Robust Hollow Spheres Consisting of Alternating Titania Nanosheets and Graphene Nanosheets with High Photocatalytic Activity for CO₂ Conversion into Renewable Fuels

Abstract

Robust hollow spheres consisting of molecular‐scale alternating titania (Ti0.91O2) nanosheets and graphene (G) nanosheets are successfully fabricated by a layer‐by‐layer assembly technique with polymer beads as sacrificial templates using a microwave irradiation technique to simultaneously remove the template and reduce graphene oxide into graphene. The molecular scale, 2D contact of Ti0.91O2 nanosheets and G nanosheets in the hollow spheres is distinctly different from the prevenient G‐based TiO2 nanocomposites prepared by simple integration of TiO2 and G nanosheets. The nine times increase of the photocatalytic activity of G‐Ti0.91O2 hollow spheres relative to commercial P25 TiO2 is confirmed with photoreduction of CO2 into renewable fuels (CO and CH4). The large enhancement in the photocatalytic activity benefits from: 1) the ultrathin nature of Ti0.91O2 nanosheets allowing charge carriers to move rapidly onto the surface to participate in the photoreduction reaction; 2) the sufficiently compact stacking of ultrathin Ti0.91O2 nanosheets with G nanosheets allowing the photogenerated electron to transfer fast from the Ti0.91O2 nanosheets to G to enhance lifetime of the charge carriers; and 3) the hollow structure potentially acting as a photon trap‐well to allow the multiscattering of incident light for the enhancement of light absorption.

Understanding the Reaction Mechanism of Photocatalytic Reduction of CO2 with H2O on TiO2-Based Photocatalysts: A Review

“…Second, electron transfer could be promoted in the direction from TiO 2 CB to external trappers or carriers. The electron trappers can be noble metals (e.g., Pt, Pd, Au, Ag) (Sasirekha et al, 2006;Iizuka et al, 2011;Yui et al, 2011;An et al, 2012;Uner and Oymak, 2012;Wang et al, 2012b) or metal oxides (e.g., CuO, FeO x , CeO 2 ) (Tseng et al, 2004;Qin et al, 2011;Srinivas et al, 2011;Wang et al, 2011b;Truong et al, 2012;Zhao et al, 2012b), and the electron carriers are often carbon materials (e.g., graphene) (Liang et al, 2011(Liang et al, , 2012Tu et al, 2012). Third, the incorporation of TiO 2 with another semiconductor, i.e., photo-sensitizer (e.g., AgBr, CdSe, PbS) Asi et al, 2011;Wang et al, 2011a;An et al, 2012) or n-type semiconductor (e.g., ZnO) (Xi et al, 2011), promotes electron transfer between the CB of the second semiconductor and the CB of TiO 2 .…”

“…The visible light response of TiO 2 /graphene is attributed to the chemical bonding between TiO 2 and the specific sites of carbon that leads to the narrowing of TiO 2 band gap (Zhang et al, 2010). To date, only a few studies have been conducted on TiO 2 /graphene for CO 2 photoreduction (Liang et al, 2011;Liang et al, 2012;Tu et al, 2012). The defects in graphene and the contact between graphene and TiO 2 will significantly affect the charge transfer and determine the activity and selectivity of CO 2 conversion.…”

“…The electrons on graphene diffused very quickly due to its larger surface area, which restrained the accumulation of electrons and decreased the local electron density. Consequently, the process of two-electron reduction of CO 2 to CO was facilitated as compared with the process of eight-electron reduction of CO 2 to CH 4 (Tu et al, 2012).…”

Hollow metal nanostructures for enhanced plasmonics: synthesis, local plasmonic properties and applications

“…In both of these approaches, the void size and the shape of the hollow structure are determined by the nature of the template while the morphology and composition of the shell (material, thickness and porosity) is mainly defined in the coating and removal processes. As a result, a wide variety of hollow NCs with different sizes, shapes, composition, and structures have been produced, using hard templates, such as polymers (polystyrene [54,55], formaldehyde resin [56,57], and poly(methyl)methacrylate [58], silica [59,60] and carbon particles [61]), or soft ones, such as emulsion block copolymer micelles [62], and even gas bubbles [63]. For a comprehensive and up-to-date review on templating approaches see Ref.…”

Research Advances in Photocatalysis of Inorganic Hollow Spheres

“…In the process of constant exploring emission reduction technology, it has been found that photogenerated electronics can change CO 2 to organic compounds with high application value such as methane and methanol. Tu et al [94] prepared hollow spheres consisting of alternating Ti 0.91 O 2 nanosheets and graphene nanosheets with polymer beads as sacrificial templates and via a microwave irradiation technique. The ultrathin Ti 0.91 O 2 nanosheets allow charge carriers to move rapidly onto the surface to take part in the photoreduction reaction and the alternating compact stacking structure allow the photogenerated electron to transfer fast from Ti 0.91 O 2 nanosheets to graphene nanosheets to extend lifetime of the charge carriers.…”