A novel coral-like porous SnO₂ hollow architecture: biomimetic swallowing growth mechanism and enhanced photovoltaic property for dye-sensitized solar cell application

Abstract

A unique coral-like porous SnO₂ hollow architecture with enhanced photovoltaic property for dye-sensitized solar cell application was prepared, and a biomimetic swallowing growth mechanism for the formation of the special structure was also proposed for the first time.

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Porous materials have received increasing attention due to their fascinating potential applications ranging from electronics and energy to catalysis.^1–5 The extraordinarily high surface area and special adsorption property make them ideal building blocks for fabricating various devices, e.g., lithium ion batteries, dye-sensitized solar cells (DSCs), separators, and gas sensors.^6,7 During the past few years, many types of porous materials have been prepared by various physical and chemical techniques.^8–11 In spite of these significant achievements, exploring a facile approach for fabricating porous architectures with controllable morphology is still a challenge. Recently, solution-phased processes imitating the synthesis of inorganic materials in a living body has become a noteworthy method for preparing functional materials.^12,13 It is expected that the biomimetic technique would be an effective strategy for preparing specific porous architectures through the growth of inorganic materials under mild conditions.

As an n-type semiconductor with a wide band gap (E_g = 3.6 eV, at 300 K), SnO₂ has been widely applied, such as in gas sensors, lithium ion batteries, DSCs, catalysts, and antistatic coatings.^14,15 In these applications, the performance is greatly influenced by the structure and morphology of SnO₂, as also shown by our previous works on porous and hollow architectures.^16,17 In the case of the application of porous materials in DSCs, dye adsorption and light absorption are restricted seriously due to the aggregation of porous materials in the constructing process of DSCs. In this condition, a great portion of surface area is lost, and the solar energy conversion is less than optimal. So, it is significant to design novel structures used in DCSs, which not only possess a high surface area for efficient dye adsorption, but also provide a suitable optical penetration for light-absorption and utilization.

Herein, we report a unique coral-like porous SnO₂ hollow architecture prepared via a facile wet-chemical approach combining with an annealing process (see ESI† for details). A biomimetic swallowing growth mechanism is proposed for the first time to demonstrate the formation of such a special structure. By constructing a DSC, the photovoltaic property of the as-prepared SnO₂ architectures is investigated. The results show that the DSC based on coral-like SnO₂ exhibits remarkably enhanced photovoltaic properties compared with that based on spherical SnO₂. Our findings not only create new opportunities for designing functional materials with specific structure for applications, but also shed a significant emphasis on the role of internal and external structures for performance enhancement, which has been neglected before whereas simply pursuing a large surface area.

The X-ray diffraction (XRD) pattern (Fig. S2a, ESI†) of the as-fabricated products can be indexed to the rutile structure of SnO₂ with lattice constants of a = 3.435 Å and c = 3.186 Å (JCPDS card 41-1445), which is further confirmed by the energy-dispersive XRD (EDXRD) pattern (Fig. S2b, ESI†). The typical morphology of the as-prepared SnO₂ architectures is shown in Fig. 1. As can be seen from field-emission scanning electronic microscopy (FESEM) images (Fig. 1(a) and (b)), each SnO₂ architecture exhibits a coral-like morphology with many tentacle-like structures with a diameter of ca. 300 nm and a length of 2 μm grown on the parent structure. The transmission electron microscope (TEM) images (Fig. 1(c) and (d)) reveal that the architectures are porous and hollow with a shell thickness of ca. 20 nm. Further investigation was performed by high-resolution TEM (HRTEM) and selective area electron diffraction (SAED), as shown in Fig. 1(e). In the HRTEM image a porous structure with a pore size of ca. 15 nm can be observed clearly. The size of SnO₂ particles ranges from 10 to 15 nm. Moreover, the ring-like SAED pattern (Fig. 1(e), inset) reveals that the SnO₂ architectures are polycrystalline. The obvious discrete spots in the SAED pattern and clear lattice fringes indicate the good crystallinity of the SnO₂. Energy dispersive X-ray spectroscopy (EDS) (Fig. 1(f)) demonstrates that the as-prepared products are composed of Sn and O elements in a molar ratio (Sn : O) of ca. 1 : 1.96, which is close to the stoichiometry of SnO₂.

Fig. 1 Typical morphology of the as-prepared SnO₂ architectures is shown in (a) low- and (b) high-magnification FESEM images. (c) Low- and (d) high-magnification TEM observations reveal that the SnO₂ architectures are porous and hollow. (e) HRTEM image and corresponding SAED pattern (inset) show the crystalline lattice and polycrystalline structure clearly. The composition of samples is presented in the EDS pattern (f).

A primary understanding on the formation mechanism of the special coral-like porous SnO₂ hollow architectures is crucial for both controlling synthetic reaction and further exploring potential applications of this approach to design other functional materials with specific structures. A series of time-dependent reactions were carried out to investigate the growth mechanism. Fig. 2 shows the typical morphology of products obtained by annealing the samples synthesized at different stages of hydrothermal reaction (from 1 to 5 h at a determined synthetic temperature of 170 °C). As can be seen from Fig. 2(a), products at the initial synthetic stage are nanospheres displaying aggregation. Increasing the time for hydrothermal reaction, the morphology becomes elliptical, as displayed in Fig. 2(b). Interestingly, it can be clearly observed from the inset TEM image that two nanospheres stick together and grow up into a whole unit. As the reaction proceeds further, first, the stuck nanospheres at the tip are packed into the whole sample gradually, as shown in Fig. 2(c). In other words, external nanospheres are swallowed by the aggregated parent structure. Secondly, during the process of growth, the structure of the swallowed nanospheres is destroyed (Fig. 2(d)), and nanospheres are assimilated by the parent structure. Finally, as shown in Fig. 2(e), coral-like structures with many radial tentacles is formed.

Fig. 2 FESEM images with inset TEM photographs of products prepared by annealing the samples obtained at different hydrothermal synthetic stages: (a) 1 h, (b) 2 h, (c) 3 h, (d) 4 h, and (e) 5 h.

On the basis of investigation on the products presented above, a biomimetic swallowing growth mechanism is proposed, as shown in Fig. 3. At the initial stage of synthetic process, sucrose dehydrates and aromatizes under hydrothermal conditions, resulting in the formation of carbonaceous spheres; similar to the reports on the synthesis of carbonaceous spheres by hydrothermal treatment of other saccharide solutions.^18,19 The surface of carbonaceous spheres are hydrophilic due to functionalization with –OH groups. The functional –OH groups are able to incorporate Sn(OH)₄ nuclei hydrolyzed from Sn⁴⁺ through coordination interaction, leading to the formation of Sn⁴⁺/carbonaceous sphere composites, as illustrated by model (a) in Fig. 3. As the reaction continues, contiguous composites congregate to form an initial parent structure via hydrolyzation between remaining –OH groups of composites. Subsequently, parent structures will adsorb individual composites in solution further, as shown in model (b) and Fig. 2(b). While the entire architecture grows up, hydrolyzation between adsorbed composites and internal layers of parent structure occurs, resulting in the encapsulation of the adsorbed composites. During this period, especially in the later stage, the adsorbed composites are disassembled gradually, and finally are combined with parent structure, as shown in model (c)–(e). After removing carbonaceous materials by annealing treatment, final products with coral-like porous hollow structure were fabricated.

Fig. 3 Schematic illustration of the proposed biomimetic swallowing formation process of coral-like porous SnO₂ hollow architectures.

Photovoltaic properties of the as-obtained SnO₂ architectures were studied by constructing DSCs. Fig. 4 shows the I–V characteristics of DSCs based on SnO₂ with two different structures: spherical (Fig. 2(a)) and coral-like (Fig. 1(a)). For the DSC based on coral-like SnO₂, the solar energy conversion efficiency (η), open-circuit voltage (V_oc), short-circuit photocurrent (J_sc) and fill factor (FF) are 1.04%, 0.52 V, 3.60 mA cm⁻² and 0.56, respectively. As compared with spherical SnO₂-based DSC, the coral-like SnO₂ architectures exhibit a higher solar energy conversion efficiency. Interestingly, the Brunauer–Emmett–Teller (BET) surface area of the coral-like porous SnO₂ hollow architectures is 35.6 m² g⁻¹ (Fig. S3a, ESI†), which is smaller than that of spherical SnO₂ materials (43.5 m² g⁻¹, Fig. S3b, ESI†). The particle-size effect is thus not an important factor of the photovoltaic characterization result due to the similar particle sizes of the two different structural products. Therefore, we propose that both internal and external structures of SnO₂ materials would play significant roles on their photovoltaic properties. During the fabrication of traditional DSCs, semiconductors are impacted tightly, resulting in narrow spaces among them which are disadvantageous for dye adsorption and light absorption. In our study, besides the porous hollow structure which is advantageous for dye adsorption and light absorption inside SnO₂ architectures, the unique radial coral-like structure enables it to keep larger spaces among SnO₂ architectures for external dye adsorption and light absorption compared with regular spherical structures (Fig. S4, ESI†). In this case, the effective surface area of coral-like architectures is thus larger than that of spherical particles, leading to the enhancement of both internal and external dye adsorption and light absorption. Consequently, the photovoltaic property is improved as reflected in a higher solar energy conversion efficiency. In further study, no degradation of photocurrent and photovoltage of solar cells was observed during many measurements, and the reproducibility of the cell was also good. This indicates that the coral-like SnO₂ materials possess good stability of photovoltaic properties in organic and aqueous redox electrolytes under irradiation. It is significant to address the challenge for developing novel DSCs with long-term stability to effectively avoid the degradation of dye in traditional TiO₂-based solar cells.

Fig. 4 I–V Characteristics of DSCs based on coral-like SnO₂ architectures and spherical SnO₂ materials.

In summary, we report a unique coral-like porous SnO₂ hollow architecture prepared via a facile wet-chemical approach combined with an annealing process. A novel biomimetic swallowing growth mechanism for the formation of such a special structure is proposed. Furthermore, we found that DSCs based on coral-like SnO₂ exhibited an enhanced photovoltaic performance compared with that based on spherical architectures. The significant role of the radial coral-like structure is demonstrated as providing a larger effective surface area for dye adsorption, light absorption and utilization than for a spherical structure. These findings create new opportunities for designing functional materials with specific structures for applications.

This work was financially supported by the “973” State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2007CB936603 and 2009CB939902), the National Natural Science Foundation of China (10635070, 60604022 and 60574094), the Anhui Provincial Natural Science Foundation (090412036), and the Dean’s Fund from Hefei Institute of Physical Science, Chinese Academy of Sciences (0721H11141). Moreover, we are grateful to Dr Linhua Hu of Institute of Plasma Physics, Chinese Academy of Sciences for constructing dye-sensitized solar cells and carrying out solar energy conversion tests.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Structure of DSC; XRD and EDXRD patterns; BET isotherm and schematic illustration of light absorption. See DOI: 10.1039/b915650j

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