A Facile Synthetic Method to Obtain Bismuth Oxyiodide Microspheres Highly Functional for the Photocatalytic Processes of Water Depuration

This protocol provides a reliable method to synthesize bismuth oxyiodide microspheres which are photocatalytically active under visible light irradiation. Here we present the key parameters that result in a successful synthesis of this kind of 3D structure via the solvothermal method. One of the main advantages of this method is that by varying key parameters such as temperature and time of heating, it is possible to obtain different structures.

Microsphere leaf and flower-like structures have been obtained for instance. Through this method, it is possible to obtain photocatalytically active materials which have demonstrated to efficiently remove organic pollutants, bacteria and even heavy materials in polluted waters. To a higher scale, it is possible to use photocatalysis to clean water for human consumption.

This method can provide some insights on the synthesis of other bismuth-based materials which have been reported as efficient in other catalysis reactions such as the oxidation of carbon monoxide as well as in artificial photosynthesis. To start preparing solution one, dissolve 2.9 grams of bismuth nitrate pentahydrate in 60 milliliters of ethylene glycol in a glass beaker. For solution two, dissolve one gram of potassium iodide in 60 milliliters of ethylene glycol in a glass beaker.

Use a micropipette to add solution two to solution one drop by drop at a flow rate of one milliliter per minute to create a yellowish suspension. Abrupt addition of solution two will create a black color due to formation of bismuth tetraiodide anion. In such case, the synthesis must be aborted and started again.

While making the precursor, it is necessary to wait until the disappearance of the yellow color from around the dripping solution before adding the next step of the potassium iodide to the bismuth solution. After stirring the mixture for 30 minutes at room temperature, transfer the mixture to a 150 milliliter autoclave reactor. Use ethylene glycol to rinse the beaker, swirl the beaker to remove the remaining suspension from the sidewalls, and tightly close the reactor.

Place the autoclave reactor in a furnace. Set the temperature to 126 degrees Celsius at a temperature ramp of two degrees per minute and keep the reactor in the furnace for 18 hours. After that, take the autoclave reactor out of the furnace to cool down.

Do not open hot reactor to avoid release of iodine gas. Adhere a 0.8 micrometer filter paper to the walls of a glass funnel. Pour the suspension from the reactor into the funnel and use ethylene glycol to rinse the reactor.

Use deionized water and absolute ethanol to wash the solid retained on the filter for removing inorganic ions and ethylene glycol respectively. Alternate the washing solvent until the leachate is colorless. Use deionized water as the last washing step to remove any trace of ethanol.

Place the product in the oven and set at 80 degrees Celsius for 24 hours. After that, separate the powder material from the filter to homogenize in an agate mortar. Then transfer the material into amber glass bottles in a desiccator.

Put 30 milligrams of the samples into the sample port of the praying mantis accessory of the spectrophotometer. With a light source of 200 to 800 nanometers, irradiate the powder samples and continue as described in the manuscript. Band gap value obtained via this characterization would be around 1.8 electron volts.

To make the 30 parts per million test solution, dissolve 7.5 milligrams of ciprofloxacin in 250 milliliters of distilled water. Then transfer the test solution to the photocatalytic reactor and thoroughly stir the solution at 25 degrees Celsius on a magnetic stir. From a pipeline, bubble dry air from a tank to the solution at 100 milliliters per minute to maintain air saturation.

Position a 70 watt lamp at a five centimeters distance above the photoreactor. Add 62.5 milligrams of the bismuth oxyiodide microspheres to the test solution to achieve a load of 0.25 grams per liter. Immediately, use a glass syringe to take eight milliliters of the sample.

Use a glass syringe to take the second eight milliliter sample for measuring the absorption of the organic molecule on the bismuth oxyiodide surface and then turn on the light. Take 12 samples after irradiation at the desired irradiation periods. Filter all the withdrawn samples by passing them through a nylon membrane.

Store the filtered samples in glass vials at four degrees Celsius. Load the glass vials into the TOC device. This equipment analyzes the concentration of total and inorganic carbon remaining in the liquid samples through an infrared detector.

3D microstructures of bismuth oxyiodide were successfully synthesized by the proposed synthetic method. SEM images show perfectly shaped spherical structures obtained by the solvothermal treatment at 126 degrees Celsius for 18 hours. Amorphous structures were observed when the solvothermal treatment was performed at 130 degrees Celsius for only 12 hours.

Mesoporous microspheres of bismuth oxyiodide were achieved when a treatment was performed at 160 degrees for 18 hours. X-ray diffraction patterns of the bismuth oxyiodide microspheres obtained with 18-hour thermal treatment at 126 degrees Celsius and 160 degrees Celsius as well as a 0D bismuth oxyiodide material were compared. The decay in the peak intensity along with the broadening of the diffraction patterns showed the loss of orientation of the crystals when microspheres were obtained.

The photocatalytic activity of the microspheres was assessed through the degradation of ciprofloxacin in pure water under UVA visible light irradiation. The bismuth oxyiodide washed with both ethanol and water showed higher mineralization rate than bismuth oxyiodide washed only with water. Photolysis was unable to completely oxidize the organic molecule.

Please remember bismuth oxyiodide precursors are very unique. Thus, the addition of the iodide to the bismuth solution must be slow giving us a result the formation of the microsphere structures. Microspheres obtained by this method can be used in other environmental photocatalysis lines.

It is necessary to test their capacity introduction reactions to produce hydrogen and also to reduce heavy metals in water. The development of this procedure paved the way to synthesize other 3D bismuth oxyiodide such as bismuth oxychloride or bismuth oxybromide which are known to potentially produce highly oxidative species.