A carbon nanowire-promoted Cu₂O/TiO₂ nanocomposite for enhanced photoelectrochemical performance

Abstract

The use of semiconductors as photoelectrochemical electrodes and photocatalysts has been studied extensively over the last few decades; however, it is challenging to design one single material meeting a wide range of requirements for applications such as photostability, wide light absorbance and earth abundance. In this study, we started with earth-abundant materials, namely TiO₂, Cu₂O and carbon, to design a nanocomposite with TiO₂ nanofibers (NFs) and Cu₂O nanocubes (NCs) connected by carbon nanowires (NWs). The nanocomposite (C/TiO₂/Cu₂O) demonstrated excellent photo(electro)activity under simulated visible light as well as enhanced durability. Morphologies imaged by electron microscopes showed that TiO₂ NFs and Cu₂O NCs were well connected, while thin and long carbon NWs were present throughout the samples and connected the NFs and NCs to form a compact composite. Electrochemical analysis revealed that the band alignment between TiO₂ and Cu₂O was typical for a type-II heterojunction, which is favourable for the separation of photogenerated charge carriers. Moreover, carbon NWs act as bridges, thus facilitating more efficient charge transfer. Overall, this research showed the design and preparation of a nanostructured composite with improved charge transfer and photostability for solar photoelectrochemical applications.

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1. Introduction

Technologies based on photon and semiconductor interactions, e.g. photoelectrochemical and photocatalysis water splitting, have become attractive research areas in recent decades because they can offer a sustainable roadmap towards clean fuels and the environment. An important factor for advancing the research is to obtain catalytic/electrode materials with suitable properties, such as wide light absorbance, photostability, high efficiency, environment friendliness and earth abundance. An extensive range of materials have been studied over the last few decades,^1–10 but each has its own limitations.^11–15

Titanium dioxide (TiO₂) is a well-known stable n-type semiconductor used over the last few decades with a large band gap of 3.2 eV;¹¹ thus, it is mainly active in the ultraviolet (UV) region, which is only 4% of the solar spectrum. Efforts have been devoted to expanding the spectral response of TiO₂ in the visible light region using various band engineering methods, such as surface modification, metal doping, creating heterojunction, co-catalyst loading and coupling with narrow band gap semiconductors.¹⁶

A highly attractive narrow band gap semiconductor, Cu₂O, is found in abundant, low-cost and non-toxic p-type semiconductor with a band gap between 2.0 and 2.2 eV.¹⁷ However, Cu₂O is prone to photocorrosion, which is attributed to the oxidation and reduction potential of Cu₂O lying within the band gap.¹⁸

TiO₂–Cu₂O as a binary heterojunction/composite has demonstrated synergistic photoelectrochemical and photocatalytic performance based on efficiency and stability.¹⁹ However, there are inevitable defects or poor connections at the interface between the two different components, which act as charge carrier sinks and hinder efficient charge carrier migration. A strategy to improve the interface and provide better connections between the two different components is highly desirable to fully unleash the synergy between TiO₂ and Cu₂O, thus providing an ideal catalyst for photoelectrochemical and photocatalytic applications.

There are attempts to utilise carbon nanomaterials in composite photoelectrochemical electrode materials and photocatalysts as charge transfer promoters and structure stabilisers. For example, carbon nanowires (NWs) are known to have excellent mechanical and electron transport properties.²⁰

In addition to the physical and chemical properties of TiO₂ and Cu₂O, their structural properties (e.g. morphology, surface area and crystallinity) also play vital roles in their photoelectrochemical and photocatalytic properties. In our previous study, it is already revealed that high crystalline TiO₂ nanofibers (NFs) and Cu₂O nanocubes (NCs) can offer optimised charge transfer efficiency and photostability compared to other structures.^19,21–23

In this study, we designed a nanocomposite with TiO₂ NFs and Cu₂O NCs, which was further connected and enhanced by carbon NWs. The synthesis procedure was simple and required inexpensive solvents and operating conditions. The photoelectrochemical (PEC) performance of the ternary C-NW/TiO₂ NF/Cu₂O composite showed improved charge carrier separation, superior PEC activity and enhanced stability.

2. Experimental

2.1. Synthesis of C-NW/TiO₂ NF

A single crystalline 1D TiO₂ NF was fabricated as described in our previous study via topotactic transformation by ion-exchange and dehydration.²² Subsequent C-NW/TiO₂ NF was prepared by a hydrothermal synthesis using gelatine as the carbon precursor as it is a cheap polymer obtained from animal skin, bones and tissues.²⁴ It is known to have strong coating interactions with metal nanoparticles.²⁵ Gelatine was dissolved in a mixture of TiO₂ NFs in 20 ml water and stirred at 40 °C. The sample was placed in a stainless-steel autoclave and heated at 250 °C for 4 h. After the hydrothermal treatment, the sample was centrifugated at 7000 rpm for 10 min with several washes of water and dried in the oven. Gelatine is abundant in amino and carboxyl groups, so it can be hydrothermally treated with only pure water. Hence, it is an inexpensive water-soluble polymer. An advantage of this procedure is that no strong acid or surface passivation reagent or post-treatment is needed to fabricate C-NW/TiO₂ NFs.²⁶

2.2. Synthesis of C-NW/TiO₂ NF/Cu₂O

C-NW/TiO₂ NF/Cu₂O was synthesised by solution phase chemistry under ambient conditions, as described previously.¹⁶ The TiO₂ to Cu₂O ratio was targeted at 1 : 1 by controlling the dosages of corresponding precursors in the synthesis process. In a typical process, PEG was ultrasonicated in 100 ml of water for 30 min. C-NW/TiO₂ NF was mixed with 0.2 M CuCl₂ and 0.01 M PEG-600 before ultrasonication for 10 min. This mixture was then heated to 50 °C, forming a blue solution. 2 M NaOH was poured into the blue solution and 0.2 M N₂H₄·H₂O was added dropwise in the stirred mixture to generate Cu(I) ions and further heated for 10 min. The resulting mixture was purged in N₂ to form C-NW/TiO₂ NF/Cu₂O, followed by centrifugation at 7000 rpm with several washes with H₂O and ethanol to remove any residual PEG.

2.3. Characterisation methods

TEM images were taken at 200 kV on a JEM-2100Plus microscope (Warwick University, UK). Samples were prepared by ultrasonication and drop-casted on a Cu grid coated with a carbon film. Powder X-ray diffraction (XRD) was operated on a Bruker-AXS D8 ADVANCE diffractometer at 40 kV, 40 mA and Cu Kα radiation (λ = 0.15418 nm) between 2θ 10 and 80° in 0.02° steps. Brunauer–Emmett–Teller (BET) surface areas were calculated at 77 K by N₂ physisorption using a Quantachrome NOVA 4000e porosimeter on degassed samples at 120 °C for 4 h. Diffuse reflectance UV-Vis Spectra (DRUVS) were recorded on a Thermo Scientific Evo220 Spectrometer using KBr as a standard in an absorbance range between 200 and 700 nm. The ratio of TiO₂ to Cu₂O in TiO₂ NF/Cu₂O and C-NW/TiO₂ NF/Cu₂O samples was quantified by ICP-OES (Optima2000DV, USA).

2.4. Photoelectrochemical methods

A homogenous colloid was formed within 30 min of sonication of the as-prepared samples (5 mg), and Nafion (10 μL, 5 wt%) was dispersed in a water/ethanol mixture (1 ml, 3 : 1 v/v). 5 μL colloid was deposited on a glassy carbon electrode (3 mm dia.) used as the working electrode. A Pt wire was used as the counter electrode, and Hg/Hg₂SO₄ was used as the reference electrode in a three-electrode photoelectrochemical cell. The electrolyte (0.5 M Na₂SO₄) was purged in N₂ for 30 min. A 200 W Hg–Xe arc lamp (Oriel Instruments 66002, with UV cutoff filter, λ > 420 nm) was used as the light source. The light intensity was adjusted and kept at 100 mW cm⁻² for all experiments. Measurements were performed using Autolab potentiostat with Nova software. Mott–Schottky plots were recorded under DC polarisation at a potential range of −1 to 0 V with a potential step of 10 mV at a frequency of 1000 Hz. Nyquist plots were recorded under an AC signal of 10 mV at a frequency range of 100 kHz–0.1 Hz.

3. Results and discussion

TEM and SEM images in Fig. 1 show the morphology and size of the individual components and C-NW/TiO₂ NF/Cu₂O as a nanocomposite. Fig. 1a demonstrates the 1D TiO₂ NF with lengths between 500 nm and 5 μm and diameters between 100 and 200 nm. Fig. 1b illustrates the Cu₂O nanocubes with sizes ranging between 150 and 250 nm. After the hydrothermal synthesis, amino acids were carbonised to form carbon NWs, and thus the ternary Cu₂O, C-NW and TiO₂ nanocomposite was formed, as shown in Fig. 1c and d. In these two images, cubic Cu₂O, TiO₂ NF and C-NW were all clearly visible. TiO₂ NFs were intimately in contact with Cu₂O nanocubes, with C-NWs occupying the spaces and further promoting the Cu₂O–TiO₂ connections.

Fig. 1 TEM images of (a) TiO₂ NF, (b) Cu₂O nanocubes, (c) C-NW/TiO₂ NF/Cu₂O, and (d) SEM images of C-NW/TiO₂ NF/Cu₂O.

XRD patterns confirmed the characteristics of Cu₂O and TiO₂ peaks in Fig. 2a, indicating that the heterojunction nanocomposite consists of Cu₂O and TiO₂, which is in good agreement with the TEM and SEM images. The diffraction peaks for TiO₂ NF were well defined and corresponded to the crystal planes of (101), (004), (200), (105), (211) and (204) at 2θ = 25.3°, 37.8°, 48.0°, 53.8°, 55.4° and 62.5°, respectively, which confirmed anatase TiO₂ NF (tetragonal, JCPDS 21-1272).²⁷ The XRD patterns illustrated the formation of crystalline Cu₂O by 2θ peaks at 29.6°, 36.4°, 42.3°, 61.4°, 73.5° and 77.4°, which are associated with characteristic (110), (111), (200), (220), (311), and (222) peaks of Cu₂O, respectively (cubic, JCPDS 73-0687).²⁸ The ratio of TiO₂ to Cu₂O in TiO₂ NF/Cu₂O and C-NW/TiO₂ NF/Cu₂O samples was quantified as 0.9 : 1 by ICP-OES. There were weak traces of CuO in both Cu₂O and TiO₂ NF/Cu₂O, which is in agreement with JCPDS 45-0937²⁹ but not observed in the C-NW/TiO₂ NF/Cu₂O sample. There is a weak but a broad pattern at 25°, suggesting either a low concentration of C-NWs or the amorphous carbon.

Fig. 2 (a) XRD patterns of (i) Cu₂O, (ii) TiO₂ NF, (iii) TiO₂ NF/Cu₂O and (iv) C-NW/TiO₂ NF/Cu₂O. (b) TGA and (c) Raman spectrum of C-NW/TiO₂ NF/Cu₂O. (d) N₂ adsorption–desorption isotherms of TiO₂ NF, Cu₂O nanocubes, TiO₂ NF/Cu₂O, C-NW/TiO₂ NF/Cu₂O.

The carbon content in C-NW/TiO₂ NF/Cu₂O was investigated by thermogravimetric analysis (TGA), as shown in Fig. 2b. There are three significant weight losses at 100 °C, 200 °C and 400 °C, respectively. The initial loss for the samples at 100 °C was due to the adsorbed moisture (2.1%), while the 2.9% weight loss until 200 °C was attributed to the removal of residual PEG, which was the structuring agent used to fabricate Cu₂O nanocubes. The weight loss above 200 °C was 4.3%, which accounted for the total loss of C-NWs in the heterojunction. Different C-NW contents (i.e., in the range from 2% to 10%) were prepared. Within this range, there was no clear difference in various photoelectrochemical performances. This 4.3% sample was presented as a representative. Fig. 2c illustrates the nature of carbon in C-NW/TiO₂ NF/Cu₂O using Raman spectroscopy. The Raman spectrum shows two peaks of 1370 cm⁻¹ and 1593 cm⁻¹ for the D and G bands, respectively, which is characteristic of amorphous carbon.

Surface areas varied between the different samples (Fig. 2d), with Cu₂O nanocubes having the lowest BET surface area of 22.9 m² g⁻¹. TiO₂ NF exhibited the highest BET surface area of 93.5 m² g⁻¹. Binary TiO₂ NF/Cu₂O had a surface area of 53.8 m² g⁻¹ reasonably between the two individual components. C-NW/TiO₂ NF/Cu₂O had a higher BET surface area than TiO₂ NF/Cu₂O, which was due to the integration of C-NWs as it has a high surface area. This may have further implications for photoelectrochemical performance because C-NWs may act as an electron anchor by improving electron mobility and transportation in the heterojunction.^30,31

The optical properties of different samples were analysed and presented in Fig. 3, whereby the absorbance spectra ranged from 200 to 600 nm (Fig. 3a), consistent with previously reported.³²

Fig. 3 (a) DRUVS and (b) Tauc plots of TiO₂ NF, Cu₂O, TiO₂ NF/Cu₂O and C-NW/TiO₂ NF/Cu₂O.

The optical band gaps E_BG were calculated from the Tauc plots (Fig. 3b) using eqn (1).

αhv = C(hν − E_BG)ⁿ (1)

where C is the proportionality constant and ‘α’ is the absorption coefficient determined from the Kubelka–Munk formula (eqn (2)).The direct band gaps of TiO₂ NF, Cu₂O, TiO₂ NF/Cu₂O and C-NW/TiO₂ NF/Cu₂O were 3.28, 2.36, 2.38 and 2.21 eV, respectively, as presented in Table 1. The tested samples containing Cu₂O absorbed light in the visible region at the wavelength of 650 nm. Two absorbance peaks are present at 300 nm and 480 nm, confirming both components were present in the system, which is in agreement with a previous report.³³ C-NW/TiO₂ NF/Cu₂O showed a red shift to TiO₂ NF/Cu₂O, which indicates C-NWs have strong visible light absorption properties. It may also promote interfacial contact between C-NWs and the two semiconductors.

Table 1 Photophysical properties of TiO₂ NF, Cu₂O, TiO₂ NF/Cu₂O and C-NW/TiO₂ NF/Cu₂O

Sample	BET surface area^a (m^2 g^−1)	Band gap^b (eV)
a N2 porosimetry. b DRUVS.
TiO2 NF	93.5	3.28
Cu2O	22.9	2.36
TiO2 NF/Cu2O	53.8	2.38
C-NW/TiO2 NF/Cu2O	66.9	2.21

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The effect of C-NW enhancement on the interfacial charge transfer between the TiO₂ NF–Cu₂O heterojunction was investigated using PEC measurements under chopped visible light illumination at 0 V vs. RHE. PEC measurements were studied in a three-electrode system, with Pt wire as the counter electrode, Hg/Hg₂SO₄ as the reference electrode, and different samples were drop-casted onto a glassy carbon electrode as the working electrode. Fig. 4a shows C-NW/TiO₂ NF/Cu₂O > TiO₂ NF/Cu₂O > Cu₂O > TiO₂ NF in terms of photoresponse. In order to enable comparison with other reported and relevant TiO₂-based materials, the widely used benchmark P25 TiO₂ was also tested, showing photoresponse similar to TiO₂ NF (data did not show as it overlapped with that of TiO₂ NF), which was consistent with our previous study.¹⁹ The low photocurrent density of TiO₂ NF was attributed to poor visible light response, and Cu₂O had photostability issues during charge transfer. The photocurrent density of C-NW/TiO₂ NF/Cu₂O was approximately 6 times higher than TiO₂ NF/Cu₂O and above the factor of 8 times higher than both Cu₂O and TiO₂ NF. The C-NW promoted the heterojunction nanocomposite produced a higher photocurrent response, indicating more efficient charge transfer, where C-NWs can act as a carrier mobility layer at the TiO₂ NF/Cu₂O interface.³⁴ Cu₂O was difficult to use in practical applications as it has low photostability; therefore, the impact of C-NWs on catalyst stability (Fig. 4b) was studied. The photocurrent generated by Cu₂O decreased at a faster rate than C-NW/TiO₂/Cu₂O over a time period of 24 hours, which suggests that the addition of TiO₂ NF and C-NWs increased photostability. The advantage of C-NW as a conductive network was more profound when we consider that the samples were dropcasted on a glassy carbon electrode. The sample particles formed a dense layer with the C-NW conductive network embedded, which could not only promote the charge mobility but also enhance the Cu₂O stability.

Fig. 4 (a) Transient photocurrent and (b) photostability (0 V vs. RHE) of TiO₂ NF, Cu₂O, TiO₂ NF/Cu₂O and C-NW/TiO₂ NF/Cu₂O under visible light irradiation. (c) I–V curve and (d) chopped I–V curves of TiO₂ NF, Cu₂O, TiO₂ NF/Cu₂O and C-NW/TiO₂ NF/Cu₂O under visible light irradiation at a scan rate of 10 mV s⁻¹, (reaction conditions: 200 W Hg–Xe arc lamp, 0.5 M Na₂SO₄ electrolyte).

Bare TiO₂ NF showed a low photocurrent density vs. voltage curves (Fig. 4c), but Cu₂O nanocubes exhibited a higher photocurrent density under visible light due to its narrow band gap. With the introduction of the heterojunction nanocomposite, TiO₂ NF/Cu₂O, the photocurrent density increased to 60.3 μA cm⁻² at 1.23 V vs. RHE. The integration of C-NWs yielded a significant increase in the photocurrent density of 139.8 μA cm⁻², which was almost 4 times higher than TiO₂ NF. The sharp increase in the photocurrent density indicates high charge transfer and separation efficiency with C-NWs in C-NW/TiO₂ NF/Cu₂O.³⁵ The results were similar in the chopped photocurrent density vs. voltage curve, as shown in Fig. 4d, where C-NW/TiO₂ NF/Cu₂O reached photocurrent densities of 122.4 μA cm⁻² at −0.3 V vs. RHE, which was 2 times higher than TiO₂ NF/Cu₂O.

The interfacial properties of C-NW/TiO₂ NF/Cu₂O were further studied by electrochemical impedance spectroscopy (EIS) in the form of a Nyquist plot. A Nyquist plot was used to investigate the characteristics of the charge transfer process, whereby the semicircle diameter indicates the charge transfer resistance. In Fig. 5a, the semicircle diameter of Cu₂O and TiO₂ NF is slightly larger than that of TiO₂ NF/Cu₂O, which shows they have similar charge transfer resistance. C-NW/TiO₂ NF/Cu₂O exhibited the lowest charge transfer resistance, which is attributed to the integration of C-NWs. This indicates that C-NWs reduced the resistance to charge carrier transport due to the efficient separation of charge carriers in the nanocomposite.³⁵Fig. 5b and c show the Mott–Schottky plots of TiO₂ NF, C-NW/TiO₂ NF and Cu₂O, which were utilised to calculate the flat band potential and majority carrier densities of each semiconductor in the heterojunction. The positive and negative slopes are indicative of n-type and p-type semiconductors, respectively. This type of p–n heterojunction is a favourable structure for enhanced charge separation.³⁶ The flat band potentials are shown by the x-intercept of the linear section, which are −0.76 V, −0.72 V and 0.54 V, vs. RHE for TiO₂ NF, C-NW/TiO₂ NF and Cu₂O, respectively. The anodic shift of 40 mV between TiO₂ NF and C-NW/TiO₂ NF is in agreement with previous reports.³⁷

Fig. 5 (a) EIS (Nyquist plot at 0 V vs. RHE) of TiO₂ NF, Cu₂O, TiO₂ NF/Cu₂O and C-NW/TiO₂ NF/Cu₂O under visible light irradiation; Mott–Schottky plot of TiO₂ NF (b) and Cu₂O (c); (200 W Hg-Xe arc lamp, 0.5 M Na₂SO₄ electrolyte); (d) Schematic for the energy band structure of C-NWs/TiO₂/Cu₂O nanocomposite.

The gradient of the linear portion of the plot was used to calculate the majority carrier density (N_A) calculated from eqn (3).

where C is the capacitance, ε is the dielectric constant of TiO₂ and Cu₂O, which is 170³⁸ and 7.60,³⁹ respectively, ε₀ is the permittivity of free space, V. is the applied potential, E_fb is the flat band potential, e is the electron charge, k_B is the Boltzmann's constant and T is the temperature. The N_A values of Cu₂O, TiO₂ NF and TiO₂ NF/C-NWs were 5.36 × 10¹⁸ cm⁻³, 1.06 × 10¹⁹ cm⁻³ and 1.30 × 10¹⁹ cm⁻³, respectively. This showed that 1D TiO₂ NF had more charge carrier activity, but more studies are needed to gain conclusive results.

The valence band edge of Cu₂O, E_V, was calculated from eqn (4):

where the effective density of states, . The effective hole mass is denoted as m_h = 0.58m_e for Cu₂O,⁴⁰ where m_e is the mass of a free electron. N_V for Cu₂O was 1.10 × 10¹⁹ cm⁻³. The valence band edge (E_V) was 0.56 V for Cu₂O. Thus, as Cu₂O has a band gap of 2.36, the valence and conduction band positions were at 0.56 V and −1.80 V vs. RHE, respectively. Assuming the distance between the flat band potential of TiO₂ NF and the lowest potential of the conduction band (CB) is negligible,⁴⁰ the CB of TiO₂ NF was −0.76 V vs. RHE. Thus, the band gap of TiO₂ NF was 3.28, so the valence band and conduction band were positioned at 2.52 V and −0.76 V vs. RHE, respectively.

A schematic of the energy band structure is proposed in Fig. 5d. The enhanced photoelectrochemical performance of C-NW/TiO₂ NF/Cu₂O nanocomposite was attributed to a few reasons. As Cu₂O has a narrow bandgap energy of 2.36 eV, it can be excited by visible light. The conduction band of TiO₂ lies at a more positive potential than that of Cu₂O, and the electrons from Cu₂O can inject into the conduction band of TiO₂. This prevents the recombination of photogenerated electrons and holes in Cu₂O.⁴¹ Accumulated electrons in the conduction band of TiO₂ can then transfer to the glassy carbon electrode.⁴² The addition of C-NWs further broadens the solar absorption to the visible light region. Also, the impedance spectra show that the C-NW/TiO₂ NF/Cu₂O nanocomposite has improved charge carrier density and least charge transfer resistance compared to bare TiO₂ NF and Cu₂O. This suggests that the efficient charge transfer was further assisted by the increased conductivity with C-NWs. The addition of C-NWs improved the charge carrier density; so it is proposed that the C-NWs promote electron transfer between CB of Cu₂O and CB of TiO₂, enhancing the separation of photogenerated electron-hole pairs. Overall, the introduction of C-NWs in the nanocomposite of C-NWs/TiO₂/Cu₂O enhances the PEC performance, increases charge transport, and the nanocomposite increases photostability and extends the light absorption region.

4. Conclusions

In summary, we fabricated a C-NW/TiO₂ NF/Cu₂O heterojunction nanocomposite via a hydrothermal treatment and subsequent solution phase method, which provided an intimate contact between 1D TiO₂ NF and Cu₂O nanocubes surrounded by C-NWs. The carbon content in the prepared samples was confirmed by both TGA and Raman spectroscopy, and DRUVS indicated that C-NWs have strong absorption properties as there was superior absorption at higher wavelengths. C-NW/TiO₂ NF/Cu₂O recorded enhanced PEC activity with higher photocurrent density, and impedance spectra illustrated less charge transfer resistance than bare TiO₂ NF and Cu₂O. After calculating the charge carrier densities of TiO₂ NF and C-NW/TiO₂ NF, it was shown that the addition of C-NWs improved the charge carrier density by promoting electron transfer. Therefore, C-NW/TiO₂ NF/Cu₂O provided superior light absorption properties, efficient charge transfer, separation of electron–hole pairs and the integration of C-NWs, which promote further electron transport within the heterojunction nanocomposite. All these properties contribute towards fabricating a nanocomposite with superior PEC activity. This work is an example of designing and fabricating effective heterojunction nanocomposite materials for solar water splitting.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This research was supported by the European Commission H2020 Marie S Curie Research and Innovation Staff Exchange (RISE) award (Grant No. 871998).

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