Indium hydroxide to bixbyite-type indium oxide transition probed in situ by time resolved synchrotron radiation

Polyoxometalate-Complexed Indium Hydroxide: Atomically Homogeneous Impregnation via Countercation Exchange

Metal hydroxides catalyze organic transformations and photochemical processes and serve as precursors for the oxide layers of functional multicomponent devices. However, no general methods are available for the preparation of stable water-soluble complexes of metal hydroxide nanocrystals (NCs) that might be more effective in catalysis and serve as versatile precursors for the reproducible fabrication of multicomponent devices. We now report that In^III-substituted monodefect Wells-Dawson (WD) polyoxometalate (POM) cluster anions, [α₂-P₂W₁₇O₆₁In^IIIOH)]^8-, serve as ligands for stable, water-soluble complexes, 1, of platelike, predominantly cubic-phase (dzhalindite) In(OH)₃ NCs that after optimization contain ca. 10% InOOH. Images from cryogenic tranmsission electron microscopy reveal numerous WD ligands at the surfaces of platelike NCs, with average dimensions of 17 × 28 × 2 nm, each complexed by an average of ca. 450 In^III-substituted WD cluster anions and charge-balanced by 3600 Na⁺ countercations. Facilitated by the water solubility of 1, countercation exchange is used to stoichiometrically disperse ca. 1800 Cu²⁺ ions in an atomically homogeneous fashion around the surfaces of each NC core. The utility of this impregnation method is illustrated by using the ion-exchanged material as an electrocatalyst that reduces CO₂ to CO 15 times faster per milligram of Cu than does K₆Cu[P₂Cu^II(H₂O)W₁₇O₆₁] (control) alone. More generally, the findings point to POM complexation as a promising method for stabilizing and solubilizing reactive d-, p-, and f-block metal hydroxide NCs and for enabling their utilization as versatile components in the fabrication of functional multicomponent materials.

In situtailoring the morphology of In(OH)3nanostructures via surfactants during anodization and their transformation into In2O3nanoparticles

The present work reports the effect of various surfactants on the morphology of In(OH)₃nanostructures prepared via anodization. In-sheets were anodized in an environmentally benign electrolyte containing a small quantity of CTAB, CTAC, and PDDA surfactants at room temperature. The produced nanostructures were characterized using XRD, HRTEM, SAED, and EDAX. The morphology of indium hydroxide (In(OH)₃) nanostructures was successfully tailoredin situwith the help of surfactants in 1 M KCl aqueous electrolyte. XRD results confirmed the formation of In(OH)₃and indium oxyhydroxide (InOOH) nanostructures in the pristine form which were transformed into single-phase cubic In₂O₃nanoparticles (NPs) after calcination. HRTEM analyses showed that the morphology and size of the In(OH)₃nanostructures can be tuned to form nanorods, nanosheets and nanostrips using different surfactants. The results revealed that CTAC and PDDA surfactants have a profound effect on the morphology of In(OH)₃nanostructure compared to CTAB due to the higher concentration of Cl^-ion. The possible mechanism of surfactants effect on the morphology is proposed. Furthermore, annealing converted the In(OH)₃nanostructures into spherical In₂O₃NPs with uniform and homogeneous size. We anticipate that the morphology of other metal-oxides nanostructure can be tuned using this simple, facile and rapid technique. In₂O₃NPs prepared without and with CTAB surfactant were further explored for the non-enzymatic detection of hydrogen peroxide (H₂O₂). Electrochemical measurements showed enhanced electrocatalytic performance with fast electron transfer (∼2s) between the redox centers of H₂O₂and electrode surface. The In₂O₃NPs prepared using CTAB/Au electrode exhibited about 4-fold increase in sensitivity compared to the bare Au electrode. The biosensor also demonstrated good reproducibility, higher selectivity, and increased shelf life.

Phase control for indium oxide nanoparticles

The indium oxides, c-In2O3, h-In2O3, InOOH and In(OH)3, constitute an important class of wide band gap semiconductors. Synthesis of any indium oxide phase involves manoeuvring in a complex matrix of process parameters, and some phases are only obtained through controlled phase transformations. Considering the widespread use of indium oxide semiconductors it is restrictive that no coherent picture exists of the formation mechanisms of individual phases and phase transformations between them. Here we access the indium oxide system through solvothermal synthesis and/or powder calcinations, and we use in situ X-ray scattering in combination with thermal analysis to investigate the complex phase relations. This allows us to unravel synthesis pathways for the different indium oxide phases, and the insight is used to develop procedures for scalable continuous flow solvothermal synthesis. Direct formation of crystalline nanomaterials from precursor solutions was observed for In(OH)3, InOOH and cubic c-In2O3, while formation of hexagonal h-In2O3 requires thermal decomposition of InOOH. The in situ X-ray scattering data reveal new phase transformations from In(OH)3 to InOOH, and from InOOH to c-In2O3. Interestingly, solvothermal synthesis conditions facilitate different reactions mechanisms than dry powder calcinations, and both In(OH)3 and InOOH have different transformations under dry and wet conditions.

Synergistic Coupling of Photo and Thermal Conditions for Enhancing CO2 Reduction Rates in the Reverse Water Gas Shift Reaction

The photocatalytic activity of nanostructured In₂O_3-x(OH)_y for the reverse water gas shift (RWGS) reaction CO₂ + H₂ → CO + H₂O can be greatly enhanced by substitution of Bi(III) for In(III) in the lattice of Bi_zIn_2-zO_3-x(OH)_y. This behavior was hypothesized as the effect of the population and location of Bi(III) on the Lewis acidity and Lewis basicity of proximal hydroxide and coordinately unsaturated metal surface sites in Bi_zIn_2-zO_3-x(OH)_y acting synergistically as a frustrated Lewis acid-base pair reaction. Nonetheless, such photocatalytic activity is usually optimized in a specific batch reactor setup sequence, with H₂ as an initial gas input under photo and thermal conditions before introducing CO₂. Hence, the chemical interplay between environment parameters such as photoillumination, thermal input, and gas reactant components with the effects of Bi substitution is unclear. Reported herein, glovebox-protected X-ray photoelectron spectroscopy (XPS) interrogates this photochemical RWGS reaction transiting from vacuum state to similar conditions in a photocatalytic reactor, under dark and ambient temperatures, 150°C in dark and 150 °C under photoillumination. Binding energy shifts were used to correlate the material system's Lewis basicity response to these acidic probe gases. In-situ gas electronic sensitivity and in-situ UV-vis-derived band-gap trends confirm the trends shown in the XPS results, hence showing its equivalency with in situ methods. The enhanced photocatalytic reduction rate of CO₂ with H₂ with a low doped 0.05% a.t Bi system is thus associated with an increased gas sensitivity in H₂ + CO₂, a greater expansion in the OH shoulder than that of the undoped system under heat and light conditions, as well as a greater thermal stability of dissociated H adatoms. The photoinduced expansion of the OH shoulder and the increased positive binding energy shifts show the important role of photoillumination over that of thermal conditions. The poor catalytic performance of the high doped system can be attributed to a competing H₂ reduction of In³⁺. The results provide new insight into how pairing photo and thermal conditions with the methodical tuning of the Lewis acidity and Lewis basicity of surface frustrated Lewis acid-base pair sites by varying z amount in Bi_zIn_2-zO_3-x(OH)_y enables optimization of the rate of the photochemical RWGS.

Scaled-up solvothermal synthesis of nanosized metastable indium oxyhydroxide (InOOH) and corundum-type rhombohedral indium oxide (rh-In2O3)

Phase pure metastable indium oxyhydroxide (InOOH) with crystallite size in the range ca. 2–7 nm is synthesized by a nonaqueous solvothermal synthesis route in ethanol. The influence of synthesis parameters such as temperature, basicity (pH), synthesis time, and water content is carefully addressed. T-pH maps summarize the impact of synthesis temperature and pH and reveal that phase pure InOOH is obtained in water-free solutions at mild temperatures (150–180°C) in highly basic conditions (pH>12). Subsequent calcination of InOOH at 375–700°C in ambient air atmosphere results in metastable nanoscaled rhombohedral indium oxide (rh-In2O3). The synthesis protocol for phase pure nanocrystalline InOOH material was successfully upscaled allowing for obtaining ca. 3 g of phase-pure InOOH with a yield of ca. 78%. The upscaled InOOH and rh-In2O3 batches are now available for a detailed in-situ characterization of the mechanism of decomposition of InOOH to rh-In2O3 to c-In2O3 as well as for the characterization of the functional properties of InOOH and rh-In2O3 materials.

Compact low power infrared tube furnace for in situ X-ray powder diffraction

We describe the development and implementation of a compact, low power, infrared heated tube furnace for in situ powder X-ray diffraction experiments. Our silicon carbide (SiC) based furnace design exhibits outstanding thermal performance in terms of accuracy control and temperature ramping rates while simultaneously being easy to use, robust to abuse and, due to its small size and low power, producing minimal impact on surrounding equipment. Temperatures in air in excess of 1100 °C can be controlled at an accuracy of better than 1%, with temperature ramping rates up to 100 °C/s. The complete "add-in" device, minus power supply, fits in a cylindrical volume approximately 15 cm long and 6 cm in diameter and resides as close as 1 cm from other sensitive components of our experimental synchrotron endstation without adverse effects.

Spatial Separation of Charge Carriers in In2O3-x(OH)y Nanocrystal Superstructures for Enhanced Gas-Phase Photocatalytic Activity

The development of strategies for increasing the lifetime of photoexcited charge carriers in nanostructured metal oxide semiconductors is important for enhancing their photocatalytic activity. Intensive efforts have been made in tailoring the properties of the nanostructured photocatalysts through different ways, mainly including band-structure engineering, doping, catalyst-support interaction, and loading cocatalysts. In liquid-phase photocatalytic dye degradation and water splitting, it was recently found that nanocrystal superstructure based semiconductors exhibited improved spatial separation of photoexcited charge carriers and enhanced photocatalytic performance. Nevertheless, it remains unknown whether this strategy is applicable in gas-phase photocatalysis. Using porous indium oxide nanorods in catalyzing the reverse water-gas shift reaction as a model system, we demonstrate here that assembling semiconductor nanocrystals into superstructures can also promote gas-phase photocatalytic processes. Transient absorption studies prove that the improved activity is a result of prolonged photoexcited charge carrier lifetimes due to the charge transfer within the nanocrystal network comprising the nanorods. Our study reveals that the spatial charge separation within the nanocrystal networks could also benefit gas-phase photocatalysis and sheds light on the design principles of efficient nanocrystal superstructure based photocatalysts.

Synthesis and formation mechanisms of morphology-controllable indium-containing precursors and optical properties of the derived In2O3 particles

Indium-containing precursors with three morphologies were synthesized, and the formation mechanisms were analyzed.

Plasma-assisted electrolytic synthesis of In(OH)3 nanocubes for thermal transformation into In2O3 nanocubes with a controllable Sn content

Single-crystal In(OH)₃ nanocubes were synthesized through a novel wet-chemical route of plasma-assisted electrolytic process and further thermally transformed into polycrystalline bixbyite-type c-In₂O₃ nanocubes with a controlled Sn content.

The Rational Design of a Single-Component Photocatalyst for Gas-Phase CO2 Reduction Using Both UV and Visible Light

The solar-to-chemical energy conversion of greenhouse gas CO2 into carbon-based fuels is a very important research challenge, with implications for both climate change and energy security. Herein, the key attributes of hydroxides and oxygen vacancies are experimentally identified in non-stoichiometric indium oxide nanoparticles, In2O3-x(OH)y, that function in concert to reduce CO2 to CO under simulated solar irradiation.