Synthesis and internal electric field dependent photoreactivity of Bi₃O₄Cl single-crystalline nanosheets with high {001} facet exposure percentages

Abstract

We prepared Bi₃O₄Cl single-crystalline nanosheets with high {001} facet exposure percentages and demonstrated that their photoreactivity strongly depended on the magnitude of the internal electric field (IEF), which was correlated with the {001} facet exposure percentage. More {001} facet exposure could induce the generation of stronger IEF, which favored the photogenerated charge separation and transfer, and thus enhancing the photoreactivity.

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Since the first report of anatase TiO₂ single-crystals with exposed highly reactive {001} facets in 2008,¹ crystal facet engineering of semiconductors has attracted worldwide attention, and has become a pivotal strategy to finely tailor the physicochemical properties and enhance the reactivity of photocatalysts.² Generally, the conventional understanding of the crystal facet effect on the photoreactivity mainly relies on facet-related surface properties: plenty of atomic steps, dangling bonds, kinks and ledges on reactive facets with high surface energy can act as active sites to facilitate the adsorption of reactant molecules, the surface transfer between photoexcited electrons and reactant molecules, and the desorption of product molecules.³ However, this theory became ambiguous because some recent studies demonstrated that the facet-dependent band-structure and/or the charge transfer orientation played significant roles in determining the photoreactivity, even more important than those played by the surface energy.⁴ On the basis of a systematic literature study, we think that the previous conflicting phenomena of crystal facet effects on the photoreactivity might have arisen due to the following three reasons. (1) The exposure degree of the targeted facets was still low and the effect of coexisting facets could not be ruled out; (2) most previous studies mainly focused on surface properties, while the corresponding bulk properties on the photoreactivity were usually neglected; (3) many conclusions were drawn by comparing the photoreactivity of the samples with different facet exposure percentages, but how an individual facet would affect the photoreactivity was still elusive. Therefore, it is of great importance but a challenge to clarify the origin of the facet effect of semiconductors on their photoreactivity.

In this communication, we first demonstrate that the photoreactivity of Bi₃O₄Cl single-crystalline nanosheets with high {001} facet exposure percentages strongly depends on the magnitude of the internal electric field (IEF), which is correlated with the percentage of {001} facets. More exposure of {001} facets can induce the generation of a stronger internal electric field, which favors the photogenerated charge separation and transfer, and thus enhances the photoreactivity.

Bi₃O₄Cl nanosheets were hydrothermally synthesized with bismuth nitrate pentahydrate and sodium chloride. The detailed synthesis procedure was provided in the ESI.† The as-synthesized product was of a pure monoclinic Bi₃O₄Cl crystal phase (space group I2/a, JCPDS no. 86-2221) with layered structure according to the X-ray diffraction pattern and X-ray photoelectron spectroscopy analysis (Fig. S1 and S2, ESI†). SEM images revealed that the as-synthesized product consisted of large numbers of sheets with 60 nm thickness and micrometer-sized width and length (Fig. S3, ESI†). The individual nanosheet was transparent under the electron-beam illumination during transmission electron microscopy (TEM) measurements (Fig. 1a), confirming that the as-synthesized nanosheet was very thin. High-resolution TEM (HRTEM) analysis revealed the high crystalline nature of the nanosheets with two sets of lattices perpendicular to each other with an equal fringe spacing of 4.01 Å, corresponding to (110) and (11̄0) planes of the monoclinic Bi₃O₄Cl crystal (Fig. 1b). The corresponding selected-area electron diffraction (SAED) pattern could be indexed as the diffraction spots of the [001] zone of a Bi₃O₄Cl single crystal (Fig. 1c). On the basis of the above results and the symmetry of Bi₃O₄Cl, we conclude that the as-synthesized Bi₃O₄Cl nanosheets are exposed with {001} facets on both the top and the bottom surfaces and four {110} facets on the lateral surfaces (Fig. 1d). A detailed statistical analysis revealed that the estimated percentage of {001} facets of the as-synthesized Bi₃O₄Cl nanosheet was as high as 91% (Fig. S3 and S4, ESI†), which we called BOC-91. Subsequently, we employed a liquid phase exfoliation and hydrothermal recrystallization method to finely tune the {001} facet exposure percentage of Bi₃O₄Cl nanosheets from 91 to 95 and 86% (Fig. 1e), respectively. The two new samples were correspondingly called BOC-95 and BOC-86.

Fig. 1 (a) TEM image, (b) HRTEM image, (c) SAED pattern of the as-prepared Bi₃O₄Cl nanosheets. (d) Schematic illustration of the crystal orientation of the nanosheet. (e) Schematic illustration of the liquid-exfoliation and hydrothermal-induced recrystallization process for tuning the {001} facet percentages of Bi₃O₄Cl nanosheets.

The success in controlled synthesis of Bi₃O₄Cl nanosheets with high and different {001} facet exposure percentages allowed us to investigate the origin of the facet effect on the photoreactivity by minimizing the influence of coexisting facets as low as possible. The photocatalytic performances of the three samples were evaluated by using salicylic acid (SA) as the probe molecule under visible light irradiation (λ > 420 nm). As expected, the photoreactivity of Bi₃O₄Cl nanosheets increased monotonously with the increased percentages of exposed {001} facets (Fig. 2a), indicating that the exposed {001} facets contribute positively to the photocatalytic degradation of SA over Bi₃O₄Cl nanosheets under visible light. In general, the surface properties of the semiconductor are crucial for their photoreactivity because a high density of surface unsaturated-coordinated atoms would cause a high surface energy of the crystal facet and a crystal with a higher percentage of the reactive facet is usually more reactive in heterogeneous reactions.^2a,3c,4a Therefore, we normalized photocatalytic degradation rate constants of BOC-86, BOC-91 and BOC-95 simultaneously with their {001} facet percentages and surface areas to check if the surface property of Bi₃O₄Cl nanosheets is the major factor to determine their photoreactivity, and surprisingly found that the normalized rate constants still increased monotonously with increasing {001} facet percentages (Table S1 and Fig. S10, ESI†). This result suggests that the photoreactivity of Bi₃O₄Cl nanosheets with high {001} facet exposure percentages is not mainly dominated by the surface properties of {001} facets in this study, but possibly by some other {001} facet-related bulk properties.

Fig. 2 (a) The variation of apparent reaction rate constants of Bi₃O₄Cl nanosheets on photocatalytic degradation of salicylic acid under visible-light irradiation along with changing percentages of {001} facets. (b) Photoluminescence spectra, (c) electrochemical impedance spectroscopy and (d) transient photocurrent responses of Bi₃O₄Cl nanosheets with different percentages of exposed {001} facets under visible light (λ > 420 nm).

It is well known that the photocatalytic process with semiconductor materials involves light photoexcitation, the subsequent separation and transfer of photogenerated charge carriers, and the final redox reactions between the charge carriers and adsorbate on the semiconductor surface, and thus the charge dynamics property plays an important role in determining the ultimate photoreactivity.⁵ We therefore investigated the separation and transfer of photogenerated charge carriers in the Bi₃O₄Cl nanosheets by photoluminescence (PL) emission and electrochemical impedance spectroscopy (EIS) as well as transient photocurrent responses.⁶ The PL intensity and the Nyquist plot diameter were found to be inversely proportional to the percentages of exposed {001} facets (Fig. 2b and c), indicating the separation and transfer efficiency of photogenerated electron–hole pairs highly depends on the percentages of exposed {001} facets and more {001} facet exposure favors the separation and transfer efficiency of photogenerated electron–hole pairs, which was further confirmed by transient photocurrent responses (Fig. 2d). It is highly possible that the charge dynamic property is the primary factor to govern the {001} facet-dependent photoreactivity of Bi₃O₄Cl nanosheets in this study.

As mentioned previously, high {001} facet exposure percentages of Bi₃O₄Cl nanosheets might minimize the influence of coexisting facets as little as possible. If the contribution of coexisting facets on the separation and transfer efficiency of photogenerated electron–hole pairs was negligible, how could the exposed {001} facets of Bi₃O₄Cl nanosheets affect the separation and transfer efficiency of photogenerated electron–hole pairs and thus influence the photoreactivity without considerably changing the exposure percentages (86% → 91% → 95%)?

To answer this question, we need to recall our recent discovery: BiOCl single-crystalline nanosheets with exposed {001} facets were more capable of efficient photoinduced charge separation and transfer, and thus exhibited higher activity for direct semiconductor photoexcitation pollutant degradation under UV light because of the cooperative effect between the surface atomic structure and suitable internal electric fields.⁷ Similar to BiOCl, Bi₃O₄Cl also possesses a layered crystal structure with the [Bi₃O₄] and [Cl] slices stacked together by the nonbonding interaction through the Cl atoms along the c-axis to form the unique layered structure (Fig. 3a), indicative of the existence of the self-induced electric field in Bi₃O₄Cl.⁸ As aforementioned, the photoreactivity of Bi₃O₄Cl with high {001} facet exposure percentages is not dominated by the surface atomic structure of {001} facets, we therefore employed density functional theory (DFT) to calculate charge density in the {110} and {11̄0} lateral surfaces of Bi₃O₄Cl exposed with {001} facets on the top and the bottom surfaces to check how {001} facet exposure could influence the IEF magnitude of Bi₃O₄Cl. From the charge density contour plots viewed from either {110} or {11̄0} facets of Bi₃O₄Cl (Fig. 3b), it can be seen clearly that the charge density surrounding Bi atoms is higher than that of Cl atoms. The non-uniform charge distribution between [Bi₃O₄] and [Cl] slices in the nanosheets would polarize the related atoms and orbitals to form permanent static electric fields along the [001] orientation perpendicular to the [Cl] and [Bi₃O₄] layers, indicating that the emergence of IEF is strongly related to the preferential {001} facet exposure of Bi₃O₄Cl. It was well established that the IEF in materials could afford fast separation, reduced diffusion resistance, and improved accessibility of the photogenerated charge carriers to reactants, which consequently promote the photocatalytic activity.⁹ Hence, we propose that the IEF magnitude of Bi₃O₄Cl nanosheets might be the right important factor to affect {001} facet-dependent charge dynamic properties and thus influence their photoreactivity. Then, we calculated the variation of the IEF magnitude with the {001} facet exposure percentages of Bi₃O₄Cl nanosheets by using the model (eqn (1)) developed by Kanata et al.¹⁰

F_s = (−2V_sρ/εε₀)^1/2 (1)

where F_s is the IEF magnitude, V_s is the surface voltage, ρ is the surface charge density, ε is the low-frequency dielectric constant, and ε₀ is the permittivity of free space. Eqn (1) reveals that the IEF magnitude is mainly determined by the surface voltage and the charge density because ε and ε₀ are two constants here. In order to measure the IEF magnitude of the three Bi₃O₄Cl samples, we carefully figured out their surface voltages and charge densities by open-circuit potential measurements as well as the simultaneous potentiometric and conductometric titration method, respectively.^11,12 The surface voltages of Bi₃O₄Cl nanosheets with a {001} facet exposure of 86%, 91%, and 95% were respectively 0.041, 0.044 and 0.049 V at pH 6, corresponding to the initial pH value of SA solution (Table S2, ESI†). However the surface charge densities were respectively 0.23, 0.28 and 0.30 C m⁻² for BOC-86, BOC-91 and BOC-95 (Table S3, ESI†). These experimental data showed that the surface voltage and the charge density of Bi₃O₄Cl nanosheets increased along with improving the {001} facet percentage, demonstrating that the IEF magnitude of Bi₃O₄Cl nanosheets is positively correlated with the percentage of exposed {001} facets.

Fig. 3 (a) Crystal structure of Bi₃O₄Cl nanosheets; (b) charge density contour plots viewed from {110} and {1̄10} facets of Bi₃O₄Cl nanosheets.

To further clarify the effect of IEF on the photoreactivity, the IEF magnitude, the efficiency of charge separation and transfer, and the photoreactivity versus the {001} facet percentages of Bi₃O₄Cl nanosheets are qualitatively correlated and shown in Fig. 4. Herein the charge separation and transfer efficiency was expressed by the photocurrent intensity qualitatively because the transient photocurrent response could reflect the amount of surviving electrons during the competitive processes of separation and recombination of photogenerated electrons and holes.⁶ We set all the IEF magnitude, the efficiency of charge separation and transfer and the photoreactivity values of BOC-86 as “1”, the normalized IEF magnitude, the charge separation and transfer efficiency, and the photoreactivity of BOC-91 were calculated to be 1.22, 1.17 and 1.26, respectively, and the corresponding values of BOC-95 were 1.58, 1.43 and 1.52, respectively. Obviously, with the increase of the {001} facet exposure percentage of Bi₃O₄Cl nanosheets, the IEF magnitude, the efficiency of charge separation and transfer, and the photoreactivity increased. The enhancement times of the IEF magnitude, the efficiency of charge separation, and transfer and the photoreactivity for either BOC-91 or BOC-95 are almost identical in comparison with those of BOC-86, strongly indicating that the {001} facet exposure percentage change would induce the IEF variation, then determine the efficiency of charge carrier separation and transfer, and finally affect the photoreactivity. It can be seen that the slight increase (9%) of the {001} facet exposure percentage from 86% to 95% would strengthen the IEF magnitude more remarkably by 58%, thus speed up the efficiency of charge separation and transfer by 43%, and finally improve the photoreactivity of Bi₃O₄Cl nanosheets by 52%. Therefore, we conclude that it is not the surface atomic structure of {001} facets, but the IEF magnitude change induced by the {001} facet exposure is the main factor to determine the photoreactivity of Bi₃O₄Cl nanosheets with high {001} facet exposure percentages.

Fig. 4 Variation of the IEF magnitude, the efficiency of charge separation and transfer, and the photoreactivity versus the percentages of {001} facets of Bi₃O₄Cl nanosheets. All the IEF magnitude, the efficiency of charge separation and transfer and photoreactivity values of BOC-86 were set as “1”.

In light of the above results, we would like to explain the {001} facet dependent photoreactivity of Bi₃O₄Cl nanosheets with high {001} facet exposure percentages as follows. Upon visible-light excitation of Bi₃O₄Cl nanosheets by adsorbing photons with energy equal to or greater than the band gap of the semiconductor, electrons are excited from the valence band to the conduction band, leaving holes in the valence band. The polarization-induced IEF along the [001] crystal orientation of nanosheets would drive the photoinduced charge separation and transfer from the bulk to the surface for the subsequent photocatalytic reactions on the surface. Higher exposure of {001} facets would significantly increase the IEF magnitude and thus further improve the separation and transfer efficiency of photogenerated electron–hole pairs to enhance the photoreactivity of Bi₃O₄Cl nanosheets with high {001} facet exposure percentages.

Besides the IEF magnitude, the thickness of the nanosheets might also influence the charge separation rate and the photocatalytic activity of the Bi₃O₄Cl nanosheets, but we think the influence might be tiny. This is because only these photogenerated charge carriers near the surface of photocatalysts could reach the surface to initiate the subsequent redox reactions.¹³ Meanwhile, the transfer orientation of photogenerated electrons and holes highly depended on the facet exposure. However the transfer of photogenerated electrons and holes might not be reverse. For instance, the transfer of photogenerated electrons in BiVO₄ is inclined to the {010} facets, but the corresponding holes prefer to migrate to the {110} facets, not to the {01̄0} facets.^6a This means that the photogenerated electrons and holes need not traverse the full distance of the Bi₃O₄Cl nanosheets along the [001] direction. We therefore conclude that the distinct migration paths caused by the different thicknesses of the Bi₃O₄Cl nanosheets would not strongly affect the charge separation rate and the photocatalytic activity.

Conclusions

In conclusion, we have prepared Bi₃O₄Cl single-crystalline nanosheets with high {001} facet exposure percentages and demonstrated that their photoreactivity strongly depends on the IEF magnitude, other than the surface atomic structure. The IEF magnitude is found to be correlated with the {001} facet exposure percentage. More {001} facet exposure can induce the generation of stronger IEF, which favors the photogenerated charge separation and transfer and thus enhances the photoreactivity. This interesting finding not only provides new insight into the origin of the crystal facet effect on the photoreactivity of semiconductors, but also highlights the importance of the IEF magnitude in the photoreactivity of layered materials. To our knowledge, this is the first report on tuning the reactivity of photocatalysts with the IEF magnitude. We believe that this strategy maybe applicable to other layered photocatalysts also.

Acknowledgements

This work was supported by National Basic Research Program of China (973 Program) (Grant 2013CB632402), and National Science Foundation of China (Grants 21073069, 91023010, 21177048, and 21377044).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3nr05246j

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