Zinc-Based Nanomaterials for Solar Energy Conversion

Abstract

The world’s population and the demand for energy are increasing. Fossil fuels are in limited supply, and their combustion creates carbon dioxide, the most abundant greenhouse gas. These conditions have driven interest in and progress toward the development of alternative energy technologies. Solar energy is an inexhaustible resource and is in abundant supply on all continents of the world. Finding a viable way to tap solar energy is urgently needed. Photocatalytic or photoelectrochemical reactions and materials for solar energy conversion have been studied extensively. Visible light accounts for 44–47% of the solar energy spectrum, whereas UV light accounts for 3–5%. Therefore, efficient use of visible light is an essential prerequisite for the efficient use of solar energy. This study reported some strategies for the efficient use of solar energy based on the zinc-related nanomaterials.In chapter 1, a very simple and mild wet-chemical synthesis of ZnO nanorods to fabricate sub-20 nm diameter ZnO nanorod arrays was described. These large scale arrays could be obtained by immersing a Zn sheet in an ammonia aqueous solution containing Al3+ ion at room temperature (25 °C) and normal atmospheric pressure (1 atm). This method uses little energy and requires no complex experimental procedures or equipment. The Zn ions were provided by the Zn sheet

as a consequence, the growth process is self-limiting. The ZnO nanorods produces were ~250 nm long and ~18 nm in diameter. On the basis of the results, we propose a mechanism for the growth of the ZnO structures on the Zn sheet with Al3+ ion-containing ammonia aqueous solution. The high surface-to-volume ratio of the sub-20 nm diameter ZnO nanorod arrays results in enhanced photocatalytic activity.In chapter 2, a method for synthesizing exposed crystal face-controlled 3D ZnO superstructures under mild conditions (at room temperature or 90°C under 1 atm) without organic additives was reported. The exposed crystal faces of the building blocks of the 3D structures were controlled by varying the reactant concentrations and the reaction temperatures. On the basis of the experimental results, we speculated a possible mechanism for the formation of the four distinct 3D ZnO superstructures (Structures I, II, III, and IV) under the different growth conditions. The optical properties of the 3D ZnO superstructures were probed by UV-Vis diffuse reflectance spectroscopy. The spectra were shifted depending on the dimensions and sizes of the building blocks of the 3D superstructures. The photocatalytic activities of the 3D superstructures varied according to the exposed crystal faces, which could be controlled by this method (Structure I > Structure IV > Structure III > Structure II).In chapter 3, a method for synthesizing quasi-single crystal mesoporous ZnO nanostructures via a self-generated templating approach was described. ZnO-carbonaceous species nanocomposites (average diameter of ~500 nm) were synthesized using a microwave-assisted hydrothermal reaction. The nanocomposites consisted of quasi-single crystalline ZnO regions and amorphous carbonaceous regions, which formed a self-generated nanotemplate. These nanocomposites were converted into quasi-single crystalline mesoporous ZnO nanostructures during combustion of the amorphous carbonaceous species (the template removing step). The adsorption properties and photocatalytic activities of the mesoporous ZnO structures were evaluated. The mesoporous ZnO structures showed better adsorption than the commercial ZnO nanostructures with similar dimensions or the 3D ZnO structures composed of nanoplates with {2-1-10} planar surfaces (benchmark materials) even though the BET surface area of the 3D ZnO structures was larger than that of the mesoporous ZnO structures. The quasi-single crystal mesoporous ZnO structures exhibited a high photocatalytic activity.In chapter 4, the synthesis of carbon-doped zinc oxide nanostructures using vitamin C, and their visible light photocatalytic activity was reported. Amorphous/crystalline vitamin C–ZnO (VitC–ZnO) structures were obtained from a solution of zinc nitrate hexahydrate, HMT, and vitamin C through heating at 95°C for 1 h. VitC–ZnO structures were calcined in air at 500°C for 2 h to create C-doped ZnO nanostructures. Calcined structures were polycrystalline, with an average crystal domain size of 7 nm. EDS, XPS, and XRD analysis revealed the substitution of oxygen with carbon and the formation of Zn-C bonds in the C-doped ZnO nanostructures. The carbon concentrations, in the form of carbide, were controlled by varying the concentrations of vitamin C (more than 1 mM) added to reaction solutions. On the basis of these experimental results, we propose a possible formation mechanism for C-doped ZnO nanostructures. The C-doped ZnO nanostructures exhibited visible light absorption bands that were red-shifted relative to the UV exciton absorption of pure ZnO nanostructures. The visible light photocatalytic activities of C-doped ZnO nanostructures were much better than the activities of pure ZnO nanostructures.Chapter 5 reported a method for synthesizing three distinct type II 3D ZnO/ZnSe heterostructures through simple solution-based surface modification reactions in which polycrystalline ZnSe nanoparticles formed on the surfaces of single crystalline ZnO building blocks of 3D superstructures. The experimental results suggested a possible formation mechanism for these heterostructures. The formation of the ZnO/ZnSe heterostructures was assumed to result from a dissolution–recrystallization mechanism. The optical properties of the 3D ZnO/ZnSe heterostructures were probed by UV-Vis diffuse reflectance spectroscopy. The 3D ZnO/ZnSe heterostructures exhibited absorptions in the visible spectral region. The visible photocatalytic activities of 3D ZnO/ZnSe heterostructures were much higher than those of the 3D pure ZnO structures. The activities of the 3D ZnO/ZnSe heterostructures varied according to the structures under visible light. The morphologies and exposed crystal faces of pure ZnO building blocks prior to surface modifications had a significant effect on the visible light photocatalytic processes of ZnO/ZnSe heterostructures after surface modification.Chapter 6 reported a post-treatment method for modifying crystalline nature and texture of ZnO nanostructure surfaces. The single crystalline ZnO structures were transformed using a solution-based reaction in an aqueous sodium selenite and hydrazine solution into single crystalline ZnO/polycrystalline ZnSe (core/shell) heterostructures (as an intermediate material), which were then used to texture the surfaces of ZnO nanostructures. The ZnO/ZnSe nanostructures were calcined to obtain pure ZnO nanostructures with a variety of surface crystalline natures and textures under controlled calcination conditions. Single crystalline ZnO core/polycrystalline ZnO shell nanostructures, single crystalline ZnO with wavy surfaces/ZnO nanoparticles, or single crystalline ZnO nanostructures with wavy surfaces were obtained by calcination at 550°C, 700°C, or 800°C, respectively. The experimental results suggest a possible mechanism by which the surface of the ZnO nanostructures was modified. This method may potentially be extended to any single crystalline ZnO nanostructures.In chapter 7, a method for synthesizing ZnO/ZnSe heterostructure nanowire arrays for use in photoelectrochemical (PEC) water splitting was described. The surfaces of ZnO nanowires immobilized on a conducting glass substrate were modified to form ZnO/ZnSe heterostructure nanowire arrays through a reaction with an aqueous sodium selenite and hydrazine solution. ZnO/ZnSe heterostructure nanowires with different morphologies were synthesized by varying solution concentrations and reaction times. The ZnO nanowire/ZnSe nanoparticle heterostructures (ZS1) were synthesized by a dissolution-recrystallization mechanism. At longer reaction times and higher solution concentrations, the nanostructure arrays transformed into ZnO nanowire/ZnSe nanosphere heterostructure arrays (ZS2) via Ostwald ripening. ZnO/ZnSe heterostructure arrays (ZS1 and ZS2) yielded higher photocurrents than the pristine ZnO nanowire arrays in a PEC water splitting test under AM 1.5G simulated solar light. The ZnO/ZnSe heterostructure array photoanodes exhibited absorption in the visible spectrum (<550 nm in wavelength) with a high incident-photon-to-current-conversion efficiency (IPCE) of up to 47% (ZS1) or 57% (ZS2) at 0.0 V vs. Ag/AgCl. The photoanode yielded a relatively high photocurrent density of 1.67 mA/cm2 (ZS1) or 2.35 mA/cm2 (ZS2) at 0.3 V compared to the ZnO nanowire arrays (0.125 mA/cm2). Structural differences between ZS1 and ZS2 yielded different PEC performances. A comparison to ZS2 revealed that ZS1 exhibited a higher photocurrent density under a low applied potential (from –0.78 V to –0.07 V) and a lower photocurrent density under a high applied potential (above –0.07 V).