Hollow Cu₂O microspheres with two active {111} and {110} facets for highly selective adsorption and photodegradation of anionic dye

Abstract

Hollow Cu₂O microspheres (0.7 to 4 μm in diameter) with two active {111} and {110} facets have been prepared in water/ethylene glycol (H₂O/EG) solution via a fast hydrothermal route in only 1 h. Due to the dangling “Cu” atoms in the highly active {111} and {110} facets, the microspheres demonstrate preferential selective adsorption and photodegradation for negatively charged methyl orange (MO), comparing to cationic rhodamine B (RhB) and neutral phenol. The 0.7 μm hollow Cu₂O microspheres show the best adsorption capacity and photodegradation performance for MO removal: 49% MO can be adsorbed in 60 min and 99.8% MO can be fully removed under visible light illumination in 80 min, owing to the two active {110} and {111} facets and hollow structure. To exactly evaluate the photocatalytic efficiency, a new methodology is proposed by deducting the adsorption effect. The results show that in spite of 99.2% MO is removed from the solution under visible light illumination in 60 min, 14% MO is still adsorbed on the catalyst, which can be totally removed under further 20 min illumination. Our synthesis strategy presents a new opportunity for the preparation of hollow structures with high active facets. And the proposed accurate evaluation methodology may be extended to other photocatalysts with high adsorption capability for organic pollutants.

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Introduction

Recently, much attention has been paid to the synthesis of metal oxides in nanoscale or microscale because of their potential applications in pollutants removal, lithium ion battery and so on.^1,2 Hollow structured metal oxides, owing to the low density and high surface area, demonstrate a wide range of interesting properties that are superior to their solid counterparts. Thus various methods have been developed to achieve hollow nano/micro-structures.^3–6

Cuprous oxide (Cu₂O), a typical 3d transition metal oxide and a p-type semiconductor with a direct band gap of 2.17 eV, has been widely researched as adsorbent or photocatalyst due to its low cost and environment friendly feature.^7–10 Among the various structures, hollow Cu₂O nano/micro-structures and special morphologies of Cu₂O nanostructures have been aroused much attention recently due to their unique physicochemical properties.^11–23 However, special additives or template-reagents are usually required for synthesizing such unique structures. This results in tedious purification procedures. In addition, synthesis of these unique structures often needs a long time. It is desirable to develop a fast single-step and template-free synthetic approach for large-scale synthesis of hollow Cu₂O nano/micro-structures with special morphologies.

In the study of adsorption or photocatalysis, an interesting aspect research for the exploration of the Cu₂O micro/nanostructures with high active facets is less popular.^13,19 In this context, Huang and co-workers have conducted experiments on the comparative photocatalytic activity for the well-shaped Cu₂O structures with high active facets.^21–23 The results show that the octahedral Cu₂O crystals with {111} facets demonstrate higher photocatalytic activity than truncated cubic crystals with mostly {100} facets, due to the higher surface energy of {111} facets than that of {100} facets. Xu and co-workers have found that the adsorption and photocatalytic activities from mixed 26-facet and 18-facet polyhedra with dominant {110} facets are higher than those of Cu₂O octahedra with dominant {111} surfaces and cubes with {100} surfaces.¹⁹ These results indicate that the {110} and {111} facets in Cu₂O demonstrate good adsorption and photodegradation performance for pollutants removal. Hence, it seems that integrating the abovementioned two active {110} and {111} facets in one Cu₂O structure is a very meaningful demonstration of designing novel structures for enhanced adsorption capability and photocatalytic activity.

On the basis of our previous work on CuO and Mn₃O₄ nanostructures with high energy facets for enhanced photocatalytic degradation of pollutants,^24–28 in this work, we reported the hollow Cu₂O microspheres with a controllable diameter via a very simple one-pot template-free fast hydrothermal synthesis approach for pollutants removal. Under our reaction system, the hollow Cu₂O microspheres could be prepared in only 1 h. As expected, the hollow microspheres have two active {110} and {111} facets. To the best of our knowledge, this is the first time to report such a “fast growth hydrothermal method” via a H₂O/EG solution for synthesizing hollow Cu₂O microspheres with two active {110} and {111} facets. Preferential selective adsorption and enhanced photocatalysis for organic pollutant removal have been achieved because of the synergy of hollow structure and high active {110} and {111} facets. Further, the adsorption influence on the photocatalytic activity is realized to exactly evaluate the photocatalytic efficiency of the special hollow Cu₂O microspheres.

Experimental section

Synthesis of hollow Cu₂O microspheres

All analytical grade chemicals were used as received. For a typical synthesis, 0.002 mol Cu(NO₃)₂·3H₂O (Sinopharm Chemical Reagent Limited Corporation, China) was firstly dissolved in deionized water. Next, 40 ml ethylene glycol (EG) (Aladdin Industrial Corporation, China) was added into the above blue solution. After stirring for 2 h, the transparent pale blue solution was transferred into a 50 ml Teflon-lined stainless steel autoclave, sealed and transferred to oven. Then the oven was heated from room temperature to 180 °C and maintained at 180 °C for 1 h. The final brick-red solid product was filtrated and washed with ethanol for several times to remove the unreacted EG and dried at 40 °C in air. In this experiment, different volume of water, 1 ml, 2 ml, 4 ml, 6 ml and 10 ml were used and the products were designated as S1, S2, S3, S4 and S5, respectively. For the time-dependent process, after the autoclave was taken out, water with temperature at ∼15 °C was used to quickly cool down the autoclave.

Material characterizations

Structural analysis was performed by powder X-ray diffraction (XRD) on a Bruker D8 Advanced diffractometer with Cu Kα radiation. The morphologies and microstructures of the samples were studied with field emission scanning electron microscopy (FESEM) on a Hitachi S-4800, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) on a JEM-2100F. The Brunauer–Emmett–Teller (BET) specific surface areas of the samples were analyzed on a Micromeritics Tristar II 3020 nitrogen adsorption–desorption apparatus according to the Barret–Joyner–Halenda (BJH) method from the N₂ adsorption isotherms.

Measurement of organic pollutants removal

The anionic methyl orange (MO), cationic rhodamine B (RhB) and neutral phenol were used as the organic pollutants. Typically, 50 mg of the powders was added in 100 ml of organic aqueous solution with a concentration of 15 mg L⁻¹ in a 250 ml reactor. After a continuous magnetic stirring in dark for 1 h, a xenon lamp (PLS-SXE300C) was used as light source with a UV cutoff filter to provide visible light illumination (λ ≥ 420 nm). The distance between the lamp and the reactor is 30 cm. At given intervals, 1.5 ml aliquots were taken out and centrifuged to remove the powders to determine the pollutants concentration by a UV-visible spectrophotometer (UV-2550, SHIMADZU, Japan). The desorption experiment was carried out according to the previous report:²⁹ a 1.5 ml 1 M NaCl aqueous solution was added in the centrifuged powders to desorb MO. After 6 h desorption, the suspension was centrifuged and the solution was detected by the same UV-visible spectrophotometer to determine the MO content. The photocatalytic reaction based on first-order, −ln(C₀/C) = kt, where C₀ and C are the initial and actual pollutant concentration respectively, and k is the apparent rate constant of the degradation. The pH value of MO aqueous solution was monitored using pH meter.

Results and discussion

XRD characterization of the products

The XRD measurement was carried out to determine the crystal structure of the as-prepared products. Fig. 1a–d displays the XRD patterns of the as-prepared S1 to S4, respectively. All the peaks match very well with cubic Cu₂O (JCPDS no. 065-3288). However, when the water volume is 10 ml, the as-prepared S5 shows Cu phase (JCPDS no. 001-1241) in the Cu₂O product. This is likely due to excessively reducing Cu₂O to Cu under such reaction condition (Fig. 1e).

Fig. 1 The XRD results of the products with different volume of water: (a) S1, (b) S2, (c) S3, (d) S4 and (e) S5.

SEM characterization of the Cu₂O microspheres

FESEM was conducted to investigate the morphology of S1 and S2. Fig. 2a and c present the typical images of the microspherical Cu₂O with uniform diameter around 0.7 and 1.6 μm for S1 and S2, respectively. Fig. 2b and d show the individually broken microsphere of S1 and S2 respectively, indicating the hollow structured Cu₂O microspheres. Further, Fig. 2a–d clearly displays that their surface is composed by randomly cross-linked polyhedrons. The polyhedron size of S1 is smaller than that of S2, resulting in thinner shell of S1. This is helpful for light penetrating into the hollow structure. Closer observations reveal that two active {111} and {110} facets are exposed at the surface of the microspheres (Fig. 2e and f). The insets in Fig. 2e and f are the schemed (110) and (111) crystal plane, respectively. Interestingly, the internal surface of the hollow microsphere is composed by many small irregular nanoparticles (Fig. 2b and d), which is quite different from the external polyhedrons with high active {110} and {111} facets. Our results show that the hollow Cu₂O microspheres with two active {110} and {111} facets have been successfully synthesized by a fast template-free method in 1 h.

Fig. 2 SEM images of S1 and S2: (a) a typical SEM image from S1, (b) one broken hollow microsphere from S1, (c) a typical SEM image from S2, (d) one broken hollow microsphere from S2, e and f the surface images clearly displaying the {110} and {111} facets of the hollow microspheres. The insets in (e) and (f) are the schemed {110} and {111} facets, respectively.

TEM and HRTEM characterizations of the Cu₂O microspheres

TEM and HRTEM were further employed to confirm the hollow structure and the exposed {110} and {111} facets. The TEM images clearly show the hollow structure of S1 (Fig. 3a and b) and S2 (Fig. 3d and e) respectively, and the shell thickness of S1 is thinner than that of S2. The lattice fringes can be clearly observed from the HRTEM images (Fig. 3c and f), corresponding to the (001) or (002), (110) and (111) crystal planes, respectively. The insets in Fig. 3c and f are the electron diffraction patterns from the fast Fourier transform. These observations are consistent with the SEM observations, indicating two active {110} and {111} facets exposed in the surface of our hollow Cu₂O microspheres.

Fig. 3 TEM and HRTEM images of the hollow Cu₂O microspheres of S1 (a–c) and S2 (d–f), respectively. The electron diffraction patterns in (c) and (f) are from the fast Fourier transform.

Water effect on the preparation of the hollow Cu₂O microspheres

We found that the volume ratio of H₂O and EG is crucial to the desired size and morphology. Without water added, only some irregular Cu₂O nanoparticles can be obtained (Fig. S1†). When 4 and 6 ml of H₂O is added, the sphere sizes are increased to 3 and 4 μm, respectively (Fig. 4a and b). Both the products show brick-red color and Cu₂O phase according to the XRD pattern (Fig. 1c and d) except for the whole size and the polyhedron size increase. However, the final product displays a cubic morphology (Fig. 4c) with Cu phase (Fig. 1e) when 10 ml water is added. As no other reducing reagent added in our reaction system, EG likely works as a reducing reagent in this synthesis process. According to the literature,³⁰ the following reactions may occur:

2HOCH₂CH₂OH → 2CH₃CHO + 2H₂O (1)

CH₃CHO + 2Cu(NO₃)₂ + 2H₂O → CH₃COOH + 4HNO₃ + Cu₂O (2)

CH₃CHO + Cu₂O → CH₃COOH + 2Cu. (3)

Fig. 4 SEM images of the products with different volume of water: (a) S3, (b) S4 and (c) S5.

It is clear that water volume increase accelerates the (2) and (3) reduction reactions, leading to Cu appearance in the Cu₂O product when 10 ml water is added (Fig. 1e). This is further convinced by much more water added in the reaction system. With 20 ml water added, the final product is only irregular Cu particles in microscale (Fig. S2†).

Growth mechanism of the hollow Cu₂O microspheres with {110} and {111} facets

To clearly show the growth of the hollow Cu₂O microspheres, a time-dependent process was monitored on S2. When Cu(NO₃)₂·3H₂O is completely dissolved in H₂O/EG at room temperature, the solution displays a transparent pale blue color. With the reaction proceeding, the pale blue solution gradually turns into transparent dark blue solution from 0 to 50 min, indicating the interaction between EG and Cu²⁺. The transparent dark blue solution changes to dark green at 52 min. When the transparent dark green solution is dried in air, many small nanoparticles are found (Fig. 5a), indicating very tiny nanocrystallites formation. From 52 to 60 min, the transparent dark green solution changes to brick-red and dark brick-red suspension (the inserted photographs in Fig. 5a–f). At 53 min, 1 μm hollow structured microspheres are formed (Fig. 5b and the inset), which are further grown to 1.5 μm at 54 min owing to the nanoparticles aggregation to reduce their surface energy (Fig. 5c). The small irregular nanoparticles at the surface grow to large polyhedron shaped particles (Fig. 5d–f). The products from 54 to 60 min show that the size of the microspheres is almost unchanged. This reveals that the polyhedron formation is directed by EG accompanying with crystallization of the microspheres via Ostwald ripening. However, when the reaction time is prolonged to 120 min, the polyhedrons grow larger and Cu appears in the product (Fig. S3†).

Fig. 5 The formation process of the hollow Cu₂O microspheres: (a) 52 min, (b) 53 min, (c) 54 min, (d) 56 min, (e) 58 min, (f) 60 min and (g) schematic formation mechanism. The inserted photographs are the corresponding photographic images of the samples, respectively. The scale bar of the inserted TEM image in (b) is 500 nm.

The possible formation mechanism is then proposed as illustrated in Fig. 5g, which is explained as a self-transformation process of the metastable nanoparticles aggregation accompanying with the localized Ostwald ripening in the H₂O/EG dual solvent system.^1,3 First, the Cu²⁺ coordinating with hydrophobic alkyl group of EG and water forms a water molecules surrounding micelle-like structure.^30,31 With the reaction proceeding, tiny Cu₂O nanocrystallites form and aggregate at the interface of the micelle-like structure, leading to the spherical hollow structure formation.^12,32 As the water viscosity is lower than EG, the newly formed tiny Cu₂O nanocrystallites can move and grow faster in water than in EG. Thus the Cu₂O nanoparticles grow faster at outside, resulting in the outer larger particles and inner small nanoparticles in the hollow microspheres. As a structure directing reagent, EG adsorbs on the surface of the Cu₂O nanoparticles then directs the formation of cross-linked polyhedron with two active {110} and {111} facets. However, when the reaction continues, the adsorbed EG direct the polyhedron to irregular shape and further reduce Cu₂O to Cu (Fig. S3†). Generally, the diffusion rate (D) of the nanoparticles will increase when the solution viscosity (η) decreases according to the Stokes-Einstein equation: D = constant/η.²³ With the water volume increase (e.g. 4 and 6 ml), the solution viscosity decreases, leading to the crystalline growth velocity and sphere size increase. At a given amount of Cu²⁺, slightly increasing the volume ratio of H₂O and EG induces disturbance to the balanced state of the micelle-like structure, the tiny nanocrystallites aggregation and EG molecules directing the nanocrystallites, resulting in size and morphology change.^33,34 Consequently, when the water volume increases to certain value, such as 10 ml, the decreasing viscosity reduces the micelle-like structure stability. The spherical structure is destroyed, leading to only micro-cubic particles formation (Fig. 4c). In addition, the decreasing viscosity in H₂O/EG system may excessively reduce Cu₂O to Cu. Indeed, only irregular Cu particles can be obtained with 20 ml water added (Fig. S2†). However, without water in the reaction system, there are only irregular Cu₂O nanoparticles due to no micelle-like structure formation (Fig. S1†).

The pollutants removal properties of the hollow Cu₂O microspheres with {110} and {111} facets

As a narrow band-gap semiconductor with large light absorption coefficient, Cu₂O nano/micro-structures have been widely used as adsorbents and/or photocatalysts for organic pollutants removal.^{7–9,19–23,35–39} Particularly, our hollow Cu₂O microspheres integrate two active {111} and {110} facets in the surface. Fig. 6 shows the Cu₂O crystal lattice with the surface atomic species and the state of {100}, {110} and {111} facets. It clearly displays that the {100}, {110} and {111} facets are different from each other. The {100} facets possess 100% saturated oxygen. However, the {110} and {111} facets are alternatively arranged “Cu” or “O” atoms. According to previous reports, the {110} and {111} facets with dangling “Cu” atoms are beneficial for pollutants removal.^{10,11,16,21–23}

Fig. 6 Crystal structures of Cu₂O: (a) (100), (b) (110) and (c) (111) planes. Surface O atoms on the {100} faces and Cu atoms on the {110} and {111} faces are shown with yellow circles.

To verify the effects of two active {110} and {111} facets on organic pollutants photodegradation, three typical pollutants, negatively charged MO, positively charged RhB and neutral phenol, were carried out on S1. The experiments were first performed using single pollutant of MO, RhB and phenol, respectively. Then a mixture of the three organic pollutants with equal concentrations was performed. The proceeding changes of the static adsorption–desorption equilibrium (0–60 min) and photodegradation curves (60–120 min) of the organic pollutants are shown in Fig. S4.† The characteristic absorption peaks of the three organic pollutants are clearly displayed, indicating no interaction between the pollutants in the mixture. Fig. 7a presents the results of the single pollutant of MO, RhB and phenol on S1. The adsorption capacity of S1 in dark for 60 min towards organic molecules adsorption–desorption equilibrium decreases from 49% of anionic MO to 10% of neutral phenol and to 8% of cationic RhB, indicating preferential selective adsorption for MO. This reveals that the dangling “Cu” atoms in the {110} and {111} facets make the surface positively charged, consistent with the previous work.^21–23 The removal efficiency of organic molecules decreases from 99.2% for MO to 41.2% for RhB and to 26.2% for phenol under visible light illumination for 60 min. In the mixed pollutants, the same adsorption sequence is observed with a large decrease of adsorption capacity in dark for 27% of MO and only 3% of phenol and 2% of RhB (Fig. 7b). Although all the adsorption capacities for these organic pollutants are decreased, the relative adsorption capacity for MO is largely increased compared to phenol and RhB. This indicates that S1 possesses higher adsorption selectivity towards MO than phenol and RhB, and the adsorption of RhB on S1 is less favorable. Under visible light illumination, the removal efficiency trend is similar to the single pollutant. Particularly, in the mixed pollutants, the degradation efficiency of RhB and phenol is largely decreased to 8.4% and 3.3%, respectively. In fact, after 10 min illumination, there is almost no degradation of RhB and phenol. Further, compared to the efficiency in single pollutant, the MO degradation on S1 in mixed pollutants exhibits a little higher efficiency after 20 min illumination. Table 1 lists the photodegradation rates of the single pollutant and the mixed pollutants on S1 (See ESI†). It clearly shows that S1 displays the best degradation rates on MO. The results clearly demonstrate that the preferential sequence of pollutant adsorption capacity and removal property on S1 is anionic MO, neutral phenol and cationic RhB, indicating that the dangling “Cu” atoms in the {110} and {111} facets can largely influence the interaction between our hollow Cu₂O microspheres and the organic molecules. It is worth to note that although the phenol adsorption is higher than that of RhB, the photodegradation rates of phenol both in single pollutant and mixed pollutants are lower than those of RhB (Fig. 7a and b). This is reasonable as RhB has conjugated groups, being more easily damaged compared to the quite stable single benzene ring in phenol.

Fig. 7 The removal performance of MO, RhB and phenol on S1: (a) the single pollutant and (b) the mixed pollutants.

In order to reveal more information of our hollow Cu₂O microspheres for MO removal, S1, S2 and S3 were selected as photocatalysts in this experiment. Fig. S5† displays the progressive spectral changes for the static adsorption in dark (0–60 min) and photodegradation curves (60–140 min) of MO on S1, S2 and S3. Fig. 8a presents the MO removal results on S1, S2 and S3. It can be seen that the MO adsorption reaches 49%, 33% and 26% for S1, S2 and S3 in dark at 60 min, respectively, indicating a high static adsorption capability for MO. After 80 min illumination, 99.8%, 98% and 93% of MO are removed from the solution by S1, S2 and S3 respectively, being superior to the previous reported results with {110} or {111} facets.^19,21,35

Fig. 8 The removal performance of MO on S1, S2 and S3: (a) the removal plots and (b) the adsorption and photocatalytic degradation efficiency plots under visible light illumination.

Since the hollow Cu₂O microspheres exhibit high adsorption and photodegradation performance for MO, it is worth to know whether the MO pollutant has been totally photodegraded although its content in solution is not detected. Herein, for the first time, a very simple and useful method is employed to evaluate the Cu₂O photocatalytic degradation and adsorption efficiencies under visible light illumination: the photocatalytic degradation efficiency (E_p) is equal to the removal efficiency (E_r) minus the adsorption efficiency (E_a), i.e. E_p = E_r − E_a. It is noted that the adsorption process is a dynamic adsorption under visible light illumination, being quite different from the static adsorption in dark. The dynamically adsorbed MO was desorbed in 1 M NaCl aqueous solution according to the previous work.²⁹ Fig. S6† shows the progressive spectral changes of MO from desorption in 1 M NaCl solution for the three samples. Fig. 8b presents the dynamic adsorption and photodegradation efficiencies from 0 to 80 min under illumination. It displays that the photodegradation efficiencies always increase with reaction proceeding. Although 99.2%, 87% and 76% MO are removed from the solution after 60 min reaction (Fig. 8a), 14%, 11% and 9% MO are still adsorbed by S1, S2 and S3 respectively, indicating the strong adsorption ability of the hollow Cu₂O microspheres (Fig. 8b). Namely, after 60 min illumination, the photodegradation efficiency is 85.2% for S1, 76% for S2 and 67% for S3, respectively. After further 20 min illumination, no MO is detected on S1 (Fig. S6a† and 8b). However, there are still 5% and 8% MO adsorbed on S2 and S3 respectively, verifying the superior performance of S1 for MO photodegradation compared to S2 and S3.

As MO can hydrolyze in water with alkalescence, during the photodegradation, the pH values of its aqueous solution should be decreased under visible light illumination. Table 2 presents the pH values (See ESI†). As expected, it clearly shows the pH values of MO aqueous solution quickly decrease from 8.82 to 7.04 in 60 min. This indicates that MO is totally degraded in 60 min, in agreement with the results from UV-visible spectrophotometer.

The SEM and EDX characterizations were further employed to investigate the structure stability and adsorbed MO of S1. With static adsorption of MO for 60 min, the EDX result shows N and S elements, verifying MO adsorbed on the surface of S1 (Fig. S7a†). However, under visible light illumination for 80 min, no N or S is detected, indicating the fully degradation of MO (Fig. S7b†). The inserted SEM images display that the morphology is well reserved under visible light illumination for 80 min, indicating the very stable hollow structure and polyhedron surface.

The results show that S1 has the best performance for MO removal compared to S2 and S3. As we know, the specific BET surface area is very important for organic pollutants removal. The BET surface area analysis of S1, S2 and S3 were then carried out. Generally, the smaller size is, the higher BET surface area is. However, the three samples demonstrate similar BET surface areas of 7.1 m² g⁻¹ for S1, 6.9 m² g⁻¹ for S2 and 6.4 m² g⁻¹ for S3, respectively. Thus, the excellent photocatalytic and selective adsorption activities of the samples are significantly attributed to the atoms arrangement in the high active facets. As mentioned above, the {110} and {111} facets with dangling “Cu” atoms make them positively charged, leading to electrostatic attraction with negatively charged MO and electrostatic repulsion with positively charged RhB. On the other hand, the hollow structure also contributes to the enhanced efficiency. The thinner shell of S1 is beneficial for visible light penetration, resulting in the inner nanoparticles taking part in the photodegradation of MO.⁴⁰ Therefore, the synergy of the hollow structure and two high active {111} and {110} facets leads to S1 with excellent performance for MO removal.

Conclusions

We developed a fast template-free hydrothermal method for the preparation of hollow Cu₂O microspheres with two active {110} and {111} facets. The hollow Cu₂O microspheres show preferential selective adsorption and enhanced photocatalytic performances for anionic MO removal owing to the dangling “Cu” atoms in the {110} and {111} facets. This affects the interaction between the hollow Cu₂O microspheres and the differently charged organic molecules. A new methodology is proposed to exactly evaluate photocatalytic efficiency of the hollow structured Cu₂O microspheres via deducting the adsorption influence on MO under visible light illumination. Our hollow Cu₂O microspheres are envisioned to have more potential applications in gas sensors, photocatalytic water-splitting, solar cells and other optoelectronics due to the hollow structure and the highly active {110} and {111} facets. In addition, our strategy for fast synthesis of the hollow Cu₂O microspheres with highly active facets may be extended to other metal oxides and the accurate evaluation methodology may be employed to other photocatalytic degradation systems.

Acknowledgements

This work is realized in the frame of a program for Changjiang Scholars and Innovative Research Team (IRT1169) of Chinese Ministry of Education. B. L. Su acknowledges the Chinese Central Government for an “Expert of the State” position in the Program of the “Thousand Talents”. Y. Li acknowledges Hubei Provincial Department of Education for the “Chutian Scholar” program. This work is also financially supported by the Ph.D. Programs Foundation of Ministry of Education of China (20120143120019) and Hubei Provincial Natural Science Foundation (2014CFB160) and Self-determined and Innovative Research Funds of the SKLWUT (2015-ZD-7). The authors also thank J. L. Xie, X. Q. Liu and T. T. Luo for the TEM testing at Centre for Materials Research and Analysis of Wuhan University of Technology.

Notes and references

1. H. Cölfen and S. Mann, Angew. Chem., Int. Ed., 2003, 42, 2350.
2. M. V. Reddy, G. V. Subba Rao and B. V. R. Chowdari, Chem. Rev., 2013, 113, 5364.
3. J. G. Yu, H. T. Guo, S. A. Davis and S. Mann, Adv. Funct. Mater., 2006, 16, 2035.
4. X. Wu, Y. Zeng, H. Gao, J. Su, J. Liu and Z. Zhu, J. Mater. Chem. A, 2013, 1, 469.
5. J. Sun, G. Chen, J. Wu, H. Dong and G. Xiong, Appl. Catal., B, 2013, 132, 304.
6. X. Lai, J. Li, B. A. Korgel, Z. Dong, Z. Li, F. Su, J. Du and D. Wang, Angew. Chem., Int. Ed., 2011, 50, 2738.
7. J. Kondo, Chem. Commun., 1998, 357.
8. H. Y. Jing, T. Wen, C. M. Fan, G. Q. Gao, S. L. Zhong and A. W. Xu, J. Mater. Chem. A, 2014, 2, 14563.
9. S. Yoon, M. Kim, I. S. Kim, J. H. Lim and B. Yoo, J. Mater. Chem. A, 2014, 2, 11621.
10. S. D. Sun, H. J. You, C. C. Kong, X. P. Song, B. J. Ding and Z. M. Yang, CrystEngComm, 2011, 13, 2837.
11. C. H. Kuo and M. H. Huang, Nano Today, 2010, 5, 106.
12. Q. D. Chen, X. H. Shen and H. C. Gao, J. Colloid Interface Sci., 2007, 312, 272.
13. H. Xu and W. Wang, Angew. Chem., Int. Ed., 2007, 46, 1489.
14. Y. Chang, J. J. Teo and H. C. Zeng, Langmuir, 2005, 21, 1074.
15. W. Wang, Y. Tu, P. Zhang and G. Zhang, CrystEngComm, 2011, 13, 1838.
16. Y. Sui, W. Fu, Y. Zeng, H. Yang, Y. Zhang, H. Chen, Y. Li, M. Li and G. Zou, Angew. Chem., Int. Ed., 2010, 49, 4282.
17. M. D. Susman, Y. Feldman, A. Vaskevich and I. Rubinstein, ACS Nano, 2014, 8, 162.
18. C. Lu, L. Qi, J. Yang, X. Wang, D. Zhang, J. Xie and J. Ma, Adv. Mater., 2005, 17, 2562.
19. Y. Zhang, B. Deng, T. R. Zhang, D. M. Gao and A. W. Xu, J. Phys. Chem. C, 2010, 114, 5073.
20. C. H. Kuo and M. H. Huang, J. Am. Chem. Soc., 2008, 130, 12815.
21. J. Y. Ho and M. H. Huang, J. Phys. Chem. C, 2009, 113, 14159.
22. C. H. Kuo and M. H. Huang, J. Phys. Chem. C, 2008, 112, 18355.
23. C. H. Kuo, C. H. Chen and M. H. Huang, Adv. Funct. Mater., 2007, 17, 3773.
24. S. Z. Huang, J. Jin, Y. Cai, Y. Li, H. Y. Tan, H. E. Wang, G. Van Tendeloo and B. L. Su, Nanoscale, 2014, 6, 6819.
25. Y. Li, H. Tan, X. Y. Yang, B. Goris, J. Verbeeck, S. Bals, P. Colson, R. Cloots, G. Van Tendeloo and B. L. Su, Small, 2011, 7, 475.
26. Y. Li, X. Y. Yang, J. Rooke, G. Van Tendeloo and B. L. Su, J. Colloid Interface Sci., 2010, 348, 303.
27. J. Liu, J. Jin, Z. Deng, S. Z. Huang, Z. Y. Hu, L. Wang, C. Wang, L. H. Chen, Y. Li, G. Van Tendeloo and B. L. Su, J. Colloid Interface Sci., 2012, 384, 1.
28. Y. Li, H. Tan, O. Lebedev, J. Verbeeck, E. Biermans, G. Van Tendeloo and B. L. Su, Cryst. Growth Des., 2010, 10, 2969.
29. J. Liu, Z. Gao, H. Han, D. Wu, F. Xu, H. Wang and K. Jiang, Chem. Eng. J., 2012, 185–186, 151.
30. M. H. Kim, B. Lim, E. P. Lee and Y. N. Xia, J. Mater. Chem., 2008, 18, 4069.
31. Ö. Çelik and Ö. Dag, Angew. Chem., Int. Ed., 2001, 40, 3799.
32. Q. Zhao, H. P. You, W. Lü, N. Guo, Y. C. Jia, W. Z. Lv, B. Q. Shao and M. M. Jiao, CrystEngComm, 2013, 15, 4844.
33. F. Luo, D. Wu, L. Gao, S. Y. Lian, E. B. Wang, Z. H. Kang, Y. Lan and L. Xu, J. Cryst. Growth, 2005, 285, 534.
34. J. S. Chen, Y. N. Liang, Y. Li, Q. Yan and X. Hu, ACS Appl. Mater. Interfaces, 2013, 5, 9998.
35. Y. Shang, D. Sun, Y. M. Shao, D. F. Zhang, L. Guo and S. H. Yang, Chem.–Eur. J., 2012, 18, 14261.
36. W. C. Huang, L. M. Lyu, Y. C. Yang and M. H. Huang, J. Am. Chem. Soc., 2012, 134, 1261.
37. L. Huang, F. Peng, H. Yu and H. J. Wang, Mater. Res. Bull., 2008, 43, 3047.
38. D. F. Zhang, H. Zhang, L. Guo, K. Zheng, X. D. Han and Z. Zhang, J. Mater. Chem., 2009, 19, 5220.
39. D. Le, S. Stolbov and T. S. Rahman, Surf. Sci., 2009, 603, 1637.
40. Y. Li, Z. Y. Fu and B. L. Su, Adv. Funct. Mater., 2012, 22, 4634.

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Footnotes

† Electronic supplementary information (ESI) available: The photodegradation rates of the single pollutant and the mixed pollutants on S1; pH values of MO aqueous solution during photodegradation process on S1; SEM image (a) and XRD patterns (b) of the product without EG, the product with 20 ml water and the product at 120 min with 2 ml water, respectively. The proceeding changes of the static adsorption and photodegradation curves of the organic pollutants on S1, S2 and S3, respectively. See DOI: 10.1039/c5ra06083d

‡ These authors contributed equally to this work.

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