TiO2-based heterojunctions for photocatalytic hydrogen evolution reaction
0 Abstract
Solar-driven photocatalysis hydrogen evolution is a promising method to generate hydrogen from water, a green and clean energy source, using solar and semiconductors. Up to now, TiO2 still represents the most inexpensive and widely studied metal oxide semiconductors for photocatalysis. TiO2 coupling with other semiconductors to form heterojunctions is considered an efficient way to improve photocatalytic performances. In this review, TiO2-based heterojunctions are classified into conventional, p-n type, Z-scheme, S-scheme, and other heterojunctions based on their band structures. The photocatalytic mechanisms of various types of heterojunctions are described in detail. In order to rationally design and better synthesize heterojunctions with excellent performance, the contribution of theoretical calculations to the field of TiO2-based heterojunction photocatalysts and the key role of theoretical prediction are also discussed. Finally, the opportunities and current challenges to promote photocatalytic performance are provided to assist the design of TiO2-based heterojunction photocatalysts with superior performance.
Keywords
INTRODUCTION
Energy and environmental crises are the two key global issues in the wake of rapid industrial development and population growth[1-3]. Developing new and cleaner energy technologies to address the problems is extremely necessary[4,5]. Among the numerous emerging new energy technologies, photocatalysis hydrogen evolution technology that mainly utilizes solar energy to generate hydrogen from water is a clean, green, and environmentally friendly way and thus has been favored by increasing researchers[6-8].
Since the discovery by Fujishima in 1972, water could be decomposed into oxygen and hydrogen on TiO2 electrodes under light[9]. TiO2 represents a typical photocatalyst that is an inexpensive, environmentally friendly, and stable n-type semiconductor [Figure 1][10]. Currently, TiO2-based photocatalysts play a pivotal role in the fields of pollutant degradation treatment, hydrogen evolution, and oxygen evolution [Figure 1]. Due to the high specific surface area of anatase TiO2, more active electrons are generated more easily by photoexcitation. So, anatase TiO2 is the most studied photocatalyst[11]. However, due to the wide bandgap of TiO2, it is only responsive to ultraviolet (UV) light, limiting its development in photocatalysis[12,13]. To address this problem, it has been reported that coupling TiO2 with other semiconductors to form heterojunctions can effectively improve the light absorption range and promote the separation of photogenerated electron-hole(e-/h+) pairs to enhance the photocatalytic activity. At present, researchers have developed a large number of TiO2-based heterojunction photocatalysts, such as g-C3N4/TiO2[14], b-N-TiO2/Ag3PO4[15], and TiO2/FePS3[16].
Figure 1. The advantages and applications of TiO2-based photocatalysis heterojunctions.
In recent decades, TiO2-heterojunction photocatalysts have achieved substantial advancements in photocatalytic H2 evolution. This review primarily examines the progress of TiO2 heterojunction modifications for H2 evolution photocatalysis since 2019. Given the growing interest in photocatalytic H2 evolution, summarizing recent studies on TiO2-based heterojunction photocatalysts is crucial to propel practical applications. Herein, heterojunctions are categorized as Type-I, II, III, p-n, S-scheme, Z-scheme, and other semiconductor types based on energy band arrangement. Emphasis will be placed on (1) the mechanism of TiO2 photocatalysis hydrogen evolution; (2) details of the mechanism of action of various types of heterojunctions and recent advances in the field of photocatalysis hydrogen evolution; (3) a summary and comparison of the hydrogen evolution rates for different types of heterojunction photocatalysts; (4) the importance of density-functional theory (DFT) calculation in the field of heterojunction photocatalysis is outlined; and (5) challenges and prospects for the construction of advanced TiO2-based heterojunctions.
FUNDAMENTAL UNDERSTANDING OF PHOTOCATALYSIS HYDROGEN EVOLUTION
It is well known that the bandgap and position of band edges of a semiconductor are very important for photocatalysts. The bandgap needs to be at least 1.23 eV for water splitting. Moreover, the Conduction band (CB) edge should be more negative than the reduction potential of H2 (
Figure 2. Schematic diagram of photocatalysis water splitting mechanism.
TiO2-BASED HETEROJUNCTION PHOTOCATALYSTS
The coupled heterojunctions could facilitate the separation of photogenerated carriers, reduce the recombination of e-/h+, and improve the photocatalysis efficiency. Heterojunctions can be classified into traditional type-I, type-II, type-III, p-n heterojunction, Z-scheme, and emerging S-scheme heterojunction according to their energy band structures. Among them, the energy band structures of type I, type II, and type III are the straddling[24], staggered[25], and broken gaps[26], respectively. The p-n heterojunction composed of p- and n-type semiconductors with type-II energy band arrangement driven by built-in electric field[27], the Z-scheme heterojunction with electron-dielectric-driven charge separation[28], and the emerging S-scheme heterojunction formed by band bending due to the difference in Fermi energy levels[29].
Conventional semiconductor heterojunction photocatalyst
Conventional heterojunctions can be categorized into type-I, type-II, and type-III [Figure 3]. In straddling gap type-I heterojunction, the CB of semiconductor A and VB of semiconductor B are more negative than the CB of semiconductor B and VB of semiconductor A. In staggered gap type-II heterojunction, the CB and VB of semiconductor A are more negative than those of semiconductor B, respectively. In type-III heterojunction with a broken gap, both CB and VB of semiconductor A are more negative than CB of semiconductor B.
Figure 3. Schematic illustration of (A) type-I, (B) type-II, and (C) type-III heterojunctions. Quoted with permission from Low et al.[30].
In type-I heterojunction, the electrons of both semiconductors transition from VB to CB and leave holes in the VB under the photoexcitation [Figure 3A]. Based on the characteristics of the band edge position, the h+ will gather in the VB edge of semiconductor B, and photoelectrons will accumulate in the CB of semiconductor B. Among the three conventional heterojunctions, Type-II is one of the most popular photocatalysis heterojunctions studied due to its suitable energy band structure, band-edge position, and high photogenerated carrier separation efficiency. As shown in Figure 3B, when the two semiconductors are in contact, under light excitation, the e- on the CB of semiconductor A will migrate to the CB of semiconductor B, and the holes on the VB of semiconductor B will gather on the VB of semiconductor A. The oxidation-reduction reaction will be conducted in the CB of semiconductor B and the VB of semiconductor A, respectively, so that the photogenerated e-/h+ pairs will be effectively separated, thus improving the photocatalysis activity[31]. Figure 3C demonstrates the structural features of type-III heterojunction photocatalysts. Obviously, in type-III heterojunction, both CB and VB of semiconductor B are more positive than those of semiconductor A. Such energy band relationship is not conducive to separating photogenerated electron-hole pairs[32], and the photocatalytic performance is inferior. Therefore, this review will not further elaborate on the TiO2-based type-III heterojunction.
Table 1 summarizes the efforts and progress of TiO2-based photocatalysis type-I and type-II heterojunctions for photocatalysis hydrogen evolution from 2019 to 2023. Cao et al. prepared Cu2O/D-TiO2 type-I heterojunctions, and a series of composites with Cu2O contents ranging from 1% to 10% were synthesized[33]. Among them, the heterojunction with 5 wt% Cu2O loading showed the highest H2 evolution rate of 4.81 mmol-1h-1 under UV-visible light [Figure 4A-C]. The work functions of D-TiO2 and Cu2O were 4.69 and 5.46 eV, respectively. Only the e- of the CB of D-TiO2 flowed to the CB of Cu2O when contacted in the dark environment. Under photoexcitation, both photogenerated e-/h+ pairs of D-TiO2 transfer towards CB and VB of Cu2O, respectively. Luo et al. prepared CdS quantum dots and ultrathin TiO2(B) nanosheets by photodeposition and hydrothermal methods[34], respectively, where the heterojunction (PCT) prepared by photodeposition belongs to the type-II heterojunction and the heterojunction (HCT) prepared by hydrothermal method belongs to the type-I heterojunction [Figure 4D]. Under visible light excitation,
Comparison of typical type-I and type-II TiO2-semiconductor heterojunctions in the last five years
| Photocatalyst | Method | Light source | H2-production rate (µmolg-1h-1) | IPCE/photocurrent density | Year | Ref. |
| TiO2/UiO-66-NH2 | Solvothermal | UV light | 593.53 | / | 2022 | [27] |
| Cu2O/D-TiO2 | Solvothermal, photodeposition | Xe lamp | 4,810 | / | 2023 | [33] |
| CdS/TiO2(B) | Hydrothermal | Xe lamp | 1,776 | / | 2020 | [34] |
| In2S3/TiO2 | Hydrothermal | Xe Arc lamp | / | 2.82 mAcm-2 | 2021 | [35] |
| g-C3N4 quantum dots/a-TiO2/r-TiO2 | Heat treatment | Simulated sunlight | 49.3 | / | 2020 | [36] |
| TiO2 (A)/TiO2 (R)/In2O3 | One-step in situ calcination | Xe lamp | 268 | / | 2022 | [37] |
| TiO2@ZnIn2S4 nanospheres | Hydrothermal | Xe lamp | 4,958 | / | 2019 | [38] |
| ZnO/ZnCr2O4@TiO2-NTA | Electrochemical reduction-oxidation | Xe lamp | 1,680 | C | 2019 | [39] |
| Ti3C2@TiO2/ZnIn2S4 | Two-step hydrothermal | Xe lamp | 1,185.8 | / | 2020 | [40] |
| BiVO4-TiO2/rGO | Template | Xe lamp (420 nm) | 6,998 | 20 mAcm-2 | 2020 | [41] |
| CdS/Ti3+/N-TiO2 | Self-assembly, hydrothermal | Xe lamp (420 nm) | 1,118.5 | 0.77 mAcm-2 | 2020 | [42] |
| GaAs (QD)/TiO2 | DFT | / | / | / | 2021 | [43] |
| 3-Cd0.5Co0.5S/SN-TiO2 | Electrospinning | Xe lamp | 4,550 | / | 2022 | [44] |
| TiO2 nanotubes/ZB CdS-CH3COO-NPs | Electrospinning- solvothermal, chemical deposition | / | 15,025.38 | / | 2022 | [45] |
| TiO2/ZnIn2S4/Co-Pt | Hydrothermal, annealing process | / | / | 1.82 mAcm-2 | 2022 | [46] |
| 3DOM CdS/In2O3-TiO2(Pt) | Colloidal crystal template | Xe lamp | 3,428 | / | 2023 | [47] |
| 4.216TiO2/CdS/g-C3N4 | Hydrothermal, wet chemical, ultrasonic sonication processes | Xe lamp | 116.5 | 4.16%/10.17 mAcm-2 | 2023 | [48] |
Type-I heterojunction effectively reduces the band gap and accelerates the photoexcited electron and hole transfer, However, it is rarely reported due to the difficulty of type-I heterojunction to separate the photogenerated e-/h+ pairs.
TiO2/UiO-66-NH2 type-II heterojunction was constructed by Kuang et al. by photo-induced activation treatment [Figure 5A and B][27]. Before contact, the CB and VB of UiO-66-NH2 were more negative than those of TiO2, respectively. After contact, under photoexcitation, in situ X-ray photoelectron spectroscopy (XPS) characterization revealed that UiO-66-NH2 acted as an electron donor to transfer photogenerated electrons from the Lowest Unoccupied Molecular Orbital (LOMO) to CB of TiO2, while h+ of TiO2 was transferred to the Highest Occupied Molecular Orbital (HOMO) of UiO-66-NH2, which led to the separation of the photogenerated electron-hole pairs, the inhibition of the combination of photogenerated carriers, and the improvement of photocatalytic performance, with the photocatalytic hydrogen evolution rate reaching 593.53 μmolg-1h-1. Park et al. synthesized In2S3/S, N-doped TiO2 nanostructures by a hydrothermal method[35]; it has been observed that S, N doping significantly decreases the work function of TiO2, enhances the electron concentration, effectively reduces the distance between CB and Ef, and increases the carrier concentration. Furthermore, the formation of In2S3/TiO2 heterojunctions can effectively inhibit carrier combination, and its carrier lifetime is increased by a factor of 20 compared with that of the monomer. Specifically, the carrier lifetimes of TiO2, S- and N-doped TiO2, and S, N-doped TiO2/In2S3 are 72.4, 50.3 ns, and 1.16 µs. The band edge positions [Figure 5C] show that the VB and CB of the S, N-doped TiO2 are more positive than those of In2S3. This conforms to the type-II energy band arrangement, and the bandgap of the heterojunction is only 2.27 eV. Under the photoexcitation [Figure 5D], h+ will accumulate on the VB of In2S3, while e- will gather on the CB of S, N-doped TiO2. Ultimately, the oxygen evolution (OER) and hydrogen evolution reactions will be performed on the VB of In2S3 and the CB of S, N-doped TiO2, respectively, thus realizing photocatalytic water splitting.
Figure 5. (A and B) Schematic diagram of UiO-66-NH2 heterojunctions. (A and B) is quoted with permission from Kuang et al.[27]. (C) Energy band position of Pristine TiO2, S, N-doped TiO2, In2S3 NPs on TiO2, and In2S3, (D) S, N-doped TiO2/In2S3 heterostructure and photocatalytic mechanism diagrams. (C and D) is quoted with permission from Park et al.[35].
In addition, the excellent photocatalysis properties of binary type-II heterojunctions have led to great interest in ternary and even multicomponent type-II heterojunctions. Recently, Zhou et al. have designed a Type-II/Type-II energy band arrangement of g-C3N4 quantum dots/a-TiO2/r-TiO2 heterojunctions for photocatalysis total water splitting[36]. Figure 6A-F shows the Type-II/Type-I and Type-II/Type-II energy band arrangement and photocatalytic mechanism diagrams. In addition, its photocatalysis hydrogen and oxygen evolution efficiencies reached 1,526.4 and 198.8 μmol h-1, respectively, and it can be extended to the decomposition of methylene blue (MB). Yang et al. prepared Mxene-derived anatase-TiO2/
Figure 6. (A-F) Energy band arrangements and photocatalytic mechanisms diagrams of T-II/T-II, T-II/T-I, T-II/T-II Stairs, T-II/T-II Concave and T-II/T-II convex. Quoted with permission from Zhou et al.[36].
Although type II heterojunctions can effectively inhibit the compounding of photogenerated electron-hole pairs, their study is limited by the fact that the original electrons inside the semiconductor will impede the foreign electron transport, which results in a significant reduction of their oxidation-reduction capability.
p-n heterojunction photocatalysts
The reaction mechanism of p-n heterojunction photocatalysis differs from that of type-II heterojunctions. In p-n heterojunctions, photocatalytic efficiency enhancement primarily stems from energy band engineering and improved carrier separation driven by internal electric fields (IEF)[31]. As illustrated in Figure 7A and B, p-n heterojunctions have two different type-II energy band arrangements[31]. Both generate an IEF pointing from the n- to the p-type semiconductor in the heterojunction formation[49]. In
Figure 7. Type-II energy band arrangement of (A) the first type and (B) the second type of p-n heterojunction. (A, B, I) is quoted with permission from Ding et al.[49]. (C) catalysis mechanism of Co3O4/TiO2. (D and E) GH of TiO2 and Co3O4 and (F and G) work function for the TiO2 and Co3O4 facets. (C-G) is quoted with permission from Wang et al.[54]. (H) band structure diagram for Ag3PO4/b-N-TiO2. (H) is quoted with permission from Zhou et al.[15]. (I) The charge segregation mechanism of Cu2O/Ni(OH)2/TiO2.
The above two p-n heterojunctions with different action mechanisms have been reported and synthesized experimentally in many cases [Figure 7C-G]. Wang et al. synthesized 2D/1D and 3D/1D Co3O4/TiO2 composites by a hydrothermal method with a hydrogen evolution rate of up to 3,460 µmolg-1h-1[54], and DFT calculations were used to explore the reason for their high activity. Theoretical calculations show that the Fermi level of Co3O4 is significantly higher than that of TiO2 [Figure 7D and E], resulting in a greater e- transfer to TiO2 and a significant increase in electron concentration. Additionally, due to the difference in Fermi energy levels, an IEF is generated within TiO2 towards Co3O4, accelerating the transfer of e- from the CB of Co3O4 to that of TiO2 and h+ from the VB of TiO2 to that of Co3O4. This effectively promotes photogenerated carrier separation, increases carrier concentration, and improves catalytic activity.
Table 2 summarizes some of the applications of p-n type heterojunctions for photocatalytic hydrogen evolution from 2019 to 2023. Most current research has focused on synthesizing a single p-n heterojunction, which is certainly effective, but the utilization efficiency of photogenerated e-/h+ pairs is still poor. Accordingly, researchers have begun to turn their attention to exploring dual p-n type heterojunctions. As presented in Figure 7I, Ding et al. adopt chemical precipitation and ultrasound-assisted glucose reduction to synthesize Ni(OH)2/TiO2,Cu2O/TiO2 and Cu2O/Ni(OH)2/TiO2 photocatalysts[49]. It is found that the hydrogen evolution rate of Cu2O/Ni(OH)2/TiO2 is 6,145 µmolg-1h-1 which is much higher than that of Ni(OH)2/TiO2 (3,265 µmolg-1h-1) and Cu2O/TiO2 (2,285 µmolg-1h-1). The excellent photocatalytic activity is attributed to the synthesized Cu2O/Ni(OH)2/TiO2 being a fully depleted p-n junction with IEF. Under photoexcitation, the e- on the CB of Ni(OH)2 and Cu2O will be transferred to the CB of TiO2 because the CB of TiO2 is more positive than the CB of Ni(OH)2 and Cu2O, while the h+ will be transferred from the VB of TiO2 to the VB of Ni(OH)2 and Cu2O, respectively, because the VB of TiO2 is more positive. The direction of the IEF is directed from TiO2 to Ni(OH)2 and Cu2O, respectively, which is opposite to the motion direction of the e- and the same as that of the h+, thus promoting the carrier migration and effectively improving the separation of the photogenerated e-/h+ pairs, which effectively improves the photocatalytic activity. In addition, Chen et al. prepared a 3D g-C3N4-Cu2O-TiO2 by a sacrificial template strategy and photo-deposition with a maximum hydrogen evolution rate of 12,108 μmol g-1h-1[55]. Overall, p-n heterojunction plays an indispensable role in heterojunction catalysts due to their unique energy band properties and excellent photocatalytic activity, but it does not overcome the disadvantages of the type II heterojunction.
Comparison of selected research results on TiO2-based p-n heterojunction photocatalytic hydrogen evolution from 2019 to 2023
| Photocatalyst | Method | Light source | H2-production rate (µmolg-1h-1) | IPCE/photocurrent density | Year | Ref. |
| Cu2O/Ni(OH)2/TiO2 | Hydrothermal, calcination | Xe lamp | 6,145 | / | 2021 | [49] |
| CuS/TiO2 | Solvothermal | Xe lamp | 705.8 | / | 2023 | [50] |
| 0D Co3O4/1D TiO2 | Hydrothermal, electrospinning | Xe lamp | 3,460 | / | 2022 | [54] |
| g-C3N4-Cu2O-TiO2 | Sacrificial template, photodeposition | / | 12,108 | / | 2021 | [55] |
| Cu2O/TiO2 (P25) | One-pot hydrothermal | Xe lamp (λ > 420 nm) | 2,550 | 0.75 mAcm-2 | 2021 | [56] |
| NiO-TiO2 | Sol-gel, hydrothermal | Xe lamp | 23,500 | / | 2021 | [57] |
| Co0.85Se/TiO2 | Two-step hydrothermal | Xe lamp (365 nm) | 2,312.5 | 10 mAcm-2 | 2022 | [58] |
| Cu3Mo2O9/TiO2 | Mechanical mixing | Xe lamp | 3,401.9 | 6.4 mAcm-2 | 2022 | [59] |
| Co3O4/Ti3+-TiO2/NiO | Chemical-hydrothermal-annealing-reduction | / | 2,134.63 | / | 2022 | [60] |
| Cu2NiSnS4/TiO2(B) | Hydrothermal | Direct sunlight | 7,144 | / | 2023 | [61] |
| Li@g-C3N4/F@TiO2-B(001) | DFT | / | / | / | 2023 | [62] |
| Ni(OH)2-TiO2-Cu2O | Hydrothermal-calcination | Xe lamp | 8,384.84 | / | 2023 | [63] |
| NiO-TiO2 | Hydrothermal | Xe lamp | 8,000 | / | 2023 | [64] |
Z-scheme heterojunction photocatalysts
Comparing the limited carrier oxidation-reduction capacity of type-II and p-n heterojunctions, the
The liquid-phase Z-scheme heterojunction [Figure 8A] is characterized by a pair of electron acceptor-donor (A-D) between the two semiconductors as a transport medium for e-[65]. At this time, the e- on CB of semiconductor 1 and the h+ on VB of semiconductor 2 will react within A and D, respectively. After the formation of the heterojunction, the oxidation reaction is carried out on the VB of semiconductor 1, and the reduction reaction is conducted on the CB of semiconductor 2, which effectively separates the photogenerated carriers. However, the existence of the reverse reaction and the light shielding effect will reduce the number of e- and h+. In addition, the liquid medium is easy to deactivate, and all of these uncertainties will affect the photocatalytic activity to a certain extent[17].
Figure 8. (A) liquid-phase Z-scheme heterojunction. Quoted with permission from Bai et al.[65]. (B) Au@TiO2-12%TrTh all-solid-state
The all-solid-state Z-scheme heterojunction is proposed to circumvent the disadvantages of liquid-phase
In 2009, Wang et al. addressed the challenge of costly all-solid-state Z-scheme heterojunctions by introducing electron-mediator-free, direct-contact Z-schemes[67]. In 2013, Yu et al. further proposed direct Z-scheme heterojunctions enhanced by IEF[68]. The key distinction of this design from liquid Z-schemes lies in its elimination of the need for an electron transfer medium. As illustrated in Figure 8C, when TiO2 and ZnIn2S4 form a heterojunction, their energy bands align in a staggered configuration due to disparities in Fermi energy levels and work functions, causing band bending at the contact interface. IEF direct from ZnIn2S4 towards TiO2, and coupled with band bending, facilitates electron transfer from the CB of TiO2 to the VB of ZnIn2S4 to recombine with h+, while simultaneously impeding the reverse flow of electrons from the CB of ZnIn2S4 and holes from the VB of TiO2. This mechanism both guarantees sufficient charge carriers for redox reactions and spatially separates photogenerated carriers, thereby contributing to enhanced photocatalytic performance.
The direct Z-scheme has been widely studied in academia due to its unique energy band structure and efficient photocatalytic ability. Consequently, we will focus on the direct Z-scheme heterojunctions in photocatalysis hydrogen evolution. Ran et al. used liquid exfoliation, precipitation-hydrothermal methods to prepare the ReSe2/TiO2 direct Z-scheme heterojunctions[69]. These prepared heterojunctions exhibited a photocatalysis hydrogen evolution rate of up to 2,081 µmolg-1h-1 and were combined with DFT to show that the presence of an IEF promotes the electron transfer from TiO2 to ReSe2. Moon et al. successfully synthesized the Pt/g-C3N4/TiO2/IrOx heterojunctions [Figure 8D] through the classical Stöber method[70]. The formation of Z-scheme heterojunctions [Figure 8E] accelerated the surface charge separation and reaction kinetics, resulting in catalysts with apparent quantum yields, hydrogen evolution rates, and oxygen evolution rates as high as 24.3%, 8.15 mmolg-1h-1 [Figure 8F], and 443.9 μmolg-1h-1, respectively. Photogenerated e- was reduced to H2 on the CB of Pt/g-C3N4 and h+ was oxidized to oxygen on the VB of TiO2/IrOx to achieve total water splitting.
Double Z-scheme heterojunction can further enhance the separation of photogenerated e-/h+ pairs and improve the photocatalytic activity. As shown in Figure 8G, MP-1, MP-2, and MP-3 are contacted to form a double Z-scheme heterojunction. After photoexcitation, the e- on the CB of MP-2 will combine with the h+ on the VB of MP-1 and MP-3, respectively. The remaining photogenerated e- on the CB of MP-1 and MP-3 will participate in the reduction reaction, and the h+ on the VB of MP-2 will engage in the oxidation reaction. It should also be noted that the double Z-scheme heterojunction is not a simple superposition of the three components but rather utilizes the synergistic effect among them to accelerate the electron transfer and the separation of the photogenerated carriers to extend the lifetime of the carriers, thus improving the photocatalytic activity.
Table 3 demonstrates some research results of TiO2-based Z-scheme heterojunctions in photocatalysis hydrogen evolution from 2019 to 2023.
Some research results of Z-scheme TiO2-based heterojunction for photocatalytic hydrogen evolution from 2019 to 2023
| Photocatalyst | Method | Light source | H2 production rate (µmolg-1h-1) | IPCE/photocurrent density | Year | Ref. |
| Au@TiO2-X%TrTh | Situ polymerization | Visible light | 4,288.54 | 0.13 µAcm-2 | 2022 | [66] |
| ReSe2/TiO2 | Liquid exfoliation, hydrothermal, DFT | Xe lamp | 2,081 | / | 2023 | [69] |
| Pt/g-C3N4/TiO2/IrOx | Classical Stöber | Xe lamp | 8,150 | / | 2022 | [70] |
| TiO2-ZnIn2S4 | Hydrothermal, DFT | Xe lamp | 18,077.2 | / | 2021 | [71] |
| TMOP Tricolor-typed microfiber | Tri-axial parallel electrospinning technology | Simulated sunlight | 536.7 | / | 2023 | [72] |
| 3D/2D TiO2 /g-C3N4 | Hydrothermal | Xe lamp | 4,128 | / | 2019 | [73] |
| fluorinated-TiO2 /CdSe-DETA | Mild solvothermal | Xe lamp (λ ≥ 420 nm) | 12,381 | / | 2020 | [74] |
| ZrO2@TiO2 | Solvothermal, calcinating | Xe lamp | 39,700 | / | 2020 | [75] |
| Bi2S3/MoS2/TiO2 | Facile microwave-assisted hydrothermal | Xe lamp (λ ≥ 420 nm) | 2,195 | / | 2020 | [76] |
| Ni (OH)2-CuxO-TiO2 | Hydrothermal | tungsten halogen lamp | 15,789 | / | 2020 | [77] |
| g-C3N4/TiO2@Pt | Hydrothermal | Xe lamp | 15,360 | 15 µAcm-2 | 2021 | [78] |
| Cu2O/TiO2 | Hydrothermal, DFT | Xe lamp | 14,020 | 20 µAcm-2 | 2021 | [79] |
| MoS2 /TiO2 nanosheets | Spin coating | Xe lamp (420 nm) | 5,423.77 | / | 2022 | [80] |
| 2D/2D TiO2/g-C3N4 | Thermal polymerization, colloidal, electrostatic self-assembly | Xe lamp (380 nm) | 3,875 | / | 2022 | [81] |
| ZnIn2S4/TiO2(MOFs) | Hydrothermal, calcination | Xe lamp (400 nm) | 2,451.5 | 1.42 µAcm-2 | 2023 | [82] |
| Bi2WO6/TiO2 | One-step solvothermal | Xe lamp | 12,900 | / | 2023 | [83] |
| g-C3N4/TiO2 nanotube | Electrospinning | / | 4,122 | / | 2023 | [84] |
| TiO2 (116)/red phosphorus (001) | Chemical vapor deposition | / | 12.9(μmol·h-1) (λ > 300 nm) | / | 2023 | [85] |
S-scheme heterojunction photocatalysts
Xu et al. reported the first S-scheme heterojunction photocatalysts in 2019[29]. Zhang et al. extended the
Figure 9. (A) n-n junction, (B) p-p junction, (C) n-p junction, (D) p-n junction (E-G) Photogenerated carrier migration process and photocatalytic mechanism before and after contact. Quoted with permission from Li et al.[87].
Comparison of some research results of S-scheme TiO2-based heterojunction photocatalysis hydrogen evolution from 2020 to 2024
| Photocatalyst | Method | Light source | H2-production rate (µmolg-1h-1) | IPCE/photocurrent density | Year | Ref. |
| α-Fe2O3/TiO2-Pd | Impregnation | UV lamp | 3,490.54 | / | 2021 | [88] |
| 3D/2D/0D TiO2/ | Solvothermal reaction, DFT | Xe lamp (420 nm) | 5,540.21 | / | 2021 | [89] |
| Co2P/PC-b-TiO2 | Pyrolyzing a mixture of cobalt phosphonate and TiO2 under H2 atmosphere | Xe lamp | 1,530 | / | 2022 | [90] |
| ZnCo2S4/TiO2 | Solvothermal | Xe lamp (420 nm) | 5,580 | / | 2022 | [91] |
| Co3Se4/TiO2 | Hydrothermal | Xe lamp (350-780 nm) | 6,065 | / | 2022 | [92] |
| 1D/2D TiO2/ZnIn2S4 | Hydrothermal | LED lamp (365 nm) | 6,030 | / | 2022 | [93] |
| O-ZnIn2S4/TiO2-x | Hydrothermal, liquid assembly, DFT | Xe lamp (420 nm) | 2,584.9 | / | 2022 | [94] |
| TiO2-CeO2/g-C3N4 | Thermal calcination, DFT | / | / | 10 mAcm-2 | 2022 | [95] |
| porous ZnS/TiO2 | One-pot hydrothermal | Xe lamp | 1,718 | / | 2023 | [96] |
| 20 wt% Co9S8/TiO2 | In-situ deposition hydrothermal | Xe lamp (350-780 nm) | 3,982 | / | 2023 | [97] |
| g-C3N4/TiO2 | Time-domain ab initio analysis | / | / | / | 2023 | [98] |
| TiO2/ZnS | DFT, NAMD | / | / | / | 2023 | [99] |
| Mxene@CdS/TiO2 | / | / | 16,200 | 9.03 µAcm-2 | 2023 | [100] |
| CdS/g-C3N4/TiO2 | Self-assemble, solvothermal, DFT | Xe lamp | 26,840 | 40.2% | 2023 | [101] |
| Cu3P/TiO2 | Microwave hydrothermal, DFT | Xe lamp | 5,830 | / | 2023 | [102] |
| C3N5/TiO2 | Sol-gel, thermally assisted in situ growth | Xe lamp (λ > 420 nm) | 1,833.86 | / | 2024 | [103] |
Other TiO2-based photocatalytic heterojunctions
In addition to the common type-II, p-n, Z-scheme, and S-scheme heterojunctions discussed above, researchers have also devoted themselves to studying 2D van der Waals heterojunction formed by the IEF dominated by van der Waals forces, phase heterojunction formed by the same semiconductor that exists only in several different crystalline phases, facet heterojunction dominated by exposed surfaces, and Schottky heterojunction formed by semiconductor interacting with metal[104]. However, these heterojunctions are mostly used for the degradation of organic matter and the photoreduction of CO2, and fewer of them have been applied to the photocatalytic hydrogen evolution [Table 5], so they will not be specifically discussed in this review.
Comparison of selected research results on Other TiO2-based heterojunction photocatalytic hydrogen evolution from 2019 to 2023
Application of DFT calculation to investigate TiO2-based photocatalytic heterojunctions
In recent years, theoretical calculations based on DFT can provide accurate predictions of the electronic structure, optical properties, and photocatalytic activity of materials[19]. The first is the binding energy of the heterojunction. In DFT calculations, the positive and negative values of the binding energy of the heterojunction can determine whether the two semiconductors need external energy injection in the coupling process. Next, the value of the binding energy can qualitatively determine the difficulty of forming heterojunctions for different semiconductors. Secondly, DFT calculations can confirm the amount of electron transfer in the heterojunction and the direction of charge transfer. Wang et al. constructed 0D/2D
Figure 10. (A) D-RuO2/TiO2 nano-heterostructure, (B) Electron localization function analysis mapped, (C) ΔGH of TiO2, RuO2/TiO2, Pt/C and D-RuO2/TiO2. (A-C) is quoted with permission from Li et al.[107] Band offsets of (D) SrTiO3 and TiO2 slabs (E) SrTiO3/TiO2 heterostructure. (D and E) is quoted with permission from Di Liberto et al.[108] work function of (F) TiO2(101), (G) ZnS (110), (H)TiO2/ZnS. (I) Planar-averaged charge density difference for the TiO2/ZnS heterojunction. (J) Absorption spectra, (K) STH efficiency of the TiO2 (101), ZnS (110), and TiO2/ZnS heterojunction. (F-K) is quoted with permission from Li et al.[99].
On the other hand, with the rapid development of DFT calculation and machine learning (ML) in the field of materials, DFT calculation can predict new heterostructures, provide new heterostructures for experiments, and allow rational design of efficient photocatalytic heterojunctions. For example, Li et al. used it to obtain the Gibbs free energy of RuO2/TiO2 as only -0.1 eV and then guided the experimental synthesis of RuO2/TiO2 for photocatalytic hydrogen evolution[107]. However, due to the large number of atoms (> 200) in TiO2-based heterojunctions, the traditional DFT calculation for heterostructure prediction is highly costly and inefficient, and thus, less research has been carried out so far. Conversely, ML has the characteristics of high efficiency, strong model generalization ability, and self-learning ability, which solves the problem of low efficiency of DFT prediction, and can be used for high-throughput screening of heterostructures according to the set conditions[6,108]. Currently, ML is in the stage of rapid development; in future research, it can reveal the physicochemical properties of materials, quickly screen the materials that meet the conditions for the construction of heterostructures, optimize the design of catalysts, quickly find the location of active sites and factors affecting the activity of catalysts, and modulate the composition of the material, so as to guide the experiments to synthesize the higher-efficiency heterostructures.
SUMMARY AND OUTLOOK
This review systematically discusses the research progress of TiO2-based heterojunction photocatalysts in photocatalytic hydrogen evolution since 2019. Different heterojunctions, including type-I, type-II, type-III, p-n type, Z-scheme, and the S-scheme heterojunctions, and their photocatalytic mechanisms are discussed in detail, along with their advantages and disadvantages. Direct Z- and S-scheme heterojunctions have optimal photocatalytic activity. Both generate a strong IEF at the interface to promote the separation of photogenerated carriers while maintaining a good oxidation-reduction capacity. The construction of TiO2-based heterojunctions for photocatalytic hydrogen evolution is a promising approach to solving the energy crisis. In addition, some challenges of TiO2-based heterojunctions still exist, outlined as follows:
(1) Among the conventional heterojunctions, type-II heterojunctions have a weak oxidation reduction capacity due to the carriers in the original catalysts that hinder the electron transfer. These problems may be overcome by modifying type-II heterojunctions into Z-type or S-type heterojunctions.
(2) The p-n heterojunctions have two types of energy band arrangements and charge transfer, in which the classification and difference between the second type of p-n junction and the Z-scheme remain greatly controversial, and more precise characterization and calculations should be employed to reveal the difference in the mechanism.
(3) The p-n heterojunction inherits most of the qualities of type-II, and the oxidation-reduction capacity of the catalyst has been improved, but it is still insufficient and hinders the development of the p-n heterojunction. Although the double p-n heterojunction can further improve the oxidation-reduction ability of the catalyst, it requires the synergistic action of three semiconductors.
(4) Direct Z-scheme heterojunction accelerates surface charge separation and reaction dynamics, but the exact mechanism of the carrier transfer in direct Z-scheme heterojunctions remains controversial currently. In addition, the bandgap of the two semiconductors in the Z scheme is easily mismatched, severely affecting the catalytic performance. In further studies, tuning the bandgap and band edge positions of the two semiconductor monomers is essential for Z-scheme heterojunctions, and the experimental characterization should be closely coupled with DFT to accurately explore the mechanism of Z-scheme heterojunctions.
(5) The S-scheme heterojunction realizes photogenerated charge separation, reduces photogenerated electrons and hole recombination through IEF and band bending, and improves the photoelectric conversion efficiency, and the generated electrons and holes have strong oxidation-reduction ability. However, it has been proposed for a short time, and its deeper action mechanism remains to be explored. Furthermore, it is unsuitable for photoelectrochemistry and is limited to powder photocatalysts. In the future, we should focus on improving the catalytic activity by adjusting the Fermi level of RP and OP and designing its surface morphology reasonably.
Developing novel and advanced photocatalysts is important to boost the photocatalytic hydrogen evolution rate. Well-constructed heterojunctions could greatly enhance the promotion of photogenerated electron-hole pair separation; thus, designing highly efficient photocatalysts with good charge separation is extremely important. To better design and synthesize high performance TiO2-based heterojunctions, some suggestions can be considered:
(1) Current experimental characterization mostly illustrates charge transfer using XPS and photoluminescence spectroscopy, but it is not accurate to indicate whether the electron transfer is the contribution of the heterojunction or a single catalyst. Therefore, it is important to develop more advanced characterization techniques to explore the charge transfer pathway, such as high spatial and temporal in situ detection at the atomic level, high precision time-resolved surface photovoltage (SPV) imaging and scanning tunneling microscopy.
(2) Employing more accurate DFT computation methods, such as time-containing DFT, transient DFT, and transient charge-carrier dynamics, can provide more precise evidence for the charge-transfer paths and migration dynamics of heterojunctions.
(3) The rapid development of ML provides more accurate predictions for the experimental synthesis of semiconductor heterojunctions. Conventional DFT is inefficient and time-consuming because it can only couple heterojunctions one by one and then simulate and predict their catalyst performance. However, ML is an efficient and economical approach to synthesizing thousands of heterojunctions and obtaining semiconductor heterojunctions with excellent catalyst performance through the high-throughput screening. In a word, the advancement of ML will further promote the development of photocatalysts.
DECLARATIONS
Authors’ contributions
Literature search and organization and manuscript drafting: Yang N, He T
Manuscript revision: Zhang G, Chen X, He Y, Zhou T
Project supervision: He T, Liu Q
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work received funding from the National Key Research and Development Program of China (2022YFB3803600), the National Natural Science Foundation of China (22378346, 22368050), and the Key Research and Development Program of Yunnan Province (202302AF080002).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2024.
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Yang N, He T, Chen X, He Y, Zhou T, Zhang G, Liu Q. TiO2-based heterojunctions for photocatalytic hydrogen evolution reaction. Microstructures 2024;4:2024042. http://dx.doi.org/10.20517/microstructures.2024.06
AMA Style
Yang N, He T, Chen X, He Y, Zhou T, Zhang G, Liu Q. TiO2-based heterojunctions for photocatalytic hydrogen evolution reaction. Microstructures. 2024; 4(4): 2024042. http://dx.doi.org/10.20517/microstructures.2024.06
Chicago/Turabian Style
Nan Yang, Tianwei He, Xinqi Chen, Yijun He, Tong Zhou, Genlin Zhang, Qingju Liu. 2024. "TiO2-based heterojunctions for photocatalytic hydrogen evolution reaction" Microstructures. 4, no.4: 2024042. http://dx.doi.org/10.20517/microstructures.2024.06
ACS Style
Yang, N.; He T.; Chen X.; He Y.; Zhou T.; Zhang G.; Liu Q. TiO2-based heterojunctions for photocatalytic hydrogen evolution reaction. Microstructures. 2024, 4, 2024042. http://dx.doi.org/10.20517/microstructures.2024.06
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