Wenliang Gao

Open-Framework Gallium Borate with Boric and Metaboric Acid Molecules inside Structural Channels Showing Photocatalysis to Water Splitting

An open-framework gallium borate with intrinsic photocatalytic activities to water splitting has been discovered. Small inorganic molecules, H3BO3 and H3B3O6, are confined inside structural channels by multiple hydrogen bonds. It is the first example to experimentally show the structural template effect of boric acid in flux synthesis.

PKU-3: An HCl-Inclusive Aluminoborate for Strecker Reaction Solved by Combining RED and PXRD

A novel microporous aluminoborate, denoted as PKU-3, was prepared by the boric acid flux method. The structure of PKU-3 was determined by combining the rotation electron diffraction and synchrotron powder X-ray diffraction data with well resolved ordered Cl(-) ions in the channel. Composition and crystal structure analysis showed that there are both proton and chlorine ions in the channels. Part of these protons and chlorine ions can be washed away by basic solutions to activate the open pores. The washed PKU-3 can be used as an efficient catalyst in the Strecker reaction with yields higher than 90%.

Ga4B2O9: An Efficient Borate Photocatalyst for Overall Water Splitting without Cocatalyst

Borates are well-known candidates for optical materials, but their potentials in photocatalysis are rarely studied. Ga(3+)-containing oxides or sulfides are good candidates for photocatalysis applications because the unoccupied 4s orbitals of Ga usually contribute to the bottom of the conducting band. It is therefore anticipated that Ga4B2O9 might be a promising photocatalyst because of its high Ga/B ratio and three-dimensional network. Various synthetic methods, including hydrothermal (HY), sol-gel (SG), and high-temperature solid-state reaction (HTSSR), were employed to prepare crystalline Ga4B2O9. The so-obtained HY-Ga4B2O9 are micrometer single crystals but do not show any UV-light activity unless modified by Pt loading. The problem is the fast recombination of photoexcitons. Interestingly, the samples obtained by SG and HTSSR methods both possess a fine micromorphology composed of well-crystalline nanometer strips. Therefore, the excited e(-) and h(+) can move to the surface easily. Both samples exhibit excellent intrinsic UV-light activities for pure water splitting without the assistance of any cocatalyst (47 and 118 μmol/h/g for H2 evolution and 22 and 58 μmol/h/g for O2 evolution, respectively), while there is no detectable activity for P25 (nanoparticles of TiO2 with a specific surface area of 69 m(2)/g) under the same conditions.

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.

Syntheses and luminescence of complete solid solutions Gd1−xEux[B6O9(OH)3] and α-Gd1−xEuxB5O9

There are very few luminescence studies for rare earth borates with hydroxyl or crystalline water molecules, which were believed to have a low luminescence efficiency because the vibrations of –OH or H2O may lead to quenching of the emission. We were motivated to study the luminescence properties of Gd1−xEux[B6O9(OH)3] (x = 0.10–1) and their dehydrated products, α-Gd1−xEuxB5O9. Efficient energy transfer from Gd3+ to Eu3+ was found in all of the studied polyborates. By TG-DSC and powder XRD experiments, we observed the dehydration of Eu[B6O9(OH)3], the re-crystallization to α-EuB5O9, and further decomposition to α-EuB3O6. During those processes, the Eu3+ luminescence spectra show interesting variations, meaning it is a good medium to understand the coordination environment evolution of Eu3+. It is observed that the symmetry of the Eu3+ coordination environment is the lowest in the amorphous state. Interestingly, this amorphous phase possesses a high efficiency of f–f transitions and a large R/O value (4.0), which implies its potential as a good red-emitting UV-LED phosphor. Anhydrous α-EuB5O9 shows the highest luminescence efficiency when excited by Eu3+ CT transition. For the first time, complete solid solutions α-Gd1−xEuxB5O9 were synthesized directly by the sol–gel method, and their luminescence properties were also studied.

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.

ZnGa(2-x)In(x)S4 (0 ≤ x ≤ 0.4) and Zn(1-2y)(CuGa)(y)Ga(1.7)In(0.3)S4 (0.1 ≤ y ≤ 0.2): optimize visible light photocatalytic H2 evolution by fine modulation of band structures.

Band structure engineering is an efficient technique to develop desired semiconductor photocatalysts, which was usually carried out through isovalent or aliovalent ionic substitutions. Starting from a UV-activated catalyst ZnGa2S4, we successfully exploited good visible light photocatalysts for H2 evolution by In(3+)-to-Ga(3+) and (Cu(+)/Ga(3+))-to-Zn(2+) substitutions. First, the bandgap of ZnGa2-xInxS4 (0 ≤ x ≤ 0.4) decreased from 3.36 to 3.04 eV by lowering the conduction band position. Second, Zn1-2y(CuGa)yGa1.7In0.3S4 (y = 0.1, 0.15, 0.2) provided a further and significant red-shift of the photon absorption to ∼500 nm by raising the valence band maximum and barely losing the overpotential to water reduction. Zn0.7Cu0.15Ga1.85In0.3S4 possessed the highest H2 evolution rate under pure visible light irradiation using S(2-) and SO3(2-) as sacrificial reagents (386 μmol/h/g for the noble-metal-free sample and 629 μmol/h/g for the one loaded with 0.5 wt % Ru), while the binary hosts ZnGa2S4 and ZnIn2S4 (synthesized using the same procedure) show 0 and 27.9 μmol/h/g, respectively. The optimal apparent quantum yield reached to 7.9% at 500 nm by tuning the composition to Zn0.6Cu0.2Ga1.9In0.3S4 (loaded with 0.5 wt % Ru).

Systematic Study of Cr3+ Substitution into Octahedra-Based Microporous Aluminoborates

Single crystals of pure aluminoborate PKU-1 (Al3B6O11(OH)5·nH2O) were obtained, and the structure was redetermined by X-ray diffraction. There are three independent Al atoms in the R3 structure model, and Al3 locates in a quite distorted octahedral environment, which was evidenced by (27)Al NMR results. This distortion of Al3O6 octahedra release the strong static stress of the main framework and leads to a symmetry lowering from the previously reported R3 to the presently reported R3. We applied a pretreatment to prepare Al(3+)/Cr(3+) aqueous solutions; as a consecquence, a very high Cr(3+)-to-Al(3+) substitution content (∼50 atom %) in PKU-1 can be achieved, which is far more than enough for catalytic purposes. Additionally, the preference for Cr(3+) substitution at the Al1 and Al2 sites was observed in the Rietveld refinements of the powder X-ray data of PKU-1:0.32Cr(3+). We also systematically investigated the thermal behaviors of PKU-1:xCr(3+) (0 ≤ x ≤ 0.50) by thermogravimetric-differential scanning calorimetry, in situ high-temperature XRD in vacuum, and postannealing experiments in furnace. The main framework of Cr(3+)-substituted PKU-1 could be partially retained at 1100 °C in vacuum. When 0.04 ≤ x ≤ 0.20, PKU-1:xCr(3+) transferred to the PKU-5:xCr(3+) (Al4B6O15:xCr(3+)) structure at ∼750 °C by a 5 h annealing in air. Further elevating the temperature led to a decomposition into the mullite phase, Al4B2O9:xCr(3+). For x > 0.20 in PKU-1:xCr(3+), the heat treatment led to a composite of Cr(3+)-substituted PKU-5 and Cr2O3, so the doping upper limit of Cr(3+) in PKU-5 structure is around 20 atom %.

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.