A p–n Junction by Coupling Amine-Enriched Brookite–TiO₂ Nanorods with Cu_xS Nanoparticles for Improved Photocatalytic CO₂ Reduction

Abstract

Photocatalytic CO₂ reduction is a promising technology for reaching the aim of “carbon peaking and carbon neutrality”, and it is crucial to design efficient photocatalysts with a rational surface and interface tailoring. Considering that amine modification on the surface of the photocatalyst could offer a favorable impact on the adsorption and activation of CO₂, in this work, amine-modified brookite TiO₂ nanorods (NH₂-B-TiO₂) coupled with Cu_xS (NH₂-B-TiO₂-Cu_xS) were effectively fabricated via a facile refluxing method. The formation of a p–n junction at the interface between the NH₂-B-TiO₂ and the Cu_xS could facilitate the separation and transfer of photogenerated carriers. Consequently, under light irradiation for 4 h, when the Cu_xS content is 16%, the maximum performance for conversion of CO₂ to CH₄ reaches at a rate of 3.34 μmol g⁻¹ h⁻¹ in the NH₂-B-TiO₂-Cu_xS composite, which is approximately 4 times greater than that of pure NH₂-B-TiO₂. It is hoped that this work could deliver an approach to construct an amine-enriched p–n junction for efficient CO₂ photoreduction.

1. Introduction

The excessive emission of CO₂ resulting from the acceleration of industrialization and use of fossil fuel has caused global warming and serious environmental problems [1]. Photocatalytic CO₂ reduction, denoted as “artificial photosynthesis”, is a promising technology for CO₂ conversion [2,3,4]. It is critical to develop efficient photocatalysts through surface and interface engineering [5,6]. Semiconductor TiO₂ has attracted extensive attention concerning CO₂ photoreduction owing to its stability, low cost, and low toxicity. However, TiO₂ is an n-type semiconductor with a wide band gap, and the disadvantages of limited light harvesting and poor electron–hole pair separation are not conducive to highly efficient photocatalytic CO₂ reduction [7,8,9].

Construction of a p–n junction is one promising approach to facilitate the separation of photogenerated carriers and improve the utilization of solar energy [10,11,12,13]. Fan et al. reported that 3D CuS@ZnIn₂S₄ p–n heterojunctions with 2D/2D nanosheet subunits can promote the separation of photogenerated carriers and accelerate carrier transfer [14]. Yu et al. suggested that a CuO/TiO₂ p–n heterojunction can improve the separation efficiency of photogenerated electron–hole pairs [15]. It is well known that CuS and Cu₂S are p-type semiconductors with narrow bandgaps [16,17]. When coupling Cu_xS with TiO₂, upon light irradiation, an internal electric field is established with the formation of a p–n heterojunction. Accordingly, the lower flat band potential of Cu_xS allows for the transfer of photoexcited electrons from Cu_xS to TiO₂, while the holes diffuse from TiO₂ to Cu_xS [18,19], which could suppress the charge recombination to achieve an efficient separation of electrons and holes during the photocatalytic process. In this regard, the formation of a TiO₂/Cu_xS p–n junction has potential for improving photocatalytic CO₂ reduction. However, most of the studies upon TiO₂/CuS p–n junctions focus on the applications of photocatalytic pollution degradation and hydrogen (H₂) energy generation from water [19,20,21,22]. There are few works relevant to photocatalytic CO₂ reduction by designing TiO₂/CuS composites. Recently, Lee et al. [23] reported a CuS_x-TiO₂ film could effectively promote photogenerated charge separation for CO₂ photoreduction. It is still a challenge to rationally design CuS_x-TiO₂ p–n junctions for efficient CO₂ photoreduction.

Due to the unique surface state and higher conduction band position of brookite TiO₂ compared to anatase and rutile [24,25], great potential has emerged in the field of photocatalytic CO₂ reduction. Liu et al. reported that defective brookite TiO₂ had the highest yield for CO and CH₄ production among the three TiO₂ polymorphs [26]. Subsequently, Peng’s group studied exposed-crystal-face controlling [27], the construction of heterojunctions [28], and supported metal cocatalysts and dual cocatalysts [29,30] to improve the CO₂ photoreduction activity of brookite TiO₂. In fact, the activation and adsorption of CO₂ are significant factors to enhance CO₂ photoreduction [31,32,33]. Surface amine modification has attracted great attention in this issue, because the amine groups can not only promote the adsorption and activation of CO₂, but also coordinate with other metal ions to bind closely. Jin et al. [32] reported that surface amine modification enhances the activity of metal@TiO₂ photocatalysts. On the basis of the above backgrounds, in this work, amine-modified brookite TiO₂ nanorods coupled with Cu_xS nanoparticles has been successfully fabricated. A significant p–n junction is formed between the NH₂-B-TiO₂-Cu_xS interface, which effectively improves the transfer and separation of charge carriers. The composition and morphology of NH₂-B-TiO₂ are characterized and the improved performance of photocatalytic CO₂ reduction is also discussed.

2. Experimental

2.1. Reagents

All the analytical reagents were used without advance refinement. Tetrabutyl titanate (TBOT), thioacetamide (TAA) and copper (II) acetate monohydrate (Cu(CH₃COO)₂·H₂O) were purchased from Aladdin in China. Ethanediamine (EDA), ethylene glycol (EG) and absolute ethyl alcohol were provided by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the experiments were conducted with deionized water.

2.2. Materials Synthesis

Synthesis of amine-modified brookite TiO₂ (NH₂-B-TiO₂): First, titanium glycolate precursor was synthesized based on our previous reports [34,35,36]; 5 mL tetrabutyl titanate (TBT) was placed into a round-bottomed flask containing 180 mL ethylene glycol (EG) and swirled with magnetic force for 1 h at 120 °C. The white powder material was washed five times with deionized water and once with anhydrous ethanol before being dried at 60 °C for 48 h. To make amine-modified brookite TiO₂, 0.5 g of the as-prepared titanium glycolate precursor was disseminated in 35 mL of deionized water and 35 mL of ethylenediamine by ultrasonic treatment, and this mixture was uniformly transferred to a 100 mL Teflon-lined stainless-steel autoclave and then placed in an oven at 180 °C for 12 h. Following filtration and washing with deionized water 5 times and with anhydrous ethanol once, finally, the product was gathered after drying at 60 °C for 12 h.

Synthesis of brookite TiO₂ (B-TiO₂): Thus method is similar to the method used to prepare amine-modified brookite TiO₂. Firstly, titanium glycolate precursor was synthesized and then 0.5 g of the as-prepared titanium glycolate precursor was dispersed in 64 mL of deionized water and 6 mL NaOH (1 mol/L) using ultrasonic treatment, which was evenly transferred to a 100 mL Teflon-lined stainless-steel autoclave and later heated at 180 °C for 12 h. Following filtration and washing with deionized water and anhydrous ethanol to pH = 7, the product was gathered and dried at 60 °C for 12 h.

Synthesis of Cu_xS particles: First, 100 mg Cu(CH₃COO)₂·H₂O (0.5 mmol) was added to a round-bottomed flask containing 20 mL of anhydrous ethanol in an oil bath held at 80 °C under magnetic stirring, to which 42 mg of TAA was added, and kept at reflux for 4 h. The obtained product was centrifuged and washed numerous times with distilled water and anhydrous alcohol.

Synthesis of NH₂-B-TiO₂-Cu_xS: As-synthesized NH₂-B-TiO₂ (12.5, 25, 50 mg, respectively) and 10 mg Cu(CH₃COO)₂·H₂O were added to a round-bottomed flask containing 20 mL of anhydrous ethanol in an oil bath held at 80 °C under magnetic stirring, then we added 42 mg of TAA and kept at reflux for 4 h. Then, the suspension was washed with distilled water and distilled alcohol several times via centrifugation. Finally, the obtained product was dried at 60 °C for 12 h. Different ratios of NH₂-B-TiO₂-Cu_xS composites were denoted as NH₂-B-TiO₂-Cu_xS-n, where n represented the molar ratio of Cu_xS, and the values of n were 8, 16, and 32, respectively.

2.3. Characterization of Photocatalysts

X-ray powder diffraction (XRD) measurements were carried out on an Ultima IV X-ray diffractometer with Cu Kα radiation in a range of 10–80° and the scan rate was 10°/min at 40 kV and 30 mA. Fourier-transform infrared (FT-IR) spectra were collected on a Nicolet iS10 IR spectrometer to analyze the chemical bonds and functional groups of the material. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) techniques on a JSM 2100 electron microscope operating at a 200 kV accelerating voltage were used to display the morphologies and elemental dispersion of the samples. X-ray photoelectron spectroscopy (XPS) spectra were measured using Thermo Scientific K-Alpha with an Al Kα X-ray sources (hν = 1486.6 eV, 12 kV 6 mA). All binding energies were calibrated through the C 1s peak at 284.8 eV. UV–vis diffuse reflectance spectrum (UV–vis DRS) was investigated on a Shimadzu UV-2550 spectrometer in a range of 200–800 nm. The photocurrent and electrochemical impedance measurements were performed on a CHI 760E electrochemical workstation, including a standard three-electrode system in 0.2 M Na₂SO₄ solution, where Pt wire, Hg/HgCl₂, and the as-prepared product were used as counter electrode, reference electrode, and working electrode, respectively. Mott–Schottky (M-S) and photocurrent decay plots were also carried out on a CHI 760E electrochemical workstation in 0.2 M Na₂SO₄ solution. Among them, the preparation process of the working electrode involved weighing 10 mg of the sample in the sample tube and then adding 1 mL N, N dimethylformamide (DMF) and 20 μL Dupont Nafion membrane solution, which was then stirred for 30 min; after it was completely uniform, we used a pipette gun to transfer 30 μL of the mixed solution coated on the FTO conductive glass. Finally, it was dried in a vacuum oven.

2.4. Evaluation of Photocatalytic CO₂ Reduction

The photocatalytic reduction of CO₂ was evaluated under irradiation of 300 W xenon lamp and its wavelength was used to simulate sunlight in a gas-closed quartz reactor with a volume of 200 mL. Typically, 50 mg of the as-prepared catalyst was dispersed completely in 1 mL of deionized water in a glass Petri dish, which was then transferred to a quartz reactor with a bottom of 10 mL of deionized water. The reactor was bubbled with high-purity CO₂ gas for half an hour and the air inside was exhausted prior to illumination. Then, the reaction system was sealed and we turned on the xenon lamp. Subsequently, 1 mL of gaseous product was extracted from the glass reactor by a sampling needle every 1 h, and an irradiation duration of 4 h was applied and analyzed using a gas chromatograph (GC-7900, CEAULIGHT Beijing, China) equipped with an FID detector while N₂ gas served as the carrier gas. The reactor temperature was maintained at 25 °C and atmospheric pressure after starting the photocatalytic reaction.

3. Results and Characterization

The formation process of Cu_xS particles supported on amine-modified brookite nanorods is shown in Figure 1. Firstly, the titanium glycolate precursor is synthesized by a simple reflux method. Subsequently, ethylenediamine is introduced to prepare amine- modified brookite TiO₂ nanorods via a hydrothermal method. Finally, Cu_xS nanoparticles are deposited on the as-synthesized brookite TiO₂ nanorods by a refluxing method using copper acetate monohydrate and thioacetamide (TAA) as the precursors.

Figure 2a shows the X-ray diffraction (XRD) pattern of the as-prepared samples to analyze the composition of the sample. It can be seen that the prepared pure brookite TiO₂ matches well with the standard one (JCPDF No. 15-875), while the obtained Cu_xS corresponds to a mixture of the CuS phase (JCPDF No.65-3588) and low-chalcocite Cu₂S phase (JCPDF No. 65-3816). It can be found that the peaks of brookite TiO₂ appear in all the as-prepared composites. However, the peaks of Cu_xS can only be found in the NH₂-B-TiO₂-Cu_xS 32% sample, which may be due to the strong diffraction peaks of brookite TiO₂ and low contents of Cu_xS particles in this composite. The UV-Vis absorption (UV-DRS) spectra of the as-synthesized materials are displayed in Figure 2b. It became apparent that the pristine NH₂-B-TiO₂ has an absorption edge at around 390 nm. However, the NH₂-B-TiO₂-Cu_xS composites show red shift to the visible light region (400–800 nm), and the light absorption intensity gradually increases with the increase in Cu_xS loading. Further, the Fourier-transform infrared (FT-IR) is used to analyze the chemical bonds and functional groups of the sample. Figure 2c shows FT-IR spectra of the NH₂-B-TiO₂ and NH₂-B-TiO₂-Cu_xS-16% samples, in which the brookite TiO₂ (B-TiO₂) (XRD in Figure 2d) is also prepared in the presence of NaOH for comparison. Indeed, the B-TiO₂ has no amine modification, while the NH₂-B-TiO₂ and NH₂-B-TiO₂-Cu_xS samples have two new peaks located at ~1620 and ~3400 cm⁻¹, respectively, which may be attributed to the N–H bending vibration and N–H stretching vibration, respectively [37,38].

To further confirm the composition of the sample, X-ray photoelectron spectroscopy (XPS) analysis was also performed to study the composition and surface chemical status of samples. Figure 3a shows the full XPS survey spectra of the NH₂-B-TiO₂, Cu_xS, and NH₂-B-TiO₂-Cu_xS samples. Among them, the full XPS survey spectra of the NH₂-B-TiO₂-Cu_xS composite confirms the existence of N, Ti, O, Cu, and S elements. In Figure 3d, for N 1s in NH₂-B-TiO₂, the binding energy peak at 400 and 401.2 is attributed to the NH₂ group and N-H bonds [37,39], respectively, which suggests amine modification was achieved. After loading Cu_xS, the N 1s peak shifted positively and it was proposed that the amines donate their lone pair of electrons on N atoms to Cu. For NH₂-B-TiO₂, the high-resolution XPS spectra of Ti 2p are shown in Figure 3b, and two tiny peaks observed at 458.6 and 464.3 eV are related to Ti 2p_3/2 and Ti 2p_1/2, respectively, which indicates the presence of Ti⁴⁺ [40,41]. Furthermore, Figure 3c exhibits two peaks located at 529.8 and 530.7 eV, which correspond to lattice oxygen and chemisorbed and dissociated oxygen, respectively [42,43,44]. For Cu_xS, as shown in Figure 3e, the peaks at 932.6 and 952.7 eV belong to Cu 2p_1/2 and Cu 2p_3/2 of Cu⁺ species, respectively [45,46], while the peak positions at 935.6 and 955.6 eV are consistent with Cu 2p_3/2 and Cu 2p_1/2 of Cu²⁺ [47]. These characteristic peaks with a spin–orbit separation of 20 eV indicate the presence of Cu²⁺ ions [16]. In addition, the other two satellite peaks of Cu⁺ and Cu²⁺ appear at 940.7 and 944.7 eV, respectively. Figure 3f displays the high-resolution spectrum at the S 2p region. The typical peak at 162.3 eV is from S 2p of S²⁻ [48,49]; moreover, a small peak at 169.1 eV is ascribed to SO₄²⁻, which is due to S²⁻ being oxidized partially [50,51]. However, in the NH₂-B-TiO₂-Cu_xS composite, all the binding energies of Ti, O, and N shift to higher regions, whereas Cu and S shift to lower binding energies compared with the pristine TiO₂ and Cu_xS. It might be attributed to chemical interaction and electron transfer in the p–n junction formed at the interface of NH₂-B-TiO₂ and Cu_xS [52].

The morphology structures of the as-prepared materials are further determined by SEM. Figure 4a shows that the as-prepared NH₂-B-TiO₂ is a rod-like structure with a diameter of 600–800 nm and a length of 2.5–3.5 μm. It can be observed that the microrods assemble with many tiny nanoparticles distributed on the surface. Figure 4b is an SEM image of Cu_xS, in which the size and morphology are difficult to determine due to the phenomenon of particle aggregation. As shown in Figure 4c, Cu_xS particles are dispersed on the surface of NH₂-B-TiO₂ nanorods. In addition, energy-dispersive X-ray spectroscopy (EDX) mapping images of the NH₂-B-TiO₂-Cu_xS composites show the presence and even distribution of Ti, O, N, Cu, and S elements on the surface of the nanorod in Figure 4d–i, which further confirms that NH₂-B-TiO₂ and Cu_xS are hybridized uniformly.

In order to explore further CO₂ photoconversion efficiency, the photocatalytic CO₂ reduction performances of the as-obtained materials are evaluated in the presence of H₂O using a gas chromatograph. As shown in Figure 5a, under the irradiation of a 300 W xenon lamp, only CH₄ was detected in the photocatalytic process of all samples, which may be because the prepared materials only meet the reduction potential of CO₂ reduction to CH₄ (E⁰ = −0.24 eV). All the samples have the activity of photocatalytic reduction of CO₂ into CH₄, except for the bare Cu_xS. Among those samples, the brookite TiO₂ modified with amines (NH₂-B-TiO₂) has superior capability to that of B-TiO₂, indicating that amine modification has a positive effect on the reduction of CO₂ to CH₄. With loading of Cu_xS nanoparticles, the NH₂-B-TiO₂-Cu_xS composites have enhanced performances for photocatalytic CO₂ reduction into CH₄. In particular, the NH₂-B-TiO₂ shows a low CH₄ production rate of about 0.73 μmol g⁻¹ h⁻¹, while the optimized NH₂-B-TiO₂-Cu_xS-16% sample has a yield rate of 3.34 μmol g⁻¹ h⁻¹, which is 4-fold more than that of the pure NH₂-B-TiO₂. Those results suggest that the Cu_xS could act as a cocatalyst in the amine-enriched B-TiO₂-Cu_xS composite for improvements in photocatalytic CO₂ reduction compared to pristine ones. Typically, to evaluate the stability of the as-synthesized photocatalyst, the photocatalytic CO₂ reduction activity with five-run cycling of the NH₂-B-TiO₂-Cu_xS 16% sample is tested (Figure 5b). It can be found that the activity of CO₂ photoconversion to CH₄ is unstable, and the yield of CH₄ production gradually decreases, which could be attributed to the deactivation of the composite resulting from the oxidation of Cu_xS.

Figure 6 shows the XPS spectra of Cu 2p of the NH₂-B-TiO₂-Cu_xS 16% sample after the photocatalytic reaction. It can be observed that the valence state of copper displays obvious changes. The binding energies of 932.7 and 952.7 eV are ascribed to Cu 2p_3/2 and Cu 2p_1/2, respectively, a typical peak location of Cu²⁺ in CuS [14,20]. A weak satellite peak around 944 eV further indicates the presence of Cu²⁺ [53]. These results indicate that Cu⁺ was completely oxidized to Cu²⁺ after the photocatalytic reaction in this photocatalysis system, which might weaken the cycling performance of the NH₂-B-TiO₂-Cu_xS composite.

As a matter of fact, as shown in the Mott–Schottky curve in Figure 7a–c, the slope of NH₂-B-TiO₂ is positive, suggesting an n-type semiconductor [54]. At the same time, the slope of Cu_xS is negative, suggesting that Cu_xS is an p-type semiconductor. However, the NH₂-B-TiO₂-Cu_xS-16% composite exhibits an inverted “V- shape”, being a symbol of a typical p–n junction [55,56], which indicates that a p–n heterojunction could be constructed between the NH₂-B-TiO₂ and Cu_xS. In addition, the formation of the p–n heterojunction is further confirmed by valence band (VB)-XPS and core-level spectrum analyses. As shown in Figure 7d–f, the band alignment of NH₂-B-TiO₂ and Cu_xS between the NH₂-B-TiO₂-Cu_xS heterojunction interface can be calculated according to the following: Equations (1)–(3) [57].

$$\Delta {\mathrm{E}}_{\mathrm{VBO}}=({\mathrm{E}}_{\mathrm{CL}}^{{\mathrm{Cu}}_{\mathrm{x}}\mathrm{S}}-{\mathrm{E}}_{\mathrm{VBM}}^{{\mathrm{Cu}}_{\mathrm{x}}\mathrm{S}})-({\mathrm{E}}_{\mathrm{CL}}^{{\mathrm{TiO}}_2}-{\mathrm{E}}_{\mathrm{VBM}}^{{\mathrm{TiO}}_2})+\Delta {\mathrm{E}}_{\mathrm{CL}}^{\mathrm{Int}}$$

(1)

$$∆{\mathrm{E}}_{\mathrm{CL}}^{\mathrm{Int}}=({\mathrm{E}}_{\mathrm{CL}}^{{\mathrm{TiO}}_2}-{\mathrm{E}}_{\mathrm{CL}}^{{\mathrm{Cu}}_{\mathrm{x}}\mathrm{S}}){ }^{{\mathrm{NH}}_2-\mathrm{B}-{\mathrm{TiO}}_2-\mathrm{CuxS}-16\%}$$

(2)

$$∆{\mathrm{E}}_{\mathrm{CBO}}={\mathrm{E}}_{\mathrm{g}}^{\mathrm{CuxS}}-{\mathrm{E}}_{\mathrm{g}}^{{\mathrm{TiO}}_2}$$

(3)

In the above equations, the $\Delta$E_VBO represents the valence band offset, which is the energy difference between the core energy level (E_CL) and the valence band maximum (E_VBM) in a pure material; meanwhile, $\Delta {\mathrm{E}}_{\mathrm{CL}}^{\mathrm{Int}}$ illuminates the energy difference between the core levels. ∆E_CBO represents the conduction band offset. Further, the band gap of the as-synthesized materials is calculated according to the Kubelka–Munk function in Figure 8a,b [58,59], and the band gap energies of pure NH₂-B-TiO₂ and Cu_xS are 3.14 and 2.04 eV, respectively. Based on the information reflected from the XPS and DRS analyses, Figure 8e reveals$\Delta$E_VBO = 2.38 eV and ∆E_CBO = 1.28 eV for the NH₂-B-TiO₂-Cu_xS nanocomposite. The VB-XPS spectra are verified, as shown in Figure 8c,d, as the valence band position for NH₂-B-TiO₂ and Cu_xS is 2.74 and 1.48 eV, respectively [60]. Therefore, the conduction band position of NH₂-B-TiO₂ and Cu_xS can be calculated as −0.4 and −0.56, respectively. In this regard, the formation of such a p–n junction and the resulting charge transfer are shown in Figure 8e. After contact, the Fermi levels of NH₂-B-TiO₂ and Cu_xS move down and up, respectively, until an equilibrium state is reached. When a built-in electric field between the NH₂-B-TiO₂ and Cu_xS interface is established, this allows for the electrons in Cu_xS to migrate to NH₂-B-TiO₂ while the holes in NH₂-B-TiO₂ are transferred to the Cu_xS. It is proposed that Cu_xS as a cocatalyst promotes the efficient separation of photogenerated charge carriers.

Generally, the separation efficiency of photogenerated electrons and holes has a significant impact on the photocatalytic performance [61,62,63]. Herein, photo/electrochemical measurements are carried out to study the charge transfer of the as-synthesized materials. In Figure 9a, the photocurrent density of the NH₂-B-TiO₂-Cu_xS-16% composite is higher than pure NH₂-B-TiO₂, which indicates that the loading of Cu_xS can effectively prevent recombination of the photogenerated electrons and holes. As shown in Figure 9b, the semicircle radius of the NH₂-B-TiO₂-Cu_xS-16% sample is smaller than that of pure NH₂-B-TiO₂, implying that the charge carriers have a rapid transfer rate on the composite.

Based on the above results and discussions, a possible CO₂ photoreduction process is proposed in Figure 9c. Before the photoreduction reaction, the surface amine modification is helpful for the adsorption and activation of CO₂ [31,37]. Under irradiation of a light source, n-type (NH₂-B-TiO₂) and p-type semiconductors (Cu_xS) generate photogenerated electrons in the conduction band (CB) and holes in the valence band (VB). The CB of NH₂-B-TiO₂ is more positive than that of Cu_xS, and the VB of Cu_xS is more negative than that of NH₂-B-TiO₂. After contact between NH₂-B-TiO₂ and the Cu_xS interface, the built-in electric field is established, which promotes the migration of photoexcited electrons from the CB of Cu_xS to NH₂-B-TiO₂ and the migration of holes from the VB of NH₂-B-TiO₂ to Cu_xS, and these facilitate the separation and transfer of photogenerated electrons and holes. Further, the rate of reduction of CO₂ to CH₄ by the photoinduced electrons in CB of NH₂-B-TiO₂ is improved.

As shown in

Table 1. A comparative study on photocatalytic CO₂ reduction upon different photocatalysts.

Photocatalyst	Light Source	Reaction Condition	CH4 Production Rate [μmol/g/h]	References
NH2-B-TiO2-CuxS	300 W Xe lamp	H2O vapor	3.34	This work
Pt-Cu2O/TiO2	300 W Xe lamp	H2O vapor	1.42	[64]
TiO2/g-C3N4	300 W Xe lamp	H2O vapor	2.50	[28]
Au@TiO2	300 W Xe lamp	H2O vapor	2.52	[65]
CdS/rGO/TiO2	300 W Xe lamp	H2O vapor	0.063	[66]
Mg-TiO2	300 W Xe lamp	H2O vapor	1.0	[67]

, the CO₂ photoreduction activity of the NH₂-B-TiO₂-Cu_xS composite is higher than that of other TiO₂-based binary and ternary composites reported previously.

4. Conclusions

In summary, an amine-enriched p–n junction upon the NH₂-B-TiO₂-Cu_xS composite was successfully prepared. The modification of amine on the surface of the photocatalyst has a positive effect on the enhancement of CO₂ activity. The photocatalytic CO₂ reduction activity of amine-modified brookite TiO₂ is higher than that of amine-free modified brookite TiO₂. Further, coupling different contents of Cu_xS with NH₂-B-TiO₂, the NH₂-B-TiO₂-Cu_xS-16% composite exhibits the greatest CH₄ yield rate of 3.34 μmol g⁻¹ h⁻¹ following 4 h of lighting, which is 4 times higher than that of pure NH₂-B-TiO₂. Combining the valence band XPS spectra with photo/electrochemical measurements, the formation of a p–n junction between the NH₂-B-TiO₂-Cu_xS interface was confirmed. With the formation of such a heterojunction, the recombination of photogenerated electrons and holes is inhibited, thereby greatly improving the photocatalytic CO₂ reduction activity. It is hoped that this work could provide an approach to construct amine-enriched p–n junctions for efficient CO₂ photoreduction.