suggests that the photocatalytic activity can be enhanced appreciably by precise cutting of the specific crystal facets.…”