The chemistry of ZnWO4 nanoparticle formation

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.

POMFinder: identifying polyoxometallate cluster structures from pair distribution function data using explainable machine learning

Characterization of a material structure with pair distribution function (PDF) analysis typically involves refining a structure model against an experimental data set, but finding or constructing a suitable atomic model for PDF modelling can be an extremely labour-intensive task, requiring carefully browsing through large numbers of possible models. Presented here is POMFinder, a machine learning (ML) classifier that rapidly screens a database of structures, here polyoxometallate (POM) clusters, to identify candidate structures for PDF data modelling. The approach is shown to identify suitable POMs from experimental data, including in situ data collected with fast acquisition times. This automated approach has significant potential for identifying suitable models for structure refinement to extract quantitative structural parameters in materials chemistry research. POMFinder is open source and user friendly, making it accessible to those without prior ML knowledge. It is also demonstrated that POMFinder offers a promising modelling framework for combined modelling of multiple scattering techniques.

Influence of the Precursor Structure on the Formation of Tungsten Oxide Polymorphs

Understanding material nucleation processes is crucial for the development of synthesis pathways for tailormade materials. However, we currently have little knowledge of the influence of the precursor solution structure on the formation pathway of materials. We here use in situ total scattering to show how the precursor solution structure influences which crystal structure is formed during the hydrothermal synthesis of tungsten oxides. We investigate the synthesis of tungsten oxide from the two polyoxometalate salts, ammonium metatungstate, and ammonium paratungstate. In both cases, a hexagonal ammonium tungsten bronze (NH4)0.25WO3 is formed as the final product. If the precursor solution contains metatungstate clusters, this phase forms directly in the hydrothermal synthesis. However, if the paratungstate B cluster is present at the time of crystallization, a metastable intermediate phase in the form of a pyrochlore-type tungsten oxide, WO3·0.5H2O, initially forms. The pyrochlore structure then undergoes a phase transformation into the tungsten bronze phase. Our studies thus experimentally show that the precursor cluster structure present at the moment of crystallization directly influences the formed crystalline phase and suggests that the precursor structure just prior to crystallization can be used as a tool for targeting specific crystalline phases of interest.

A reactor for time-resolved X-ray studies of nucleation and growth during solvothermal synthesis

Understanding the nucleation and growth mechanisms of nanocrystals under hydro- and solvothermal conditions is key to tailoring functional nanomaterials. High-energy and high-flux synchrotron radiation is ideal for characterization by powder X-ray diffraction and X-ray total scattering in real time. Different versions of batch-type cell reactors have been employed in this work, exploiting the robustness of polyimide-coated fused quartz tubes with an inner diameter of 0.7 mm, as they can withstand pressures up to 250 bar and temperatures up to 723 K for several hours. Reported here are recent developments of the in situ setups available for general users on the P21.1 beamline at PETRA III and the DanMAX beamline at MAX IV to study nucleation and growth phenomena in solvothermal synthesis. It is shown that data suitable for both reciprocal-space Rietveld refinement and direct-space pair distribution function refinement can be obtained on a timescale of 4 ms.

Atomic structural changes in the formation of transition metal tungstates: the role of polyoxometalate structures in material crystallization

Material nucleation processes are poorly understood; nevertheless, an atomistic understanding of material formation would aid in the design of material synthesis methods. Here, we apply in situ X-ray total scattering experiments with pair distribution function (PDF) analysis to study the hydrothermal synthesis of wolframite-type MWO₄ (M : Mn, Fe, Co, Ni). The data obtained allow the mapping of the material formation pathway in detail. We first show that upon mixing of the aqueous precursors, a crystalline precursor containing [W₈O₂₇]^6- clusters forms for the MnWO₄ synthesis, while amorphous pastes form for the FeWO₄, CoWO₄ and NiWO₄ syntheses. The structure of the amorphous precursors was studied in detail with PDF analysis. Using database structure mining and an automated modelling strategy by applying machine learning, we show that the amorphous precursor structure can be described through polyoxometalate chemistry. A skewed sandwich cluster containing Keggin fragments describes the PDF of the precursor structure well, and the analysis shows that the precursor for FeWO₄ is more ordered than that of CoWO₄ and NiWO₄. Upon heating, the crystalline MnWO₄ precursor quickly converts directly to crystalline MnWO₄, while the amorphous precursors transform into a disordered intermediate phase before the crystalline tungstates appear. Our data show that the more disordered the precursor is, the longer the reaction time required to form crystalline products, and disorder in the precursor phase appears to be a barrier for crystallization. More generally, we see that polyoxometalate chemistry is useful when describing the initial wet-chemical formation of mixed metal oxides.

A review on the applications of zinc tungstate (ZnWO4) photocatalyst for wastewater treatment

The monoclinic wolframite-phase structure of ZnWO₄ materials has been frequently synthesised, characterised, and applied in optical fibres, environmental decontamination, electrochemistry, photonics, catalysis, and not limited to magnetic applications. However, the problems of crystal growth conditions and mechanisms, growth, the crystal quality, stability, and the role of synthesis parameters of ZnWO₄ nanoparticles remain a challenge limiting its commercial applications. This review presents recent advances of ZnWO₄ as an advanced multi-functional material for Industrial wastewater treatment. The review also examines the influence of the synthesis parameters on the properties of ZnWO₄ and provides insight into new perspectives on ZnWO₄-based photocatalyst. Many researches have shown significant improvement in the efficiency of ZnWO₄ by mixing with polymers and doping with metals, nonmetals, and other nanoparticles. The review also provides information on the mechanism of doping ZnWO₄ with metals, non-metals, metalloids, metals oxides, and polymers based on different synthesis methods for bandgap reduction and extension of its photocatalytic activity to the visible region. The doped ZnWO₄ photocatalyst was a more effective and environmentally friendly material for removing organic and inorganic contaminants in industrial wastewater than ordinary ZnWO₄ nanocrystalline under suitable growth conditions.

Effects of Voigt diffraction peak profiles on the pair distribution function

Powder diffraction and pair distribution function (PDF) analysis are well established techniques for investigation of atomic configurations in crystalline materials, and the two are related by a Fourier transformation. In diffraction experiments, structural information, such as crystallite size and microstrain, is contained within the peak profile function of the diffraction peaks. However, the effects of the PXRD (powder X-ray diffraction) peak profile function on the PDF are not fully understood. Here, all the effects from a Voigt diffraction peak profile are solved analytically, and verified experimentally through a high-quality X-ray total scattering measurement on Ni powder. The Lorentzian contribution to the microstrain broadening is found to result in Voigt-shaped PDF peaks. Furthermore, it is demonstrated that an improper description of the Voigt shape during model refinement leads to overestimation of the atomic displacement parameter.

Unravelling the complex formation mechanism of HfO2 nanocrystals using in situ pair distribution function analysis

Hafnia, HfO₂, which is a wide band gap semiconducting oxide, is much less studied than the chemically similar zirconia (ZrO₂). Here, we study the formation of hafnia nanocrystals from hafnium tetrachloride in methanol under solvothermal conditions (248 bar, 225-450 °C) using complementary in situ powder X-ray diffraction (PXRD) and Pair Distribution Function (PDF) analysis. The main structural motif of the precursor solution (HfCl₄ dissolved in methanol) is a Hf oxide trimer with very similar local structure to that of m-HfO₂. Different measurements on precursor solutions show large intensity variation for the Hf-Cl correlations signifying different extents of HCl elimation. A few seconds of heating lead to a correlation appearing at 3.9 Å corresponding to corner-sharing Hf-polyhedra in a disordered solid matrix. During the next minutes (depending on temperature) the disordered structure rearranges and the nearest neighbour Hf-Hf distance contracts while the Hf-O coordination number increases. After approximately 90 seconds (at T = 250 °C) the structural rearrangement terminates and 1-2 nm nanocrystals of m-HfO₂ nucleate. Initially the m-HfO₂ nanocrystals have significant disorder as reflected in large Hf atomic displacement parameter (ADP) values, but as the nanocrystals grow to 5-6 nm in size during extended heating, the Hf ADPs decrease toward the values obtained for ordered bulk structures. The nanocrystal growth is not well modelled by the Johnson-Mehl-Avrami expression reflecting that multiple complex chemical processes occur during this highly nonclassical nanocrystal formation under solvothermal conditions.

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

Understanding the mechanisms for nanoparticle nucleation and growth is crucial for the development of tailormade nanomaterials. Here, we use X-ray total scattering and Pair Distribution Function analysis to follow the formation and growth of niobium oxide nanoparticles. We study the solvothermal synthesis from niobium chloride in benzyl alcohol, and through investigations of the influence of reaction temperature, a formation pathway can be suggested. Upon dissolution of niobium chloride in benzyl alcohol, octahedral [NbCl6-xOx] complexes form through exchange of chloride ligands. Heating of the solution results in polymerization, where larger clusters built from multiple edge-sharing [NbCl6-xOx] octahedra assemble. This leads to the formation of a nucleation cluster with the ReO3 type structure, which grows to form nanoparticles of the Wadsley-Roth type H-Nb2O5 structure, which in the bulk phase usually only forms at high temperature. Upon further growth, structural defects appear, and the presence of shear-planes in the structure appears highly dependent on nanoparticle size.

Structural evolution in thermoelectric zinc antimonide thin films studied by in situ X-ray scattering techniques

Zinc antimonides have been widely studied owing to their outstanding thermoelectric properties. Unlike in the bulk state, where various structurally unknown phases have been identified through their specific physical properties, a number of intermediate phases in the thin-film state remain largely unexplored. Here, in situ X-ray diffraction and X-ray total scattering are combined with in situ measurement of electrical resistivity to monitor the crystallization process of as-deposited amorphous Zn-Sb films during post-deposition annealing. The as-deposited Zn-Sb films undergo a structural evolution from an amorphous phase to an intermediate crystalline phase and finally the ZnSb phase during heat treatment up to 573 K. An intermediate phase (phase B) is identified to be a modified β-Zn8Sb7 phase by refinement of the X-ray diffraction data. Within a certain range of Sb content (∼42-55 at%) in the films, phase B is accompanied by an emerging Sb impurity phase. Lower Sb content leads to smaller amounts of Sb impurity and the formation of phase B at lower temperatures, and phase B is stable at room temperature if the annealing temperature is controlled. Pair distribution function analysis of the amorphous phase shows local ordered units of distorted ZnSb4 tetrahedra, and annealing leads to long-range ordering of these units to form the intermediate phase. A higher formation energy is required when the intermediate phase evolves into the ZnSb phase with a significantly more regular arrangement of ZnSb4 tetrahedra.

Controlling the Self-Assembly of New Metastable Tin Vanadium Selenides Using Composition and Nanoarchitecture of Precursors

In solid-state chemistry, the direct reaction of elements at low temperatures is limited by low solid-state interdiffusion rates. This and the limited number of processing parameters often prevent the synthesis of metastable compounds. Precisely controlling the number of atoms and nanoarchitecture of layered elemental precursors enabled the selective synthesis of two closely related metastable tin vanadium selenides via near-diffusionless reactions at low temperatures. Although the nanoarchitectures of the precursors required to form [(SnSe₂)_0.80]₁(VSe₂)₁ and [(SnSe)_1.15]₁(VSe₂)₁ are very similar, controlling the local composition of the Sn|Se layers in the precursors enables the selective synthesis of either compound. The metastable alloy Sn_xV_1-xSe₂ was preferentially formed over [(SnSe₂)_0.80]₁(VSe₂)₁, which has the identical composition, by modifying the nanoarchitecture of the precursor. Ex situ in-plane X-ray diffraction and X-ray reflectivity collected as a function of annealing temperature provided information on lateral and perpendicular growth of [(SnSe₂)_0.80]₁(VSe₂)₁. The presence of Laue oscillations throughout the self-assembly provided atomic-scale information on the thickness of the [(SnSe₂)_0.80]₁(VSe₂)₁ domains, giving insights into the self-assembly process. A reaction mechanism is proposed and used to rationalize how composition and nanoarchitecture control the reaction pathway through the free energy landscape.

There's no place like real-space: elucidating size-dependent atomic structure of nanomaterials using pair distribution function analysis

The development of new functional materials builds on an understanding of the intricate relationship between material structure and properties, and structural characterization is a crucial part of materials chemistry. However, elucidating the atomic structure of nanomaterials remains a challenge using conventional diffraction techniques due to the lack of long-range atomic order. Over the past decade, Pair Distribution Function (PDF) analysis of X-ray or neutron total scattering data has become a mature and well-established method capable of giving insight into the atomic structure in nanomaterials. Here, we review the use of PDF analysis and modelling in characterization of a range of different nanomaterials that exhibit unique atomic structure compared to the corresponding bulk materials. A brief introduction to PDF analysis and modelling is given, followed by examples of how essential structural information can be extracted from PDFs using both model-free and advanced modelling methods. We put an emphasis on how the intuitive nature of the PDF can be used for understanding important structural motifs, and on the diversity of applications of PDF analysis to nanostructure problems.

Mapping the redox chemistry of common solvents in solvothermal synthesis through in situ X-ray diffraction

Solvothermal technology shows great promise in "green" materials synthesis, processing, and recycling. The outcome of a specific solvothermal reaction depends strongly on the solvent properties, and the versatility of solvothermal synthesis hinges on the very large changes in solvent properties as a function of temperature and pressure. Here, six simple 3d transition metal nitrate salts (Cu(ii), Ni(ii), Co(ii), Fe(iii), Mn(ii), Cr(iii)) were dissolved in five common solvents (water, ethanol, ethylene glycol, glycerol, and 10% hydrogen peroxide solution) and heated stepwise up to 450 °C at a pressure of 250 bar using an in situ reactor while X-ray scattering data was recorded. A range of crystalline phases were observed in the form of metallic phases, metal oxides, and other ionic compounds. These data by themselves provide simple recipes for synthesis of many technologically important 3d transition metal nanomaterials. However, more generally the oxidation states of the metals in the synthesized materials can be used to map the solvent redox properties under solvothermal conditions. It is found that glycerol and ethylene glycol are strongly reducing, ethanol is moderately reducing, while water is weakly oxidizing. The behavior of the hydrogen peroxide solution is more complex including both oxidization and reduction. Furthermore, it is observed that the reducing powers of ethanol, ethylene glycol, and glycerol are enhanced with increasing temperature. The mapping of the redox properties of these common solvents provides a method for tailoring a given reaction through choice of solvent and reaction temperature. Solvothermal processes represent an environmentally benign alternative to the use of toxic reducing agents in chemical reactions, and quantification of the redox chemistry is a first step in rational materials design.

Intrinsic Defect Engineering in Eu3+ Doped ZnWO₄ for Annealing Temperature Tunable Photoluminescence

Eu³⁺ doped ZnWO₄ phosphors were synthesized via the co-precipitation technique followed by subsequent thermal annealing in the range of 400–1000 °C. The phase, morphology, elemental composition, chemical states, optical absorption, and photoluminescence (PL) of the phosphors were characterized by X-ray diffraction, scanning electron microscopy, dispersive X-ray spectroscopy, X-ray photoelectron spectrometry, diffuse UV–vis reflectance spectroscopy, PL spectrophotometry, and PL lifetime spectroscopy, respectively. It is found that the PL from Eu³⁺ doped ZnWO₄ is tunable through the control of the annealing temperature. Density functional calculations and optical absorption confirm that thermal annealing created intrinsic defects in ZnWO₄ lattices play a pivotal role in the color tunable emissions of the Eu³⁺ doped ZnWO₄ phosphors. These data have demonstrated that intrinsic defect engineering in ZnWO₄ lattice is an alternative and effective strategy for tuning the emission color of Eu³⁺ doped ZnWO₄. This work shows how to harness the intrinsic defects in ZnWO₄ for the preparation of color tunable light-emitting phosphors.

The Chemistry of Nucleation: In Situ Pair Distribution Function Analysis of Secondary Building Units During UiO-66 MOF Formation

The concept of secondary building units (SBUs) is central to all science on metal-organic frameworks (MOFs), and they are widely used to design new MOF materials. However, the presence of SBUs during MOF formation remains controversial, and the formation mechanism of MOFs remains unclear, due to limited information about the evolution of prenucleation cluster structures. Here in situ pair distribution function (PDF) analysis was used to probe UiO-66 formation under solvothermal conditions. The expected SBU-a hexanuclear zirconium cluster-is present in the metal salt precursor solution. Addition of organic ligands results in a disordered structure with correlations up to 23 Å, resembling crystalline UiO-66. Heating leads to fast cluster aggregation, and further growth and ordering results in the crystalline product. Thus, SBUs are present already at room temperature and act as building blocks for MOF formation. The proposed formation steps provide insight for further development of MOF synthesis.

Eu2+ and Eu3+ Doubly Doped ZnWO₄ Nanoplates with Superior Photocatalytic Performance for Dye Degradation

Eu²⁺ and Eu³⁺ doubly doped ZnWO₄ nanoplates with highly exposed {100} facets were synthesized via a facile hydrothermal route in the presence of surfactant cetyltrimethyl ammonium bromide. These ZnWO₄ nanoplates were characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectrometry, diffuse UV-vis reflectance spectroscopy, photoluminescence spectrophotometry, and photoluminescence lifetime spectroscopy to determine their morphological, structural, chemical, and optical characteristics. It is found that Eu-doped ZnWO₄ nanoplates exhibit superior photo-oxidative capability to completely mineralize the methyl orange into CO₂ and H₂O, whereas undoped ZnWO₄ nanoparticles can only cleave the organic molecules into fragments. The superior photocatalytic performance of Eu-doped ZnWO₄ nanoplates can be attributed to the cooperative effects of crystal facet engineering and defect engineering. This is a valuable report on crystal facet engineering in combination with defect engineering for the development of highly efficient photocatalysts.

Pitfalls and reproducibility ofin situsynchrotron powder X-ray diffraction studies of solvothermal nanoparticle formation

In situpowder X-ray diffraction (PXRD) is a powerful characterization tool owing to its ability to provide time-resolved information about phase composition, crystal structure and microstructure. The application of high-flux synchrotron X-ray beams and the development of custom-built reactors have facilitated second-scale time-resolved studies of nanocrystallite formation and growth during solvothermal synthesis. The short exposure times required for good time resolution limit the data quality, while the employed high-temperature–high-pressure reactors further complicate data acquisition and treatment. Based on experience gathered during ten years of conductingin situstudies of solvothermal reactions at a number of different synchrotrons, a compilation of useful advice for conductingin situPXRD experiments and data treatment is presented here. In addition, the reproducibility of the employed portablein situPXRD setup, experimental procedure and data analysis is evaluated. This evaluation is based on repeated measurements of an LaB6line-profile standard throughout 5 d of beamtime and on the repetition of ten identicalin situsynchrotron PXRD experiments on the hydrothermal formation of γ-Fe2O3nanocrystallites. The study reveals inconsistencies in the absolute structural and microstructural values extracted by Rietveld refinement and whole powder pattern modelling of thein situPXRD data, but also illustrates the robustness of trends and relative changes in the extracted parameters. From the data, estimates of the effective errors and reproducibility ofin situPXRD studies of solvothermal nanocrystallite formation are provided.

ZnWO4 nanocrystals: synthesis, morphology, photoluminescence and photocatalytic properties

The best photocatalytic properties for monoclinic ZnWO₄ nanocrystals are related to the surface energy and the types of clusters formed on their surface.