Issue |
Natl Sci Open
Volume 2, Number 2, 2023
Special Topic: Chemistry Boosts Carbon Neutrality
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Article Number | 20220028 | |
Number of page(s) | 30 | |
Section | Chemistry | |
DOI | https://doi.org/10.1360/nso/20220028 | |
Published online | 30 November 2022 |
REVIEW
Functional customization of two-dimensional materials for photocatalytic activation and conversion of inert small molecules in the air
1
Hefei National Research Center for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China
2
Institute of Energy, Hefei Comprehensive National Science Center, Hefei 230031, China
* Corresponding authors (emails: cxiao@ustc.edu.cn (Chong Xiao); yxie@ustc.edu.cn (Yi Xie))
Received:
23
April
2022
Revised:
8
August
2022
Accepted:
8
August
2022
Air has the advantage of abundance and easy availability, so it is suitable to be used as a synthetic raw material and energy source. However, the triggering of inert small molecules in the air, like O2, N2, and CO2, is a kinetically complex and energetically challenging multistep reaction. Photocatalysis brings hope for this challenge, but obstacles remain in many aspects. Here, aiming at the key difficulties of the photocatalytic activation and conversion of these three inert small molecules, i.e., regulating electronic structure, active sites, charge carrier separation and mobility, and reaction energy barrier, we propose the concept of functional customization strategy of ultrathin two-dimensional materials for achieving more efficient activation and better performance, including thickness control, vacancy engineering, doping operation, single-atom site fabrication, and composite construction. The in-depth understanding of the functional customization will provide more profound guidance for designing photocatalysts that specialize in activating and converting inert small molecules.
Key words: inert small molecules / photocatalysis / functional customization / dinitrogen / dioxygen / carbon dioxide
© The Author(s) 2023. Published by China Science Publishing & Media Ltd. and EDP Sciences.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Introduction
Air contains various important small molecules closely related to our daily life. The O2 in the air is necessary for all aerobic organisms, which need to breathe to live. Besides, O2 is widely used as oxidants in various practical applications. The N2 is the most abundant gas occupying 78% of the volume in the air. It has been used as the key building block for industrial chemical syntheses, especially high-volume chemicals like fertilizers [1]. Further, plants use CO2 in the air for photosynthesis to maintain their growth and development, where CO2 is the only source of carbon for almost all plants. However, the CO2 released into the atmosphere has reached an unprecedented level due to fossil-fuel use, causing a negative influence on the global environment [2–4]. So it is of important significance for research on converting these small molecules to products that are more practical and environment-friendly. Until now, through the photocatalytic conversion route, we can change O2 into strongly oxidizing species [5], use N2 as raw material to generate nitrogen-containing compounds with high industrial value (such as ammonia and nitrate) [6–8], and produce CO, methane, methanol, formic acid, ethanol, ethylene, aromatics, etc. by transforming CO2 [9–12]. In a word, the appropriate utilization of small molecules, mainly O2, N2, and CO2, is essential to the daily functioning of human society. However, there remain many challenges and obstacles in the inert small molecule utilization process.
The dioxygen, on which we rely for survival, is one of the most typical small molecules. Reactive oxygen species (ROS), obtained by reduction of O2, play an important role in many chemical reactions. ROS mainly include superoxide radical (·O2–), hydrogen peroxide (H2O2), hydroxyl radical (·OH), and singlet oxygen (1O2) [13–16]. These highly reactive oxygen species can be used as green oxidants to oxidize pollutants, biological macromolecules, organic chemicals and other substances, and have important application prospects in pollutant treatment, photodynamic therapy, and selective oxidation reactions. Therefore, O2 activation has become an indispensable link in the process of utilizing O2 resources efficiently. However, triplet dioxygen is the ground state of O2, which has a special configuration that the most stable form has two unpaired electrons with parallel spins in the highest electron occupied molecular orbital, namely degenerate π* anti-bond orbital. Due to the electronic transition rule, the transition from the singlet state to the triplet state is a spin-forbidden process and thus the application of O2 is restricted. Meanwhile, the dinitrogen is another small molecule that is difficult to be activated due to its strong N≡N triple bond and non-polarity [17]. Both thermodynamic and kinetic factors prevent the reaction from proceeding [18,19]. In industry, the Harbor-Bosch process is conducted under high reaction temperature and pressure, generating huge energy consumption and serious environmental pollution [20]. Thus, it is highly desirable to develop a less energy-consuming, more eco-friendly and efficient nitrogen fixation method. By utilizing such method, we could make the active sites of the catalysts, which contain empty orbitals and orbital occupied electrons, accept the electrons given by the bonding orbital of N2 and back-donate to the π* antibonding orbital of N2, forming coordination with N2 to complete chemical adsorption and then achieve the activation of N2 by increasing the bond length and reducing the bond energy [21–25]. Furthermore, CO2 could be converted into various valuable carbon-containing products through different pathways [26–29]. At present, the difficulties of activating CO2 mainly lie in low solubility [30], thermodynamic limit [31], and kinetic effects [32,33]. Therefore, the functional customization of photocatalysts to activate and convert inert small molecules becomes urgent and inevitable.
To deal with the intractable problems above, appropriate photocatalysts of advantages are supposed to be designed and synthesized. Recently, atomically ultrathin two-dimensional materials have attracted plenty of attention because of the large specific surface area, abundant active sites, short charge carrier migration distances, and other favorable properties, which are suitable for photocatalytic research [34–37]. More importantly, due to the simplified and manipulable structure, it is convenient to study the photocatalysis mechanism with the aid of ultrathin two-dimensional materials. Therefore, they have been widely used in inert small molecule photocatalytic activation and conversion [38–40].
In this review, aiming at the key difficulties of the activation and conversion of these three inert small molecules, we have developed a description system similar to the fourth-order tensor to enumerate, generalize, and integrate various photocatalytic research on inert small molecule activation and conversion. In particular, we summarize the recent progress in the functional customization of ultrathin two-dimensional materials in activating and converting the three typical kinds of inert small molecules (O2, N2, CO2), and we introduce various regulations of two-dimensional materials in detail. In light of the main photocatalytic reaction process (adsorption, activation, dissociation, formation and desorption of intermediates, and product distribution), we present some opinions about strategies including thickness regulation, vacancy introduction, doping operation, single-atom catalyst construction, and interface engineering, to direct at specific functions, such as electronic structure, active sites, charge carrier separation and mobility, and reaction energy barriers. The research object (inert small molecules), specific modification functions, functional customization strategies, and photocatalytic process, as four dimensions, organically combine to form the description system. Finally, we put forward some prospects that may further improve the performance of two-dimensional materials, hoping to promote their application in the field of inert small molecule activation and conversion research.
Photocatalysis process and corresponding performance-enhancing strategies
Photocatalysis uses light as an energy source to increase the rate of photoreactions by proper catalysts. A typical mechanism of photocatalysis contains five main steps: (1) The photocatalysts harvest light to produce electron-hole pairs. (2) The photogenerated charge carriers transfer to the surface from the bulk (or recombine halfway). (3) Reactants adsorb at the active sites on the surface of photocatalysts. (4) The surface-reached charge carriers trigger redox reactions from adsorbed reactants to products at the corresponding active sites. (5) Products desorb to restore the active site state. It is worth noting that the band structure plays a crucial role in the catalytic process, because only the light whose energy is larger than the bandgap energy (Eg) between the highest completely filled valence band (VB) and the lowest empty conduction band (CB) could be harvested, and the redox reactions whose redox potential matches the band structure of photocatalysts could be activated. In general, there are four kernel points worthy of consideration, including yield (rate), efficiency, selectivity, and stability of the whole catalytic reaction. To achieve high performance for the four main purposes above, several properties and functions of photocatalysts could be regulated by seeking out appropriate materials and implementing functional customization.
The substantial process of activating inert small molecules is weakening or directly cleaving the existing bonds in the molecules after adsorption. Considering the steps of photocatalysis, it is consequential to think that several key directions could be taken into account for functional customization, including: (1) Regulating the electronic structure of the photocatalysts to enhance the light harvest and ensure more reductive electrons or oxidative holes to be quickly trapped in the active centers of the photocatalysts. (2) Introducing more catalytic active sites into the photocatalysts, while two-dimensional materials have exceptional advantages to obtain an extremely high ratio of catalytic active sites on the surface. (3) Searching for suitable strategies which can construct a proper surface structure to tighten the adsorption of reactant molecules, because the charge carriers could only transfer to the inert small molecules which are adsorbed onto the photocatalyst surface. (4) Accelerating the efficient charge exchange between the photocatalysts and adsorbates to lift the activating rate of inert small molecules. (5) Suppressing the recombination of charge carriers to gain higher activating efficiency, making the best use of the energy input. (6) Lowering the overall reaction energy barrier to make the redox reaction more feasible and regulate the reaction pathway to expected products. (7) Facilitating the products to desorb easily and swiftly from the active sites avoiding the poisoning of photocatalysts, to finally complete the conversion process of inert small molecules. With all the factors combined synergically, it could thereby increase the degree of inert small molecule activation, and reduce the reaction activation energy for the more thorough conversion of inert small molecules.
Along with the discovery of graphene, due to the huge potential for manipulating the intrinsic properties in two-dimensional materials without destroying pristine lattices while creating a high exposed proportion of surface atoms, two-dimensional materials construct a suitable model platform for photocatalysis research. It is possible to use the advantages and characteristics to precisely customize each step of the photocatalytic reaction: (1) The energy band structure of two-dimensional nanomaterials is different from the corresponding three-dimensional bulk materials, which is the essence of two-dimensional nanomaterials. Therefore, it is possible to regulate the energy band structure of the photocatalyst to adjust the light absorption properties, and at the same time adjust the corresponding redox ability of the photogenerated carriers. (2) The thickness of two-dimensional nanomaterials is ultrathin, usually at the level of several atomic layers. After the photogenerated carriers are generated from the bulk phase of the material, the distance they need to transfer to the surface of the photocatalyst is very short, which is conducive to photogenerated charge carriers. High-speed transport could also reduce the probability of photogenerated carriers recombining halfway. (3) Two-dimensional materials have a large specific surface area and abundant atoms with low coordination numbers on the surface. These atoms could be used as efficient reactant adsorption sites and photocatalytic active sites triggering the redox reactions. (4) The abundant surface active sites of two-dimensional materials enrich photogenerated carriers, regulate the electronic structure of the material surface, and at the same time reduce the energy barrier of catalytic reactions by promoting the activation of reactants. (5) According to the scaling relationship in adsorption research, the surface modification and electronic structure control of two-dimensional nanomaterials could effectively promote the desorption of products and avoid the poisoning of photocatalysts.
Through the functional customization of two-dimensional materials (Figure 1), it is very promising to clarify the nature of photocatalytic reactions and improve the performance of inert small molecule activation and conversion. In fact, many approaches have been implemented in two-dimensional materials to resolve those key difficulties of photocatalysis. Defect energy level caused by introducing vacancies not only alters the electronic structure of the photocatalysts but also prevents the recombination of carriers. At the same time, the vacancy can cooperate with the nearby low-coordinated atoms to promote the transport of electrons to adsorbates and complete an efficient activation and conversion process [41]. Besides, the introduction of heteroatoms, namely doping operation, can provide more active sites for inert small molecules to be adsorbed and activated, where constructing heterostructured composites could achieve the same effect and promote charge carriers separating and transferring [42–46]. In addition, single-atom sites are widely utilized for constructing good adsorbing locations in photocatalysis research [47]. But it is worth noting that single atoms cannot exist independently, while two-dimensional materials are ideal platforms for supporting single atoms to realize the utmost potential of single-atom sites. The mutual benefits between two-dimensional materials and single atoms make their composites proper models for adsorbing sites research, which is helpful for elucidating the catalytic mechanism objectively [48]. In a word, two-dimensional materials are appropriate platforms for functional customization to enhance photocatalytic activation and conversion of inert small molecules.
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Figure 1 Schematic illustration of functional customization of two-dimensional materials for activation and conversion of inert small molecules in the air. |
Approaches for functional customization of two-dimensional materials
Considering the current problems of photocatalytic activation of inert small molecules, we could start from addressing the performance constraints in photocatalysis, by using thickness regulation, vacancy engineering, doping operations, composite construction, and single-atom sites introduction, to enhance the photocatalytic performance in a targeted manner. Then, we will ultimately actualize functional customization through regulating the electronic structure, active sites, charge carrier separation and mobility of ultrathin two-dimensional materials, and lowering the reaction energy barriers of related photocatalytic reactions. And the functional customization of two-dimensional materials photocatalysts will be a promising direction to achieve higher yield, efficiency, selectivity, and stability of inert small molecule conversion process.
Electronic structure regulation
The electronic structure, including band structure, bandgap width, density of states (DOS), Fermi level, and spin ordering, plays an essential role in the photocatalysis process, to which the ultrathin thickness of the two-dimensional materials is crucial. The electronic structure is very relevant to the electronic interaction in the given lattice. The change of structural lattice will strongly influence the charge carrier concentration, charge carrier mobility, and conductivity of the catalysts. When the thickness is reduced, the wave propagation of two-dimensional materials will be confined, and the bandgap will be altered correspondingly. For instance, the multilayered MX2 (M = Mo, W; X = S, Se, Te) are indirect band gap semiconductors, while the single-layered MX2 are direct band gap semiconductors with a large gap because the changed thickness totally converted the order at the top of the VB and the bottom of the CB of MX2 [49]. In addition, two-dimensional materials with atomic thickness show obviously enhanced DOS near the Fermi level than the bulk materials, leading to higher electrical conductivity and catalytic performance. Furthermore, spin ordering could generate a large variety of novel quantum phenomena through lattice coupling. Given the interconnection between lattice and spin ordering, the electronic configuration could be modulated to a certain degree by tuning the surface lattice environment of two-dimensional materials [50]. Therefore, it is of great significance to regulate the electronic structures of ultrathin two-dimensional materials for controllable properties by functional customization.
For instance, Li prepared hexagonal sheetlike metallic CuS atomic layers, using oleylamine and octylamine as structure-directing agents and reducing agents, respectively [51]. Benefiting from the existence of additional partially occupied band (CB) in the conductor, both bulk CuS and CuS atomic layers could absorb infrared (IR) light by intraband transition where electrons in the CB are excited to the Fermi level. The difference was that the bandgap between the lowest unoccupied band (B1) and CB in bulk CuS was too large for it to absorb the IR light, which means interband transition did not happen. While the bandgap in CuS atomic layers was appropriate and thus interband transition occurred. So in the CuS atomic layers, electrons in the CB could experience intraband transition first and be excited to the Fermi level, then they could suffer from interband transition and be excited to B1. The two transitions could facilitate each other for the electrons consumed in the interband transition, which promoted the intraband transition process, and vice versa. It was the holes and electrons produced in the two cooperative transitions that were capable of converting the CO2 and H2O into CO and O2 for their suitable redox potential. And the yield reached 14.5 μmol g−1 h−1 under IR light irradiation, while the bulk CuS with only intraband transition showed no activity. IR light occupies about 50% of sunlight, which would greatly improve photocatalytic efficiency based on utmost use. Given the variety of conductors, combined with the thickness control strategy, more possibilities will be provided for activating and converting CO2 to obtain expected products.
In addition to thickness control, vacancy engineering also deserves more attention in regulating electronic structure. When vacancies are included in a photocatalyst with suitable reduction and oxidation ability, they will bring defect energy levels and generate intermediate bands. If the new band structure is proper, it will be possible to greatly expand the light absorption range to the IR region to achieve inert small molecule activation [52]. For example, after introducing oxygen vacancies (VO••) into the ultrathin WO3 atomic layers, the intermediate band (IB) emerged at approximately 0.63 eV below the Fermi level, which was evidenced by various characterizations [53]. The reduction of CO2 to produce CO at 800 nm was achieved with a quantum efficiency of 0.0274%. With the increase of the VO•• concentration, both the yield and the quantum efficiency were improved, while the products could hardly be detected in the initial WO3 atomic layers absent of VO••. It demonstrated that the oxygen-deficient WO3 atomic layers could potentially realize carrier transitions from the VB to the empty IB or from the occupied IB to the CB, as well as transitions within the partially filled IB through harvesting IR light, and hence achieve IR light-driven CO2 overall splitting (Figure 2A–2F).
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Figure 2 (A) Atomistic model, (B) computed band structure, and (C) density of states of the WO3 slab with three kinds of surface VO•• (the ratio of VO•• and O atoms is 7.3%). (D) Atomistic model, (E) computed band structure, and (F) density of states for the WO3 slab without surface VO••. (G) The calculated density of states, (H) distribution of charge density, and (I) charge density contour plots at the CB edge for BiOBr atomic layers. (J) The calculated density of states, (K) distribution of charge density, and (L) charge density contour plots at the CB edge for VO••-rich BiOBr atomic layers. (A–F) Adapted with permission from Ref. [53], copyright 2018 Elsevier; (G–L) adapted with permission from Ref. [55], copyright 2018 Wiley-VCH. |
Bismuth oxyhalide is a promising type of semiconductor material with a unique electronic structure, good optical properties and catalytic performance, and it responds well to visible light, leading to high photocatalytic activity [54]. For example, Wu introduced VO•• into the synthesized BiOBr ultrathin nanosheets through ultraviolet light radiation [55]. The CO yield of VO••-rich ultrathin nanosheets reached 20 times that of ultrathin nanosheets with lean VO•• and 24 times that of bulk counterpart. The ultraviolet (UV)-visible diffuse reflection absorption spectra showed that the introduction of VO•• improved the absorption of visible light. Combined with density functional theory (DFT) calculations, it was believed that the new energy level caused by the VO•• reduced the bandgap and increased the delocalization of charge near VO••, which endowed the catalysts with better visible light catalytic performance (Figure 2G–2L). At the same time, BiOBr nanosheets with VO•• could bring on efficient fixation of N2 to NH3 under room temperature and atmospheric pressure in water [56]. The electronic structure change, which is related to localized electrons for π-back-donation caused by designed catalytic VO•• of BiOBr nanosheets on the exposed {001} facets, plays an important role in activating the adsorbed N2 and converting it to NH3 by the interfacial electrons transferred from the excited BiOBr nanosheets.
Localized surface plasmon resonance (LSPR) is a near-field enhancement phenomenon, which can collect nanoscale light with maximum light absorption by adjusting the electronic structure of the material [57]. For instance, plasmonic MoO3−x-TiO2 composites with regulated LSPR were synthesized for visible-light-driven photocatalytic CO2 reduction [58]. The well-structured nanocomposite combining two-dimensional MoO3−x nanosheets and one-dimensional TiO2 nanotubes showed an LSPR absorption band in the visible region, and the participation of TiO2 obviously increased the LSPR absorption band, boosting visible light harvesting and enhancing the performance for visible-light-driven CO2 photoreduction. This work has increased the possibility of catalyst design using LSPR for the development of high-performance photocatalysts for inert small molecules activation and conversion.
Furthermore, it was reported that the scarce single Cu atoms on the defects of polymeric carbon nitride (p-CN) ultrathin nanosheets (Cu-CN) had achieved a photocatalytic nitrogen fixation yield of NH3 up to 186 μmol g−1 h−1 under visible light irradiation [59,60]. According to the extended X-ray absorption fine structure (EXAFS) measurement and the DFT calculation results, it could be concluded that Cu2+ was coordinated in a triangle defect formed by the edge N atoms of the triazine patch (T-defect) and one Cu atom was bound to three N atoms through lone-pair electrons to form a triangular pyramid configuration with three Cu–N bonds (Figure 3A–3C). The presence of Cu single atoms narrowed the band gap of the catalysts, which was beneficial for the harvest of visible light. The electron spin resonance (ESR) results showed that a larger concentration of extra free electrons was activated in Cu-CN than in p-CN under light irradiation (Figure 3D and 3E). The electron density distribution and the calculated projected density of states (PDOS) for local T-defect in Cu-CN proved that the π-conjugated electron cloud was deformed and the single valence electron of the edge N atom was delocalized even isolated from the previous π electron cloud due to the coordination effect between the edge N and Cu in p-CN, so that these isolated valence electrons could be easily activated and then promptly transferred from the interface of the metal ion to the N2 (Figure 3F–3I). In short, the coordination effect of the single Cu atom with the N atom in the support could manipulate the electronic structure, such as increasing the lone-pair electrons of the defects and enlarging the quantity of available active electrons, thereby achieving the efficient activation and conversion of N2.
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Figure 3 (A) EXAFS curves of Cu foil and Cu-CN. (B) The data of EXAFS curves and the fit of Cu-CN. (C) The raised configuration of Cu-CN in a T-defect. ESR of p-CN and Cu-CN (D) before and (E) during illumination. (F) Top view of the distribution of electron density. (G) Side view of the distribution of electron density. The yellow and green isosurfaces represent an augment in the number of electrons and the depletion zone, respectively. The isosurfaces are 0.003 e Å−3. (H) PDOS of C and N atoms of T-defect in Cu-CN. (I) PDOS of Cu atoms, donor N atoms, and C atoms bound with the donor N atoms. Adapted with permission from Ref. [59], copyright 2018 Springer Nature. |
Besides, doping operation has a great effect on the band structure of the materials. Mao et al. [61] replaced the position of some O atoms with C atoms in BiO2, causing VO•• to appear on the surface of the material at the same time, and with two kinds of defects interacting simultaneously, which efficiently lowered the CB, serving as the capturing center for electrons, and thus facilitating the adsorption, activation, and conversion of O2. Moreover, Li increased the bottom of the CB and the top of the VB by introducing Ni2+ into BiO2−x nanosheets hence introducing impurity levels into the bismuth oxide forbidden band, where the enhanced light absorption range of the catalysts was achieved [62]. Similarly, the incorporation of Br in (BiO)2CO3 also changed the energy level of (BiO)2CO3, making its CB position more negative, leading to more efficient O2 activation and conversion [63].
Besides, in the strategies of electronic structure tuning for improving activity, the stability of catalysts could also be altered, which is another important factor in evaluating photocatalytic performance [64]. To investigate this issue, a composite catalyst with nano iron oxide hosted in a graphene template material, with Al2O3 as the structural promoter, demonstrated high activity and stability for the ammonia synthesis [65]. The stability of the photocatalyst was due to the prevention of iron particle aggregation during the reaction by the structural promoter Al2O3. Furthermore, a detailed first-principles calculation has been conducted to study the effect of the monolayer MoS2/SnO2 nanocomposite on the enhanced photocatalytic performance of its nanocomposites [66]. The results showed the dual role of monolayer MoS2 as a sensitizer and a co-catalyst in MoS2-based nanocomposites for improved photocatalytic activity and stability. During the photocatalytic process, the Mo 4d electrons were excited from MoS2 to SnO2. Afterward, the Mo atoms would transport to the catalytic active sites, making the monolayer MoS2 a highly active co-catalyst in the nanocomposite, enhancing both the photocatalytic activity and stability.
In summary, approaches such as thickness regulation, vacancy engineering, doping operations, composites construction, and single-atom sites introduction could be implemented to regulate the specific function of the electronic structure to enhance inert small molecule activation and conversion by two-dimensional material catalysts, thus achieving the high photocatalytic performance.
Active site construction
The active sites, which mainly contain the low coordinated steps, edges, terraces, kinks, and corner atoms in catalysts, are the places at which all the reactants adsorb and reactions occur. The reactant adsorption and product desorption, which to a large extent influence the feasibility and rate of the catalytic reactions, need to be enhanced for better photocatalytic performance. It is generally accepted that the sites with more dangling bonds possess higher reactive activity, so the sites with lower coordination numbers inducing more dangling bonds could be regarded as catalytic active sites. The atomic thickness and ultrahigh specific surface area of atomically two-dimensional materials could increase the number of their low coordinated sites, namely active sites, compared with the total number of atoms, leading to the extremely high ratio of active sites for better catalytic efficiency [67]. In addition, the more exposed surface atoms in the two-dimensional materials could easily escape from the lattice and hence inevitably result in the formation of defect structure with low coordination number, which creates even more active sites including inaccessible vacancies in three-dimensional bulk materials and also strongly affects the electronic structure of catalysts, lowers their surface energy, enhances the adsorption of reactants, and hence endows them with better stability [68]. Therefore, by functional customization, the two-dimensional materials could serve as an ideal model to investigate the atomic level interplay between active sites and catalytic activity, namely the active sites–catalytic activity relationship.
Thickness control alone could increase the active sites to a certain degree. Liang et al. [69] synthesized single-unit-cell Bi2WO6 layers by a lamellar Bi-oleate intermediate for the first time. The CO2 adsorption isotherms and UV-visible diffuse reflectance spectra revealed that Bi2WO6 atomic layers had 3 times higher CO2 adsorption capacity relative to bulk Bi2WO6. When the thickness is down to a single-unit-cell, the ultra-large surface area for the single-unit-cell Bi2WO6 layers with sufficient active sites favored the enhancement of CO2 adsorption capacity, thus providing the prerequisite to participate in the following reactions of CO2 activation and conversion.
In addition, due to the diversity of CO2 reduction products, selectivity is also an important issue in the conversion of CO2. However, due to the scaling relationships, which means a positive correlation of the binding abilities between the same active site and multiple intermediates, an increase in reactivity may lead to a decrease in selectivity [70]. In order to solve this dilemma, Li et al. [71] constructed atomically ultrathin CuIn5S8 layers with rich sulfur vacancies (VS••). The introduction of VS•• led to the accumulation of the charge on Cu and In atoms around the vacancies, and thus formed Cu-In bimetal active sites. According to the calculated results shown in Figure 4, it is suggested that the formation of COOH* intermediates was the rate-limiting step for both samples, and the VS-CuIn5S8 single-unit-cell layers had a lower COOH* formation energy than the pristine CuIn5S8 single-unit-cell layers. Due to the enhanced hybridization ability between the d orbitals of the charge-rich Cu and In atoms and the p orbitals of C and O atoms, the CHO* intermediates tended to bind atoms more easily with Cu–In double sites than with Cu single sites, which decreased CHO* formation energy in VS-CuIn5S8 single-unit-cell layers. As shown in Figure 4A (outlined by the red dashed box) and Figure 4C, the Gibbs free energy of CHO* formation (∆G(CHO*)) was smaller than the desorption energy of the CO molecules (∆G(* + CO)). That is, the formation of CHO* radical was an exothermic and spontaneous process, while the CO* desorption process was endothermic and involved a large activation energy barrier. This means that the VS-CuIn5S8 single-unit-cell layers were more beneficial for the protonation of CO* to produce CHO* than for the desorption of CO* molecules from their surface, which accounted for their near 100% product selectivity for visible-light-driven CO2 reduction to CH4. Contrastingly, for pristine CuIn5S8 single-unit-cell layers (Figure 4B and 4D), ∆G(* + CO) was lower than ∆G(CHO*), while both the desorption and the hydrogenation of the CO* group on the Cu single sites still needed to overcome a certain potential barrier. Therefore, the CO* molecules could be more easily desorbed from the surface of the pristine CuIn5S8 single-unit-cell layers to form free CO molecules. Thus, for the layers with the bimetallic sites, not only did the yield of CH4 increase to 5.4 times over the original, but also the selectivity of CH4 reached nearly 100%. Besides, the presence of In metal sites led to the formation of In–O bonds, and the C–O bond was thus weakened, which was conducive to the desorption of CH4, enhancing the process of generating CH4 and finally achieving high CH4 selectivity (Figure 5A).
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Figure 4 Free energy diagrams of photocatalytic CO2 reduction to CH4 for (A) the VS-CuIn5S8 single-unit-cell layers and (B) the pristine CuIn5S8 single-unit-cell layers. The blue line represents the more facile way while the black line shows the less favorable way. Steps in the red-dashed box are determining factors of the reaction selectivity. Key steps of photocatalytic CO2 reduction to CH4 for (C) the VS-CuIn5S8 single-unit-cell layers and (D) the pristine CuIn5S8 single-unit-cell layers, in which the charge-enriched Cu-In dual sites convert the endoergic protonation to an exoergic process, and hence change the reaction routes to product CH4 instead of CO. Adapted with permission from Ref. [71], copyright 2019 Springer Nature. |
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Figure 5 (A) CO2 photoreduction into fuels such as CH4 and CO through the use of dual-metal-site catalytic systems (“H+ + e−” refers to the proton-coupled electron transfer process and “−H2O” means the desorption of H2O molecules after the intermediates react with the proton-electron pair). (B) 3D colormap surface with the projection of in situ Fourier transformed infrared spectroscopy (FTIR) spectra for coadsorption of a mixture of CO2 and H2O vapor on the mildly oxidized SnS2 atomic layers. (C) CO TPD spectra for the SnS2 atomic layers, the poorly oxidized and the mildly oxidized SnS2 atomic layers. (A) Adapted with permission from Ref. [71], copyright 2019 Springer Nature; (B and C) adapted with permission from Ref. [73], copyright 2017 American Chemical Society. |
The strategy of interface engineering by constructing composites also plays an important role in increasing the number of active sites [72]. For example, Jiao et al. [73] synthesized Sn oxide in situ on the SnS2 ultrathin nanosheets, creating considerable active sites in the locally oxidized domains. In a series of contrast samples, with the increase of the Sn oxide content, the performance of photocatalytic reduction of CO2 for CO production gradually improved, and moderately oxidized SnS2 ultrathin nanosheets reached high yields, which was approximately 2.3 and 2.6 times that of the slightly oxidized and unoxidized SnS2 ultrathin nanosheets, respectively. Considering that the CO desorption process is an important factor affecting the entire CO2 reduction process, for the slightly oxidized SnS2 atomic layer, the lowest initial CO desorption temperature and the highest CO detection amount mean that the formed CO* molecules are easier to dissociate from the surface than the counterparts revealed by the temperature programmed desorption (TPD) measurement (Figure 5B and 5C). In heterostructures, a sudden change in the energy band structure at the interface can lead to the bending of its energy band and the formation of an internal electric field. These band characteristics can effectively promote the effective separation of charge carriers at the interface.
For nitrogen fixation, Zhao et al. [74] synthesized a series of ultrathin layered-double-hydroxide (LDH) nanosheets by simple co-precipitation routes, which contained an abundance of VO•• that enhanced the adsorption and activation of N2, endowing them with high photocatalytic activity for transforming N2 to NH3. The excellent activity can be attributed to the surface composition, distorted structure and compressive strain in the LDH nanosheets, which significantly enhanced N2 adsorption and thereby promoted NH3 formation. The adsorption energy for N2 on CuCr-VO•• and CuCr-VO••-Strain have obviously increased compared with defect-free counterparts by a factor of nearly two times. The presence of VO•• and strain in CuCr-LDH nanosheets weakened the N≡N bond on chemisorption, obviously increasing the N–N distance compared with free molecular N2 (Figure 6A). By adjusting the types of metal ions to construct specific active sites, the CuCr-LDH was given the ability to achieve visible light nitrogen fixation at 500 nm light irradiation. The photogenerated electrons in the CB of CuCr-LDH could donate to an antibonding orbital of strongly chemisorbed N2, thereby weakening the N≡N bond and thereby facilitating NH3 synthesis. Graphitic carbon nitride (g-C3N4), a novel metal-free semiconductor material, has been widely utilized in many fields. Hu et al. [75] synthesized a kind of honeycombed iron doped graphitic carbon nitride with enhanced N2 photofixation ability. The Fe3+ was inserted in the gap position and stabilized in the electron-rich g-C3N4 by coordinating Fe–N bonds. Fe3+ can chemically adsorb and activate N2 molecules and transfer photogenerated electrons from g-C3N4 to adsorbed N2 molecules. The density functional theory simulation proved that the N2 activation effect at the Fe3+ site was caused by high adsorption energy and long N≡N bond. When N2 was adsorbed on the Fe3+ doping site, the electrons in the σg2p orbital (the highest occupied molecular orbital, HOMO) of the N atom were obviously delocalized, and the orbital energy was almost the same as that of the πg2p orbital (the lowest unoccupied molecular orbital, LUMO), which confirmed that doping as an effective method could improve the photocatalytic N2 activation and conversion.
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Figure 6 (A) N–N distance of free N2, N2 on CuCr-VO••, N2 on CuCr-VO••-Strain, N2H2, and N2H4. (B) 2D structural model of TiO2 nanosheets with VO•• and engineered strain. (C) The O2 chemisorbed at a coordinatively unsaturated W site of defective WO3, and (D) the O2 adsorbed at the surface of perfect WO3. The purple and olive colors represent increase and decrease in electron density, respectively, revealed by first-principle calculations. (A) Adapted with permission from Ref. [74], copyright 2017 Wiley-VCH; (B) adapted with permission from Ref. [76], copyright 2019 Wiley-VCH; (C and D) adapted with permission from Ref. [77], copyright 2016 American Chemical Society. |
Besides, Zhao et al. [76] reported that ultrathin TiO2 nanosheets with an abundance of VO•• through a facile Cu doping strategy and intrinsic compressive strain exhibited remarkable and stable performance for photocatalytic reduction of N2 to NH3 in water. It demonstrated that the superior activity was attributed to Jahn-Teller distortions and the Cu dopant, which created abundant VO•• and introduced compressive strain into the TiO2 nanosheets, synergistically promoting N2 adsorption and activation (Figure 6B). For quantitative analysis, the adsorption energy trend is TiO2-VO••-Strain (−0.37 eV) > TiO2-VO••(−0.25 eV) > TiO2-Pure (−0.17 eV), supporting the conclusion above, which corresponded to the nitrogen adsorption-desorption isotherms and N2 TPD results. However, the formation of extra phases (such as CuOx) on the TiO2 surface may cover defect sites, reducing the concentration of VO•• available for N2 adsorption, which proved the function of doping operation from the negative aspect and gave us proper advice on regulation limitation. It indicates that functional customization, such as introducing defects appropriately, is expected to provide more active sites for inert small molecule adsorption and thereby enhance photocatalytic performance.
The enhanced chemical adsorption capacity of O2 on the material surface reduces the energy of photons required for O2 activation. To gain insights into the adsorbate-catalyst interaction, Zhang et al. [77] synthesized the WO3 nanosheets with numerous coordinatively unsaturated sites on the surface for photocatalytic O2 activation. The chemisorption is accompanied by a slight electron transfer from WO3 active sites to the adsorbed O2. In comparison, a perfect WO3 surface can hardly supply active sites for the chemisorption of O2 so as to hinder their electronic coupling (Figure 6C and 6D). It demonstrated that the strong coupling between defective WO3 and O2 would serve as a bridge to enable energy transfer from excitons to adsorbed O2 once WO3 is photoexcited to achieve high reactivity. The chemisorption not only facilitates the transfer of photogenerated electrons to reactants, but also lowers the photon energy requirement for reactions by altering the form of active species.
Single-atom doping is also an efficient method for introducing active sites into photocatalysts. To explore the relationship between the amount of single atom loading and the coordination environment and photocatalytic performance, Ma et al. [78] synthesized a kind of Co/g-C3N4 single-atom catalysts (SACs) with a high surface metal loading of 24.6 wt%, which could offer numerous uniform coordination N2C sites for Co-anchoring. The formation of the ultrahigh density of single dispersed Co-N2C active sites promoted the migration of photogenerated carriers and helped for the localization of photogenerated electrons under illumination. As a result, the as-prepared Co/g-C3N4 SAC was validated to be an effective photocatalyst for CO2 reduction to CH3OH. It demonstrated that the high amount of single atom loading could greatly improve the photocatalytic efficiency. Moreover, a single La atom catalyst with a specific coordination structure for photocatalytic N2 fixation was synthesized [79]. The specific coordination of single La atom is identified to be O2c-La-O2c. The chemicophysics experiments and DFT calculations suggest that the single La atoms optimized the local/intrinsic electron properties, and favored the transfer of electrons into the π*2p antibonding orbital of the adsorbed N2. It was proposed that the coordination environment of single atom could act as specific active sites, thus promoting the photocatalytic performance.
Therefore, the inert small molecule activation process is regulated by the active sites being introduced on ultrathin two-dimensional materials through functional customization. The ultrahigh specific surface area of two-dimensional materials could bring about a high ratio of active sites. And vacancy introduction plays a vital role in importing active sites with higher reactivity, which could increase the yield and selectivity of the photocatalytic reaction.
Charge carrier separation and mobility enhancement
The charge carrier mobility from the bulk to the surface of the catalysts directly relates to the rate and efficiency of catalytic reactions, so it requires necessary improvement for high photocatalytic performance. It is also needful to avoid charge carrier recombination halfway through the reaction, while fortunately the intrinsic ultrathin two-dimensional architecture could shorten the charge diffusion distance and thus reduce the recombination possibility of electron-hole pairs [80,81]. However, the relationship between active sites and charge carrier mobility is usually conflicting. The mobility of two-dimensional materials is often hindered by the charge scattering mechanism from defects or grain boundaries [82]. Moreover, larger specific surface areas and abundant active sites are prone to appear with thinner two-dimensional construction, while the overall mobility in this kind of material is usually lower due to the poor interlayer charge carrier transport [83]. Thus, balancing the benefits between rich active sites and high mobility is highly desirable to achieve efficient photocatalytic properties. So the functional customization, controllable disorder engineering with element incorporation, was proposed to synergistically modulate the charge carrier behavior for obtaining promoted photocatalytic performances.
For example, the ZnIn2S4 atomic-level nanosheets with different vacancies were constructed for photoreduction of CO2, where the defect type was determined to be zinc vacancies (VZn″), showing a 3.6 times yield improvement than the VZn″-poor sample [84]. The difference in the time signal corresponding to the positron captured by VZn″ from the results of the positron annihilation spectra confirmed the relatively high level of the vacancy concentration. A higher concentration of VZn″ increased the electron affinity and brought more negative zeta potential by bringing more dangling bonds. Ultrafast transient absorption spectra (TA) gave the real-time information about the charge carrier dynamics (Figure 7A and 7B). When excited electrons returned from the CB, they were captured by the defect states brought from the VZn″, and then came back to the VB. In the VZn″-rich sample, the time parameters associated with the final process were longer, which meant that the carrier separation efficiency was improved. DFT calculations showed that the introduction of VZn″ would cause electrons to be enriched near the S atoms, which promoted the separation and transport of charge carriers on the two-dimensional nanosheets, leading to high CO2 activation and conversion efficiency (Figure 7C–7G). For the approach by composite construction, an ultrathin 2D/2D Ti3C2/g-C3N4 heterojunction was synthesized to show enhanced CO2 photoreduction activity than the pure Ti3C2 or g-C3N4 [85]. The enhanced CO2 photoreduction activity was mainly attributed to the combined effects of (1) improved CO2 adsorption and activation, and (2) the construction of heterojunction, where the intimate contact stimulated an efficient spatial separation of photo-excited charge carriers.
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Figure 7 Distribution of partial charge density close to the edge of conduction band for (A) VZn″-rich one-unit-cell ZnIn2S4 layers slab and (B) VZn″-poor one-unit-cell ZnIn2S4 layers slab in the [001] orientation. The unit cell structure of (C) VZn″-rich one-unit-cell ZnIn2S4 layers slab and (D) VZn″-poor one-unit-cell ZnIn2S4 layers slab in the [001] orientation. (E) Scheme for the photocatalytic CO2 reduction into CO on the VZn″-rich one-unit-cell ZnIn2S4 layers. Ultrafast TA spectroscopy of (F) VZn″-rich one-unit-cell ZIS layers and (G) VZn″-poor one-unit-cell ZIS layers. (H) Surface photovoltage spectra and (inset) corresponding phase spectra of BiVO4 layers and (I) the field-induced surface photovoltage spectra under different external electric fields. (A–G) Adapted with permission from Ref. [84], copyright 2017 American Chemical Society; (H–I) adapted with permission from Ref. [86], copyright 2017 American Chemical Society. |
In the case of BiVO4 single-unit-cell layers, samples with different vanadium vacancy (VV″‴) concentrations were synthesized by simply changing the reaction temperature and time [86]. The element ratio provided by the X-ray fluorescence analysis, along with the positron annihilation spectra, proved the difference in vacancy concentrations. The time-resolved fluorescence emission decay spectra showed that the carrier lifetime had increased from 74.5 ns at low vacancy concentration to 143.6 ns at high vacancy concentration. The surface photovoltage (SPV) test demonstrated that the signal of the latter was likewise stronger, which indicated that a higher VV″‴ concentration promoted the separation of photogenerated charge carriers. It also showed that the electrons and holes in the BiVO4 layers transferred to the surface and inside of the catalyst, respectively (Figure 7H). Therefore, electrons enriched on the surface would adsorb H2O and CO2 molecules, and these molecules would be subjected to a polarization effect on the catalyst surface atoms. Field-induced surface photovoltage spectra (FISPS) indicated that in BiVO4, a higher VV″‴ concentration corresponded to a stronger polarization effect, which was more favorable for charge carrier separation, higher CO2 absorption capacity and hydrophilicity (Figure 7I). Finally, the methanol yield of the VV″‴-rich sample reached 398.3 μmol g−1 h−1, which was 1.4 times that of the comparative sample.
Moreover, a model atomically thin structure of single-unit-cell Bi3O4Br nanosheets with surface defects was proposed to boost photocatalytic nitrogen reduction efficiency by simultaneously promoting charge separation in bulk and on the surface [87]. The longer lifetime detected by the TA spectra of defect-deficient Bi3O4Br relative to bulk Bi3O4Br could be ascribed to the ultrathin thickness, which provided high charge carrier mobility from the interior to the surface of catalysts. The surface defects can serve as surface separation centers for charge carriers and further promote the charge carrier separation, therefore prolonging the charge carrier lifetime. The steady-state photoluminescence (PL) spectra and transient photocurrent have been employed to further confirm the above results. The atomically ultrathin configuration and confined surface defects have a synergistic effect, promoting the bulk phase and surface charge separation, thereby boosting the N2 activation and conversion performance.
Exciton is formed by excited electrons bounding to holes in the valence band by Coulomb interaction. This bound has limited the freedom of charge carriers to participate in the photocatalysis process [88]. Considering electrons play an indispensable role in the process of O2 activation, the effect of exciton in photocatalysis cannot be ignored. Consequently, reducing exciton annihilation and further increasing the charge carrier concentration are very important for improving the efficiency of O2 activation. Compared with semiconductor bulk materials, in two-dimensional electronic structures, the binding energy of excitons is much greater, so the exciton effect is enhanced and more stable at higher temperatures or under the action of an electric field. Wang et al. [89] confirmed the existence of excitons in BiOBr nanosheets and interrogated the influence of excitonic effects on photocatalytic processes for the first time, showing that the structure of materials has a great effect on the concentration of excitons. It demonstrated that the spatial-confinement-induced excitonic effects played crucial roles in photocatalytic processes on account of the unique facet-dependent feature of photocatalytic O2 activation behavior by functional customization of the BiOBr nanosheets (Figure 8A).
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Figure 8 (A) Optical excitation and relaxation processes involved in the confined layered structure, where PF and PH represent prompt fluorescence and phosphorescence, respectively. (B) Scheme of the photophysical processes involving exciton- and VO••-mediated trap states. (C) SPV spectra of VS••-rich and VS••-poor In2S3. (A) Adapted with permission from Ref. [89], copyright 2017 American Chemical Society; (B) adapted with permission from Ref. [90], copyright 2018 American Chemical Society; (C) adapted with permission from Ref. [93], copyright 2019 American Chemical Society. |
Later, by introducing VO•• into the BiOBr nanosheets, the separation of excitons was greatly improved, and the efficiency of VO••-BiOBr for activating O2 to generate ·O2– was improved [90]. Theoretical simulations suggested VO•• were able to markedly distort the localization of the band-edge states around the defective sites, leading to instability of excitons, ultimately resulting in promoted charge carrier generation of the catalysts (Figure 8B). Besides, Wang et al. [91] also utilized the special structure of semi-crystalline heptazine-based melon (SC-HM) to separate excitons efficiently, thereby promoting the excited electron concentration of the material to be 7 times higher than that before modification. By regulating the exciton content, other steps in the reaction process were also modulated. The increase of triplet excitons results in a high concentration of oxygen-containing groups, which have effects on the activated oxygen properties of the material. Experiments on introducing carbonyl groups into g-C3N4 have shown that if there are fewer carbonyl groups, the 1O2 production will be reduced [92].
Moreover, Sun et al. [93] introduced VS•• in ultrathin In2S3 to increase the concentration of free electrons participating in the surface O2 activation reaction, which can further improve the efficiency of O2 activation to gain ·O2−. Due to the existence of surface VS••, electrons in VB could transfer more easily to the CB or defect energy levels. The VS••-rich In2S3 exhibits a higher SPV response intensity than the VS••-poor sample, suggesting efficient transfer and redistribution of space-separated photogenerated charge carriers due to abundant VS••, and thus reducing the energy of photons required for O2 activation (Figure 8C).
To sum up, the two-dimensional structure decreases the bulk vacancies, where most charge carrier recombination occurs, while increasing the surface vacancies which have many advantages for making the utmost of photogenerated charge carriers. Besides vacancy introduction, other approaches like the exciton effect could be utilized to regulate the charge carrier separation and mobility to enhance inert small molecule activation and conversion, leading to high photocatalytic performance.
Reaction energy barrier reduction
The reaction energy barrier, which is obtained by subtracting the free energy of the reactant structure from that of the transition state structure using transition state theory, is essential to the rate and selectivity of catalytic reactions. The word “activation energy” is widely used in the catalysis research as well, which we could treat approximately as the same as the reaction energy barrier. The appropriately regulated configuration of two-dimensional materials could reduce the overall reaction energy barrier and even convert the endoergic step to an exoergic reaction process under certain circumstances, thus changing the reaction pathway to produce the expected substance with high selectivity.
Similar to surface vacancy defects, the surface pothole structure can also facilitate the activation of N2. The surface pothole can be regarded as the uneven distribution of atoms or the vacancies of multiple atoms, so many dangling bonds will emerge around the pothole, which could form a localized electron enrichment area spontaneously to adsorb N2 directly and influence the reaction path and thus the reaction energy barrier. Liu et al. [94] successfully synthesized the pothole-rich WO3 nanosheets by a chemical topological transformation strategy and achieved the direct nitrate synthesis from N2 under mild conditions. Without any sacrificial agent or precious-metal co-catalysts, the average yield of nitrate was 1.92 mg g−1 h−1 at room temperature. It was found from the DFT calculation that the nitrogen fixation process on WO3 nanosheets followed the photogenerated hole oxidation mechanism. Pothole-free WO3 is needed to overcome the considerable reaction energy barriers during the adsorption and activation processes of N2 (Figure 9A and 9B), while the pothole-rich WO3 nanosheets could motivate charge carrier transfer spontaneously due to the pothole structure (reaction state A in Figure 9C). The dangling O atoms around the potholes trapped electrons and injected them into the empty π* antibonding orbital of N2, thereby reducing the bond level of N2 and achieving strong chemisorption of N2 to form a metastable intermediate state (reaction state B in Figure 9C). Because the activation energy of the subsequent multi-electron reactions was significantly reduced (Figure 9D), N2 was easily activated and converted to NO. Then NO desorbed into the solution and was translated into nitrate with O2 and H2O.
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Figure 9 (A) The multistep process in pothole-free WO3. (B) Calculated Gibbs free energy diagram of reaction transformation process over pothole-free WO3. (C) DFT calculation results indicated the strong chemisorption behavior of N2 and the obvious charge transfer behavior on the pothole-rich surface with dangling bonds. (D) The specific Gibbs free energy diagram of the stepwise of activation and cleavage of N≡N bonds of N2. Adapted with permission from Ref. [94], copyright 2019 Wiley-VCH. |
For constructing single-atom catalysts, single-atom Pt anchored at the –N3 sites of stable and ultrathin covalent triazine framework nanosheets (Pt-SACs/CTF) have been successfully synthesized [95]. In general, the photocatalytic N2 reduction reaction takes two associative mechanisms, the distal mechanism and the alternating mechanism, in which nitrogen is consecutively protonated via the proton-coupled electron process (PCET) without breaking the N≡N until the release of the first NH3 (Figure 10A). The DFT results indicated the alternating mechanism is slightly favored for the N2 fixation in the Pt-SACs/CTF catalyst. The calculated Gibbs adsorption energy of NH3 is 1.4 eV, which was too high for the produced NH3 to desorb in the general case. Affected by regulation, the ammonia is not detected by the gaseous NH3 but by the NH4+ ions. Therefore, the last step for N2 photocatalytic fixation to ammonia is NH3* + H+ → NH4+. Although the extensive computational cost of using the explicit solvent model brings about the impracticability of DFT calculation, it is expected that the formation of NH4+ is feasible at room temperature in terms of the considerable yield obtained, leading to a reaction energy barrier decrease from the beginning to the end of the whole photocatalytic process (Figure 10B and 10C).
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Figure 10 (A) Scheme and (B) free energy diagrams of the distal and the alternating mechanisms for the nitrogen reduction process on the Pt-SACs/CTF catalyst. (C) Optimized structures of reaction intermediates in the distal and the alternating mechanisms. (D) Free energy diagrams of photocatalytic CO2 reduction to CO by the Bi3O4Br and Co-Bi3O4Br. (E) Free energy diagram for the photocatalytic CO2 reduction to CO on LZS (001). The local configurations of the adsorbates on the LZS are shown as inserts at the initial state, the transition state, and the final state along the minimum-energy pathway (C, O, and H atoms of the adsorbates are represented by gray, light-green, and light-blue spheres, respectively). (A–C) Adapted with permission from Ref. [95], copyright 2020 American Chemical Society; (D) adapted with permission from Ref. [96], licensed under a Creative Commons Attribution 4.0 International License; (E) adapted with permission from Ref. [97], copyright 2019 Wiley-VCH. |
Furthermore, isolated single-atom Co incorporated into Bi3O4Br atomic layers was successfully prepared, performing light-driven CO2 reduction with a selective CO formation rate about 4 and 32 times higher than that of atomic layers and bulk counterparts, respectively [96]. The Co single atoms could lower the CO2 activation energy barrier by stabilizing the COOH* intermediates and tuning the rate-determining step from the formation of COOH* to the desorption of CO* (Figure 10D). Then, as shown from CO TPD, the single-atom sample showed lower initial desorption temperature and higher total CO detection, indicating that the incorporation of Co single atoms is conducive to CO desorption, and this result is verified by calculating the free energy of CO desorption, demonstrating that this functional customization helps to reduce the reaction energy barrier in the photocatalytic process.
Without loading heteroatom, layer-structured zinc silicate (LZS) nanosheets have been successfully synthesized by a liquid-phase epitaxial growth route for the efficient conversion of CO2 into CO [97]. DFT calculation based on the LZS structure model was used to investigate the possible reaction pathway along with the local configurations. As mentioned above, the formation of adsorbed COOH* is usually a key step in the reduction process of CO2 to CO. The corresponding free energy barrier of 1.01 eV for COOH* was overcome in the CO2 reduction reaction over LZS, which demonstrated that the two-dimensional structure had the potential to surmount the reaction energy barrier, obtaining high photocatalytic performance (Figure 10E).
Moreover, an efficient, pure water CO2-to-CO conversion photocatalyzed by sub-3-nm-thick BiOCl nanosheets with van der Waals gaps (VDWGs) on the two-dimensional facets was proposed [98]. Compared with bulk BiOCl, the VDWGs-rich atomic layers possessed a weaker excitonic confinement power to decrease exciton binding energy from 137 to 36 meV, consequently yielding a 50-fold enhancement in the bulk charge separation efficiency. In addition, the VDWGs facilitated the formation of the VDWG-Bi-VO″-Bi defect, a highly active site to accelerate the CO2-to-CO transformation via the synchronous optimization of CO2 adsorption, CO2 activation, COOH* splitting, and CO* desorption. These results suggest that increasing VDWG exposure is a way for enhancing the adsorption of reactant molecules and facilitating the products to desorb, thus improving the photocatalytic performance.
Besides, Han et al. [99] synthesized InVO4 layers of three unit cells thick which mainly exposed (110) facet, performing CO generation rate about 6 times higher than that of InVO4 nanocubes enclosed with (100) facet (Figure 11A and 11B). DFT calculation of the thermodynamic process of CO2 reduction to CO showed that the free-energy barrier of the rate-determining step on (110) was 0.76 eV lower than that of (100) facet. Furthermore, the CO desorption process on (110) facet was exothermic by −0.11 eV, while endothermic by 0.45 eV on (100) facet (Figure 11C). Thus, the energy barrier reduction of CO formation and desorption on (110) facet led to the promotion of photocatalytic activity.
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Figure 11 Top and side views of the optimized (A) (110) and (B) (100) facet of InVO4. In, V, O, C, and H atoms are represented by light green, violet, red, brown, and white spheres, respectively. The black dashed circle represents active sites. (C) Computed Gibbs free energy diagram of CO2 reduction reaction into CO catalyzed by (110) for black line and (100) for orange line, respectively. Adapted with permission from Ref. [99], copyright 2019 American Chemical Society. |
To summarize, thickness regulation, vacancy engineering, and single-atom sites introduction of two-dimensional materials could be carried out to regulate the specific function, that is, reducing the reaction energy barrier, to facilitate inert small molecule activation and conversion, leading to high photocatalytic performance, especially expected high selectivity.
Summary and outlook
Atomically ultrathin two-dimensional materials, benefiting from their unique structure, bridge the study of the relationship between structure and activity, which is convenient for guiding the design of photocatalysts and opens a new way for the functional customization of materials with better photocatalytic performance. Therefore, it has great potential in the field of inert small molecule activation and conversion. We here summed up concisely the functions available to be regulated as electronic structure, active sites, charge carrier separation and mobility, and reaction energy barriers. We then summarized the progress in the functional customization of two-dimensional materials for activating and converting inert small molecules in the air, including the preparation of atomically ultrathin two-dimensional nanosheets based on the principle of thickness control, the introduction of anion or cation vacancies, heteroatom engineering containing doping or loading of heteroatoms to fabricate single-atom catalysts, and composite construction with other materials. It is foreseeable that through exploration and innovation of construction strategies, characterization technology, and dynamics research with the development of theoretical calculations, we could make considerable progress in understanding the mechanism of inert small molecule activation and conversion in two-dimensional materials and regulating their photocatalytic performance. Finally, how to further improve the yield, efficiency, selectivity, and stability is an urgent subject to be addressed.
Despite the fact that much progress has been made in the research of functional customization of ultrathin two-dimensional materials for activating and converting inert small molecules, there are still many challenges.
(1) How to realize the industrial-scale production of two-dimensional materials and gradually narrow the gap between laboratory research and practical applications needs continuous and intensive exploration. Although a number of top-down and bottom-up methods have been employed for the synthesis of ultrathin two-dimensional materials, it is still challenging to prepare two-dimensional materials on a large scale. The mass production of ultrathin two-dimensional materials with specific surface defects will be the pivotal issue in the photocatalytic application. More diverse and abundant synthetic strategies should be explored to prepare defect-rich two-dimensional materials with the ultrathin thickness on a large scale.
(2) Further synthesizing two-dimensional materials with improved photocatalytic activity is still urgent. Firstly, constructing two-dimensional semiconductor heterostructures is a promising approach and several methods have been investigated, like epitaxial growth [100]. It is necessary to develop the technique of constructing high-quality two-dimensional semiconductor heterostructures with different functional characteristics assembled into a single nanosheet, and the effective combination of different multifunctional materials, which shows a wide range of application prospects in solar energy conversion, such as photocatalysis. Secondly, the Z-scheme heterojunction with atomically ultrathin two-dimensional materials could promote charge carrier separation and retain the oxidation and reduction ability of components, thus receiving widespread attention [101–103]. However, this strategy is mainly applied in water splitting, organics reduction, and lithium-ion batteries [104–107]. In the future, by integrating photocatalysts with the strong oxidizing capacity to promote the oxidization part and photocatalysts with the strong reducing ability to carry out the reduction part, it may further play a role in the more extensive inert small molecule activating and converting ability.
(3) The adsorption and activation of inert small molecules are still difficult in the real photocatalytic process. Physical adsorption is similar to the phenomenon of agglomeration of molecules on a surface, which is not selective. Physical adsorption and chemical adsorption often coexist, which leads to the formation of multilayer molecular adsorption layers on the photocatalyst surface due to the van der Waals attraction between adsorbed phase molecules and gas phase molecules. The adsorption layer will affect the desorption of the products produced on the photocatalyst surface and hinder the continuous progress of the photocatalytic reaction [108,109]. More strategies are needed to enhance the adsorption of reactant molecules and facilitate the desorption of the products for better photocatalytic performance. In addition, more effective control measures are needed to reduce the reaction energy barrier to solve the activation problem of the inert small molecules.
(4) It is uneasy to establish a clear structure-activity relationship. From the perspective of structure, the structural analysis of the active sites of photocatalysts has always been an impediment in this field, which severely limits the study of the photocatalytic mechanism and structure-activity relationship, and hinders the rational design of new highly efficient photocatalysts for inert small molecule activation and conversion. This dilemma is mainly due to the complexity of the composition and structure of the photocatalysts prepared by traditional methods. At the same time, effective and accurate structural characterization methods are urgently needed for a clearer or even quantitative structure-activity relationship in the future. Atomic-scale defects can notably alter the local vibrational responses of materials and thus their macroscopic properties. Up to now, the precise signal could be detected by using techniques such as high-resolution electron energy-loss spectroscopy in the electron microscope [110]. Extensive ab initio calculations revealed that the measured spectroscopic signature arose from defect-induced pseudo-localized phonon modes with energies that can be directly matched to the experiments. This single-atom level sensitivity could be very helpful for functional customization of two-dimensional materials by subtle regulation, and to make proper adjustments corresponding to the detection results.
(5) From the perspective of activity, a proper descriptor is highly urgent during the process of deducing quantitative structure-activity relationships. Although photocatalysts may have many useful properties related to the activity for activating inert small molecules, there are usually very few available parameters used for the construction of quantitative structure-activity relationships in an extremely diverse variable space. Many descriptor-involved tentative fitting operations are required to cover the entire variable space, and the required fitting technique emphasizes not only derivation fitting, but also the quality of the prediction of the fitting. Although these methods have not established a primary quantitative structure-activity relationship, they are helpful in examining the large amounts of data generated in research. With the development of specific methods, their applications in the field of photocatalysis will become much more widespread.
Ultimately, we hope that the experimental strategies and mechanistic understandings provided in this review can serve as a resource or starting point for researchers who want to develop the various regulations to gain better performance of inert small molecule activation and conversion. In the foreseeable future, the structure-activity relationships and mechanistic understandings will be constantly refined and developed as the functional customization experimental capabilities and computational insights are advanced.
Funding
This work was financially supported by the Strategic Priority Research Program of Chinese Academy of Sciences (XDB36030300), the National Natural Science Foundation of China (21890750, U2032212), the Youth Innovation Promotion Association CAS (202092), and the Fundamental Research Funds for the Central Universities (WK2340000094).
Conflict of interest
The authors declare no conflict of interest.
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All Figures
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Figure 1 Schematic illustration of functional customization of two-dimensional materials for activation and conversion of inert small molecules in the air. |
In the text |
![]() |
Figure 2 (A) Atomistic model, (B) computed band structure, and (C) density of states of the WO3 slab with three kinds of surface VO•• (the ratio of VO•• and O atoms is 7.3%). (D) Atomistic model, (E) computed band structure, and (F) density of states for the WO3 slab without surface VO••. (G) The calculated density of states, (H) distribution of charge density, and (I) charge density contour plots at the CB edge for BiOBr atomic layers. (J) The calculated density of states, (K) distribution of charge density, and (L) charge density contour plots at the CB edge for VO••-rich BiOBr atomic layers. (A–F) Adapted with permission from Ref. [53], copyright 2018 Elsevier; (G–L) adapted with permission from Ref. [55], copyright 2018 Wiley-VCH. |
In the text |
![]() |
Figure 3 (A) EXAFS curves of Cu foil and Cu-CN. (B) The data of EXAFS curves and the fit of Cu-CN. (C) The raised configuration of Cu-CN in a T-defect. ESR of p-CN and Cu-CN (D) before and (E) during illumination. (F) Top view of the distribution of electron density. (G) Side view of the distribution of electron density. The yellow and green isosurfaces represent an augment in the number of electrons and the depletion zone, respectively. The isosurfaces are 0.003 e Å−3. (H) PDOS of C and N atoms of T-defect in Cu-CN. (I) PDOS of Cu atoms, donor N atoms, and C atoms bound with the donor N atoms. Adapted with permission from Ref. [59], copyright 2018 Springer Nature. |
In the text |
![]() |
Figure 4 Free energy diagrams of photocatalytic CO2 reduction to CH4 for (A) the VS-CuIn5S8 single-unit-cell layers and (B) the pristine CuIn5S8 single-unit-cell layers. The blue line represents the more facile way while the black line shows the less favorable way. Steps in the red-dashed box are determining factors of the reaction selectivity. Key steps of photocatalytic CO2 reduction to CH4 for (C) the VS-CuIn5S8 single-unit-cell layers and (D) the pristine CuIn5S8 single-unit-cell layers, in which the charge-enriched Cu-In dual sites convert the endoergic protonation to an exoergic process, and hence change the reaction routes to product CH4 instead of CO. Adapted with permission from Ref. [71], copyright 2019 Springer Nature. |
In the text |
![]() |
Figure 5 (A) CO2 photoreduction into fuels such as CH4 and CO through the use of dual-metal-site catalytic systems (“H+ + e−” refers to the proton-coupled electron transfer process and “−H2O” means the desorption of H2O molecules after the intermediates react with the proton-electron pair). (B) 3D colormap surface with the projection of in situ Fourier transformed infrared spectroscopy (FTIR) spectra for coadsorption of a mixture of CO2 and H2O vapor on the mildly oxidized SnS2 atomic layers. (C) CO TPD spectra for the SnS2 atomic layers, the poorly oxidized and the mildly oxidized SnS2 atomic layers. (A) Adapted with permission from Ref. [71], copyright 2019 Springer Nature; (B and C) adapted with permission from Ref. [73], copyright 2017 American Chemical Society. |
In the text |
![]() |
Figure 6 (A) N–N distance of free N2, N2 on CuCr-VO••, N2 on CuCr-VO••-Strain, N2H2, and N2H4. (B) 2D structural model of TiO2 nanosheets with VO•• and engineered strain. (C) The O2 chemisorbed at a coordinatively unsaturated W site of defective WO3, and (D) the O2 adsorbed at the surface of perfect WO3. The purple and olive colors represent increase and decrease in electron density, respectively, revealed by first-principle calculations. (A) Adapted with permission from Ref. [74], copyright 2017 Wiley-VCH; (B) adapted with permission from Ref. [76], copyright 2019 Wiley-VCH; (C and D) adapted with permission from Ref. [77], copyright 2016 American Chemical Society. |
In the text |
![]() |
Figure 7 Distribution of partial charge density close to the edge of conduction band for (A) VZn″-rich one-unit-cell ZnIn2S4 layers slab and (B) VZn″-poor one-unit-cell ZnIn2S4 layers slab in the [001] orientation. The unit cell structure of (C) VZn″-rich one-unit-cell ZnIn2S4 layers slab and (D) VZn″-poor one-unit-cell ZnIn2S4 layers slab in the [001] orientation. (E) Scheme for the photocatalytic CO2 reduction into CO on the VZn″-rich one-unit-cell ZnIn2S4 layers. Ultrafast TA spectroscopy of (F) VZn″-rich one-unit-cell ZIS layers and (G) VZn″-poor one-unit-cell ZIS layers. (H) Surface photovoltage spectra and (inset) corresponding phase spectra of BiVO4 layers and (I) the field-induced surface photovoltage spectra under different external electric fields. (A–G) Adapted with permission from Ref. [84], copyright 2017 American Chemical Society; (H–I) adapted with permission from Ref. [86], copyright 2017 American Chemical Society. |
In the text |
![]() |
Figure 8 (A) Optical excitation and relaxation processes involved in the confined layered structure, where PF and PH represent prompt fluorescence and phosphorescence, respectively. (B) Scheme of the photophysical processes involving exciton- and VO••-mediated trap states. (C) SPV spectra of VS••-rich and VS••-poor In2S3. (A) Adapted with permission from Ref. [89], copyright 2017 American Chemical Society; (B) adapted with permission from Ref. [90], copyright 2018 American Chemical Society; (C) adapted with permission from Ref. [93], copyright 2019 American Chemical Society. |
In the text |
![]() |
Figure 9 (A) The multistep process in pothole-free WO3. (B) Calculated Gibbs free energy diagram of reaction transformation process over pothole-free WO3. (C) DFT calculation results indicated the strong chemisorption behavior of N2 and the obvious charge transfer behavior on the pothole-rich surface with dangling bonds. (D) The specific Gibbs free energy diagram of the stepwise of activation and cleavage of N≡N bonds of N2. Adapted with permission from Ref. [94], copyright 2019 Wiley-VCH. |
In the text |
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Figure 10 (A) Scheme and (B) free energy diagrams of the distal and the alternating mechanisms for the nitrogen reduction process on the Pt-SACs/CTF catalyst. (C) Optimized structures of reaction intermediates in the distal and the alternating mechanisms. (D) Free energy diagrams of photocatalytic CO2 reduction to CO by the Bi3O4Br and Co-Bi3O4Br. (E) Free energy diagram for the photocatalytic CO2 reduction to CO on LZS (001). The local configurations of the adsorbates on the LZS are shown as inserts at the initial state, the transition state, and the final state along the minimum-energy pathway (C, O, and H atoms of the adsorbates are represented by gray, light-green, and light-blue spheres, respectively). (A–C) Adapted with permission from Ref. [95], copyright 2020 American Chemical Society; (D) adapted with permission from Ref. [96], licensed under a Creative Commons Attribution 4.0 International License; (E) adapted with permission from Ref. [97], copyright 2019 Wiley-VCH. |
In the text |
![]() |
Figure 11 Top and side views of the optimized (A) (110) and (B) (100) facet of InVO4. In, V, O, C, and H atoms are represented by light green, violet, red, brown, and white spheres, respectively. The black dashed circle represents active sites. (C) Computed Gibbs free energy diagram of CO2 reduction reaction into CO catalyzed by (110) for black line and (100) for orange line, respectively. Adapted with permission from Ref. [99], copyright 2019 American Chemical Society. |
In the text |
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