Nontraditional template synthesis of microjagged bismuth oxide: a highly efficient visible light responsive photocatalyst

Abstract

A free-standing 3D microjagged Bi₂O₃ was firstly prepared through a nontraditional template method and its visible light responsive photoactivity is highly related to its surface phase under H₂O₂ treatment.

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Tailoring the architecture of 3D superstructures has been of great research interest due to their unique shape and size-dependent properties.¹Bi₂O₃ is an interesting material with semiconductor characters such as high refractive index and dielectric permittivity as well as marked photoconductivity and photoluminescence properties.² However, to the best of our knowledge, efforts were almost limited to the architecture of Bi₂O₃ nanomaterials with 0D, 1D morphology.^3–5 Until now, it remains a significant challenge to develop facile, solution-based and shape controlled self-assembly routes for the formation of hierarchical architectures of Bi₂O₃-based materials. Furthermore, Bi₂O₃ has proven to be a good visible light driven photocatalyst for water splitting and pollutant decomposition.^6–8 The exploration of such bulk Bi₂O₃ material with superior photocatalytic activity under visible light is another important motive in this work.

In the present work, we report on the first discovery of the formation of Bi₂O₃ nanocomposites with 3D jagged morphology through hydrolysis of bismuth nitrate in the presence of tungsten trioxide (WO₃) (see Experimental section in ESI†). The addition of WO₃ mediated the nucleation and growth of Bi₂O₃, leading to the in situself-assembly of jagged component units which are composed of WO₃ and Bi₂O₃. Herein, WO₃ works as the nontraditional template and pH buffering agent, which can be later dissolved by NaOH solution to finally obtain free-standing 3D microjagged Bi₂O₃. The microjagged Bi₂O₃ was subsequently found to undergo self-oxidation under simulated sunlight irradiation. This self-oxidation feature reflects in the transition of the surface phase of the bulk Bi₂O₃. The surface phase of Bi₂O₃, which is directly exposed to light and the reactants, contributes to photocatalysis and solar energy conversion because the photocatalytic reaction or photoelectron conversion takes place only when photoexcited electrons and photogenerated holes are available on the surface. The photocatalytic activity of the as-prepared Bi₂O₃ is directly related to its surface phase. On the basis of the understanding thus obtained, surface modification possibility of Bi₂O₃ samples with different bulk and surface crystalline phases could be easily achieved by H₂O₂ treatment. The photocatalytic activity of the bulk Bi₂O₃ under visible light irradiation can be remarkably enhanced through introducing a surface crystalline phase on the bulk Bi₂O₃ after H₂O₂ treatment.

The structural details of the final products were investigated using a scanning electron microscope (SEM). To investigate the growth mechanism of the Bi₂O₃ nanocomposites, the growth processes were systematically studied by analyzing the samples at different growth stages. Fig. 1a–c show a series of SEM images of the precursor and products at 2 h, 12 h and 24 h at 180 °C. As shown in Fig. 1a, the precursor is composed of irregular nanoparticles. After 12 h of reaction, uniform tooth units can be found (Fig. 1b), meanwhile the WO₃ templates were gradually etched. Prolonging the reaction time to 24 h gave the assembly of relatively uniform tooth units forming the 3D jagged superstructure as the dominant product (Fig. 1c). High-magnification SEM images of a typical 3D superstructure from different directions of view are shown in Fig. 1d and e. Every tooth unit is made up of two parts from the side-looking view (Fig. 1d). The coronal part with a smooth surface attached the peak part with a rough surface, which composited of the completed unit just like a tooth of animals. The coronal parts of these component units were observed to attach each other by self-assembly and the peak parts then intermeshed with other peak parts by replacement forming the final 3D jagged superstructure. From the overlook view (Fig. 1e), the coronal part was not solid but etched into a tubular shape. The whole component unit is made up of WO₃ in the coronal (EDS1) and Bi₂O₃ in the peak part (EDS2) as shown in Fig. S1 in ESI.† The temperature effect was also investigated (Fig. S2 in ESI†), our results showed that reaction at 180 °C gave the optimal 3D microjagged Bi₂O₃ nanocomposites.

Fig. 1 SEM images of Bi₂O₃ products prepared at 180 °C in various reaction stages: (a) 2 h, (b) 12 h, (c) 24 h. (d, e) SEM images of Bi₂O₃ nanocomposites with 3D jagged morphology in different views. (f) SEM image of free standing Bi₂O₃ with 3D jagged morphology.

In this respect, we try to demonstrate a plausible formation mechanism of the 3D jagged superstructure. The existence of WO₃ was crucial in the formation of the jagged component units and their self-assembly. The 3D jagged superstructure could only be obtained in the presence of an appropriate amount of WO₃. Other metal oxides, such as TiO₂ and ZnO, have also been tested in this work. Our results show that the addition of the above oxides only led to the irregular Bi₂O₃ particles or mixture. Therefore, to explore the effect of WO₃ on the product, controlled experiments were conducted in the presence of different amounts of WO₃ while keeping other conditions unchanged. If no WO₃ was introduced in the reaction system, only irregular Bi₂O₃ microrods can be observed. Only the amount range of 1–2 mmol gave the type of 3D superstructure. Exceeding the amount range couldn't take on the 3D jagged appearance. The appropriate addition amount of WO₃ which can be slightly soluble under acidic conditions may have an important influence on the pH value of the solution when a bismuth salt was strongly hydrolyzed in water. In our case, WO₃ is used as both a non-traditional template and a pH buffering agent. Bi₂O₃ firstly deposited on the WO₃ template through the hydrolyzation of bismuth nitrate, meanwhile, partial WO₃ was involved in the reaction system and induced the pH change in the reaction solution. The pH value of the residue solution after the reaction in the absence of WO₃ was measured to be 2.5, while the value range was 3.5–4.0 when containing 1–2 mmol WO₃. The pH change induced by the addition of WO₃ affected the hydrolysis rate of the bismuth salt which could further affect the deposition and final morphology of Bi₂O₃ on WO₃. Therefore, the pH buffering action plays decisive role in ensuring the formation of jagged component units and their assembly. This is also the reason why the composition-based superstructures didn't occur in the case of TiO₂ and ZnO, which represent acid-insoluble oxide and acid-soluble oxide, respectively. Moreover, the tubular shape on the coronal WO₃ part was formed by acid etching with extending the reaction time. This is a novel kind of preparation method which is totally different from the traditional template-based method. The final sample was subsequently treated by a 5 M NaOH solution for 24 h in order to completely remove the coronal WO₃ template and finally yielded Bi₂O₃ with free-standing 3D jagged structures, as shown in Fig. 1f. Based on the above SEM observation and analysis, the whole process can be illustrated in Fig. 2.

Fig. 2 Schematic illustration of the proposed formation mechanism of 3D microjagged Bi₂O₃.

For most semiconductors used as photocatalysts, a question that arises is why not to use a thicker metal oxide to observe a much higher photocatalytic activity? In fact, the reported literature have indicated that a thick semiconductor is opaque like a mirror and an appropriate thickness for the semiconductor layer in order to have an overall high transmission is estimated to be always less than 100 nm; in other words, it is impossible to enhance photocatalytic activity with a thick semiconductor. This is the reason why the bulk microjagged Bi₂O₃ shows so low photocatalytic efficiency in whole photocatalytic reaction under visible light irradiation (λ > 400 nm) and in the initial reaction period under simulated sunlight (λ > 320 nm), as shown in Fig. 3aand b. CO₂ generation from an acetaldehyde solution over the as-prepared Bi₂O₃ photocatalyst is almost negligible under visible light. The rate of CO₂ evolution did not evidently change within 3 h under simulated sunlight irradiation. However, after that, an interesting phenomenon contradicted the above discussion. A remarkable increase in the rate of acetaldehyde decomposition can be observed. Traces of surface active species Bi₂O_4−x were later revealed by XRD in the Bi₂O₃ sample. It is obvious that the overall photocatalytic activity is highly related to its surface phase as the Bi₂O₃ phase always exists in the surface region for the samples during irradiation. The change trend of photocatalytic activity can be thus interpreted by assuming that, within the first 3 h of irradiation, a great deal of highly active species can be yielded due to the capture of photoexcited electrons produced on the Bi₂O₃ surface by oxygen either adsorbed or present in solution. Among them, the superoxide radical has the highest oxidation ability and the sufficiently long lifetime.⁹ Most of the photogenerated superoxide radicals primarily tend to oxidize Bi³⁺ to Bi⁵⁺ with formation of additional Bi⁵⁺–O bonds at the surface, thus leading to the production of Bi₂O_4−x, rather than attacking the pollutant in solution. With the formation of a significant amount of surface active species, the oxygen and photoexcited electrons become available for the production of sufficient oxidizing species, which lead to a significant increase of the decomposition rate.

Fig. 3 Comparison of photocatalytic degradation of acetaldehyde over microjagged Bi₂O₃ under (a) visible light (λ > 400 nm), (b) simulated sunlight (λ > 320 nm) and (c) H₂O₂-treated Bi₂O₃ (immersed for 30 min) under visible light radiation (λ > 400 nm).

With essential understanding of the self-oxidation feature of the Bi₂O₃ phase, in order to quickly and precisely introduce the surface active species on Bi₂O₃, H₂O₂ treatment is proven an ideal way for surface modification of Bi₂O₃ with the possibility of controlling the amount of surface active species by an appropriate selection of the immersion time, while it has no effect on the morphology change. The surface phase of Bi₂O₃ is differentiated by XRD, XPS and its optical property is investigated by UV-visible spectroscopy (Fig. S3–S5 in ESI†). As shown in Fig. 3c, the H₂O₂-treated sample exhibited remarkably higher photocatalytic activity than that of pure Bi₂O₃ for first 3 h visible light irradiation. This result suggests that the presence of the surface Bi₂O_4−x on the bulk Bi₂O₃ can keep a relatively high overall photocatalytic activity. From the intrinsic view, the high photocatalytic efficiency can be attributed to the phase junctions formed between the bulk Bi₂O₃ and surface Bi₂O_4−x when the Bi₂O₃ sample is treated by H₂O₂ because the formation of the surface phase junction could promote the spatial charge separation in the surface region.^10–12 In this case, the surface phase junctions may facilitate transfer of the photogenerated electron from the conduction band of the Bi₂O₃ phase to the trapping sites on the surface active phase of Bi₂O_4−x, thereby improving the charge separation efficiency and thus enhancing the photocatalytic activity of the bulk Bi₂O₃.

To further confirm the surface phase effect of Bi₂O₃, the overall photocatalytic activity of the samples obtained from H₂O₂ treatment with different immersion times was investigated because the surface phase composition in the Bi₂O₃ sample can be precisely controlled by the immersion time. The Bi₂O_4−x content in the surface region, estimated from XRD spectra (Fig. S3, ESI†), is shown in Fig. 4a and the corresponding photocatalytic activity is shown in Fig. 4b. The Bi₂O₃ samples that exhibit the surface modification can be accelerated considerably by H₂O₂ treatment. Compared with the pure Bi₂O₃, the photocatalytic activity is remarkably increased by supporting a small amount of Bi₂O_4−x on the surface of the bulk Bi₂O₃ after H₂O₂ treatment. For instance, the Bi₂O_4−x/Bi₂O₃ samples obtained from H₂O₂ treatment for 10 min exhibit much higher photocatalytic activity despite their low surface Bi₂O_4−x content (8 wt%). H₂O₂-treated samples for 30 min (the surface Bi₂O_4−x content reaches 46 wt%) show the photocatalytic activity about 60 times higher than that of pure Bi₂O₃ for 3 h irradiation. The photocatalytic activity rapidly decreased somewhat because the surface of the bulk Bi₂O₃ may be almost covered by the Bi₂O_4−x species, which decreases the amount of exposed Bi₂O_4−x/Bi₂O₃ phase junction on the Bi₂O₃ surface. Accordingly, a direct correlation between surface phase junctions of Bi₂O₃ and its photoactivity is of great significance, but has remained unclear mainly owing to the difficulty in characterizing the surface phase junctions of Bi₂O₃, which has been used as catalysts or photocatalysts.

Fig. 4 (a) Dependence of the surface Bi₂O_4−x content on the immersion time. (b) Bi₂O₃ samples treated in a H₂O₂ solution at different immersion times and their corresponding overall photocatalytic activity under visible light irradiation for 3 h.

To visualize the surface phase junction, H₂O₂-treated Bi₂O₃ samples were investigated by HRTEM. Fig. S6 in ESI† shows the HRTEM results of the object and firstly gives the direct evidence that the junction structure between the bulk Bi₂O₃ and surface Bi₂O_4−x phase is formed. Notably, designing such a surface phase junction effectively contributes to the photocatalytic activity of the as-prepared Bi₂O₃ photocatalyst with large size (less than 1 m² g⁻¹).

In conclusion, a free-standing 3D microjagged Bi₂O₃ was prepared through a nontraditional template synthesis method, in which WO₃ was used as the template and functioned as the pH buffering agent. This free-standing bulk Bi₂O₃ photocatalyst could self-oxidate under simulated sunlight. On the basis of understanding its self-oxidation feature, H₂O₂ treatment was found to be an ideal way that could accelerate to induce the surface modification of the as-prepared Bi₂O₃ sample. The photocatalytic activity of bulk Bi₂O₃ material is directly related to the surface-phase structure. The surface phase junction formed between the surface active species (Bi₂O_4−x) and bulk Bi₂O₃ can greatly enhance the photocatalytic activity for acetaldehyde decomposition. From this work, it can thus be concluded that the surface phase junction of a semiconductor photocatalyst can greatly contribute to photocatalytic reactions. Therefore, it provides another possible way to develop high performance photocatalysts by designing and introducing the surface phase junctions.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, EDXA data, SEM images, TEM images, XRD data, XPS data and UV-vis spectral data of as-prepared samples. See DOI: 10.1039/c1cy00069a

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