Semiconductor facet junctions for photocatalytic CO₂ reduction

Introduction

Combustion of fossil fuels such as petroleum and coal, could provide enormous energy for sustaining the daily life and development of human society [1, 2]. Yet, alongside with energy release, most of the carbon atoms in fossil fuels end up coupling with two oxygen atoms to form carbon dioxide, a greenhouse gas [3, 4]. Therefore, with the rapid civilization of humans, the dependence of the human society on the fossil fuels has caused depletion of fossil fuel reserves and increased atmospheric greenhouse gas concentration, igniting serious energy and environmental issues [5, 6]. In this regard, it is important to search for a renewable energy source to substitute fossil fuels and reduce the carbon dioxide concentration in the atmosphere. With this aim in mind, the utilization of CO₂ as the carbon source to produce various valuable fuels (e.g., methanol, ethanol and ethylene), has been known as one of the most promising strategies [7, 8]. In fact, nature has long been achieved this target. Specifically, the green plants utilize their chlorophyll and energy provided by sunlight to convert the CO₂ and water into the energy source (i.e., glucose) to sustain their life [9]. However, these natural processes could not alleviate the increasingly serious greenhouse effect.

Imitating the natural process, Halmann proposed the photocatalytic CO₂ conversion technology which could utilize solar energy to drive the conversion of CO₂ and H₂O into various valuable organic compounds [10]. In his work, he has successfully converted the CO₂ into formic acid and formaldehyde, tentatively proving the viability of this technology. Since then, various semiconductors have been demonstrated to be effective for such a reaction [11], [12], [13], [14], [15], [16], [17]. However, the photocatalytic CO₂ conversion efficiency remains unsatisfactory for the practical application mainly due because of the prompt recombination of the photogenerated charge carrier on the single photocatalyst [18, 19]. To this context, different strategies have been employed to overcome this limitations, such as facets engineering [20, 21], morphology tuning [22, 23], cocatalyst loading [24, 25], impurity doping [26, 27], heterojunction construction [28], and defect engineering [29, 30]. Among them, facets engineering has been widely adopted due to its simplicity and high effectiveness [31].

The facet engineering for photogenerated charge carriers has been first demonstrated by Li and coworkers in 2013 [32]. In their work, they found that the photogenerated electrons and holes on the BiVO₄ could be separated on the different facets, allowing the spatial separation of photogenerated charge carriers and thus enhancing its photocatalytic performance. With the increasing report on the facets engineering, such a phenomenon has been systematically studied and understood [33, 34]. Specifically, due to the different atomic arrangements of different crystals facets of a semiconductor, band alignment can be formed between these facets, resulting in the formation of a junction (also known as facet junction) [35], [36], [37]. Facet junctions can be defined as single crystals containing more than two facets, which present evident facet-crosslinking effects due to the electronic structure difference of different faces in the same crystal. Such an electronic difference can allow the charge carrier separation across the different facets of a semiconductor crystal, thereby enhancing photocatalytic performance. Such a facet junction can guide the photogenerated electron–hole pairs to separate to different facets for enhancing photocatalytic performance [38, 39].

Since the pioneering report, the semiconductor facet junctions have been broadly applied for photocatalytic CO₂ conversion. This perspective aims for concisely overviewing the recent development of the semiconductor facet junctions for photocatalytic CO₂ conversion application. The unique features of the semiconductor facet junction for photocatalytic application will be first discussed. Then, the recent reports on the facet junction for photocatalytic CO₂ conversion application will be outlined. Lastly, the challenges and perspectives for the facet junction will be presented. This perspective is expected to give a timely report on the semiconductor facet junctions for photocatalytic applications.

Unique features of semiconductor facet junctions for photocatalytic CO₂ conversion

Over the past few years, the facet junctions have been substantially used and applied in photocatalytic CO₂ conversion. And it has been widely accepted to be one of the most potential ways for strengthening the photocatalytic CO₂ conversion performance. Generally, the facet junction system demonstrated various unique advantages, including spatial separating photogenerated charge carrier, minimizing the potential loss of a junction system and boosting the photogenerated charge carrier separation across the junction, for such a reaction [40], [41], [42].

Semiconductors facet junctions for photocatalytic CO₂ reduction

The utilization of facet junction photocatalysts for photocatalytic CO₂ conversion can be traced be as early as 2014 [21]. Based on theoretical simulations results, Yu et al. discovered that the {001} and {101} facets owned significantly different band structures (Fig. 1a and b). In detail, the {001} facets owned a significantly larger Fermi level than the {101} facets of the anatase TiO₂. Therefore, under light irradiation, the photogenerated electrons on {001} facets tended to migrate to the {101} facets; meanwhile, the photogenerated holes on the {101} facets inclined to transfer to the {001} facets, achieving the spatial separation of photogenerated charge carriers. Then, they systematically prepared anatase TiO₂ exposed with different {101} and {001} ratios (Fig. 1c–e). They found that anatase TiO₂ with a balance exposed ratio of {001} (58%) and {101} (42%) demonstrated the highest photocatalytic CO₂ conversion performance toward CH₄ production. This finding opens up a new vision for the modification of single crystals for photocatalytic CO₂ conversion and attracts broad attention from researchers.

Fig. 1:

General mechanism and properties of the TiO₂ facet junction.

(a, b) Density of states plots of {101} and {001} facets of anatase TiO₂ (a) and schematic illustration for the corresponding facet junction. (c) TEM image of the TiO₂ exposed with 89% {101} and 11% {001} facets (d, e) SEM images of the TiO₂ exposed with 42% {101} and 58% {001} facets (d) and with 17% {101} and 83% {001} facets. Adapted from [21].

It is well-studied that the adsorption capability of the photocatalysts toward reactant molecules is another important issue in facilitating the photocatalytic reaction [47]. In view of this, Xu et al. meticulously tuned the TiO₂ to expose with {001} and {100} facets (Fig. 2a) [46]. According to the theoretical simulations results, the co-exposed of the {001} and {100} facets could also allow the formation of the facet junction on the TiO₂. In addition, the {100} facets owns the highest CO₂ adsorption energy among the common exposed facets on anatase TiO₂, including {001} (1.820 eV), {100} (0.311 eV) and {101} (0.224 eV), enhancing the CO₂ adsorption capability of the TiO₂. As such, the optimized sample demonstrated a superior photocatalytic CO₂ conversion performance toward CH₄ (4.56 mmol g⁻¹ h⁻¹) and CH₃OH (1.48 mmol g⁻¹ h⁻¹) production (Fig. 2b).

Fig. 2:

Photocatalytic pollutant degradation performance of the TiO₂ facet junction.

(a) TEM image of TC. (b) The photocatalytic CO₂ conversion performance of the commercial TiO₂ (P25) and anatase with different {101}, {001} and {100} facets ratios of 100:26:39 (TC) and 100:34:33 (TW). Adapted from [46].

Truong et al. reported rutile TiO₂ facet junction with high-index facets for photocatalytic CO₂ conversion [48]. Generally, they prepared the high-index facets rutile TiO₂ via hydrothermal method using the titanium-glycolate complex as precursors and picolinic acid as structure-directing and shape controlling agent. As shown in Fig. 3a, the prepared samples demonstrated a multi-armed starlike structure. Based on the HRTEM and structural illustration, they confirmed that the rutile TiO₂ with {331} facets was successfully prepared. The {331} facets are periodically constituted by two (110) terraces and one (111) step (Fig. 3). Therefore, such a rutile TiO₂ dominated with {331} facets not only owned a large amount of stepped surface for providing the active surface site for catalytic reaction, but also owned facet junction composed of {110} facets and {111} facets for facilitating the photogenerated charge carrier separation. As a consequence, their prepared rutile TiO₂ exposed with {331} facets demonstrated a high photocatalytic CO₂ conversion for methanol production. This work demonstrated that the preparation of the semiconductor with high-index facets could be also a feasible way for synthesizing semiconductor facet junction.

Fig. 3:

Schematic illustrations of the rutile TiO₂ dominated with high-index {331} facets including its three-dimensional structure (a), [110] projection with the description of {331} facets (b) and [010] projection of the tetrahedral crystals. Adapted from [48].

Although many works have been done on preparing anatase TiO₂ with {001} and {101} facets, the prepared samples normally demonstrated disorder morphology, which greatly inhibits the photogenerated charge carrier between {001} and {101} facets. Against this background, Li and co-workers meticulously controlled the adsorption of HF during the growth of anatase TiO₂ to ensure the well-defined {001} and {101} facets on its surface (Fig. 4) [49]. Specifically, different from the other works, they employed the nanotube titanic acid (NTA) as the Ti precursors, which own a layered structure with an interlayer distance of ca. 0.8 nm (larger than the diameter of fluorine ions), allowing the quick embedding of fluorine ions during the growth of anatase TiO₂. Therefore, well-defined {001} and {101} facets can be formed on the TiO₂, facilitating the rapid photogenerated electron–hole pair separation between them. As a result, the optimized samples achieved a superior photocatalytic CO₂ conversion performance for CH₄ production.

Fig. 4:

Morphology of the TiO₂ facet junction.

(a, b) TEM (a) and HRTEM (b) images of TiO₂ prepared using 0.2 mL of HF (c, d) TEM (c) and HRTEM (d) images of TiO₂ prepared without HF. Adapted from [49].

Furthermore, Meng et al. demonstrated that the facet junction could be also formed on the BiOBr [50]. They scrupulously prepared the BiOBr facet junction with different facets pairs, including {001}/{110} and {010}/{102}. Based on the theoretical simulations test, they discovered that the {001} and {110} facets of the BiOBr have a large band structure difference, resulting in a slow photogenerated charge carrier transfer efficiency. In contrast, the band structure difference between {010} and {102} facets was relatively small, thus the photogenerated charge carriers could transfer rapidly across their junction. Accordingly, the photocatalytic CO₂ conversion performance of the former samples was significantly higher than the latter. This work clearly demonstrated that the rigorous selection of the target exposed facets on a facet junction is critical for optimizing the photocatalytic performance of a semiconductor.

In addition, Chen et al. methodically tuned the photogenerated electron–hole pair separation efficiency across the semiconductor facet junction by changing the thickness of the semiconductor crystals [51]. In detail, they tuned the layered thickness of the BiOlO₃ to optimize the photogenerated charge carrier separation across the {010} and {100} facets. As shown in Fig. 5a–d, more junctions could be formed between the {010} and {100} facets with the reduction in the thickness of BiOIO₃, resulting in a higher photogenerated charge carrier separation efficiency (Fig. 5e). As a result, the BiOIO₃ with the thinnest structure owned the highest photocatalytic CO₂ conversion performance for CO production. This result further confirms the importance of the optimization of the structure of the semiconductor facet junction in enhancing photocatalytic CO₂ conversion performance.

Fig. 5:

Morphology of the BiOIO₃ facet junction.

(a–d) SEM images of BiOIO₃ with different thicknesses, including 150 (a), 75 (b), 35 (c) and 15 nm (d). (e) The photogenerated charge carrier separation pathway on the BiOIO₃ facet junction. Adapted from [51]. (f–k) SEM images of the MIL-125-NH₂(Ti) with disk shaper (f), rhombic dodecahedron (g), octahedron (h), rectangle-like shape (i), truncated cube (j) and truncated octahedron (k). Scale bar: 500 nm. Adapted from [52].

Furthermore, Zhou and co-workers reported ZnSn(OH)₆ facet junction for photocatalytic CO₂ conversion [53]. Specifically, they meticulously tuned the exposed facets on the ZnSn(OH)₆ to have different {100} and {111} facets exposed ratio. Similar to the TiO₂ crystals, they found that the ZnSn(OH)₆ with moderate exposed {100} and {111} facets could form the facet junction, thereby boosting the photogenerated electron–hole pair separation efficiency. On the basis of the theoretical simulations results, the {100} facets own higher conduction and valence band than the {111} facets. Therefore, during the photocatalytic reaction, the photogenerated electrons and holes tend to accumulate on the {111} and {100} facets of ZnSn(OH)₆, respectively, resulting in spatial separation of the charge carriers. Therefore, the photocatalytic CO₂ conversion of the ZnSn(OH)₆ was significantly enhanced.

Furthermore, Sun and co-workers systematically investigated the photogenerated charge carrier separation mechanism on the MIL-125-NH₂ (Ti) by preparing a series of MIL-125-NH₂ (Ti) exposed with single and mixed low-index facets, including {001}, {110}, {111}, {001}/{110}, {001}/{111} and {110}/{111} (Fig. 5f–k) [52]. They found that, among the three exposed facets, the {001} played the role of the recombination center for the photocatalytic reaction, thereby it is unfavorable for the reaction. In addition, due to the band structure difference between all the facets on MIL-125-NH₂ (Ti), a facet junction could be formed by co-exposing any two of them on MIL-125-NH₂ (Ti). As such, the MIL-125-NH₂ (Ti) demonstrated the highest photocatalytic CO₂ conversion performance for CO and CH₄ production. Such work suggested that the meticulously chosen of exposed facets on the semiconductor is critical for optimizing its photocatalytic performance.

Different from the other facet junction works, which fabricate two or more facets on single semiconductor crystals, Zou and co-workers combined CeO₂ hexahedron prism with exposed {100} facets and CeO₂ octahedron with exposed {111} facets to build facet junction photocatalysts [54]. Specifically, they used phosphate ions (H₂PO₄⁻) as mineralizers to allow the growth of the CeO₂ hexahedron prism on the surface of the CeO₂ octahedral. As shown in Fig. 6, the content of the CeO₂ hexahedron prism increased with the increase in the phosphate ions concentration used during the sample preparation. Given that the CeO₂ hexahedron prism was in situ growth from the CeO₂ octahedral, they had an intimate contact interface, which allowed the rapid photogenerated charge carrier migration between them. Upon light irradiation, the photogenerated electrons could rapidly migrate from the CeO₂ hexahedron prism with exposed {100} facets to CeO₂ octahedron with exposed {111} facets because of the Fermi level difference between {100} and {111} facets, achieving spatial separation of photogenerated electron–hole pairs. As a consequence, the optimized sample achieved a superior photocatalytic CO₂ conversion performance toward both CO and CH₄ production.

Fig. 6:

SEM images of CeO₂ prepared using (a₁, a₂) 0% (CP0), (b₁, b₂) 0.5% (CP1), (c₁, c₂) 1% (CP2) and (d₁, d₂) 2% (CP3) H₂PO₄⁻ and corresponding illustrated structure of (a₃) CP0, (b₃) CP1, (c₃) CP2 and (d₃) CP3. Adapted from [50].

Although the photogenerated charge carrier separation across two different facets of a semiconductor has been proven to be effective, the potential in enhancing the photocatalytic CO₂ conversion performance is limited. As such, Wang et al. coupled the BiOCl, which owns well-defined {010} and {001} facets [55], with the CdS to form S-scheme heterojunction to boost the photocatalytic CO₂ conversion performance. An S-scheme heterojunction is mainly composed of reduction photocatalysts and oxidation photocatalysts, and the internal electric field formed at the interfaces, which compels the electrons in the conduction band of one semiconductor to transform to the valance band of the other semiconductor. In such a system, the electrons with weak reduction capability and holes with weak oxidation capability are recombined and eliminated, preserving electrons and holes with strong redox ability. They precisely loaded the CdS on the {010} facets of BiOCl, which accumulated photogenerated electrons on the BiOCl, allowing the photogenerated electrons with weak reduction capability on BiOCl to recombine with the photogenerated holes with weak oxidation capability of the CdS to optimize the redox capability of the system. As a result, the photocatalytic of the BiOCl with facet junction was significantly enhanced after the formation of the S-scheme with the CdS, achieving a photocatalytic CO and CH₄ production rates of 2.0 and 6.85 μmol g⁻¹, respectively. Such a work systematically revealed that the formation of the heterojunction using the semiconductor with well-defined exposed facets could be an ideal method for optimizing the photocatalytic CO₂ conversion performance.

Furthermore, Zhou et al. built the Z-scheme photocatalytic system using the BiVO₄ facet junction as oxidation semiconductor [56], Au as electron mediator and Cu₂O as reduction semiconductor. To demonstrate the importance of the loading of Au and Cu₂O on the proper facets of the BiVO₄, they intentionally loaded them on the {010} facets and {110} facets of the BiVO₄ and compared the physicochemical characters and photocatalytic CO₂ conversion performance of the obtained samples (Fig. 7). Given the presence of BiVO₄ facet junction, the photogenerated electrons and holes tended to accumulate on the {010} and {110} facets, respectively. On the Z-scheme system, the weak photogenerated electrons on the BiVO₄ should be eliminated with the weak photogenerated holes on the Cu₂O. Therefore, to facilitate this process, the Au and Cu₂O should be loaded onto the {010} facets of BiVO₄. In contrast, the loading of Au and Cu₂O on {110} facets of BiVO₄ could retard the process. Hence, the photocatalytic CO₂ conversion performance of the BiVO₄{010}–Au–Cu₂O was determined to be far higher than that of the BiVO₄{110}–Au–Cu₂O. Such a work showed that well-defined exposed facets of the semiconductor with facet junction could be employed for the preparation of highly efficient heterojunction system.

Fig. 7:

Preparation procedures and general properties of the BiVO₄ facet junction.

(a) The preparation procedures of the BiVO₄–Au–Cu₂O with different structures. (b) XRD patents of the prepared samples including BiVO₄, Cu₂O, Au loaded on {010} facets of BiVO₄ (BiVO₄{010}–Au), Au loaded on {110} facets of BiVO₄ (BiVO₄{110}–Au), Cu₂O loaded on {010} facets of BiVO₄ (BiVO₄{010}–Cu₂O), Au–Cu₂O loaded on {010} facets of BiVO₄ (BiVO₄{010}–Au–Cu₂O) and Au–Cu₂O loaded on {110} facets of BiVO₄ (BiVO₄{110}–Au–Cu₂O), (c–g) SEM images of BiVO₄ (c), BiVO₄{010}–Au (d), BiVO₄{110}–Au (e), BiVO₄{010}–Au–Cu₂O (f) and BiVO₄{010}–Au–Cu₂O (g), (h, i) TEM (h) and elemental mapping (i) images of the BiVO₄{010}–Au–Cu₂O. Adapted from [56].

Co-catalyst loading is another common strategy that has been widely applied in photocatalytic reactions. For example, Xiong et al. further enhanced the TiO₂ with facet junction through Pt loading [57]. They compared the Pt loading strategies on the TiO₂ with facet junction and demonstrated that the chemical deposition strategy is more beneficial for depositing Pt on the surface of the TiO₂ facet junction compared to the photodeposition strategy. This result was because the chemical deposition could result in a high Pt⁰/Pt^II ratio of Pt nanoparticles on the TiO₂ which could effectually improve the photogenerated electron–hole pair separation on TiO₂; in contrast, the photodeposition strategy could only produce a relatively low Pt⁰/Pt^II ratio of Pt nanoparticles. Therefore, the photocatalytic performance of the sample produced through the chemical deposition strategy was significantly higher than that through the photodeposition strategy. In addition, the same group of researchers employed metal-free carbon nanofibers to improve the photocatalytic CO₂ conversion performance of the TiO₂ facet junction [58]. The results showed that the carbon nanofibers could also boost the photogenerated charge carrier separation over the TiO₂ facet junction, thereby enhancing its photocatalytic CO₂ conversion performance for CO generation.

Typically, the co-catalysts can be categorized into reduction co-catalysts, which accept photogenerated electrons for reduction reaction, and oxidation co-catalysts, which accept photogenerated holes for the oxidation reaction. For the typical photocatalyst, the reduction and oxidation co-catalysts can hardly be simultaneously employed due to the unclear reduction and oxidation sites. For semiconductors with well-defined facets, such a problem can be overcome. For example, Meng et al. simultaneously loaded reduction co-catalysts (i.e., Pt) and oxidation co-catalysts (MnO_x) onto the TiO₂ exposed with {101} and {001} facets, respectively (Fig. 8) [59]. During the photocatalytic reaction, the photogenerated electrons and holes were first migrated from the bulk to the {101} and {001} facets, and further to Pt and MnO_x, respectively. Attributed to the effective separation of the photogenerated electron–hole pairs, the photocatalytic CO₂ conversion performance of the TiO₂ could be momentously enhanced. As a result, the TiO₂ loaded with Pt and MnO_x reached a CH₄ and CH₃OH production rates of 104 and 91 μmol m⁻², respectively.

Fig. 8:

Morphologies and chemical compositions of the TiO₂ facet junction loaded with Pt and MnO_x.

(a, b) SEM (a) and EDS spectrum (b) of TiO₂, (c, d) SEM (c) and EDS spectrum (d) of MnO_x loaded TiO₂, (e, f) SEM (e) and EDS spectrum (f) of Pt loaded TiO₂, (g, h) SEM (g) and EDS spectrum (h) of Pt and MnO_x co-loaded TiO₂. Adapted from [59].

Furthermore, Khodakov and co-workers employed the unique feature of BiVO₄ facet junction (Fig. 9a and b) in separating electrons and holes to selectively load the CuO_x and CoO_x onto its {010} and {110} facets, respectively [60]. Specifically, through the photoreduction reaction, the CuO_x could be deposited onto the {010} facets using Cu(NO₃)₂·3H₂O as precursors; meanwhile, through the photooxidation reaction, the CoO_x could be deposited onto the {110} facets using Co(NO₃)₃·6H₂O as precursors (Fig. 9c–e). As such, the CuO_x could form the direct Z-scheme heterojunction with BiVO₄ by accepting photogenerated electrons from the BiVO₄ and the CoO_x could act as co-catalysts for accepting photogenerated holes from the BiVO₄. Clearly, the simultaneous loading of the CuO_x and CoO_x could effectively accelerate the spatial separation of photogenerated electron–hole pairs. Consequently, the photocatalytic CO₂ performance of the BiVO₄ was significantly enhanced after loading of both CuO_x and CoO_x.

Fig. 9:

General information for the BiVO₄ facet junction loaded with CuO_x and CoO_x.

(a, b) The crystal model of monoclinic BiVO₄ with truncated tetragonal bipyramid shape. (c) The preparation procedure for the BiVO₄ loaded with CuO_x and CuO_x, (d–f) SEM images of the BiVO₄ loaded with CuO_x (d), BiVO₄ loaded with CoO_x (e) and BiVO₄ co-loaded with CuO_x and CoO_x. Adapted from [60].

In the whole incident solar spectrum, visible light accounts for more than 40%. Therefore, the photo-response of a semiconductor toward incident light in the visible light range is an important aspect of determining its practical applicability in photocatalytic reactions [62]. In this regard, Liu et al. tuned the light-responsive range of the anatase TiO₂ exposed with {001} and {101} facets via the defect engineering [61]. Specifically, after obtaining the anatase TiO₂ exposed with {001} and {101}, they introduced the oxygen vacancy on the sample through the solid-state method using NaBH₄ as the reducing agent. Based on the EPR characterization, it was confirmed that the oxygen vacancy was successfully introduced, showing the increase in the Ti³⁺ density on the sample. Such an increase in Ti³⁺ density could be also confirmed via the XPS characterization (Fig. 10a). In addition, as shown in Fig. 10b and c, the introduction of oxygen defects could significantly enhance the light absorption of the sample in visible light range, improving the light utilization capability of the sample. Then, they utilized their optimized sample for photocatalytic CO₂ conversion under visible light irradiation. The results showed that the photocatalytic performance of the TiO_2−x{001}–{101} was ca. 3 times higher than that of TiO₂{001}–{101} for CO production. This work evidently manifested that the introduction of defects is an effective strategy for enhancing the utilization capability of the semiconductor with facet junction. Such a strategy has been also employed by Xue and co-workers to enhance the photocatalytic CO₂ conversion performance and their obtained oxygen vacancies-rich black TiO₂ facet junction reached a CO production rate of 128.5 μmol g⁻¹ h⁻¹ [63].

Fig. 10:

Structural and optical properties of the defect TiO₂ facet junction.

(a) High-resolution Ti 2p XPS spectra of the different prepared samples. (b, c) Light absorption spectra of TiO₂ dominated with {101} facets (TiO₂{101}), TiO₂ dominated with {001} facets (TiO₂{001}) and TiO₂ with balance between exposed {101} and {001} facets, before (b) and after (c) introduction of oxygen vacancies. Adapted from [61].

In addition, Zhang et al. introduced the oxygen vacancies on the anatase TiO₂ facet junction via an atmospheric pressure dielectric barrier discharge cold plasma (DBDCP) strategy (Fig. 11a) [64]. Different from the above-mentioned solid-state method strategy, no environmentally hazardous reducing agents were required during DBDCP, allowing it to be a green method for introducing oxygen vacancies on the TiO₂ facet junction. More importantly, such a strategy allowed the introduction of the oxygen vacancies on the TiO₂ attached to the Ti foil (Fig. 11b). Compared to the particulate system, this film structured photocatalysts could be easily recycled and employed for long-term applications. The EPR characterization was used to confirm the successful introduction of Ti³⁺ species on the TiO₂ (Fig. 11c). Similarly, they found that the presence of the oxygen vacancies could greatly enhance the light absorption of the TiO₂ facet junction in the visible light range, thereby boosting the light utilization capability of the samples and enhancing the photocatalytic CO₂ conversion efficiency.

Fig. 11:

Preparation procedures for defect TiO₂ facet junction.

(a) Schematic illustration of the preparation procedure of the oxygen vacancies-modified TiO₂ facet junction via atmospheric pressure dielectric barrier discharge cold plasma strategy. (b) Schematic illustration of the crystal models of the TiO₂ facet junction with {001} and {101} facets (i), TiO₂ with {001} facets and a rough surface (ii) and TiO₂ film attached on the Ti-foil (iii). (c) EPR spectra of the pristine TiO₂ facet junction and oxygen vacancies-modified TiO₂ facet junction. Adapted from [64].

Impurity doping is another effective method to improve the light absorption capability of the semiconductor toward the visible light range. For instance, Liu et al. employed lanthanum (La) doping for intensifying the visible light photocatalytic performance of the TiO₂ facet junction (Fig. 12a and b) [65]. Specifically, the light absorption edge of the TiO₂ red-shifted after the impurity doping (Fig. 12c), implying the enhanced light-responsive range of the TiO₂ facet junction into the visible light range. In addition, the La doping could provide the additional surface active site for the photocatalytic reaction. Therefore, the photocatalytic CO₂ conversion performance of the TiO₂ facet junction was significantly improved for CO production after La doping.

Fig. 12:

Morphologies and optical properties of La-doped TiO₂ facet junction. (a, b) SEM images of the pure TiO₂ (a) and 1.0% La-doped TiO₂ (1.0%La TiO₂) (b). (c) Light absorption spectra of pure TiO₂ and TiO₂ doped with different concentration of La including 0.5% (0.5%La TiO₂), 1.0%, 1.5% (1.5%La TiO₂) and 2.0% (2.0%La TiO₂). Adapted from [65].

Conclusion and future perspectives

In short, this perspective aims for providing a timely highlight for the recent development in semiconductor facet junction for photocatalytic CO₂ conversion. We have summarized the unique features of the facet junction for photocatalytic applications and the recent works on the facet junction in photocatalytic CO₂ conversion. Obviously, the recent bloom of the reports on semiconductor facet junction can greatly push forward the development of photocatalytic applications. Although significant progress has been made in semiconductor facet junction, there are enormous challenges ahead.

1. The report on meticulous control of the exposed facets of a semiconductor remains scarce. Different from the other junction systems, the precise control of the exposed facets on a semiconductor is a very tedious job. Therefore, limited semiconductors have been applied to build junctions. Some common semiconductors, such as CdS and Cu₂O, which own relatively high photocatalytic CO₂ conversion performance, have not been applied to build facet junctions.

2. Although semiconductor facet junctions have already proven their potential, their photocatalytic CO₂ conversion performance remains unfavorable. Specifically, the photocatalytic CO₂ conversion efficiency and the selectivity toward C₂₊ compound production are far lower compared to the conventional thermocatalytic CO₂ conversion. Therefore, many works should be performed to combine such a facet junction system with other photocatalyst modification strategies, such as impurity doping, co-catalyst loading and functional grafting to enhance the photogenerated charge carrier separation for improving photocatalytic CO₂ conversion efficiency and promoting intermediate (e.g., C–O, CH₂ and –COOH) stabilization capability for facilitating C–C coupling toward C₂₊ compound production.

3. The facet junction on a single semiconductor is normally in a very small geometry (normally in the 50–100 nm scale). Therefore, the identification of the facet junctions on a semiconductor is a complicated task. Currently, the common strategy for facet junction identification is to load reduction and oxidation co-catalysts on the facet junction to confirm the photogenerated electron and hole accumulations sites on different facets of a semiconductor. Yet, such a strategy confronts several problems due to its indirect process. Therefore, the novel direct characterization technique for confirming the photogenerated charge carrier migration pathway is highly sought for the further development of facet junction photocatalyst.