Environmentally persistent free radicals, EPFRs, exist in significant concentration in atmospheric particulate matter (PM). EPFRs are primarily emitted from combustion and thermal processing of organic materials, in which the organic combustion byproducts interact with transition metal-containing particles to form a free radical-particle pollutant. While the existence of persistent free radicals in combustion has been known for over half-a-century, only recently that their presence in environmental matrices and health effects have started significant research, but still in its infancy. Most of the experimental studies conducted to understand the origin and nature of EPFRs have focused primarily on nanoparticles that are supported on a larger micrometer-sized particle that mimics incidental nanoparticles formed during combustion. Less is known on the extent by which EPFRs may form on engineered nanomaterials (ENMs) during combustion or thermal treatment. In this critical and timely review, we summarize important findings on EPFRs and discuss their potential to form on pristine ENMs as a new research direction. ENMs may form EPFRs that may differ in type and concentration compared to nanoparticles that are supported on larger particles. The lack of basic data and fundamental knowledge about the interaction of combustion byproducts with ENMs under high-temperature and oxidative conditions present an unknown environmental and health burden. Studying the extent of ENMs on catalyzing EPFRs is important to address the hazards of atmospheric PM fully from these emerging environmental contaminants.

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In this work, anatase nanorods (ANR) of TiO 2 with active facet {100} as the major facet were successfully synthesized, and reducing the ANR by NaBH 4 led to the formation of gray colored oxygen deficient TiO 2-x (ReANR). On the surface of ReANR, a thin layer of Al 2 O 3 was deposited using atomic layer deposition (ALD), and the thickness of Al 2 O 3 varied by the number of ALD cycles (1, 2, 5, 10, 50, 100, or 200). The growth rate of Al 2 O 3 was determined to be 0.25 Å per cycle based on high-resolution TEM analysis, and the XRD result showed the amorphous structure of Al 2 O 3. All the synthesized photocatalysts (ANR, ReANR, and Al 2 O 3 coated ReANR) were tested for CO 2 photocatalytic reduction in the presence of water vapor, with CO detected as the major reduction product and CH 4 as the minor product. Compared with ANR, ReANR had more than 50% higher CO production and more than ten times higher CH 4 production due to the oxygen vacancies that possibly enhanced CO 2 adsorption and activation. By applying less than 5 cycles of ALD, the Al 2 O 3 coated ReANR had enhanced overall production of CO and CH 4 than uncoated ReANR, with 2 cycles being the optimum, about 40% higher overall production than ReANR. Whereas, both CO and CH 4 production decreased with increasing number of ALD cycles when more than 5 cycles were applied. Photoluminescence (PL) analysis showed an ultrathin layer of Al 2 O 3 (2 cycles of ALD) coating on the ReANR was able to reduce the charge carrier recombination rate, likely because of the passivation of surface states. On the other hand, a relatively thick layer of Al 2 O 3 may act as an insulation layer to prohibit electron migration to the catalyst surface. This work gives valuable insights on the application of ALD coating on photocatalysts to promote CO 2 photoreduction to fuels.

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toughened alumina.18 Zirconia is also known to exhibit excellent mechanical properties and superior fracture tough-Nanocrystalline MgAl 2 O 4 and ZrO 2 -MgAl 2 O 4 ness because of its unique phase transformations occurring powders were synthesised by combustion and convenover stress or temperature.19 According to Fujita et al., an tional solid state reaction routes. T he synthesised addition of 20 wt-%ZrO 2 to MgAl 2 O 4 followed by hot powders were processed, dry pressed, and sintered for pressing may increase fracture toughness and bend strength 3 h at temperatures ranging f rom 1550 to 1625°C. T he to 6•3 MPa m1/2 and 400 MPa respectively.10-12 In general, sintered pellets were then characterised in terms of ZrO 2 -MgAl 2 O 4 composite material exhibits better thermal phase (XRD), microstructure (SEM), relative density, shock resistance and chemical inertness than pure MgAl 2 O 4 , apparent porosity, water absorption, hardness, three and in the form of a honeycomb structure this composite point bend strength, and f racture toughness. T he XRD material nds application for molten metal ltration of studies revealed that ZrO 2 was present in tetragonal superalloys and cast steels.20 form in the case of combustion synthesised powders The most common method used for preparation of (CSP), whereas in powders obtained by solid state ceramic powders involves the solid state reaction of comreaction (SSP) it was present in the monoclinic ponent oxides at elevated temperatures. This method suVers form. T his study also revealed that the addition of from several limitations and may not produce a material ZrO 2 improved the mechanical properties of sintered with compositional homogeneity.21-28 It is also known that MgAl 2 O 4 samples: 20 wt-%ZrO 2 -MgAl 2 O 4 composites powders with particle size å 200 nm can be sintered to prepared f rom CSPs and conventional SSPs and sinproduce materials with ne grained microstructure and tered at 1625°C for 3 h had f racture toughness of 5•96 desirable properties. In order to produce ZrO 2 -MgAl 2 O 4 and 4•33 MPa m 1 / 2 and three point bend strength of 269 and 98 MPa respectively. Higher sintered density, composites with good compositional homogeneity, they the presence of tetragonal zirconia as a major phase, have also been prepared by methods including coprecipiand the ner microstructure are probably responsible tation,21 vapour phase oxidation,22 spray pyrolysis,23 freeze for the superior mechanical properties exhibited by drying,24 and sol-gel processing.25 Recently, Hwang and sintered CSP materials as compared with the sintered Kim have adopted the reaction sintering method for the SSPs. BCT /577 preparation of 10 wt-% cubic stabilised zirconia/spinel composites with relative density of around 98%, fracture tough-T he authors are in the Ceramic Materials Division, ness of 12 MPa m1/2, and strength of 240 MPa from a International Advanced Research Centre for Powder pressed compact mixture of monoclinic zirconia (baddeley-Metallurgy and New Materials, Balapur PO, ite) and Mg-Al metal...

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