Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO₂-Ag Nanowires Made by Templated Electrodeposition

The overall goal of this procedure is to explain the steps that are taken to make segmented nanowires by an easy and reproducible method called templated electro deposition, and to show an easy method for the detection of hydrogen gas in photo catalytic water splitting experiments. This is accomplished by first preparing the membrane that serves as a template for the deposition of nano wires with a specific shape and size. This preparation includes sputtering of a gold layer for electrical contact and a glass slide for isolation.

The second step is to electrode deposit the desired materials inside the template pores, and by performing multiple depositions steps inside the same template, segmented nanowires can be made. Next, the nanowires are released from the template and transferred to the solution used for photo catalytic experiments. The final step is to prepare the setup used for photo catalytic hydrogen evolution.

Ultimately, a hydrogen gas sensor in combination with UV irradiation is used to show that these nanowires are photo cataly active and can be used for autonomous hydrogen formation. One of the main advantages of making nanowires by templated electro deposition is that many different compositions can be made. The process is carried out in Acquia solutions under mouth conditions, and does not require Expensive equipment.

In this video, we show how to make photo catalytic nano wires for hydrogen formation, but the same method can also be applied to make nano wires for photovoltaic, thermoelectrics fuel cells, and many other applications. Begin this procedure by choosing a polycarbonate track etch or PCTE membrane with an outer pour diameter of 200 nanometers and a thickness of six microns. The diameter of the membrane used here is 25 millimeters.

A gold layer is then sputtered at the backside of the membrane using a commercially available sputtering system. In this case, a deposition pressure of two times 10 to the minus second millibar was used with argon as sputtering gas and a slow deposition rate of about 13 nanometers per minute. This gold layer will be used as electrical contact during electro deposition.

After sputtering the gold layer on the PCTE membrane, the next step is to attach a small glass slide on top of the gold coated side of the membrane using four small strips of double-sided tape along the edges of the glass slide. This glass slide is used to ensure selective electro deposition inside the membrane. Pours for mechanical stability.

Stick a small piece of copper tape on the part of the membrane that sticks out from the glass slide. Since copper tape is conducting, the crocodile clip of the working electrode can be attached to the copper tape. Improve the adhesion of the membrane to the glass slide by putting Teflon tape around the edges.

This is recommended for depositions at elevated temperatures. Prior to making the silver segment, prepare an aqueous solution containing silver nitrate and boric acid and adjust the pH to 1.5. Using nitric acid, put the prepared PCTE membrane together with a platinum counter electrode and a silver, silver chloride reference electrode.

In the prepared solution, apply a potential of positive 0.1 volts versus the silver, silver chloride reference electrode. For 30 seconds following the directions from the potential stat manufacturer, remove the electrodes from the solution and rinse them with Milli Q water. To make the zinc oxide segment first, prepare an aqueous solution containing 0.1 molar zinc nitrate hexahydrate.

Next, heat the solution to 60 degrees Celsius using a water bath and put the membrane containing the silver segment. Together with a platinum counter electrode and a silver, silver chloride reference electrode in the heated solution, apply a potential of negative one volt versus the silver, silver chloride reference electrode for 20 minutes. Following the potential stat manufacturer's directions, It is important to check the IT curve during electrode deposition since an illogical or zero current indicates a bad contact.

After 20 minutes, remove the electrodes from the solution and rinse them with milli Q water. This entire procedure for electro deposition of silver and zinc oxide nanowire segments must be repeated four times to obtain enough nanowires for significant signal from the hydrogen sensor to extract the silver zinc oxide. Nano wires cut the membrane containing the nano wires from the glass slide and transfer part of the membrane to a polypropylene centrifuge tube.

Add about two milliliters of chloro methane to dissolve the PCTE membrane and release the nanowires into the solution. After about 30 minutes, the membrane should be completely dissolved. Apply a small droplet of the DI chloro methane solution containing nanowires on a small silicon wafer for scanning electron microscopy analysis.

Centrifuge the obtained solution at about 19, 000 times G for five minutes, remove the excess di chloro methane and add fresh di chloro methane. Repeat the process at least three times to make sure all polycarbonate has been removed. After the final wash with di chloro methane and removal of the excess chloro methane, add UE water to the nano wires centrifuge, discard the water and add fresh milli Q water.

Repeat this wash at least three times again to completely replace all di chloro methane with milli Q water. The hydrogen sensor used in the hydrogen formation experiments is prepared from a palladium based hydrogen sensor. The sensor is inside an NS plug that fits on top of a quartz tube.

Connect the sensor to a standard Wheatstone bridge circuit. This schematic illustrates a typical setup for the detection of hydrogen gas, evolved from photo catalytic nano wires. To begin photo catalytic hydrogen formation, put the aqueous nanowire solution in a 72 milliliter quartz tube.

Add more water until a total of 10 milliliters of water is inside the quartz tube. Then add 40 milliliters of methanol. Start recording the signal from the palladium based hydrogen sensor before placing it on top of the quartz tube and monitor the variation in signal.

After about 200 seconds of stable signal, put the hydrogen sensor on top of the quartz tube while simultaneously turning on the UV light source to start the actual measurement. Here you can see hydrogen gas bubbles evolving from the nano wires when dispersed in a methanol water solution During deposition, the current that is measured between the working and counter electrodes can be visualized in an IT curve. Since the current is directly related to the amount of deposited material via Faraday's law.

The observed current is an important indication of how the deposition proceeds a typical IT curve. For the deposition of silver zinc oxide nanowires is shown in the left panel. The deposition of titanium dioxide silver nanowires was not demonstrated in this video, but a typical IT curve is shown in the right panel.

Typical axially segmented silver zinc oxide nanowires are shown in this scanning electron microscopy image. This next set of scanning electron microscopy images show empty titanium dioxide nanotubes, a coaxial titanium dioxide, silver nanowire, and titanium dioxide nanotubes with silver nanoparticles. These graphs show the signal as detected by the sensor and the same signal after transformation to the timeframe of actual hydrogen gas formation.

The red lines represent the response from the sensor during UV irradiation of silver zinc oxide. Nano wires in a methanol water solution and the blue lines represent a reference experiment without nano wires. When the UV light source was turned on at 17.5 minutes, the signal drops substantially due to the light sensitivity of the sensor.

Right after this drop in signal, the reaction starts and consequently, this moment was defined as T equals zero minute in panel B, and the corresponding signal was defined as zero volts. Since the sensor used is slightly cross sensitive to methanol, the measurement of a reference sample without nanowires was also included as shown in these results during UV illumination, the signal from the sample with nanowires was higher than the signal from the reference sample hydrogen gas formation using silver's zinc oxide nanowires typically ceased after about 48 hours of UV illumination as evidenced by termination of gas bubble formation. The reason for this loss of activity is photo corrosion of zinc oxide.

This scanning electron microscopy image shows a photo corroded silver zinc oxide nanowire. After 48 hours of UV illumination, the surface of the zinc oxide segment is much rougher than that of a newly synthesized nanowire. In the literature, several methods are reported for inhibition of the photo corrosion process of zinc oxide.

Alternatively, please refer to the accompanying manuscript for a synthesis method of coaxial titanium dioxide, silver nanowires, which can be used for autonomous splitting without any sign of photo corrosion. Additional functionalities like autonomous nanowire movement or external magnetic steering can be realized by incorporating additional segments like platinum, gold or nickel. In this way, multifunctional nanowires can be made.

After watching this video, you should have a good understanding of how to make nanowires including a simple way for isolation of the membrane. You should also be able to test their photo catalytic activity using a simple hydrogen sensor.