Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO<sub>2</sub>-Ag Nanowires Made by Templated Electrodeposition

A. Wouter Maijenburg; Eddy J.B. Rodijk; Michiel G. Maas; Johan E. ten Elshof

doi:10.3791/51547

JoVE Journal > Engineering

Ingegneria

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

Published: May 02, 2014

doi:

10.3791/51547

A. Wouter Maijenburg¹, Eddy J.B. Rodijk¹, Michiel G. Maas¹, Johan E. ten Elshof¹

¹MESA+ Institute for Nanotechnology,University of Twente

Summary

Procedures are outlined to prepare segmented and coaxial nanowires via templated electrodeposition in nanopores. As examples, segmented nanowires consisting of Ag and ZnO segments, and coaxial nanowires consisting of a TiO₂ shell and a Ag core were made. The nanowires were used in photocatalytic hydrogen formation experiments.

Abstract

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and reduction half reactions, and fast electron (hole) diffusion and charge separation. Nanowires present suitable architectures to meet these requirements. Axially segmented Ag|ZnO and radially segmented (coaxial) TiO₂-Ag nanowires with a diameter of 200 nm and a length of 6-20 µm were made by templated electrodeposition within the pores of polycarbonate track-etched (PCTE) or anodized aluminum oxide (AAO) membranes, respectively. In the photocatalytic experiments, the ZnO and TiO₂ phases acted as photoanodes, and Ag as cathode. No external circuit is needed to connect both electrodes, which is a key advantage over conventional photo-electrochemical cells. For making segmented Ag|ZnO nanowires, the Ag salt electrolyte was replaced after formation of the Ag segment to form a ZnO segment attached to the Ag segment. For making coaxial TiO₂-Ag nanowires, a TiO₂ gel was first formed by the electrochemically induced sol-gel method. Drying and thermal annealing of the as-formed TiO₂ gel resulted in the formation of crystalline TiO₂ nanotubes. A subsequent Ag electrodeposition step inside the TiO₂ nanotubes resulted in formation of coaxial TiO₂-Ag nanowires. Due to the combination of an n-type semiconductor (ZnO or TiO₂) and a metal (Ag) within the same nanowire, a Schottky barrier was created at the interface between the phases. To demonstrate the photocatalytic activity of these nanowires, the Ag|ZnO nanowires were used in a photocatalytic experiment in which H₂ gas was detected upon UV illumination of the nanowires dispersed in a methanol/water mixture. After 17 min of illumination, approximately 0.2 vol% H₂ gas was detected from a suspension of ~0.1 g of Ag|ZnO nanowires in a 50 ml 80 vol% aqueous methanol solution.

Introduction

Owing to their small dimensions and large surface-to-volume ratio, nanowires are very promising one-dimensional objects that can be used in a wide range of biomedical and nanotechnological applications¹. In the literature, many nanowires containing a single component with functional properties have been reported^2-7. But when multiple materials (metals, polymers and metal oxides) are incorporated sequentially within a single nanowire, multifunctional nanowires can be made⁸^,⁹. When several segments are connected inside a single nanowire, functional properties may appear that were not present when only the individual segments were used. For instance, nanomotors containing Au and Pt segments within a single nanowire were reported that moved autonomously when placed in hydrogen peroxide⁴. Suitable techniques for the formation of multisegmented nanowires are infiltration and templated electrodeposition⁸^,⁹.

In 1987, Penner and Martin were the first to publish the use of templated electrodeposition for the formation of Au nanowires in polycarbonate membranes¹⁰. Since then, many other researchers have started using templated electrodeposition for the synthesis of nanowires with different dimensions, using either polycarbonate track-etched membranes (PCTE) or anodized aluminum oxide (AAO) membranes and templates¹¹. The advantages of using templated electrodeposition for nanowire synthesis are its cost-effective nature as electrodeposition is usually performed under mild conditions, the possibility to form nanowires from either metals, metal oxides and/or polymers, and its ability to create an exact negative replica of the template used¹¹. Furthermore, segmented nanowires can be formed by sequential deposition of two or more different phases, and when a nanotube of one of the two phases can be made by templated electrodeposition, coaxial nanowires containing two different phases can be made.

Metal oxides can be electrodeposited when the respective metal ions are insoluble in aqueous solutions at high pH. For the necessary oxygen, three different precursors can be used, i.e. nitrate ions^12-15, hydrogen peroxide¹³^,¹⁶^,¹⁷, and molecular oxygen¹⁸. With the use of nitrate ions, as in this protocol, application of a potential more negative than -0.9 V vs. Ag/AgCl leads to a locally increased pH by reduction of nitrate at the cathode¹⁹^,²⁰:

NO₃^– + H₂O + 2e^– → NO₂^– + 2OH^–. (1)

When the electrolyte solution is heated to 60-90 °C, ZnO nanowires will form from precipitated zinc hydroxide:

Zn²⁺ + 2OH^– → ZnO + H₂O. (2)

Upon application of a potential to the working electrode, which is positioned at the pore bottom in templated electrodeposition, the pH inside the pore is locally increased resulting in local nanowire formation. Since ZnO is an n-type semiconductor, reactions (1) and (2) can continue at the ZnO/electrolyte interface, resulting in the formation of a crystalline and dense ZnO nanowire²¹^,²².

Several methods exist for the synthesis of TiO₂ nanotubes, but for the formation of a coaxial structure using a sequential electrodeposition process, the electrochemically induced sol-gel method is most suitable. This method for cathodic electrodeposition of TiO₂ films was first introduced by Natarajan et al. in 1996 ²³. and was further improved by Karuppuchamy et al. in 2001 ¹⁹^,²⁴. Using this method, titanium oxysulfate (TiOSO₄) powder is dissolved in an aqueous solution of hydrogen peroxide (H₂O₂) upon the formation of a peroxotitanate complex (Ti(O₂)SO₄):

TiOSO₄ + H₂O₂ → Ti(O₂)SO₄ + H₂O. (3)

At potentials more negative than -0.9 V vs. Ag/AgCl, the pH at the electrode surface is increased by reduction of nitrate (reaction (1)), forming a titanium hydroxide gel¹⁹^,²⁰:

Ti(O₂)SO₄ + 2OH^– + (x+1) H₂O → TiO(OH)₂.xH₂O + H₂O₂ + SO₄^2-. (4)

Natarajan et al. used differential thermal analysis to find that water is removed from the gel around 283 °C during thermal annealing, which results in the formation of an amorphous TiO₂ phase²³. For a planar film, crystallization into the anatase phase occurs when the temperature is increased above 365 °C²³^,²⁵, while crystallization occurs at a temperature between 525 and 550 °C when an AAO template is used²⁵.

TiO(OH)₂·xH₂O → TiO₂ + (x+1)H₂O. (5)

The pore diameter of the AAO template used determines whether a solid nanowire or open nanotube will be formed. Deposition in a template with a small pore diameter (~50 nm) results in nanowire formation²⁰^,²⁶, while applying the same method inside a pore with larger diameter (~200 nm) results in nanotube formation²⁵. This is because gel collapse can take place upon removal of excess water.

In the early 1970s, Fujishima and Honda were the first to publish a system for direct water splitting under UV light, which was accomplished by a rutile electrode coupled to a platinum electrode²⁷^,²⁸. Since then, over 130 semiconductor materials were identified as photocatalysts^29-31. Of these, titanium dioxide^32-36, zinc oxide^37-40, and iron oxide⁴¹^,⁴² are among the most intensively studied materials. The surface-to-volume ratio of these materials can be increased drastically when nanoparticles or nanowires are used, leading to improved photocatalytic efficiencies²⁹^,³⁰^,^43-49.

For the construction of photocatalytic Ag|ZnO nanowires, ZnO, which is a photoactive n-type semiconductor, was connected with Ag via sequential electrodeposition inside the same template⁵⁰. Within such a single nanowire, the ZnO photoanode and Ag cathode are directly coupled without the need of an external circuit connecting the electrodes, which is in contrast to the situation in conventional photo-electrochemical cells. This simplifies device architecture considerably and increases the efficiency by reduction of Ohmic losses in the system. ZnO and Ag segments were coupled since the electron affinity of ZnO (4.35 eV vs. vacuum) is very close to the work function of Ag (4.26 eV vs. vacuum). This induces the formation of a Schottky barrier between both phases⁵¹, which allows excited electrons in the conduction band of ZnO to flow to Ag, but not vice versa, thus prohibiting the chance of electron-hole recombination⁵². The active wurtzite phase of ZnO can be formed already at 60-90 °C, which provides an easy and cost effective way of nanowire formation. This is in contrast to most other photoactive oxides that require an intermediate annealing step at high temperatures when made via cathodic electrodeposition.

The conversion of methanol and water into hydrogen and carbon dioxide was used as a model reaction to demonstrate the use of a segmented nanowire containing a metal and a metal oxide phase for autonomous H₂ formation under the influence of UV light. In this experiment, methanol is used as a hole scavenger which is oxidized to CO₂ at the ZnO segment, following the net reaction

CH₃OH + H₂O + 6h⁺ → CO₂ + 6H⁺, (6)

where h⁺ represents an electron hole. The protons formed at the ZnO segment are reduced to H₂ at the Ag surface, following the reaction

2H⁺ + 2e^– → H₂. (7)

Since the total energy required for reactions (6) and (7) is much smaller than the band gap of ZnO (0.7 and 3.2 eV, respectively), this process can take place without the need for an external power source. This process is schematically illustrated in Figure 1.

In this protocol, the experimental procedures of templated electrodeposition for the formation of segmented and coaxial nanowires containing both a metal and a semiconductor phase are explained. A procedure for the formation of segmented Ag|ZnO nanowires is outlined, as well as the formation of TiO₂ nanotubes and their subsequent filling with Ag to yield coaxial TiO₂-Ag nanowires. Furthermore, the photocatalytic activity of the Ag|ZnO nanowires is demonstrated by converting a methanol/water mixture into H₂ and CO₂ gas upon irradiation with UV light employing a Pd-based sensor for H₂ detection. The emphasis of this protocol is on the preparation and photocatalytic characterization of two differently segmented metal oxide|metal nanowire modules, and a more in-depth treatment and an example of a multifunctional nanowire can be found elsewhere⁵³. The water splitting reaction that was employed using the coaxial TiO₂-Ag nanowires can also be found elsewhere²⁵.

Protocol

Segmented Ag|ZnO Nanowire Formation in PCTE Membranes

1. PCTE Membrane Preparation for Templated Electrodeposition

Choose a track-etched polycarbonate membrane with an outer pore diameter of 200 nm and a thickness of 6 µm (Figure 2a). The diameter of the membrane used here is 25 mm.
Sputter a gold layer at the backside of the membrane (Figure 2b). In this case, a deposition pressure of 2 x 10^-2 mbar was used with Ar as sputtering gas. Use a slow deposition rate of ~13 nm/min. NOTE: This Au layer will be used as electrical contact during electrodeposition.
Use double-sided sticky tape to attach a small glass slide (1.4 x 2.1 cm) on top of the gold-coated side of the membrane. For this, put four small strips of double-sided tape along the edges of the glass slide (Figure 2c). NOTE: Make sure the membrane is as smooth as possible, without any folds or wrinkles. This glass slide is used to ensure selective electrodeposition inside the membrane pores.
Stick a small piece of copper tape on the part of the membrane that sticks out from the glass slide for mechanical stability. Since copper tape is conducting, the crocodile clip of the working electrode can be attached to the copper tape.
If necessary, improve the adhesion of the membrane to the glass slide by putting Teflon tape around the edges. NOTE: For depositions at room temperature the adhesion of double-sided tape is usually strong enough, but at elevated temperatures it is recommended to use Teflon tape as well.

2. Electrodeposition of Ag|ZnO Nanowires

Preparation of the Ag segment
1. Prepare an aqueous solution containing 0.20 M AgNO₃ (1.70 g per 50 ml) and 0.10 M H₃BO₃ (0.31 g per 50 ml). Adjust the pH to 1.5 using HNO₃.
2. Put the prepared membrane together with a Pt counter electrode and an Ag/AgCl (3 M KCl) reference electrode in the as-prepared solution.
3. Apply a potential of +0.10 V vs. the Ag/AgCl reference electrode for 30 sec (Figures 2d and 2e). NOTE: Although every potentiostat software will be different, all programs should have input lines like “set potential” and “duration”, where these values can be filled in. Please refer to the potentiostat manual and included software for more details.
4. Take the electrodes from the solution and rinse them with milli-Q water.
Preparation of the ZnO segment
1. Prepare an aqueous solution containing 0.10 M Zn(NO₃)₂·6H₂O (1.49 g per 50 ml).
2. Heat the solution to 60 °C using a water bath, and put the membrane containing the Ag segment together with a Pt counter electrode and an Ag/AgCl reference electrode in the heated solution.
3. Apply a potential of -1.00 V vs. the Ag/AgCl reference electrode for 20 min (Figures 2d and 2e). NOTE: Although every potentiostat program will be different, all should have input lines like “set potential” and “duration”, where these values can be filled in. Please refer to the potentiostat manual and included software for more details.
4. Take the electrodes from the solution and rinse them with milli-Q water.
Repeat this procedure 4x to obtain enough nanowires for significant signal from the H₂ sensor.

3. Extraction of the Nanowires and Transfer to Aqueous Solution

Cut the membrane containing the nanowires from the glass slide.
Transfer this part of the membrane to a polypropylene centrifuge tube.
Add ~2 ml of CH₂Cl₂ to dissolve the PCTE membrane and release the nanowires into the solution. After ~30 min, the membrane should be completely dissolved (Figures 2f and 2g).
Apply a small droplet of the CH₂Cl₂ solution containing nanowires on a small Si wafer for SEM analysis.
Centrifuge the obtained solution at ~19,000 x g for 5 min, remove the excess CH₂Cl₂, and add fresh CH₂Cl₂. Repeat the process at least 3x to make sure all polycarbonate has been removed.
After all polycarbonate has been removed, add milli-Q water to the nanowires after removal of the excess CH₂Cl₂. Repeat the centrifugation at least 3x again to completely replace all CH₂Cl₂ by milli-Q water.

Coaxial TiO₂-Ag Nanowire Formation in AAO Membranes

4. AAO Membrane Preparation for Templated Electrodeposition

Take an AAO membrane with a pore size of 200 nm and thickness of 60 µm (Figure 2a). The diameter of the membrane used here is 13 mm.
Sputter a gold layer on the backside of the membrane (Figure 2b). In this case a deposition pressure of 2 x 10^-2 mbar was used with Ar as sputtering gas. Use a slow deposition rate of ~13 nm/min. NOTE: This Au layer will be used as electrical contact during electrodeposition.
Attach the AAO membranes to an Au-coated glass slide in a configuration as in Figure 2h using Teflon tape. NOTE: To ensure selective electrodeposition inside the membrane pores, the AAO membrane needs to be attached to a small glass slide in a different configuration than the PCTE membranes, because the AAO membranes are too brittle for connection with a crocodile clip. When a glass slide of 3.0 x 2.5 cm is used, two membranes can be used at once.
Put a small piece of copper tape on the Au coated part of the glass slide for easy handling when connecting the electrodes.

5. Electrochemical Deposition of TiO₂-Ag Nanowires

Preparation of a TiO₂ gel
1. Prepare an aqueous solution containing 0.02 M TiOSO₄ (0.16 g per 50 ml), 0.03 M H₂O₂ (0.13 ml per 50 ml), 0.05 M HNO₃ (0.15 ml per 50 ml), and 0.25 M KNO₃ (1.26 g per 50 ml).
2. Put the prepared membrane together with a Pt counter electrode and an Ag/AgCl (3 M KCl) reference electrode in the as-prepared solution.
3. Apply a potential of -1.0 V vs. the Ag/AgCl reference electrode for 3.5 hr (Figures 2d and 2e). NOTE: Although every potentiostat software will be different, all programs should have input lines like “set potential” and “duration”, where these values can be filled in. Please refer to the potentiostat manual and included software for more details.
4. Take the electrodes from the solution and DO NOT rinse the membrane with milli-Q water, because the TiO₂ gel is still water soluble. The other electrodes can be rinsed with milli-Q water.
Preparation of coaxial TiO₂-Ag nanowires
1. Thermally anneal the membranes with the TiO₂ gel in an oven at 650 °C for 2 hr in air.
2. Reattach the membranes to a gold coated glass slide.
3. Prepare an aqueous solution containing 0.20 M AgNO₃ (1.70 g per 50 ml) and 0.10 M H₃BO₃ (0.31 g per 50 ml). Adjust the pH to 1.5 using HNO₃.
4. Put the prepared membrane together with a Pt counter electrode and an Ag/AgCl (3 M KCl) reference electrode in the as-prepared solution.
5. Apply a potential of +0.10 V vs. the Ag/AgCl reference electrode for 1.5 min (Figures 2d and 2e). NOTE: Although every potentiostat software will be different, all programs should have input lines like “set potential” and “duration”, where these values can be filled in. Please refer to the potentiostat manual and included software for more details.
6. Take the electrodes from the solution and rinse them with milli-Q water.
Preparation of Ag nanoparticles incorporated in TiO₂ nanotubes
1. Heat the membranes with the TiO₂ gel overnight at 100 °C.
2. Prepare an aqueous solution containing 0.20 M AgNO₃ (1.70 g per 50 ml) and 0.10 M H₃BO₃ (0.31 g per 50 ml). Adjust the pH to 1.5 using HNO₃.
3. Put the prepared membrane together with a Pt counter electrode and an Ag/AgCl (3 M KCl) reference electrode in the as-prepared solution.
4. Apply a potential of +0.10 V vs. the Ag/AgCl reference electrode for 1.5 min (Figures 2d and 2e). NOTE: Although every potentiostat software will be different, all programs should have input lines like “set potential” and “duration”, where these values can be filled in. Please refer to the potentiostat manual and included software for more details.
5. Take the electrodes from the solution and rinse them with milli-Q water.
Repeat this procedure to obtain at least 10 membranes filled with nanowires/nanotubes to obtain enough material for significant signal from the H₂ sensor.

6. Extraction of Nanotubes and Nanowires

Cut the membrane containing the nanotubes or nanowires from the glass slide.
Transfer this part of the membrane into a polypropylene centrifuge tube.
Add ~2 ml of an aqueous solution containing 1.0 M NaOH to dissolve the AAO membrane and release the nanotubes or nanowires into the solution. After ~2 hr, the membrane should be completely dissolved (Figures 2f and 2g).
Centrifuge the obtained solution at ~19,000 x g for 5 min, remove the excessive NaOH solution, and add fresh milli-Q water. Repeat the process at least 3x to make sure all NaOH has been removed.
After all NaOH has been removed, the aqueous suspension can be used for H₂ formation experiments.
Alternatively, add CH₂Cl₂ or another volatile solvent to the nanotubes and nanowires after removal of excess water for visualization of the prepared nanotubes or nanowires with SEM. Repeat the centrifugation at least 3x to completely replace all water by the volatile solvent. Deposit a small droplet of the solution containing nanotubes or nanowires onto a small Si wafer.

H₂ Formation Experiments

7. Preparation of the Hydrogen Sensor

Take a Pd-based hydrogen sensor.
Place the sensor inside an NS plug that fits on top of a quartz tube.
Connect the sensor to a standard Wheatstone bridge as illustrated in Figure 3.

8. Photocatalytic Hydrogen Formation

Put the aqueous nanowire solution in a 72 ml quartz tube. Add more water until a total of 10 ml water is inside the quartz tube. Then add 40 ml methanol.
Start recording the signal from the Pd based H₂ sensor before placing it on top of the quartz tube and monitor the variation in signal.
After ~200 sec of stable signal, put the H₂ sensor on top of the quartz tube while simultaneously turning on the UV light source to start the actual measurement. NOTE: In these experiments, the UV source was placed approximately 10-15 cm away from the sample.

Representative Results

During electrodeposition, the current that is measured between the working and counter electrodes can be visualized in an I-t 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. Typical I-t curves for the deposition of Ag|ZnO and TiO₂-Ag nanowires are shown in Figure 4. Typical SEM images of Ag|ZnO nanowires, TiO₂ nanotubes, a coaxial TiO₂-Ag nanowire and TiO₂/Ag nanotubes can be found in Figure 5 and Figure 6, respectively.

Using the electrochemically induced sol-gel method for deposition of a titania gel inside the template and sequential electrodeposition of Ag can result in two different structures depending on the temperature used to dry the gel. Drying of the gel overnight at 100 °C results in condensation of the gel, preventing it to redissolve in water. Since no dense tubular shape has yet formed at this temperature, Ag nuclei are deposited inside the titania gel. Subsequent annealing at 650 °C results in the formation of Ag nanoparticles incorporated in a TiO₂ nanotube (Figure 6c), since the collapse of the titania gel causes the Ag nanoparticles to be transported to the pore walls. In contrast, high temperature annealing of the titania gel prior to Ag electrodeposition leads to the formation of solid TiO₂ nanotubes. In this case, Ag nanowires could be deposited inside these tubes, leading to the formation of TiO₂-Ag nanowires with a coaxial architecture (Figure 6b).

The activity of the segmented Ag|ZnO nanowires in photocatalytic water splitting can be investigated using a methanol/water solution under UV illumination, where methanol acts as a hole scavenger. A technically simple method to detect gaseous hydrogen evolving from the solution is obtained by placing a H₂ sensor directly above the solution (Figure 7). This experiment only detects the amount of H₂ reaching the sensor, so the actual amount of formed H₂ may be higher as some H₂ will remain dissolved in the methanol/water phase. The signal as detected by the sensor is shown in Figure 8a. Figure 8b shows the same signal after transformation to the timeframe of actual H₂ formation. When the UV light source was turned on (t = 17.5 min in Figure 8a), 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 = 0 min in Figure 8b, and the corresponding signal was defined as 0 V. During UV exposure of the test tube, it was also visible that small gas bubbles were formed. Since the sensor used is slightly cross-sensitive to methanol, the measurement of a reference sample without nanowires was also included. During UV illumination, Figure 8 shows that the signal from the sample with nanowires is higher than the signal from the reference sample.

The increase in potential is a relative measure for the amount of gaseous H₂ that forms and evolves from the solution. In order to give a quantitative estimate for the amount of evolved H₂, the potential response of the sensor from the photocatalytic experiments was compared with its response in a 4 vol% H₂ in N₂ gas stream. From the comparison, it was estimated that 17 min of UV illumination of the Ag|ZnO nanowires resulted in the formation of approximately 0.2 vol% H₂ in the gas volume above the solution. Since ~0.1 g of nanowires was used, this equals to a H₂ evolution rate of 6.92 x 10^-6 mol/hr·g. As reference, experiments with single-phase ZnO or Ag nanowires were also performed. These experiments, not shown here, did not give any indication of H₂ formation; neither from gas bubble formation nor from sensor signal.

Figure 1. Working principle of segmented Ag|ZnO nanowire in photocatalytic water splitting: (a) schematic representation, and (b) energy diagram. When UV light is absorbed by the ZnO segment, an electron-hole pair is formed. The as-formed electrons flow to the Ag phase where they are consumed in an electrochemical reduction half-reaction. The hole stays in the ZnO segment where it is consumed in an oxidative half-reaction. Click here to view larger image.

Figure 2. Schematic representation of the consecutive steps taken for nanowire synthesis.

Figure 3. Typical operating circuit of the H₂ sensor with Wheatstone bridge. In this scheme, pin 1 to 4 refer to the wiring of the sensor (pin 1 is black, pin 2 is blue, pin 3 is white, pin 4 is brown), R_h is the resistance of the heater (150 ± 50 Ω), R_r is the resistance of the reference (1,500 ± 500 Ω), R_s is the resistance of the sensor (1,000 ± 250 Ω). The sensor is connected to a 12 V power source so that 0.5 to 1.0 V is applied to the heater and 2.7 V is applied to the Wheatstone bridge. V_out is connected to the multimeter/potentiostat. The resistance next to pin 2 is variable and can be adjusted in order to obtain an appropriate baseline.

Figure 4. Typical I-t curves of (a) Ag|ZnO nanowire deposition, and (b) TiO₂-Ag nanowire deposition. The insets show an enlarged curve of the deposition of the Ag segment (a) or Ag core (b). Click here to view larger image.

Figure 5. Scanning Electron Microscopy (SEM) picture of axially segmented ZnO|Ag nanowires.

Figure 6. SEM pictures of (a) TiO₂ nanotubes, (b) coaxial TiO₂-Ag nanowire and (c) TiO₂/Ag nanotubes. Click here to view larger image.

Figure 7. Typical setup for the detection of H₂ gas evolved from photocatalytic nanowires. The Pd based H₂ sensor is placed in the NS plug of a quartz cuvette, and connected to an amplifier (see Figure 3). The amplifier is operated by a 12 V power source and the signal from the sensor is read by a multimeter (or potentiostat) connected to a computer for graphical representation of the obtained signal. Click here to view larger image.

Figure 8. Response from the H₂ sensor during UV irradiation of Ag|ZnO nanowires in a methanol/water solution (red line) and reference experiment without nanowires (blue line). (a) Signal as measured by the sensor; (b) Signal during H₂ formation, where the data point at t = 17.5 min of (a) was defined as the start of the reaction in (b). Click here to view larger image.

Figure 9. SEM image of photocorroded Ag|ZnO nanowire after 48 hr of UV illumination.

Discussion

Very important in the templated electrodeposition of nanowires is isolation of the back side of the gold electrode sputtered on top of the membrane. Without isolation, the material would preferentially deposit on the gold surface at the back side of the membrane instead of inside the pores. This is because the diffusion of ions to a flat electrode is much faster than diffusion into membrane pores. Another disadvantage of deposition on both sides of the gold electrode is that the obtained I-t curve cannot be related to the amount and length of deposited nanowires. In Figure 4, several stages can be identified for the deposition of the Ag segment (a) or Ag core (b). The first stage of every electrodeposition experiment is charging of the electrical double layer, which is accompanied by a sudden increase in current that slowly decreases as the electrical double layer reaches its equilibrium. As the PCTE membrane pores from Whatman have a cigar-shape, the current increases in the second stage as the surface area of deposition increases, leading to deposition of more material at the same time, and faster supply of reactants since the surface of the nanowire gets closer to the entrance of the membrane pores. In the third stage, the change in surface area is minimal, leading to a smaller slope of increasing current since only the effect of faster reactant supply is visible in this stage.

Please note that in the case of depositing segmented nanowires containing both a metal and an oxide segment, the order of electrodeposition inside the pores should be determined by taking the solubility of the deposited phases in each other’s solution explicitly into account. In this case, the Ag segment was deposited before the ZnO segment as ZnO would dissolve in the acidic AgNO₃ solution. In the case of forming a segmented nanowire containing a noble metal and a less noble one, e.g. Pt and Ni, the galvanic replacement reaction of Ni by Pt should be taken into account. This galvanic replacement reaction can be suppressed by using a larger overpotential as discussed in a previous publication⁵⁴.

The choice for using either PCTE or AAO membranes for nanowire or nanotube synthesis is usually based on whether or not a thermal annealing step is desired for the material of choice. Without the necessity of an annealing step, PCTE membranes are easier to handle and relatively good membranes can be obtained commercially. For high temperature annealing, the use of AAO membranes is required. These membranes are not as flexible as the polycarbonate membranes and are very brittle. Some commercial AAO membranes are available, but the quality of homemade AAO membranes using a 2-step anodization is much better. For this, several recipes are available^55,56.

The Pd-based H₂ sensor used in this study is an easy and relatively cheap method for determining whether H₂ has formed or not. Unfortunately, it is not suitable for quantitative measurements due to its cross-sensitivity to volatile solvents like methanol, the intrinsic inability to detect dissolved H₂ in the methanol/water solution, and its non-linear response as seen in the shape of the curves in Figure 8. Quantitative measurements could be performed in a setup with a GC inlet connected to the head space above the methanol/water mixture, which is specialized equipment that is not available in every lab.

H₂ formation using Ag|ZnO nanowires typically ceased after ~48 hr of UV illumination as evidenced by terminated gas bubble formation. The reason for this loss of activity is photocorrosion of ZnO according to the following reaction^57-60:

ZnO + 2h⁺ → Zn²⁺ + 1/2 O₂ (8)

An SEM image of photocorroded Ag|ZnO nanowires is shown in Figure 9. As can be seen from this figure, the surface of the ZnO segment became much rougher upon UV illumination as compared to the as-synthesized wires of Figure 5. When suspending another batch of Ag|ZnO nanowires in the same solution in the dark for 48 hr, no sign of corrosion was found. This confirmed that the observed corrosion indeed resulted from photocorrosion and not from electrolytic corrosion. In the literature, several methods have been reported for inhibition of ZnO photocorrosion, including hybridization of ZnO nanoparticles with a monolayer of polyaniline or C₆₀ and grafting of ZnO nanorods on TiO₂ nanotubes^59,61,62.

Templated electrodeposition of axially or radially segmented nanowires is a perfect platform for the deposition of multisegmented nanowires that are able to carry out more than one function at once, in which Ag|ZnO segments can be applied as photocatalytic elements. In a previous publication, an SEM image of a single nanowire containing six segments was introduced: Pt|Au|Pt|Ni|Ag|ZnO. Such a nanowire could be used for autonomous movement (Pt|Au|Pt), magnetic steering (Ni) and photocatalytic H₂ formation (Ag|ZnO)⁵³.

In summary, a simple protocol for the synthesis of segmented Ag|ZnO nanowires and coaxial TiO₂-Ag nanowires by templated electrodeposition is provided. A semi-quantitative method to determine the photocatalytic activity of such nanowires was demonstrated using the photocatalytic conversion of methanol and water into H₂ and CO₂ under UV illumination. It is envisaged that these metal oxide-metal nanowires can be used in multifunctional nanowires and other nanowire devices.

Divulgazioni

The authors have nothing to disclose.

Acknowledgements

Financial support from the Chemical Sciences division of the Netherlands Organization for Scientific Research (NWO-CW) in the framework of the TOP program is acknowledged.

Materials

Silver Nitrate (AgNO₃)	Acros Organics	419351000	99+%
Boric Acid (H₃BO₃)	Sigma-Aldrich	202878-500G	99.99%
Nitric Acid (HNO₃)	Acros Organics	124660010	65%
Zinc Nitrate Hexahydrate (Zn(NO₃)₂·6H₂O)	Sigma-Aldrich	228737-500G	98%
Dichloromethane (CH₂Cl₂)	Merck (Boom)	51006050100	99%
Titanium oxysulfate (TiOSO₄)	Sigma-Aldrich	333980-500G	Synthesis grade
Hydrogen peroxide (H₂O₂)	Sigma-Aldrich	349887-500ML	35%
Nitric acid (HNO₃)	Acros Organics	124660010	65%
Potassium nitrate (KNO₃)	Acros Organics	P/6040/60	>99%
Sodium hydroxide (NaOH)	Sigma-Aldrich	20606-0025	>98%
Methanol (CH₃OH)	Merck	1060121000	Dried ≥99.9%
Polycarbonate membranes 200 nm	Fisher Scientific	09-300-61
Anopore AAO membranes 200 nm	VWR	514-0523
Sputtering system	Perkin-Elmer	Model 2400
Microscope glass slides (Menzel)	VWR	631-0704
Autolab potentiostat with	Metrohm-Autolab	PGSTAT 128N
– Pt sheet counter electrode		PT.SHEET
– Ag/AgCl in 3 M KCl reference electrode		60,733,100
Polypropylene Nunc centrifuge tubes	Fisher Scientific	12-565-286C
Centrifuge	Hermle	Z36HK
Pd-based hydrogen sensor	Kebaili	KHS-100
4x 15W Hg lamp UV source	Philips	Philips original home solaria

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Maijenburg, A. W., Rodijk, E. J., Maas, M. G., ten Elshof, J. E. Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO₂-Ag Nanowires Made by Templated Electrodeposition. J. Vis. Exp. (87), e51547, doi:10.3791/51547 (2014).