Hierarchical assembly of TiO2–SrTiO3 heterostructures on conductive SnO2 backbone nanobelts for enhanced photoelectrochemical and photocatalytic performance
Graphical abstract
Introduction
Semiconductor-based photosynthetic and catalytic reactions using a photoelectrochemical (PEC) cell have been considered as an ideal, renewable system for green technology, as they can provide the clean H2 fuel from water as well as the purification of polluted water [1], [2]. Since the PEC system was first demonstrated using TiO2 thin film in 1972 by Fujishima and Honda [3], extensive studies have focused on improving the efficiency of PEC devices [4], [5], [6], [7], [8]. During the last decade, the efficiency per unit area was rapidly enhanced by high integrated electrode designs from the development of nano-engineering technology [4], [5], [6], [7], [8], [9], [10], [11], enabling an increase in the number of reactive sites, control of the charge transfer/transport, and a reduction in the number of electron/hole recombinations [10], [12]. In recent years, three-dimensional (3D) nanostructures have attracted attention, as they can provide an efficient current path and a large contact area for the electrolyte [5], [6], [7], [8], [9], [10], [13]. Cho et al. synthesized a branched TiO2 nanorod structure with 0.49% efficiency [6], and Noh et al. reported Sn:In2O3(ITO)@TiO2 nanowire arrays that offer a large surface area and effective light-trapping characteristics [7]. Thus, 3D nanoarchitecturing is a fascinating route for efficient photoelectrochemical and photocatalytic performance.
Another important approach for enhancing the performance of a PEC cell is to construct a heterostructure composed of two different semiconductors. These structures are advantageous for retarding the recombination of photogenerated electron/hole pairs by promoting a charge carrier transfer at the interface of the semiconductors due to a cascade-type band alignment [14]. Moreover, the synergistic effect of two compounds can be achieved [15], [16]. A combination of strontium titanate (SrTiO3) and TiO2 has been regarded as a typical heterostructure, and this type has been widely studied in relation to photoelectrodes [14], [17], [18], [19], [20], [21], [22], [23], [24]. Titanium oxide (TiO2) has the advantages of high electron/hole mobility, strong photocorrosion resistance, a low cost, and non-toxicity [25]. However, the relatively low conduction band edge (ECBE = 0 V vs. RHE) of the material limits its use in direct solar-driven applications because external bias is required to both produce the hydrogen and decompose the pollutants. This problem can be resolved by a combination with SrTiO3, which has a conduction band edge that is 200 mV more negative than that of TiO2 [26], inducing the shift in the Fermi level to a more negative potential [14], [17], [18], [19], [20], [21], [22], [23], [24]. Also, SrTiO3 has good photocatalytic degradation ability for organic pollutants compared with TiO2 [27], [28], [29]. Therefore, the heterostructure can also provide enhanced photochemical and catalytic activity.
Hence, 3D heterostructures induced by the convergence of 3D nanoarchitecturing and heterojunctions is expected to contribute to the superior performance of PEC devices. Very recently, we synthesized the TiO2–Sb:SnO2 (ATO) rutile–rutile nanostructure, and recorded higher PEC efficiency as compared with other TiO2 based nanostructures such as nanowire, nanotube or 3D branched structure, etc. [30]. It is attributed to the fast charge transport by conductive path of 1D ATO nanobelts (NB), low interface resistance by epitaxial relationship between rutile TiO2 and rutile ATO, and high surface area by branched TiO2 NRs arrays. Moreover, when CdS quantum dots are decorated on this structure as a visible sensitizer, its photocurrent levels is relatively high in comparison with other CdS-sensitized structures [31]. It is due to the maximization of the geometrical and structural effects. Thus, it is expected that the heterostructure obtained from 3D ATO@TiO2 nanostructure could attain superior PEC performance and photocatalytic activity.
In this study, we demonstrate that 3D heterostructures enhance the photochemical and catalytic performance of a PEC device. As a proof-of-concept, we selected the TiO2-SrTiO3 heterostructure and designed a 3D hierarchical TiO2-SrTiO3 heterostructure (Fig. 1). The TiO2-based 3D nanostructure is synthesized by branching TiO2 nanorods (NRs) on a Sb:SnO2 (ATO) transparent conducting oxide (TCO) nanobelt (NB) array. Then, SrTiO3 nanoparticles are decorated onto the ATO@TiO2 nanostructure via a simple hydrothermal method. Therefore, ATO@TiO2–SrTiO3 3D heterostructures were successfully fabricated. Interestingly, in our synthetic approach, it is possible that small TiO2 NRs with length of 150 nm are only participated in the hydrothermal reaction of SrTiO3 with maintaining the 1D ATO backbone, inducing the well-distributed and hierarchically assembled SrTiO3 NPs on the ATO@TiO2, whereas TiO2 1D materials such as nanofiber, nanowires or nanotubes with length of over 1 μm had acted as roles of both the 1D backbone and the sacrificial template to fabricate the TiO2–SrTiO3 heterostructure in previously reports, inducing the random formation of SrTiO3 NPs on the 1D TiO2 materials, the irregular distribution and the particle size increase [14], [17], [18], [19], [20], [21], [22], [23], [32], [33]. As a result, the onset potential is increased by the shifting of the Fermi level and the photocurrent is enhanced by the blocking layer effect. Moreover, the photocatalytic degradation of methylene blue was increased in the ATO@TiO2–SrTiO3 3D heterostructure as compared with the ATO@TiO2 nanostructure.
Section snippets
Materials and synthesis
Sb:SnO2 NBs arrays are deposited onto an F-doped SnO2 glass substrate (FTO; TEC 8, Pilkington) by a thermal evaporation method [34]. Sn (99.5%, Samchun, Korea) and Sb (99.9%, High-Purity Chemicals, Japan) powders are mixed thoroughly by ball-milling for 24 h at a ratio of 3:1. The mixed powder is loaded onto a quartz boat and is inserted into a dual-zone horizontal tube furnace. A FTO glass substrate coated with an Au catalyst (3 nm thickness) is also placed in the tube furnace 20 cm from the
Material characterization
ATO NBs arrays grown vertically on the FTO substrate to a length of 10 μm are used to synthesize the ATO@TiO2 and ATO@TiO2–SrTiO3 (Fig. S1). Fig. 2 shows XRD graphs of the ATO@TiO2 nanostructure and ATO@TiO2–SrTiO3 heterostructures with a reaction time of 45 min to 2 h. Fig. 2 indicates that the ATO NBs and TiO2 NRs have rutile structures, whereas the SrTiO3 has a perovskite structure. The sample before the hydrothermal treatment exhibited XRD peaks corresponding to rutile SnO2 and TiO2; however,
Conclusion
Sb:SnO2@TiO2–SrTiO3 is synthesized for the first time as a conceptual design of a 3D heterostructure for enhanced photo-chemical and catalytic performances. The synthesis process provided an efficient charge-transport pathway (1D Sb:SnO2 nanobelts), a superior surface area (branched TiO2 NRs), and an enhanced reduction potential (heterojunction of SrTiO3). The Sb:SnO2@TiO2 nanostructure is converted to Sb:SnO2@TiO2–SrTiO3 heterostructure via the substitution of strontium in the TiO2 NRs through
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2012R1A2A2A01045382). This work was also supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Education, Science and Technology, Korea (0420-20110156).
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