Article Cite This: ACS Appl. Nano Mater. 2019, 2, 4180−4192
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Low-Temperature Photocatalytic Hydrogen Addition to TwoDimensional MoO3 Nanoflakes from Isopropyl Alcohol for Enhancing Solar Energy Harvesting and Conversion Soheil Razmyar,† Tao Sheng,‡ Manira Akter,† and Haitao Zhang*,† Department of Mechanical Engineering and Engineering Science, and ‡Department of Physics and Optical Science, and Optical Science and Engineering Program, The University of North Carolina at Charlotte, 9201 University City Boulevard, Charlotte, North Carolina 28223, United States
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ABSTRACT: Recently, research has been focused on the plasmon resonances in semiconductors (e.g., metal oxides) with tunable resonance absorption covering visible, near-infrared, and mid-infrared regions to enhance the efficiency in the utilization of solar energy. Two-dimensional (2D) molybdenum trioxide (MoO3) nanoflakes were studied to form substoichiometric oxides with strong tunable plasmon resonance via photocatalytic hydrogen addition. Despite the classic concept that the hydrogen addition to MoO3 from pure alcohols requires elevated temperatures more than 200 °C, we report a low-temperature hydrogen addition reaction of 2D MoO3 nanoflakes with pure isopropyl alcohol under visible light irradiation. The nanoflakes were produced by a facile liquid exfoliation of the MoO3 whiskers from chemical vapor deposition. Using alcohol−water solutions at different mixing ratios, the exfoliation process can tune the dimensions of the nanoflakes, as well as their defect structures. For the photocatalytic hydrogen addition of nanoflakes with pure alcohol, the suspension temperature is below 50 °C throughout the process. Material structure characterization, optical property measurement and analysis, and photothermal study were performed thoroughly to reveal the reaction mechanism. Our study shows the reaction is initiated by the visible light absorption from their sub-bandgap defect tails and is expedited by photothermal heating without providing any additional heating. The low reaction temperature provides a new low-cost method to produce substoichiometric semiconductors with tunable plasmonic behaviors. The reaction mechanism can be extended to other photocatalytic processes of MoO3 nanostructures to improve their efficiencies in utilizing solar energy. KEYWORDS: two-dimensional (2D) nanomaterials, molybdenum trioxide, photocatalysis, low-temperature hydrogen addition, semiconductor plasmonic nanostructures, Urbach defect tail conductor.5 Although it is still under investigation that under what conditions which process would dominate the enhancement through LSPR, these multiple processes provide a broad range of opportunities for the enhancement of semiconductor photocatalysis using plasmonic nanoparticles. Intensive studies of LSPRs have been focused on noble metals, such as Au and Ag, because of their excellent chemical stability and strong light absorption in the visible region. The resonance frequency is also tunable by adjusting the size and shape of the nanoparticles.2,4 However, due to their high carrier density, the resonant absorption of these metal nanoparticles only cover a narrow region in the visible range, and hence the major portion of the solar spectrum cannot be fully utilized.2 Recently, research results have demonstrated strong LSPRs in nonmetal materials, i.e., heavily doped
1. INTRODUCTION Recently, an efficient way to enhance the photocatalytic efficiency in semiconductors has been developed. Metal nanoparticles are attached to semiconductor nanostructures to employ plasmon resonances in photocatalysis.1 Plasmon resonances are the collective oscillation of free carriers in response to the electromagnetic field of light irradiation. When the plasmon resonances are confined to the surface of metallic nanoparticles, they are called localized surface plasmon resonances (LSPRs).2−4 With the LSPRs excited at resonance frequency, metal nanoparticles can dramatically enhance the light absorption, light scattering, and charge separation and transfer in the semiconductors. The exact mechanism of the LSPR enhancement is still not clear. Several modes have been proposed including: (1) direct electron transfer of hot electrons from metal to the semiconductor, (2) local electromagnetic field enhancement of the carrier generation in the semiconductor, and (3) resonant energy transfer from the LSPR dipole to the electron−hole pair in the semi© 2019 American Chemical Society
Received: April 8, 2019 Accepted: June 6, 2019 Published: June 6, 2019 4180
DOI: 10.1021/acsanm.9b00645 ACS Appl. Nano Mater. 2019, 2, 4180−4192
Article
ACS Applied Nano Materials
Scheme 1. Synthesis and Processing Procedures of the 2D MoO3 Nanoflakes: (I) CVD Growth of MoO3 Whiskers, (II) Liquid Exfoliation of Whiskers Using Ultrasonication, (III) Collection of Nanoflake Suspension Using Centrifuging, and (IV) Resulting Nanoflakes and Their Layered Crystalline Structure
classic concept of the high reaction temperature requirement beyond 200 °C. The 2D MoO3 nanoflakes were produced using a facile liquid exfoliation of the CVD grown high-quality MoO3 whiskers (shown in Scheme 1). Comprehensive material structure characterization, optical property measurement and analysis, and photothermal study were performed to reveal and understand its reaction mechanism.
semiconductors (e.g., metal chalcogenides and metal oxides).2−4,6 With their tunable carrier density by adjusting doping or stoichiometry, the resulting resonance absorption can be expanded to cover visible, near-infrared (NIR), and mid-infrared regions. Therefore, this approach opens up new opportunities for the plasmonic enhancement in solar energy utilization.6 While most studies have been conducted on widely used conducting oxides such as indium tin oxide and aluminum-doped zinc oxide with plasmon resonances at NIR region,7,8 we will focus on transition metal oxides (TMOs, e.g., molybdenum oxide) for their visible range plasmon resonances.4,9−11 TMOs have unique properties in metal−insulator transition, high-temperature superconductivity, fast ionic transport, and colossal magnetoresistance resulting from their outer-d valence electrons.4 They are widely employed in applications in microelectronics, quantum computing, spintronics, and energy conversion and storage.4 With anisotropic structures and wide range of substoichiometric compositions, the LSPRs in TMO nanostructures can be finely tailored by controls in dimension and composition. Molybdenum oxides have multiple varieties of compositions and crystalline structures, including molybdenum trioxide (MoO3) and different substoichiometric forms, such as Mo4O11, Mo5O14, Mo8O23, and Mo9O26, etc.12 The most common phase of MoO3 is α-MoO3 having an layered orthorhombic structure (shown in Scheme 1) with strong ionic and covalent bonding within the (010) layers but weak van der Waals interactions between the (010) layers.13 Computational calculations show the α-MoO3 (010) surface has the lowest surface energy of 0.19−0.31 J/m2, while the (100) surface has a surface energy of 0.7 J/m2.12 No surface energy value was reported for (001) surface, but it is expected to be slightly higher than the one for (100) surface according to the crystal structure. This highly anisotropic structure makes α-MoO3 a natural two-dimension (2D) layered material. Therefore, it is easy to form 2D nanostructures of MoO3 with a (010) basal plane by dry14 or wet15 exfoliations or by direct growth using chemical vapor deposition (CVD).13 Here, we report the discovery of low-temperature hydrogen addition of 2D MoO3 nanoflakes with pure isopropyl alcohol (IPA) under visible light irradiation below 50 °C, significantly lower than the
2. EXPERIMENTAL SECTION 2.1. Synthesis of MoO3 Whiskers. The synthesis of MoO3 whiskers was performed using a home-built hot-wall CVD system based on resistance heating, which has been employed previously for the growth of other nanostructures.13,16−18 About 2 g of Mo powders was loaded in a quartz boat, which was placed at the center of the reaction chamber. The reaction chamber was first sealed and pumped down to ∼20 mTorr and then brought up to atmospheric pressure by purging UHP (ultrahigh purity) Ar. Constant flows of 10 sccm (standard cubic centimeter per minute) UHP Ar as the carrier gas and 10 sccm UHP O2 as the reactant gas were then supplied during the synthesis. The heating temperature at the center of the reaction chamber was ramped up to 800 °C in 30 min and maintained for 120 min, followed by a natural cooling down to room temperature. A large amount of light yellow crystals of MoO3 whiskers was collected downstream in a temperature zone of ∼770−700 °C. 2.2. Whisker Exfoliation. Liquid phase exfoliation method was employed to produce 2D MoO3 nanoflakes from as-synthesized whiskers. Anhydrous IPA (Millipore Sigma) with extremely low H2O content (
