Foreign Metal Ions to Control the Morphology of Solution−Liquid−Solid

Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

pubs.acs.org/crystal

Foreign Metal Ions to Control the Morphology of Solution−Liquid−Solid Reaction Guanwei Jia‡ and Jiang Du*,†,§ †

Crystal Growth & Design Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 11/02/18. For personal use only.

Henan Province Industrial Technology Research Institute of Resources and Materials, Zhengzhou University, 450001 Zhengzhou, People’s Republic of China ‡ School of Physics and Electronics, Henan University, Kaifeng 475004, People’s Republic of China § Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: Morphology plays a decisive role in the properties of nanomaterials. In this paper, foreign metal ions have been proved to be an effective morphology controller for solution−liquid−solid (SLS) reaction. Multipod and branched II−VI semiconductor nanowires are formed in the presence of foreign metal ions. On the basis of the advantage of SLS mechanism, we explore Bi-seeded CdSe/CdS multipod heterojuntions and demonstrate their potential to produce more sophisticated and multicomponent nanomaterials.

1. INTRODUCTION Morphology is a key parameter for nanomaterial properties.1−3 For nanocrystals, morphology control mainly relies on the selection and optimization of precursors, surfactants, stoichiometric ratio, and other physical reaction parameters.4,5 For higher-dimensional (two-dimensional (2D) or three-dimensional (3D)) nanostructure, to achieve morphology control and innovation is still a sophisticated challenge.6 The addition of impurities, or foreign species, to nanomaterials syntheses can change the morphology of the nanostructures rather dramatically. For instance, Al3+ ions were found to influence the shape of CuxSe from nanospheres to nanocubes, directly influencing their plasmonic optical properties.7 Tungsten,8 iron,9 cobalt,10 and chromium ions11 have also been used to produce Pt-based nanocubes by manipulating nanocrystal nucleation and growth rate in the different crystallographic directions without relevant metal impurity. Additionally, organic impurities have also been found to be an effective morphology controller for nanocrystal synthesis.12 In the synthesis of CdSe nanocrystal, different phosphorus-containing impurities in tri-n-octylphosphine oxide (TOPO) exhibit different morphology control force. Di-n-octylphosphine oxide (DOPO) is shown to assist CdSe quantum-dot growth; di-n-octylphosphinic acid (DOPA) and mono-n-octylphosphinic acid (MOPA) are shown to assist CdSe quantum-rod growth, and DOPA is shown to assist CdSe quantum-wire growth.13 Multidimensional nanostructure can be regarded as zerodimensional (0D) building blocks rationally assembled into higher dimension.14 Impurity species should be considered as a potential morphology controller for higher-dimensional nanomaterial by the general or new mechanism. © XXXX American Chemical Society

3D branched semiconductor nanowire structures with narrow radii, direct transport pathway, and high surface area are attractive in optical and electrical research fields and also in the energy-harvesting, conversion, and storage applications.15,16 For instance, in solar cells, 3D branched nanowires can improve the light absorption significantly due to the increased optical path as well as additional light trapping through reduced reflection and multiscattering.17,18 Furthermore, the direct charge carrier transport pathway in both the stems and branches boosts the charge collection efficiency.19 These fascinating properties of 3D branched nanowire structures have therefore stimulated widespread interests in fabricating them. There have been many demonstrations of branched nanowire in recent literature. For instance, Kuno’s group synthesized branched CdSe nanowires by manipulating the Cd/Se/trioctylphosphine ratio.20 Meanwhile, the most common method might be the sequential deposition of a suitable metal catalyst, followed by vapor−liquid−solid (VLS), solution−liquid−solid (SLS), or other catalyst-driven nanowire growth. This process can be cycled as often as a new generation of branches is desired. Early examples include vapor-phase synthesis of branched nanowires of Si,21 GaN,21 GaP,22 InAs,23,24 and tin-doped indium oxide (ITO).25 Furthermore, continuous in situ generation of a VLS catalyst during nanowire growth provides the source for multiple generations of branches to nucleate and grow at the same time.26−28 This allows the growth of PbSe(PbS)29 and SnO228 Received: August 27, 2018 Revised: October 20, 2018 Published: October 23, 2018 A

DOI: 10.1021/acs.cgd.8b01289 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Figure 1. TEM images of multipod Bi-seeded CdSe nanowires synthesized in the presence of In(C2H3O2)3. The mole ratio of In(C2H3O2)3/Cd is (a) 0.0 (b) 0.3, (c) 0.6, and (d) 1.2.

with a very dense hierarchical branched structure. Additionally, phase-induced branch growth of multipod nanocrystals (NCs) has been synthesized for a variety of semiconductors such as tetrapod CdS,30 CdSe,31 and CdTe.32 The growth of multipod NCs started with zinc blende nuclei and grew arms in a wurtzite manner.32 Meanwhile, other methods such as solution growth on preformed nanowires and screw dislocation in combination with VLS have also been used to produce branched ZnO,33 Co3O4,34 and PbS.35 Here, we report an example of foreign metal ions to control the SLS reaction to form 2D and 3D nanomaterial. Previous studies show that solution−liquid−solid (SLS) reaction is widely used in the synthesis of nanowires due to the facile reaction conditions, inexpensive apparatus, and simple operation process.36 Optimization of catalyst seeds, surfactants, and chemical precursors are the key parameters to control the morphology, composition, and crystalline phase.37 Except above parameters, our new findings indicate that certain foreign metal ions can suppress the nanowire growth in the solvent and thus promote the precursor solubility in the Bi seeds and result in a multipod growth. On the basis of this preliminary mechanism, we detail a synthetic route to produce multipod CdSe, CdS, and ZnSe nanowires by the presence of In3+, Al3+, Ga3+, Co3+, Fe3+, and Sn4+ metal ions. By utilizing the general underlying mechanism, we also synthesized branched CdSe nanowires by varying the precursor’s injection time and In3+ ion concentration. Compared to previous methods,21,22,38 branched II−VI semiconductor nanowires can be achieved in one step and easier to control the degree of branching by foreign metal ions. At the same time, this method has the potential be widely applied in the SLS reaction. Furthermore, since we use Bi as the seeds of multipod nanowires, there is an enormous potential to make multipod multicomponent materials by sequentially using SLS process with different precursors. This is exemplified by preparing Biseeded CdSe/CdS multipod heterojuntions.

Figure 2. XRD patterns of multipod Bi-seeded CdSe synthesized in the presence of In(C2H3O2)3. The mole ratio of In(C2H3O2)3/Cd is (a) 0.0, (b) 0.3, (c) 0.6, and (d) 1.2. The peak position of CdSe is consistent with hexagonal CdSe JCPDS, 01-071-4772. The red reference pattern corresponds to hexagonal Bi JCPDS, 98-000-0118.

10 mL chloroform was injected. The nanowires were isolated and purified by standard centrifugation procedure (experimental details are contained in the Supporting Information). Figure 1a shows a representative transmission electron microscopy (TEM) micrograph of the CdSe nanowires. We initially attempted to prepare CdInSe2 nanowires, by mixing In(C2H3O2)3 with TOP/Se precursor. All other parameters were maintained the same. Figure 1b−d shows TEM images of V-shaped bipod, T-shaped tripod, and Xshaped tetrapod nanowires obtained in the presence of 0.11, 0.22, and 0.44 mmol of In(C2H3O2)3. Additionally, further increasing the In(C2H3O2)3 to 0.88 mmol will produce more pods but lead to a nonuniform pods diameter (Figure S1). Energy-dispersive X-ray spectroscopy (EDS) showed that In did not incorporate into the nanowires and that the Cd/Se molar ratio was close to 1:1. No compositional variation was observed from pod to pod or nanowire to nanowire within the error of the EDS detector. X-ray diffraction (XRD) analysis (Figure 2) evidenced that the crystallographic structure of the CdSe nanowires was not

2. RESULTS AND DISCUSSION In a typical preparation of CdSe nanowires, 0.05 g of CdO (0.39 mmol), 0.22 g of n-tetradecylphosphonic acid (TDPA; 0.79 mmol), and 10 g of TOPO were loaded in a 100 mL flask and then heated under N2. The mixture turned clear at ∼330 °C. Bi seeds solution (0.1 mL; 5 mg/mL) was injected followed by 0.1 mol of trioctylphosphine (TOP)/Se precursor. The reaction proceeded for 1 min and then cooled to ∼70 °C; B

DOI: 10.1021/acs.cgd.8b01289 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Figure 3. TEM images of multipod Bi-seeded CdSe nanowires synthesized in the presence of different metal ions, as noted.

Figure 5. (a) TEM image of Bi-seeded CdSe nanowires synthesized in the presence of CuCl2. Red circles show the tails, while blue circles show the Bi seeds. (b) STEM and EDS elemental mapping of the irregularly shaped tail.

To explore the universality influence of metal ions and anions on the CdSe nanowire growth, various experiments were performed with different metal salts: Al(C2H3O2)3, FeCl3, Co2O3, gallium(III) acetylacetonate (Ga(acac)3), InCl3, Sn(CH3CO2)4, and CuCl2. Except CuCl2 (discussed later), those metal salts demonstrated similar properties to form multipod structure (Figure 3). EDS and XRD data (Figure 4) show no relevant metal impurities or crystallographic structure change. Interestingly, those metal salts exhibit different morphology control capability: InCl3 > FeCl3 > Co2O3 > Ga(acac)3 > Al(C2H3O2)3 > In(C2H3O2)3 > Sn(CH3CO2)4. This activity series is based on the requirements amount of metal salts to achieve the similar multipod morphology. The amount variation of Ga(acac)3, Al(C2H3O2)3, and Sn(CH3CO2)4 was also performed and showed that higher amount of the metal salt resulted in more pods (Figures S2−S4), which is consistent with the In(C2H3O2)3.

Figure 4. XRD patterns of multipod Bi-seeded CdSe nanowires synthesized in the presence of different metal ions. The peak position of CdSe is consistent with hexagonal CdSe JCPDS, 01-071-4772. The red reference pattern corresponds to hexagonal Bi JCPDS, 98-0000118.

substantially modified by the presence of In(C2H3O2)3. Meanwhile, all XRD patterns nearly exactly match the hexagonal CdSe (JCPDS, 01-071-4772) and hexagonal Bi (JCPDS, 98-000-0118). With the increase of the In(C2H3O2)3 added amount, a peak at 22.2° is raised and can be assigned to Bi (003) facet, which is attributed to the increased Bi sizes (Figure 1). C

DOI: 10.1021/acs.cgd.8b01289 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Figure 6. Branched Bi-seeded CdSe nanowires (a, c, d) are obtained when the injection time of TOP/Se/In(C2H3O2)3 precursor was extended to 3 min. (inset) The enlarged area near Bi seeds; the scale bar is 50 nm. (b) HRTEM image of the branch region; the scale bar of inset is 50 nm. The mole ratio of In(C2H3O2)3/Cd is 1:2.

Figure 8. TEM images of multipod Bi-seeded CdS nanowires synthesized in the presence of different metal ions, as noted.

serrated body (Figure 5a, inset). Further analysis by EDS mapping (Figure 5b) evidenced that a Cu tip was formed in the irregularly shaped tail, which indicates that Cu2+ ions might be mainly consumed and reacted at the beginning of the reaction. Meanwhile, the rest of the SLS reaction was performed under a significantly low or zero Cu2+ ions concentration and thus maintained the monopod structure. Additionally, the remaining anion Cl− ions in the solution did not show the capability of forming multipod structure, which confirms that the functional part of the metal salts (such as InCl3 and FeCl3) is the metal ions rather than anions. The serrated nanowire morphology might be formed during the SLS growth process due to the trace amount of Cu2+ ion effect on the Bi seeds or inherited from the irregularly shaped tail. Furthermore, Cl− ion has been proved to strongly influence the

Figure 7. (a, b) TEM images of heavy branched Bi-seeded CdSe nanowires when the TOP/In(C2H3O2)3 was first injected to the reaction system. Then, the Se/TOP precursor was injected within 3 min. (c, d) TEM images of branched Bi-seeded CdSe nanowires when the injection time of TOP/Se/In(C2H3O2)3 precursor was extended to 7 min.

CuCl2 is an exception metal salt to form multipod CdSe nanowires. With the presence of 0.22 mmol of CuCl2, monopod nanowires were obtained (Figure 5a) rather than multipod structure. Interestingly, each CdSe nanowire has an irregularly shaped tail (Figure 5a, within red circles) and a D

DOI: 10.1021/acs.cgd.8b01289 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Figure 9. XRD patterns of multipod Bi-seeded CdS nanowires synthesized in the presence of different metal ions. The peak position of CdS is consistent with wurtzite CdS JCPDS, 01-080-0019. The red reference pattern corresponds to hexagonal bismuth JCPDS, 98-0000118).

Figure 11. Multipod Bi-seeded CdSe/CdS heterostructure nanorods: (a) STEM dark field image, (b) HRTEM image, and (c) EDS line scan.

CdSe nanowires were formed (Figure 7c,d). This reveals that, except a certain concentration of In3+, the instantaneous concentration of Se precursor is also a necessary condition to form branched CdSe nanowires, because extended injection time also results in a reduction of reactant instantaneous concentration, which cannot meet the multipod growth demands. On the basis of the above results, to make heavier branched CdSe nanowires, the reaction was improved by injecting TOP/ In(C2H3O2)3 first to achieve a higher In3+ concentration before the growth of CdSe nanowires. Meanwhile, TOP/Se injection time remained at 3 min to maintain the reactant instantaneous concentration. As we expect, the product has a much heavier branched structure (Figure 7a) that even part of one nanowire shows more than 30 branches (Figure 7b). By analyzing the growth trajectory, there is an alternate growth of convergence or branch, which might be caused by the fluctuation of local Se concentration. EDS and XRD data (Figure S5) also confirm the purity of those heavier branched CdSe nanowires. Auxiliary experiments were performed to explore the mechanism. When the amount of InCl3 (highest activity metal salt) is increased to 0.22 mmol, we can only get Bi seeds after the reaction, which indicates that higher In3+ concentration can totally suppress the pod growth (Figure S8). Similar results can be also found in the synthesis of those II− VI nanoparticles (NPs), foreign metal ions can significantly reduce the NPs size (Figure S9). Additionally, EDS data evidenced that, with the presence of foreign metal ions, the proportion of Cd and Se in the Bi seeds is higher than the control reaction. From the experimental results obtained, we speculate that the role of foreign metal ions is probably to suppress the CdSe growth in the solvent and thus promote the precursor solubility in the Bi seeds and result in a multipod growth. Thus, when we increase the amount of foreign metal ions, they have the same trend to form more and thinner pods.

Figure 10. Multipod Bi-seeded CdS NPs are obtained when the injection time of TOP/S/In(C2H3O2)3 precursor was extended to 3 min.

growth of II−VI semiconductors.39 Previous studies have shown that Cu+ ion can exchange Cd2+ in both CdSe and CdS nanostructure via cation exchange reactions.40−42 But we did not observe the similar results in our reaction, which might be due to the different valence states between Cu2+ and Cu+. We are currently studying in detail the influence of the Cu2+ ions on the SLS reaction. When the injection time of TOP/Se/In(C2H3O2)3 was extended to 3 min, branched CdSe nanowires (∼7 branches) were formed (Figure 6). Growth trajectory (the nanowires are grown from Bi seed36) indicates that only monopod or bipod nanowires were formed at the beginning of the reaction, because the concentration of In3+ ions at that moment was relatively lower than the rapid injection. As In3+ ions were not consumed, the concentration of In3+ ions was increased along with the TOP/Se/In(C2H3O2)3 injection and led to a tripod or tetrapod CdSe nanowire growth at the end of the reaction. High-resolution transmission electron microscopy (HRTEM; Figure 6b) further detailed the growth trajectory of bipod nanowires convergent into monopod. With further extension of the TOP/Se/In(C2H 3O 2 ) 3 injection time to 7 min, longer, thinner, and less-branched E

DOI: 10.1021/acs.cgd.8b01289 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

systematic and comprehensive experiments are still needed to perfect this method and achieve a general application.

To further explore the universality of foreign metal ions to control the morphology of II−VI semiconductors, CdS nanowires were synthesized by the similar recipe (Supporting Information) as that for CdSe. Interestingly, the activity series of the metal salts is consistent (Figure 8): InCl3 > Al(C2H3O2)3 > In(C2H3O2)3. EDS and XRD data (Figure 9) show that there are no relevant residual metal impurities. Additionally, CdS is more sensitive to foreign metal ions than CdSe. With the extension of the injection time of TOP/S/ In(C2H3O2)3 to 3 min, multipod CdS NCs (Figure 10) were formed rather than branched nanowires. On the basis of the color change observations after the injection, the CdS reaction speed is much slower than that of CdSe, especially when we extend the injection time. Thus, we speculate that, when the injection time is extended, the In3+ suppression effect is prevailed at the beginning of the reaction and can totally block the pod growth. CdS pods start growing simultaneously, until the concentration of S precursors is increased to a certain value along with the injection. Thus, branched structure was not formed. Again, compared to rapid injection (Figure 8), these multipod CdS nanowires are smaller and possess more pods. As pointed out above, multipod NCs have been synthesized for a variety of II−VI, IV−VI semiconductors.30,32 However, the limitation of those techniques is that the zinc blende seed cannot be used as a new growing point to synthesize more sophisticated structures, which possess unique or advanced properties. On the contrary, Bi-seeded multipod NCs has an enormous potential to construct multicomponent structure by alternate injection of different precursors. Here, Bi-seeded CdSe/CdS tripod heterojunctions (Figure 11) were successfully synthesized by making multipod CdS NCs first and subsequently injecting TOP/Se/In(C2H3O2)3 precursor. Each CdSe segment grows along CdS pods regularly and maintains the same structure. It is important to note that In3+ ion is also a key factor to maintain the second segment the same structure, because without In3+, the second segment will grow randomly (Figure S6). The CdSe segment is larger and brighter than CdS, which makes the heterojunction very obvious (Figure 11a). The lattice spacing of CdS and CdSe segments are ∼0.335 and 0.351 nm, respectively, which indicates the growth directions are both perpendicular to the CdS and CdSe hexagonal (002) lattice planes (Figure 11b). EDS line scan further exhibited that the Se and S switched sharply within a narrow range (Figure 11c). Since this was a one-pot reaction, the S residual in CdSe segment might be derived from the unreacted S remains in the previous CdS reaction. ZnSe was also chosen to verify the universal applicability of this method. Preliminary experiment shows that In3+ shows the similar effects on forming multipod ZnSe without any indium impurity (Figures S7 and S8). Additionally, ZnSe is more sensitive to metal ions compared to CdSe and CdS. In summary, we described how foreign metal ions catalyzed the formation of multipod CdSe, CdS, and ZnSe nanowires. Branched CdSe nanowires have been successfully synthesized, while the branch degree can be controlled by varying the injection time or the metal ion concentration. Furthermore, multipod Bi-seeded CdSe/CdS heterojunction nanowires have been synthesized by alternate injection of different precursors within one-pot reaction. Those Bi-seeded multipod structures opened a new platform to design and implement more sophisticated and multicomponent nanostructures. More

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b01289.

■

TEM images and relevant XRD of branched CdSe, ZnSe, CdS/CdSe heterojunctions, and CdTe nanoparticles synthesized at different conditions (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Guanwei Jia: 0000-0003-3752-8522 Jiang Du: 0000-0001-8949-6230 Author Contributions

All of the authors supervised the research project, participated in discussions, and made a conclusion. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support of the research was provided by the National Key R&D Program of China (Nos. 2016YFB0301101 and 2016YFB0301000), the Robert A. Welch Foundation (Grant No. F-1464), and the National Science Foundation through its Industry/University Cooperative Research Centers program (Grant No. IIP-1134849). C.J.S. acknowledges funding by a National Science Foundation Graduate Research Fellowship (Grant No. DGE-1110007).

■

REFERENCES

(1) Lim, B.; Jiang, M.; Camargo, P. H. C.; Cho, E. C.; Tao, J.; Lu, X.; Zhu, Y.; Xia, Y. PdPt Bimetallic Nanodendrites with High Activity for Oxygen Reduction. Science 2009, 324, 1302−1305. (2) Thurn-Albrecht, T.; Schotter, J.; Kastle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Ultrahigh-density nanowire arrays grown in self-assembled diblock copolymer templates. Science 2000, 290, 2126−2129. (3) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Semiconductor Nanocrystals as Fluorescent Biological Labels. Science 1998, 281, 2013−2016. (4) Sun, Y.; Xia, Y. Shape-Controlled Synthesis of Gold and Silver Nanoparticles. Science 2002, 298, 2176−2179. (5) Panthani, M. G.; Akhavan, V.; Goodfellow, B.; Schmidtke, J. P.; Dunn, L.; Dodabalapur, A.; Barbara, P. F.; Korgel, B. A. Synthesis of CulnS2, CulnSe2, and Cu(InxGa(1‑x))Se2 (CIGS) nanocrystal ″inks″ for printable photovoltaics. J. Am. Chem. Soc. 2008, 130, 16770− 16777. (6) Miszta, K.; Bertoni, G.; Dorfs, D.; Brescia, R.; Marras, S.; Ceseracciu, L.; Cingolani, R.; Dijkstra, M.; et al. Hierarchical selfassembly of suspended branched colloidal nanocrystals into superlattice structures. Nat. Mater. 2011, 10, 872−876. (7) Li, W.; Zamani, R.; Ibáñez, M.; Cadavid, D.; Shavel, A.; Morante, J. R.; Arbiol, J.; Cabot, A. Metal ions to control the morphology of semiconductor nanoparticles: copper selenide nanocubes. J. Am. Chem. Soc. 2013, 135, 4664. (8) Zhang, J.; Fang, J. A General Strategy for Preparation of Pt 3dTransition Metal (Co, Fe, Ni) Nanocubes. J. Am. Chem. Soc. 2010, 41, 18543. F

DOI: 10.1021/acs.cgd.8b01289 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

(9) Wang, C.; Daimon, H.; Lee, Y.; Kim, J.; Sun, S. Synthesis of monodisperse Pt nanocubes and their enhanced catalysis for oxygen reduction. J. Am. Chem. Soc. 2007, 129, 6974−6975. (10) Lim, S. I.; Ojeajiménez, I.; Varon, M.; Casals, E.; Arbiol, J.; Puntes, V. Synthesis of platinum cubes, polypods, cuboctahedrons, and raspberries assisted by cobalt nanocrystals. Nano Lett. 2010, 10, 964. (11) Loukrakpam, R.; Chang, P.; Luo, J.; Fang, B.; Mott, D.; Bae, I. T.; Naslund, H. R.; Engelhard, M. H.; Zhong, C. J. Chromiumassisted synthesis of platinum nanocube electrocatalysts. Chem. Commun. 2010, 46, 7184−7186. (12) Wang, F.; Tang, R.; Kao, J. L. F.; Dingman, S. D.; Buhro, W. E. Spectroscopic Identification of Tri-n-octylphosphine Oxide (TOPO) Impurities and Elucidation of Their Roles in Cadmium Selenide Quantum-Wire Growth. J. Am. Chem. Soc. 2009, 131, 4983−4994. (13) Wang, F.; Tang, R.; Buhro, W. E. The Trouble with TOPO; Identification of Adventitious Impurities Beneficial to the Growth of Cadmium Selenide Quantum Dots, Rods, and Wires. Nano Lett. 2008, 8, 3521. (14) Wang, X.; Li, Y. Selected-Control Hydrothermal Synthesis of αand β-MnO2 Single Crystal Nanowires. J. Am. Chem. Soc. 2002, 124, 2880−2881. (15) Bierman, M. J.; Jin, S. Potential applications of hierarchical branching nanowires in solar energy conversion. Energy Environ. Sci. 2009, 2, 1050−1059. (16) Cheng, C.; Fan, H. J. Branched nanowires: Synthesis and energy applications. Nano Today 2012, 7, 327−343. (17) Gu, F.; Gai, L.; Shao, W.; Li, C.; Schmidt-Mende, L. Heteroepitaxial growth of ZnO branches selectively on TiO2 nanorod tips with improved light harvesting performance. Chem. Commun. 2011, 47, 8400−8402. (18) Lin, Y.; Zhou, S.; Liu, X.; Sheehan, S.; Wang, D. TiO(2)/TiSi(2) heterostructures for high-efficiency photoelectrochemical H(2)O splitting. J. Am. Chem. Soc. 2009, 131, 2772−2773. (19) Sun, X.; Li, Q.; Lü, Y.; Mao, Y. Three-dimensional ZnO@ MnO2 core@shell nanostructures for electrochemical energy storage. Chem. Commun. 2013, 49, 4456−4458. (20) Kuno, M.; Ahmad, O.; Protasenko, V.; Bacinello, D.; Kosel, T. H. Solution-Based Straight and Branched CdTe Nanowires. Chem. Mater. 2006, 18, 5722−5732. (21) Wang, D.; et al. Rational Growth of Branched and Hyperbranched Nanowire Structures. Nano Lett. 2004, 4, 871−874. (22) Dick, K. A.; Deppert, K.; Larsson, M. W.; Mårtensson, T.; Seifert, W.; Wallenberg, L. R.; Samuelson, L. Synthesis of branched ’nanotrees’ by controlled seeding of multiple branching events. Nat. Mater. 2004, 3, 380−384. (23) Dick, K. A.; Deppert, K.; Karlsson, L. S.; Seifert, W.; Wallenberg, L. R.; Samuelson, L. Position-controlled interconnected InAs nanowire networks. Nano Lett. 2006, 6, 2842−2847. (24) Dick, K. A.; Geretovszky, Z.; Mikkelsen, A.; Karlsson, L. S.; Lundgren, E.; Malm, J. O.; Andersen, J. N.; Samuelson, L.; Seifert, W.; Wacaser, B. A.; et al. Improving InAs nanotree growth with composition-controlled Au In nanoparticles. Nanotechnology 2006, 17, 1344. (25) Wan, Q.; Wei, M.; Zhi, D.; Macmanus-Driscoll, J. L.; Blamire, M. G. Epitaxial Growth of Vertically Aligned and Branched SingleCrystalline Tin-Doped Indium Oxide Nanowire Arrays. Adv. Mater. 2006, 18, 234−238. (26) Zhu, J. Hyperbranched Lead Selenide Nanowire Networks. Nano Lett. 2007, 7, 1095−1099. (27) Fardy, M.; Hochbaum, A. I.; Goldberger, J.; Zhang, M. M.; Yang, P. Synthesis and Thermoelectrical Characterization of Lead Chalcogenide Nanowires. Adv. Mater. 2007, 19, 3047−3051. (28) Chen, Y. X.; Campbell, L. J.; Zhou, W. L. Self-Catalytic Branch Growth of SnO2Nanowire Junctions. J. Cryst. Growth 2004, 270, 505−510. (29) Bierman, M. J.; Lau, Y. K.; Jin, S. Hyperbranched PbS and PbSe nanowires and the effect of hydrogen gas on their synthesis. Nano Lett. 2007, 7, 2907−2912.

(30) Talapin, D. V.; Nelson, J. H.; Shevchenko, E. V.; Aloni, S.; Sadtler, B.; Alivisatos, A. P. Seeded growth of highly luminescent CdSe/CdS nanoheterostructures with rod and tetrapod morphologies. Nano Lett. 2007, 7, 2951. (31) Pang, Q.; Zhao, L.; Cai, Y.; Regnault, N.; Wang, N.; Yang, S.; Ge, W.; Ferreira, R.; Bastard, G.; et al. CdSe Nano-tetrapods: Controllable Synthesis, Structure Analysis, and Electronic and Optical Properties. Chem. Mater. 2005, 17, 5263−5267. (32) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Controlled growth of tetrapod-branched inorganic nanocrystals. Nat. Mater. 2003, 2, 382−385. (33) Cheng, C.; Liu, B.; Yang, H.; Zhou, W.; Sun, L.; Chen, R.; Yu, S. F.; Zhang, J.; Gong, H.; Sun, H.; et al. Hierarchical assembly of ZnO nanostructures on SnO(2) backbone nanowires: low-temperature hydrothermal preparation and optical properties. ACS Nano 2009, 3, 3069−3076. (34) Liu, J.; Jiang, J.; Cheng, C.; Li, H.; Zhang, J.; Gong, H.; Fan, H. J. Energy Storage: Co3O4 Nanowire@MnO2 Ultrathin Nanosheet Core/Shell Arrays: A New Class of High-Performance Pseudocapacitive Materials (Adv. Mater. 18/2011). Adv. Mater. 2011, 23, 2075− 2075. (35) Bierman, M. J.; Lau, Y. K.; Kvit, A. V.; Schmitt, A. L.; Jin, S. Dislocation-driven nanowire growth and Eshelby twist. Science 2008, 320, 1060−1063. (36) Trentler, T. J.; et al. Solution-Liquid-Solid Growth of Crystalline III-V Semiconductors: An Analogy to Vapor-LiquidSolid Growth. Science 1995, 270, 1791−1794. (37) Steinhagen, C.; Akhavan, V. A.; Goodfellow, B. W.; Panthani, M. G.; Harris, J. T.; Holmberg, V. C.; Korgel, B. A. Solution−Liquid− Solid Synthesis of CuInSe2 Nanowires and Their Implementation in Photovoltaic Devices. ACS Appl. Mater. Interfaces 2011, 3, 1781− 1785. (38) Dong, A.; et al. Solution-Based Growth and Structural Characterization of Homo- and Heterobranched Semiconductor Nanowires. J. Am. Chem. Soc. 2007, 129, 12254−12262. (39) Kim, M. R.; Miszta, K.; Povia, M.; Brescia, R.; Christodoulou, S.; Prato, M.; Manna, L.; et al. Influence of chloride ions on the synthesis of colloidal branched CdSe/CdS nanocrystals by seeded growth. ACS Nano 2012, 6, 11088−11096. (40) Li, H.; Zanella, M.; Genovese, A.; Povia, M.; Falqui, A.; Giannini, C.; Manna, L. Sequential cation exchange in nanocrystals: preservation of crystal phase and formation of metastable phases. Nano Lett. 2011, 11, 4964−4970. (41) Li, H.; Brescia, R.; Krahne, R.; Bertoni, G.; Alcocer, M. J.; D’Andrea, C.; Scotognella, F.; Tassone, F.; Zanella, M.; et al. BlueUV-emitting ZnSe(dot)/ZnS(rod) core/shell nanocrystals prepared from CdSe/CdS nanocrystals by sequential cation exchange. ACS Nano 2012, 6, 1637−1647. (42) Miszta, K.; Dorfs, D.; Genovese, A.; Kim, M. R.; Manna, L. Cation exchange reactions in colloidal branched nanocrystals. ACS Nano 2011, 5, 7176−7183.

G

DOI: 10.1021/acs.cgd.8b01289 Cryst. Growth Des. XXXX, XXX, XXX−XXX