Preparation and characterization of SrTiO₃–ZnTe nanocomposites for the visible-light photoconversion of carbon dioxide to methane

Abstract

Limited fossil fuel resources and increasingly stringent requirement of environmental protection from major greenhouse gases, including carbon dioxide (CO₂), which results directly from the burning of fossil fuels, energy savings and greenhouse-effect alleviation have emerged as major global concerns. The development of an “artificial photosynthetic system” (APS) having both the analogous important structural elements and the reaction features of photosynthesis to achieve solar-driven water splitting and CO₂ reduction is highly challenging. Herein, it has been demonstrated that SrTiO₃–ZnTe can be utilized as an efficient APS for the photoreduction of CO₂ into methane (CH₄) under visible-light irradiation (≥420 nm). The results indicate that the combination of ZnTe with SrTiO₃ visibly increases the formation of CH₄ by efficiently promoting electron transfer from the conduction band of ZnTe to that of SrTiO₃ under visible-light irradiation, and thereby demonstrate this to be a promising candidate for the photocatalytic conversion of CO₂ into hydrocarbon fuels.

---

Introduction

To date, hydrocarbon fuels (petroleum, natural gas, and coal) have been considered as the most important source of energy due to their ready availability in nature, high energy content, stability, and ease of storage and transportation. However, the foreseen shortage of fossil fuels has become a global concern as global energy consumption has tremendously increased since the start of the industrial revolution, leading toward the depletion of these energy resources. Other than their depletion, their combustion also has a significant drawback of environmental pollution by CO₂ emissions on an alarmingly larger scale, which directly contributes to global warming.¹ To address this major problem, a solar-energy-based system for recycling CO₂ into some readily transportable and stable fuels, i.e., solar fuels, could help to control and reduce atmospheric CO₂ levels and to partly fulfil the energy requirements in the current hydrocarbon-based fuel infrastructure.

Solar energy can be efficiently utilized for the fixation of CO₂ by its photoreduction with H₂O. Ever since the pioneering work by Honda and Fujishima in the early 1970s,² the photochemical conversion of solar energy into different forms of chemical energy has been intensively studied. Considerable efforts have been made in this regards to explore the photoreduction of CO₂ into useful chemical fuels such as CH₄, methanol (CH₃OH), and carbon monoxide (CO).^3–13 A variety of photocatalysts, such as metal oxides,^3–6 tantalates,⁷ ZnGa₂O₄,⁸ Bi₂WO₆,⁹ Zn₂GeO₄,¹⁰ and titanium-based materials^11–13 have been investigated for the efficient conversion of CO₂ in both gaseous and liquid phases. However, the utilization of such a single semiconductor material as a photocatalyst is often restricted due to short lifetime of the photogenerated electron–hole pairs and/or the large band-gap, which is the cause of its limited visible-light response.¹⁴ On the other hand, narrow-band-gap semiconductors can absorb the visible-light photons for the generation of electron–hole pairs, but unfortunately they suffer from photocorrosion¹⁵ and the fast recombination of the charge carriers. It has therefore been a vital concern to develop an efficient design for the fabrication of nanostructured photocatalysts that can exhibit high activity and stability for use in practical applications.

Strontium titanate (SrTiO₃), an n-type semiconductor with a simple perovskite structure, has been a renowned photocatalyst applied in the field of hydrogen energy;^16,17 but there are only a very few reports about the use of SrTiO₃ as a photocatalyst for the photoreduction of CO₂.^18,19 Unfortunately, owing to its large band-gap, it responds only to the UV light, which accounts for only 4% of the solar spectrum. In order to improve its photocatalytic activity under visible-light, which accounts for about 43% of the solar spectrum, many researchers have tried different dopants, such as Fe, C, N, and Cr, to enhance its photocatalytic activity in the visible region.^20–23 However, doping may also create some defects in the crystal structure, which may facilitate the recombination of the charge carriers, leading to a poor photocatalytic activity of SrTiO₃. To prevent this recombination of the charge carriers, researchers have also tried loading some transition and noble metals, including Pt,²⁴ Pd,²⁵ and Cu,²⁶ over the single large band-gap semiconductors. Such a scheme could inhibit the electron–hole pair recombination, but does not shift its response from UV to visible light. Thus, in contrast to the single semiconductors, the combination of two different semiconductors would facilitate the separation of charge carriers to minimize their recombination and, thereby, would enhance the photocatalytic activity.

In the present work, n-type SrTiO₃ has been combined with different amounts (%) of p-type ZnTe, for the first time, to form the nanocomposite systems. ZnTe is a p-type II–VI semiconductor with a direct band gap of ∼2.26 eV at 300 K and is a promising material, due to its applications in solar cells,²⁷ blue-green light-emitting diodes,²⁸ and optoelectronic devices in the visible region.²⁹ Because of its very negative conduction band position, ZnTe has a large driving force for the interfacial electron transfer from the semiconductor to the acceptor.³⁰ As a result, ZnTe is considered an attractive material for designing novel photocatalysts. SrTiO₃–ZnTe nanocomposites have been synthesized via the hydrothermal method. The photoreduction of CO₂ using bare SrTiO₃–ZnTe nanocomposites with different amounts of ZnTe (i.e., 0% (bare SrTiO₃), 25%, 50%, 75%, and 100% (pure ZnTe)) has been studied under visible-light irradiation (≥420 nm), with CH₄ as the major product for all of the photocatalysts. The obtained CH₄ can be used as a renewable solar fuel, and will not be released into the atmosphere directly as a greenhouse gas. The results indicate that this work may help the development of high-efficiency photocatalysts, to help decrease the CO₂ content in the environment via photoreduction, and, thereby, to help decrease the greenhouse effect.

Experimental

Chemicals

The chemicals used in our experiments were Ti(SO₄)₂ (>96%), Sr(NO₃)₂ (>99.5%), zinc powder (99.8%), tellurium powder (5 N), KOH (analytical grade, >85%), and NaOH (>96%). All of these chemicals were purchased from Sinopharm Chemicals and were used as such without any further treatment.

Synthesis of SrTiO₃

SrTiO₃ nanoparticles were successfully synthesized using the sonochemical method at room temperature.³¹ Titanium was used in the form of titanium hydroxide suspension, denoted as TOH. First, a clear aqueous solution of 0.05 M Ti(SO₄)₂ was prepared by dissolving it in 50 mL of 0.1 M HNO₃ solution. This solution was then added dropwise into 0.5 M KOH solution under constant stirring, and white precipitates of titanium hydroxide were formed. For the removal of SO₄²⁻ and NO₃⁻, these precipitates were then centrifuged and washed copiously with Milli-Q water. For the preparation of TOH, the precipitates of titanium hydroxide were dispersed in 50 mL of Milli-Q water and collected for further use. For the synthesis of SrTiO₃, equal volumes (50 mL) of TOH and aqueous solution of 0.05 M Sr(NO₃)₂ were added to a beaker containing 50 mL of 0.5 M KOH solution. This mixture was then sonicated for 2.5 hours under an ambient atmosphere. The resultant white precipitates of SrTiO₃ were washed several times with Milli-Q water and finally with absolute ethanol, and then dried in an oven at 70 °C under vacuum overnight. Finally, the dried SrTiO₃ nanoparticles were annealed at 650 °C for 4 hours at a heating rate of 5 °C min⁻¹, and were saved for further characterizations.

Synthesis of ZnTe

ZnTe nanoparticles were successfully synthesized by the hydrothermal process.³² In a typical procedure, 0.52 g of zinc powder, 0.51 g of tellurium powder, and 4.6 g of NaOH were put into a Teflon tube with 50 mL capacity. The Teflon tube was then filled with Milli-Q water up to 80% of its capacity, which was then sealed in a stainless steel autoclave. The autoclave was kept in an oven for two hours at 100 °C. After it cooled down to room temperature, the resulting brown ZnTe precipitates were then centrifuged, washed several times with Milli-Q water and finally with absolute ethanol, and were then dried in oven for overnight at 70 °C under vacuum. The dried ZnTe nanoparticles were ground down and stored for further use.

Synthesis of SrTiO₃–ZnTe nanocomposites

In a typical hydrothermal method, stoichiometric amounts of the obtained SrTiO₃ (0.15, 0.1, and 0.05 g) and ZnTe (0.05, 0.105, and 0.16 g) with a respective molar ratio of 3 : 1 (25% ZnTe), 1 : 1 (50% ZnTe), and 1 : 3 (75% ZnTe) were taken in a 50 mL Teflon tube, which was then filled to 80% of its capacity with Milli-Q water. The mixture, after being stirred for 2 hours, was then sealed in a stainless steel autoclave, which was then kept in an oven at 100 °C for 10 hours. After the autoclave was cooled down at room temperature, the precipitates of SrTiO₃–ZnTe nanocomposites were collected via centrifuge and washed several times with Milli-Q water and finally with absolute ethanol. The resultant precipitates were then dried at 70 °C under vacuum for 8 hours, followed by thermal annealing at 200 °C for two hours under ambient conditions, and finally saved for further use.

Characterization

A Bruker D8 focus diffractometer with Ni-filtered Cu-Kα radiation was used for the X-ray diffraction (XRD) analysis to probe the crystal structure of the as-synthesized products. The XRD patterns were recorded in the 2θ range of 20° to 80° at a scan rate of 0.1° min⁻¹. A Lambda 750 UV/visible/NIR spectrophotometer was employed to collect the UV-visible diffuse reflectance spectra with wavelengths in the range of 200–800 nm at room temperature, with BaSO₄ as the reference. An HITACHI S4800 FESEM with energy dispersive X-ray (EDX) spectrometer and HITACHI H-7700 transmission electron microscope (TEM) at 100 kV were used to analyze the morphology and composition of the as-synthesized powders. An LS55 fluorescence spectrometer was used to record the photoluminescence (PL) spectra. X-ray photoelectron spectroscopy (XPS) was performed to determine the composition and valance band position of the catalysts using an ESCALAB 250Xi X-ray photoelectron spectrometer.

Photocatalytic reduction of CO₂

A schematic illustration of the photocatalytic reaction system for the reduction of CO₂ is shown in Fig. 1. To produce a mixture of the CO₂ and water vapor at atmospheric pressure, a high purity CO₂ gas (≥99.999%) was bubbled through Milli-Q water in a quartz reactor attached with the photoreaction system. It was assured before the irradiation that there was no leakage in the photoreaction system and hence the positive pressure of CO₂ inside the system was maintained at 25 kPa. Wet CO₂ was allowed to flow through the photoreaction system loaded with 20 mg of the photocatalyst with different amounts of ZnTe (i.e., 0%, 25%, 50%, 75%, and 100%) for about 40 minutes before the irradiation, to ensure the complete removal of air from the system, as confirmed from the GC results, followed by the maximum adsorption of gas molecules on the photocatalysts. After the CO₂-flow was stopped, a xenon lamp (300 W) equipped with a cut-off filter (≥420 nm) was turned on and the production of CH₄ as a function of irradiation time was measured for a total time period of 8 hours. An approximate distance of 14–15 cm was maintained between the light source and the surface of the photocatalyst suspension in the photoreactor. Tap water (15 °C) was constantly recycled during the entire course of the reaction to maintain the temperature of the photoreaction system. An Agilent 7890A gas chromatograph (GC) was used to analyze the obtained CH₄ via an automated gas valve using helium (He) as the carrier gas. The GC was equipped with one thermal conductivity detector (TCD) and two (front and back) flame ionization detectors (FID). In addition, four different control experiments were conducted, i.e., a dark experiment with the photocatalysts, a blank reactor with and without irradiation, and finally with exactly the same experimental conditions but using N₂ instead of CO₂. There was no gas product like CH₄ detected for all these control experiments, indicating that the CH₄ is produced only from the photoreduction of CO₂ over the photocatalysts under visible-light irradiation and not from any other external surrounding factors. An ILT 950 spectrodiometer (International Light Technologies) was used to measure the intensity of the incident light, so as to determine the solar energy conversion efficiency.

Fig. 1 Schematic illustration of the experimental setup using a photoreactor connected with a GC for the photoreduction of CO₂.

Calculations

The XRD data were used to calculate the different crystal parameters, such as crystallite size (D) via the Scherrer formula, lattice constant (a), and cell volume (V) by using eqn (1)–(3),³³ where K is the Scherrer constant (0.89),³⁴ λ is the wavelength for the X-ray radiation (0.15418 nm for Cu-Kα), β is the full width at half maxima (FWHM) for the diffraction peak measured at 2θ, and θ is the diffraction angle. Diffuse reflectance spectroscopy was used to determine the band gap energy (E_g, eV) by using the Tauc equation (eqn (4)), where h is Plank's constant, v is the frequency of vibration, α is the absorption coefficient, E_g is the band gap (eV), and K is the proportional constant.

D = Kλ/β cos θ (1)

1/d² = (h² + k² + l²)/a² (2)

V = a³ (3)

αhν^1/2 = K(hν − E_g) (4)

Results and discussion

XRD analysis

The crystal structure of the synthesized materials was verified by XRD (Fig. 2). In the case of SrTiO₃, it was found to be phase pure in accordance with the available diffraction patterns (powder diffraction file (PDF) number 35-0734, a = 3.905 Å), where all the diffraction peaks confirmed the formation of a perovskite-type structure with cubic symmetry. The diffraction patterns for ZnTe, on the other hand, could be indexed for cubic ZnTe, in accordance with the reported data (PDF # 15-0746, a = 6.103 Å). It was evident from the ZnTe diffraction patterns that there were no characteristic peaks for impurities, such as elemental Zn or Te, which confirms the purity of the synthesized sample. The strong and sharp ZnTe peaks also indicate that the photocatalyst is highly crystalline in nature. To obtain a viable catalyst for the photoreduction of CO₂ under visible light, the synthesized SrTiO₃ and ZnTe were combined together via a modified hydrothermal reaction approach for the synthesis of SrTiO₃–ZnTe nanocomposites with different amounts of ZnTe (i.e., 25%, 50%, and 75%). The XRD pattern (Fig. 2) confirms the formation of the SrTiO₃–ZnTe nanocomposites, as all the peaks from SrTiO₃ and ZnTe are present in the composites. It is noted that the intensity of the (111) diffraction peak corresponding to ZnTe increases with the increase of the amount of ZnTe in the nanocomposites (Fig. 2b–d), leading to a decrease in the intensity of the (110) diffraction peak for SrTiO₃.

Fig. 2 XRD patterns of (a) SrTiO₃, (b–d) SrTiO₃–ZnTe nanocomposites with 25%, 50%, and 75% ZnTe, respectively, and (e) ZnTe.

The XRD data were also used to calculate the crystallite size (D), lattice constant (a), and cell volume (V) for SrTiO₃ and ZnTe, and the values are given in Table 1. All the calculated parameters for the synthesized products were in good agreement with the reported data, as already mentioned above. For ZnTe and SrTiO₃, the crystal domain sizes were determined, respectively, to be around 36 nm and 26 nm using the diffraction peak width of (111) and (110) (at half maxima) in terms of the Scherrer equation (eqn (1)).

Table 1 Crystalline parameters derived from the XRD patterns for SrTiO₃ and ZnTe

Sample	Crystallite size (D) (nm)	Lattice constant (a) (Å)	Cell volume (V) (Å^3)
SrTiO3	26	3.8987	59.299
ZnTe	36	6.0958	226.513

---

Morphological and EDX analysis

Morphological analyses for all the synthesized photocatalyst powders using SEM are presented in Fig. 3. It is evident from Fig. 3a that the SrTiO₃ particles are uniformly worm-like in shape, with a size distribution in the range of around 20–26 nm, which agrees well with the crystallite sizes obtained from XRD. For bare ZnTe, it can be clearly observed that the particles are spherical in shape, with a uniform size distribution around 30–40 nm (Fig. 3e). In the case of the nanocomposite with 25% ZnTe (Fig. 3b), the spherical ZnTe nanoparticles and worm-like SrTiO₃ are well dispersed with each other, thereby, leading to close contact. As the amount of ZnTe was increased to 50% (Fig. 3c), apart from that the majority of the ZnTe nanoparticles are still well dispersed with SrTiO₃, some of the small ZnTe nanoparticles start to be decorated on the surface of the large SrTiO₃ worm-like structures. Upon further increasing the amount of ZnTe to 75% (Fig. 3d), the majority of the ZnTe nanoparticles appear to agglomerate on the surface.

Fig. 3 SEM images of SrTiO₃–ZnTe nanocomposites with different amounts of ZnTe: (a) 0% (i.e., SrTiO₃), (b) 25%, (c) 50%, (d) 75%, and (e) 100% (i.e., pure ZnTe).

Moreover, the morphology was further analyzed using TEM and the results are shown in Fig. 4. In the high magnification TEM image for bare SrTiO₃ (Fig. 4a), it can be seen that the small worm-like nanoparticles combine together to form a large cage-like structure. For the nanocomposite with 25% ZnTe (Fig. 4b), some of the spherical ZnTe particles are diffused into the cavities of the worm-like SrTiO₃ to some extent. Upon further increasing the amount of ZnTe to 50% and 75% (Fig. 4c and d), most of the ZnTe appear to be decorated in the form of agglomerated clusters on the surface of the perovskite base material. For bare ZnTe (Fig. 4e), it can also be seen that the particles are spherical in shape, for which the size is in good agreement with the XRD and SEM results. To further probe the microstructure of the nanocomposite and the formation of the interface between the two semiconductors, HRTEM was also carried out (Fig. 4f). The results clearly reveal the formation of a close interface between ZnTe, with exposed (220) crystal plane, and SrTiO₃, with exposed (110) crystal plane. The formation of this close interface between the two materials may facilitate the efficient transfer of electrons, thereby, hindering the recombination of the charge carriers.

Fig. 4 TEM images of SrTiO₃–ZnTe nanocomposites with different amounts of ZnTe: (a) 0%, (b) 25%, (c) 50%, (d) 75%, (e) 100%, and (f) HRTEM image of SrTiO₃–ZnTe with 25% ZnTe.

Energy Dispersive X-ray (EDX) elemental mapping also reveals that the SrTiO₃–ZnTe nanocomposite with 25% ZnTe was formed (Fig. 5) by SrTiO₃ acting as the base material, with Sr, Ti, and O uniformly distributed all over the surface. The micrograph further indicates that the spherical ZnTe nanoparticles were well dispersed with this base perovskite SrTiO₃. The elemental maps of Zn and Te also support the claim that some of the ZnTe diffused into the cavities of the worm-like SrTiO₃ to some extent. The results of the EDX elemental mapping for the nanocomposites with 50% and 75% ZnTe also agree well with the aforementioned discussions and conclusions (Fig. S1 in the ESI†).

Fig. 5 EDX mapping for the SrTiO₃–ZnTe nanocomposite with 25% ZnTe.

EDX spectroscopy was used to determine the composition and stoichiometry for all the elements in the nanocomposite (Fig. S2a†). These results reveal that all the elements in the composite are in agreement with the desired stoichiometric ratios. Moreover, the EDX compositional analyses for bare SrTiO₃ and ZnTe were recorded too, and the results are shown in Fig. S2b and c,† which further indicated that all the elements in SrTiO₃ (Fig. S2b†) and ZnTe (Fig. S2c†) are in accordance with the stoichiometric ratios. This also confirmed that there were no impurities in the synthesized photocatalysts. In addition, it was noted that the excess of oxygen was observed for both samples containing SrTiO₃, which may be due to the adsorbed oxygen on the surface from the environment while preparing the sample for the analysis.

Alignment of band energy levels

The photocatalytic properties of a catalyst are closely related to the alignment of its band energy levels, which we investigated by UV-vis diffuse reflectance spectra and XPS valence band spectra in this work. Fig. 6 shows the UV-vis diffuse reflectance spectra of the as-prepared SrTiO₃ and ZnTe. The Kubelka–Munk function was applied to convert the diffuse reflectance measurements into the equivalent absorption coefficients.³⁵ It is evident from Fig. 6 that SrTiO₃ exhibits a strong absorption, with an edge in the UV region at around 345 nm; whereas, ZnTe shows an absorption peak at around 555 nm. For both SrTiO₃ and ZnTe, the band gap values (E_g) were thus estimated respectively to be 3.51 eV and 2.24 eV using the Tauc plots,³⁶ i.e., the vertical segment of each plot was extra plotted to intersect the hν axis (x-axis) (insets of Fig. 6).

Fig. 6 UV-vis diffuse reflectance spectroscopy and Tauc plots for (a) ZnTe, and (b) SrTiO₃.

In addition, the valence band maximum of the as-synthesized materials was determined by the XPS valence band spectra (Fig. 7). The work function for the XPS instrument in this study was 4.62 eV. The valence band maximum for ZnTe was calculated to be 0.42 eV (inset of Fig. 7a, 0.44 V against a standard hydrogen electrode (SHE, given as ∼4.60 eV for 0 V)). In the case of SrTiO₃, its valence band maximum was around 2.48 eV (inset of Fig. 7b, 2.50 V against SHE). Thus, the alignment of the energy levels is drawn as shown in Fig. 8, according to which the photogenerated electrons can transfer from the conduction band (CB) of ZnTe to the CB of SrTiO₃. Accordingly, the recombination of the photogenerated charge carriers can be suppressed greatly, which can be confirmed by the photoluminescence (PL) results.

Fig. 7 Valance band edge from XPS Spectra for (a) ZnTe and (b) SrTiO₃.

Fig. 8 Band alignment of the energy levels and proposed scheme for the photoreduction of CO₂ with SrTiO₃–ZnTe nanocomposite under irradiation of λ ≥ 420 nm.

The PL spectra of bare ZnTe and the nanocomposites with different amounts of ZnTe are shown in Fig. 9. The spectra for all the synthesized photocatalysts exhibit a very similar shape to each other, with a broad band of emission signal. It is noted that the PL intensity of the nanocomposites is much weaker than that of ZnTe, due to the suppression of the recombination, the change in which follows in the order of 100% ZnTe > 75% > 50% > 25%. The reason that the nanocomposite with 25% ZnTe shows the lowest PL intensity may be mainly because of the good uniform dispersion of ZnTe in SrTiO₃, as indicated by the above SEM/TEM and the mapping results (Fig. 3–5).

Fig. 9 Photoluminescence spectra of the as-prepared photocatalysts upon 500 nm excitation.

Photoreduction of CO₂

Methane (CH₄) is the major product in the gas phase for the photoreduction of CO₂, for which the production results are shown in Fig. 10. In addition, sometimes trace amounts of H₂ can be observed. However, the amount of H₂ cannot be determined, as He was used as carrier gas in our system. Upon visible-light illumination (λ ≥ 420 nm), the amount of CH₄ produced from the SrTiO₃–ZnTe nanocomposites with different amounts of ZnTe i.e., 25%, 50%, 75%, and 100% (bare ZnTe) came out to be 19.03, 14.58, 9.64, and 5.15 μmol g⁻¹ (Fig. 10), respectively. No CH₄ was observed from the bare SrTiO₃ (0% ZnTe) under light ≥420 nm. Here it is noted that only ZnTe is considered to be a photocatalyst, as SrTiO₃ has no response to visible light. By using the amount of fuel energy converted relative to the solar power illuminated on the samples (≥420 nm), the total conversion efficiency after 8 hours was calculated to be 0.36%, 0.28%, 0.18%, and 0.098% for the photocatalysts with different amounts of ZnTe, respectively. Thus, the combination of ZnTe with SrTiO₃ can greatly improve the visible-light activity for the photoreduction of CO₂. In addition, the stabilities of the photocatalysts have been demonstrated by studying the nanocomposite with 25% ZnTe. A slight decrease in the CH₄ yield (<5% for each run) is observed. Thus, the nanocomposite photocatalysts exhibit relatively good stability.

Fig. 10 Production of CH₄ as a function of time for different photocatalysts upon irradiation of visible-light with λ ≥ 420 nm.

The activity and the rate of CH₄ production for bare ZnTe were relatively high because it is active under visible-light irradiation, owing to its narrow band gap, as reported earlier; whereas, the efficiency was limited, due to recombination of the photogenerated charge carriers, as already indicated by the PL spectra (Fig. 9). In the case of the SrTiO₃–ZnTe nanocomposite with 25% ZnTe, the production rate of CH₄ and the conversion efficiency turned out to be the highest (Fig. 10). This may be ascribed mainly to the formation of nanocomposite with the close interface and uniform dispersion between ZnTe and SrTiO₃, due to the diffusion of ZnTe spheres into the cavities of the worm-like SrTiO₃ materials, as revealed by the above results. This can result in an efficient transfer of the photogenerated charge carriers from the CB of ZnTe to the CB of SrTiO₃, which will, thereby, facilitate the electron–hole separation to avoid possible recombination, giving rise to an efficient production of CH₄. Generally, this process of charge transfer from the CB of ZnTe to the CB of SrTiO₃ is believed to be faster and more efficient than the corresponding electron–hole recombination process, because of the feasible band alignment.³⁷

The mechanism for the photoreduction of CO₂ can be elucidated according to Fig. 8. First, the charge carriers (i.e., the electron–hole pairs) were generated by the photoexcitation of the catalyst.³⁸ For efficient electron transfer, the CB of ZnTe (ca. −1.80 V vs. SHE) must be more negative than the CB of SrTiO₃ (ca. −1.01 V vs. SHE). The photocatalytic reaction for the reduction of CO₂ is initiated by the absorption of visible-light photons with an energy equal or larger than the band gap energy of ZnTe (2.24 eV), resulting in the formation of charge carriers, i.e., photogenerated holes in the valence band of ZnTe semiconductor and electrons in its CB. The photogenerated electrons can then be transferred to the CB of SrTiO₃ and can move freely via the conducting network of the SrTiO₃, which can act as the electron reservoir. Hence, the charge separation is facilitated and the recombination of these photogenerated charge carriers is eventually hindered, leading to an improved photocatalytic activity of the nanocomposite.

The photogenerated holes in the valence band of ZnTe can adsorb the water molecules to produce hydroxyl radicals (˙OH), which subsequently result in the formation of H⁺ and O₂. The interaction of H⁺ with the excited electrons gives rise to the formation of ˙H radicals. The photogenerated electrons in the CB of SrTiO₃, transferred from the CB of ZnTe, can react with CO₂ to trigger the formation of very negative and metastable superoxide radicals (˙CO₂⁻),^39,40 which then react with the ˙H radicals to form CO. The CO can be converted into ˙C radicals via a carbide mechanism,^41,42 followed by the formation of a series of radicals, which eventually, leads to the production of CH₄.⁴² It is noted that it may also take place via the CO insertion mechanism, which involves the cleavage of the C–O bond of CO after incorporation into the growing hydrocarbon chain, rather than the cleavage of the C–O bond of the CO before the incorporation of a CH_x species into the growing hydrocarbon chain.⁴¹ Though it is possible to form other products (such as C₂H₄), CH₄ is the major product, possibly because the amount of ˙H is much higher than other radicals.

Conclusions

In summary, ZnTe–SrTiO₃ nanocomposite photocatalysts with different amounts of ZnTe were successfully synthesized for the photoreduction of CO₂ under visible-light illumination. CH₄ was found to be the major product. The nanocomposite photocatalysts exhibit improved efficiency for the production of CH₄, due to the efficient transfer and recombination suppression of the photogenerated charge carriers. The results in this work may facilitate further R&D into high-efficiency photocatalysts for the photoreduction of CO₂.

Acknowledgements

This work was supported by the Hundred-Talent Program of the Chinese Academy of Sciences.

Notes and references

1. R. Lal, Energy Environ. Sci., 2008, 1, 86–100.
2. A. Fujishima and K. Honda, Nature, 1972, 238, 37–38.
3. X. Feng, J. D. Sloppy, T. J. LaTempa, M. Paulose, S. Komarneni, N. Bao and C. A. Grimes, J. Mater. Chem., 2011, 21, 13429–13433.
4. C. C. Lo, C. H. Hung, C. S. Yuan and J. F. Wu, Sol. Energy Mater. Sol. Cells, 2007, 91, 1765–1774.
5. K. Teramura, T. Tanaka, H. Ishikawa, Y. Kohno and T. Funabiki, J. Phys. Chem. B, 2004, 108, 346–354.
6. K. Teramura, H. Tsuneoka, T. Shishido and T. Tanaka, Chem. Phys. Lett., 2008, 467, 191–194.
7. Z. Y. Wang, H. C. Chou, J. C. S. Wu, D. P. Tsai and G. Mul, Appl. Catal., A, 2010, 380, 172–177.
8. S. C. Yan, S. X. Ouyang, J. Cao, M. Yang, J. Y. Feng, X. X. Fan, L. J. Wan, Z. S. Li, J. H. Ye, Y. Zhou and Z. G. Zou, Angew. Chem., Int. Ed., 2010, 49, 6400–6404.
9. Y. Zhou, Z. Tian, Z. Zhao, Q. Liu, J. Kou, X. Chen, J. Gao, S. Yan and Z. Zou, ACS Appl. Mater. Interfaces, 2011, 3, 3594–3601.
10. N. Zhang, S. Ouyang, P. Li, Y. Zhang, G. Xi, T. Kako and J. Ye, Chem. Commun., 2011, 47, 2041–2043.
11. T. Inoue, A. Fujishima, S. Konishi and K. Honda, Nature, 1979, 277, 637–638.
12. M. Anpo, H. Yamashita, K. Ikeue, Y. Fujii, S. G. Zhang, Y. Ichihashi, D. R. Park, Y. Suzuki, K. Koyano and T. Tatsumi, Catal. Today, 1998, 44, 327–332.
13. Y. Li, W. N. Wang, Z. Zhan, M. H. Woo, C. Y. Wu and P. Biswas, Appl. Catal., B, 2010, 100, 386–392.
14. W. J. Ong, M. M. Gui, S. P. Chai and A. R. Mohamed, RSC Adv., 2013, 3, 4505–4509.
15. G. C. De, A. M. Roy and S. S. Bhattacharya, Int. J. Hydrogen Energy, 1996, 21, 19–23.
16. Y. Yang, Y. Sun and Y. Jiang, Mater. Chem. Phys., 2006, 96, 234–239.
17. Y. Qin, G. Wang and Y. Wang, Catal. Commun., 2007, 8, 926–930.
18. K. Xie, N. Umezawa, N. Zhang, P. Reunchan, Y. Zhang and J. Ye, Energy Environ. Sci., 2011, 4, 4211–4219.
19. D. Sui, X. Yin, H. Dong, S. Qin, J. Chen and W. Jiang, Catal. Lett., 2012, 142, 1202–1210.
20. W. Wei, Y. Dai, M. Guo, L. Yu, H. Jin, S. Han and B. Huang, Phys. Chem. Chem. Phys., 2010, 12, 7612–7619.
21. W. Wei, Y. Dai, H. Jin and B. Huang, J. Phys. D: Appl. Phys., 2009, 42, 055401 , (8pp).
22. H. Zhang, X. Wu, Y. Wang, X. Chen, Z. Li, T. Yu, J. Ye and Z. Zou, J. Phys. Chem. Solids, 2007, 68, 280–283.
23. J. W. Liu, G. Chen, Z. Li and Z. G. Zhang, J. Solid State Chem., 2006, 179, 3704–3708.
24. O. Ishitani, C. Inoue, Y. Suzuki and T. Ibusuki, J. Photochem. Photobiol., A, 1993, 72, 269–271.
25. O. Ozcan, F. Yukruk, E. U. Akkaya and D. Uner, Top. Catal., 2007, 44, 523–528.
26. C. C. Yang, Y. H. Yu, B. v. d. Linden, J. C. S. Wu and G. Mul, J. Am. Chem. Soc., 2010, 132, 8398–8406.
27. J. Schrier, D. O. Demchenko and L. W. Wang, Nano Lett., 2007, 7, 2377–2382.
28. T. Tanaka, K. Saito, Q. Guo and H. Ogawa, Appl. Phys. Express, 2009, 2, 122101.
29. Q. Wu, M. Litz and X. C. Zhang, Appl. Phys. Lett., 1996, 68, 2924–2926.
30. S. Kaniyankandy, S. Rawalekar, S. Verma and H. N. Ghosh, J. Phys. Chem. C, 2011, 115, 1428–1435.
31. Y. Wang, L. Yang and Y. Wang, Adv. Mater. Res., 2012, 424–425, 949–952.
32. B. B. Wang, M. K. Zhu, H. Wang, G. B. Dong and H. L. Xie, Mater. Sci. Semicond. Process., 2012, 15, 131–135.
33. M. N. Ashiq, M. F. Ehsan, M. J. Iqbal and I. H. Gul, J. Alloys Compd., 2011, 509, 5119–5126.
34. P. Scherrer, Mathematisch-Physikalische Klasse, 1918, 2, 98–100.
35. W. E. Vargas, J. Opt. A: Pure Appl. Opt., 2002, 4, 452–456.
36. M. E. S. Vergara, J. C. A. Huitron, A. R. Gomez and J. N. R. Burstin, Molecules, 2012, 17, 10000–10013.
37. L. Zhang, K. H. Wong, Z. Chen, J. C. Yu, J. Zhao, C. Hu, C.-Y. Chan and P.-K. Wong, Appl. Catal., A, 2009, 363, 221–229.
38. I. H. Tseng, W. C. Chang and J. C. S. Wu, Appl. Catal., B, 2002, 37, 37–48.
39. X. H. Xia, Z. J. Jia, Y. Yu, Y. Liang, Z. Wang and L. L. Ma, Carbon, 2007, 45, 717–721.
40. M. A. Asi, C. He, M. Su, D. Xia, L. Lin, H. Deng, Y. Xiong, R. Qiu and X. Z. Li, Catal. Today, 2011, 175, 256–263.
41. R. A. V. Santen, A. J. Markvoort, I. A. W. Filat, M. M. Ghouri and E. J. M. Hensen, Phys. Chem. Chem. Phys., 2013, 15, 17038–17063.
42. M. Tahir and N. S. Amin, Appl. Catal., A, 2013, 467, 483–496.

---

Footnote

† Electronic supplementary information (ESI) available: Energy Dispersive X-ray (EDX) analysis/mapping of as-prepared samples and stability study of photocatalyst. See DOI: 10.1039/c4ra06828a

---