Programmable synthesis of metal hydroxide/oxide hollow architectures: towards an efficient and robust photocatalyst for water remediation

Abstract

A general, one-pot, “soft” SiO₂-templating strategy has been developed for the synthesis of well-defined metal hydroxide/oxide hollow nanostructures (HNs) with programmable shells. The principle of this unique strategy lies in controlling the balance of the reaction kinetics between the deposition, growth and crystallization of the shells and the simultaneous etching of the SiO₂ cores by the superhot water in the hydrothermal system. As a proof-of-concept study, hydroxy iron oxide (FeOOH) HNs with tunable architecture, shape and thickness of shells have been synthesized via this proposed “soft” SiO₂-templating method. Then, several important metal oxide HNs such as α-Fe₂O₃, CeO₂, SnO₂ and TiO₂ have been successfully synthesized by simply altering the metal precursor and/or reaction temperature, highlighting the excellent versatility and reliability of the proposed strategy. Further, this novel method can be readily extended to the fabrication of multi-shelled HNs with tunable chemical compositions. The as-synthesized FeOOH HNs with hierarchical and thicker shells exhibited excellent photocatalytic activity and outstanding stability for sustainable degradation of organic pollutants in water.

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Introduction

Hollow nanostructures (HNs) have attracted tremendous interest in various fields including energy storage, catalysis and water remediation, owing to their unique merits such as large surface area, high permeability, and low density.^1–4 Engineering the shell architecture, shape and composition of HNs has been considered as one of the most promising strategies to optimize their physicochemical properties for various applications.^5–12 As such, extensive research endeavors have been devoted to the development of new synthesis methods that can facilely tune the size, shape, thickness, number, and chemical composition of the shells, thereby endowing the HNs with desired properties towards specific applications.^13–18 SiO₂ hard template method (Si-HTM) has been regarded as one of the most popular approaches for fabricating the HNs. This is because SiO₂ is easily obtainable and eco-friendly. Using the Si-HTM, Yin and co-workers synthesized well-defined TiO₂ hollow spheres with controllable crystallinity, phase, and shell thickness.^19–21 Despite such exciting progress, there still exist several difficulties in the programmable synthesis of HNs via the SiO₂-templating method. Firstly, it is challenging to achieve homogeneous deposition of the designed materials on the non-modified surfaces of SiO₂ templates in aqueous systems, especially on the high-curvature surfaces of non-spherical SiO₂ templates such as SiO₂ tetrahedra and polyhedra. Secondly, it is very difficult to maintain the reaction kinetics balance between the controlled growth of a shell with a specific composition and the synchronous etching of the SiO₂ core. This could be the reason why the conventional Si-HTM usually involves multi-step and time-consuming synthesis procedures, including coating via the sol–gel method, annealing at high temperature and etching by a strong acid/base. Thirdly, current Si-HTMs can only succeed in obtaining single-shell hollow nanospheres with a single component, but cannot meet the needs for the synthesis of complex HNs in a programmable manner, thus limiting the opportunities for modulating the properties of HNs for many applications.

Generally, SiO₂ is regarded as a hard template for the fabrication of HNs, and etching of the hard SiO₂ template is performed by using a strong acid or base solution.^22,23 However, recent research has revealed that the SiO₂ templates prepared by the classical Stöber method are not as “hard” as expected. For example, Chen et al. found that SiO₂ can be disintegrated by hot water (90 °C).²⁴ Yin and co-workers also considered that the insoluble SiO₂ can be decomposed into the soluble monosilicic acid (Si(OH)₄) by incubating it in the boiled water.^25,26 These intriguing observations suggest that the hard SiO₂ template can be softened and dissolved in heated water at a certain temperature. Therefore, based on this unique “soft” feature of SiO₂, it is highly possible to achieve the growth of a desired shell and the dissolution of SiO₂ core simultaneously in a hydrothermal system.

Inspired by the above considerations, a general, one-pot, “soft” SiO₂-templating strategy has been developed for the synthesis of well-defined metal hydroxide/oxide HNs with programmable shells. The principle of this unique strategy lies in controlling the balance of the reaction kinetics between the deposition, growth and crystallization of the shell and the dissolution of the SiO₂ core by superhot water in the hydrothermal system (Fig. 1). As a proof-of-concept study, hydroxy iron oxide (FeOOH) HNs with tunable architecture, shape and thickness of shells have been synthesized via the proposed “soft” SiO₂-templating method. Then, several important metal oxide HNs such as α-Fe₂O₃, CeO₂, SnO₂ and TiO₂ have been successfully synthesized by simply altering the metal precursor and/or reaction temperature, highlighting the excellent versatility and reliability of the proposed strategy. Furthermore, it is also worth noting that this strategy can be readily extended to the fabrication of some multi-shelled HNs with tunable chemical compositions. As an example to demonstrate the potential applications for water treatment, the hierarchical and thicker FeOOH shells exhibited excellent photocatalytic activity and outstanding stability for sustainable degradation of organic pollutants in water.

Fig. 1 Schematic illustration of the principle of the “soft” SiO₂-templating strategy. (I) Deposition of a metal precursor on the SiO₂ surface; (II) and (III) growth and crystallization of the shell and simultaneous etching of the SiO₂ core.

Results and discussion

Monodisperse SiO₂ nanospheres with an average diameter of 118 nm were prepared by the classical Stöber method (Fig. S1†). The dissolution behavior of SiO₂ was investigated under the hydrothermal treatment at different temperatures. It can be seen that the color of SiO₂ solution exhibits a tendency to fade with the increase of reaction temperature (Fig. S2a†), which is in good agreement with the change of turbidity in the SiO₂ solution (Fig. S2b†). These results indicate that the hard and insoluble SiO₂ nanospheres have been transformed to the “soft” and soluble Si(OH)₄ in the hydrothermal system. Meanwhile, the silicon concentration of the reaction system also increased from 0 to 411 mg L⁻¹ with the increase of reaction temperature from 20 to 180 °C (Fig. S2b†), further supporting the above-mentioned observation. Theoretically, the as-synthesized amorphous SiO₂ nanospheres are highly disordered and discontinuous. Thus, the Si–O–Si network structures in the amorphous SiO₂ can be easily broken when it is incubated in water at high temperature and high pressure.^24–26

Based on this intriguing property of SiO₂, it is considered that the growth of the designed materials against the SiO₂ surface and the etching of the SiO₂ template may proceed simultaneously under hydrothermal treatment, thereby giving rise to the HNs. To verify this hollowing approach, iron-based HNs were selected as the target materials due to their extensive applications in various fields, such as water remediation and energy storage.^5,27,28 Iron chloride (FeCl₃) and urea were respectively used as the metal precursor and alkaline source in this exploration because of their wide employment in the fabrication of iron-based nanomaterials. Unfortunately, the resulting nanomaterials are shown to be granular aggregations rather than the hollow architectures as we expected (Fig. S3†). This could be attributed to (1) the mismatch of reaction kinetics between the hydrolysis of Fe³⁺ ions and the dissolution of SiO₂ nanospheres; (2) the insufficient adsorption capacity of Fe³⁺ ions on the non-modified surface of SiO₂ owing to their weak electrostatic interactions (Table S1†). Therefore, ferric acetylacetonate (Fe(acac)₃) was selected as the ferric precursor instead of FeCl₃ because it possesses better stability and a higher electric potential (Table S1†). Eventually, highly uniform FeOOH hollow nanospheres with an average size of 162 nm (Fig. S4†) were successfully fabricated (Fig. 2a and S5†). It is also noted that no structural deformation or shell breakage is found for these hollow nanospheres, demonstrating the excellent reliability of this synthesis method. A higher magnification TEM image shows that the porous shells with a thickness of about 18 nm are composed of tiny nanoparticles (Fig. 2b). Interestingly, many ultrathin nanosheets (<3 nm) are randomly grafted onto the shell surface, thus leading to a hierarchical hollow architecture (Fig. 2b and d). The HAADF image and corresponding EDS mapping images further indicate that Fe and O are homogeneously distributed over the whole profile of hierarchical hollow nanospheres (Fig. 2c). The crystallographic structure of FeOOH hollow nanospheres was examined by X-ray diffraction (XRD). As shown in Fig. S6a,† all diffraction peaks in the patterns can be readily indexed to the phase of β-FeOOH (JCPDS card no. 341266). The selected-area electron diffraction (SAED) pattern (Fig. 2e) clearly displays the polycrystalline feature of these β-FeOOH hollow nanospheres. Although the crystallinity of the β-FeOOH hollow nanospheres is not that good, from an observation of XRD and SAED results, this instead may be conducive to their applications in the field of photochemistry and electrochemistry in view of some recent reports.^29–34 The Fe 2p fine spectrum (Fig. S6b†) indicates that the levels of Fe 2p_3/2 and 2p_1/2 are 710.4 and 724.2 eV, respectively, with the satellite peak at ca. 719 eV, which is assigned to Fe³⁺ in β-FeOOH. A d spacing of 0.193 nm corresponding to the (411) plane of β-FeOOH can also be discerned in the high resolution TEM image (Fig. S7†).

Fig. 2 TEM images (a, b and d), HAADF image and the corresponding EDS mapping images (c), SAED pattern (e) of the FeOOH hierarchical hollow nanospheres.

To gain an in-depth insight into this “soft” SiO₂-assisted hollowing strategy, the effects of experimental conditions on the morphology of FeOOH hollow nanospheres have been investigated systematically. Firstly, time dependent investigation has been carried out in this work. Before starting the hydrothermal reaction, the mixed solution of SiO₂ nanoparticles and Fe(acac)₃ was thoroughly stirred; after that, the SiO₂ was separated and characterized using a Fourier transform-infrared spectrophotometer (Fig. S8†). The result demonstrates that Fe(acac)₃ can be easily adsorbed onto the surface of SiO₂via the strong electrostatic interaction. When the temperature of the hydrothermal system increased from 25 to 160 °C, numerous solid SiO₂@FeOOH core–shell hybrids were produced, suggesting that the SiO₂ cores can maintain their structural integrity during the deposition of the FeOOH shells (Fig. S9a†). After hydrothermal treatment for only 0.5 h at 160 °C, a small amount of SiO₂@FeOOH yolk–shell hybrids are observed in the system, together with large amounts of FeOOH hollow nanospheres (Fig. S9b†). The rapid morphological evolution from the solid core–shell hybrids to the yolk–shell hybrids and then to the hollow nanospheres is believed to result from the fast but mild etching of SiO₂ by the superhot water under the high pressure. With the prolongation of reaction time to 6.0 h, the SiO₂@FeOOH yolk–shell hybrids disappear completely and high-quality FeOOH hierarchical hollow nanospheres are obtained finally (Fig. S9c†).

Secondly, the effect of SiO₂ dosage on the morphology of FeOOH hollow nanospheres has also been uncovered (Fig. S10†). When the dosage of SiO₂ is 40 mg, uniform hollow FeOOH nanospheres can be obtained after 6.0 h of hydrothermal reaction. When the SiO₂ dosage is increased to 80 mg, the hollow FeOOH nanospheres and SiO₂@FeOOH yolk–shell hybrids are generated simultaneously. However, when SiO₂ dosage is further increased to 120 mg, no hollow FeOOH nanospheres could be found in the typical TEM image. These observations illustrate that the dissolution of SiO₂ has been limited by the amount of water in the reaction system. To be specific, the solubility of SiO₂ in water under the given temperature and pressure should be a fixed value, and an over dosage of SiO₂ could not be further dissolved in the limited amount of water in the hydrothermal system, which is further confirmed by the experimental results (Fig. S11†). This result highlights the importance of the mass ratio between the water and the SiO₂ for the fabrication of HNs with this “soft” SiO₂-templating strategy.

Thirdly, it is also found that the shell structure of FeOOH hollow nanospheres can be easily tuned by simply adjusting the dosage of urea and Fe(acac)₃ (Fig. 3). For example, with the increase of urea dosage from 0 to 75 mg, the average thickness of the shells increased from 4.7 to 7.0 nm correspondingly. Thicker shells (∼13.2 nm) were obtained when 200 mg of urea was added. This is because urea works as the alkali source in the hydrothermal reaction, and a higher amount of urea can accelerate the hydrolysis of the ferric precursor, thus leading to the formation of thicker shells. Interestingly, the ultrathin nanosheets grafted on the shells show a growing trend in terms of amount and size with the increase of Fe(acac)₃ dosage, thereby resulting in a complex and hierarchical hollow architecture. These results indicate that the ultrafine structure of the FeOOH shells can be readily regulated through this “soft” SiO₂-templating method. More importantly, it provides a great opportunity to optimize the architecture of FeOOH HNs, thus developing a high-performance and robust photocatalyst for water remediation (detailed results as shown in Fig. 6).

Fig. 3 Morphological evolution of FeOOH hollow nanospheres with change in the dosage of Fe(acac)₃ and urea. Scale bars: 100 nm.

To further verify the reliability and versatility of this synthesis approach, SiO₂ templates with various morphologies have been prepared. First, the much larger SiO₂ spheres with an average diameter of 419 nm (Fig. S12a†) are selected as the templates. Fig. 4a and d show that the resultant products are the well-defined hierarchical δ-FeOOH hollow nanospheres with an average diameter of 457 nm. Encouraged by this exploration, non-spherical SiO₂ with high curvature surfaces (e.g., tetrahedra, polyhedra) have been synthesized as templates to further challenge this hollowing strategy (Fig. S12b and c†). Interestingly, the appealing hollow δ-FeOOH tetrahedra and polyhedra with porous shells have been successfully fabricated (Fig. 4). To the best of our knowledge, this is the first time that a non-spherical SiO₂ template has been employed for preparing anisotropic HNs.

Fig. 4 TEM images and XRD patterns of the bigger hollow FeOOH spheres (a and d), the hollow FeOOH tetrahedra (b and e) and the hollow polyhedra (c and f).

In the experiments, it is also noticed that when the hydrothermal temperature increased from 160 to 200 °C, the crystallographic phase of hierarchical HNs could be readily transformed from β-FeOOH to α-Fe₂O₃, which is corroborated by the XRD and SAED results (Fig. 5e and S13†). This could be due to that the high reaction temperature can effectively accelerate the nucleation and growth of the oxide crystals.^5,12,28 However, the unique architecture has been preserved intact during the transformation of the crystallographic structure (Fig. 5a). As a result, hierarchical α-Fe₂O₃ hollow nanospheres with porous shells are obtained. Additionally, several vital metal oxide hollow nanospheres such as CeO₂, SnO₂, and TiO₂ have also been achieved by simply altering the metal precursor and reaction temperature (Fig. 5), highlighting the promising versatility of this synthesis approach. Compared to the conventional Si-HTM that usually involves the coating, annealing and etching procedures, the “soft” SiO₂-templating method developed here exhibits significant advantages such as simple process procedure, mild reaction conditions, excellent reliability as well as promising versatility.

Fig. 5 TEM images and XRD patterns of the α-Fe₂O₃ (a and e), CeO₂ (b and f), SnO₂ (c and g) and TiO₂ (d and h) hollow nanospheres.

More impressively, the design and synthesis of multi-shelled HNs with the desired shell composition can also be realized based on this new “soft” SiO₂ template strategy. As a demonstration, double-shelled FeOOH HNs have been fabricated in this work. Firstly, single-shelled FeOOH hollow nanospheres were prepared as primary templates. Then, the surface of the primary templates are uniformly coated with the SiO₂, thus producing SiO₂@FeOOH shell-by-shell hollow nanospheres, which are named as secondary templates (Fig. S14†). By adding the secondary templates instead of the solid SiO₂ nanospheres into the synthesis process, well-defined double-shelled FeOOH HNs can be obtained as shown in Fig. 6a and b. The elemental mapping result demonstrates that Fe and O are distributed homogeneously in both the outer shell and the inner shell (Fig. 6c). The XRD result (Fig. 6d) shows that the phase of the double-shelled FeOOH HNs is a mixture of akaganeite (JCPDS card no. 341266) and iron hydroxide (JCPDS card no. 220346). This mixed phase of the double-shelled FeOOH HNs may be conducive to their applications in the field of photochemistry, just like the commercial P25 TiO₂ photocatalyst that is comprised of the anatase and rutile phases of TiO₂. Also, the composition of the outer shell can be further tuned by simply employing the desired metal precursor to replace the Fe(acac)₃. For example, when the acetylacetone tin(IV) dichloride salt is added, uniform SnO₂@FeOOH double-shelled HNs can be achieved as shown in Fig. 6e. The elemental mapping image evidently suggests that the composition of the inner shell is Fe and O whereas that of the outer shell is Sn and O (Fig. 6f). The XRD pattern further confirms the co-existence of FeOOH and SnO₂ in the products (Fig. 6g). It is noted that elaborate manipulation and integration of different materials with distinct physical/chemical properties has been rarely reported in previous studies, despite the fact that it is conceptually straightforward by a layer-by-layer templating strategy.³⁵ Importantly, the fabrication method developed in this work can provide more opportunities for modulating and optimizing the properties of HNs to satisfy the specific requirements of various industrial applications.

Fig. 6 TEM images, HAADF image and the corresponding EDS mapping images, and XRD patterns: (a)–(d) for the double-shelled FeOOH HNs; (e)–(g) for the SnO₂@FeOOH double-shelled HNs.

Recently, photo-Fenton technology has emerged as an alternative strategy for water purification, owing to its environmental friendliness and general applicability. However, traditional photo-Fenton technology usually requires highly acidic conditions (pH 2–3) and generates large amounts of residual sludge, thus leading to a high treatment cost. To address these disadvantages, as-synthesized FeOOH HNs with different thicknesses and shell structures have been employed as heterogeneous photocatalysts for the degradation of rhodamine B (RhB) under natural pH. The blank experiment reveals that RhB is barely decomposed under visible light irradiation (Fig. 7a). The control experiments show that only limited removal efficiency (<20%) can be achieved in the presence of H₂O₂ and FeOOH HNs in the dark, and 24% of the RhB can be degraded with H₂O₂ and solid FeOOH nanoparticles under visible light irradiation (Fig. 7a and S15†). However, a significant increase in RhB removal has been observed with the co-existence of H₂O₂ and FeOOH HNs under visible light irradiation. This demonstrates that all the FeOOH HNs possess excellent photocatalytic activities due to their hollow features. By further comparing the degradation results of these FeOOH HNs, three obvious differences can be clearly discerned. Firstly, the removal efficiency of RhB gradually decreased from 92 to 72% with the increase of shell thickness from 4.7 to 13.2 nm (samples a–c). Coincidentally, both the surface areas and total pore volumes of these FeOOH HNs exhibit a similar declining tendency with the increase of shell thickness (Fig. 7b and Table S3†). Thus, it can be deduced that thicker shells with smaller surface areas and lower pore volumes might result in the decrease of active sites, which should be responsible for the decrease in photocatalytic performance. Secondly, the removal efficiency of RhB gradually increased from 72 to 84% with the increase of the amount and size of the ultrathin nanosheets grafted on the shells (samples c–i). This could be attributed to the increased complexity in the shell architecture, which can enhance the light absorption, reflection and scattering (Fig. 7c), and thus deliver higher photocatalytic efficiency.^8,36 Thirdly, the cyclic degradation tests show that the RhB removal efficiency by the thinner shells (sample a) seriously decreased from 92 to 61% after three cycles of repeated use, which can be attributed to the severe collapse of the hollow architecture as observed by TEM (Fig. S16a†). However, the photocatalytic activity of the thicker hierarchical shells (sample i) has been well maintained in the cyclic tests owing to the excellent structural stability (Fig. S16b†). Besides, the thicker hierarchical FeOOH HNs are also found to be robust enough to withstand heat treatment upon converting to α-Fe₂O₃ at 700 °C, further highlighting their outstanding stability (Fig. S17†). Therefore, in view of practical application, the thicker hierarchical FeOOH shells should be a better candidate for the photocatalytic degradation of organic pollutants in water.

Fig. 7 Photocatalytic efficiencies of the samples in the degradation of RhB under visible light irradiation (a). N₂ adsorption–desorption isotherms (b) and diffuse reflectance spectra (c) of the samples. Cyclic photo-degradation tests for samples (a) and (i) (d). “B” and “S” denote “Blank” and “Solid”, respectively. The number of samples (a–i) was in agreement with Fig. 3 and the detailed sample description can be found in Table S2.†

To elucidate the mechanism involved in the photocatalytic degradation of RhB, classical quenching tests have been carried out by employing tert-butanol as the scavenger for ˙OH radicals and ascorbic acid as the scavenger for O₂˙⁻ radicals (as shown in Fig. S18†).³⁷ When tert-butanol was added in the reaction solution, the degradation effectiveness of RhB decreased from 92 to 51%. When ascorbic acid was added, only 70% of RhB could be degraded. Thus, it can be deduced that both ˙OH and O₂˙⁻ radicals generated by the FeOOH HNs under visible light irradiation make a certain contribution to the degradation of RhB.

The solution pH is generally considered as an important factor in the photo-Fenton system. Therefore, the effect of pH on the photo-Fenton performance of the FeOOH HNs (sample i) has been further investigated. As shown in Fig. S19,† although the photocatalytic activity of the FeOOH HNs decreased with the increase of pH value, 57% of RhB removal could still be achieved at a pH of 9.0, implying that the typical disadvantage of traditional Fenton and photo-Fenton technologies of requiring highly acidic conditions could be well overcome by employing the FeOOH HNs.

In addition, the colorless phenol has also been employed as a recalcitrant pollutant to further verify the photocatalytic activity of the FeOOH HNs (sample i). Fig. S20† shows that only very limited degradation of phenol (<3%) was achieved by the solid FeOOH nanoparticles. However, when the FeOOH HNs (sample i) were employed in the heterogeneous photo-Fenton process, the degradation efficiencies of phenol could reach up to 37%, demonstrating again the excellent photo-activity of the FeOOH hierarchical hollow nanospheres and their potential applications in water purification.

Conclusion

In summary, we have developed a general, one-pot, “soft” SiO₂-templating method for the programmable synthesis of metal hydroxide/oxide HNs. The principle of this strategy lies in controlling the balance of reaction kinetics between the deposition, growth and crystallization of the shells and the simultaneous etching of SiO₂ by superhot water in the hydrothermal system. The FeOOH HNs exhibited excellent photocatalytic activity and outstanding stability for water purification. We believe that the synthesis strategy proposed in this work could not only provide a novel and open-ended platform for the design and synthesis of various HNs for many applications, but also provide more possibilities for modulating and optimizing the properties of the HNs towards specific applications.

Experimental section

Synthesis

Synthesis of spherical SiO₂ templates. The SiO₂ nanospheres were prepared by the classic Stöber method with minor modifications. In a typical synthesis procedure, 0.86 mL tetraethyl orthosilicate (TEOS) was injected into a mixture of 4.3 mL deionized water, 0.6 mL ammonia with a concentration of 25–28 vol% and 23 mL ethanol under magnetic stirring at room temperature. After reacting for 1.5 h, SiO₂ nanospheres with an average diameter of 118 nm were collected by centrifugation and washed several times with absolute ethanol and deionized water. Finally, the synthesized samples were dried in an oven at 60 °C overnight. For the synthesis of SiO₂ spheres with an average diameter of 419 nm, a similar experimental method was employed, except that the volume of ammonia added was 1.5 mL and the reaction time was prolonged to 3.0 h.

Synthesis of non-spherical SiO₂ templates. For the synthesis of SiO₂ tetrahedra, F127 (0.123 g), deionized water (3.5 mL), CTAB (12.5 mL, 0.04 M), and ammonia (12 mL, 2.5 wt%) were mixed to form a clear solution, to which TEOS (0.6 mL) was added under stirring. After 3.0 min of stirring, the mixture was allowed to stand under static conditions at 35 °C for 3.0 h, thus producing a white suspension. After that, the obtained products were collected by centrifugation and washed several times with absolute ethanol and deionized water. Finally, the synthesized samples were dried in an oven at 60 °C overnight. For the synthesis of SiO₂ polyhedra, a similar experimental method was employed, except that the volume of ammonia added was 22 mL.

Synthesis of the FeOOH HNs. In a typical procedure, 40 mg SiO₂ templates were ultrasonically dispersed in 70 mL deionized water for 0.5 h. Then, known masses of Fe(acac)₃ and urea were added under stirring for 1.0 h. After that, the above suspensions were transferred into a Teflon-lined autoclave, which was sealed and maintained at 160 °C for 6 h. The obtained precipitate was collected by centrifugation and washed several times with absolute ethanol and deionized water. Finally, the synthesized samples were dried in an oven at 60 °C overnight.

Synthesis of the metal oxide HNs. For the synthesis of α-Fe₂O₃ HNs, 40 mg SiO₂ templates were ultrasonically dispersed in 70 mL deionized water for 0.5 h. Then, 200 mg Fe(acac)₃ and 200 mg urea were added under stirring for 1.0 h. After that, the above suspensions were transferred into a Teflon-lined autoclave, which was sealed and maintained at 200 °C for 12 h. The obtained precipitate was collected by centrifugation and washed several times with absolute ethanol and deionized water. Finally, the synthesized samples were dried in an oven at 60 °C overnight. For the synthesis of CeO₂ HNs, 40 mg SiO₂ templates were ultrasonically dispersed in 70 mL deionized water for 0.5 h. Then, 50 mg cerium(III) acetylacetonate was added under stirring for 1.0 h. After that, the above suspensions were transferred into a Teflon-lined autoclave, which was sealed and maintained at 180 °C for 3.0 h. For the synthesis of SnO₂ HNs, 40 mg SiO₂ templates were ultrasonically dispersed in 70 mL deionized water for 0.5 h. Then, 100 mg acetylacetone tin(IV) dichloride salts and 100 mg urea were added under stirring for 1.0 h. After that, the above suspensions were transferred into a Teflon-lined autoclave, which was sealed and maintained at 180 °C for 3.0 h. For the synthesis of TiO₂ HNs, 40 mg SiO₂ templates were ultrasonically dispersed in 70 mL deionized water for 0.5 h. Then, 50 mg titanium(IV) oxide acetylacetonate and 200 mg urea were added under stirring for 1.0 h. After that, the above suspensions were transferred into a Teflon-lined autoclave, which was sealed and maintained at 160 °C for 2.0 h.

Synthesis of the multi-shelled HNs. For the synthesis of double-shelled FeOOH HNs, 10 mg thick FeOOH HNs (sample c as depicted in Fig. 2) was ultrasonically dispersed in a mixture of 4.3 mL water and 23 mL ethanol for 0.5 h. Then, 100 mg hydroxyethyl cellulose and 0.86 mL TEOS were added in the above-synthesized suspensions under strong stirring for 2.0 h. After that, 0.2 mL ammonia with a concentration of 25–28 vol% was stirred for 1.5 h, thus producing the SiO₂@FeOOH shell-by-shell hollow nanospheres named as secondary templates (Fig. S16†). When using this secondary template instead of solid SiO₂ nanospheres in the synthesis of FeOOH HNs, well-defined double-shelled FeOOH HNs were obtained. Similarly, when another metal acetylacetonate, namely, acetylacetone tin(IV) dichloride salt was added, the resultant product was the SnO₂@FeOOH double-shelled HNs.

Characterization

The micro-morphologies of the samples were observed by using the FEI Tecnai G2 F30 TEM, FEI Talos F200x TEM and JEOL JEM-1400 TEM. XRD patterns were collected by using a Bruker D8 X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). The BET surface area measurements were performed by using an ASAP2020 instrument. The FTIR spectra were recorded on a Perkin-Elmer Spectrum One B spectrometer. The UV-vis diffuse spectra were obtained by using a Shimadzu UV2550 spectrophotometer. X-ray photoelectron spectroscopy measurements were performed using a Thermo ESCALAB 250 with Al Kα X-ray (hν = 1486.6 eV) radiation.

Photocatalytic activities of HNs

The photocatalytic activities of HNs were evaluated by measuring the decomposition of RhB. A 300 W short-arc xenon lamp (Perkin-Elmer, PE300BF) equipped with a 400 nm cut-off filter was used as the light source. The reactor was made of open cylindrical Pyrex with diameter 4 cm and height 7 cm. The distance between the light source and the surface of the solute was approximately 20 cm. For photocatalytic degradation of RhB, 10 mg of the sample was suspended in 50 mL of RhB solution (C_o = 20 mg L⁻¹, pH = 6.1), and the mixture was stirred for 30 min to reach an adsorption equilibrium in the dark. 0.2 mL of H₂O₂ aqueous solution (1%) was added to the reaction solution at the beginning of irradiation. The temperature of the reactor was maintained at 25 °C by continuously circulating water. About 2.0 mL of the suspension was collected at regular intervals for the analysis of residual contaminant concentration. The concentrations of RhB were determined by the absorption peak at 554 nm, using a UV-vis spectrophotometer (Hach, DR5000).

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51408162), Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. QA201524), the Heilongjiang Postdoctoral Special Fund (No. LBH-TZ0409), and the Postdoctoral Scientific Research Developmental Fund of Heilongjiang Province (No. LBH-Q14070).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ta08042a

‡ These authors contributed equally to this work.

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