Formation and growth mechanism for niobium oxide nanoparticles: atomistic insight from in situ X-ray total scattering

Exploring the Effects of the Photochromic Response and Crystallization on the Local Structure of Noncrystalline Niobium Oxide

Niobium oxide (Nb2O5) is a versatile semiconductor material with photochromic properties. This study investigates the local structure of noncrystalline, short-range-ordered niobium oxide synthesized via a sol-gel method. X-ray atomic pair distribution function analysis unravels the structural arrangements within the noncrystalline materials at a local scale. In the following, in situ scattering and diffraction experiments elucidate the heat-induced structure transformation of the amorphous material into crystalline TT-Nb2O5 at 550 °C. In addition, the effect of photocatalytic conditions on the structure of the material was investigated by exposing the short-range-ordered and crystalline materials to ultraviolet light, resulting in a reversible color change from white to dark brown or blue. This photochromic response is due to the reversible elongation of the nearest Nb-O neighbors, as shown by local structure analysis based on in situ PDF analyses. Optical band gap calculations based on the ultraviolet-visible spectra collected for both the short-range-ordered and crystalline materials show that the band gap values reduced for the darkened materials return to their initial state after bleaching. Furthermore, electron energy loss spectroscopy reveals the reduction of Nb5+ to Nb4+ centers as a persistent effect. The study establishes a correlation between the band gap and the structure of niobium oxide, providing insights into the structure-performance relation at the atomic level.

From Gel to Crystal: Mechanism of HfO2 and ZrO2 Nanocrystal Synthesis in Benzyl Alcohol

Nonaqueous sol-gel syntheses have been used to make many types of metal oxide nanocrystals. According to the current paradigm, nonaqueous syntheses have slow kinetics, thus favoring the thermodynamic (crystalline) product. Here we investigate the synthesis of hafnium (and zirconium) oxide nanocrystals from the metal chloride in benzyl alcohol. We follow the transition from precursor to nanocrystal through a combination of rheology, EXAFS, NMR, TEM, and X-ray total scattering (PDF analysis). Upon dissolving the metal chloride precursor, the exchange of chloride ligands for benzylalkoxide liberates HCl. The latter catalyzes the etherification of benzyl alcohol, eliminating water. During the temperature ramp to the reaction temperature (220 °C), sufficient water is produced to turn the reaction mixture into a macroscopic gel. Rheological analysis shows a network consisting of strong interactions with temperature-dependent restructuring. After a few minutes at the reaction temperature, crystalline particles emerge from the gel, and nucleation and growth are complete after 30 min. In contrast, 4 h are required to obtain the highest isolated yield, which we attribute to the slow in situ formation of water (the extraction solvent). We used our mechanistic insights to optimize the synthesis, achieving high isolated yields with a reduced reaction time. Our results oppose the idea that nonaqueous sol-gel syntheses necessarily form crystalline products in one step, without a transient, amorphous gel state.

The Chemistry of Spinel Ferrite Nanoparticle Nucleation, Crystallization, and Growth

The nucleation, crystallization, and growth mechanisms of MnFe2O4, CoFe2O4, NiFe2O4, and ZnFe2O4 nanocrystallites prepared from coprecipitated transition metal (TM) hydroxide precursors treated at sub-, near-, and supercritical hydrothermal conditions have been studied by in situ X-ray total scattering (TS) with pair distribution function (PDF) analysis, and in situ synchrotron powder X-ray diffraction (PXRD) with Rietveld analysis. The in situ TS experiments were carried out on 0.6 M TM hydroxide precursors prepared from aqueous metal chloride solutions using 24.5% NH4OH as the precipitating base. The PDF analysis reveals equivalent nucleation processes for the four spinel ferrite compounds under the studied hydrothermal conditions, where the TMs form edge-sharing octahedrally coordinated hydroxide units (monomers/dimers and in some cases trimers) in the aqueous precursor, which upon hydrothermal treatment nucleate through linking by tetrahedrally coordinated TMs. The in situ PXRD experiments were carried out on 1.2 M TM hydroxide precursors prepared from aqueous metal nitrate solutions using 16 M NaOH as the precipitating base. The crystallization and growth of the nanocrystallites were found to progress via different processes depending on the specific TMs and synthesis temperatures. The PXRD data show that MnFe2O4 and CoFe2O4 nanocrystallites rapidly grow (typically <1 min) to equilibrium sizes of 20-25 nm and 10-12 nm, respectively, regardless of applied temperature in the 170-420 °C range, indicating limited possibility of targeted size control. However, varying the reaction time (0-30 min) and temperature (150-400 °C) allows different sizes to be obtained for NiFe2O4 (3-30 nm) and ZnFe2O4 (3-12 nm) nanocrystallites. The mechanisms controlling the crystallization and growth (nucleation, growth by diffusion, Ostwald ripening, etc.) were examined by qualitative analysis of the evolution in refined scale factor (proportional to extent of crystallization) and mean crystallite volume (proportional to extent of growth). Interestingly, lower kinetic barriers are observed for the formation of the mixed spinels (MnFe2O4 and CoFe2O4) compared to the inverse (NiFe2O4) and normal (ZnFe2O4) spinel structured compounds, suggesting that the energy barrier for formation may be lowered when the TMs have no site preference.

Sub-second pair distribution function using a broad bandwidth monochromator

Here the use of a broad energy bandwidth monochromator, i.e. a pair of B₄C/W multilayer mirrors (MLMs), is demonstrated for X-ray total scattering (TS) measurements and pair distribution function (PDF) analysis. Data are collected both on powder samples and from metal oxo clusters in aqueous solution at various concentrations. A comparison between the MLM PDFs and those obtained using a standard Si(111) double-crystal monochromator shows that the measurements yield MLM PDFs of high quality which are suitable for structure refinement. Moreover, the effects of time resolution and concentration on the quality of the resulting PDFs of the metal oxo clusters are investigated. PDFs of heptamolybdate clusters and tungsten α-Keggin clusters from X-ray TS data were obtained with a time resolution down to 3 ms and still showed a similar level of Fourier ripples to PDFs obtained from 1 s measurements. This type of measurement could thus open up faster time-resolved TS and PDF studies.

Beam-induced redox chemistry in iron oxide nanoparticle dispersions at ESRF-EBS

The storage ring upgrade of the European Synchrotron Radiation Facility makes ESRF-EBS the most brilliant high-energy fourth-generation light source, enabling in situ studies with unprecedented time resolution. While radiation damage is commonly associated with degradation of organic matter such as ionic liquids or polymers in the synchrotron beam, this study clearly shows that highly brilliant X-ray beams readily induce structural changes and beam damage in inorganic matter, too. Here, the reduction of Fe³⁺ to Fe²⁺ in iron oxide nanoparticles by radicals in the brilliant ESRF-EBS beam, not observed before the upgrade, is reported. Radicals are created due to radiolysis of an EtOH-H₂O mixture with low EtOH concentration (∼6 vol%). In light of extended irradiation times during insitu experiments in, for example, battery and catalysis research, beam-induced redox chemistry needs to be understood for proper interpretation of insitu data.

Chemical Insights into the Formation of Colloidal Iridium Nanoparticles from In Situ X-ray Total Scattering: Influence of Precursors and Cations on the Reaction Pathway

Iridium nanoparticles are important catalysts for several chemical and energy conversion reactions. Studies of iridium nanoparticles have also been a key for the development of kinetic models of nanomaterial formation. However, compared to other metals such as gold or platinum, knowledge on the nature of prenucleation species and structural insights into the resultant nanoparticles are missing, especially for nanoparticles obtained from Ir_xCl_y precursors investigated here. We use in situ X-ray total scattering (TS) experiments with pair distribution function (PDF) analysis to study a simple, surfactant-free synthesis of colloidal iridium nanoparticles. The reaction is performed in methanol at 50 °C with only a base and an iridium salt as precursor. From different precursor salts─IrCl₃, IrCl₄, H₂IrCl₆, or Na₂IrCl₆─colloidal nanoparticles as small as Ir_∼55 are obtained as the final product. The nanoparticles do not show the bulk iridium face-centered cubic (fcc) structure but show decahedral and icosahedral structures. The formation route is highly dependent on the precursor salt used. Using IrCl₃ or IrCl₄, metallic iridium nanoparticles form rapidly from Ir_xCl_y^n- complexes, whereas using H₂IrCl₆ or Na₂IrCl₆, the iridium nanoparticle formation follows a sudden growth after an induction period and the brief appearance of a crystalline phase. With H₂IrCl₆, the formation of different Ir_n (n = 55, 55, 85, and 116) nanoparticles depends on the nature of the cation in the base (LiOH, NaOH, KOH, or CsOH, respectively) and larger particles are obtained with larger cations. As the particles grow, the nanoparticle structure changes from partly icosahedral to decahedral. The results show that the synthesis of iridium nanoparticles from Ir_xCl_y is a valuable iridium nanoparticle model system, which can provide new compositional and structural insights into iridium nanoparticle formation and growth.