Nanoparticles are a class of materials with properties distinctively different from their bulk and molecular counterparts. A critical review of the very broad topic of environmental nanoparticles is presented. Because of the vast nature of the topic, the review is focused primarily on gas-borne nanoparticles. The "life history of nanoparticles" is presented, tracking it from its formation to its potential use and eventual fate in the environment. Nanoparticle sources, anthropogenic emissions from industrial and occupational settings, and conversion and formation in the atmosphere are discussed. The ability to characterize and capture these nanoparticles (as would be necessary in a nanoparticle production system), as well as their control (of emissions from an industrial source) is discussed. A description on the use of nanoparticles in environmental technologies and the potential impact on the energy sector is provided. The potential effects on human health and the environment, both adverse and beneficial, are important aspects that need to be considered. As will be evident, the study of "environmental nanoparticles" is a new and fast-growing field. Much work remains to be done before we can fully harness the advantages of nanoparticles and ensure that there are no potential adverse consequences. A set of recommendations for additional work in each area is provided.

https://www.tandfonline.com/doi/pdf/10.1080/10473289.2005.10464656

Nanoparticles and the Environment

Recent advances in aerosol and combustion science and engineering now allow scalable flame synthesis of mixed oxides, metal salts and even pure metals in the form of nanoparticles and films with closely controlled characteristics. In this way, high purity materials with novel metastable phases are made that are not accessible by conventional wet-phase and solid state processes. Here, flame processes are classified into vapour-fed and liquid-fed ones depending on the employed state of the metal precursor. Liquid-fed flame processes are distinguished for their flexibility in producing materials of various compositions and morphologies that result in unique product functionalities. Parameters controlling the characteristics of flame-made particles and films are summarized and selected classes of materials are reviewed focusing on catalysts, sensors, biomaterials (orthopaedic, dental or nutritional), electroceramics (fuel cells, batteries) and phosphors exhibiting superior performance over conventionally made ones. Just a few years ago it seemed impossible to make these materials in the gas phase. Finally, health effects of such particles are discussed while future challenges and opportunities for flame-made materials are highlighted.

Flame aerosol synthesis of smart nanostructured materials

Fe 3+ -doped anatase nanosized TiO 2 photocatalysts have been prepared by combining sol–gel method with hydrothermal treatment. The samples were characterized by UV–vis diffuse reflectance spectroscopy, X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET)-specific surface area ( S BET ), transmission electron microscopy (TEM), atomic absorption flame emission spectroscopy (AAS), electron paramagnetic resonance (EPR) spectroscopy and X-ray photoelectron spectroscopy (XPS). From results of UV–vis diffuse reflectance spectroscopy, Fe 3+ -doped TiO 2 extends its absorption to longer than 500 nm, which leads to an obvious photocatalatic activity under visible irradiation. From XRD, EPR, AAS and XPS, it was found that Fe exist in trivalent ionic state substituting Ti 4+ in TiO 2 lattice and its concentration decreases from the surface to the center of doped TiO 2 . The photocatalytic activity of prepared samples was investigated for the photocatalytic degradation of active yellow XRG dye. The photocatalytic activity of TiO 2 doped with appropriate content of Fe 3+ exceeded those of non-doped TiO 2 and P25 both under UV and visible light irradiation.

Fe3+-TiO2 photocatalysts prepared by combining sol-gel method with hydrothermal treatment and their characterization

Cr3+-doped anatase titanium dioxide photocatalysts were prepared by the combination of sol–gel process with hydrothermal treatment. The samples were characterized by UV–vis diffuse reflectance spectroscopy, X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) specific surface area (SBET), transmission electron microscopy (TEM), atomic absorption flame emission spectroscopy (AAS), electron paramagnetic resonance (EPR) spectroscopy and X-ray photoelectron spectroscopy (XPS). It was confirmed that Cr substitutes Ti4+ in TiO2 lattice in trivalent ionic state, and the concentrations of dopants Cr3+ decrease from the exterior to the interior of doped TiO2. The photocatalytic activity of Cr-TiO2 was investigated for the photocatalytic degradation of XRG aqueous solution both under UV and visible light irradiation. Due to the excitation of 3d electron of Cr3+ to the conduction band of TiO2, Cr-TiO2 shows a good ability for absorbing the visible light to degrade XRG. Doping of chromium ions effectively improves the photocatalytic activity under both UV light irradiation and visible light irradiation with an optimal doping concentration of 0.15% and 0.2%, respectively. The special distribution of dopants Cr3+ seems having a good effect on enhancing the photocatalytic activity of Cr-TiO2.

Hydrothermal doping method for preparation of Cr3+-TiO2 photocatalysts with concentration gradient distribution of Cr3+

Role of Synthesis Method and Particle Size of Nanostructured TiO2 on Its Photoactivity

Processing of iron-doped titania powders in flame aerosol reactors

Characterization and sinterability of nanophase titania particles processed in flame reactors

A premixed flame aerosol reactor was used to produce titania particles by oxidation of titanium isopropoxide vapor. The growth, aggregation of particles, and the agglomerate structure were determined as a function of height in the flame using in situ light scattering and transmission electron microscopy (TEM) measurements. A methodology to determine the sintering characteristic time using light scattering data was established. The light scattering data provided the evolution of the fractal dimension which was then related to the normalized surface area change using a computer simulation. The sintering equation was redeveloped in terms of the normalized surface area, thus not having to account for coagulation effects. Experimental results indicate that isolated titania particles were observed at the high temperatures due to fast sintering. An agglomerate was obtained at downstream locations with an associated change in fractal dimension due to sintering.

https://www.tandfonline.com/doi/pdf/10.1080/02786829708965491

Study of the sintering of nanosized titania agglomerates in flames using in situ light scattering measurements

A random particle computer simulation (in three dimensions) was conducted to investigate the structure and aggregation characteristics of small clusters (containing limited numbers of primary particles per cluster). Though an individual cluster of small size does not satisfy the power law, it was found that these aggregated clusters are fractal-like and comply with the fractal power law form in a statistical sense. This statistically averaged fractal dimension decreases as the clusters become smaller. A cluster-restructuring model is further developed to simulate the topological evolution of dendritic structured materials due to sintering at high temperatures. Results indicate that the cluster fractal dimension increases as sintering proceeds for small clusters, in contrast to results for large clusters wherein the topology is retained and the fractal dimension remains constant. A relationship of the fractal dimension change with the normalized surface area of dendrites for different-sized clusters is established.

Computer simulation of the aggregation and sintering restructuring of fractal-like clusters containing limited numbers of primary particles

Aerosol coating processes were developed to deposit titania ceramic films onto steel and silica substrates. In situ light-scattering measurements were used to understand the deposition mechanisms in different system configurations. The conditions that were necessary to obtain uniform, nonporous, and well-adhered titania films on steel substrates were established. The as-coated films had excellent anticorrosion characteristics at room temperature, as established by the standard salt-fog test. Film crystallinity and morphology were examined using X-ray diffractometry and scanning electron microscopy; these analysis methods revealed an oriented, nanocrystalline anatase phase. Film composition was established, as a function of film thickness, using Auger electron spectroscopy and was confirmed to be stoichiometric (Ti:O = 1:2). The optical band gap and optical phonons of the deposited films were probed using spectrophotometry and Raman scattering, respectively; these analysis methods revealed a blue shift of the gap, relative to bulk anatase, and a localization of carriers in the nanometer-sized crystallites.

Guixiang Yang

Papers

Processing of iron-doped titania powders in flame aerosol reactors

Characterization and sinterability of nanophase titania particles processed in flame reactors

Study of the sintering of nanosized titania agglomerates in flames using in situ light scattering measurements

Computer simulation of the aggregation and sintering restructuring of fractal-like clusters containing limited numbers of primary particles

Deposition of Multifunctional Titania Ceramic Films by Aerosol Routes