领域：学术论文 · 自然科学

Synthesis and characterization of BTO photocatalysts
As a layered structural two-dimensional material, BTO is characterized by regular stacking of bismuth oxygen [Bi2O2]2+ slabs and perovskite-like [Bi2Ti3O10]2– blocks, in which Bi–O bond can be etched via the interaction of proton and halide ion. Briefly, pristine single-crystal BTO was firstly prepared by a salt-assisted solid-state reaction. Due to the different structural stability of exposed crystal facets, the [Bi2O2]2+ structure of {001} crystal facet and Bi–O bond of {010} crystal facet of BTO can be selectively preferentially etched (Supplementary Fig. 1 and Supplementary Note 1). Also, the Ti–O bond of BTO {001} will prevent further etching of {001} crystal facet after etching [Bi2O2]2+, ultimately leading to the formation of a hollow structure along the edge region. Scanning electron microscopy (SEM) images show the pristine BTO with an evident nanosheet morphology. When HCl etching was conducted, the etched edge gradually became highly visible with the extension of the etching time to 2 h (Fig. 1a and Supplementary Fig. 2). However, the particle size or crystallite size of samples had no significant change, but the obvious difference of specific surface area occurs before and after acid etching (Supplementary Fig. 3 and Supplementary Table 1). When the etching time extended to 4 h, the BTO-4 shows that the nanosheets are broken with impurities due to nucleation of TiO2 (Supplementary Fig. 4). X-ray diffraction (XRD) pattern indicates the same crystal structure between the pristine and the etched BTO within 2 h (Fig. 1b). However, the crystallinity of the etched BTO is gradually weakened with further extension of etching time (Fig. 1c). Unsurprisingly, the over-etched BTO-4 shows a mixed structure of BTO and TiO2 (Supplementary Figs. 5 to 6 and Supplementary Note 2). Similarly, under the same concentration as HCl, the HBr-treated BTO sample shows similar crystal structure and morphology (Supplementary Figs. 7 and 8). The control experiments were performed with HNO3 and H2SO4 treated pristine BTO under the same concentration as HCl. The XRD results indicate the same crystal structure as BTO (Supplementary Fig. 9). And the edge region of BTO has not been etched (Supplementary Fig. 10). Further, an atomic force microscope (AFM) was used to investigate the thickness change before and after etching for different periods. No significant change appears in the overall thickness before and after etching and between the center and edge of the etched BTO (Supplementary Fig. 11). Furthermore, a highly hollow edge structure was observed by the SEM image, and the corresponding elemental mapping reveals the uniform distributions of Bi and Ti elements (Supplementary Fig. 12). Other SEM images, the corresponding elemental mapping, and linear scan elemental analysis are provided in Supplementary Figs. 13–15, and the results proved the uniform distributions of Bi, Ti, and O elements in both the center and the edge region of BTO with and without HCl treatment.
Further characterization of the local structure of the etched BTO was carried out under high-resolution transmission electron microscopy (HRTEM). An evident thickness variation contrast occurred between the central and the edge regions of transmission electron microscopy (TEM) image because of the etching effect (Fig. 2a). The HRTEM image exhibits the same lattice orientation and lattice fringes between the edge and the central regions of the etched BTO (Figs. 2b,c), indicating the single crystalline nature of the structure. Furthermore, the same diffraction patterns further confirm the single-crystal structure by selected area electron diffraction (SAED) at different locations (Figs. 2d and 2e). Accordingly, the chemical valence states of elements were also characterized by X-ray photoelectron spectroscopy (XPS). Due to the gradually etched depth of the structure, the chemical states of the surface elements change in a dynamically evolving manner, especially for Ti and Bi. Evidently, as shown in Fig. 2f, the binding energy of Ti 2p gradually shifts towards a higher value, which comes from the increasing proportion of the Ti–O structures exposed on the surface because of the Ti element binding more high-energy O (hydroxyls and water molecules). Due to the change of lattice oxygen (Bi–O and Ti–O) structure on the exposed surface, the binding energy of O 1 s shifts to higher value at the same time (Supplementary Fig. 16a). In contrast, the corresponding Bi 4 f peak slightly shifts downward in binding energy with increasing extent of the acid etching, because the outside high binding energy Bi of [Bi2O2]2+ layer and [Bi2Ti3O10]2– was etched away. Although the binding energies of Bi and Ti evolve, the main structure of samples keeps unchanged during the etching process (Fig. 2g). No Cl element was observed during the etching process, indicating that the nucleation of bismuth oxyhalide was inhibited (Supplementary Fig. 16b). In addition, the structural change should be accompanied by the evolution of the band structure. The band gap slightly increases with the extension of the etching, which is due to the change of the band structure by the spatial size effect, which is mainly from the ultrathin hollow edge (Supplementary Figs. 17–19). With the increase of edge etching depth, the trend of band structure change is more obvious. Meanwhile, the difference in the band structures between the unetched part and adjacent etched BTO will form “homojunction”, which are beneficial to the transfer of electrons to the surface of the photocatalyst. Also, due to TiO2 impurities resulting from structural destruction caused by the excessive etching, the BTO-4 further shows the bigger band gap and evident differences from BTO in light absorption behavior. However, such a small change of bandgaps is not expected to influence the photocatalytic performance.
Further understanding the charge distribution on the crystal surface is helpful to explore the process of photocatalytic water splitting. SPVM is an effective tool for spatial correlation between photogenerated charge distribution and structural morphology, as well as the dependence between light absorption and carrier migration and transfer46,47. As shown in Figs. 3a–f, photogenerated electrons were imaged in the surface due to negative SPV. With the increasing wavelength of illumination light, the intensity of the SPV decreases, which is consistent with the behavior of the light absorption spectrum, which proves that the charge generation is driven by light (Supplementary Fig. 17 and Fig. 3g). The difference is that when the light irradiated the surface of photocatalyst, the charge distribution intensity varies between the edge and the middle of the sample. This phenomenon is very interesting and differs from most of the results reported in the literatures. We speculate the carrier distribution is dependent on the space geometry. Differential SPV analysis was performed at different positions of a given facet and the differences were compared at the three positions. The corresponding results are shown in Figs. 3a–f. The SPV is significantly different with the change of light wavelength at the three locations of α1, α2 and α3. Therefore, the intensity of SPV varies from the edge to the middle position, which induces a directional movement of surface-generated electrons, leading to effective photogenerated carrier separation compared with pristine BTO (Supplementary Fig. 20). Combined with the previous results, we find that the electron transfer is related to the spatial morphology and structure. The hollow regions with etched edges possess abundant photogenerated electrons, which is mainly because the regions with thin edges can accelerate electron migration to the surface from the bulk and center. In order to further determine the transfer and the regional distribution of electrons, the deposition of Au on BTO was performed with and without acid treatment. The random distribution of Au nanoparticle can be observed on the surfaces of pristine BTO in Fig. 3h. Not surprisingly, it is found to be preferentially deposited on the edge area of surface crystal facets of BTO-2 in Fig. 3i, which shows a special behavior, that is, spatially differentiated structure induces the differentiated distribution of electrons. A fast electron transfer dynamic will be promoted due to the electron differentiated distribution, which is beneficial to photogenerated charge separation for efficient OWS (Fig. 3j).
The process of photocatalysis is largely limited by the separation and transfer of photogenerated charge carriers. The lifetime and transfer dynamic were characterized by photoluminescence (PL) and time-resolved photoluminescence (TRPL). Both pristine BTO and etched BTO show an evident PL peak at 485 nm (Supplementary Fig. 21). The PL intensity of the etched BTO gradually decreases with etching time, indicating a decrease in the number of excitons due to carries’ effective separation. We carried out TRPL to monitor the charge carrier dynamics on the nanosecond time-scale. As shown in the Fig. 4a, the carrier lifetime curve conforms to a multi-exponential decay function, which can be fitted by different parameters. The average lifetime is 3.64 ns for pristine BTO, which is longer than that of the etched BTO. And the average carrier lifetime gradually reduces with the duration of HCl treatment (3.28 ns for BTO-0.25, 2.71 ns for BTO-0.5, 2.34 ns for BTO-1, and 1.69 ns for BTO-2), representing that charge carriers’ transfer and lifetime is dependent on spatial morphology. Specifically, the faster decay lifetime of τ1 of etched BTO is longer than that of pristine BTO and the contribution of fast decay of τ1 increases largely from 34% for BTO to 85% for BTO-2, confirming the suppression of the inter-band recombination and fast electron transfer of the etched BTO due to the homojunction between the edge and the center. Also, combined with the morphological analysis, the decreased lifetime is gentle at the initial stages of etching, which is because the etching time is relatively short and only the surface Bi–O layer is etched (Fig. 4b). With increasing duration of the etching, the carrier’s lifetime of the sample rapidly declines, which is positively correlated to the etching depth due to the formation of the homojunction and the ultrathin edge, where the electrons of the etched edge will be accelerated and transferred to the surface compared with the center part of the BTO (Fig. 4c). Moreover, we further demonstrated the rapid interfacial electron transport performance. The etched BTO shows a larger photocurrent than that of the pristine BTO. And it is found that a positively correlation of the photocurrent with the etching depth for etched BTO (Supplementary Fig. 22a). The electrochemical impedance spectroscopy (EIS) measurements and analyses also reveal fast electron transport of the etched BTO. The radius of the fitted curve reflects the strength of the transfer of semiconductor electrons, that is, the smaller the radius indicates the faster the electron transfer. As shown in the impedance curves (Supplementary Fig. 22b), the BTO-2 shows the smallest electrochemical impedance, implying the fastest electron transport among the photocatalysts.



翻译考点：
考点1：“Homojunction” 必须译为 ”同质结”
考点2：“Salt-assisted solid-state reaction” 推荐译为 ”熔盐辅助固相反应”
考点3：“Over-etched” 必须译为 ”过度刻蚀”
考点4：“Carrier migration and transfer” 必须译为”载流子迁移与传输”
考点5：“Electron differentiated distribution” 必须译为 ”电子差异化分布”
考点6：“Particle size” 推荐译为 ”粒径/颗粒尺寸”
考点7：“Control experiments” 必须译为 ”对照实验”
考点8：“Dynamically evolving manner” 必须译为 ”动态演化方式”