Direct Z-scheme photocatalytic systems based on vdW heterostructures for water splitting and CO2 reduction: fundamentals and recent advances
Abstract
Photocatalytic water splitting and CO2 reduction are conducive to alleviating the increasingly serious environmental problems and ever-tightening energy problems. Among various modification strategies, constructing Z-scheme heterostructures and direct Z-scheme heterostructures, in particular, by mimicking natural photosynthesis, has been widely researched for the effective separation of photogenerated electrons and holes with strong redox ability. However, a low lattice matching degree of different semiconductors often results in serious crystal defects in the composite. Fortunately, van der Waals (vdW) heterostructures constructed through interlayer weak vdW interactions provide a remedy, which not only can ensure the high quality of Z-scheme heterostructures but also preserve the original properties of individual components and induces new properties at the heterogeneous interfaces. Herein, we introduce the fundamentals of direct Z-scheme vdW heterostructure and review the last five-year progress of direct Z-scheme vdW heterostructures for photocatalytic water splitting and CO2 reduction, highlighting the characteristics and fundamental modification principles of different heterostructures, aiming to provide informative principles for the design of advanced heterostructure photocatalysts for solar energy conversion.
Keywords
INTRODUCTION
Due to the rapid development of modern industrial civilization, the consumption of fossil energy is increasing day by day. According to the 2023 edition of Statistical Review of World Energy[1], traditional fossil fuels still account for a large proportion of primary energy consumption, comprising 82%, and the non-renewability of fossil fuels leads to the aggravation of the energy crisis. Compared with the previous year, CO2 emissions from energy utilization and industrial processes increased by 0.8%, indicating that the imbalance of the carbon cycle is further aggravated and environmental problems are deteriorating. In recent years, in order to cope with the energy crisis and solve environmental problems, the development and utilization of renewable energy, such as solar energy and wind energy, came into being. Among them, solar energy has been widely studied because of its advantages of sustainability, pollution-free nature, and richness, but its utilization is limited by time and space dimensions[2]. Since the pioneering discovery of photocatalytic water splitting over TiO2 by Fujishima and Honda in 1972[3], many developments on advanced photocatalytic systems have been continuously reported. Due to the advantages of H2, including high energy storage density, high calorific value, non-pollution, and sustainability[4,5], and the fact that the reduction products of CO2 can be used as high value-added chemicals and fuels[6-8], the research on photocatalytic water splitting into H2 and CO2 reduction has become a hot spot in the energy field. Converting solar energy into chemical energy and storing it via photocatalysis in a zero-carbon way can not only solve the intermittent problem of renewable energy but also reduce carbon emissions and the excessive reliance on traditional fossil energy, which is of great significance for solving environmental pollution problems, accelerating the realization of green and low-carbon circular economic development system and carbon peaking and carbon neutrality goals.
As shown in Figure 1, the energy band of a semiconductor photocatalyst is divided into conduction band (CB) and valence band (VB), and the difference between CB minimum (CBM) and VB maximum (VBM) is energy band gap (Eg). Under the irradiation of sunlight, photons with energy greater than the Eg of a semiconductor are absorbed, and the electrons located in the VB of the semiconductor can be stimulated to the CB to become photogenerated electrons (e-); meanwhile, the corresponding positions in the VB generate photogenerated holes (h+). Then, the photogenerated electrons and holes migrate to the surface of the photocatalyst, participating in reduction and oxidation reactions, respectively. During the migration process of photogenerated carriers from the bulk to the surface of the photocatalyst, bulk and surface recombination of carriers occurs, accompanied by the release of absorbed energy in the form of light, which is detrimental to the photocatalytic efficiency. For photocatalytic water splitting, the reduction reaction reduces protons to H2, and the oxidation reaction oxidizes OH- to O2; for CO2 reduction reaction (CO2RR), the reduction reaction reduces CO2 to substances such as CO, CH4 and other hydrocarbons, and the oxidation reaction still oxidizes OH- to O2.
Figure 1. Principles of photocatalytic water splitting reaction & CO2RR (i) the process of photo-excitation to generate photogenerated electrons and holes, (ii) separation and migration of photogenerated charges; (ii’) bulk recombination of photogenerated charges; (ii’’) surface recombination of photogenerated charges; (iii) O2 evolution reaction (OER); (iii’) H2 evolution reaction (HER); (iii’’) CO2RR.
To date, numerous modification strategies have been reported to improve the photocatalytic performance, such as solid solution construction[9], nanomaterials development[10,11], morphology control[12,13], heteroatoms doping[14], defect engineering[15], loading cocatalysts[16,17], constructing heterostructures[18,19], etc. Among these strategies, mimicking natural photosynthesis by coupling two dissimilar semiconductors to construct a
FUNDAMENTALS OF DIRECT Z-SCHEME vdW HETEROSTRUCTURE
Z-scheme heterostructures
Inspired by the natural photosynthesis, the Z-scheme artificial photocatalytic system was proposed, which is named after the charge transfer path of letter “Z” type between the two semiconductors. So far, the
Figure 2. Evolution of Z-scheme heterostructures. (A) The 1st generation liquid-mediator Z-scheme heterostructures; (B) the 2nd generation solid-mediator Z-scheme heterostructures; and (C) the 3rd generation mediator-free Z-scheme heterostructures.
Compared with the liquid-mediator Z-scheme photocatalytic system, the 2nd generation solid-mediator
To address the problems of liquid-mediator Z-scheme heterostructures and solid-mediator Z-scheme heterostructures, Wang et al. proposed a 3rd generation mediator-free Z-scheme system based on ZnO and CdS[24], which was subsequently named the direct Z-scheme system by Xu et al. in 2013[25] and is currently commonly used in the field of photocatalysis. In a direct Z-scheme system, photogenerated electrons located in the lower CB position and photogenerated holes located in the higher VB position of the composites can directly recombine without an additional electron transfer mediator. As a result, the strong redox capability is retained, accompanied by discarded reverse reaction, light shielding effect and reduced system cost, which make the direct Z-scheme system attract constant and extensive research, including the S-scheme heterostructure proposed by Fu et al. in 2019, which is also an interpretation of the mechanism of the 3rd generation mediator-free Z-scheme heterostructure[26]. For example, Chen et al. successfully prepared a C,N co-doped TiO2/g-C3N4 direct Z-scheme heterostructure and found that the H2 production rate of 3 wt% C,N-TiO2 NPs/g-C3N4 was increased by 20.3-fold compared to that of pure g-C3N4, with a significant improvement in light absorption properties[27]. Low et al. synthesized a TiO2/CdS direct
vdW Heterostructures
The fabrication of conventional heterostructures with interfacial chemical bonds often requires considering the lattice matching degree of different semiconductors to minimize crystal defects in the composite, whereas lattice matching is less restrictive for vdW heterostructures formed through weak vdW forces between the coupled semiconductors. In 1984, Koma et al. successfully synthesized a high-quality heterostructure of sub-nanometer thickness by vdW epitaxy for the first time[29], which relies on weak vdW interactions to release the stresses stemming from the lattice mismatch, thus minimizing crystal defects. In the case of vdW heterostructures, the weak vdW forces formed between the layers of semiconductor materials, as opposed to strong chemical bonds, offer the flexibility of stacking layers with different lattice constants and orientations without significant strain, leading to novel crystal phases and properties (e.g., tunable chemical and physical properties, adjustable energy band structure with different stacking sequences, etc.); the photogenerated carriers are produced in atomically thin layers[30], shortening the carrier migration distance from the body to the surface of the photocatalyst, ensuring maximal participation of photogenerated carriers in surface reactions. When different semiconductors come into contact, the difference in their Fermi levels causes electrons to spontaneously flow from the semiconductor with a higher Fermi level to that with a lower Fermi level. This results in the accumulation of electrons at the interface of the semiconductor with a lower Fermi level, thereby creating a built-in electric field directed from the high Fermi level semiconductor to the low Fermi level semiconductor. The photogenerated electrons at the CB and photogenerated holes at the VB of both semiconductors migrate under the driving of this built-in electric field. Figure 3 provides a simple schematic of a Z-scheme heterostructure based on vdW forces, where photogenerated carriers transfer follows a Z-scheme pathway driven by the built-in electric field, resulting in the accumulation of electrons and holes in the more negative CB and more positive VB, respectively. Therefore, it is beneficial to construct direct Z-scheme heterostructures based on vdW forces to break the limitation of lattice mismatch of different materials and to further accelerate the interlayer charge transfer. For example, Xu et al. constructed Bi3O4Cl/g-C3N4 vdW heterostructures for
Figure 3. Schematic diagram of Z-scheme heterostructure based on vdW forces.
APPLICATIONS OF DIRECT Z-SCHEME vdW HETEROSTRUCTURES
Photocatalytic water splitting
Graphitic carbon nitride-based materials
Since the discovery of graphene with its unique hexagonal structure, the applications of graphene-like materials have been constantly developed. Additionally, 2D graphitic carbon nitride (CN) materials, such as g-C3N4[32] and g-C6N6[33], are increasingly gaining ground due to their high stability, low cost, and non-toxicity. However, most CN materials find it challenging to split pure water alone, limited by the thermodynamic contradiction between optical absorption and redox potentials and a high recombination rate of photogenerated carriers. To solve the problem mentioned above, some CN materials-based 2D/2D direct Z-scheme vdW heterostructures have recently been developed for photocatalytic water splitting. For instance, Zhang et al. reported a PtS2/g-C3N4 vdW heterostructure for photocatalytic water splitting through first principles calculation[34]. Calculation results show that this heterostructure is a direct Z-scheme heterostructure with an indirect interlayer band gap of 1.98 eV [Figure 4A and B]. The heterostructure has a high light absorption rate of 5.82 × 105 cm-1 at 400 nm, which could induce photocatalytic water splitting when pH = 0. When the biaxial strain changes in the range of -6% to 6%, especially when tensile strain is applied, the absorption of visible light by the composite photocatalytic system is significantly redshifted [Figure 4C], which is beneficial for the utilization of visible light. More strikingly, the theoretical solar-to-H2 (STH) efficiency of a PtS2/g-C3N4 heterostructure reaches 31.64% [Figure 4D], making it a promising heterostructure for photocatalytic water splitting. Yang et al. reported a g-C6N6/WTe2 vdW heterostructure based on single-layer WTe2 with excellent photoelectric properties [Figure 4E] and used the formation energy Ef to evaluate the stability of the heterostructure[35]. Ef can be obtained from Equation Ef = Eg-C6N6/WTe2-EWTe2-Eg-C6N6, in which Eg-C6N6/WTe2, and Eg-C6N6 denote the total energies of the g-C6N6/WTe2 heterostructure, isolated WTe2 and g-C6N6 monolayers, respectively. The smaller the Ef value, the more stable the heterostructure is. Calculation results show that the g-C6N6/WTe2 heterostructure is a direct Z-scheme heterostructure in which the reduction reaction and oxidation reaction respectively occur on WTe2 and
Figure 4. (A) Band structures of the PtS2/g-C3N4 vdW heterostructure. (Reproduced with permission[34]. Copyright 2023, Elsevier); (B) Projected density of states (DOS) of the PtS2/g-C3N4 vdW heterostructure. (Reproduced with permission[34]. Copyright 2023, Elsevier); (C) Absorption spectrums of the PtS2/g-C3N4 vdW heterostructure under various biaxial strains. (Reproduced with permission[34]. Copyright 2023, Elsevier); (D) The STH efficiency of other 2D materials compared with PtS2/g-C3N4 vdW heterostructure. In plot (A), the purple and gray lines indicate the contributions from the monolayers PtS2 and g-C3N4, respectively, and the horizontal black dashed line set to zero energy represents the Fermi level. (Reproduced with permission[34]. Copyright 2023, Elsevier); (E) The direct Z-scheme photocatalytic mechanism for water splitting in the g-C6N6/WTe2 heterostructure. (Reproduced with permission[35]. Copyright 2022, Elsevier); (F) The variation of driving forces for OER and HER with pH value. (Reproduced with permission[35]. Copyright 2022, Elsevier).
Several studies have discussed the effects of interlayer interactions and intralayer electric fields on photocatalytic performance of direct Z-scheme vdW heterostructures, which are essential for guiding the optimization of heterostructure photocatalysts. For example, Zhang et al. constructed a 2D C-doped boron nitride (BCN)/C2N direct Z-scheme vdW heterostructure[36]. From the transient plots of excited electrons and holes [Figure 5A and B], the authors estimated that electron-hole recombination in the heterostructure occurs within 2 ps, including up to 85% through interlayer transfer between BCN and C2N. This suggests that most recombination of photogenerated carriers occurs between BCN and C2N layers, effectively weakening the recombination within BCN and C2N themselves. It was proposed that if the interlayer band gap of a 2D vdW heterostructure was smaller than that of each individual component, i.e., the intralayer band gap, then ultrafast interlayer electron-hole recombination can be realized in vdW heterostructures due to the intralayer optical phonon modes and interlayer shear phonon mode induced by vdW interaction so that holes and electrons with relatively stronger redox ability can be retained, which is of enlightenment significance for constructing direct Z-scheme vdW heterostructures[36]. Wang et al. constructed a direct
Figure 5. (A) The time-dependent electron and (B) hole population of BCN/C2N heterostructure. (Reproduced with permission[36]. Copyright 2018, American Chemical Society); (C and D) 2D plane-integrated electron density difference along the Z direction for the C7N6/PSn-GaS and C7N6/SGa-SnP heterostructures. 3D isosurfaces of the electron density difference of the two heterostructures are plotted in the insets of this figure. The isosurface level is 0.0001e Å–3. The orange and green regions represent electron accumulation and depletion, respectively. (Reproduced with permission[37]. Copyright 2023, American Chemical Society); (E and F) Electrostatic potentials of C7N6/PSn-GaS and C7N6/SGa-SnP heterostructures. The horizontal arrows indicate the directions of the electric fields. The blue, yellow, and red arrows stand for the Janus layer, interfacial, and total effective electric fields, respectively. (Reproduced with permission[37]. Copyright 2023, American Chemical Society).
In addition to interlayer interactions and intralayer electric fields, heteroatoms doping plays a synergistic role in improving the photocatalytic properties of direct Z-scheme vdW heterostructures. For example, Dong et al. constructed a Z-scheme heterostructure by combining CN with a small amount of BCN[39] and verified the successful formation of vdW heterostructure by comprehensive analysis of
Figure 6. (A) TEM images of (a) BCN, (b) CN and (c) CN/BCN-0.5% samples; (d-g)TEM EDX elemental mappings of CN/BCN-0.5% sample. (Reproduced with permission[39]. Copyright 2020, Elsevier); (B) H2 production rate of different samples of CN/BCN heterostructures under the visible light (λ > 420 nm). (Reproduced with permission[39]. Copyright 2020, Elsevier); (C) Wavelength-dependent AQE of CN/BCN-0.5% sample. (Reproduced with permission[39]. Copyright 2020, Elsevier); (D) The electron-hole recombination and electron and hole transfer dynamics in C7N6/Sc2CCl2 heterostructures, where the time constants are fitted by the exponential function corresponding to three processes. (Reproduced with permission[40]. Copyright 2022, American Chemical Society); (E) Charge transfer in C7N6/Sc2CCl2 heterostructure. (Reproduced with permission[40]. Copyright 2022, American Chemical Society); (F) Processes occurring on the C7N6/Sc2CCl2 surface at pH 7. The blue and orange short lines stand for intermediate states in HER/OER processes on the undoped heterostructure with or without light-induced bias potential U, whereas the green and purple short lines stand for intermediate states in the OER processes occurring on the doped S atom. (Reproduced with permission[40]. Copyright 2022, American Chemical Society).
Transition-metal dichalcogenides-based materials
Transition-metal dichalcogenides (TMDs) MX2 (M is a transition metal atom, and X is a sulfur atom) have been extensively studied in the photocatalysis field due to the graphene-like structure with excellent optoelectronic properties and mechanical properties[41-43]. Most TMDs can be used for visible-light photocatalytic H2 production due to their suitable band gaps and energy band positions, but they cannot be used for O2 production as their VBM cannot cross the oxidation potential of water. Therefore, coupling TMDs with other O2-evolving photocatalysts to form direct Z-scheme vdW heterostructures is of great importance for expanding the response range of photocatalysts that can facilitate overall water splitting driven by visible light.
Ren et al. constructed a PtS2/arsenene vdW heterostructure and investigated its structural stability, photocatalytic activity, light absorption properties, and STH efficiency based on first-principles calculations[44]. Calculation results show that the heterostructure is a direct Z-scheme system and can achieve an ultrahigh STH efficiency of 49.32%. Zhang et al. studied a CdO/HfS2 vdW heterostructure, and the calculated charge density difference between the interfaces of CdO and HfS2 indicates that the CdO/HfS2 heterostructure is a direct Z-scheme heterostructure, and the heterostructure has an excellent visible-light absorption performance, with the absorption peak reaching up to 7.21 × 104 cm-1 at 465 nm[45]. Li et al. respectively constructed 2D C3N/WTe2 and C3N/WS2 vdW heterostructures, and the comprehensive analysis on electronic properties, Bader charge and charge density difference distributions reveals that the energy bands of C3N/WTe2 are arranged in a Type-I configuration, which is unfavorable for the separation of photogenerated electrons and holes[46]. On the contrary, the C3N/WS2 heterostructure is a Z-scheme configuration, which has excellent visible light response, and its band edge position covers the redox potential of water. Strikingly, this study found carbon atoms to be reasonable sites for HER and OER by calculating the Gibbs free energy for H2 precipitation reactions and the overpotential for O2 precipitation reactions. Singh et al. investigated a series of direct Z-scheme vdW heterostructures by coupling MoS2, Janus MoSSe with common 2D materials such as TMDs and transition metal oxides (TMOs), and
Figure 7. (A) The optimized geometries of MoTe2/XS2 (X = Hf, Sn, Zr) heterostructures. Claybank, ultramarine, yellow, blue, cyan, and emerald balls represent Te, Mo, S, Hf, Sn, and Zr atoms, respectively, as labeled in the picture above. Left: the top view of the heterostructures; right: the side view of the heterostructures. (Reproduced with permission[48]. Copyright 2020, Elsevier); (B) The schematic diagram of photocatalysis of MoTe2/BAs heterostructure Z-scheme mechanism. (Reproduced with permission[49]. Copyright 2021, Elsevier); (C) DOS and PDOS of MoTe2/BAs heterostructure using HSE06 functional. (Reproduced with permission[49]. Copyright 2021, Elsevier); (D) Average charge density difference of the heterostructure along the Z-axis direction. The blue area (positive value) and the yellow area (negative value) represent the accumulation and consumption of electrons, respectively (the inset image shows 3D isosurface of the heterostructure, with the level at 0.0005 e Å−3). (Reproduced with permission[49]. Copyright 2021, Elsevier).
Several studies have investigated the effects of built-in electric field, biaxial strain and pH on photocatalytic water splitting reactions. For instance, based on the characteristics of transition metal chalcogenides and arsenene, Zhu et al. constructed arsenene/HfS2 direct Z-scheme vdW heterostructures, whose band edge position satisfies the conditions of overall water splitting[50]. Due to the Fermi level difference between arsenene and HfS2, the redistribution of charges near the contact interface forms a built-in electric field from arsenene to HfS2 [Figure 8A], then the photogenerated electrons at the CB of HfS2 will transfer through the interface to the VB of arsenene and recombine with the photogenerated holes, i.e., a Z-scheme path [Figure 8B], which improves the separation rate of photogenerated carriers with strong redox ability. On this basis, the influence of biaxial strain and pH on the heterostructure is explored. As shown in Figure 8C, it is found that when the biaxial strain changes in the range of -5%~5%, the band gap of arsenene /HfS2 heterostructure decreases with the increase of compressive strain, but it is not affected by tensile strain. In addition, the heterostructure keeps its photocatalytic activity stable in both neutral (pH = 7) and acidic (pH = 0) environments [Figure 8D]. Similarly, Wang et al. built a MoSe2/HfS2 direct Z-scheme vdW heterostructure for overall water splitting[41]. The light absorption range of this heterostructure was significantly widened compared with that of the original monolayer MoSe2 and HfS2, with an obvious absorption peak at a wavelength larger than 400 nm, which mainly benefits from the newly generated interlayer band gap between the CBM of HfS2 and the VBM of MoSe2 in the heterostructure. Additionally, it is verified that the heterostructure manages to maintain its band edge positions in a near-perfect manner under applied strains of -6% to 6%. Relying on first principles, Luo et al. investigated the photoelectric properties of GaN/boron sulfide vdW heterostructures and showed that these heterostructures are direct
Figure 8. (A) Planar-averaged charge density difference of arsenene/HfS2 heterostructure along the Z direction. The blue and yellow regions represent charge depletion and charge accumulation, respectively. (Reproduced with permission[50]. Copyright 2021, Elsevier); (B) Band edge position of arsenene/HfS2 heterostructure after contact. (Reproduced with permission[50]. Copyright 2021, Elsevier); The band gap (C) and band edge positions (D) of arsenene/HfS2 heterostructure under various biaxial strains from -5 % to 5 % by HSE06. (Reproduced with permission[50]. Copyright 2021, Elsevier). The optical adsorption coefficients of the GaN/BS heterostructures under different (E) biaxial strains and (F) external electric fields (where the value of wavelength ranges from 280 to 1,240 nm). (Reproduced with permission[51]. Copyright 2022, Elsevier).
The role of energy band bending in direct Z-scheme vdW heterostructures has also been studied. Cao et al. constructed a 2D WSeTe/XS2 (X = Hf, Sn, Zr) vdW heterostructure[53] and indicated that the heterostructure accelerates the separation of photogenerated carriers due to the energy band bending and built-in electric field induced by the Fermi level equilibrating between WSeTe and XS2 [Figure 9A]. In addition, the absorption spectrum of the heterostructure covers the entire visible light range, significantly improving the light absorption performance of a single XS2 material. Zhang et al. constructed a GeC/HfS2 direct Z-scheme vdW heterostructure, in which the CBM of GeC is higher than the water oxidation potential and the VBM of HfS2 is lower than the water reduction potential, boasting carrier mobilities up to 5,823 cm2 V-1 s-1[54]. A work function difference of 0.4 eV between GeC and HfS2 exists in this heterostructure [Figure 9B], and resultant interfacial energy band bending leads to an accelerated interlayer charge transfer along a Z-scheme path [Figure 9C].
Figure 9. (A) The schematic illustration of the charge transfer paths of direct Z-scheme WSeTe/XS2 (X = Hf, Sn, Zr) heterostructures: (left) before contact, (right) after contact. (Reproduced with permission[53]. Copyright 2022, Elsevier); (B) The electrostatic potential diagrams of the GeC/HfS2 heterostructure. (Reproduced with permission[54]. Copyright 2023, AIP Publishing); (C) Band arrangement and Z-scheme photocatalytic mechanism of the GeC/HfS2 heterostructure. (Reproduced with permission[54]. Copyright 2023, AIP Publishing).
In addition, a number of other approaches have been used to enhance the photocatalytic performance of TMDs, including passivation by Al2O3 to reduce surface defects and thus inhibit carrier recombination, introducing MXenes (a tunable family of 2D Carbides and Nitrides[55]) with strong antioxidant capacity to inhibit the photocorrosion of TMDs, using the interactions of intrinsic dipole of the polar material with the built-in electric field of the heterostructure to enhance carriers separation, introducing defects or doping, etc. For example, Lu et al. successfully developed a direct Z-scheme vdW heterostructure of SnSe2/SnSe by a two-step vapor phase deposition method [Figure 10A][56] and indicated that the heterostructure has better photocatalytic performance than the original monolayer SnSe2 and SnSe. The stability of the SnSe2/SnSe heterostructure can be further improved by Al2O3 passivation, mainly because Al2O3, as a dense oxide film, can play a role in reducing the surface defects of the heterostructure, thereby inhibiting the carrier recombination with the defect as the recombination center, which is of great significance for improving the stability of photocatalysts. Fu et al. proposed a direct Z-scheme vdW heterostructure by combining TiCO2 (MXenes) with MoSe2 to solve the limitations of high carrier recombination rate and photocorrosion originating from the oxidation of sulfur group elements by photogenerated holes in monolayer MoSe2
Figure 10. (A) Schematic diagram of the physical vapor deposition growth process, showing that SnSe nanosheets can be synthesized onto the ITO substrate with the source-substrate distance of 13-16 cm with the temperature of 860 K, and SnSe2 nanosheets can be synthesized on the ITO substrate with the source-substrate distance of 17-20 cm with the temperature of 610 K. (Reproduced with permission[56]. Copyright 2023, American Chemical Society); (B) Schematic diagram of the photogenerated carrier transfer pathway for the MoSe2/Ti2CO2 heterostructure. (Reproduced with permission[57]. Copyright 2021, Royal Society of Chemistry); (C and D) The time-dependent hole and electron population. (Reproduced with permission[57]. Copyright 2021, Royal Society of Chemistry); (E) Calculated conduction and valence band edge positions for S-substituted SL MoO3 and 1-6L MoS2 with respect to the vacuum level and the standard H2 electrode. The lower edge of the conduction band (orange color) and the upper edge of the valence band (blue color) are presented along with the band gap in electron volts. The dashed black lines indicate the water stability limits for H2 and oxygen evolution. The absolute potential of the standard H2 electrode was taken as 4.44 eV at pH = 0. (Reproduced with permission[61]. Copyright 2021, American Chemical Society); (F) Evolution of the band gap and the Bader charge transfer (from MoS2 layer to MoO3-xSx layer) of the S-doped MoO3/MoS2 heterostructure as a function of the sulfur concentration in the MoO3 layer. (Reproduced with permission[61]. Copyright 2021, American Chemical Society).
Arsenene-based materials
Arsenene, possessing a graphene-like structure, has attracted much attention for its outstanding advantages such as high carrier mobility, anisotropy of heat transfer, and an indirect band gap of ~2.2 eV[62-64]. In recent years, some arsenene-based direct Z-scheme vdW heterostructures have been reported.
Lu et al. constructed a heterostructure of InSe2/arsenene[65], and different configurations were proposed using supercell matching (2 × 2 × 1 arsenene supercells and 3 × 3 × 1 α-In2Se3 supercells) [Figure 11]. The high structure stability shown in Figure 11A and F at room temperature was demonstrated by calculating the electron binding energy (Eb) and AIMD simulations. Thanks to the high interlayer potential drop, the formed built-in electric field can effectively accelerate the separation of photogenerated carriers in the heterostructure. Finally, the two heterostructures achieve a high STH efficiency of 57.08 % and 24.67 %, respectively, which are higher than that of InSe2 and arsenene.
Figure 11. Two different stacking configurations are used in the construction of the α-In2Se3/arsenene vdW heterostructures because the α-In2Se3 layer has a different ferroelectric polarisation orientation, where A, B and C belong to Configuration I and D, E and F belong to Configuration II. The spontaneous out-of-plane electrode polarisation P in Configuration II points in the arsenene direction, whereas P in Configuration I points in the opposite direction. (Reproduced with permission[65]. Copyright 2022, Elsevier).
Photocatalytic CO2RR
In response to growing environmental concerns and energy scarcity, research on converting CO2 to solar fuel through photoreduction has emerged. Similar to the photocatalytic water splitting, direct Z-scheme vdW heterostructures based on graphitic CN have been studied quite extensively, in addition to Bi-based heterostructures.
Graphitic carbon nitride-based materials
Fan et al. investigated a 2D C2N/aza-CMP direct Z-scheme heterostructure consisting of an ultrathin aza-fused conjugated microporous polymer (aza-CMP) and C2N[66]. Since the reduction potential of this heterostructure is not sufficient to drive CO2RR, a Cu dimer is introduced to be anchored on the C2N monolayer. The results show that the Cu2@C2N/aza-CMP heterostructure can significantly lower the reaction energy barrier, allowing CO2RR to proceed spontaneously, and that the Cu-dimer is favorable for promoting the separation of photogenerated carriers at the interface of the heterostructure. Figure 12 illustrates the reaction pathways for CO2RR and OER, with the generation of CH4 being the most favorable from a thermodynamic point of view. The introduction of Cu-dimer improves the conversion rate of
Figure 12. (A) CO2RR and (B) OER photocatalytic pathways within C2N/aza-CMP heterostructure. Red balls stand for O atoms, while the white balls indicate H atoms. (Reproduced with permission[66]. Copyright 2020, Elsevier); (C) The diagrams regarding free energy for the 8e- pathways of CO2RR within C2N/aza-CMP heterostructure at diverse voltages. (Reproduced with permission[66]. Copyright 2020, Elsevier); (D) The diagrams regarding free energy for the 4e- pathways of OER within C2N/aza-CMP heterostructure at diverse voltages. The photogenerated holes and electrons provide the potentials of Uh = 2.24 V and Ue = 0.53 V, separately. (Reproduced with permission[66]. Copyright 2020, Elsevier).
Based on the weak affinity and high activation energy for CO2 of g-C3N4, Wang et al. introduced covalent organic frameworks (COFs) to modulate the photocatalytic CO2RR performance of g-C3N4, mainly depending on the effective CO2 adsorption and enhanced migration of photogenerated carriers through the off-domain of p-electrons supported by imide-conjugated COFs[67]. The performance of the g-C3N4/COF heterostructure is further improved by introducing N vacancies into g-C3N4. Photocatalytic performance test results show that the g-C3N4(NH)/COF heterostructure exhibits a high CO selectivity of 90.4%, with a high CO production rate of up to 11.25 μmol/h under visible light, which is 44 and 14 times higher than that of the pristine g-C3N4 and the heterostructure without N defects, respectively.
Aiming at the low solar-to-fuel energy conversion efficiency (SFE) in the photocatalytic CO2RR process, Bian et al. proposed to improve SFE by introducing a wide band gap oxide semiconductor as an energy platform for the reduction photocatalyst to further enhance the separation of photogenerated carriers[68]. Specifically, a (001)TiO2-g-C3N4/BiVO4 (T-CN/BVNS) 2D heterostructure was constructed [Figure 13A], with the CBM of the heterostructure mainly contributed by g-C3N4 and the VBM mainly contributed by BiVO4, while the (001)TiO2 acts as an energy platform to converge electrons from the CB of g-C3N4 to its own CB, where the reduction reaction takes place. Consequently, the photocatalytic activity of the 5T-15CN/BVNS heterostructure was boosted greatly compared to 15CN/BVNS for both CO2RR and pure water splitting [Figure 13B and C]. More importantly, this study provides a new guideline for developing advanced heterostructures based on narrow-band gap photocatalysts (e.g., WO3, Fe2O3, etc.) by introducing wide band gap oxide semiconductors (e.g., SnO2, etc.) as energy platforms. He et al. designed and fabricated zero-dimensional Ti3C2 quantum dots-decorated TiO2/C3N4 (T-CN-TC) to precisely address the low utilization of photogenerated carriers for CO2RR due to the slow reaction dynamics on the TiO2/C3N4
Figure 13. (A) Schematic representation of the proposed cascade Z-scheme mechanism of photogenerated charge transfer under visible light for efficient photocatalysis. T refers to (001)TiO2, which can feasibly be replaced by other wide band gap semiconductors such as SnO2. (Reproduced with permission[68]. Copyright 2021, Wiley-VCH); (B) Visible light irradiation for 4h of BVNS, 15CN/BVNS, and 5T-15CN/BVNS for CO2RR. For yT-xCN/BVNS, x and y represent the mass percentage of CN and T to BVNS, respectively. (Reproduced with permission[68]. Copyright 2021, Wiley-VCH); (C) Visible light irradiation of BVNS, 15CN/BVNS, and 5T-15CN/BVNS for water splitting. The overall water splitting was carried out with Pt as the cocatalyst and without any sacrificial agents. (Reproduced with permission[68]. Copyright 2021, Wiley-VCH); (D) Schematic of the synthesis of ultrathin Ti3C2 quantum dots anchored TiO2/C3N4 core-shell nanosheets. (Reproduced with permission[69]. Copyright 2020, Elsevier); (E) PL spectra of the samples of CN,T-CN and T-CN-TC. (Reproduced with permission[69]. Copyright 2020, Elsevier); (F) Photocatalytic CO2 reduction performance of T, CN,T-CN, T-CN-TC and CN-TC after irradiation for 1h. (Reproduced with permission[69]. Copyright 2020, Elsevier); (G) The stability test of CO2 reduction over T-CN-TC. (Reproduced with permission[69]. Copyright 2020, Elsevier).
Bi-based materials
Beyond the widely used graphite carbon nitride-based materials, Bi-based materials have also been studied in the field of photocatalytic CO2RR. Since BiOI has a strong reduction capacity and a narrow band gap suitable for visible-light photocatalysis, Wang et al. designed and fabricated a 2D Bi2MoO6/BiOI heterostructure in response to the high carrier conformality and excessive energy barrier of CO2RR[70]. Thanks to the interleaved [Bi2O2]2+ and MoO42-/I- ionic layers, both Bi2MoO6 and BiOI in Bi2MoO6/BiOI heterostructure possess spontaneous polarization and internal electric field, which are beneficial for accelerating the separation of photogenerated carriers. As shown in Figure 14, the heterostructure can effectively promote charge transfer due to the large interaction surface at the interfaces. What is more, the energy barrier of the heterostructure for photocatalytic CO2RR was reduced by 0.35 eV compared with that of BiOI. In addition, this study found that the product generation rate was determined by CO2 hydrogenation through density functional theory calculations[65]. Based on this study, it is expected to further develop direct Z-scheme vdW heterostructures constructed by Bi-based materials with other materials, especially oxide semiconductors, to optimize the advantages of the low CBM position and narrow band gap of Bi-based materials.
Figure 14. Work function profiles of (A) BiOI and (B) Bi2MoO6; (C) Charge density difference of Bi2MoO6/BiOI composite with an isosurface of 3 × 10-4 e/Å3. (Reproduced with permission[70]. Copyright 2022, Elsevier).
CONCLUSIONS AND OUTLOOK
A direct Z-scheme vdW heterostructure is a promising photocatalytic system for solar–chemical energy conversion, which has some prominent advantages, mainly including effective separation of photogenerated carriers, strong redox driving force, simple construction, and minimized crystal defects. In the past decade, a variety of advanced direct Z-scheme vdW heterostructures have been developed for artificial photosynthesis. In this work, we briefly introduce the application of direct Z-scheme vdW heterostructures for photocatalytic water splitting and CO2RR in the last five years. It is found that graphitic CN-based direct Z-scheme vdW heterostructures have exhibited excellent application fundamentals for both photocatalytic water splitting and CO2RR. Meanwhile, heterostructures based on TMDs and arsenene are primarily utilized in water splitting applications, and Bi-based heterostructures exhibit promising potential for
DECLARATIONS
Authors’ contributions
Conceptualization and writing-original draft: Guan X, Hu K, Tian J
Writing-review and editing, supervision, and funding acquisition: Guan X, Zhao D, Zhou Z
Availability of data and materials
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Financial support and sponsorship
This work was supported by the National Key Research and Development Program of China (No. 2022YFB3803600); the National Natural Science Foundation of China (No. 52172248); China Postdoctoral Science Foundation (No. 2020M673386 and No. 2020T130503); and China Fundamental Research Funds for the Central Universities.
Conflicts of interest
All authors declared that there are no conflicts of interest.
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Copyright
© The Author(s) 2024.
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Hu, K.; Tian, J.; Zhou, Z.; Zhao, D.; Guan, X. Direct Z-scheme photocatalytic systems based on vdW heterostructures for water splitting and CO2 reduction: fundamentals and recent advances. Microstructures 2024, 4, 2024021. http://dx.doi.org/10.20517/microstructures.2023.76
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