Critical size of crystalline ZrO(2) nanoparticles synthesized in near- and supercritical water and supercritical isopropyl alcohol

Stabilizing tetragonal ZrO2 nanocrystallites in solvothermal synthesis

Phase-pure tetragonal ZrO2 nanoparticles have been prepared under simple solvothermal synthesis conditions using different types of alcohols as solvents and studied using in situ X-ray scattering. The variation of tetragonal/monoclinic phase ratios within the produced powders was directly correlated with the amount of in situ generated water from solvent dehydration during the syntheses. By controlling the dehydration kinetics, either choosing primary alcohols of varying thermal stability or by changing synthesis temperatures, it is possible to selectively tune this tetragonal/monoclinic phase ratio.

CFD-PBM Simulation for Continuous Hydrothermal Flow Synthesis of Zirconia Nanoparticles in a Confined Impinging Jet Reactor

Computational fluid dynamics (CFD) and population balance models (PBM) were coupled together for the first time to simulate the synthesis of zirconia nanoparticles in a continuous hydrothermal flow synthesis (CHFS) system with a self-designed confined impinging jet mixing (CJM) reactor. The hydrodynamic and thermodynamic behaviors within the CJM reactor strongly influenced the formation of the ZrO₂ nanoparticles. Crucial parameters, such as velocities, temperatures, mixing conditions, and reaction rates, were analyzed under various supercritical conditions. Temperature and velocity measurements as functions of distance were also investigated. Normal particle size distribution (PSD) patterns were observed in all cases. The mean particle sizes in this study were calculated and compared using PBM aggregation analysis.

Why Does Bi2WO6 Visible-Light Photocatalyst Always Form as Nanoplatelets?

Bi₂WO₆ nanocrystals exhibit excellent photocatalytic properties in the visible range of the solar spectrum, and intense efforts are directed at designing effective synthesis processes with control of size, morphology, and hierarchical structure. All known hydrothermal syntheses produce either nanoplatelet morphology or hierarchical structures based on such primary entities. Here we investigate the nucleation and growth of Bi₂WO₆ nanocrystals under hydrothermal conditions using in situ X-ray total scattering (TS) and powder X-ray diffraction (PXRD) measurements. It is shown that the preferential growth of Bi₂WO₆ nanoplates is due to the presence of disordered layers of Bi₂O₂²⁺ molecular complexes in the precursor solution with an approximate length of 13 Å. These layers interact with tetrahedral WO₄^2- molecular units and eventually form the disordered cubic (Bi_0.933W_0.067)O_1.6) crystalline phase. When enough tungsten units are intertwined between Bi₂O₂²⁺ layers formation of Bi₂WO₆ pristine nanoplates takes place by necessary sideways addition of units in the ac plane. The experimentally observed formation mechanism suggests that the Bi/W atomic ratio must play a central role in the nucleation (assembly of initial crystal layers). Indeed, it is observed in separate continuous flow supercritical synthesis that for a stoichiometric (Bi/W = 2:1) precursor, a (Bi_0.933W_0.067)O_1.6) impurity phase is always observed together with the main Bi₂WO₆ product. Excess tungsten is required in the precursor to form phase-pure Bi₂WO₆ material. Thus, the present study also reports a fast, scalable, and green method for production of this highly attractive photocatalyst.

Detrimental Effect and Neutralization of in Situ Produced Water on Zirconia Nanoparticles Obtained by a Nonaqueous Sol-Gel Method

In this work, the phase purity and size of zirconia nanocrystals samples were studied in terms of zirconium concentration, added water content, and subsequent use of a post solvothermal treatment. The progressive tetragonal-to-monoclinic transformation of zirconia sample was observed to be strongly related to the water content of the alcoholic medium. But more surprisingly, it has been shown that even under initially anhydrous conditions and for particle size below 5 nm, the phase purity of the samples was deteriorated by a side-reaction of alcohol dehydration catalyzed by the surface of the nanoparticles (NPs). Since the phase transformation is essentially driven by the water content of the reaction mixture, we have shown that it was possible to recover an excellent phase purity without the help of any usual dopants by adding a strong alkaline desiccating agent. Provided that a sufficient sodium to zirconium ratio was ensured, the formation of the monoclinic phase was not observed whatever the zirconium precursor concentration. The effectiveness of this cure was related to the ability of sodium metal to generate reactive alkoxide able to neutralize water and to catalyze an alternative sol-gel mechanism leading to the formation of the t-ZrO₂ NPs.

Three-dimensional morphology of anatase nanocrystals obtained from supercritical flow synthesis with industrial grade TiOSO4 precursor

Anatase TiO2 (a-TiO2) nanocrystals are vital in catalytic applications both as catalysts (e.g. photodegradation) and as a carrier material (e.g. NOx removal from exhaust). The synthesis of a-TiO2 nanocrystals and their properties have been heavily scrutinized, but there exists a clear gap between the scientific literature, and the scale and price expectation of industrial application. Here it is demonstrated that the industrially most attractive Ti precursor, titanyl sulfate (TiOSO4), can be combined with the green, scalable and fast supercritical flow method to produce phase pure and highly crystalline a-TiO2 nanoparticles with high specific surface area. Control of the nanocrystal morphology is important since it is known that certain facets substantially promote catalytic activity. It is, however, in itself challenging to determine nanocrystal morphology to provide a rational basis for the synthesis control. Here we advocate the use of advanced Rietveld refinement of powder X-ray diffraction data including anisotropic size broadening models in aiding to establish the sample three-dimensional morphology. This relatively quick and robust method assists in overcoming the often encountered ambiguity inherent in two-dimensional to three-dimensional reconstruction of selected particle morphologies with transmission electron microscopy and tomography techniques.

Direct Observation of Enhanced Raman Scattering on Nano-Sized ZrO2 Substrate: Charge-Transfer Contribution

Direct observation of the surface-enhanced Raman scattering (SERS) of molecules adsorbed on nano-sized zirconia (ZrO₂) substrates was first reported without the need for the addition of metal particles. It was found that ZrO₂ nanoparticles can exhibit unprecedented Raman signal enhancements on the order of 10³ for the probe molecule 4-mercaptobenzoic acid (4-MBA). The dramatic effect of the calcination temperature on the ZrO₂ nanoparticles was also investigated. The ZrO₂ nanoparticles with the particle diameter of 10.5 nm, which were prepared by calcination at a temperature of 500°C, have the highest SERS activity. A comparison between the experimental and calculation results indicates that charge transfer (CT) effects dominate the surface enhancement. The plentiful surface state of ZrO₂ active substrate that is beneficial to CT resonance occurs between molecules and ZrO₂ to produce a SERS effect. The CT process depends, to a large extent, on the intrinsic properties of the modifying molecules and the surface properties of the ZrO₂. This is a new SERS phenomenon for ZrO₂ that will expand the application of ZrO₂ to microanalysis and is beneficial for studying the basic properties of both ZrO₂ and SERS.

Continuous Hydrothermal Synthesis of Inorganic Nanoparticles: Applications and Future Directions

Nanomaterials are at the leading edge of the emerging field of nanotechnology. Their unique and tunable size-dependent properties (in the range 1-100 nm) make these materials indispensable in many modern technological applications. In this Review, we summarize the state-of-art in the manufacture and applications of inorganic nanoparticles made using continuous hydrothermal flow synthesis (CHFS) processes. First, we introduce ideal requirements of any flow process for nanoceramics production, outline different approaches to CHFS, and introduce the pertinent properties of supercritical water and issues around mixing in flow, to generate nanoparticles. This Review then gives comprehensive coverage of the current application space for CHFS-made nanomaterials including optical, healthcare, electronics (including sensors, information, and communication technologies), catalysis, devices (including energy harvesting/conversion/fuels), and energy storage applications. Thereafter, topics of precursor chemistry and products, as well as materials or structures, are discussed (surface-functionalized hybrids, nanocomposites, nanograined coatings and monoliths, and metal-organic frameworks). Later, this Review focuses on some of the key apparatus innovations in the field, such as in situ flow/rapid heating systems (to investigate kinetics and mechanisms), approaches to high throughput flow syntheses (for nanomaterials discovery), as well as recent developments in scale-up of hydrothermal flow processes. Finally, this Review covers environmental considerations, future directions and capabilities, along with the conclusions and outlook.

The chemistry of ZnWO4 nanoparticle formation

The need for a new approach to describing nanoparticle nucleation and growth different from the classical models is highlighted. In and ex situ total scattering experiments combined with additional characterization techniques are used to unravel the chemistry dictating ZnWO4 formation.

The need for a change away from classical nucleation and growth models for the description of nanoparticle formation is highlighted. By the use of in situ total X-ray scattering experiments the transformation of an aqueous polyoxometalate precursor mixture to crystalline ZnWO₄ nanoparticles under hydrothermal conditions was followed. The precursor solution is shown to consist of specific Tourné-type sandwich complexes. The formation of pristine ZnWO₄ within seconds is understood on the basis of local restructuring and three-dimensional reordering preceding the emergence of long range order in ZnWO₄ nanoparticles. An observed temperature dependent trend in defect concentration can be rationalized based on the proposed formation mechanism. Following nucleation the individual crystallites were found to grow into prolate morphology with elongation along the unit cell c-direction. Extensive electron microscopy characterization provided evidence for particle growth by oriented attachment; a notion supported by sudden particle size increases observed in the in situ total scattering experiments. A simple continuous hydrothermal flow method was devised to synthesize highly crystalline monoclinic zinc tungstate (ZnWO₄) nanoparticles in large scale in less than one minute. The present results highlight the profound influence of structural similarities in local structure between reactants and final materials in determining the specific nucleation of nanostructures and thus explains the potential success of a given synthesis procedure in producing nanocrystals. It demonstrates the need for abolishing outdated nucleation models, which ignore subtle yet highly important system dependent differences in the chemistry of the forming nanocrystals.

In situ studies of solvothermal synthesis of energy materials

Solvothermal and hydrothermal synthesis, that is, synthesis taking place in a solvent at elevated temperature and pressure, is a powerful technique for the production of advanced energy materials as it is versatile, cheap, and environmentally friendly. However, the fundamental reaction mechanisms dictating particle formation and growth under solvothermal conditions are not well understood. In order to produce tailor-made materials with specific properties for advanced energy technologies, it is essential to obtain an improved understanding of these processes and, in this context, in situ studies are an important tool as they provide real time information on the reactions taking place. Here, we present a review of the use of powder diffraction and total scattering methods for in situ studies of synthesis taking place under solvothermal and hydrothermal conditions. The experimental setups used for in situ X-ray and neutron studies are presented, and methods of data analysis are described. Special attention is given to the methods used to extract structural information from the data, for example, Rietveld refinement, whole powder pattern modelling and pair distribution function analysis. Examples of in situ studies are presented to illustrate the types of chemical insight that can be obtained.

Pulsed supercritical synthesis of anatase TiO₂ nanoparticles in a water-isopropanol mixture studied by in situ powder X-ray diffraction

A new step in supercritical nanoparticle synthesis, the pulsed supercritical synthesis reactor, is investigated in situ using synchrotron powder X-ray diffraction (PXRD) to understand the formation of nanoparticles in real time. This eliminates the common problem of transferring information gained during in situ studies to subsequent laboratory reactor conditions. As a proof of principle, anatase titania nanoparticles were synthesized in a 50/50 mixture of water and isopropanol near and above the critical point of water (P = 250 bar, T = 300, 350, 400, 450, 500 and 550 °C). The evolution of the reaction product was followed by sequentially recording PXRD patterns with a time resolution of less than two seconds. The crystallite size of titania is found to depend on both temperature and residence time, and increasing either parameter leads to larger crystallites. A simple adjustment of either temperature or residence time provides a direct method for gram scale production of anatase nanoparticles of average crystallite sizes between 7 and 35 nm, thus giving the option of synthesizing tailor-made nanoparticles. Modeling of the in situ growth curves using an Avrami growth model gave an activation energy of 66(19) kJ mol(-1) for the initial crystallization. The in situ PXRD data also provide direct information about the size dependent macrostrain in the nanoparticles and with decreasing crystallite size the unit cell contracts, especially along the c-direction. This agrees well with previous ex situ results obtained for hydrothermal synthesis of titania nanoparticles.

Ultrafast and continuous synthesis of crystalline ferrite nanoparticles in supercritical ethanol

Magnetic nanoparticles (NPs) are of increasing interest in various industrially relevant products. For these, the development of greener and faster approaches facilitating scaling-up production is of paramount importance. Here, we report a novel, green and potentially scalable approach for the continuous and ultrafast (90 s) synthesis of superparamagnetic ferrite NPs (MnFe(2)O(4), Fe(3)O(4)) in supercritical ethanol (scEtOH) at a fairly moderate temperature (260 °C). ScEtOH exhibits numerous advantages such as its production from bio-resources, its lack of toxicity and its relatively low supercritical coordinates (p(c) = 6.39 MPa and T(c) = 243 °C), being therefore appropriate for the development of sustainable technologies. The present study is completed by the investigation of both in situ and ex situ NP surface functionalization. The as-obtained nanoparticles present good crystallinity, sizes below 8 nm, superparamagnetic behavior at room temperature and high saturation magnetization. Moreover, depending on the capping strategy, the ferrite NPs present extended (for in situ coated NPs) or short-term (for ex situ coated NPs) colloidal stability.

Simple method for manufacturing Pt counter electrodes on conductive plastic substrates for dye-sensitized solar cells

A novel, facile, and low-cost method was developed for manufacturing Pt counter electrodes (CEs) of dye-sensitized solar cells (DSCs) on the indium tin oxide-coated polyethylene terephthalate (ITO-PET). This press-transferring method reconciled the temperature conflict between the sintering process of thermal decomposition of H(2)PtCl(6) and plastic substrates. Cyclic voltammograms, electrochemical impedance spectroscopy, transmittance spectra and photovoltaic performance were characterized to investigate the transferred Pt CEs. It was found that the transferred Pt CEs on ITO-PET exhibited an excellent catalytic activity comparable with traditional electrodes on FTO glasses. On the front-side, an illuminated conversion efficiency of 7.21% was reached with more than 94% efficiency of conventional thermally deposited Pt CEs on FTO glasses, and on the back-side, the illuminated conversion efficiency was 4.86%, which was higher than that for conventional electrodes.

High-temperature and high-pressure pulsed synthesis apparatus for supercritical production of nanoparticles

In materials science continuous flow supercritical fluid reactors are widely used for highly controlled synthesis of nanoparticles. The major limitation of continuous flow reactors is that the inherent distribution of residence times leads to broadening of the corresponding size distribution of the nanoparticles, and in addition it is not possible to carry out synthesis with very short or very long reaction times. Here, we report a new synthesis concept that we call pulsed synthesis, which removes the limitations of flow synthesis at the expense of a more complex reactor design and extensive computer control. Another limitation of flow synthesis is that it is largely a black box, where limited direct information is available of the specific chemical reactions taking place, the particle nucleation, the particle growth, etc. Such information is commonly obtained from in situ synchrotron and neutron scattering studies, but transfer of information from in situ studies with static reactors to laboratory flow reactor conditions is highly non-trivial. The new pulse reactor provides superior heating rates, arbitrary residence times with narrow distribution limited only by the pulse duration, and the ability of using the same reactor both for nanoparticle production and in situ synchrotron studies; thus eliminating the need for transfer of in situ information to laboratory reactor designs.

Hydrothermal Synthesis of Metal Oxide Nanoparticles in Supercritical Water

This paper summarizes specific features of supercritical hydrothermal synthesis of metal oxide particles. Supercritical water allows control of the crystal phase, morphology, and particle size since the solvent's properties, such as density of water, can be varied with temperature and pressure, both of which can affect the supersaturation and nucleation. In this review, we describe the advantages of fine particle formation using supercritical water and describe which future tasks need to be solved.