Atomically Surficial Modulation in Two-Dimensional Semiconductor Nanocrystals for Selective Photocatalytic Reactions

Photocatalysis, directly converting solar energy into chemical energy, is identified as an ideal strategy to reduce the increasing consumption of fossil fuels and facilitate carbon neutralization. In the past few years, a great number of endeavors have been devoted to developing photocatalysts with a high conversion efficiency and selectivity. Atomically surficial modulation strategies, including surface vacancies, single-atom modification, and dual-site components, exhibited positive impacts on tuning key steps of photocatalytic reactions. In this mini-review, we focus on the latest progress of the atomically surficial modulations on two-dimensional semiconductor photocatalysts and their role in enhancing selectively photocatalytic performance. We hope that this mini-review could provide new insights for researchers on nanosynthesis and photocatalysis.


INTRODUCTION
Photocatalysis is regarded as an efficient strategy to directly convert solar energy into chemical energy, which provides a solution to release the increasingly serious shortage of natural resources (Bai et al., 2017;Gao et al., 2020). Photocatalytic reactions are mainly driven by photo-induced charge carriers, and the reaction process could be divided into charge generation, separation, migration, and surficial reactions (Li et al., 2016;Gao et al., 2017;Yang et al., 2021). The utilization efficiency of carriers in each step determines the overall performance of photocatalysis (Li A et al., 2019). The two-dimensional (2D) nanocrystals with thin thicknesses are attractive candidates for photocatalysts by virtue of the short carrier transport distance and thus allow for improved charge separation (Tan et al., 2017;Wang H. et al., 2020). Moreover, the large specific surface area of 2D nanocrystals could expose more active sites, which could provide a large parameter space for the establishment of surficial modulations. The organization of surficial vacancies, single-atom modification, and dual-site components (as illustrated in Figure 1) are effective strategies for surficial modulation for enhanced photocatalytic performance, such as enlarging the light absorption region, facilitating electron-hole separation and transportation, and improving gas adsorption and activation properties. In this mini-review, the latest progress of the surficial modulations on 2D semiconductor photocatalysts with emphasis on their enhanced photocatalytic performance will be introduced.

SURFICIAL VACANCIES
Normally, high-crystalline nanocrystals with a suitable band gap could feature a high build-in electronic field for a remarkable density of photo-induced charge carriers. By further modulating their surficial active sites, the light absorption and photo-induced charge transportation and separation processes could be further improved for enhanced photocatalytic performance (Sun et al., 2020). Two-dimensional semiconductor photocatalysts with atomically surficial modulation for selective photocatalytic reactions are introduced in Table 1. The surface modulation methods, active sites, photocatalyst conditions, and selectivity of the 2D photocatalysts described in the mini-review are highlighted. Among other surficial modulation strategies, the surficial vacancies ( Figure 1A) could significantly change the local chemical coordination and surficial valence of the catalyst, which could not only mediate the surficial band gap but also enhance the activation of reactive molecules and charge transfer. Vacancies and adjustment of the surface band gap had a synergistic effect to reduce the recombination rate of photo-   (Jo et al., 2019;Kong et al., 2020;Aggarwal et al., 2021). For example, the Chai group synthesized atomic-layered g-C 3 N 4 with nitrogen vacancies (V N -g-C 3 N 4 ), which exhibited high performance in converting CO 2 to CH 4 with high selectivity (Tang et al., 2019). The π-conjugated aromatic ring structure of g-C 3 N 4 was distorted after the introduction of V N . At the same time, the introduced vacancy energy level changed the band gap. With increasing V N density, the catalyst exhibited increased visible light absorption and enhanced reduction ability. In addition, the introduced V N could suppress the recombination of electron-hole pairs because the vacancy level could act as a reservoir for trapping electrons. Therefore, the CH 4 yield was 3.16 times and 5.14 times higher than its pristine ultrathin material and bulk material, respectively. The Hu group reported the preparation of 2D-TiO 2 nanosheets having adjustable oxygen vacancies (V O ) for selectively photocatalytic amine oxidation (Chen et al., 2020). Another representative study by the Chen and Han group reported that ZrS 3 nanoribbons (NBs) containing disulfide (S 2 2− ) vacancies and sulfide anion (S 2− ) vacancies as a photocatalyst exhibited good photocatalytic performance for the formation of H 2 O 2 and simultaneously selective conversion of benzylamine to benzonitrile, and the selectivity exceeded 99% (Tian et al., 2021). Similarly, they demonstrated that S 2 2− vacancies in ZrS 3 NBs accelerated the separation of photo-induced carriers, hole extraction, and kinetics of benzylamine oxidation. In another impressive study by the Xie group, cubic-phase In 2 S 3 nanosheets were applied as a template, and S vacancies were introduced on their surface, which could enlarge the density of charge carriers involved in this reaction (Sun et al., 2019). It was indicated that the surficial S vacancies could promote the carrier separation and transfer and perform highly selective photocatalytic oxidation reactions by enhanced production of O 2 •− as well. Theoretical simulations exhibited that the density of states of Valence Band Maximum (VBM) was significantly enhanced, which would inhibit electron-hole recombination. Compared to the aforementioned anion vacancies, cation vacancies were relatively difficult to control, which could closely affect the conductivity and further affect the charge carrier mobility of the 2D nanocrystals in determining their separation of charge carriers (Hui et al., 2017). In addition, compared with the ideal vacancies, the metal vacancies on the engineering surface had an obvious electronic structure change due to the diversity of the electronic configuration and orbital of metal cations. The Sun and Xie group synthesized the one-unitcell ZnIn 2 S 4 layers with rich Zn vacancies (V Zn ) to realize efficient and selective solar CO 2 reduction performances (Jiao et al., 2017). The results of PL, SPV, and TA showed that the introduction of Zn vacancies into the ZnIn 2 S 4 layer could significantly accelerate the charge transport, improve the electron-hole separation efficiency, and further increase the CO 2 photoreduction efficiency. Additionally, DFT results of the density of states (DOS) indicated that the presence of zinc vacancy led to notable enhancement in the charge density of nearby sulfur atoms, which displayed that the electrons could be easily photoexcited to the conduction band and thus clustered around the zinc vacancies nearby the sulfur atoms and promoted enhanced carrier separation and transport. The Liu and Xia group synthesized BiOBr ultrathin nanosheets with Bi vacancies (V Bi -BiOBr) to optimize CO 2 photoreduction performance (Di et al., 2019b). DFT calculations indicated that an increased charge density around the Fermi level due to the engineered Bi vacancies and V Bi -BiOBr could be endowed with increased charge carriers to participate in light-driven CO 2 conversion with increased DOS. Therefore, the selectively photocatalytic CO yield of V Bi -BiOBr nanosheets in pure water was 20.1 μmol g −1 h −1 , which was much higher than that of BiOBr nanosheets (5.3 μmol g −1 h −1 ).

SINGLE-ATOM MODIFICATION
Compared to surficial vacancies, single-atom modification ( Figure 1B) enabled by introducing guest atoms was another efficient strategy for surficial modulation of 2D semiconductor nanocrystals. The organized single-atom-modified photocatalysts could exhibit the advantages of single-atom catalysts, such as unsaturated coordination, a unique electronic structure, maximum atom utilization, and clear catalytic sites (Li et al., 2019a). Similar to surficial vacancies, the deposited guest atoms on semiconductor surfaces could act as active sites that could trap photo-induced charge carriers, thereby inhibiting charge carrier recombination and accelerating photocatalytic reaction . Hence, the single-atom modifications could be applied to regulate the dynamics of photogenerated charge transfer by shortening the charge migration distances and endowing strong interactions between metals and carriers . For example, the Zeng and Li group designed the single-atom Au-modified black phosphorus (Au 1 / BP) nanosheet catalyst for the selective oxidation process of methane to methanol . Under light irradiation, 1 O 2 generated by Au 1 /BP could react with water, forming adsorbed P-OH and P-OOH. P-OOH could be easily decomposed to generate •OH, while methane could react with P-OH to be dehydrated to form CH 3 • and be adsorbed on Au single atoms. Subsequently, •OH reacted with CH 3 • to generate CH 3 OH regardless of steric hindrance. In addition, Au single atoms stabilized CH 3 • against deeper dehydrogenation. Hence, Au 1 /BP can catalyze the partial oxidation of methane in an aqueous solution with selectivity >99%. The Liu, Song, and Xia group successfully incorporated Co single atoms into the Bi 3 O 4 Br atomic layer to construct a Co-Bi 3 O 4 Br catalyst (Di et al., 2019a). The introduced Co single atoms were beneficial to charge transfer, carrier separation, adsorption, and activation of CO 2 . Moreover, they could stabilize the COOH • intermediate and modulate the rate-limiting step from COOH • formation to CO • desorption, which could lower the CO 2 activation energy barrier. Utilizing single-atom Co and 2D ultrathin Bi 3 O 4 Br atomic layers, the photocatalyst could achieve a highly efficient photocatalytic CO 2 conversion with a selective CO generation rate of 107.1 μmol g −1 h −1 , which was approximately 4 and 32 times higher than that of atomic layer Bi 3 O 4 Br and bulk Bi 3 O 4 Br, respectively.
Meanwhile, the tunable surficial structure of the support materials could provide different coordination environments for guest atoms so that the geometric and electronic structures of these active sites would be tuned through the metal-support interaction, thereby changing the adsorption and strength of reactants, intermediates, or products (Gawande et al., 2020;Mitchell and Pérez-Ramírez, 2020). The Xiong group designed the partially oxidized graphene nanosheets (Co 1 -G) with singleatom Co sites and realized the highly selective photocatalytic conversion of CO 2 to CO (Gao et al., 2018). The strong interaction between single-atom Co sites and supports could change the charge state of Co single atoms, thereby affecting the ability to activate CO 2 molecules. Under the assistance of [Ru(bpy) 3 ]Cl 2 , the turnover number (TON) for CO production reached a high value of 678 and the turnover frequency reached an unprecedented 3.77 min −1 . TON was almost identical when the Co loading varied from 0.3 to 1.2 wt%, which indicated that each individual Co atom had been fully utilized as a photocatalytic site for CO 2 reduction. The Fu, Jiang, and Zhang group reported the coordination of Cu atoms with N in C 3 N 4 (Cu-N x ) by thermal polymerization (Xiao et al., 2020). X-ray absorption spectroscopy combined with theoretical simulation displayed that each Cu atom could coordinate with three N atoms in one C 3 N 4 layer or four N atoms in two adjacent C 3 N 4 layers, forming two different types of Cu-N x as efficient charge transport channels to promote rapid charge transfer. The catalyst exhibited an excellent visible light catalytic hydrogen evolution performance (212 mol h −1 /0.02 g catalyst), which was 30 times higher than that of bulk C 3 N 4 . Additionally, the yield of benzene oxidation under visible light catalysis was 92.3%, and the selectivity reached 99.9%.

DUAL-SITE COMPONENTS
Compared to the aforementioned single-site modulation by vacancies and single atomic modification, surficial active sites coupling with other coordinated sites ( Figure 1C), namely, dualsite component catalysts, were also reported by many groups, which generally exhibited higher selectivity because of the mediated work function, Fermi energy, and synergistic effects between the two components (Sankar et al., 2012). In this section, two main dual-site components were introduced, including metal-metal sites and metal-vacancy sites.
As for metal-metal sites, the engineered interatomic distances and reduced orbital overlap could induce the modulation in the band structure, affecting the density of states of the d-band and its position relative to the Fermi level (Rosseler et al., 2015). Therefore, the adsorption properties and reactivity of dualmetal component catalysts were different from those of singleatom modifications. Additionally, the dual-metal structure facilitates electron transfer between the two metal moieties, thereby further improving electron-hole separation (Ding et al., 2018). A representative study by the Sun and Xie group reported the synthesis of dual-metal site CuIn 5 S 8 ultrathin nanosheets, which changed the intermediate configuration of the key reaction and adjusted the reaction barrier, thereby changing the reaction path and realizing the highly selective conversion of CO 2 to CH 4 (Li et al., 2019b). Both theoretical simulations and in situ infrared spectroscopy confirmed that the low-coordinated Cu and In sites could interact with CO 2 molecules to form the extremely stable Cu-CO-In intermediate. Breaking the Cu-C bond and In-O bonds simultaneously needed to overcome a high reaction energy barrier to form free-state CO molecules. However, the reaction of hydrogenation on the C atom of this intermediate to form the CHO intermediate was exothermic and could proceed spontaneously. Photocatalytic experiments confirmed that the sulfur-defective CuIn 5 S 8 ultrathin nanosheets could reduce CO 2 to CH 4 with close to 100% selectivity and an average yield of 8.7 μmol g −1 h −1 driven by visible light. Dual-metal alloymodified photocatalysts could also exhibit improved catalytic performance (Tahir et al., 2017;Majeed et al., 2018), such as Pd-Au-modified g-C 3 N 4 reported by the Feng group (Wang Z. et al., 2020). By regulating the proportion of Pd and Ag, the yields of CO and CH 4 were adjusted, and selective conversion characteristics and high productivity could be achieved. The Santo group synthesized the dual-metal Au-Pt/TiO 2 photocatalyst. Using ethanol as a sacrificial reagent, the obtained materials exhibited good hydrogen yields under both ultraviolet A and more sustainable simulated sunlight (Gallo et al., 2012). The interaction between Pt and Au led to a reduction in the strength of the metal-hydrogen bond, which means easier desorption of H 2 from the metal surface and an enhanced electron capture capability of the materials.
Different from dual-metal components, single-atom modifications by introducing guest atoms could simultaneously generate surficial vacancies, forming metaldefect sites, which could further improve the performance and selectivity of photocatalysis due to the coupling effects. A recent study by the Zhang group reported that an engineered sulfide photocatalyst with surficial single atoms and vacancies exhibited a high-efficiency overall water splitting performance, which was obtained through a simple aqueous cation-exchange reaction between a 2D ZnIn 2 S 4 monolayer and Ag + (Pan et al., 2021). The vacancies in the Ag-ZnIn 2 S 4 photocatalyst could act as the active centers, and the surficial isolated Ag atoms and their surrounding coordination environment could provide higher adsorption energy of H 2 O and OH − , promote the water oxidation process, and help to inhibit the light corrosion of the sulfide photocatalyst through holes. The dual-site Frontiers in Chemistry | www.frontiersin.org April 2022 | Volume 10 | Article 890287 synergistic effect served as the active site for overall water oxidation and reduction and realized extremely stable performance of photocatalytic overall water splitting without a co-catalyst. The Deskins group investigated the effects of surface oxygen vacancies and photoexcited electrons on the photocatalytic performance of TiO 2 -loaded Cu single atoms by DFT calculations and experiments . The singleatom Cu centers near the V O s had high selectivity for bent CO 2 formation, which was beneficial to CO 2 activation. In addition, the electron-rich nature of the single-atom Cu centers near the V O s increased the adsorption energy of CO 2 at these sites, thereby accelerating the dissociation of CO 2 . However, Cu single atoms supported on the vacancy-free TiO 2 surface did not exhibit the above properties. The presence of photoexcited electrons significantly increased the reaction rate on the surface of the TiO 2 photocatalyst with Cu single atoms. In the absence of photoexcited electrons, the Cu/TiO 2 surface was essentially inert. These results demonstrated that Cu single atoms, surface V O s, and photoexcited electrons work together to achieve highefficiency and high-selectivity CO 2 reduction.

CONCLUSION AND PERSPECTIVE
In this mini-review, three surficial modulation strategies, namely, surficial vacancies, single-atom modulation, and dual-site components, are introduced for 2D semiconductor nanocrystal photocatalysts. These surficial modulation strategies have a positive impact on tuning key steps of photocatalytic reactions, such as enlarging the light absorption region, promoting electron-hole separation and transport, and facilitating gas adsorption and activation processes. However, there are still many challenges for surficial modulation strategies for 2D semiconductor nanocrystal photocatalysts: 1) the leading reason for the enhancement of photocatalytic performance of 2D nanocrystals after surficial modulation needs more systematic and accurate test methods. Modulation of nanocrystals will cause changes in the overall properties of nanocrystals, but how to demonstrate the dominant reason needs more accurate research. 2) The explorations of the active sites and intermediate products require more detailed in situ characterizations such as in situ infrared, X-ray photoelectron spectroscopy, surface-enhanced Raman spectroscopy, etc., to further investigate the mechanism of photocatalysis. 3) A high density of surficial active sites could bring high catalytic activity, but the high loading of single atoms or defects could inevitably lead to aggregate or decomposition. Hence, how to maintain the stability of the photocatalyst is another important scientific issue. 4) Taking advantage of the photogenerated free radicals during photocatalytic reactions, realizing catalytic reactions which are difficult to be realized by conventional industrial catalysis could be an important direction for the future.

AUTHOR CONTRIBUTIONS
XL and JZ supervised the project. SZ wrote and edited the manuscript. XL revised the manuscript and provided financial support. All authors listed approved it for publication and had made a substantial and intellectual contribution to this work.