La Photocatalyst Research Articles

Carbon monoxide as an intermediate product in the photocatalytic steam reforming of methane with lanthanum-doped sodium tantalate

A NaTaO3:La photocatalyst without a cocatalyst can produce carbon monoxide as a partially oxidized product in photocatalytic steam reforming of methane (PSRM) at around room temperature.

TiO2 nanocrystalline for enhanced hydrogen and oxygen generation of thin film photocatalyst: from catalytic mechanism and microstructural analysis

Anatase TiO2 nanocrystalline film prepared by using sol–gel method was assembled on TiO2 nanotubes to produce composite thin film photocatalyst and investigated as interfacial layer for the catalytic performance of Ti/TNT/N–P/NaTaO3:La (the nanostructures is Ti/TiO2 nanotubes/TiO2 nanocrystalline film/NiO film/NaTaO3:La) photocatalyst to generate hydrogen and oxygen under UV irradiation. The as-synthesized samples were characterized by FESEM, XRD, Element Mapping, UV–Vis, PL, I–V, and EIS. Characteristic analyses show that the interface between the TNT layer (TiO2 nanotubes) and P layer (NiO film) can be optimized by adjusting the number of N layers (TiO2 nanocrystalline film), thus affecting the efficiency of the photocatalyst. Compared with unformed TiO2 film, the Ti/TNT/N–P/NaTaO3:La photocatalyst with four TiO2 layers enhanced the H2 and O2 generation rate from 43.7 to 87.1 μmol/h which realized almost 2.0 times. The enhanced photocatalytic performance can be attributed to the low defect density and the effective separation of photo-generated electron–hole pairs. Furthermore, the performance stability of thin film photocatalyst was favorably showcased through multiple water splitting reactions, and the rational mechanism of improved photocatalytic property was interpreted systematically. In summary, such results have certain guiding effect on solving the weakness of composite film catalyst of interface defects and carrier recombination.

Photocatalytic activity of NaTaO3:La prepared under different atmospheres

NaTaO3:La photocatalysts were prepared under air, O2 and H2/Ar atmospheres by solid-state reaction method. The photocatalytic activities of these samples were evaluated from the photocatalytic hydrogen evolution from the methanol solution and pure water, and decomposition of Rhodamine B (RhB). NaTaO3:La prepared under air atmosphere showed the best photocatalytic performance in all the evaluation of photocatalytic activity. The sample prepared under H2/Ar atmosphere showed higher activity in pure water splitting than that under O2 atmosphere, which was opposite in H2 evolution reaction and photocatalytic degradation of RhB.

Nature of Catalytic Active Sites Present on the Surface of Advanced Bulk Tantalum Mixed Oxide Photocatalysts

The most active photocatalyst system for water splitting under ultraviolet (UV) irradiation (270 nm) is the promoted 0.2% NiO/NaTaO3:2% La photocatalyst with an optimized photonic efficiency of 56%, but fundamental issues about the nature of the surface catalytic active sites and their involvement in the photocatalytic process still need to be clarified. This is the first study to apply cutting-edge surface spectroscopic analyses to determine the surface nature of tantalum mixed oxide photocatalysts. Surface analysis with high- resolution X-ray photoelectron spectroscopy (1−3 nm) and high-sensitivity low-energy ion scattering spectroscopy (0.3 nm) indicates that the NiO and La2O3 promoters are concentrated in the surface region of the bulk NaTaO3 phase. The NiO is concentrated on the NaTaO3 outermost surface layers, while La2O3 is distributed throughout the NaTaO3 surface region (1−3 nm). Raman and UV−vis spectroscopy revealed that the bulk molecular and electronic structures, respectively, of NaTaO3 were not modified by the addition of the La2O3 and NiO promoters, with La2O3 resulting in a slightly more ordered structure. Photoluminescence spectroscopy reveals that the addition of La2O3 and NiO produces a greater number of electron traps resulting in the suppression of the recombination of excited electrons and holes. In contrast to earlier reports, La2O3 is only a textural promoter (increasing the BET surface area by ∼7-fold by stabilizing smaller NaTaO3 particles) and causes an ∼3-fold decrease in the specific photocatalytic TORs (micromoles of H2 per square meter per hour) rate because surface La2O3 blocks exposed catalytic active NaTaO3 sites. The NiO promoter was found to be a potent electronic promoter that enhances the NaTaO3 surface-normalized TORs by a factor of ∼10−50 and turnover frequency by a factor of ∼10. The level of NiO promotion is the same in the absence and presence of La2O3, demonstrating that there is no promotional synergistic interaction between the NiO and La2O3 promoters. This study demonstrates the important contributions of the photocatalyst surface properties to the fundamental molecular/electronic structure−photoactivity relationships of promoted NaTaO3 photocatalysts that were previously not appreciated in the literature.

Doping La into the depletion layer of the Cd0.6Zn0.4S photocatalyst for efficient H2 evolution

We report a novel strategy for the enhancement of photocatalytic H2 evolution by doping La into the depletion layer of Cd0.6Zn0.4S (CZS: x% La). The apparent quantum yield of the CZS: 2% La photocatalyst at 350 nm is up to 93.3%, which is extremely high for solar water splitting even compared with the noble metal cocatalyst systems. This work may contribute to the design and construction of materials with outstanding capability for charge separation and hence improve the properties of the materials for various applications.

Contror of Surface Structure and Effect of Cocatalyst Aiming at Water Splitting over Photocatalyst

Activities of photocatalytic water splitting were improved by control of surface morphologies and loading of new cocatalysts. Doping of Ca, Sr and Ba into NaTaO3 photocatalysts led to formation of fine crystals and control of surface morphologies such as construction of nano-step structure. The photocatalytic activities for water splitting were improved by the doping. Photocatalytic overall water splitting on K4Nb6O17, Sr2Nb2O7, KTaO3, NaTaO3, and NaTaO3 doped with La photocatalysts were promoted by loading of nano-sized gold particles. The nano-sized Au worked as an active site for H2 evolution. An IrO2 cocatalyst which was photodeposited from an aqueous solution containing nitrate ions was also effective for overall water splitting. IrO2 functioned as an active site for O2 evolution.

Highly efficient water splitting into H2 and O2 over lanthanum-doped NaTaO3 photocatalysts with high crystallinity and surface nanostructure.

NiO-loaded NaTaO(3) doped with lanthanum showed a high photocatalytic activity for water splitting into H(2) and O(2) in a stoichiometric amount under UV irradiation. The photocatalytic activity of NiO-loaded NaTaO(3) doped with lanthanum was 9 times higher than that of nondoped NiO-loaded NaTaO(3). The maximum apparent quantum yield of the NiO/NaTaO(3):La photocatalyst was 56% at 270 nm. The factors affecting the highly efficient photocatalytic water splitting were examined by using various characterization techniques. Electron microscope observations revealed that the particle sizes of NaTaO(3):La crystals (0.1-0.7 microm) were smaller than that of the nondoped NaTaO(3) crystal (2-3 microm) and that the ordered surface nanostructure with many characteristic steps was created by the lanthanum doping. The small particle size with a high crystallinity was advantageous to an increase in the probability of the reaction of photogenerated electrons and holes with water molecules toward the recombination. Transmission electron microscope observations and extended X-ray absorption fine structure analyses indicated that NiO cocatalysts were loaded on the edge of the nanostep structure of NaTaO(3):La photocatalysts as ultrafine particles. The H(2) evolution proceeded on the ultrafine NiO particles loaded on the edge while the O(2) evolution occurred at the groove of the nanostep structure. Thus, the reaction sites for H(2) evolution were separated from those of O(2) evolution over the ordered nanostep structure. The small particle size and the ordered surface nanostep structure of the NiO/NaTaO(3):La photocatalyst powder contributed to the highly efficient water splitting into H(2) and O(2).