Hierarchical Shape Evolution of Cuprous Oxide Micro- and

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Hierarchical Shape Evolution of Cuprous Oxide Micro- and Nanocrystals by Surfactant-Assisted Electrochemical Deposition Sanghwa Yoon, Sung-Dae Kim, Si-Young Choi, Jae-Hong Lim, and Bongyoung Yoo Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00873 • Publication Date (Web): 26 Aug 2015 Downloaded from http://pubs.acs.org on August 31, 2015

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Crystal Growth & Design

Hierarchical Shape Evolution of Cuprous Oxide Micro- and Nanocrystals by Surfactant-Assisted Electrochemical Deposition Sanghwa Yoona, Sung-Dae Kimb, Si-Young Choib, Jae-Hong Lim*c and Bongyoung Yoo*a a

Department of Materials Engineering, Hanyang University, Ansan-si, Gyeonggi-do 426-791, Republic of Korea b

Materials Modeling & Characterization Department, Korea Institute of Materials Science, Changwon-si, Gyeongnam 641-831, Republic of Korea

c

Electrochemistry Research Group, Materials Processing Division, Korea Institute of Materials Science, Changwon-si, Gyeongnam 641-831, Republic of Korea

KEYWORDS: Cuprous oxide, electrochemical deposition, surfactant, shape evolution

ABSTRACT: Cu2O microcrystals (MCs) and nanocrystals (NCs) of various shapes were potentiostatically deposited on Ti substrates by exploiting the capping effects of differently charged surfactants on certain surfaces. Positively charged hexamethylenetetramine was

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adsorbed onto the {111} and {100} planes of the Cu2O crystals (copper and oxygen terminated surface) while negatively charged poly(vinylpyrrolidone) was adsorbed onto the {111} planes (Cu terminated surface due to coulomb interaction). The difference in the adsorption behaviors of the two surfactants resulted in the synthesis of differently shaped Cu2O MCs. We were also able to systematically control the shape evolution of Cu2O NCs synthesized as cubes, truncated cubes, truncated octahedrons, and stepped octahedrons into perfect octahedrons by adding or removing surfactants successively during the electrochemical deposition process. This method is a simple, flexible, and cost-effective one and should aid in the elucidation of the shapetransformation phenomenon in other semiconductors as well as their facet-dependent properties.

1. INTRODUCTION The shape evolution of microcrystals (MCs) and nanocrystals (NCs) of semiconducting materials is an interesting issue in nanotechnology, biology, chemistry, and materials engineering, because the surface morphology of semiconducting crystals is related to their surface energy, electronic structure, bonding ability, and chemical reactivity.1-7 Cuprous oxide (Cu2O), which is a typical p-type semiconductor with a direct band gap of 2.1 eV, is an attractive material for use in photocatalysis,8-11 fuel cells,12 solar cells,13-17 gas sensors,18,

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lithium

batteries20-22, and water-splitting systems.23-25 It is also an ideal material for studying the shape evolution of semiconducting MCs and NCs. In recent years, various methods for synthesizing Cu2O NCs have been developed, such as chemical vapor deposition,26 sol-gel techniques, spraying methods,27 hydrothermal methods,8,

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and electrochemical deposition (ED).15,

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Of

these, solution-based processes are the best suited for practical use, because they involve low


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temperatures and can be used to produce MCs and NCs in large volumes. In particular, ED, which involves the application of an external potential or current, is fast, flexible, and allows for the synthesis of MCs and NCs at a low cost. In the case of ED, several parameters such as the electrolyte concentration, bath temperature, applied voltage, bath pH, and choice of surfactant have an effect on the shape of the synthesized MCs and NCs.28, 29 There are several reports on the synthesis and shape control of Cu2O NCs using ED. Siegfried and Choi reported the shape transformation of Cu2O NCs by ED while using sodium dodecyl sulfate as a surfactant.30 Liu et al. electrochemically fabricated ordered Cu2O NCs on n-InP substrates by controlling the bath pH.31 Radi et al. synthesized Cu-Cu2O core-shell nanoparticles of different shapes on a Hterminated Si(100) substrate by controlling the electrolyte concentration with precision.32 Ng et al. could produce octahedral nano- and micron-sized Cu2O single crystals by low-potential ED in the absence of additives.33 Hong et al. synthesized sub-10-nm-sized Cu2O nanowires on anodic aluminum oxide membranes by poly(vinyl pyrrolidone) (PVP)-assisted ED .34 Fu et al. could fabricate variously shaped Cu2O particles with pores using polymer beads.35 Although there have been numerous reports on the electrochemical synthesis and shape control of Cu2O NCs using surfactants, studies on the nucleation and growth mechanisms of Cu2O MCs and NCs grown by ED are relatively few. Because the ED method involves electrons from an external power source, the heterogeneous nucleation and growth of the NCs and MCs occurs on the conducting substrate; these phenomena are different from those occurring in other solution-based processes that do not require the application of an external voltage or current. No differences have been reported in the shape evolution behaviors corresponding to different surfactants and the terraces observed on the {111} planes in the case of Cu2O crystals synthesized by solution-based processes.36


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In this study, we investigated the growth mechanism of Cu2O MCs and NCs synthesized by ED on 300-nm-thick-Ti-layer-coated Si wafers while using different surfactants. While positively charged hexamethylenetetramine (HMT) was adsorbed on negatively charged surfaces, negatively charged PVP was adsorbed on positively charged surfaces. The growth mechanism of the Cu2O crystals was affected by the differences in the adsorption behaviors of the two surfactants, with the resulting micron-sized Cu2O crystals exhibiting very different shapes. On the nanoscale, the shape of the Cu2O crystals could be controlled and the crystals transformed from cubes into octahedrons or from octahedrons into cubes by adding or removing a particular surfactant from the solution. We could also synthesize stepped pyramidal structures by controlling the surface energy of the Cu2O crystals.

2. EXPERIMENTAL SECTION To form the working electrode, a 600-nm-thick layer of Ti was deposited on a clean Si wafer by electron beam evaporation. Cu2O NCs and MCs were potentiostatically electrodeposited using 0.1 M copper sulfate (Daejung, 99.0%), 0.2 M sodium citrate (Daejung, 99.0%), and 4 M KOH (pH 12, Daejung, 85.0%) at -0.3 V (vs. Ag/AgCl reference electrode).16,

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A three-

electrode system (Princeton Applied Research, VersaSTAT 4) with a platinum plate as the counter electrode was employed for the purpose. HMT (14–56 g/l, Daejung, 98.5%) or PVP (20– 40 g/l, Aldrich, Mw = 55,000) was added as the surfactant to the Cu2O solution. The temperature of the bath was maintained at 60°C using a circulating chiller (Labkorea Inc., HLTC08). The shapes of the Cu2O crystals were characterized using field-emission scanning electron microscopy (FESEM; TESCAN, MIRA3), and the growth direction was investigated using transmission electron microscopy (TEM; JEOL, JEM-2100F) after making cross-sectional


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samples using a focused ion beam system (FEI, Quanta 3D FEG). The crystalline structures of the NCs and MCs were studied using X-ray diffraction (XRD) analysis (Rigaku, D/MAX2500/PC).

3. RESULTS AND DISCUSSION Figure 1 shows scanning electron microscopy (SEM) images indicating the shapes of the Cu2O MCs synthesized by ED using HMT and PVP in three different concentrations. In the presence of 14 g/L HMT, cubic Cu2O MCs with diameters of approximately 2.5 µm were formed; the sides of these cubes were {100} planes (Figure 1a). When the concentration of HMT was increased to 42 g/L, the crystals changed from cubes to vertex-truncated octahedrons (Figure 1b). Further, when the concentration of HMT was increased to 56 g/L, the {100} planes were not observed, and the MCs were octahedrons with {111} planes (Figure 1c). When the amount of PVP used was varied, the shape evolution behavior was different from that observed after varying the HMT concentration. In the presence of 20 g/L PVP, {111}-corner-truncated, sunken cubes were produced (Figure 1d). When the concentration of PVP was increased to 30 g/L, the synthesized Cu2O MCs were vertex-truncated octahedrons (Figure 1e). Finally, when 40 g/L PVP was added to the Cu2O solution, the products formed were branched octahedrons. As shown in Figures 1c and f, terraces appeared on the surfaces of the Cu2O MCs formed in the surfactantcontaining solution.


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Figure 1. Field-emission SEM (FESEM) images of the Cu2O MCs formed using the two surfactants in different concentrations. Scale bar is 2 µm. To investigate the effects of HMT and PVP on the shape evolution behavior of the Cu2O crystals further and to study the formed terraces using FESEM, the imaging samples were tilted by 45° in the SEM chamber, as shown Figure 2. The Cu ions migrate to the surfaces of the Cu2O crystals fairly freely during deposition in the absence of a surfactant, as shown in Figure 2 (a). The concentration of Cu ions on the surfaces of the Cu2O crystals becomes high, and a layer of Cu2O layer is formed on the entire surface. At high pH values, cubic crystals are formed preferentially during ED, because of the low surface energy of the {100} planes.31, 37 When the Cu2O MCs were synthesized in the presence of HMT, perfect octahedrons with {111} planes were obtained. On the other hand, in the presence of PVP, the Cu2O MCs formed were octahedrons with branched structures at their vertices. It was confirmed that terraces were formed on the {111} planes of the octahedrons (Figures 2b, c). The XRD peaks of the samples corresponded to those of Cu2O (JCPDS # 65-3288); this was true for the samples synthesized with and without a surfactant (Figure 2d). Because the Cu2O crystals were randomly distributed


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over the substrate, all the XRD peaks related to Cu2O were observed, regardless of whether a surfactant was used or not.

Figure 2. FESEM images of the Cu2O MCs tilted at 45°: MCs synthesized (a) without a surfactant and (b) with HMT and (c) PVP. (d) X-ray diffraction patterns of the three types of Cu2O MCs. To investigate the growth directions and structures of the Cu2O crystals produced using the different surfactants, high-resolution TEM (HR-TEM) measurements were performed. Figure 3 shows TEM micrographs of the Cu2O MCs grown using (3a-d) HMT and (3e-h) PVP as the surfactant. Figures 3a and 3e are low-magnification TEM images of the MCs and show the shape of their cross-sections. High-magnification images of the same are shown in Figures 3b and 3f. The SAED patterns of the MCs (see inset of Figures 3b and 3f) proved that the viewing direction for the TEM observations for both types of MCs was parallel to the direction. Figures 3c and 3d are selected examples showing the stepped surfaces of Cu2O MCs synthesized using


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HMT as the surfactant; these MCS contain {001} and {111} surfaces. However, in the case of the MCs grown using PVP, the terrace contained just {111} surfaces (Figures 3g-h). Interestingly, a wide (100) plane was observed on the top surface of the MCs, while {100} and {111} planes were exposed at their sides (Figures 3b-d). In the case of the MCs produced using HMT, the growth direction was from the bottom to top, and the growth did not start from the vertexes; this is what resulted in the observed terraces (Figure 3b). Figures 3e and f show the cross-section of a Cu2O MC synthesized using PVP. Terraces were observed in the case of these MCs, which were octahedrons with branched structures (Figure 3f). Further, their growth started from the vertexes because PVP was adsorbed only on the {111} planes (Figure 3h).


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Figure 3. TEM images of the Cu2O MCs synthesized using (a-d) HMT and (e-h) PVP. The insets in Figures 3b and 3f are the SAED patterns of the corresponding Cu2O MCs. The insets in Figures 3c-d and 3h are the fast Fourier transform (FFT) patterns of the corresponding highresolution TEM images. We propose the following mechanism to explain the growth of Cu2O MCs by surfactantassisted ED (see Figure 4, Figure S3). When a surfactant is present, the surfaces of the Cu2O crystals adsorb the surfactant; this phenomenon depends on the polarity of the surfactant. This significantly hinders growth normal to the surfaces, owing to a decrease in the diffusion of Cu ions as well as in the supply of electrons. Positively charged HMT and negatively charged PVP are adsorbed onto the negatively and positively charge surfaces, respectively, of the Cu2O crystals (Fig S1). The polar {100} planes of Cu2O crystals have perfect oxygen-terminated surfaces. This means that the {100} surfaces are negatively charged. On the other hand, the terminated layer of the nonpolar {111} planes is composed of both copper and oxygen atoms,


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which have positive and negative polarities, respectively.8 HMT forms the complex (CH2)6N44H+ in aqueous solutions.38 Thus, HMT is adsorbed onto both the {100} and the {111} surfaces of Cu2O crystals, owing to coulombic interactions. In this case, the surface energy of the {111} planes of Cu2O crystals is lower than that of the {100} planes.39 Because the crystal facets that were exposed were the ones with the lowest total surface energy, the Cu2O crystals synthesized in the presence of HMT were stepped octahedrons with {111} and {100} facets (Figure 4a). Meanwhile, PVP contains the polarized functional group "-C=O"; the negatively charged "O" ions interact with the copper atoms, because of the positive charge of the {111} surfaces.8 The resulting Cu2O crystals are octahedrons. Further, there are branches at the vertices of these octahedrons because of fast growth in the directions (Figure 4b). In the surfactant-free copper citrate solution, Cu2O cubes are synthesized because of the attachment of [Cu2H2-Cit2]4ions and the formation of Cu2O, which occurs as per the following chemical reaction.40

[Cu2H2-Cit2]4- + 2e + H2O → Cu2O + [Cit] 3-

(1)

The differences in the growth direction and the exposed surfaces in the cases of the crystals synthesized using HMT and PVP are shown in Figure S2. The atomic arrangement of Cu2O was consistent with that suggested by the TEM images.


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Figure 4. Schematic illustrations of the growth mechanism of Cu2O MCs using different surfactants: (a) HMT and (b) PVP.

Figure 5. FESEM images of Cu2O NCs of various morphologies formed by two-step electrodeposition. (a) An octahedron electrodeposited in the presence of HMT, (b)-(e) the shape evolution of an as-synthesized octahedron with time; the octahedron was electrodeposited in the absence of HMT, (f) a cube electrodeposited in the absence a surfactant, and (g)–(j) the shape


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evolution of an as-synthesized cube with time; the cube was electrodeposited in the presence of HMT. The scale bars are 200 nm. We grew Cu2O NCs using a two-step ED process, in order to prove that their growth mechanism was the one proposed, as well as to be able to control their growth direction in the presence of a surfactant. Figure 5 shows the degree of shape control that could be achieved by adding or removing a surfactant. Nano-sized octahedral crystals were deposited on the substrate in the presence of HMT, as shown in Figure 5a. Subsequently, Cu2O NCs continued to be formed in the solution even in the absence of HMT (Figures 5b-e). After the initial 50 s, {100} planes appeared at the vertices of the octahedrons. Further, pyramidal steps were formed on the {111} and {110} planes, such that the {111} planes could no longer be seen. After 150 s, {100} planes could be seen clearly, and the steps on the {110} planes widened (Figure 5c). After a while, these steps disappeared almost completely, and the Cu2O NCs became edge-truncated cubes (Figures 5d and e). On the other hand, nano-sized cubes were deposited on the substrate in the absence of a surfactant (Figure 5f). Next, HMT was added to the solution; this resulted in the continuous deposition of Cu2O NCs (Figures 5g–j). No other steps were involved, unlike in the previous case. With an increase in the synthesis time, the Cu2O NCs changed from cornertruncated cubes (Figures 5g and h) to vertex-truncated octahedrons (Figure 5i). Ultimately, the Cu2O NCs became perfect octahedrons (Figure 5j). This shape evolution also occurred at the microscale (Figure S4). The TEM images obtained and corresponding SAED patterns showed the surfaces of the pyramidal Cu2O MCs with {100} planes (Figure S5).


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Figure 6. Schematic representation showing the shape evolution of the Cu2O NCs synthesized by a two-step electrochemical deposition process. (a) The transformation of an octahedron with the electrodeposition time in absence of a surfactant and (b) that of a cube in the presence of HMT. A mechanism for the shape evolution of the Cu2O NCs under the successive addition and removal of a surfactant is proposed (see Figure 6). After octahedral Cu2O NCs are formed in the presence of the surfactant HMT, they continue to be electrodeposited on the substrate even in the absence of the surfactant. As the ED process progresses continuously in the absence of the surfactant, the adsorbed surfactant molecules get detached from the surfaces of the octahedrons. As a result, the {111} planes, which have a high surface energy, are exposed to the Cu ions in the electrolyte. This leads to the gradual shrinkage of the {111} planes and continuous growth on the {100} planes, resulting in the formation of edge-truncated cubes (Figure 6a). In contrast, in


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the opposite process, that is, the growth of Cu2O in the absence of HMT and the subsequent addition of HMT, the surfactant molecules get attached at the corners of the cubes with Cu+ termination sites. As a result, the area of the {111} planes increases in the presence of the surfactant. Thus, the Cu2O cubes grow in the directions and become octahedrons. However, because there are no nucleation sites on the {100} planes, which only have an oxygenterminated layer, steps are not observed, in contrast to the first process (Figure 6b).

3. CONCLUSION In summary, we investigated the capping effects of differently charged surfactants during the ED of Cu2O crystals. HMT and PVP were the oppositely charged surfactants investigated. It was found that, on the microscale, the positively charged HMT gets adsorbed on both the {111} and the {100} planes of Cu2O crystals, resulting in the formation of stepped octahedrons, while the adsorption of negatively charged PVP on the {111} planes leads to the formation of branched octahedrons. Further, on the nano-scale, the Cu2O NCs undergo shape evolution when a surfactant is either added or removed from the solution. Thus, it was possible to control the geometries of the NCs with precision. This phenomenon of shape evolution represents a new way of readily controlling the morphology of semiconductor materials.

ASSOCIATED CONTENT Supporting Information Characterization details and additional SEM images. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author * Bongyoung Yoo, E-mail: [email protected], Tel: +82-31-400-5229, Fax: +82-31-4173701 *Jae-Hong Lim, E-mail: [email protected], Tel: +82-55-280-3523 ACKNOWLEDGMENT This research was mainly supported by the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) project of the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT & Future Planning (2013M3A6B1078870). This work was partially supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy (No. 20123010010160), the MOTIE (Ministry of Trade, Industry & Energy (10048778) and KSRC (Korea Semiconductor Research Consortium) support program for the development of the future semiconductor device.


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For Table of Contents Use Only

Hierarchical

Shape

Evolution

of

Cuprous

Oxide

Micro-

and

Nanocrystals by Surfactant-Assisted Electrochemical Deposition Sanghwa Yoona, Sung-Dae Kimb, Si-Young Choib, Jae-Hong Lim*c and Bongyoung Yoo*a

The shape evolution of Cu2O micro- and nano-crystals was investigated by controlling the surface energy using capping effects of different charged surfactants.


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Hierarchical Shape Evolution of Cuprous Oxide Micro- and

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