Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE
Article
Facile synthesis of well-aligned ZnO nanowires on various substrates by MOCVD for enhanced photoelectrochemical water-splitting performance Mostafa Afifi Hassan, Muhammad Ali Johar, Sou Young Yu, and Sang-Wan Ryu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02392 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 28, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Sustainable Chemistry & Engineering
Facile synthesis of well-aligned ZnO nanowires on various substrates by MOCVD for enhanced photoelectrochemical water-splitting performance
Mostafa Afifi Hassan, Muhammad Ali Johar, Sou Young Yu, and Sang-Wan Ryu* Department of Physics, Chonnam National University, Yongbong-ro 77, Buk-gu, Gwangju 61186, Republic of Korea *corresponding
author (e-mail):
[email protected] Abstract: We present results related to the growth of high-aspect-ratio ZnO nanowires (NWs) using metalorganic chemical vapor deposition based on the study of the NW growth mechanism on various substrates. The ZnO NWs on Si, sapphire, and GaN substrates were optically and structurally characterized, and the effect of a low-temperature ZnO buffer layer was investigated. It was demonstrated that NWs deposited under the same conditions could be randomly oriented or vertically aligned. Well-aligned ZnO NWs were grown on Si (100) substrates with a ZnO buffer layer which was grown via atomic layer deposition. The NWs are single-crystalline wurtzite structures with a preferential growth in the (002) direction. Room temperature photoluminescence measurements exhibited a strong ultraviolet emission and suppressed visible emission. This result affirms the absence of defects related to ZnO or oxygen vacancies. On changing the substrate, different morphologies of the ZnO NWs can be attained, which present different photoelectrochemical (PEC) performances. Among these, ZnO NWs grown on GaN and Si substrate (with a buffer layer) exhibited a superior photocurrent of 4.75 mA cm-2 and 3.24 mA cm-2 at 1.5 V (vs. Ag/AgCl), respectively. This excellent PEC performance can be attributed to the enhancement of light absorption intensity and the increased photogenerated carrier lifetime, implying that well-aligned ZnO NWs are suitable for PEC water splitting. Keywords: MOCVD; ZnO nanowires; Photoelectrochemistry; Water splitting.
ACS Paragon Plus Environment
ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Introduction During the past decade, one-dimensional (1D) semiconductor nanostructures such as rods, wires, and tubes have exhibited distinctive and unique properties that often differ from those of bulk and thin-film materials [1, 2]. Hence, semiconductor nanowires (NWs) have been widely studied to understand the underlying basic science as well as to investigate potential applications in photonics, electronics, and sensors [3]. Owing to their interesting properties and wide variety of applications, these structures have become the focus of intensive research with respect to the fabrication of nanoscale devices. Among the excellent candidates for the manipulation of one-dimensional nanostructure systems, ZnO NWs have been intensively investigated due to their amazing intrinsic properties and nanotechnological importance [4-8]. ZnO is regarded as one of the most promising and efficient II–VI semiconductors with a wide direct band gap (Eg=3.37 eV) and exhibiting a high exciton binding energy (60 meV). These properties facilitate efficient excitonic emission at room temperature (RT) [9-12]. Furthermore, ZnO is inexpensive, environmentally benign, and a nontoxic source material. Moreover, it can be fabricated via a simple fabrication process. Consequently, ZnO NWs are considered as a competitive candidate for a massive variety of astonishing applications such as light emitting diodes [13-15], solar cells [16-18], ultraviolet (UV) photodetectors [19-21], UV lasers [22], gas sensors [23-26], and piezoelectric nanogenerators [27, 28]. Several methods have been adopted to synthesize ZnO NWs including hydrothermal synthesis [29-31], thermal oxidation [32, 33], electro-deposition [34, 35], thermal metal evaporation [36], metal-organic chemical vapor deposition (MOCVD) [37], chemical solution routes [38], pulsed laser deposition (PLD) [39], CVD [40, 41], etc. Among these techniques, advanced growth methods such as MOCVD and molecular beam epitaxy can provide good uniformity, high crystallinity, and precise thickness control in the nanometer range required for current electronic and photonic device applications [42, 43]. In particular, there is special interest in MOCVD since it has numerous advantages such as mechanical stability, excellent crystallinity, and reproducibility. Till date, well-aligned ZnO NWs have been fabricated with a high surface-tovolume ratio, and excellent morphological, structural and optical properties [44]. As the growth of ZnO NWs by MOCVD is a bottom-up technique, the choice of the substrate material plays an essential role in determining the dimension and alignment of the resulting nanowires. Based on lattice misfit evaluation, the most suitable substrate for ZnO growth is ScAlMgO4, which is
ACS Paragon Plus Environment
Page 2 of 29
Page 3 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Sustainable Chemistry & Engineering
expensive and technologically inconvenient [45]. Interestingly, C-plane sapphire overcomes some of these limitations. In addition, silicon is considered the best substrate for microelectronics and micro-electro-mechanical-systems (MEMS), and it serves as a good integration platform. However, it is difficult to obtain well-aligned ZnO NWs on silicon substrates because of the formation of an interfacial layer (SiO2) and a large lattice misfit [46, 45]. To overcome this problem and to achieve an improved morphology, the introduction of a buffer layer of ZnO thin film will be beneficial. It is assumed that a ZnO seed layer is expected to introduce uniform growth of well-aligned NWs. After optimization of the ZnO NWs, their application to highly efficient water-splitting photoanodes is explored to investigate the role of morphology and crystallinity in photoelectrochemical (PEC) process. The optimized ZnO NWs are prepared as single crystalline (defect-free) structures, so they offer fewer sites for electron trapping and enhanced carrier transport toward conductive electrodes. Consequently, electron transport in single-crystalline ZnO NWs is expected to be several orders of magnitude faster than transport in particulate films where diffusion is dictated by percolated random walk. Herein, we thoroughly investigate the growth of highly oriented ZnO NWs via MOCVD growth technique on various substrates (silicon, sapphire, and GaN) with and without a buffer layer. Welldefined, vertically oriented, highly dense, and uniform ZnO NWs with a wurtzite crystal structure were obtained. One of the purposes of this investigation is showing how the existence of a ZnO buffer layer influences the ZnO nanostructures' crystallinity and morphology. The ZnO NWs with different morphologies were then applied to PEC water splitting.
Experimental method Well-aligned ZnO NWs were grown on different substrates (e.g., silicon, sapphire, and GaN) using MOCVD technique with Zn(C2H5)2 [diethyl zinc (DEZn)] as a zinc precursor. Our growth procedure is schematically shown in Figure 1 and it consists of three steps outlined in the following sections. Substrate preparation & cleaning procedure. Three different substrates were prepared for highly oriented ZnO NWs: boron doped (100) Si substrate with a resistivity of
