Graphene Oxide Regulated Tin Oxide Nanostructures: EngineeringComposition, Morphology, Band Structure, and PhotocatalyticPropertiesXiaoyang Pan and Zhiguo Yi*

Key Laboratory of Design and Assembly of Functional Nanostructures & Fujian Provincial Key Laboratory of Nanomaterials, FujianInstitute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China

*S Supporting Information

ABSTRACT: A facile, one-step hydrothermal method has been developed tofabricate tin oxide−reduced graphene oxide (Sn−RGO) nanocomposites withtunable composition, morphology, and energy band structure by utilizinggraphene oxide (GO) as a multifunctional two-dimensional scaffold. By adjustingthe GO concentration during synthesis, a variety of tin oxide nanomaterials withdiverse composition and morphology are obtained. Simultaneously, the varyingof GO concentration can also narrow the bandgap and tune the band edgepositions of the Sn−RGO nanocomposites. As a result, the Sn−RGOnanocomposites with controllable composition, morphology, and energy bandstructure are obtained, which exhibit efficient photoactivities toward methylorange (MO) degradation under visible-light irradiation. It is expected that ourwork would point to the new possibility of using GO for directing synthesis ofsemiconductor nanomaterials with tailored structure and physicochemicalproperties.

KEYWORDS: graphene oxide, tin oxide, visible-light photocatalysis, morphology control, composition modulation

1. INTRODUCTION

Recent years have witnessed an ever-growing interest infabrication of metal oxide semiconductor photocatalysts withdesirable morphologies and distinct structures, due to theirstructure-dependent optical, electrical, and catalytic proper-ties.1−6 In particular, the synthesis of tin oxide (SnO2, SnO, andso on) nanostructures for solar energy conversion has beengaining immense attention.7−14 It is reported that theconstruction of tin oxide with desirable architectural structure,such as one-dimensional (1D) nanorods, 2D nanosheets, or 3Dhierarchical nanostructures, would be beneficial for high light-collection efficiency and fast charge carriers transport,15 whichprovides an effective strategy for improving the photoactivitiesof tin oxide.16−19 Unfortunately, the reported synthesis of tinoxide with tunable nanostructures often requires multistepsynthetic procedures involving the manipulation of a variety ofexperimental parameters, which is time-consuming and lessproductive.20,21

In addition to controlling the morphology of tin oxides, theintroduction of graphene to fabricate tin oxide−graphene (Sn−GR) nanocomposites has also been demonstrated as aneffective strategy to enhance their performance.22−24 However,despite numerous efforts on synthesis of tin oxide nanostruc-tures, tin oxides in most of the Sn−GR hybrids are in the formof simple nanoparticles.25−30 Although several reports havesuccessfully grown SnO2 nanorods/nanosheets onto GRsubstrate,31−34 the organic structure-directing agents are often

required during the synthetic procedure, which can stronglyadsorb onto the surface of SnO2 and block the surface activesites 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 for only 4% of the solar spectrum, due to theirintrinsically wide bandgap.37,38 Therefore, band structureengineering strategy needs to be utilized for enhancing thevisible-light absorption of tin oxide photocatalysts. Moreimportantly, considering that design strategy alone leads toonly limited improvement of the photocatalyst, it is highlydesirable to develop Sn−GR nanostructures which integrate allof the aforementioned design principles. However, it is still asignificant challenge to achieve this goal by a simple and one-step process without organic species since simultaneouslycontrol of these factors generally requires organic structure-directing agents, time-consuming synthetic steps, and complexexperimental parameters manipulation.Herein, we highlight a facile and one-step strategy by utilizing

graphene oxide (GO) as a multifunctional 2D scaffold to realizecomposition modulation, morphology control, and bandstructure engineering of tin oxides without organic species.Our inspiration is derived from the following idea: As the most

Received: August 23, 2015Accepted: November 18, 2015Published: November 18, 2015

Research Article

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© 2015 American Chemical Society 27167 DOI: 10.1021/acsami.5b07858ACS Appl. Mater. Interfaces 2015, 7, 27167−27175

often used precursor for GR, GO with abundant oxygenatedgroups would provide multivalent interaction or even involve inthe redox reaction with the metal ions precursors ofsemiconductors. Therefore, it is possible to use GO for tuningthe relative amount of Sn2+ and Sn4+ ions in the resulting tinoxides through the redox reaction between Sn2+ and GO.Moreover, the composition modulation of tin oxide by GOwould in turn engineer their energy band structures.Considering that the functional group on GO also plays akey role in nucleation and growth of nanomaterials, we couldalso utilize GO to control the morphology of tin oxides. As aresult, a variety of visible-light-driven photocatalysts: Sn3O4,binary Sn3O4−RGO, SnO2 (Sn2+ doping)/RGO and ternarySn3O4/SnO2/RGO with diverse morphology and bandstructure are obtained by simply changing the GO concen-tration during synthesis.

2. EXPERIMENTAL SECTIONMaterials. Tin(II) fluoride (SnF2), tin fluoride (SnF4), tin(II)

chloride dehydrate (SnCl2), anhydrous ethanol (EtOH), phosphoruspentoxide (P2O5), hydrochloric acid (HCl), graphite powder,potassium persulfate (K2S2O8), hydrogen peroxide (30%H2O2),potassium permanganate (KMnO4), methyl orange (MO), sodiumfluoride (NaF), potassium dichromate (K2Cr2O7), benzyl alcohol(C6H5CH2OH), benzyl aldehyde (C6H5CHO), benzoic acid(C6H5COOH), and sodium sulfate (Na2SO4) were purchased fromSinopharm Chemical Regent Co., Ltd. (Shanghai, China). Deionizedwater was supplied from local sources. All of the materials were used asreceived without further purification.Synthesis. Graphene oxide was synthesized from graphite powder

according to the modified Hummers method (for details see theSupporting Information).39 The synthesis of the Sn−RGO nano-composites was carried out as follows: 0.9 g of SnF2 powder wasdissolved in 80 mL of distilled water. After stirring for 40 min, a givenamount of GO was dispersed in the SnF2 aqueous solution. Theresulting suspension was stirred for 1 h and then transferred to a 100mL stainless steel autoclave and kept at 453 K for 24 h in an electricoven. The products were filtered and washed with distilled water forseveral times and then dried in an oven at 353 K overnight. The blanktin oxide was synthesized using the same procedure for Sn−RGOnanocomposites without adding GO scaffold.Characterization. the crystal structures of the as-prepared samples

were analyzed on a Rugaku Miniflex II X-ray diffractometer with CuKα radiation. The optical properties of the samples were characterizedby a Cary 500 UV−visible ultraviolet/visible diffuse reflectancespectrophotometer (DRS), during which BaSO4 was employed asthe internal reflectance standard. A field-emission scanning electronmicroscope (JSM-6700F) and transmission electron microscope(JEM-2010, FEI, Tecnai G2 F20 FEG TEM) were used to determinethe morphology and microscopic structure of the as-synthesizedsamples. X-ray photoelectron spectroscopy (XPS) measurements wereperformed using a Thermo Scientific ESCA Lab250 spectrometerwhich consists of a monochromatic Al Kα as the X-ray source, ahemispherical analyzer, and sample stage with multiaxial adjustabilityto obtain the composition on the surface of samples. All of the bindingenergies were calibrated by the C 1s peak of the surface adventitiouscarbon at 284.6 eV. Photoluminescence analysis was carried out with aVarian Cary Eclipse spectrometer at an excitation wavelength of 325nm.Photo-electrochemical Measurements. The photo-electro-

chemical analysis was carried out in a conventional three-electrodecell. A Ag/AgCl electrode was used as reference electrode and Ptelectrode acted as the counter electrode. Fluoride−tin oxide (FTO)glass was used to prepare the working electrode, which was firstcleaned by ultrasound in ethanol for 30 min and dried at 80 °C. Thesample powder (10 mg) was ultrasonicated in 1 mL of anhydrousethanol to obtain an evenly dispersed slurry. Then, the slurry wasspreaded onto the FTO glass whose side part was protected in advance

using Scotch tape. The working electrode was dried overnight underambient conditions. A copper wire was connected to the side part ofthe working electrode using a conductive tape. Uncoated parts of theelectrode were isolated with epoxy resin. The exposed area of theworking electrode was 0.25 cm2. The irradiation source was a 300 WXe lamp (CEL-HXF300) system with UV-CUT filter (λ > 420 nm).The photocurrent measurements and Mott−Schottky analyses wereperformed in a homemade three-electrode quartz cell with a CHI660Dworkstation. The electrolyte was 0.2 M aqueous Na2SO4 solution (pH= 6.8) without additive.

Photocatalytic Activities. For photocatalytic degradation methylorange (MO), 30 mg of photocatalyst was dispersed into 60 mL ofMO solution (20 ppm) in a quartz vial. The resulting suspension wasstirred in the dark for 1 h to ensure the establishment of anadsorption−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 reactionsproceed, 3 mL of the suspension was taken at a certain time intervaland was centrifuged to remove the catalyst. Afterward, the residualamount of MO in the solution was analyzed on the basis of itscharacteristic optical absorption at 470 nm, using a UV/vis/near-IRspectrophotometer (PerkinElmer Lambada 900) to measure thechange of MO concentration with irradiation time based on Lambert−Beer’s law. The percentage of degradation is denoted as C/C0. Here Cis the absorption of MO solution at each irradiation time interval ofthe main peak of the absorption spectrum, and C0 is the absorption ofthe initial concentration when the absorption−desorption equilibriumwas achieved.

3. RESULTS AND DISCUSSION

The synthesis of the tin oxide and tin oxide reduced grapheneoxide nanocomposites (Sn−RGO) is achieved by a facile one-step hydrothermal method using SnF2 and GO40 (SupportingInformation Figures S1−S3) as precursors. As shown in Figure1a, blank tin oxide synthesized from hydrothermal treatment of

Figure 1. XRD patterns (a) and structure cartoon (b) of the Sn3O4and tin oxide reduced graphene oxide nanocomposites (Sn−RGO-x%). Here x denotes the weight ratio of RGO.

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SnF2 solution is mixed-valence Sn3O4. All of the diffractionpeaks match the standard pattern of a triclinic Sn3O4 structure(JCPDS 16-0737).41 The Sn−RGO nanocomposites with lowRGO contents (1, 2, and 5%) exhibit XRD patterns similar tothat of blank Sn3O4 (Supporting Information Figure S4a).When the RGO content increased to 10% (Sn−RGO-10%),some new diffraction peaks corresponding to SnO2 emergedand dominated the XRD patterns (Supporting InformationFigure S4b). When RGO content reached 20%, all of thediffraction peaks of Sn−RGO nanocomposite (Sn−RGO-20%)could be assigned to the rutile SnO2.Raman spectroscopy analysis further confirms the phase

transformation from Sn3O4 to SnO2 in the presence of GO. Asshown in Supporting Information Figure S5, Sn3O4 features theRaman peaks at 143 and 170 cm−1,41 RGO features the Ramanpeaks at 1344 and 1596 cm−1,42−44 whereas SnO2 features theRaman peaks at 600 cm−1.41 The evolution of Raman modes ofthe Sn−RGO composites have clearly shown that theintroduction of GO can modulate the composition of tinoxide to obtain (Figure 1b): Sn3O4, binary Sn3O4−RGO(Sn−RGO-1%, Sn−RGO-2%, and Sn-RGO-5%), ternarySn3O4/SnO2/RGO (Sn−RGO-10%), and binary SnO2/RGO(Sn−RGO-20%).Accompanying the changes of composition and structure of

the tin oxides, morphology of the as-obtained Sn−RGOnanocomposites is modulated as well. The blank Sn3O4 shows aflower-like morphology composed of Sn3O4 nanosheets, asshown in Figure 2a. Sn−RGO-1% and Sn−RGO-2% sampleshave hierarchical structures, in which Sn3O4 nanosheets arevertically grown on the RGO surface (Figure 2b,c). For theSn−RGO-5%, both Sn3O4 nanosheets and nanoparticles coexiston the surface of RGO (Figure 2d), while for Sn−RGO-10%and Sn−RGO-20%, the RGO surface is densely covered bymostly SnO2 nanoparticles (Figure 2e,f). These results indicatethat the introduction of GO can tune the growth and assemblyof tin oxide nanosheets and the morphology of the tin oxidecan be further tuned by varying the addition amount of GO.To further identify the microscopic morphology and

structure of the Sn−RGO nanocomposite, Sn−RGO-2%sample is investigated by TEM analysis. As shown in Figure3a, Sn3O4 nanosheets are mostly grown upright with a random

orientation on the RGO support, which is consistent with theaforementioned SEM observation. The thickness of the Sn3O4nanosheets is determined to be ca. 9 nm. The RGO layers canalso be clearly identified in the edge area of Sn−RGOnanostructures (Figure 3a). The high resolution TEM(HRTEM) image in Figure 3b displays a single Sn3O4nanosheet with distinct lattice fringes of 0.334 nm, which isattributed to the (111) plane of the Sn3O4.

41 Thecorresponding EDX spectrum (Figure 3c) of Sn−RGO-2%confirms the element composition of C, O, and Sn.X-ray photoelectron spectroscopy (XPS) is utilized to

investigate the chemical states of Sn−RGO nanocomposite.The survey spectrum of Sn−RGO-2% shown in SupportingInformation Figure S6a further confirms the existence of Sn, O,and C. The high resolution spectrum of C 1s (Supporting

Figure 2. SEM images of the Sn3O4 (a). Tin oxide reduced graphene oxide nanocomposites (Sn−RGO-x%): Sn−RGO-1% (b), Sn−RGO-2% (c),Sn−RGO-5% (d), Sn−RGO-10% (e), and Sn−RGO-20% (f). Here x denotes the weight ratio of RGO.

Figure 3. TEM (a) and HRTEM (b) images, EDX spectrum (c), andXPS spectrum (d) of Sn−RGO-2%.

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Information Figure S6b) reveals that the oxygenation specieson the GO are remarkably reduced in comparison to theoriginal GO (Supporting Information Figure S7). This resultindicates that the original GO is effectively converted toreduced GO (RGO) during the synthesis of the Sn−RGOnanocomposites. The two characteristic peaks at 530.4 and531.9 eV shown in the O 1s spectra can be assigned to the Sn−O and hydroxyl species, respectively (Supporting InformationFigure S6c). The prominent peak of Sn 3d5/2 shown in Figure3d can be deconvoluted into two subpeaks centered at 486.3and 487.0 eV, which is assigned to the Sn2+ and Sn4+,respectively.41

Figure 4a shows the UV−vis diffuse reflectance spectra(UV−vis DRS) of the samples. It is shown that the Sn3O4exhibits obvious visible-light absorption with a band gap of 2.75eV (Figure 4b). As compared to Sn3O4, Sn−RGO nano-composites show obviously enhanced visible-light absorption inthe range of 500−800 nm, which can be attributed to intrinsicabsorption of black colored graphene.45−47 The band gaps ofthe Sn−RGO samples are determined based on the Tauc plotsshown in Supporting Information Figure S8. The results showthat the RGO decoration narrows the band gap of the tin oxide.This can be attributed to the electronic interaction between tinoxide and RGO, which is also observed on other metal oxides−RGO nanocomposites.48−50

By using the Mott−Schottky analysis (Figure 4c andSupporting Information Figure S9), the band edge positionsof the samples are determined (Figure 4d and SupportingInformation Table S1). For Sn3O4, its conduction bandminimum is determined to be −0.94 V vs NHE and thecorresponding valence band maximum is calculated to be +1.81V vs NHE. With the increase of RGO weight content, a positiveshift of the conduction band of Sn3O4 in the Sn3O4−RGOnanocomposites (Sn−RGO-1%, -2%, and -5%) is realized. It isreported that Sn2+ cations in Sn3O4 and SnO are responsible for

more negative conduction bands than that of SnO2.51

Therefore, the positive shift of the Sn3O4 conduction bandcan be explained by the decrease of Sn2+ cations in the Sn−RGO nanocomposites, which is caused by the redox reactionbetween Sn2+ and GO. As compared to the Sn−RGOnanocomposites with relatively low RGO content, Sn−RGO-10% and Sn−RGO-20%, in which the primary component isSnO2, possess more positive conduction band minima. This canbe ascribed to the more positive conduction band of SnO2

when compared to Sn3O4.51 These results indicate that the

introduction of GO can effectively engineer the energy bandstructure (band gap narrowing and band edge position tuning)of tin oxide in the resultant Sn−RGO nanocomposites.Notably, although SnO2 is a wide band gap semiconductor

(3.4 eV), the band gap of SnO2 in Sn−RGO-20% is narrowedto 2.69 eV (Supporting Information Figure S8e). The band gapnarrowing of SnO2 may be attributed to two possible reasons:one is the result of electronic interaction between RGO andSnO2.

50 The other is band gap narrowing of SnO2 by Sn2+

doping,7 since Sn2+ is used as a precursor, and not all of themare oxidized to Sn4+. To examine what reason is accounted forthe band gap narrowing of SnO2, we have then prepared SnO2

and SnO2−RGO-20% samples (Supporting Information FigureS10) using SnF4 and GO as precursors with the sameprocedures for preparing the Sn−RGO nanocomposites. TheUV−vis DRS analysis (Supporting Information Figure S11)indicates that the band gap narrowing of SnO2 in Sn−RGO-20% is mainly ascribed to aliovalent Sn2+ doping. This is furtherconfirmed by XPS analysis (Supporting Information FigureS12). It is worth noting that the ion radius of Sn2+ (0.62 Å) issimilar to that of Sn4+ (0.69 Å).38 Therefore, Sn2+ doping in theSnO2 nanostructure is more favorable on preserving theintrinsic crystal structure and thus decreasing the chargerecombination centers.

Figure 4. UV−vis diffuse reflectance spectra (DRS) (a), Tauc plots of Sn3O4 (b), Mott−Schottky plot of Sn3O4 (c), and band edge positions (d) ofthe Sn3O4 and tin oxide reduced graphene oxide nanocomposites (Sn−RGO-x%). Here x denotes the weight ratio of RGO.

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From the preceding results, it is clearly seen that the additionof GO during the synthetic procedure has a significant effect onengineering the composition, morphology, and energy bandstructure of the tin oxides. To clarify the underlyingmechanism, a variety of controlled experiments have beencarried out. First, the mechanism of composition modulation byGO is investigated. From the aforementioned results, we canconclude that the phase transformation from Sn3O4 to SnO2occurred in the presence of GO, indicating that GO additionwould facilitate the oxidation of Sn2+ into Sn4+ during thesynthesizing procedure. This can be explained by the redoxreaction between Sn2+ and the oxygenated groups on GO, inwhich the Sn2+ is oxidized to Sn4+ while the oxygenated groupson GO are consumed.26,27 To further examine this point,graphene (GR) with almost no oxygenated functional groupshas been used to fabricate tin oxide−graphene nanocomposite(Sn−GR-20%), it can be seen that the Sn−GR-20% possessesthe same XRD patterns as the blank Sn3O4, indicating that GRwithout oxygenated groups cannot modulate the compositionof tin oxide (Supporting Information Figure S13).In addition, it should be noted that the dissolved oxygen can

also oxidize Sn2+ to Sn4+ during the synthesis.38 As a result, inthe absence of GO, part of the Sn2+ ions are oxidized bydissolved oxygen and thus a mixed-valence Sn3O4 is obtained.In the presence of GO, the dissolved oxygen and GO bothcontribute to the oxidation of the Sn2+ and thus most of theSn2+ are oxidized to Sn4+. Thus, the composition modulation oftin oxide by GO can be explained by the following equations:

+ + → + ↑3SnF 3H O 12O Sn O 6HF2 2 2 3 4 (1)

+ + + → + + ↑SnF GO H O 12O SnO RGO 2HF2 2 2 2

(2)

It should be noted that a small amount of GO is insufficientto change the bulk composition of tin oxide (Sn−RGO-1%,-2%, and -5%). Therefore, Sn−RGO nanocompsites (Sn−RGO-10% and -20%) with SnO2 as primary component canonly be obtained at higher GO concentration. Moreover, theinvolvement of F− is also important. If the raw material SnF2 isreplaced by SnCl2, only SnO2−RGO composite materials canbe obtained.26,27,52 This is because when SnCl2 is used asprecursor, the Sn2+ will be oxidized easily to Sn4+ and thenhydrolyzed to form SnO2. In contrast, the F− ions in SnF2 canretard the oxidation of Sn2+ and thus Sn3O4 is obtained duringhydrothermal treatment.41 With the addition of GO, theoxygenated groups on it endow GO with oxidizing capacity tofacilitate the oxidation of Sn2+. Therefore, the combination ofGO and F− ions have enabled flexible control of tin oxidecomposition.To investigate the role of the GO on controlling the

morphology of tin oxide nanostructures, the formation processof the Sn−RGO nanocomposites has been further studied bytime-dependent experiments. As displayed in SupportingInformation Figure S14a, the samples with time duration of 1h are RGO sheets with small nanoparticles on their surface.With the reaction proceeding to 6 h (Supporting InformationFigure S14b), a large amount of small particles are decorated onthe surface of the RGO sheets while a small number of tin oxidenanosheets emerge. With a longer reaction time of 12 h, thesurface of the RGO substrate is almost covered by the tin oxidenanosheets while tin oxide nanoparticles can still be observedon the surface of RGO (Supporting Information Figure S14c).

After reaction for 24 h, the RGO surface is completely coveredby the tin oxide nanosheets (Supporting Information FigureS14d).Furthermore, the formation mechanism of the tin oxide

nanosheets is also investigated. As shown in SupportingInformation Figure S15, the result that only irregularnanoparticles are obtained when SnCl2 is used as precursorsuggests that the F− ions derived from SnF2 precursor play animportant role on directing the growth of 2D sheet-likemorphology. F− ions would favorably interaction with the tinoxide surface and thus facilitate the formation of the 2Dstructure with specific exposed facets.38

Based on the aforementioned results, the growth mechanismof tin oxide nanosheets on the RGO surface at low RGOcontent is proposed as follows: At the initial stage, Sn2+ ionstend to adsorb on the negative charged GO surface (−38 mV,Supporting Information Figure S16) while F− ions withnegative charge could not access the GO surface due toelectrostatic repulsion.53 Therefore, only particle-like tin oxideis formed on the surface of GO at the initial stage (Scheme 1).

During this process, the Sn2+ ions could react with theoxygenated functional groups on GO and result in the removalof these functional groups on GO. Since the negative charge ofGO is derived from these functional groups;54 the removal ofthese groups and the adsorption of Sn2+ ions on it would resultin a positively charged RGO surface, which would favor theadsorption of F− ions. As a result, the residual Sn2+ and F− ionsin the solution would subsequently interact with the tin oxidenanocrystals on RGO and grow into 2D nanostructure throughsurface fluorine passivation.Considering that the morphology of tin oxide is also

influenced by the pH values,8 the possibility of GO onchanging the pH values of the reaction solution is alsoexamined. For the SnF2 aqueous solution, the pH value isdetermined to be 3.00, owing to the hydrolysis of Sn2+. Withthe addition of GO, the pH value of the solution is almostconstant (2.97). Therefore, the morphology evolution of tinoxide from nanosheets to nanoparticles in the presence of GOcan be explained as follows (Supporting Information SchemeS1): With the increase of the GO concentration in the reaction

Scheme 1. Schematic Illustration of the Growth Mechanismof Tin Oxide Nanosheets on RGO Surface at Low GOConcentration: (a) Sn2+ Ions Adsorbing onto the NegativelyCharged GO Surface; (b) Growth of Tin OxideNanoparticles on the Surface of GO and Consumption ofOxygenation Functional Groups on GO; (c−e)Heterogeneous Nucleation and Growth of Tin OxideNanosheets

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solution, more Sn2+ ions are adsorbed on the surface of the GO,which cannot be accessed by the F− ions, and consequently lessor almost no Sn2+ ions are present in the solution. Theadsorbed Sn2+ ions on GO are then involved in the redoxreaction with GO and hydrolyzed under hydrothermalcondition, which leads to the formation of tin oxidenanoparticles on the surface of RGO. Due to a shortage ofthe reaction precursors (Sn2+ ions) in solution, the growth oftin oxide nanosheets on the RGO substrate is thus restrained.The morphological transformation is accompanied by corre-sponding changes in the composition of tin oxide, i.e., theoxidation of Sn2+ and the reduction of GO. As a result, the tinoxide nanoparticles−reduced graphene oxide nanocompositesare obtained at higher GO addition.As shown in Supporting Information Figure S7, there are

three main types of oxygenated functional groups on the GOsurface: hydroxyl group, aldehyde groups, and carboxyl groups.To understand which group is more active for structureregulation, GO is replaced by organic molecules with differentoxygenated groups (benzyl alcohol, benzyl aldehyde, andbenzoic acid). Supporting Information Figures S17 and S18show that morphology control and composition modulationcan only be realized in the presence of benzoic acid, suggestingthat the carboxyl groups are more active for structureregulation.From the preceding discussion, we can conclude that the GO

addition during synthesis can effectively tune the composition,morphology, and band structure of tin oxide. Meanwhile, theinsulating GO can be converted to reduced GO (RGO)possessing electronic conductivity. The resultant composite willfavor the transfer of the electrons photogenerated from tinoxide to the RGO surface and thus promote chargeseparation,55−57 which can be evidenced by the photo-luminescence (PL) analysis. As shown in Figure 5a, blankSn3O4 exhibits a broad PL emission centered at ca. 450 nm,which can be attributed to the charge recombination near the

band gap. In contrast, Sn−RGO-2% exhibits almost no PLemission, indicating more efficient charge separation over Sn−RGO-2%.39

Photoelectrochemical analysis is further performed to provethe role of RGO on promoting the charge separation over theSn−RGO nanocomposites. As shown in Figure 5b, under thevisible-light irradiation, the photocurrent of the Sn−RGO-2% ismuch higher than that of Sn3O4, indicating more efficientcharge separation over Sn−RGO-2% than Sn3O4.

58,59 Thedecreased photocurrent of Sn−RGO-2% with the increase ofthe irradiation time is attributed to the fact that no sacrificialagent for photogenerated hole is used in the photo-electrochemical experiments. Under the irradiation of visiblelight, the electrons photogenerated from tin oxide aretransferred to the Pt electrode, and the photogenerated holesare accumulated in the valence bands of tin oxides since theycan hardly be scavenged by the water molecule (please notezero bias voltage is applied in the experiment). Theaccumulation of holes will result in the decrease of photo-current with time. The enhanced charge carrier transfer can alsobe evidenced by electrochemical impedance spectroscopy (EIS)analysis. As compared to Sn3O4, the smaller semicircle (Figure5c) at high frequencies for the Sn−RGO-2% suggests a fastercharge−carrier transfer rate in Sn−RGO-2%.58As a result, the Sn−RGO nanocomposites with tunable

composition, morphology, and energy band positions as well asefficient charge separation may act as promising candidates forsolar energy conversion. As a proof-of-concept, we takephotocatalytic methyl orange (MO) degradation as a typicalexample to demonstrate the application of Sn−RGO nano-compsites. As shown in Figure 5d and Supporting InformationFigure S19, the Sn−RGO nanocomposites with low RGOcontent (1%, 2%, and 5%) exhibit obviously enhancedphotocatalytic performance as compared to that of Sn3O4under visible-light irradiation (λ > 420 nm). In particular, theSn−RGO-2% nanocomposite shows the best photoactivity,

Figure 5. Photoluminescence spectra (a) of the Sn3O4 and Sn−RGO-2% nanocomposite; transient photocurrent response (b) and electrochemicalimpedance spectroscopy (EIS) Nyquist plots (c) of the sample electrodes of the Sn3O4 and Sn−RGO-2% nanocomposite under visible-lightirradiation (λ > 420 nm); photocatalytic degradation (d) of methyl orange (MO) by Sn3O4 and tin oxide reduced graphene oxide nanocomposites(Sn−RGO-x%) under visible-light irradiation (λ > 420 nm). Here x denotes the weight ratio of RGO.

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which can completely decompose MO within 4 min (Figure 5dand Supporting Information Figure S20).Since the bulk compositions of Sn−RGO-10% and Sn−

RGO-20% (SnO2 as the primary component) are different fromthe other samples, their photoactivities are compared to theSnO2−RGO composites prepared using SnF4 and GO asprecursors (Supporting Information Figures S10 and S11). Asshown in Supporting Information Figure S21, the Sn−RGO-20% with Sn2+ doping shows much higher activity than that ofSnO2−RGO-20% without Sn2+ doping toward MO degradationunder visible-light irradiation. These results show unambigu-ously the dominant role of compositional modulation on thephotoactivity. The fact that Sn−RGO-20% shows lowerphotoactivity than Sn−RGO-10% (Supporting InformationFigure S21) can be attributed to two reasons: One is thedecreased concentration of tin oxide photoactive component aswell as the increased light shielding effect of black RGO.39,60

The other involves synergistic effect between the mixed tinoxide in Sn−RGO-10% considering that it is comprised ofSn3O4 and SnO2 with matched energy band structure(Supporting Information Figure S22). The photogeneratedholes of SnO2 would transfer to the valence band of Sn3O4while the photoexcited electrons of Sn3O4 may also transfer tothe conduction band of SnO2.

51 As a result, the chargeseparation efficiency is promoted and thus Sn−RGO-10%shows higher photoactivity than that of Sn−RGO-20%.To investigate the morphology effect on the photoactivity of

tin oxide, we also prepared SnO2 nanosheets (SnO2−NS) usingSnCl2 and NaF as precursors (Supporting Information FigureS23a) and prepared SnO2 nanoparticles (SnO2−NPs) usingSnCl2 as precursor (Supporting Information Figure S15). Allthe samples possess a rutile SnO2 phase (SupportingInformation Figure S23b) and exhibit identical UV−vis DRSspectra (Supporting Information Figure S23c), implying a samecontent of Sn2+ doping. Therefore, the distinct photoactivity(Supporting Information Figure S23d) implies the SnO2nanosheets with specific facets exposure are more active forphotocatalytic reaction than that of SnO2 nanoparticles.

61

The durability of the catalyst is also investigated. As shown inSupporting Information Figure S24a, from the first to secondrun there is an obvious decrease of photoactivity; however, theactivity remains stable from the second to the fifth run. Toclarify the possible reasons, a variety of characterizations havebeen employed. We first examined the crystal structure of thesamples after the photoreaction. As shown in SupportingInformation Figure S24b, the crystal structure and crystallinityof tin oxide before and after the reaction remain unchanged.The Raman spectroscopy analysis further reveals no obviousdegradation of RGO (Supporting Information FigureS24c).62,63 However, an apparent morphology change of thesample is found after the photocatalytic reaction (SupportingInformation Figure S24d). That is, part of the tin oxidenanosheets peeled off from the RGO substrate after thephotoreaction. Since the interfacial interaction between tinoxide and RGO is a prerequisite for high efficient chargecarriers’ separation,42 the exfoliation of the tin oxide from RGOsurface would account for the decrease of the photoactivity.To determine the quantum efficiency of the Sn−RGO

nanocomposite, we perform a photoreduction test of Cr(VI)upon the Sn−RGO-2% sample under visible-light irradiation.As shown in Supporting Information Figure S25, the Sn−RGO-2% sample shows obviously enhanced visible-light photoactivity

for Cr(VI) reduction and has a quantum efficiency of 2.04% at420 nm (Supporting Information Figure S26).64

4. CONCLUSIONIn conclusion, graphene oxide (GO) is utilized as a structure-directing agent to control the composition, morphology, andband structure of tin oxide−reduced graphene oxide (Sn−RGO) nanocomposites while neither organic surfactants nororganic solvents have been used. By adjusting the additionamount of GO during synthesis, tin oxide nanostructure in Sn−RGO evolves from hierarchical Sn3O4 nanosheet arrays toultrafine Sn2+ self-doped SnO2 nanoparticles. The Sn−RGOnanocomposites with narrowed band gap and tunable energyband structure show efficient photocatalytic activities for MOdegradation under visible-light irradiation.It is known that tremendous efforts have been devoted to

synthesizing graphene-based nanocomposites for diverseapplications. In most cases, graphene oxide is used as precursorfor graphene (GR). Notably, during the synthesis of suchcomposite materials, GO is most often introduced merely forthe sake of GR without putting it into full play with elaboratedesign and is unable to utilize the structure advantage of GO.This work provides valuable guidance for further design andsynthesis of graphene-based photofunctional materials.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.5b07858.

XRD, SEM, and TEM images of graphene oxide,additional XRD patterns, UV−vis diffuse reflectancespectra (DRS), XPS spectra, Tauc plots, Mott−Schottkyplots, the energy band gap and conduction band andvalence band positions, and additional SEM and TEMimages of the samples, a scheme of the morphologyevolution mechanism, and photocatalytic performance ofthe samples. (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: zhiguo@fjirsm.ac.cn.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by the National KeyProject on Basic Research (Grant No. 2013CB933203), theNatural Science Foundation of China (Grant Nos. 21373224and 21577143), the Natural Science Foundation of FujianProvince (Grant Nos. 2014H0054 and 2015J05044), and theOne Hundred Talents Program of the Chinese Academy ofSciences.

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