Mufei Yue

Coordination environment evolution of Eu3+ during the dehydration and re-crystallization processes of Sm1−xEux[B9O13(OH)4]·H2O by photoluminescent characteristics

There are limited photoluminescence (PL) studies for rare earth borates with crystalline water molecules, which are usually supposed to have low PL efficiency because the vibrations of H2O or -OH may lead to emission quenching. We investigated the PL properties of Sm(1-x)Eu(x)[B9O13(OH)4]·H2O (x = 0-1.00) and their dehydrated products α-Sm(1-x)Eu(x)B5O9. There is no quenching effect in those studied polyborates because the large borate ionic groups isolate the Eu(3+) activators very well. Sm(3+) and Eu(3+) are basically separated luminescent activators. Comparatively, Sm(3+) shows a very small emission intensity, which can be almost ignored, therefore our interest is focused on the Eu(3+) luminescence. By TG-DSC and powder XRD experiments, we defined three weight-loss steps for Eu[B9O13(OH)4]·H2O and a re-crystallization process to α-EuB5O9, during which luminescent spectra of Eu(3+) are recorded. It shows an interesting variety and therefore is a good medium to understand the coordination environment evolution of Eu(3+), even for the intermediate amorphous phase. In fact, the coordination symmetry of Eu(3+) in the amorphous state is the lowest. The high efficiency of the f-f transitions and large R/O value (3.8) imply this amorphous phase is potentially a good red-emitting UV-LED phosphor. Anhydrous α-EuB5O9 shows the highest luminescent efficiency excited by Eu(3+) CT transition. In addition, α-Sm(1-x)Eu(x)B5O9 was synthesized by a sol-gel method directly for the first time, and α-EuB5O9 shows superior PL properties due to its better crystallinity. A lot of hydrated polyborates with crystalline water molecules remain unexplored and our study shows their potential as good phosphors.

ZnCr2S4: Highly effective photocatalyst converting nitrate into N2 without over-reduction under both UV and pure visible light

We propose several superiorities of applying some particular metal sulfides to the photocatalytic nitrate reduction in aqueous solution, including the high density of photogenerated excitons, high N2 selectivity (without over-reduction to ammonia). Indeed, ZnCr2S4 behaved as a highly efficient photocatalyst, and with the assistance of 1 wt% cocatalysts (RuOx, Ag, Au, Pd, or Pt), the efficiency was greatly improved. The simultaneous loading of Pt and Pd led to a synergistic effect. It offered the highest nitrate conversion rate of ~45 mg N/h together with the N2 selectivity of ~89%. Such a high activity remained steady after 5 cycles. The optimal apparent quantum yield at 380 nm was 15.46%. More importantly, with the assistance of the surface plasma resonance effect of Au, the visible light activity achieved 1.352 mg N/h under full arc Xe-lamp, and 0.452 mg N/h under pure visible light (λ > 400 nm). Comparing to the previous achievements in photocatalytic nitrate removal, our work on ZnCr2S4 eliminates the over-reduction problem, and possesses an extremely high and steady activity under UV-light, as well as a decent conversion rate under pure visible light.

Superior performance of CuInS2 for photocatalytic water treatment: full conversion of highly stable nitrate ions into harmless N2 under visible light

Photocatalytic nitrate removal is potentially a green and low-cost technique for water purification; however, no effective visible light photocatalyst has been developed during the past thirty years. Here, we prove that CuInS2 (CIS) is a highly efficient visible light photocatalyst for nitrate removal. By the simultaneous loading of Pt, Ru and Au, it exhibited a high record of nitrate conversion rate of 8.32 mg N h−1 under pure visible light (λ > 400 nm). Highly concentrated NO3− (100 ppm N) can be completely converted into N2 in a few hours, without any over-reduction to ammonia nor production of H2. Under the irradiation of a monochromatic beam from 400 to 650 nm, CIS was most sensitive at 500 nm with an optimal apparent quantum yield of 23.85%. The mechanism was proposed as adsorption–reduction reactions and supported by the fact that the photocatalytic efficiency decreased when halogen ions were added as competing anions. The necessity of using a sacrificial agent during nitrate reduction was validated by a thorough discussion. Herein CIS remained effective when various or harmless sacrificial agents were used. Our work presents a new generation of photocatalysts for nitrate removal and pushes it forward to possible applications in industry.

Photocatalytic H2 evolution for α-, β-, γ-Ga2O3 and suppression of hydrolysis of γ-Ga2O3 by adjusting pH, adding a sacrificial agent or loading a cocatalyst

In contrast to α- and β-Ga2O3 which have already been studied as photocatalysts for pure water splitting, there has been no report on the γ-phase. A comparative study on α-, β- and γ-Ga2O3 all prepared by a precipitation method was therefore performed. The as-prepared gallium oxides were phase-identified by powder X-ray diffraction, where γ-Ga2O3 possessed the most broad reflection peaks due to poor crystallization. Scanning electron microscopy and N2 adsorption–desorption experiments confirmed the morphology, and the specific surface areas were 144.3, 30.7, and 77.3 m2 g−1 for γ-, β- and α-Ga2O3, respectively. Photocatalytic H2 evolution efficiency in pure water was determined to be in the order of γ-Ga2O3 > α-Ga2O3 > β-Ga2O3, and the efficiencies were all much higher than that of P25–1 wt% Ag. A slight hydrolysis process was observed for γ-Ga2O3. Both lowering the pH value (∼4.5) by H2SO4 and adding sacrificial agent (CH3OH) were applied to prohibit the hydrolysis completely. Eventually, 1 wt% Ag was loaded as a cocatalyst in order to not only improve the stability but also to increase the H2 generation rate to 742 μmol h−1 g−1 in pure water. In addition, for this particular photocatalyst, the optimal apparent quantum yield achieved at 254 nm was 8.34%. Our work represents the first study of γ-Ga2O3 in the application of photocatalytic water splitting, and indeed it might have a high potential in solar energy conversion.

Optimizing the performance of photocatalytic H2 generation for ZnNb2O6 synthesized by a two-step hydrothermal method

Semiconductor-based photocatalytic H2 generation is a promising technique and the development of efficient photocatalysts has attracted great attention. Columbite-ZnNb2O6 is a wide-bandgap semiconductor capable of photocatalytic water splitting. Here we employed a two-step hydrothermal method to first dissolve Nb2O5 with a highly basic aqueous solution and further react it with Zn2+ to form nanosized ZnNb2O6. The reaction time plays an important role on its morphology and photocatalytic performance in water reduction. The sample synthesized through 7 days of reaction was the optimal one with an appropriate crystallinity and a large specific surface area, however the severe surficial defects prohibited its photocatalytic activity in pure water. The H2 generation at a rate of 23.6(5) μmol h−1 g−1 emerged when 20 vol% methanol was used as the hole-sacrificial agent. Most remarkably, once metal or metal oxide cocatalysts, including Pt, Au, NiO, RuO2, Ag2O, and Pd/PdO, were loaded appropriately, the photocatalytic H2 generation rate ultimately achieved 3200(100) or 680(20) μmol h−1 g−1 with or without using methanol, respectively. Apparent quantum yields (AQYs) at 295 nm were investigated by changing the experimental parameters, and the optimal AQYs are 4.54% and 9.25% in water and methanol solution, respectively. Further post-modifications like bandgap engineering may be performed on this highly efficient nano-ZnNb2O6.

Strong Lewis Base Ga4B2O9: Ga–O Connectivity Enhanced Basicity and Its Applications in the Strecker Reaction and Catalytic Conversion of n-Propanol

Heterogeneous solid base catalysis is valuable and promising in chemical industry, however it is insufficiently developed compared to solid acid catalysis due to the lack of satisfied solid base catalysts. To gain the strong basicity, the previous strategy was to basify oxides with alkaline metals to create surficial vacancies or defects, which suffers from the instability under catalytic conditions. Monocomponent basic oxides like MgO are literally stable but deficient in electron-withdrawing ability. Here we prove that a special connectivity of atoms could enhance the Lewis basicity of oxygen in monocomponent solids exemplified by Ga4B2O9. The structure-induced basicity is from the μ3-O linked exclusively to five-coordinated Ga3+. Ga4B2O9 behaved as a durable catalyst with a high yield of 81% in the base-catalyzed synthesis of α-aminonitriles by Strecker reaction. In addition, several monocomponent solid bases were evaluated in the Strecker reaction, and Ga4B2O9 has the largest amount of strong base centers (23.1 μmol/g) and the highest catalytic efficiency. Ga4B2O9 is also applicable in high-temperature solid-gas catalysis, for example, Ga4B2O9 catalyzed efficiently the dehydrogenation of n-propanol, resulting in a high selectivity to propanal (79%). In contrast, the comparison gallium borate, Ga-PKU-1, which is a Brönsted acid, preferred to catalyze the dehydration process to obtain propylene with a selectivity of 94%.