Xiaoyang Pan, and Zhiguo Yi*

Introduction 

 Recent years have witnessed an ever-growing interest in fabrication of metal oxide semiconductor photocatalysts with desirable morphologies and distinct structures, due to their structure-dependent optical, electrical and catalytic properties.1-6 In particular, the synthesis of tin oxides (SnO2 , SnO, etc.) nanostructures for solar energy conversion has been gaining immense attention.7-14 It is reported that the construction of tin oxide with desirable architectural structure, like 1D nanorods, 2D nanosheets, or 3D hierarchical nanostructures, would be beneficial for high light-collection efficiency and fast charge carriers transport,15 which provides an effective strategy for improving the photoactivities of tin oxide.16-19 Unfortunately, the reported synthesis of tin oxide with tunable nanostructures often requires multi-step synthetic procedures involving the manipulation of a variety of experimental parameters, which is time-consuming and less productive.20-21 In addition to control the morphology of tin oxides, the introduction of graphene to fabricate tin oxide-graphene (SnGR) nanocomposites has also been demonstrated as an effective strategy to enhance their performance.22-24 However, despites numerous efforts on synthesis of tin oxide nanostructures, tin oxides in most of the Sn-GR hybrids are in the form of simple nanoparticles.25-30 Although several reports have successfully grown SnO2 nanorods/nanosheets onto GR substrate,31-34 the organic structure-directing agents are often required during the synthetic procedure, which can strongly adsorb onto the surface of SnO2 and block the surface active sites for photocatalytic reactions.35-36 Notably, as the mostly investigated tin oxide photocatalysts, SnO2 and SnO can only be activated by UV light irradiation, which accounts only 4% of the solar spectrum, due to their intrinsically wide bandgap.37-38 Therefore, band structure engineering strategy needs to be utilized for enhancing the visible light absorption of tin oxide photocatalysts. More importantly, considering that either design strategy alone leads to only limited improvement of the photocatalyst, it is highly desirable to develop Sn-GR nanostructures which integrate all of the above-mentioned design principles. However, it is still a significant challenge to achieve this goal by a simple and onestep process without organic species since simultaneously control of these factors generally requires organic structuredirecting agents, time-consuming synthetic steps and complex experimental parameters manipulation. Herein, we highlight a facile and one-step strategy by utilizing graphene oxide (GO) as multifunctional 2D scaffold to realize composition modulation, morphology control and band structure engineering of tin oxides without organic species. Our inspiration is derived from the following idea: As the most often used precursor for GR, GO with abundant oxygenated groups would provide multivalent interaction or even involve in the redox reaction with the metal ions precursors of semiconductors. Therefore, it is possible to use GO for tuning the relative amount of Sn2+ and Sn4+ ions in the resulting tin oxides through the redox reaction between Sn2+ and GO. Moreover, the composition modulation of tin oxide by GO would in turn engineer their energy band structures. Considering that the functional group on GO also plays a key role in nucleation and growth of nanomaterials, we could also utilize GO to control the morphology of tin oxides. As a result, a variety of visible-light-driven photocatalysts: Sn3O4 , binary Sn3O4-RGO, SnO2 (Sn2+ doping)/RGO and ternary Sn3O4 /SnO2 /RGO with diverse morphology and band structure are obtained by simply changing the GO concentration during synthesis.

Photoelectrochemical measurements: 

the photoelectrochemical analysis was carried out in a conventional threeelectrode cell. Ag/AgCl electrode was used as reference electrode and Pt electrode acted as the counter electrode. Fluoridetin oxide (FTO) glass was used to prepare the working electrode, which was firstly cleaned by ultrasound in ethanol for 30 min and dried at 80 ℃. The sample powder (10 mg) was ultrasonicated in 1 mL anhydrous ethanol to obtain evenly dispersed slurry. Then, the slurry was spreaded onto the FTO glass whose side part was protected in advance using Scotch tape. The working electrode was dried overnight under ambient conditions. A copper wire was connected to the side part of the working electrode using a conductive tape. Uncoated parts of the electrode were isolated with epoxy resin. The exposed area of the working electrode was 0.25 cm2 . The irradiation source was a 300 W Xe lamp (CEL-HXF300) system with UV-CUT filter (λ>420 nm). The photocurrent measurements and Mott-Schottky analyses were performed in a home made three electrode quartz cell with a CHI660D workstation. The electrolyte was 0.2 M aqueous Na2SO4 solution (pH=6.8) without additive. 

Photocatalytic Activities: 

for photocatalytic degradation methyl orange (MO), 30 mg photocatalyst was dispersed into 60 mL of MO solution (20 ppm) in a quartz vial. The resulting suspension was stirred in the dark for 1 h to ensure the establishment of an adsorption-desorption equilibrium between the sample and reactant. Then the reaction system was irradiated by a 300 W Xe lamp (CEL-HXF300) system with UV-CUT filter (λ>420 nm). As the reactions proceed, 3 mL of the suspension was taken at a certain time interval and was centrifuged to remove the catalyst. Afterward, the residual amount of MO in the solution was analyzed on the basis of its characteristic optical absorption at 470 nm, using a UV/Vis/NIR sepectrophotometer (Perkin Elmer Lambada 900) to measure the change of MO concentration with irradiation time based on Lambert-Beer’s law. The percentage of degradation is denoted as C/C0 . Here C is the absorption of MO solution at each irradiation time interval of the main peak of the absorption spectrum, and C0 is the absorption of the initial concentration when the absorption-desorption equilibrium was achieved.

Conclusion 

 In conclusion, graphene oxide (GO) is utilize as structure directing agent to control the composition, morphology and band structure of tin oxide-reduced graphene oxide (Sn-RGO) nanocomposites while neither organic surfactants nor organic solvents has been used. By adjusting the addition amount of GO during synthesis, tin oxide nanostructure in Sn-RGO evolves from hierarchical Sn3O4 nanosheets arrays to ultrafine Sn2+ self-doped SnO2 nanoparticles. The Sn-RGO nanocomposites with narrowed band gap and tunable energy band structure show efficient photocatalytic activities for MO degradation under visible light irradiation. It is known that tremendous efforts have been devoted to synthesizing graphene-based nanocomposites for diverse applications. In most cases, graphene oxide is used as precursor for graphene (GR). Notably, during the synthesis of such composite materials, GO is most often introduced merely for the sake of GR without putting it into full play with elaborate design and is unable to utilize the structure advantage of GO. Our work demonstrated in this paper, provides valuable guidance for further design and synthesis graphene-based photofunctional materials.