Graphene-Templated Bottom-up Fabrication of Ultralarge Binary CdS

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Graphene-Templated Bottom-Up Fabrication of Ultralarge Binary CdS-TiO Nanosheets for Photocatalytic Selective Reduction 2

Xiaoyang Pan, and Yi-Jun Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512797t • Publication Date (Web): 10 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Graphene-Templated Bottom-Up Fabrication of Ultralarge Binary CdS-TiO2 Nanosheets for Photocatalytic Selective Reduction Xiaoyang Pan,†,‡ and Yi-Jun Xu†,‡,* †

State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry,

Fuzhou University, Fuzhou, 350002, P.R. China ‡

College of Chemistry, New Campus, Fuzhou University, Fuzhou, 350108, P. R. China

* Corresponding author. E-mail: [email protected]

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ABSTRACT

A bottom-up strategy based on graphene (GR) template is developed to synthesize free-stranding TiO2 nanosheets. By functionalizing the GR surface with benzyl alcohol, a cross-link of GR nanosheets and a homogeneous coating of TiO2 onto the self-assembled GR surfaces are simultaneously achieved. Followed by thermal treatment in air, the two dimensional (2D) TiO2 structure with ultralarge lateral size far beyond the size of original GR nanosheets (several hundred nanometers) has been built with GR as a sacrificial template. The resultant TiO2 nanosheets (TiO2-NS) are then homogeneously photo-deposited with CdS nanocrystals, and thus the CdS-TiO2 composite nanosheets (CdS-TiO2-NS) are obtained. Using simulated solar light as energy source, the CdS-TiO2-NS exhibits much higher activity than does bare TiO2-NS toward selective gas-phase reduction of CO2 and liquid-phase reduction of nitroaromatics. The improved photocatalytic activities of CdS-TiO2-NS benefits are: 1) the deposition of narrow-band-gap CdS can effectively extend the light absorption range of wide-band-gap TiO2-NS; 2) the unique 2D structure of TiO2-NS provides abundant coupling interface for CdS decoration, which is beneficial for photogenerated charge carriers transport across the interfacial domain. It is hoped that our work would promote further interest in the fabrication of new 2D materials using functional graphene as sacrificial template for diverse photoredox applications.

Keywords: TiO2; 2D nanosheets; CO2 reduction; photocatalysis; nitro reduction

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INTRODUCTION

In recent years we have witnessed an ever-increasing interest in fabrication of two-dimensional (2D) semiconductor nanosheets due to their novel electronic structures and distinctive physicochemical properties arising from their 2D geometry.1-4 The favorable characteristics of 2D semiconductor nanosheets have enabled vast application in the fields of flexible thin-film transistors, photoelectronics, energy storage and catalysis.1-3,

5-6

The

synthesis of semiconductor nanosheets is often fabricated by exfoliation of their layered compounds (top-down synthesis), and thus the product yields are relatively low and the size of the nanosheets remains in the submicrometer regime ascribing to the limitations of this fabrication method.1-4, 6-7 It is still a challenge to develop scalable approaches to fabricate ultralarge semiconductor nanosheets with a size of several micrometers or even tens of micrometers. Therefore, the development of an alternative bottom-up and easily scalable strategy is highly desirable for synthesis of semiconductor large nanosheets for potential applications. Graphene (GR) nanosheets, as the thinnest 2D nanomaterial with exceptionally intriguing properties, have been widely explored in nanoelectronics, nanodevices and nanoscale catalysis.8-15 Recently, some pioneering works also demonstrate the promising properties of GR or functionalized GR as structure directing agent for guiding material assembly.16-21 The quasi-2D structure coupled with organic functional groups imparts novel physiochemical properties to GR for constructing unique nanoarchitectures.20, 22-27 Such unique properties of GR qualify it as a reliable source for bottom-up production of other 2D nanosheets by using GR as a sacrificial 2D template. Several promising examples have been reported for 3



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fabrication of semiconductor 2D nanosheets by using GR as template.28-31 For instance, the MnO2 and FeOOH nanosheets with lateral sizes of several hundred nanometers have been fabricated from GR templates.29-30 In addition, using GR as growth templates, MoS2 layers have been successfully fabricated by the chemical vapor deposition (CVD) method.31 These pioneering works have pointed to the possibility of using GR to synthesize new 2D nanomaterials by a bottom-up technique. Notably, in these previous works, the as-obtained semiconductor 2D nanosheets are all layered compounds, which can be easily self-assembled into lamellar structures.29-31 In contrast, semiconductor materials without layered structure, like TiO2, ZnO and CdS, etc., are hard to assemble into a 2D structure during the crystal growth process.1 Although several examples about sandwich-like semiconductor-GR hybrid nanosheets have been reported,32-34 the structural stability of these hybrid nanosheets is based on the GR as 2D scaffold and the removal of GR would result in the collapse of the 2D structure. To date, 2D semiconductor nanosheets with unstratified structure, which are obtained from GR as a sacrificial template, have not yet been described. In addition, in the previous reports on 2D semiconductor nanosheets achieved by GR templating method, the lateral size of these 2D semiconductor nanosheets is limited by the lateral size of the GR template, and thus their size is similar to the size of GR or even much smaller than that of a GR template.28-31 To date, it is still difficult to breakthrough the lateral size limitation of GR template to synthesize 2D semiconductor nanosheets with lateral sizes much larger than that of an original GR sacrificial template. Recently, we have found that the controlled benzyl alcohol functionalization of the GR surface can promote the cross-link of individual GR nanosheets to the ultralarge one, with an average

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lateral size much larger than the original GR nanosheets.15 In particular, we note that TiO2 nanocrystals are densely and compactly coated onto the GR surface.15 Thus, we envisage that this chemical precedent might provide an accessible GR-templating approach to fabricate ultralarge 2D semiconductor nanosheets and derived 2D semiconductor-based composites by subsequent structure architecturing on the 2D semiconductor sheets. Herein, taking TiO2 semiconductor as a typical example, we demonstrate a graphene-template method for bottom-up growth of TiO2 2D nanostructures from molecular precursors, in which benzyl alcohol functionalized graphene (GR) 2D sheets are intelligently utilized as structure-directing agents to confine the stacking and growth of TiO2 onto the surface of GR nanosheets. Consequently, ultralarge GR based TiO2 nanosheets are obtained by such a sol-gel strategy. Thermal treatment is then carried out to induce complete condensation and crystallization of TiO2, and simultaneously remove the GR template without the collapse of TiO2 sheet structure. As a result, the ultralarge, freestanding 2D TiO2 sheets (TiO2-NS) with lateral sizes from a few micrometer to tens of micrometers are obtained, which is much larger than that of the original GR. The CdS nanoparticles are then homogeneously photo-deposited onto the TiO2-NS surface. As compared to bare TiO2-NS, the CdS-TiO2-NS heterostructure shows obviously improved photocatalytic activity for both gas-phase reduction of CO2 and liquid-phase reduction of nitroaromatics under simulated solar light irradiation.

EXPERIMENTAL SECTION

2.1 Materials. Tetrabutyl-orthotitanate (TBOT), Cd(NO3)2, thioacetamide, sodium dodecyl sulfate, 5



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benzyl alcohol (BA), anhydrous ethanol (EtOH), cadmium chloride (CdCl2·2.5H2O), sublimed sulfur (S8), graphite powder, hydrochloric acid (HCl), potassium persulfate (K2S2O8), titanium (IV) (ammonium lactato) dihydroxybis, ethylenediamine, phosphorus pentoxide (P2O5), hydrogen peroxide (30%H2O2), potassium permanganate (KMnO4), 2-nitroaniline (C6H6N2O2), 3-nitroaniline 3-nitrophenol

(C6H6N2O2), (C6H5NO3),

4-nitroaniline 4-nitrophenol

(C6H6N2O2),

(C6H5NO3),

2-nitrophenol

4-nitrotoluene

(C6H5NO3),

(C7H7NO2) and

4-nitroanisole (C7H7NO3) were purchased from Sinopharm chemical regent Co., Ltd. (Shanghai, China). Deionized water and ultrapure water were supplied from local sources. All of the materials were used as received without further purification. 2.2 Synthesis. (a) Synthesis of graphene (GR) nanosheets Graphene oxide (GO) was synthesized from graphite powder according to the modified Hummers method as shown in supporting information. The reduction of GO to GR was typically performed as follows. 40 mg of GO powder was dispersed into 100 ml of deionized water completely by ultrasonication. Then 20 ml of 50 mM NaBH4 aqueous solution was added and the suspension was stirred at room temperature for 5 hours. Finally, the suspension was vacuum-filtered and washed in ethanol and dried in air at room temperature. (b) Synthesis of TiO2 nanosheets In a typical experiment, a given amount of GR was dispersed in ethanol (EtOH) with the aid of ultrasonication for 30 minutes. Benzyl alcohol (BA) and water (H2O) were then added and the suspension was stirred at 0 °C. Tetrabutyl-orthotitanate (TBOT) was dissolved in ethanol and slowly dropped into the above GR suspension, so that the final molar ratio of a

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mixture of TBOT:BA:EtOH:H2O was 1:5:100:5. After two hours stirring, the precipitates were vacuum-filtered, washed in ethanol and dried in air at room temperature followed by calcination in air at 450 °C for 2 hours. (c) Synthesis of CdS-TiO2 nanosheets Typically, the as-obtained TiO2 nanosheets (TiO2-NS, 50mg), CdCl2·2.5H2O (40 mg) and S8 (10mg) were dispersed in 15 ml ethanol to form a stable suspension. Then the suspension was bubbled with nitrogen gas for 30 min in the dark. Subsequently, the suspension was irradiated with simulated solar light (300

Graphene-Templated Bottom-up Fabrication of Ultralarge Binary CdS

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