Studies on the substrate-dependent photocatalytic properties of Cu₂O heterojunctions

Abstract

Cu₂O is a promising material for photocatalysis because of its absorption ability in the ultraviolet (UV)-visible light range. Cu₂O deposited on conductive Ti and fluorine-doped tin oxide (FTO) substrates behaves as a photocathode. Cu₂O deposited on an n-type semiconductor such as TiO₂ nanotube arrays (TNA)/Ti behaves as a photoanode and has demonstrated better photocatalytic activity than that of TNA/Ti. The substrate-dependent photocatalytic properties of Cu₂O heterojunctions are not well studied. In this work, the photocatalytic properties of a Cu₂O/TNA/Ti junction as a photoanode and of Cu₂O/Ti and Cu₂O/FTO junctions as photocathodes without bias were systematically studied to understand their performance. The Cu₂O/TNA/Ti photoanode exhibited higher photocurrent spectral responses than those of Cu₂O/Ti and Cu₂O/FTO photocathodes. The photoanodic/photocathodic properties of those junctions were depicted in their energy band diagrams. Time-resolved photoluminescence indicated that Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO junctions did not enhance the separation of photogenerated charges. The improved photocatalytic properties of Cu₂O/TNA/Ti compared with TNA/Ti were mainly attributed to the UV-visible light absorption of Cu₂O.

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1. Introduction

Metal oxide semiconductors have attracted extensive interest because they are potentially low cost and environmentally friendly materials for photocatalytic applications. Cuprous oxide (Cu₂O), one of the metal oxide p-type semiconductors, has a direct band gap energy between 2.0 eV and 2.6 eV.^1,2 At pH = 7.0, its conduction band (−1.35 V vs. Ag/AgCl) lies at more negative potential than the reduction potential of H⁺ to H₂ (−0.61 V vs. Ag/AgCl), and its valence band (0.65 V vs. Ag/AgCl) lies at more positive potential than the oxidation potential of H₂O to O₂ (0.62 V vs. Ag/AgCl).^3,4 Cu₂O is able to absorb ultraviolet (UV)-visible light and has some tremendous characteristics, such as nontoxic nature, abundant availability, and low fabrication cost.⁵ With the appropriate positioning of energy bands and these advantages, Cu₂O is a potential candidate for photocatalyst that can generate H₂ through water splitting and degrade organic pollutants in wastewater.^6–8 Cu₂O behaves as a catalyst under dark conditions and photocathode under illumination.^4,9 Typically, Cu₂O is grown on conductive substrates, such as Cu,¹⁰ Ti,^11,12 Al,¹⁰ fluorine-doped tin oxide (FTO),^12–14 and indium tin oxide (ITO),^4,13 and can still behave as a photocathode.

To further advance this promising photocatalytic material, Cu₂O has been used as top layers on various n-type metal oxide semiconductors, such as TiO₂,¹⁵ ZnO,¹⁶ WO,^3,17 and g-C₃N₄,¹⁸ to form heterojunctions and enhance photogenerated charge separation. These junctions demonstrate better photocatalytic activity than that of bare Cu₂O. Cu₂O combined with TiO₂ is an attractive composite because of its matching energy-level alignments that thermodynamically promoting the transfer of photogenerated electrons from Cu₂O to TiO₂.¹⁹ TiO₂ is the most popular n-type metal oxide semiconductor in photocatalysis, that has a wide band gap energy (approximately 3.0 eV for rutile²⁰ and 3.2 eV for anatase²¹) and several beneficial characteristics, such as chemical stability, corrosion resistance, environmental friendliness, abundance, and cost effectiveness.²² However, the solar energy conversion efficiency of TiO₂ is limited by UV light absorption. To date, TiO₂ nanotube arrays (TNA)/Ti has been favoured because of its higher surface-area-to-volume ratio.^23,24 Cu₂O electrodeposited on TNA/Ti (Cu₂O/TNA/Ti) has been applied in the photocatalytic degradation of pollutants, such as methyl orange (dye),²⁵ CO₂ (gas),²⁶ ibuprofen (drug),^27,28 and ciprofloxacin (drug).²⁹ To this end, Cu₂O/TNA/Ti is more effective than TNA/Ti. Contrary to the Cu₂O-functionalized conductive substrates, which are utilized as photocathodes, Cu₂O/TNA/Ti frequently acts as a photoanode. However, the substrate-dependent photocatalytic properties of p-Cu₂O heterojunctions are not well studied, and the heterojunctions are a critical issue in photocatalysis. Moreover, the Cu₂O/TNA heterojunction is very important for photocatalysis. It is considered that the heterojunction enhance the separation of photogenerated charges.^25–29 In fact, the enhancement of the charge separation at Cu₂O/TNA junction to assist in photocatalysis has yet to be verified.

In this study, the photocatalytic properties of Cu₂O/TNA/Ti were compared with those of Cu₂O/Ti and Cu₂O/FTO. A p-Cu₂O was deposited on different substrates, i.e., n-TNA/Ti, Ti, and n-FTO. The results revealed that the Cu₂O/TNA/Ti photoanode exhibited higher photocurrent spectral responses, which obviously outperformed the Cu₂O/Ti and Cu₂O/FTO photocathodes. The photoanodic current of Cu₂O/TNA/Ti was twice as high as that of TNA/Ti. The higher photocatalytic performance of Cu₂O/TNA/Ti over other junctions was verified by the degradation of rhodamine 6G (R6G). The results of time-resolved photoluminescence (TRPL) for both the TNA and Cu₂O sides of Cu₂O/TNA/Ti revealed that the junction of Cu₂O/TNA did not enhance the charge separation. Both Cu₂O/Ti and Cu₂O/FTO did not enhance the charge separation from TRPL measurement, either. The improved photocatalytic properties of the Cu₂O/TNA/Ti over TNA/Ti alone were mainly ascribed to the UV-visible light absorption of Cu₂O.

2. Results and discussion

2.1 Morphology, crystalline phase, and optical absorption

The morphologies of TNA/Ti, Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO are shown in Fig. 1. A nanotube morphology of TNA with an outer diameter of approximately 90 nm could be clearly observed, as depicted in Fig. 1(a). The cross-sectional image in the inset demonstrates that TNA was vertically well-aligned on Ti, with a length of approximately 2 μm. Fig. 1(b) shows that the morphology of Cu₂O on TNA surface exhibited an octahedral shape with a side length of approximately 430 nm and a coverage of 25%. The structures of TNA retained their integrity without significant change after the deposition of Cu₂O. Cu₂O on Ti had an octahedral shape, with a side length of approximately 380 nm and total coverage of 50% [Fig. 1(c)]. Similarly, Cu₂O on FTO had an octahedral shape, with a side length of approximately 500 nm and total coverage of 60% [Fig. 1(d)]. However, some aggregations of Cu₂O were observed on the Ti and FTO surfaces. During the electrodeposition of Cu₂O, the conductivity of the substrate critically affected the morphology of the deposited Cu₂O.^13,30 Obviously, there were significant differences in the sheet resistance among TNA, FTO, and Ti, which were 2.64 ± 0.15 MΩ sq⁻¹, 7.86 ± 0.04 Ω sq⁻¹, and 1.66 ± 0.04 mΩ sq⁻¹, respectively.

Fig. 1 Top-view scanning electron microscopy (SEM) images of (a) TNA/Ti, (b) Cu₂O/TNA/Ti, (c) Cu₂O/Ti, and (d) Cu₂O/FTO. The inset in (a) is the cross-sectional SEM image of TNA/Ti.

The X-ray diffraction (XRD) patterns in Fig. 2(a) display peaks confirming the presence of Ti, TNA, Cu₂O, and FTO. The TNA/Ti and Cu₂O/TNA/Ti had peaks with 2θ values of 25.34°, 37.98°, 48.10°, and 54.10°, which were indexed to the (101), (004), (200), and (105) crystal planes of anatase TiO₂ (ICSD file no. 98-007-6028), respectively. Two peaks with 2θ values of 36.58° and 42.29°, observed on Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO, were indexed to the (111) and (200) crystal planes of Cu₂O (ICSD file no. 98-006-0719), respectively. No Cu or CuO peaks were observed in Cu₂O/TNA/Ti, Cu₂O/Ti, or Cu₂O/FTO, indicating that only Cu₂O was grown in the substrates. For comparison, the morphology and XRD pattern of the commercial Cu₂O are also presented in Fig. S1.† The optical absorption spectra of TNA/Ti, Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO are shown in Fig. 2(b). TNA/Ti demonstrated high absorption in the UV region, with an approximate band gap energy of 3.35 eV (370 nm), which is consistent with the anatase phase.³¹ Broad light scattering of TNA/Ti caused by its specific structure was observed in the visible region.³² Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO all demonstrated a visible light absorption edge at 500 nm due to the band gap of Cu₂O (2.47 eV), which is consistent with values reported in other studies.^1,2 The additional absorption edge at 560 nm observed in Cu₂O/Ti is attributable to the absorption characteristics of Ti. The absorption characteristics of Cu₂O/FTO were obtained through measurement of the reflection and transmission spectra, as shown in Fig. S2.† The optical absorption of the commercial Cu₂O is shown in Fig. S3.†

Fig. 2 (a) XRD patterns and (b) optical absorption spectra of TNA/Ti, Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO. The dashed line indicates the absorption edge of Cu₂O. An arrow denotes the additional absorption due to Ti.

2.2 Photocatalytic properties

Fig. 3 shows the spectral response with the working electrodes of Cu₂O/TNA/Ti, Cu₂O/Ti, Cu₂O/FTO, and TNA/Ti, which were obtained from the photocurrent intensity under different monochromatic wavelengths, with a light intensity of 5 mW cm⁻² (Fig. S4†). With positive spectral responses, TNA/Ti and Cu₂O/TNA/Ti are photoanodes, and with negative spectral responses, Cu₂O/Ti and Cu₂O/FTO are photocathodes. TNA/Ti exhibited a high response to UV light (10.1 μA mW⁻¹ at 365 nm and 1.8 μA mW⁻¹ at 385 nm) but a low response to visible light (0.21 μA mW⁻¹ at 415 nm). The responses of Cu₂O/TNA/Ti in the UV-visible region (4.0 μA mW⁻¹ at 385 nm and 2.2 μA mW⁻¹ at 415 nm) were approximately 2 and 10 times larger than that of TNA/Ti, and the onset of response at 590 nm (0.16 μA mW⁻¹) was consistent with the onset of absorption in Fig. 2(b). This higher spectral response observed in Cu₂O/TNA/Ti could be attributed to the absorption of Cu₂O, which is active in the UV-visible range. Cu₂O/Ti and Cu₂O/FTO also had the same onset of response (shown in the inset of Fig. 3), but their overall responses were low, indicating that Cu₂O is a poor photocathode.

Fig. 3 Spectral responses of TNA/Ti, Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO. The inset is the enlarge graph to see the onset response of Cu₂O/Ti and Cu₂O/FTO.

The photocurrent density was measured under a solar simulator. Under dark conditions, both Cu₂O/TNA/Ti and TNA/Ti exhibited no current density. Under solar light (λ > 350 nm), Cu₂O/TNA displayed a photocurrent density of 27 μA cm⁻², approximately twice that of TNA/Ti (15 μA cm⁻²) [Fig. 4(a)]. Under visible light (λ > 400 nm), Cu₂O/TNA/Ti demonstrated a photocurrent density of 14 μA cm⁻², whereas TNA/Ti had a low photocurrent density (2 μA cm⁻²) [Fig. 4(b)]. Fig. 4(c) presents the photovoltage obtained from the open-circuit potential (OCP) vs. Ag/AgCl, which is directly related to the photocurrent. Under dark conditions, the OCPs of TNA/Ti and Cu₂O/TNA/Ti were 0.1 V vs. Ag/AgCl. Subsequently, a negative photovoltage was observed when the working electrode was illuminated by solar light. A negative photovoltage indicates that electrons accumulated in the electrodes, denoting a photoanode. Cu₂O/TNA/Ti had a more negative photovoltage (−0.17 V vs. Ag/AgCl) compared with that of TNA/Ti (−0.13 V vs. Ag/AgCl), indicating that a greater number of electrons accumulated in the Cu₂O/TNA/Ti upon illumination. The photocurrent densities of Cu₂O/Ti and Cu₂O/FTO under solar light in Fig. 4(d) and visible light in Fig. 4(e) demonstrate the typical behaviour of p-type semiconductors to be photocathodes. The photocurrent densities of Cu₂O/Ti and Cu₂O/FTO under solar light were approximately −3.0 μA cm⁻² and −4.0 μA cm⁻², respectively. Under visible light, Cu₂O/Ti and Cu₂O/FTO exhibited photocurrent densities of −2.4 μA cm⁻² and −3.1 μA cm⁻², respectively. Fig. 4(f) shows OCPs vs. Ag/AgCl under solar light, which fluctuated between 0.05 V and 0.08 V and between 0.04 V and 0.09 V for Cu₂O/Ti and Cu₂O/FTO, respectively. The positive photovoltages suggest that holes accumulated in the working electrodes.

Fig. 4 (a and b) Photocurrent density of TNA/Ti and Cu₂O/TNA/Ti under solar light and visible light. (c) OCP vs. Ag/AgCl of TNA/Ti and Cu₂O/TNA/Ti under solar light. (d and e) Photocurrent density of Cu₂O/Ti and Cu₂O/FTO under solar light and visible light. (f) OCP vs. Ag/AgCl of Cu₂O/Ti and Cu₂O/FTO under solar light.

The photocatalytic activity of Cu₂O/TNA/Ti, Cu₂O/Ti, Cu₂O/FTO, and TNA/Ti was evaluated though the degradation of R6G under solar light without bias through monitoring of the change in height of the absorption peak at a wavelength of 527 nm.³³ R6G was degraded through photooxidation process. Fig. 5(a) shows the decrease of the absorption spectra of R6G degraded by the photocatalysis with Cu₂O/TNA/Ti. Fig. 5(b) presents the photocatalytic degradation in the R6G concentration with different working electrodes. Cu₂O/TNA/Ti could degrade R6G by approximately 55% in 120 min. However, TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO demonstrated smaller degradation effects on R6G than that of Cu₂O/TNA/Ti, with approximate degradation percentages of 30%, 26%, and 25%, respectively. Hence, the photooxidation of R6G is favourable with the Cu₂O/TNA/Ti photoelectrode.

Fig. 5 (a) Decrement of the R6G absorption peak by Cu₂O/TNA/Ti under solar light. (b) Degradation of the R6G concentration (C/C₀) by Cu₂O/TNA/Ti, Cu₂O/Ti, Cu₂O/FTO, and TNA/Ti under solar light.

2.3 Energy band diagram of Cu₂O heterojunctions

The photocatalytic behaviours (photoanode or photocathode) of TNA/Ti, Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO can be described by their energy band diagrams obtained upon light illumination (Fig. 6). In a state of equilibrium under dark conditions, a Schottky junction formed at the TNA/Ti interface with a built-in potential (V_bi) of approximately 0.27 V,³⁴ as shown in Fig. 6(a). When TNA absorbed UV light, electrons and holes were generated at the conduction and valence bands, respectively. The photogenerated electrons were transferred to the Ti back contact and then moved to Pt through an external circuit; subsequently, the photogenerated holes were transferred to the Na₂SO₄ electrolyte as a photoanodic current. Cu₂O/TNA created a staggered-gap (Type II) heterojunction with a built-in potential (V_bi) of 0.89 V,³⁴ as shown in Fig. 6(b). Cu₂O/TNA/Ti behaved as a photoanode analogous to TNA/Ti.^27,29 Cu₂O absorbed UV-visible light and generated electron–hole pairs. The photogenerated electrons in the conduction band of Cu₂O were transferred to the conduction band of TNA and then to the Ti back contact, and photogenerated holes in the valence band of TNA were transferred to the valence band of Cu₂O and then to the Na₂SO₄ electrolyte, serving as a photoanodic current. As seen in Fig. 3, the response of TNA/Ti at 365 nm was higher than that of Cu₂O/TNA/Ti, which indicates the TNA/Ti is favourable to the transfer of photoelectrons. In addition, in Fig. 4(a), the photocurrent of TNA/Ti under solar light was almost half the photocurrent of Cu₂O/TNA/Ti, thought that TNA/Ti only takes UV light, which is less than 10% of the solar power. Therefore the charge transfer at Cu₂O/TNA is more difficult than that at TNA/Ti; the effect of TNA/Ti can be insignificant.

Fig. 6 Energy band diagrams involving (a) TNA/Ti, (b) Cu₂O/TNA/Ti, (c) Cu₂O/Ti, and (d) Cu₂O/FTO in the Na₂SO₄ 0.05 M electrolyte upon light illumination without applied bias.

Cu₂O/Ti formed a Schottky junction with a built-in potential (V_bi) of approximately 0.45 V,³⁴ as displayed in Fig. 6(c). When Cu₂O absorbed UV-visible light, a photocathodic current was observed because the photogenerated holes were easily transferred to the Ti back contact, and the photogenerated electrons were transferred to the Na₂SO₄ electrolyte. Furthermore, FTO is an n-type degenerate semiconductor, but behaves like a metal¹⁴ and has a Fermi level above the conduction band,³⁵ as indicated in Fig. 6(d). The work function of Cu₂O is a little lower than that of FTO. At equilibrium state, Cu₂O/FTO created a broken-gap (type III) junction, yielding a band bending that demonstrated an ohmic contact³⁶ for hole transfer and a small barrier for electron transfer to FTO with a built-in potential (V_bi) of approximately 0.15 V. Similar to the case for Cu₂O/Ti, the photogenerated holes of Cu₂O were transferred to the conduction band of FTO and further transferred to Pt through an external circuit, whereas the photogenerated electrons were transferred to the Na₂SO₄ electrolyte. Hence, a photocathodic current was observed.

2.4 Charge lifetime of photogenerated charges

It is a common belief that the built-in potential could lead to charge separation enhancement at the Cu₂O/TNA junction, where the photogenerated electrons in the conduction band of Cu₂O to drift toward the conduction band of TNA and the photogenerated holes in the valence band of TNA to drift toward the valence band of Cu₂O.^25–29 Enhanced charge separation created in the junction can reduce the probability of electron–hole recombination and enhance photocatalysis. To understand the presence of charge separation enhancement at the Cu₂O/TNA junction, photoluminescence (PL) and TRPL of all samples were applied.

The commercial Cu₂O, bare TNA/Ti, Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO samples were excited by a 355 nm pulse laser under a long-pass filter (λ > 480 nm) and integration time of 1 s at room temperature to measure the PL, as shown in Fig. 7(a and b). Commercial Cu₂O exhibited PL peaks located at 620 nm and 900 nm, which is attributable to the band-to-band transition of Cu₂O and the defect states of copper vacancies.^37,38 TNA/Ti exhibited broad band PL with a peak located at 600 nm, which is consistent with the reports of other studies.^39,40 No PL of the band-to-band transition of TNA was observed because TNA is an indirect band gap semiconductor.³⁹ The 500 nm and 600 nm broad band PL of TNA originated from surface oxygen vacancies and subsurface oxygen vacancies, respectively.⁴⁰ The PL of TNA and Cu₂O in the Cu₂O/TNA/Ti sample was captured from the TNA and Cu₂O sides (an integration time of 30 s was used to measure the PL of the Cu₂O side because of the weak intensity). The Cu₂O side of Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO exhibited a broad band PL peak at 600 nm, mainly caused by band-to-band emission. Cu₂O also had defects due to oxygen vacancies with +2 and +1 charge states at 770 nm and 825 nm, respectively, and copper vacancies at 900 nm.³⁸ The TNA side demonstrated similar PL emission to that of bare TNA/Ti.

Fig. 7 (a and b) PL spectra of commercial Cu₂O, Cu₂O/TNA/Ti (Cu₂O and TNA sides), Cu₂O/Ti, Cu₂O/FTO, and TNA/Ti excited at a wavelength of 355 nm. (c) TRPL of the commercial Cu₂O, Cu₂O side of Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO measured using a band-pass filter (600 nm/45 nm). The curves were shifted vertically for clarity. (d) TRPL of TNA/Ti and TNA side of Cu₂O/TNA/Ti measured using a long-pass filter (λ > 480 nm). The TRPL results are plotted in the log scale. The solid lines in (c and d) are the fitting lines.

The charge dynamics of the photogenerated charges was further studied by TRPL. Fig. 7(c) shows the TRPL of commercial Cu₂O, Cu₂O side of Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO under a band-pass filter (600 nm ± 45 nm) because only the band-to-band transition of Cu₂O was considered. Fig. 7(d) shows the TRPL of defect states in bare TNA/Ti and on the TNA side of Cu₂O/TNA/Ti under a long-pass filter (λ > 480 nm). The TRPL results were fitted by a one-exponential-decay function for commercial Cu₂O, Cu₂O side of Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO, and a two-exponential-decay function for bare TNA/Ti and TNA side of Cu₂O/TNA/Ti, where τ₁ and τ₂ are the lifetimes of charge carriers along with the corresponding amplitudes, A₁ and A₂. The fitted results are summarized in Table 1. There is no significant difference of charge carrier lifetimes among commercial Cu₂O (0.38 ns), Cu₂O side of Cu₂O/TNA/Ti (0.36 ns), Cu₂O/Ti (0.31 ns), and Cu₂O/FTO (0.33 ns), and between TNA/Ti (0.88 ns) and TNA side of Cu₂O/TNA/Ti (0.86 ns). Thus, the Cu₂O/TNA junction did not enhance the charge separation of photogenerated charges on the Cu₂O or TNA sides. Therefore, no charge separation enhancement was observed at the Cu₂O/TNA junction, which could be attributed to a poor contact area and poor quality of the Cu₂O/TNA.

Table 1 Fitted TRPL parameters of Cu₂O and TNA from different samples

TRPL	Cu2O side				TNA side
Sample/parameter	Commercial Cu2O	Cu2O/TNA/Ti	Cu2O/Ti	Cu2O/FTO	TNA/Ti	Cu2O/TNA/Ti
A1	0.98	0.97	0.97	0.99	0.66	0.84
τ1 (ns)	0.38	0.36	0.31	0.33	0.88	0.86
A2					0.35	0.23
τ2 (ns)					5.24	5.90

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Inevitably, the contact area between the three-dimensional structures of TNA and the octahedral crystal grains of Cu₂O prepared by square wave voltammetry electrochemical (SWVE) was remarkably small. The grain size of Cu₂O was larger than the outer diameter of the TNA, as seen in Fig. 1(b). Such large grains of Cu₂O were due to the high growth rate of Cu₂O and resulted in the low contact area and low coverage at Cu₂O/TNA junction. In addition the high growth rate could lead to poor junction quality. To improve the Cu₂O/TNA junction, we suggest a lower growth rate of Cu₂O by adjusting the scan rate in the SWVE method to allow better nucleation and grow smaller grains of Cu₂O. As a result, the contact area and coverage of Cu₂O/TNA increase, and the quality of Cu₂O/TNA junction is improved. Therefore, the Cu₂O/TNA junction exhibiting better photocatalytic properties than those of TNA is only attributable to the absorption of Cu₂O, not to charge separation enhancement at the Cu₂O/TNA junction. In addition, the PL at 500–600 nm related to the oxygen vacancies indicated that the quality of the electrochemical-anodized TNA was unsuitable for the formation of a high-quality junction. Other types of TiO₂, such as sputtered TiO₂ and porous TiO₂, could be better candidates for the formation of high-quality Cu₂O/TiO₂ junctions and assist the separation of photogenerated charges. No charge separation enhancement was observed at the Cu₂O/Ti and Cu₂O/FTO junctions, because the built-in potential is not favourable to the drift of photogenerated holes toward Ti and electrons toward FTO, respectively.

3. Experimental section

3.1 Preparation of sample

TNA/Ti, Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO were prepared according to the methods described by Chang et al.²⁷ TNA was fabricated through the electrochemical anodization of Ti plate and then calcined in a tubular furnace for 3 h at 450 °C with a temperature increase of 2°C min⁻¹ to obtain a crystalline phase. Cu₂O was electrodeposited on the TNA/Ti, Ti, and FTO surfaces by SWVE deposition method (see ESI† for more details). The optical absorption spectra were measured by a UV-visible diffuse reflectance spectrometer equipped with an integrating sphere assembly. The PL spectra and TRPL were measured by a micro PL system with a 355 nm pulse laser excitation (0.15 mW μm⁻²).

3.2 Photocatalytic measurement

Photocatalytic properties were measured in a single quartz chamber with a two-electrode setup in Fig. 8. A solution of Na₂SO₄ 0.05 M (pH = 6.7) was used as the electrolyte. A calibrated solar simulator (a Xe lamp equipped with an AM1.5 filter) with a front-mounted long-pass filter (λ > 400 nm) was used as the solar light source. The photocurrent intensity was measured with the setup in Fig. 8(a), in which the sample served as a working electrode, and a Pt plate acted as a counter electrode. Fig. 8(b) displays the setup to measure OCP vs. Ag/AgCl. Spectral response was measured under the illumination of different monochromatic wavelengths (5 mW cm⁻²) from a multichannel light-emitting diode source. Degradation of R6G was used to evaluate the photocatalytic activity of the samples. The initial concentration of the R6G aqueous solution was 20 μM (0.95 mg of R6G in 100 mL of Na₂SO₄ 0.05 M). The degradation of R6G was performed in the single quartz electrolyzer and continuously stirred at room temperature under solar light. Light illumination was started after 15 min to allow for the adsorption or desorption of R6G to reach equilibrium on the surface of photocatalyst. The degradation of R6G was monitored by observing the change of the absorption peak at a wavelength of 527 nm (ref. 33) through UV-visible light spectrometry every 15 min for a total illumination time of 120 min.

Fig. 8 Schematics of the photocatalytic measurement under a two-electrode setup for (a) photocurrent intensity without bias and (b) OCP vs. Ag/AgCl.

4. Conclusions

The photocatalytic properties of Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO were studied. Without applied bias, Cu₂O/TNA/Ti acted as a photoanode, whereas Cu₂O/Ti and Cu₂O/FTO acted as photocathodes. The Cu₂O/TNA/Ti photoanode outperformed the other two Cu₂O/Ti and Cu₂O/FTO photocathodes in photocatalysis. Cu₂O/TNA/Ti demonstrated the best photocatalytic activity to R6G, with a rate twice that of Cu₂O/Ti and Cu₂O/FTO over a period of 120 min. The photoanodic and photocathodic characteristics of Cu₂O on different substrates can be described by their energy band diagrams. No charge separation enhancement at the Cu₂O/TNA/Ti, Cu₂O/Ti, and Cu₂O/FTO junctions was observed. Cu₂O/TNA/Ti demonstrated superior photocatalytic properties compared with TNA/Ti, which were mainly attributable to the UV-visible light absorption of Cu₂O.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to thank the Ministry of Science and Technology of Taiwan for their support of this research under grants MOST 109-2112-M-029-001 and 109-2112-M-029-006.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra10681j

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