From nanoparticle networks to metal-organic frameworks: synthesis, structural engineering and applications

Nanoparticle networks, self-assembled from flame generated hot aerosols consisting of ceramic nanoparticles with well-controlled particle size, are promising materials for many different applications, especially for photodetectors and VOC sensors. Furthermore, the great structural flexibilities of these self-assembled nanoparticle networks including tuneable thickness and hierarchical porosity, precisely-controlled averaged particle size as well as chemical composition make them as potential platforms for templated materials synthesis via chemical conversion. On the other hand, metal-organic framework (MOF), is a growing family of microporous materials consisting of metal cations connected by organic linkers. Their unique properties, including a narrow pore size distribution (intrinsic porosity), designable topology, high accessible surface area, and chemical mutability, make MOFs promising materials for a variety of applications including gas storage, separation, catalysis, biotechnology, optics, microelectronics and energy production/storage. However, there are still several bottlenecks hindering the structural engineering of metal-organic frameworks, especially for pure crystalline MOF materials, including limited attainable thickness, scalability, poor mechanical stability (i.e. brittle nature of MOFs), hard to realize the morphological control (e.g. tuneable extrinsic hierarchical porosity) and geometric designs on pure crystalline MOF components. Thus, a facile synthetic approach for MOF structuring is highly desirable, which could afford the fabrication of three-dimensional MOF materials with possibly unlimited thickness, free-standing feature, the control over extrinsic hierarchy as well as pre-determined designs of MOFs while maintain their crystalline property and intrinsic extreme accessible surfaces. Firstly, we started with the synthesis of pure ZnO nanoparticle networks and the optimization of their particle size. Later, using the ZnO nanoparticle networks with an optimal particle size, a high-performing UV photodetector has been prepared to show a proof of concept application of such structural engineering. After achieving the first structural control over ZnO nanoparticle networks, a multi-dimensional control has been further investigated associated with its potential use for multi-functional devices including transparent conductive oxides and gas sensors. Given the successful structural control over nanoparticle networks, considering the existing bottlenecks in current MOF fabrication, this multi-dimensional structural control has been successfully replicated to MOF preparation via a means of gas phase conversion. Therefore, in this thesis, a systematic study has been presented from the synthesis and applications of nanoparticle networks to those of metal-organic frameworks in the sequence of: (i) the synthesis of three-dimensional nanoparticle networks (i.e. ZnO-based metal oxide nanoparticle networks), (ii) the realization of a precise particle size control over the synthesized nanoparticle networks (e.g. ZnO) and the use of resulted optimal structure for photodetector application, (iii) the realization of chemical composition manipulation over the synthesized nanoparticle networks (e.g. ZnO nanoparticle networks with varied Al doping concentrations) and the use of the resulted structures as proof of concept applications for both porous conductive electrodes and VOC sensor, (iv) the establishment of a synthetic pathway from nanoparticle networks to metal-organic frameworks based on the replication of the structural control over nanoparticle networks towards metal-organic frameworks, and the proof of concept application of the resulted free-standing metal-organic frameworks monolith for effective molecular sieve in batteries, and (v) the use of the established fabrication approach (i.e. from nanoparticle networks to metal-organic frameworks) for monolithic metal-organic framework patterning

One-Step Rapid and Scalable Flame Synthesis of Efficient WO3 Photoanodes for Water Splitting

Photoelectrochemical water splitting is a promising approach for the carbon‐free production of hydrogen using sunlight. Here, robust and efficient WO3 photoanodes for water oxidation were synthesized by the scalable one‐step flame synthesis of nanoparticle aerosols and direct gas‐phase deposition. Nanostructured WO3 films with tunable thickness and band gap and controllable porosity were fabricated by controlling the aerosol deposition time, concentration, and temperature. Optimal WO3 films demonstrate superior water oxidation performance, reaching a current density of 0.91 mA at 1.24 V vs. reversible hydrogen electrode (RHE) and an incident photon‐to‐current conversion efficiency (IPCE) of ca. 61 % at 360 nm in 0.1 m H2SO4. Notably, it is found that the excellent performance of these WO3 nanostructures arises from the high in situ restructuring temperature (ca. 1000 °C), which increases oxygen vacancies and decreases charge recombination at the WO3/electrolyte interface. These findings provide a scalable approach for the fabrication of efficient photoelectrodes based on WO3 and other metal oxides for light‐driven water splitting.A.T. gratefully acknowledges the support of the Australian Research Council (AR; DP150101939, ARC DE160100569), and a Westpac 2016 Research Fellowship

Superior Self-Powered Room-Temperature Chemical Sensing with Light-Activated Inorganic Halides Perovskites

Hybrid halide perovskite is one of the promising light absorber and is intensively investigated for many optoelectronic applications. Here, the first prototype of a self-powered inorganic halides perovskite for chemical gas sensing at room temperature under visible-light irradiation is presented. These devices consist of porous network of CsPbBr3 (CPB) and can generate an open-circuit voltage of 0.87 V under visible-light irradiation, which can be used to detect various concentrations of O2 and parts per million concentrations of medically relevant volatile organic compounds such as acetone and ethanol with very quick response and recovery time. It is observed that O2 gas can passivate the surface trap sites in CPB and the ambipolar charge transport in the perovskite layer results in a distinct sensing mechanism compared with established semiconductors with symmetric electrical response to both oxidizing and reducing gases. The platform of CPB-based gas sensor provides new insights for the emerging area of wearable sensors for personalized and preventive medicine.H.C. and M.Z. contributed equally to this work. A.T. gratefully acknowledges the support of Australian Research Council (ARC) DP150101939, ARC DE160100569, and Westpac 2016 Research Fellowship. M.Z., S.H., and A.W.Y. H.-B. acknowledge the support of the Australian government via financial support from the ARC through the DP160102955 program and the Australian Renewable Energy Agency. K.R.C. acknowledges the support of an ARC Future Fellowship. The financial support from ARC through DP160102955 is also acknowledged

Three-dimensional nano-heterojunction networks: A highly performing structure for fast visible-blind UV photodetectors

Visible-blind ultraviolet photodetectors are a promising emerging technology for the development of wide bandgap optoelectronic devices with greatly reduced power consumption and size requirements. A standing challenge is to improve the slow response time of these nanostructured devices. Here, we present a three-dimensional nanoscale heterojunction architecture for fast-responsive visible-blind UV photodetectors. The device layout consists of p-type NiO clusters densely packed on the surface of an ultraporous network of electron-depleted n-type ZnO nanoparticles. This 3D structure can detect very low UV light densities while operating with a near-zero power consumption of ca. 4 × 10-11 watts and a low bias of 0.2 mV. Most notably, heterojunction formation decreases the device rise and decay times by 26 and 20 times, respectively. These drastic enhancements in photoresponse dynamics are attributed to the stronger surface band bending and improved electron-hole separation of the nanoscale NiO/ZnO interface. These findings demonstrate a superior structural design and a simple, low-cost CMOS-compatible process for the engineering of high-performance wearable photodetector

Fabrication, mechanical properties, and multifunctionalities of particle reinforced foams:A review

In the past decade, particle reinforced foams have been intensively studied and applied in diverse fields owing to their low weight-to-strength ratio, low cost, and tailorable physical properties using various matrix materials and additives. In particular, the thin-walled microstructures in foams and certain particles provide excellent energy absorption capacity compared with the solid materials. A considerable number of research findings on particle reinforced foams have been reported from various aspects, including fabrication techniques, matrices and reinforcement types, mechanical responses as well as other physical properties. Up to date, several review articles have been published to partially cover the stated aspects on hollow particles reinforced foams (i.e., syntactic foams). However, discussion on different types of nano/micro-scale solid particles and millimeter-scale porous particles reinforced foams remains insufficient. Therefore, this article aims to provide a comprehensive review on particle (e.g., solid/porous/hollow particles) reinforced foams (made up of metal/polymer/ceramic matrices) covering fabrication techniques, mechanical responses and their multifunctionalities. Particularly, different reinforcing mechanisms and modifications to physical functions of foams with different matrices using various types of particle additives are reviewed. The opportunities for future explorations of particle reinforced foams in the aspects of manufacturing, modeling and applications are discussed lastly

Low-Voltage High-Performance UV Photodetectors: An Interplay between Grain Boundaries and Debye Length

Accurate detection of UV light by wearable low-power devices has many important applications including environmental monitoring, space to space communication, and defense. Here, we report the structural engineering of ultraporous ZnO nanoparticle networks for fabrication of very low-voltage high-performance UV photodetectors. Record high photo- to dark-current ratio of 3.3×105 and detectivity of 3.2×1012 Jones at ultra-low operation bias of 2 mV and low UV-light intensity of 86 μW⋅cm-2 are achieved by controlling the interplay between grain boundaries and surface depletion depth of ZnO nanoscale semiconductors. An optimal window of structural properties is determined by varying the particle size of ultraporous nanoparticle networks from 10 to 42 nm. We find that small electron-depleted nanoparticles (≤ 40 nm) are necessary to minimize the dark-current, however, the rise in photo-current is tampered with decreasing particle size due to the increasing density of grain boundaries. These findings reveal that nanoparticles with a size close to twice their Debye’s length are required for high phototo dark-current ratio and detectivity, while further decreasing their size decreases the photodetector performanceA.T. gratefully acknowledges the support of Australian Research Council DP150101939, Australian Research Council DE160100569, and Westpac2016 Research Fellowshi

Self-assembly of Au Nano-islands with Tuneable Organized Disorder for Highly Sensitive SERS

Aggregates of disordered metallic nano-clusters exhibiting long-range organized fractal properties are amongst the most efficient scattering enhancers, and they are promising as high performance surface-enhanced Raman scattering (SERS) substrates. However, the low reproducibility of the disordered structures hinders the engineering and optimization of well-defined scalable architectures for SERS. Here, a thermophoretically driven Au aerosol deposition process is used for the self-assembly of thin films consisting of plasmonic nano-islands (NIs) with a controllable and highly reproducible degree of disorder. The intrinsic Brownian motion of the aerosol deposition process results in long-range periodicity with self-similar properties and stochastically distributed hot-spots, providing a facile means for the reliable fabrication of crystalline Au substrates with uniform disorder over large-surfaces. These morphological features result in the generation of a high density of hot-spots, benefitting their application as SERS substrates. NI substrates with an optimal uniform disorder demonstrate a SERS enhancement factor (EF) of 107–108 with nanomolar concentrations of Rhodamin-6G. These findings provide new insights into the investigation of light scattering with disordered structures, paving the way toward low-cost scalable self-assembly optoelectronic materials with applications ranging from ultrasensitive spectroscopy to random lasing and photonic devices

Structural engineering of nano-grain boundaries for low-voltage UV-photodetectors with gigantic photo- to dark-current ratios

Ultraporous networks of ZnO nanoparticles (UNN) have recently been proposed as a highly performing morphology for portable ultraviolet light photodetectors. Here, it is shown that structural engineering of the nanoparticle grain boundaries can drastically enhance the performance of UNN photodetectors leading to gigantic photo to dark current ratios with operation voltages below 1 V. Ultraporous nanoparticle layers are fabricated by scalable low-temperature deposition of flame-made ZnO aerosols resulting in highly transparent layers with more than 95% visible light transmittance and 80% UV-light absorption. Optimal thermally induced necking of the ZnO nanoparticles increased the photo- to dark-current ratio, at a low light density of 86 μW cm−2, from 1.4 × 104 to 9.3 × 106, the highest so far reported. This is attributed to the optimal interplay of surface depletion and carrier conduction resulting in the formation of an open-neck grain boundary morphology. These findings provide a robust set of guiding principles for the design and fabrication of nanoparticle-based optoelectronic devices.A.T. gratefully acknowledges the support of Australian Research Council DP150101939, Australian Research Council DE160100569, and Westpac 2016 Research Fellowship

Optoelectronics properties of tungsten oxide nanoparticle networks deposited by flame spray pyrolysis

In this work, we present the optoelectronic characterization of pure tungsten oxide nanoparticle networks synthesized and self-assembled by flame spray pyrolysis. Current-voltage measurements performed in dark conditions indicate the presence of trapping and de-trapping phenomena from defects inside the energy gap. The presence of defects is confirmed by the time evolution of the photocurrent, measured under monochromatic radiation at 420 nm. After illuminating the WO3 films with light, the current increases exponentially with a time constant independent on the applied voltage. This behavior is ascribed to the presence of defects lying at 1.1 eV below the conduction band of WO3 (bandgap of ca. 2.9 eV). When the illumination is terminated, the photocurrent shows an exponential decrease, once again independently on the applied voltage. The defect level corresponding to this relaxation process corresponds to 0.92eV below the conduction band

High‐Temperature One‐Step Synthesis of Efficient Nanostructured Bismuth Vanadate Photoanodes for Water Oxidation

Authors acknowledge the financial supports of the Australian Research Council (ARC) DP150101939, ARC DE160100569, Westpac 2016 Research Fellowship, the ActewAGL Endowment Fund, and the Research School of Engineering of the ANU. Authors also acknowledge the Centre for Advanced Microscopy (CAM) with funding through the Australian Microscopy and Microanalysis Research Facility (AMMRF), and the Australian National Fabrication Facility (ANFF), ACT Node. A.N.S. acknowledges funding by the ARC through the Centre of Excellence for Electromaterials Science (CE140100012)