Fabrication of Nanowire Arrays CuO-Al₂O₃-TiO₂ as p-Insulator-n Heterojunction for Photochemical Water Splitting

Abstract

CuO@TiO2 nanowires were prepared with the use of a porous alumina membrane (PAM). The conventional thermal chemical vapor deposition (CVD) method was used for the deposition of TiO2 by decomposing titanium isopropoxide (Ti(OiPr)4). The multilayer heterojunction is tested for possible photochemical water splitting application. The photo conversion properties of the CuO-Al2O3-TiO2 (p-insulator-n) heterojunction along with the microstructures and composition were characterized by Potentiostat, SEM and TEM, respectively. The shape of CuO nanowire arrays were hexagonal honeycombs and size was about 90 nm which depends on the uniform pore size of the PAM. The microstructural characterization showed that the nanosized CuO-Al2O3-TiO2 is a p-insulator-n heterojunction. The maximum photoconversion efficiency of 1.13 and 1.61% is obtained for CuO and CuO-Al2O3-TiO2 nanowire arrays junctions. An energy band diagram was introduced to explain the change in current during water splitting due to electron tunneling through the insulating layer.

1. Introduction

The use of TiO₂ for photoelectrochemical (PEC) water splitting applications has been attracted a great deal attention from many research groups [1, 2, 3, 4, 5, 6, 7, 8] since Fujishima et al. [9, 10] discovered that the hydrolysis of water in oxygen and hydrogen could be carried out by lighting anatase phase of titanium oxide (TiO₂). This is because TiO₂ has a wide band gap of ~3.2 eV, appropriate band-gap positions, strongly optical adsorption, and high chemical stability. However, the conversion efficiency of TiO₂ is usually limited, due to large band-gap energy, which means that UV irradiation is need for photocatalytic activity to occur. Since UV light accounts for only 5% of the sun’s energy compared to the 45% of visible, enhancing the photoconversion efficiency of TiO₂ has been a key objective of many research teams. H₂ has been used to treat TiO₂ in order to modify its optical absorption responsive band gap from UV to the visible wavelength [8, 11, 12], and narrow band gap semiconductor have also been used to avoid the problem of rapid electron-hole recombination [13, 14, 15, 16, 17]. For example, it is known that p-type copper oxide (CuO) can act as a hole conductor due to its narrow bandgap of only 1.4 eV. The use of copper oxides heterojunctions with TiO₂ have been reported for enhanced hydrogen generation, due to the resulting shift in the adsorption light wavelength [18].

p-n heterojunctions with various structures have been developed for photoconversion applications, resulting in high photocurrent efficiencies and H₂ generation [19, 20, 21, 22, 23, 24, 25, 26, 27]. The interface layers between p-type and n-type heterojunction semiconductors have significant effects on the resulting opto-electric properties, which are enhanced when the electrons at the conduction band (CB) were rapidly move from the n-type semiconductor to p-type semiconductor, or vice versa, which depends on the semiconductor band edge position. The interfaces of p-n heterojunction have recently been shown to have excellent photovoltaic properties, because the intermediate intrinsic layer prevents the injection of holes from the p-type semiconductor [28, 29]. Tian et al. [30] also indicated that p-type/intrinsic/n-type coaxial silicon nanowires have a high threshold voltage, as seen in the result of I-V characterization, due to the contributions from tunneling and avalanche mechanisms [31]. However, the influence of an insulator interface between p-n heterojunctions on the resulting water splitting behavior has not been examined.

An insulator layer between p-type and n-type semiconductors was prepared for this study by electrochemical deposition (ECD) and chemical vapor deposition (CVD), for the fabrication of the heterojunction structure. In the ECD process, the CuO/Al₂O₃ nanowire arrays were prepared with the use of template assistance process [32, 33]. The TiO₂ particles were deposited by a thermal CVD process. An energy band structural diagram is used to explain the I-V characteristics of the resulting, CuO/Al₂O₃/TiO₂ (p-insulator-n) heterojunction for water splitting applications.

2. Experimental methods

2.1 Preparation of porous alumina membranes (PAMs)

The porous alumina membranes (PAM) are prepared using high purity Al foils (99.9995%) with two step anodization process [34, 35]. The Al foil was degreased in ethanol, annealed in an argon, cleaned and electro-polished in a mixture of HClO₄-C₂H₅OH (1:4 vol %) at 10°C, with a current density of 100 mA/cm² for 1 min. The anodization was carried out in 0.3 sulfuric acid and 0.3 M oxalic acid at a constant voltage of 60 V using Pt foil as a counter electrode. After 3–6 h of anodization, the alumina film was selectively etched in a mixture of H₃PO₄-CrO₃-H₂O (2 g-3.5 mL-100 mL) at 70°C for 40 min. Afterwards second anodization was done under the same conditions for 18–24 h. The straight nano channel pore was obtained by dissolving the barrier layer using 5% H3PO4 solution at 60°C for 20 min.

2.2 Preparation of CuO/PAMs nanowire arrays

Before the deposition of the nanowire arrays, layers of Ti (10 nm) and Pt (100 nm) were coated onto one side of the membrane and stuck on the Cu substrate to serve as the working electrode (cathode). Pt foil (anode) and a saturated calomel electrode (SCE, 0.241V/NHE) served as the counter and reference electrode, respectively, in a three-electrode electrochemical deposition system. A potentiostat (263A, Princeton Applied Research) was used for electrochemical system to provide the potential. The PAMs/Ti/Pt/Cu were degassed using a mixture of 0.2 M CuSO₄ and 0.15 M H₃BO₃ solution with pH = 2 (controlled by H₂SO₄) and deposited at −0.18 V/SCE for 2 h. After electrochemical deposition, the Cu/PAMs nanowires arrays were heated in air at 400°C for 12 h to obtain the CuO/PAMs nanowires arrays.

2.3 Preparation of CuO@TiO₂ heterojunction structure by CVD

Thermal chemical vapor deposition (CVD) was used to grow the TiO₂ coatings on the CuO nanowires. Before the coating process, the CuO/PAMs compound was stuck on the FTO glass using silver paste, and the PAM was removed by 1 M NaOH at 50°C for 5 min. The CuO/PAMs sample was placed on the graphite holder in the quartz tube under reduced pressure of 10⁻⁶ Torr. The growth parameter was set at 500°C for 15 min. The precursor, titanium isopropoxide (Ti(OiPr)₄),was introduced to decompose the TiO₂ in the quartz tube at 35°C.

2.4 Characterization of the CuO/PAMs nanowires arrays and CuO@TiO₂

The CuO/PAMs was immersed into the 1 M NaOH solution at 50°C for 5 min, and then cleaned with deionized water several times before the SEM observation. The TEM sample for CuO@TiO₂ was dispersed in ethanol and subjected to ultrasonication for 10 min, then dropped on the Ni grid. To examine the photochemical response of CuO/PAM and CuO@TiO₂ p-insulator-n junction, sliver wire was stuck on the substrates, and this was then covered with a non-transparent and non-conductive epoxy-resin. The photochemical current-voltage was characterized using a potentiostat, while the counter and reference electrode were platinum and saturated calomel (SCE, 0.242V) in 1 M NaOH solution. Sunlight was simulated using a 150 W xenon arc lamp (1.5 AM filter, Oriel).

3. Results and discussion

3.1 Crystal structure and microstructure observation

Figure 1 presents the X-ray diffraction (XRD) spectra of the Cu/CuO nanowire arrays obtained after 400°C heat treatment for 12 h. It clearly shows the CuO peaks which correspond to the JCPDF card number 45-0937. The remaining Cu was due to the alumina template covering the surface of the Cu. The alumina template made it difficult for the oxygen to diffuse into it and to form CuO. Figure 2(A) presents the ordered pore array of AAO with 90–100 nm pore size obtained using two step anodination of Al foil under oxalic acid at 60 V [35]. The uniform shape of CuO nanowires were synthesized with the use of porous alumina membranes. Figure 2(B) shows the CuO nanowires after removing the PAM with NaOH solution. It is observed that the CuO nanowires were agglomerated and collapsed on both sides of AAO due to surface interaction. The SEM in Figure 2(C) represents the images of TiO₂ particles attached to the CuO nanowire after the CVD process. Figure 2(D) indicates the clear view of formation of nanosize CuO nanowires on PAM.

Figure 1.

XRD pattern of Cu/CuO after 400°C heat treatment for 12 h.

Figure 2.

SEM images of (A) porous alumina membranes, (B) CuO nanowire arrays, and (C) CuO@TiO₂ nanostructure, (D) clear view of nanosize CuO nanowire arrays.

Figure 3 shows the TEM, high-resolution TEM, and fast Fourier transformation (FFT) images of a CuO@TiO₂ nanowire. Figure 3A and C show that the particles cohered on the surface of nanowire. The d-spacings of the nanowire and particles were calculated as 0.242 nm and 0.446 nm, which confirm that the compounds were CuO (111) and TiO₂ (110), respectively. The high resolution TEM image shows that the CuO nanowire was a single crystal, and that its growth direction was (111). In addition, the CuO nanowire, CuO@TiO₂ and TiO₂ particles are also shown in the fast Fourier transformation images in Figure 3C–E. The impurity spots in Figure 3D show that the CuO nanowire was covered with TiO₂ particles. However, some indistinct regions can also be seen on the CuO nanowire in the HR-TEM and hollow ring in FFT images (Figure 3C and D), and this was due to the amorphous porous alumina membranes. Figure 3B shows the EDS analysis of a CuO@TiO₂ nanowire, which indicates the presence of the Al₂O₃ compound. This demonstrates that the insulator layer, i.e., PAM, was not completely removed during etching by 1M NaOH solution. The TEM and EDS results indicated that a p (CuO)-insulator (Al₂O₃)-n (TiO₂) junction was successfully fabricated.

Figure 3.

TEM images of a CuO@TiO₂ nanowire (A) at low magnification, (B) EDS analysis of a CuO@TiO₂ nanowire, (C) high resolution, and fast Fourier transformation images, (D) CuO/PAMs, (E) CuO@TiO₂, and (F) TiO₂ particles produced by thermal CVD.

3.2 Water splitting properties of CuO/Al₂O₃/TiO₂ p-insulator-n heterojunction

According to the water splitting theory, the optimum energy bandgap of the semiconductor should be in the range of 1.7–2.0 eV and, the band edges should be straddle water potential of 1.23 V/NHE. However, the photon adsorption of CuO is 1.4 eV, which do not provide enough energy to split water into hydrogen and oxygen. The wide band gap of TiO₂ not only provides enough band edges to straddle the H₂O reduction potentials but forms the donor level in the forbidden band to enable electron transition [13]. The photoelectrochemical and photocurrent characterization of the CuO/PAMs nanowire arrays and CuO@TiO₂ nanostructure by thermal CVD were carried out based on their current density and voltage properties under dark and UV illumination. Figure 4(A) presents the current density measurement of nanowire arrays with various applied potential under 150 W UV illumination. The CuO/PAMs nanowire arrays indicates p-type semiconductor behavior due to the signature of negative current in the figure. There is also a current density peak between 0.2 and 0.7 V/SCE due to the oxidation of the remaining Cu to Cu²⁺ as the potential increases to +1 V/SCE.

Figure 4.

(A) Variation of current density versus applied potential measured using 150 W UV illuminations (AM 1.5 filter) and (B) photocurrent density vs applied potential.

Figure 4(B) reveals the photochemical response of CuO@TiO₂ nanowire arrays under 100 mW/cm² simulated solar illumination. The threshold voltages (V_th) of the CuO/PAMs and CuO@TiO₂ samples were 0.67 and 0.89 V/SCE, while the maximum photocurrent densities were 1.12 and 1.83 mA/cm² respectively.

In order to examine the photoconversion efficiency (η) of each sample, the efficiency equation is expressed as follow [36]:

$$\eta \left(\%\right)=\frac{J_{ph}\left[\left(1.23-\left|V_{app}\right|\right)\right]}{I_0}\times 100$$E1

Where I₀ is the intensity of the incident light (100 mW/cm², V_app is the applied potential, V_mea is the measured potential (obtained with a potentiostat, and V_oc is the open circuit potential. Figure 5 presents the photo conversion efficiency of CuO/PAMs and CuO@TiO₂ samples. The conversion efficiency of 1.61% at 0.8 V/SCE, was observed for CuOs@TiO₂ heterojunction which is higher than that of 1.13 % at 0.67 V/SCE for CuO/PAMs.

Figure 5.

Plot of photoconversion efficiency of versus applied potential of Cu/PAM and CuO@TiO₂ nanowire arrays.

3.3 Interface behavior of the CuO/Al₂O₃/TiO₂ (p-insulator-n) heterojunction

Based on the photochemical properties outlined above, the results for the CuO@TiO₂ sample show that it has better performance than the CuO/PAMs sample [37]. Bandara et al. [13] reported that in a CuO-TiO₂ water system the electrons and holes are generated on the conduction band (CB) and valence band (VB) when the TiO₂ is illuminated. The rapid electrons transfer from the CB of TiO₂ to the CB of CuO occurs because the CB position of CuO is lower than that of TiO₂. The threshold voltage (V_th) of the ideal CuO-TiO₂ p-n junction should be set at 0 V/SCE. However, in our study, the photo response of CuO@TiO₂ appeared at a relatively high V_th (0.9 V/SCE). This was due to the Al₂O₃ insulation layer between the CuO and TiO₂ interlayers, which formed the heterojunctions, as shown in Figure 6. The band gaps of CuO, Al₂O₃, and TiO₂ are 1.4, 8.8, and 3.2 eV, respectively. In this heterojunction, insulator (Al₂O₃) was used as an electron blocking layer. In order to balance Fermi level, (the interface between the p- and n-type) insulators started to bend when applying a bias due to the high band gap of this layer. This caused electrons to accumulate on the barrier layer before the breakdown potential was reached. When the applied potential was increased to the breakdown level, at ~0.6 V/SCE, the rapidly increasing current was caused by electron tunneling.In this CuO-Al₂O₃-TiO₂ heterojunction, the conduction band edge of CuO is more negative than TiO₂. Upon illumination the photogenerated electrons (e⁻) and holes (h⁺) are produced at the active sites of the CuO-TiO₂ photocatalyst. The electrons in p-type CuO easily flow to the conduction band of TiO₂, whereas the photogenerated holes migrate in the opposite direction. CuO serves as electron reservoir by receiving electrons from TiO₂, which suppresses the recombination of e⁻/h⁺ and transfers the received electron.

Figure 6.

Energy band gap structural diagram of CuO-Al₂O₃-TiO₂ heterojunctions.

4. Application

CuO based photoelectrodes have great potential for applications in photo electrochemical (PEC) water splitting which give useful information on factors that affect the photo electrochemical capability. Modifying the CuO structure revealed a significant influence on the enhancement of the photocurrent density and photostability. Further, copper containing TiO₂ is cost-effective compared with noble metal loaded TiO₂, for the photocatalytic hydrogen production. The CuO-TiO₂ oxide system have promising photoactive optical and photocatalytic properties, as compared with TiO₂ or CuO alone. CuO and TiO₂ based junction have better charge mobility at the proximity to the junction of the electrode/electrolyte within the nanostructure. CuO-Al₂O₃-TiO₂ nanocomposites represent the synergy effect for hydrogen production. This nanocomposites junction is very good for design of efficient catalyst for selective alcohol conversion to other valuable chemicals. The p-i-n heterojunctions with suitable band edge positions has the improved separation of the photogenerated charge which improved the photocatalytic activity. This junction has application for improved photocurrent behavior, the H₂ generation rate by water splitting in a full PEC device (without application of a bias) and the solar-to-hydrogen efficiency.

5. Conclusions

CuO@TiO₂ nanostructures were fabricated by a template assisted (PAMs) nanowires (CuO) growth and CVD process (TiO₂). Observations of the microstructure show that the CuO@TiO₂ nanostructure was composed of p-insulator-n heterojunctions. The I-V characteristics of the CuO/PAMs and CuO@TiO₂ nanostructures show high threshold voltages of 0.67 and 0.89 V/SCE, with photoconversion efficiencies of 1.13% and 1.61%, respectively. The energy band structure diagram indicates that the rise in current during water splitting is due to electron tunneling through the insulation layer (Al₂O₃).

Acknowledgments

This work was supported by the National Science Council of Taiwan under grant NSC 101-2221-E-006-129.

Author contributions statement

DRS, SCW, JLH conceived the experiment(s), YMS conducted the experiment(s) and characterization. YMS and DRS wrote the manuscript. YMS, DRS and SCW analyzed the results.

All authors reviewed the manuscript.

Competing financial interests

The authors declare no competing financial interests.