Aerosol-spray diverse mesoporous metal oxides from metal nitrates

Transition metal oxides are widely used in solar cells, batteries, transistors, memories, transparent conductive electrodes, photocatalysts, gas sensors, supercapacitors, and smart windows. In many of these applications, large surface areas and pore volumes can enhance molecular adsorption, facilitate ion transfer, and increase interfacial areas; the formation of complex oxides (mixed, doped, multimetallic oxides and oxide-based hybrids) can alter electronic band structures, modify/enhance charge carrier concentrations/separation, and introduce desired functionalities. A general synthetic approach to diverse mesoporous metal oxides is therefore very attractive. Here we describe a powerful aerosol-spray method for synthesizing various mesoporous metal oxides from low-cost nitrate salts. During spray, thermal heating of precursor droplets drives solvent evaporation and induces surfactant-directed formation of mesostructures, nitrate decomposition and oxide cross-linking. Thirteen types of monometallic oxides and four groups of complex ones are successfully produced, with mesoporous iron oxide microspheres demonstrated for photocatalytic oxygen evolution and gas sensing with superior performances.

oxides. (c) NiFe oxides. In the leftmost column are the SEM images. In the second column are the TEM images of the single mesoporous bimetallic microspheres. In the third column are the XRD patterns of the products obtained by calcination under different conditions. The most matching JCPDS patterns are also shown. In the fourth column are the EDX spectra of the mesoporous bimetallic oxides. The molar ratios between the respective metals to Fe determined from the EDX analyses are also given. The SEM and TEM images were taken on the products that were calcined at 400 C for 8 h. with the mesoporous -Fe 2 O 3 microsphere sample, which was exposed to various types of gas molecules. The sensor was exposed to 100 ppm of (1) ethanol, (2) acetone, (3) methanol, (4) toluene, (5) benzene, (6) ammonia, (7) isopropanol, and (8) formaldehyde, respectively.
The working temperature of the sensor was maintained at 280 C. After the detection of one type of gas molecules, the measurement chamber was pumped to remove the gas molecules.

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Dry air was then supplied, followed with the injection of a different type of gas molecules for detection. Figure 15 | Temperature effect on the gas sensing. The real-time sensing response curve was measured when the mesoporous -Fe 2 O 3 microsphere sample was exposed to formaldehyde at different temperatures. The concentration of formaldehyde was kept at 100 ppm. After the detection at one temperature, the measurement chamber was pumped, followed with the supply of dry air. The working temperature of the sensor was then raised to a higher temperature, followed with the injection of formaldehyde for another measurement.   Cr were doped at 10 mol%, while Co doping was varied from 1 mol% to 10 mol%. After complete dissolution, the precursor solution was subjected to the same aerosol spray and calcination processes as described above for mesoporous monometallic oxides.

Synthesis of mesoporous mixed and bimetallic oxides.
The synthesis was similar to that of the mesoporous monometallic oxides except that two types of metal nitrate salts were used. was equipped with a UV-cutoff filter ( > 420 nm). The system was connected to a vacuum pump and a gas chromatograph for online detection of oxygen evolution. A commercial -Fe 2 O 3 product (Aldrich, product number: 544884, nanopowder, particle size: < 50 nm) was also used as the photocatalyst for comparison. All of the catalyst samples were measured under the same conditions.
Gas sensing. The structure of the gas sensing device and the electrical measurement system were identical to those described in our recent work 31,32 . The sensing device was made of a ceramic tube (Hanwei Electronics Co., Ltd., Henan Province, China). The length, outer diameter, and thickness of the tube were 5 mm, 2 mm and 0.2 mm, respectively. Two gold electrodes were printed circularly around the circumference of the tube and at the two ends, with a separation distance of 4 mm between them. For oxide coating on the tube, the mesoporous Fe 2 O 3 sample (0.5 g) was first dispersed in ethanol (5 mL). The resultant solution was then deposited on the outer surface of the tube with a brush, followed by drying in air at 60 C for 2 h and subsequently annealing at 350 C for 2 h. The obtained oxide film was several ten micrometers in thickness. A small Pt coil was inserted into the tube as a resistance heater to control the temperature of the gas sensor. In order to improve the longterm stability, the sensor was kept at the working temperature for 48 h in advance before measurement. The sensing test was carried out in a home-designed chamber of 1 L in volume with an electrical measurement system (ART-2000A, Art Beijing Science and Technology Development Co., Ltd.) that was connected to the two gold electrodes through electrical feedthroughs. The sensing signal was measured as a voltage by use of a sampling resistor. A higher measured voltage corresponded to a larger current, and therefore a smaller resistance, when the oxide was exposed to a molecular vapor. A stationary-state gas distribution method was used for testing the response of the sensing device to a gas at different concentrations in dry air. Specifically, a gas to be detected, such as toluene vapor, was injected into the chamber with a syringe and mixed automatically with air in the chamber. The concentration of the gas vapor was estimated by taking into account the saturated vapor pressure of the gas at a given temperature, the injected vapor volume, and the chamber volume. With this system, the gas measurements down to 1 ppm can be performed, with a good degree of reproducibility. For comparison, the commercial -Fe 2 O 3 product mentioned above was also tested for gas sensing.