Continuous search for materials that will be useful in eliminating contamination in the atmosphere and water supplies, as well as other applications is a very current and relevant topic– we need to get a handle on our global pollution right?. To that end, semiconductor photocatalysis and their materials useful for the application are under development as a “green” intiative, as a way to degrade organic pollutants in the atmosphere. While there has been an emphasis on UV-driven catalyst development, the visible-light driven efforts offers additional attention. A recent publication (Crystal Growth & Design 2007) indicates the need and progress made to date, and also uses microwave technology to make materials as visible light driven photocatalysts.

For a little bit of historical context:

Basically, there are two ways to exploit the photocatalysts responsive to visible-light irradiation: (i) modification of TiO2 and (ii) development of new materials. For the former, the doping or ion-implanting methods have been extensively utilized to enhance the photocatalytic activity under visible-light irradiation. Only recently have there been a few successful syntheses of new materials as visible-light photocatalysts. In this report, semiconducting chalcogenides of the formula: ABmCn (A ) Cu, Ag, Zn, Cd, etc.; B ) Al, Ga, In; C ) S, Se, Te) have been extensively studied with ZnIn2S4 the desired compund in this study due to the potential usefulnessl in photocatalysis, charge storage, electrochemical recording, and thermoelectricity.

This isn’t the first report of this specific structure as a number of conventional hydrothermal and solvothermal processes yield nanotubes, nanoribbons, nanowires and microspheres. They go on to point out a common theme about the need for a more controlled and consistent method for making these chalcogenides in a robust process that will provide testable materials in a large enough scale for impact — not just for research sake.

A fast and cost-effective route to synthesize hierarchically nanoporous ZnIn2S4 submicrospheres without using any catalyst, template, or surfactant, taking advantage of microwave irradiation and solvothermal effects. I am quickly learning that if microwaves can be used used in place of templates and surfactants it makes the overall process much more attractive moving forward, and the nice thing about the micorwave is that method development helps the time spent figuring it out.

Microwave method ( ZnIn2S4): Zn(NO3)2 · 6H2O (0.1 mmol), InCl3 (0.2 mmol), and excess thiourea (0.8 mmol) were dissolved in 30 mL of ethylene glycol. The solution was sealed in a Teflon-lined double-walled digestion vessel. After treating the solution at a controllable temperature of 200 °C for 10 min using a multimode microwave system,the vessel is cooled to room temperature. The product was collected by centrifugation, washed with deionized water and absolute ethanol, and dried in a vacuum at 40 °C for 4 h.

The morphology of ZnIn2S4 is shown in the SEM images below. It reveals that the product is composed of a large quantity of nearly monodispersed spheres with an average diameter of about 0.6 μm. Further magnification shows a rough fibrous surface of the spheres. It turns out that these nanoporous microspheres have excellent absorptive properties providing both catalytic and separation capability.

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SEM:Ternary chalcogenide ZnIn2S4 by mw solvothermal process

Depending on the synthesis, concentration and stoichiometry has a profound effect on particle structure and size whether compounds are made conventionally or in the microwave. To this end, they explored the influence of different reaction conditions on the formation of ZnIn2S4. It was found that the initial concentrations of reactants played a crucial role on the size and porosity of ZnIn2S4. If the concentration of Zn(NO3)2 was gradually increased from the typical value of 3.3 mM (sample 1) to 16.7 (sample 2), 33.3 (sample 3), or 66.7 mM (sample 4) and the molar ratios of Zn(NO3)2:InCl3:thiourea were kept at 1:2:8, the average diameter of ZnIn2S4 spheres would appreciably increase (below).

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Increasing concentration under microwave irradiation

These observations clearly indicate that a high initial concentration of reactants leads to large ZnIn2S4 particles rather than hierarchically porous spheres. We also investigated the effect of prolonged microwave irradiation but found virtually no change in sample morphology/porosity after 1 h of irradiation.

It was nice to see once again how easy the synthetic method can help push new structural developments. This group was able to develop a reliable method for making a more consistent structure with the advantages microwave irradiation has on the distribution of the energy and structural assembly in the process. Read through the paper to see the benefit on the porous structure on the photocatalytic properties. Happy Reading!