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In this paper, CuxO photocatalyst on plasmonic nanoporous Au film is proposed to enhancing the H2 evolution rate of pure water splitting. The nanoporous Au film can simultaneously provide surface-enhanced absorption and built-in potential. The reflection spectrum shows that the surface plasmon (SP) assisted absorption wavelength of the CuxO on the nanoporous Au film can be modified by changing the annealing temperature. It is found that the enhancement of the H2 evolution rate highly depends on the SP-assisted absorption. As the annealing temperature is 220°C, the H2 evolution rate is 58μmolhr−1 under the condition that the device area is 0.25cm2.
©2013 Optical Society of America
1. Introduction
Plasmonic nanostructured materials in the form of nanocomposites, where the metallic and dielectric structures are randomly mixed, represent a new type of materials with unique optical properties, which are quite different from those of the underlying components [1, 2]. For example, metallic nano holes can present strong absorption bands, which are induced by excitation of the localized surface Plasmon (LSP) resonance. The enhancement of the optical absorption, which is attributed to the enhancement of the local electromagnetic field near the metal surface, can have a great impact on applications where light / surface plasmon (SP) interaction is required. Taking the randomly distributed nanoporous Au films for instance, they have been widely applied for chemical and biological sensing applications [3, 4] because they are Surface enhanced Raman spectroscopy (SERS) active, biocompatible and reusable. Regarding metallic nanoparticles, they have been widely applied in solar cells [5], light extraction of light-emitting diodes (LED) [6], and so on. The resonant frequency of an LSP is usually a complex function of the geometry, size, and dielectric function of the metallic nanostructure as well as the surrounding dielectric matrix. Consequently, a simple fabrication method for tuning the resonance properties of the plasmonic structure is required.
Photocatalytic water splitting, in which water is separated into oxygen (O2) and hydrogen (H2) by directly utilizing the energy of light, could be a clean and renewable way to produce energy. Theoretically, only solar energy, water, and a catalyst are needed for water splitting. It provides a clean, renewable energy, without producing greenhouse gases. Since the well-known Honda–Fujishima method for water splitting was reported [7], TiO2 is the most common photocatalyst. It yields both high quantum efficiency and a high H2 evolution rate. However, TiO2 absorbs only ultraviolet light due to its large band gap (>3.0ev). In addition, aqueous solutions and sacrificial reagents are needed for a high H2 evolution rate. NiO/NaTaO3:La yields the highest water splitting rate of photocatalysts without using sacrificial reagents [8, 9]. However, the NiO/NaTaO3:La photocatalyst with nanostep is challenge to be fabricated. Up to date, the visible-light-driven photocatalysts without adding external redox agents for water splitting [10–16], such as NiOy/In1-xNixTaO4 [17] and RuO2/In1-xNixTaO4 [17], are still limited.
Cu2O, which exists in abundance as cuprite in nature, has been studied extensively since Cu2O is a simple metal oxide semiconductor with small bandgap energy of 2.0~2.2 eV [18–20]. The conduction and valence band edges of Cu2O are available for the reduction and oxidation of water, respectively. It has been shown that Cu2O is capable of decomposing water into H2 and O2 under visible light excitation [21].
In this paper, the CuxO on nanoporous Au film is fabricated for pure water splitting. The CuxO film is a photocatalyst layer and the nanoporous Au film is the plasmonic enhancing layer. No electrolyte and sacrificial reagent is needed. The structure of the photocatalyst, CuxO, is a thin film rather than a powder. Therefore, the apparatus of H2 evolution is very simple. It is shown that the SP-assisted resonance properties of the nanoporous Au film covered with CuxO layer can be modified through tuning the annealing temperature. The H2 evolution rate as a function of the annealing temperatures is measured. It is found that the H2 evolution rate at an annealing temperature of 220°C is 58 μmolhr−1 for a device area of 0.25cm2.
2. Fabrication processes
The fabrication processes are described as below [22] and the procedure is shown in Fig. 1 . A tungsten (W) plate is cleaned successively in acetone, ethanol, and de-ionized water. A Au layer and a Cu layer are deposited in sequence onto the W substrate by electron beam evaporation. The thickness of the Au film and Cu film are both 20nm. The samples are then annealed at 200 °C to 260 °C for 6 hours in a chamber with a N2 flow to facilitate Cu and Au inter-diffusion. After this process, the film becomes partially Cu-rich and partially Au-rich. It is further annealed at 190 °C for 10 hours in air for the purpose of Cu oxidation. Finally, a nanoporous Au-rich film covered with a CuxO layer can be made. The device area is fixed as 0.25cm2. Therefore, the total weight of the coated Cu is 4.5x10−6g. Assuming the Cu layer is completely oxidized to CuO/Cu2O, the weight of CuO/Cu2O is 5.7x10−6g/ 5.0x10−6g. The fabrication processes are simple, the temperature of the process is low and no lithography technique is needed.
Figures 2(a) and 2(b) show the optical microscopy (OM) pictures of the CuxO film for an annealing temperature of 220°C and 250°C, respectively. Generally, the colors of the bulk CuO and bulk CuxO are black and red, respectively. The OM pictures show that the colors of the CuxO films are green and red for 220°C and 250°C, respectively. The CuxO layer is wet etched using a 0.1M HCl solution so that the morphologies of the nanoporous Au film can be measured using a scanning electron microscope (SEM). As shown in Figs. 2(c) and 2(d), the structure of the nanoporous Au film is random. The ratio (Au/Cu) of the weight of the nanoporous Au film is around 1.7~2.6 measured by the energy dispersive spectrometer. Therefore, the nanoporous Au film is Au-Cu composite.
Fig. 2 Optical microscopy pictures of the CuxO film for an annealing temperature of (a) 220°C and (b) 250°C, respectively. SEM pictures of the nanoporous Au film for an annealing temperature of (c) 220°C and (d) 250°C, respectively.
Generally, at least two benefits of the metal/photocatalyst systems have been demonstrated. First, the nanoporous Au film plays a role of plasmonic enhancing layer for the purpose of light harvesting since the randomly distributed nanoporous Au film can increase the wavelength response range. Second, in Au-Cu/CuxO heterojunction there exists a built-in potential for separating the photo-generated electron-hole pairs. Consequently, the recombination rate can be suppressed.
Current approaches about SP-enhanced photocatalyst are mixing the photocatalyst nanoparticles with metallic nanoparticles. Therefore, a method for precisely controlling the adhesion conditions between the metallic particles and the photocatalyst is needed. The absorption is limited if too many metallic nanoparticles cover photocatalyst. In addition, the contacting surface area between photocatalyst and water with be shrunk as covering for metallic nanoparticles. On the other hand, the recombination rate is high if there are no metallic nanoparticles covered on the photocatalyst. By using the thin film coating method, the covering rate can be easily controlled which is convenient for practical applications.
3. Optical and Electric Properties of CuxO photocatalyst
The reflectance spectra of the CuxO photocatalyst layer on plasmonic nanoporous Au film are shown in Fig. 3(a) . The incident light is unpolarized. For reference, the emission spectrum of the tungsten lamp (black solid line) and the reflection spectrum of the nanoporous Au film without CuxO photocatalyst layer (red solid line) are also measured as shown in Fig. 3(b). The reflectance of the nanoporous Au film without CuxO photocatalyst layer is lower than that of flat Au film which can lead to an enhanced absorption. Compared to Fig. 3(a), it can be seen that a resonance dip occurs at 560nm for an annealing temperature of 200°C. The resonance dip is coincident with the inherent nature of Au. The larger absorbance is an indication of the plasmonic effect of the nanoporous film and is related to the conversion of incident light into SPs. The resonance dip gradually shifts to short wavelength as the annealing temperature increases. The absorption coefficient of Cu2O and CuO at λ = 551nm is 4.28x104 cm−1 and 1.34x105 cm−1, respectively. Theoretically, a pure Cu2O (CuO) film with a thickness thicker than 234nm (75nm) is thick enough for completely absorbing the incident light with a wavelength of 551nm. As shown in Fig. 3(a), the reflectance of the CuxO film on nanoporous metallic film can be as low as 20% around the green light spectral range. Therefore, we infer that the absorption is enhanced by the nanoporous metallic film.
Fig. 3 (a) Reflectance spectrum of the CuxO photocatalyst layer on plasmonic nanoporous Au film. Black, red, blue and green solid lines indicate the annealing temperature of 200 °C, 220 °C, 230 °C and 240°C, respectively. (b) Black and red solid lines respectively indicates the emission spectrum of the tungsten lamp and the reflection spectrum of the nanoporous Au film(without CuxO photocatalyst layer).
When the energy of incident light is larger than that of the bandgap of CuxO, electrons and holes are generated in the conduction and valence bands, respectively. The fabricated CuxO film is p-type. Within the Au-Cu/CuxO heterogeneous interface there exists a slightly bending barrier. The electron and hole are separated due to the built-in potential at the barrier. Therefore, the life-time of the excitons can be prolonged. The photogenerated electrons and holes cause redox reactions similar to electrolysis, oxidation at the CuxO/water interface and reduction at the metal/water interface. Thus, H2 and O2 can be generated at the metal/water and CuxO/water interfaces, respectively.
According to Ref [23], the Fermi energy (EF) of CuO and Cu2O are 0.1eV and 0.3eV above the valence band, respectively. The work function of Au and Cu is 5.15eV and 4.7eV. The electron affinity of CuO and Cu2O are 4.07eV and 3.2eV, respectively. The bandgap of CuO and Cu2O are 1.3~1.5eV and 2.0~2.2eV, respectively. The barrier height at the Au/CuO and Au/Cu2O are about 0.22eV and 0.15eV, respectively. According to the above database, we can draw the energy-band diagram of the CuxO photocatalyst on Au-Cu composite film as shown in Fig. 4 . For the Pt/TiO2 system, Anpo et. al. employed Electron Spin Resonance (ESR) signals to investigate the electron transfer from TiO2 to Pt particles. It was found that Ti3+ signals increased with the irradiation time and the loading of Pt reduced the amount of Ti3+ [24]. This observation indicates the occurrence of electron transfer from TiO2 to Pt particles. According to the energy-band diagram at the Au-Cu/ CuxO interface, the holes drift toward the CuxO.
Fig. 4 Approximate energy-band diagram of the CuxO photocatalyst on plasmonic nanoporous Au-Cu composite film for water splitting.
Additionally, it is common opinion that the metal/photocatalyst boundary is a Schottky barrier. Owing to the proposed structure being a film one, we can easily measure the IV characteristics of the Au-Cu/CuxO heterogeneous interface. As shown in Fig. 5(a) , the Au-Cu/CuxO heterogeneous interface is Ohmic with a slight bending band, i.e. quasi-ohmic behavior. The formation of the quasi-ohmic contact occurs because of the large contact area of the Au-Cu/CuxO interface. The photogenerated electrons and holes are separated due to the built-in potential at the Au-Cu/CuxO heterojunction. Therefore, the life-time of the excitons can be prolonged. These activities greatly reduce the possibility of electron-hole recombination, resulting in efficient separation and stronger photocatalytic reactions. Besides the benefit of prolonging the carrier life-time, the bandgap of CuxO matches the solar spectrum which can result in for a high throughput of the evolution of H2.
Fig. 5 (a) IV characteristic of the Au-Cu/CuxO heterogeneous interface for an annealing temperature of 240°C. (b) Ideality factor, n, as a function of annealing temperatures.
From the dark J–V characteristics, one can calculate the ideality factor, which is a fitting parameter for carrier recombination, using the following equation [25]:
where n is the ideality factor, q the elementary charge, k Boltzmann’s constant, and T temperature. The ideality factor as a function of annealing temperatures is shown in Fig. 5(b). According to the Shockley-Read-Hall (SRH) recombination theory, which assumes recombination via isolated point defect levels, the maximum of an ideality factor is n = 2. Our experimental results deviate this prediction and are much higher than 2 which might be led by the high defect concentrations and trap-assisted tunneling [26, 27].4. Enhancement of the H2 Evolution Rate
After the samples are made, they are placed directly into water without adding any electrolyte and sacrificial reagent. The photocatalytic reaction is performed in a reactor equipped with a cooling water jacketed cylindrical quartz cell. The evolved H2 and O2 are collected by displacement of water from a container. A 300 watt white light lamp (Philip) is mounted outside the above cell. The emission spectrum of the lamp is shown in Fig. 3(b). Figure 6 shows the enhancement factor of the H2 evolution rate of the CuxO photocatalyst on plasmonic nanoporous Au film. The initial evolved gas in the first 15min is not collected for avoiding the air contamination in the container and the pipe of the water splitting apparatus. The composition of the evolved gas is measured by using gas chromatograph. The ratio of the amount of evolved H2 to O2 was 1.7 during the reaction time from 15min to 30min. The H2 evolution rate is recorded and calculated by the amount of water that was displaced by the gas during the reaction time from 15min to 30min. The H2 evolution rate at each annealing temperature is normalized to CuxO film on flat Au film. The H2 evolution rate of CuxO on flat Au film is 31μmolhr−1 for a device area of 0.25cm2. The total weight of the CuxO photocatalyst is about 5.0x10−6 g. It can be seen that the H2 evolution rates are all enhanced when the plasmonic nanoporous Au film is applied. However, similar to other photocatalyst with small bandgap, the CuxO suffers from serious photocorrosion. In our case, the H2 evolution rate decay about 75% from 22.5min to 52.5min. It has been demonstrated that the photocorrosion can be suppressed by depositing NiOx onto Cu2O electrode [28]. Improving its photostability, the CuxO photocatalyst can be dramatically promoted as a promising candidate for water splitting.
Fig. 6 Enhancement factor of the H2 evolution rate of the CuxO photocatalyst on plasmonic nanoporous Au film. The H2 evolution rate at each annealing temperature is normalized to CuxO on flat Au film. The H2 evolution rate of CuxO on flat Au film is 31μmolhr−1.
It can be seen that the H2 evolution rate is gradually increased when the annealing temperature increases from 200°C to 220°C. At this time, the resonance dip gradually shifts to short wavelength, as shown in Fig. 3(a). This implies that the absorption of the shorter wavelength contributes to a higher H2 evolution rate. As the annealing temperature is 220°C, the H2 evolution rate is 58μmolhr−1, and the enhancement factor is 1.85 folds. When the annealing temperature is 240°C, the enhancement factor is as low as 1.1-fold. As shown in Fig. 3(a), the reflectance spectrum for an annealing temperature of 240°C shows that the reflectance of the resonance dip at 0.5μm is about 0.4. Therefore, the absorbance of this resonance dip is relatively low. This makes the SP-assisted absorption enhancement is low. Consequently, the H2 evolution rate is limited. In addition, the H2 evolution rate as a function of annealing temperatures is also similar to the trend of the ideality factor as a function of annealing temperatures. The smaller ideality factor the lower recombination rate is which also benefits the H2 evolution rate. Thanks to our photocatalytic system is a film type, the reflectance spectra and the IV characteristics can be more easily measured than a nanoparticle type. By analyzing the optical and electric properties, the photocatalytic performance can be roughly evaluated.
Here, the effect of the CuxO thickness is not yet investigated. A thicker CuxO film can absorbs more photons. However, a thicker CuxO layer suffers from a higher bulk recombination. Obviously, it is a trade-off between the recombination and the absorption. Therefore, the thickness is still under optimization. Here we simply choose a CuxO thickness of 20nm due to the SP extending length in the CuxO regime being about 20nm for a Au CuxO infinite flat interface.
The material of the plasmonic enhancing layer also plays crucial role. For examples, by applying a metal with a work function lower than Au, the built-in potential at the metal/photocatalyst can be increased. Noble metals with a suitable work function can help with the electron transfer, leading to higher photocatalytic activity. Additionally, Ag might perform with a higher localized electromagnetic field enhancement at a shorter wavelength than that of Au.
5. Conclusion
In this paper, the plasmonic enhanced photocatalytic water splitting, consisting of a CuxO photocatalyst layer and a plasmonic enhancing layer, is investigated. The plasmonic enhancing layer is a nanoporous Au film which can simultaneously provide surface-enhanced absorption and built-in potential. The reflection spectrum shows that the absorption wavelength of the nanoporous Au film can be modified by changing the annealing temperature. As a result, an 1.85 fold enhancement factor of the H2 evolution rate can be achieved compared to a CuxO film on a flat Au film.
Acknowledgments
The authors are grateful for the financial support of this research received from the National Science Council of Taiwan, R.O.C. under grant number NSC 101-2221-E-259-024-MY3, NSC 101-3113-P-002-021-, NSC 100-2923-M-002-007-MY3, NSC 101-2120-M-259-002 and NSC 101-2112-M-002-023.
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