Doped or Not Doped: Ionic Impurities for Influencing the Phase and

Review pubs.acs.org/cm

Doped or Not Doped: Ionic Impurities for Influencing the Phase and Growth of Semiconductor Nanocrystals Amit K. Guria and Narayan Pradhan* Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032, India ABSTRACT: Dimension, phase, and shape tunable architectures of nanostructures are of interest due to morphologydependent modulation of the properties of materials. However, the manipulation of specific structures of nanomaterials, in order to achieve unique physical and chemical properties by modulating the synthesis parameters, remains challenging. Wet chemistry has proven to be quite efficient for such shape- and size-controlled synthesis. Recent architectural progress with different nanostructures suggests that dopants or impurities present in the reaction system can affect the crystal growth and tune the shapes of crystals in ways that go beyond the classical approach where the nature of the precursors, ligands, and reaction conditions dependent facets controlling crystal growth mostly determined the shape. These foreign atoms or ions may or may not be retained in the crystal lattice and may or may not become incorporated into the crystal. This is a new era of current research on the architecture of nanostructures and involves deep fundamental insights that are unsolved or unknown to date. This review presents several reports where foreign ions or impurities influence the crystal growth, tune shapes of nanostructures, and unveil the chemistry involved in their synthesis processes. The ability of those species, either from inside the crystal (as dopants) or from bulk solution, to affect the growth and even the nucleation itself, is discussed. Furthermore, the specific roles of some of these impurities either alone or in combination with other synthesis parameters like surfactants, solvents, temperature, and precursors in forming nanostructures using a variety of inorganic compounds as precursors are also summarized. Finally, the challenges and future perspectives of these novel impuritydriven strategies are presented. wet chemical route.18−30 This method also offers easy chemical processability of the obtained nanocrystals. The solutionprocessed synthetic approach is particularly attractive to chemists as it is simple and involves fascinating chemistry. The surface chemistry also plays a key role in controlling the crystal growth and, hence, helps in designing intriguing nanostructures. In this approach, a surfactant or mixture of surfactants are generally used for the surface construction of nanocrystals.18−30 Phase and shape are also controlled by monitoring the reaction temperature and selecting suitable precursors and polarity of the solvent used. All these factors influence the composition, nucleation, and growth of nanocrystals, thus determining the surface structure of crystals and affecting their electrical, optical, and other properties.36−48 A few techniques in the wet chemical approach such as the use of adequate surfactants, proper surface binding of organic ligands, 38,39,49,50 monomer activity-mediated kinetic growth,48,51−53 catalytic routes,42,54−57 seeded growth,43,44,58,59 template synthesis,37,40,41,60−62 and oriented attachment of seed nanoparticles63−67 have been extensively studied for designing nanocrystals with anisotropic morphology.

1. INTRODUCTION Semiconductor nanocrystal photocatalysts have drawn considerable attention over the past decade due to their intriguing properties that may be of use to solve problems within the areas of energy conversion and storage and the environment. The activity and selectivity of these catalysts are dependent on a set of parameters, including composition, size, shape, and surface structures, among others; in addition, the catalytic properties show facet dependence for nanocrystals of a particular composition.1−10 Thus, the fabrication of anisotropic nanocrystals with catalytically active facets and their assembly into complex patterns is an active area of research. Spatial dimensionality also affects the degree of confinement when an electron−hole pair is confined to one or more dimensions while approaching the limit of the bulk exciton Bohr radius. Thus, the optoelectronic properties of nanocrystals are a function of nanocrystal morphology.11−17 Understanding the underlying mechanism of creating the desired morphology and manipulating the synthesis process for different functional nanomaterials with precise control over their phase and dimensions might enable these nanostructures to be optimized for many important applications. Hence, the control of size, shape, phase, and composition has occupied a central position in the field of nanocrystal research from the beginning.18−35 Furthermore, high quality nanocrystals with monodispersed size/shape (size variation < 5%) can be easily obtained using a © 2016 American Chemical Society

Received: May 18, 2016 Revised: July 5, 2016 Published: July 12, 2016 5224

DOI: 10.1021/acs.chemmater.6b02009 Chem. Mater. 2016, 28, 5224−5237

Review

Chemistry of Materials

Figure 1. (a) Schematic representation of transformation of WZ hexagonal ZnS platelet to distorted ZB ZnS platelet on Mn doping. (b) Atomic models of a WZ and a ZB phase of ZnS in their viewing directions. (c) HRTEM image of single WZ undoped ZnS and ZB doped ZnS platelets. (d) XRD patterns of undoped and doped ZnS. Parts c and d are adapted with permission from ref 100. Copyright 2011 American Chemical Society.

variations of reaction parameters, the surface ligands, their facet-dependent binding, etc. mostly control the crystal phase as well as the morphology of the nanostructures. However, impurity-induced shape tuning is a new concept for metal chalcogenides and will be focused on here. Interestingly, for a number of cases,109−128 these shape-controlling species were not inserted inside the lattices of the nanocrystals; these are discussed in a later section. In the search for shape control using dopants, it has been recently reported that dopants do, in fact, influence the morphology of metal chalcogenides.100−108 2.1.1. Thermal Incorporation of Dopant and Resulting Change of Shape/Phase. Doping of transition metal ions in semiconductor host nanocrystals has been extensively studied. From different doping processes, it is established that inserting dopants with different size and/or charge into the lattice of host nanocrystals can strongly influence the growth process. One of the most common doping systems investigated to date is Mn doping in different group II−VI semiconductor host nanocrystals. This typically leads to paramagnetic yellow-orange emitting nanocrystals. The intense emission from this Mn doping was also first reported in a sulfide (ZnS) host.68 However, when the doping process was further investigated, it was successfully extended to ZnS and ZnSe but remained difficult for CdSe.69−72,74,75,80 This created concern over the generalization of the synthesis. Soon after, it was established that facet adsorption and the crystal phase of the host were the key issues facilitating the doping. For ZnSe hosts, the zincblende (ZB) phase was observed to be energetically more favorable for Mn doping, and this was supported by both experimental and theoretical results. Based on this concept a very intriguing result on this phase-specific Mn accommodation was reported by Karan et al. for a ZnS host. It was observed that, to enable dopant retention in the crystal lattice, Mn even changed the phase of host ZnS.100 At a certain reaction temperature, Mn ions were first adsorbed onto facets of wurtzite (WZ) ZnS and nucleated the phase transformation to ZB. Furthermore, annealing propagated the entire nanocrystal into the ZB phase. The presence of Mn in the crystals was confirmed by electron paramagnetic resonance (EPR) and photoluminescence studies, and the phase change was supported with transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis. A rapid-cooling protocol was

Impurity doping has recently emerged as an important method for inducing new material properties in different host nanomaterials. Extensive studies of doping in semiconductor as well as metal oxide hosts have been carried out, and the resulting doped nanomaterials have been explored for lightemitting, photovoltaic, catalysis, and sensing applications, and also in optoelectronics for enhancing the device performance.68−81 Dopants can also change the magnetic behavior of nanocrystals.82−87 Furthermore, it has been reported very recently that dopants/impurities can influence the charge carrier density of host materials and generate localized surface plasmon resonance in semiconductor nanostructures.88−92 The chemistry of designing these materials and understanding the doping in the host lattice remain the key behind all these successes in obtaining new dopant-controlled functional materials. Published reports on doping chemistries reveal that dopants or impurities also drastically control the crystal growth and tune the phase and shape of the host materials.93−135 More intriguingly, in several cases, either cationic or anionic impurities control the crystal growth even without entering into the lattice of the host nanostructures; they appear to extend assistance even while remaining in the bulk solution.94−99,109−128 These observations were reported for metals93−99 and semiconductor metal chalcogenides100−128 as well as different lanthanide-based materials.129−135 Thus, the adsorption of foreign impurities onto the nanocrystals during their formation indeed has a significant impact on crystal growth and shape evolutions. In this review article, we focus on an overview of the most recent progress in foreign-ion-induced control of crystal phase, size, and morphology of solutionprocessed nanomaterials, mostly confined to semiconductor metal chalcogenide nanostructures.

2. PHASE, SIZE, AND MORPHOLOGY CONTROL OF SEMICONDUCTOR NANOCRYSTALS 2.1. Impurities Retained in Nanocrystals (doped). The synthesis of various metal chalcogenide nanostructures and their shape architectures has crossed several milestones, and the chemistry associated with their formation is also largely understood. The unique characteristics of these materials include the relative ease with which they can be synthesized and their applications in catalysis. Reactivity of the precursors, 5225


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Ga(III) diffused into CuInS2 nanorods, retaining their shape. However, the presence of Sb(III), which mostly adsorbed onto one of the facets of the rods, facilitated more Ga incorporation, and two rods were fused, leading to the formation of nanodumbbells (NDBs). The annular dark-field scanning transmission electron microscopy (ADF-STEM) and HRTEM images of CIGS nanocrystals in the absence and presence of Sb are shown in parts b, c and d, e, respectively, of Figure 2. 2.1.2. Dopants Affecting the Subsequent Growth Rate and Shape Change. Dopants, being different in size and charge, can change the subsequent growth rate when adsorbed onto the facets of host nanocrystals.103,104 When a crystal grows in a favorable process, the adsorption of dopants typically changes the facet energy. Accordingly, the ongoing growth process is affected. One such observation is recently reported for Mn doped ZnSe nanostructure.103 Atomic models showing the change in the growth directions in the presence and absence of Mn in ZnSe are presented in Figure 3a. In a typical

adopted to arrest the phase, and analyses were done at room temperature. Consequently, the hexagonal disk shape was also observed distorted to circular. A schematic presentation of the atomic models showing a WZ ZnS platelet changing to ZB is shown in Figure 1a, and the atomic arrangements of both phases are presented in Figure 1b. High-resolution TEM (HRTEM) images supporting the shape distortion are shown in Figure 1c, and XRD patterns of both phases are depicted in Figure 1d. The most interesting part of this experiment was the reversibility of the changes during the slow cooling process, where Mn was ejected with reversal of the shape and phase to hexagonal disk and WZ, respectively. Hence, the results strongly support the idea that the dopant insertion can change the phase composition as well as the shape of the host nanocrystals. A similar study on phase change was also performed for Co doping in a CdS host where the phase was altered from ZB to rocksalt (RS).101 Typically, bulk CdS undergoes a WZ to RS phase transition at a pressure of around 2−3 GPa, depending on the particle size. Interestingly, doping with Co2+ ions reduced the phase transition pressure (CdS = 4.89 GPa and CdS:Co = 4.06 GPa). In Co2+-doped CdS, defects were incorporated in CdS owing to Co d-orbitals mixing with their valence and conduction bands. The authors explained that stress concentrated in these defects when high pressure was applied, and sufficient local stress initiated the phase transition. Thus, the dopant had reduced the phase transition pressure. The first-principles calculations based on the density functional theory (DFT) revealed that smaller Co2+ substitution doping reduces the lattice constants and influences the volume of the ZB and RS structures which favors the phase transition. In a different aspect, Singh et al. studied the effect of dopants on the shape architecture of a multinary alloyed sulfide system.102 This was established using Sb dopant with CuIn1−xGaxS2 (CIGS) nanocrystals as the host. In the adopted synthetic protocol, in absence of Sb monodisperse nanorods (NRs) were produced, but in the presence of dopant Sb paired nanorods were formed via head-on fusion. The reaction process, starting from Cu2S followed by In(III) and Ga(III) diffusion, is schematically shown in Figure 2a. Without Sb(III),

Figure 3. (a) Schematic presentation of the effect of Mn2+ dopant on shape evolution of ZnSe nanocrystals with their atomic models. TEM images of hemisphere shaped (b) and spherical (c) ZnSe nanocrystals. Parts b and c are adapted with permission from ref 103. Copyright 2013 American Chemical Society.

synthesis in alkylamine medium, ZnSe was grown anisotropically to form hemisphere-like nanostructures with appropriate Zn and Se precursor. In contrast, the presence of Mn dopant strongly hindered the directional growth and led to smaller spherical shaped of ZnSe nanocrystals. Being in ZB phase, Mn adsorption on ZnSe crystals was facilitated, and this changed the surface energy. As a consequence, the growth along the [111] polar direction was hindered, and only spherical particles were formed. TEM images of ZnSe synthesized in the absence and presence of Mn are shown in Figure 3b,c, respectively. This result suggests that the dopants can play a crucial role in crystal growth and shape architecture of semiconductor nanomaterials in solution. The role of dopants in controlling the nucleation, growth, and also the rate of shape change was examined during the conversion of Pd(0) to palladium selenide nanocrystals.104 This was observed with Ag dopant. While Pd(0) nanocrystals, during selenization, evolve to Pd17Se15 cuboids, the presence of Ag dopant twisted the shape to hollow Ag-doped Pd17Se15 nanocubes. In addition, the rate of selenization of the doped nanocrystals was observed to be much faster. Figure 4a presents

Figure 2. (a) Schematic representation of the shape evolution. (b, d) HAADF-STEM images of CIGS NRs and Sb-doped CIGS NDBs and corresponding HRTEM images (c, e). Adapted with permission from ref 102. Copyright 2015 American Chemical Society. 5226


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Figure 4. (a) Schematic presentation of the effect of Ag+ dopant on shape evolution of Pd17Se15 nanocrystals with their atomic models. TEM images of cuboid (b) and hollow cube (c) shaped Pd17Se15 nanocrystals. HAADF-STEM image of hollow cubes is shown in the inset in (c). Schematic presentation of (d) a possible graded structure during the selenization process and (e) the selenization process approached along one direction and (f) from all directions in doped Pd(0) nanocrystals. Parts b−f are adapted with permission from ref 104. Copyright 2015 American Chemical Society.

Figure 5. (a) Representative TEM images of resulting nanostructures of ZnO obtained with 0%, 5%, and 50% Mg(St)2. Insets are the models corresponding to the obtained shapes. (b) Plot of Mg concentrations in the nanocrystals. Sections I, II, III, and IV represent the nanocrystals with faceted particles, tetrapods, nanowires, and irregular shaped particles, respectively. Regions T1 and T2 represent two shape transition zones. Adapted with permission from ref 106. Copyright 2010 American Chemical Society.

doped CdTe nanorods with variable diameter in water.105 The dipole−dipole attractions among CdTe nanoparticles had organized them, and nanorods were formed on annealing. It was found that the diameter of the CdTe nanorod increased gradually with increased Mn incorporations. Compared to undoped CdTe, Mn incorporations reduced the dipole−dipole attractions. Hence, doped CdTe nanocrystals undergo oriented attachments and change their morphology. 2.1.3. Valency of Dopant Affecting the Shape of Host Nanocrystals. Dopant inorganic ions having variable oxidation states can possess variable charges and also sizes. Accordingly, the coordination sites in the crystal lattices as well as the binding energy also changes. Accordingly, the growth process is affected and so also the shape/phase of host nancrystals. In a recent study on doping antimony in vanadium dioxide (VO2) nanostructures, the phase and shape controlled crystal growths are reported according to the valency of Sb.107 For antimony(III) (Sb 3+ ) doping, hexagonal-shaped monoclinic VO 2 nanostructures were synthesized. In addition, controllable polymorphs were designed via the hydrothermal method. Intriguingly, it was noticed that an appropriate amount of Sb3+ was required for promoting the phase transition from tetragonal to monoclinic. From these observations, the authors proposed that dopants which were larger in radius and lower in valence than V4+ ions introduced extra oxygen vacancies during the formation of VO2 nanostructures. Hence, on Sb3+ doping,

atomic models of two distinct shapes of doped and undoped Pd17Se15 obtained under identical reaction conditions but in the absence and presence of dopant Ag. The corresponding TEM images showing these structures are presented in Figure 4b. The high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image inserted in Figure 4c clearly reveals the hollow nature of the doped nanostructure. Further investigation and detailed analysis suggested that these cube structures were formed via graded structures with Pdvariation (scheme shown in Figure 4d). Interestingly, for the undoped nanocrystals, selenization is approached along one direction (Figure 4e) leading to cuboid shapes. On the other hand, for the doped nanocrystals, the approach was from all around the nanocrystals (Figure 4f). The presence of Ag causes the formation of silver selenide instantaneously, and then this propagates into the entire nanocrystal. The fast rate allowed selenization to occur all around the metal Pd(0) surfaces; this triggered the Kirkendall effect. As a consequence, the inner Pd atoms quickly diffused outward, producing a hollow nanostructure. These interesting results suggest that dopants can enhance the rate of chemical reaction, change the shape of the nanocrystal, and also induce the Kirkendall effect. Besides the crystals’ phase and shape changes, the presence of dopant can change the dipole moment of host nanocrystals and trigger their oriented attachments. In a state-of-art example, this was noticed by Ramasamy et al. during synthesizing Mn5227


hosts

without dopant

Pd17Se15 ZnO

with dopant

RS nanocrystals nanodumbbells monoclinic

Shape and/or Phase of Nanocrystals circular disk, ZB spherical particle, ZB nanorods with lower aspect ratio

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Cl− Cl− Cl− Cl−

Cl−

Cl−, CH3COO, NO3− Cl− Cl−, I− Cl−

rod-like hetero-NCs -nanoparticles

Au−CdS CdS CdX (X = S,Te) CdX (X = S,Se) CdSe CdSe CdSe/CdS CdSe

nanorods, nanoplates nanoparticles nanorods -nanorods

--

CdS

tetrapod hexagonal pyramids tetrapods pyramids

etched NCs

nanopyramids, microspheres, nanorods, and nanoparticles, respectively flower and bullet-like hetero-NCs sphere/cube/rod, nanodendrite nanopencils

cuboid hollow cube hehagonal pyramid, tetrahedron, nanorods, WZ and ZB WZ Nb5+ TiO2 nanoplatelets peanut shape Impurities in Bulk Solution That Control the Nanocrystal Shape and Phase Ca2+, Al3+ β-In2S3 nanoparticles nanobelts, nanoflakes Al3+ Bi19S27Br3 Bi2S3 nanowires Bi19S27Br3 NWs with reduced length Al3+ Cu3Se2 spherical NCs nanocubes Al3+ CuSe monoclinic CuClSe2 klockmannite CuSe nanosheets microribbons Al3+, Zn2+, Ag+, CuO elliptical nanosheets rounded nanosheets or nanoparticles Mg2+ Al3+, In3+, Ga3+, ZnO nanowires platelets (bivalent ions) and needles Cd2+, Cu2+, Mg2+ (trivalent ions) Cu+, Cu2+, Cl−, In2O3 flower-shaped NCs quasi-spherical NCs CH3COO− In3+, Cl− CdSe NCs NCs Cl− ZnS straight nanorods/ kinked nanorods/nanowires nanowires halide ions Ni9S8 and Cu2S nanorods crossed platelets

Ag+ Mg2+

Impurity That Is Retained in Crystal (dopant) for Controlling Mn2+ ZnS hexagonal disk, WZ Mn2+ ZnSe hemisphere, ZB Mn2+ CdTe nanorods with higher aspect ratio Co2+ CdS ZB nanocrystals Sb3+ CuIn1−xGaxS2 nanorods Sb3+/Sb5+ VO2 tetragonal

dopants/impurities

change in shape and phase of nanocrystals (NCs)

Cl− removes the oleate ligand from ZB seed and favors the WZ arm growth124 Cl− monitors the adsorption/desorption processes of ligands for reshaping126 Cl−, by forming complexes, decreases the concentration of reactive Cd-precursor and favors the CdS arm growth125 Cl− regulated the ligand attachment, and it also acted as a ligand and facilitated the reshaping; when present initially, nucleation and growth were affected by forming a complex with Cd2+127

Cl− etched selective facets122,123

halide ions hindering the formation of metal thiolate, thereby inhibiting nucleation events and slowing down growth118 anions affected nucleation by forming active cadmium complexes with different decomposition temperatures; adsorption of those ions onto the energetically preferred facets of NCs affected growth119 concentration of Cl− affected the CdS nucleation on Au facets128 halide ions act as ligand and affect the directional growth120 adsorption of Cl− as a ligand reshaped the NCs through Ostwald ripening121

facets specific adsorption of cations changes the growth; anions control the release rate of In3+ and affect the size of NCs116 In3+ favors the dissolution; Cl− stabilizes the magic size CdSe particles111 Cl− ions slowed down the nucleation and growth rates117

metal ions form complexes and control the release rate of OH−, thus concentration of precursor and growth rate change115 complexes formed by these ions have electrostatic interaction with NCs’ facets114

cations remove ligands from specific facets of NCs and favor their oriented attachment along that direction109 Al3+ partially inhibited the absorption of S2− by BiBr3 and hindered the growth in a specific direction110 Al3+ inhibits the binding of ligands to specific facets and controls growth to form nanocubes112 Al3+ extracts Cl− from Cu-precursor and enhances the Ostwald ripening rate113

increasing concentration of dopant gradually elongates the c-axis and reduces the dimension of ab plane108

dopants decrease the phase transition pressure by altering the crystal parameters101 facet selective dopant adsorption facilitates the nanorod end-to-end fusion102 dopants with higher cationic charge increase the growth rate; smaller nanocrystals with Sb3+ and larger nanocrystals with Sb5+107 dopants increase the rate of selenization and trigger Kirkendall effect in Pd nanocrystals104 dopant changes the initial growth stage; shape and phase both change with dopant concentration106

thermal incorporation of dopant changes the local crystallographic arrangement100 change the energy of specific facets and hinders the directional growth.103 dopants decrease the intraparticle attraction and trigger oriented attachment procedure105

plausible role of dopant

Table 1. Dopants/Impurities and Their Function During Crystal Growth in Various Host Semiconductor Nanostructures

Chemistry of Materials Review


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reported for both cationic and anionic inorganic species.109−128 Their presence may initiate new growth kinetics or affect the ongoing growth of the nanocrystals leading to a new shape or reshaping of a particular shape. These changes were observed both for inorganic cations and for anions present in the solution. 2.2.1. Cations Modulating the Morphology. Cationic impurities are the inorganic cations which temporarily adsorb on energetically favorable facets of the nucleated nanocrystals and control the growth. Also, there are reports where these cations make complexes with the reagents to decrease the monomer concentration, resulting in minimized growth rates. The major focus in the following section of this review is restricted to cations controlling the growth but do not enter to the host lattice. 2.2.1.1. Cations Influencing Activity of Surface Ligands and Shape Change. Cations present in solutions may form complexes with the used surfactant and precursor species, and thus cations can desorb ligands from specific facets of host nanocrystals. As a consequence, the growth along those particular facets may be affected. A remarkable difference in growth patterns in presence and absence of foreign cations Ca2+ and Al3+ is reported by Tian et al., for the synthesis of In2S3 nanostructures.109 Without these ions, β-In2S3 nanoparticles were obtained as per the adopted synthetic protocol. However, in the presence of these impurities in the solution, 2D β-In2S3 nanobelts and nanoflakes with ∼2 nm thicknesses respectively were the ultimate products. Interestingly, the mechanism proposed here is the ion induced triggered oriented attachments. It was explained that metal ions in the solution plausibly desorbed some of the octylamine (ligand) molecules from particular facets and favored the oriented attachments in those directions. Hence, the case here was the bulk solution impurity favored the oriented attachment which regulated the morphology of the nanostructures. In another example, Wu et al. reported the synthesis of single-crystalline Bi19S27Br3 and Bi19S27Br3−xIx nanowires with tunable lengths, in the presence of impurity metal ions (Al3+, Fe3+, Fe2+, Co3+, Co2+, and Ni2+ions).110 It was observed that foreign ions had control over composition, as Bi2S3 was formed in the absence of those ions and counteranions had no role on morphological control. Increased concentration of Al3+ ions had decreased the length of the NWs, keeping the diameters constant. On the basis of this observation, it was proposed that foreign ions probably promoted crystal growth in a specific direction. It was also proposed that Al3+ partially inhibited the absorption of S2− by BiBr3 precursor, affecting the composition. Both these examples suggest that inorganic cations without entering the crystal lattice can control the reaction kinetics and can also alter the shape of the nanostructures. Recently, shape change in copper selenide nanostructures using aluminum ions was reported.112,113 In a very interesting observation, Li et al. established that treatment of Al3+ ions externally to an ongoing reaction leading to spherical Cu3Se2 nanocrystals surprisingly resulted into highly monodisperse nanocubes.112 The shape transformation is schematically shown in Figure 6a. Figure 6b shows the TEM micrographs of quasispherical copper selenide nanocrystals obtained on reaction of CuCl with ODE-Se in the presence of hexadecylamine at 180 °C. Figure 6c presents the TEM image of copper selenide nanocrystals obtained using Al(NO3)3·9H2O under otherwise similar conditions. Using different counter cations and anions, it was established that only Al3+ ions played the key impurity

the positively charged nuclei suppressed the adsorption of hydrated VO2+ ions and, hence, inhibited the growth of doped monoclinic VO2 nanostructures. On the other hand, Sb5+ dopants which have higher cationic charge than V4+ could induce the growth of doped monoclinic VO2. As a result, with doping Sb3+ while tuning the size, limiting to 8 to 30 nm, Sb5+ doping tuned the anisotropic growth up to 500 nm (length) and 200−300 nm width. Here the nanocrystals’ growth mechanism and growth rate were strongly affected by presence of dopants and their oxidation states. 2.1.4. Effect of Dopant Concentration on the Shape/ Phase of Host Nanocrystals. Insertion of more dopants inside the lattice of a host nanocrystal continuously changes the energy of active facets and consequently the growth pattern is affected. In a very interesting study, Yang et al. reported the magnesium dopant concentration induced shape and phase evolution of colloidal zinc oxide nanocrystals.106 Varying the concentration of dopant precursor, different shapes of doped nanocrystals ranging from tetrapods to ultrathin nanowires were designed (Figure 5a). The crystal phase was also modified leading to the variable morphologies (Figure 5b). It was established that the primary growth stage was affected by Mg dopants which further tuned the morphology as well as the phase. This doping strategy was also extended to other ions, such as Cd2+, Mn2+, and Ni2+ in the same ZnO hosts. These results suggest that the foreign ions concentration in the reaction system could drastically alter both shape and phase, and in this case those were also retained in the crystal lattice of ZnO. Dopant induced phase changes of ZnO are also studied by other groups.136−139 A survey on doped ZnO systems, in those reports, reveals that the transition from wurtzite (B4) phase to rocksalt (B1) phase of ZnO is always favored with different types of transition metal (V, Cr, Mn, Fe, Co, or Ni) dopants. The shape change has also been reported for niobium (Nb) doping in metal oxide TiO2 nanocrystals.108 In the presence of dopant, the shape gradually transformed from tetragonal platelets to “peanut-like” 1D nanorods. On the contrary, under similar reaction conditions in the absence of Nb, platelet shaped nanocrystals were the only product. Increasing of dopant Nb precursor concentrations observed tuned the aspect ratio of the doped TiO2 by systematically elongating the c axis. As a consequence, stretched “peanut-like” shaped nanocrystals were obtained. While with low concentration of the dopant the crystal growth was not significantly affected, with higher Nbprecursor concentration (up to 14%), the longitudinal dimension of nanocrystals elongated up to ∼15.4 nm and resulted in anisotropic shape. The authors had established that, with increasing Nb, not only the nanocrystal elongated along the c direction but also nanocrystal dimension in the ab plane were also reduced. All these reports, on shape switching of solution processed nanocrystals by beneficial impurity species present in crystals, established that the presence of foreign ions can be regarded as a new parameter for shape control. Summarizing all these reports, the dopant induced growth activities are provided in Table 1. 2.2. Impurities in Bulk Solution that Result in Control of Nanocrystal Shape and Phase. Adsorption of impurities or foreign ions onto the surface of nanocrystals during growth or their mere presence in solution without entering the nanocrystal lattice can also change the growth pattern and alter the morphology of the nanocrystals. These were extensively 5229


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nanoplates were obtained. Hence, all these results suggest that foreign cations indeed played a crucial role in changing the growth rate and controlling the morphology of the selenide nanostructures. 2.2.1.2. Cations Modulating the Morphology by Balancing the Dissolution/Formation of Nanocrystals. On balancing the rate of dissolution/formation of nanocrystals, impurity cations, without entering into the host crystal lattice, can change the growth pattern and alter the morphology. This was reported by Chikan and co-workers, and they studied the effect of indium ions on the growth kinetics of various CdSe nanocrystals.111 Here, the size and size distributions were mostly regulated with the formation of magic size CdSe nanocrystals. In the presence of foreign agents like InCl3 and InMe3, the growth as well as dissolution rates of these nanostructures was observed to be affected. Functions of different cationic and anionic species were studied by applying different indium(III) salts and different metal chloride salts. When InMe3 was used, magic size crystals quickly disappeared, whereas the presence of NaCl slowed down that process. Using InCl3, which produces both In3+ and Cl− ions, the dissolution rate of magic size particles and hence the monomer concentration could be controlled. With the increasing amount of InCl3, which increases the concentration of Cl− more than that of In3+, the size of nanoparticles was found to be increased. Hence it was concluded that while indium ions helped in removing the magic size CdSe crystals, the chloride ions stabilized the same. Here, chloride ions were not incorporated in crystals though indium ions were assumed to be surface doped. Later, Evans et al. also reported that excess anionic surfactants prevent the fusion of the magic size CdSe clusters.140 In the presence of a stoichiometric amount of surfactants, they have observed that larger clusters were consistently formed from smaller clusters by step growth polymerization. In the presence of excess anionic ligand, CdSe nanoparticles with a better size distribution was formed through living chain-addition polymerization, where smaller nanocrystals dissolve to form monomers which were later consumed by larger ones. Hence it can be proposed here that Cl− anion might also play the same role as the anionic surfactant did with the CdSe clusters as discussed above. 2.2.1.3. Complexation of Cationic Impurities that Direct Facet Selective Growth. The complex formation ability of foreign cations also monitors the growth kinetics leading to some intriguing colloidal nanostructures.114−116 Joo et al. tuned the shape of hydrothermally grown ZnO nanowires from platelets to needles.114 The principle followed here for geometry change was the face-selective electrostatic interactions. A schematic presentation of the growth of wurtzite ZnO along [002] and [100] directions controlled in the

Figure 6. (a) Schematic presentation of the effect of Al3+ dopant on shape evolution of Cu3Se2 nanocrystals with their atomic models. TEM images of spherical (b) and cube (c) shaped Cu 3 Se 2 nanocrystals. Parts b and c are adapted with permission from ref 112. Copyright 2013 American Chemical Society.

role. Very interestingly, the final cube shaped nanocrystals obtained due to the presence of Al3+ did not have any Al in the lattice. This mystery indeed talks about the involvement of new chemistry behind the Al3+ ions. The most appropriate possibility stated here was that the Al3+ inhibited the binding of ligands to a particular group of facets of spherical particles, and those high energy facets grew further during ripening and particles turned into nanocubes. In another report, Liu et al. demonstrated that the introduction of Al3+ ions induced the structural transformation from monoclinic CuClSe2 microribbons to klockmannite CuSe nanosheets.113 The presence of Al3+ ions also supported the growth of large-sized CuSe nanosheets. In polar benzyl alcohol, reaction of CuCl2 and SeO2 resulted mostly in the kinetically controlled phased nanocrystals, the monoclinic CuClSe2. In the presence of Al3+, concentration of free Cl− ions in bulk solution was decreased due to rapid extraction of the Cl− ions by Al3+. Thus, the formation of hexagonal klockmannite CuSe was favored. Moreover, it was also speculated that foreign species could greatly enhance the Ostwald ripening rate leading to large-sized CuSe nanosheets. Those mechanisms were supported by two facts: with increased concentration of Al3+ ions, microribbons proportion was decreased, and in absence of Al3+, with chlorine free precursors, only small hexagonal

Figure 7. Atomic models presenting the rational control over zinc oxide nanowire morphology by means of face-selective electrostatic crystal growth inhibition. 5230


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Figure 8. (a) Schematic presentation of the effect of halide ions on shape evolution of Ni9S8 nanocrystals with their atomic models. TEM images of rod-like (b) and cross-like nanoplates (c) of Ni9S8 formed in the presence of Cl− ions. The inset in (c) is a HAADF-STEM image illustrating its stepterrace morphology. Adapted with permission from ref 118. Copyright 2014 American Chemical Society. (d) Schematic presentation of the effect of halide ions on shape evolution of CdSe nanocrystals with the corresponding TEM image of the formed tetrapods. Adapted with permission from ref 124. Copyright 2013 American Chemical Society. (e) Schematic presentation of the chloride ion controlled seeded growth synthesis of branched CdSe/CdS nanocrystals. Adapted with permission from ref 125. Copyright 2012 American Chemical Society. (f) Schematic presentation of reshaping on chlorine incorporation into the CdSe nanocrystals ligand shell.

copper(I) acetate, keeping all other parameters intact, formed quasi-spherical nanocrystals. However, except Cl−, other counteranions did not affect the nanocrystals morphology. Here, the authors had concluded that adsorption of copper ions onto the surface of In2O3 in the growth stage was the key factor for tuning the shape. Though anions were not involved in this shape variation, the size of these nanocrystals was monitored by their presence as the reactivity of In3+ varies with the counteranions. All these above observations strongly supported the role of impurity cations in changing the rate of crystal growth and morphology of the semiconductor nanostructures even without entering into the lattice. 2.2.2. Anions Modulating the Morphology. Apart from cations, foreign anions were also reported extensively controlling the rate of the crystal growth and also the change in shape of nanostructures. Anions not only help in binding to the surface of the nanocrystals but also control the monomer activity. As a result they can influence both the nucleation and the growth of nanocrystals. A large number of reports where impurity anions present in bulk solution control the nanocrystals synthesis but are not doped into the crystal lattices were disclosed in the past few years.117−128 2.2.2.1. Anions Modulating the Morphology by Affecting Both Nucleation and Growth of Nanocrystals. The presence of anionic impurities, likewise cations in bulk solution, is also observed to modify the shape of nanocrystals by regulating the growth and even the nucleations.117−119,128 For example, chloride ions inducing phase impure synthesis of kinked ultrathin ZnS nanorods/nanowires was reported by Zhuang et al.117 Varying metal salts and their concentrations, the authors reported that only sufficient Cl− could result in the morphology transition from straight to kinked nanorods under identical conditions and counter cations had no role. Sufficient Cl− ions slowed down the nucleation and growth rates leading to kinked morphology. Similarly, Wu et al. showed that the introduction of halide ions reduced the rate of formation of the metal thiolate precursors, thereby inhibiting nucleation events and slowing down growth kinetics.118 As a result plate-like Ni9S8

presence of different types of cations is shown in Figure 7. Those metal ions that formed predominately negatively charged complex ions at reaction pH of 11 were categorized as type-1 (trivalent cations of Al, In, Ga), whereas those ions that formed predominately positively charged complexes were categorized as type-2 (bivalent cations of Cd, Cu, Mg, Ca). The authors correlated the aspect ratio of these nanostructures with relative charge distribution of complex species. While type-1 ions promoted the formation of nanorods with high aspect ratio, type-2 ions reversed the same. It has been established that these nonzinc metal cations formed unreactive complexes in basic medium and decreased the concentration of precursor required for growth of ZnO. Among these, complexes with type-1 metal ions predominantly block the facets perpendicular to the major axis (c) and facilitated the growth along the c axis. On the other hand, type-2 metal cations reverse the growth by blocking the {002} facets along the c axis. With the support of elemental analysis of nanocrystals, it was ascertained that impurity ions were not in the lattice. This suggests again the impurity controlled growth of nanostructures with temporary outside supports. Further, Huang et al. studied the effect of heterometal cations on morphology control of CuO in aqueous solution at room temperature.115 Their synthesized CuO nanocrystals include elliptical nanosheets and rounded nanosheets or nanoparticles with various aspect ratios. Different mole percentages of nitrate salts of zinc, silver, magnesium, and aluminum were used as foreign species along with copper nitrate which formed CuO via Cu(OH)2 in alkaline solution. With increased foreign ion concentration, the reaction rate was decreased, and the morphology of CuO was tuned. Based on that observation, it was proposed that, by forming complexes, the impurity metal cations stored OH− and controlled the release of the same. As a consequence, this controlled the overall reaction kinetics. In a different study, shape modification of indium oxide using copper ions was reported by Selishcheva et al.116 While only indium(III) acetate resulted in flower-shaped In2O3 nanostructures, addition of copper (I/II) salts such as copper(II) acetylacetonate, copper(II) acetate, copper(I) chloride, or 5231


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nanocrystals. Lee and Char achieved high morphological uniformity in synthesizing CdSe tetrapods using halide ions.124 Those anions play a vital role in eliminating surface oleate ligands of initially formed ZB CdSe nanocrystals and thus favored the WZ arms growth (scheme in Figure 8d). It is widely established that coexistence of two phases is essentially required for tetrapod or octapod formation. For semiconductor nanocrystals, the seed is mostly confined to one phase (typically ZB) and arms in other phase (typically WZ). However, this may not be true in general. As for metal nanocrystals, it is established that two phases are present in initially formed single or multiple twin seeds, and one etches out or grows faster than the other, leading to such nanostructures.96,97 Further, in another report Klinke and co-workers studied the transformation of CdSe nanorods into hexagonal pyramids in the presence of halogen containing species.126 These additives facilitated the dynamic adsorption/desorption processes of ligands. As a consequence, crystal surface energy was altered, and that favored the reshaping and resulted pyramidal shaped nanocrystals. 2.2.2.3. Anions Forming Complexes To Change the Growth Rate. Anions can form complexes with metal precursors and thus monitor the concentration of active precursors and consequently the growth rate. Kim et al.125 reported Cl− ions induced seeded growth of branched CdSe/ CdS nanoheterostructures. Figure 8e schematically presents the formation of multipod shaped CdSe/CdS from Cu2−xSe seeds in the presence of excess CdCl2. First, the cation exchange converted Cu2−xSe nanocrystals to octapod shaped CdSe nanocrystals. After that, four pods of many octapods started dissolving and disappeared. The other four pods continued their growth and resulted in the tetrapod nanocrystals. It was proposed that, by forming stable complexes with Cd2+, Cl− ions had decreased the concentration of reactive Cd-precursor. Thus, the growth process was slowed down so that CdS pods could overgrow on seed CdSe. Slow growth rate favoring the different phase arms growth on initially formed seed leading to tetrapod and octapod formation is also noticed for metal nanocrystals.94 Another halogenated shape control study was reported by Palencia et al. for CdSe nanocrystals. The rod shaped CdSe here changed to Cl-capped pyramid-like nanostructures via in situ generation of chloride anions.127 In this case, also, Cl− regulated the ligand attachment by selfacting as ligand and facilitated the shape transformation (Figure 8f). Varying the Cd/Se precursor molar ratio, the concentration of Cl− was monitored, and its effect on the growth pattern was corelated. It is further established that the presence of a higher amount of Cl− significantly lowered the free Cd2+ monomer concentration by complexing with Cd2+ ions, e.g., [CdCl4]2−, which influenced nucleation and/or growth processes. From the foregoing discussion on the shape evolution of semiconductor nanocrystals by halide ions, it can be concluded that halide ions generating precursors can be added to a reaction system as a shape controlling species. From the aforementioned discussion it is also evident that foreign species may change the morphology of nanocrystals without incorporating in the formed crystals. Summarizing all these activities of dopants/ impurities and hosts, results are provided in Table 1.

and Cu2S were formed. Figure 8a presents the reaction protocol and the atomic models of evolutions of shapes in the presence and absence of halide ions for Ni9S8. Corresponding TEM images of rod-like and crossed 2d platelet-like structures are presented in Figure 8b,c, respectively. The dimension of the synthesized nanostructure was tuned from 0D to 2D by simply varying the concentration of halide ions added to the reaction system. Exploring experiments with introducing MXn salts (M = Cu, Ni; X = Cl, Br, I) indicated that higher temperature or more time was required for the reaction for completion with different halide ions down the order from chloride to iodide. This was also correlated to their size. Anion-modified shape evolution of CdS was reported by Gaur et al.119 Effects of foreign ions on synthesis of these nanocrystals in solid state and in solution were compared. CdS nanocrystals of nanoflowers, nanotubes, spheres, and irregular morphologies were derived from solid state thermal decomposition of the cadmium− thiourea complexes with nitrate, chloride, acetate, and sulfate ions, respectively. However, thermal decomposition of those complexes, in diphenyl ether solvent, with different anions like chloride, acetate, and nitrate ions, produced nanopyramids, microspheres, and a mixture of nanorods and nanoparticle morphologies, respectively. Shape controlling using ions, not in solution, are beyond the scope of this review, and hence, the role of the anions in solid state synthesis is not discussed here. Their suggested method for shape modification with various anions in solution was based on differences in the decomposition temperature of the cadmium complexes and adsorption of those ions onto the energetically preferred facets of nanocrystals. Also the formed intermediates and byproducts were found to affect the shape. Recently Hinrichs et al. also studied the role of the chloride ion concentration in synthesizing metal−semiconductor heterostructure.128 Employing seeded growth protocol where cobalt−platinum and gold nanocrystals were the metal seeds, on which CdS or CdSe was grown, they have found that the chloride ion had significantly influenced the nucleation step resulting in different heterostructures. 2.2.2.2. Anions That Act as Ligands for Controlling the Growth and Shape. Anions can act as ligands and bind on selective surfaces. Hence, this can change the facet energy and/ or play the role of facet blockers and affect the crystal growth. Following this analogy, recently, Pawar et al. reported the synthesis of sphere, cube, and rod morphology, and nanodendrite like structures of CdS from CdCl 2 and CdI2 thiosemicarbazone complexes, respectively.120 The authors proposed here that halide ions could change the energy of some of the facets of the nanocrystals through reversible binding and influenced the growth leading to different shapes of nanostructures. The Cd−X bond dissociation energy and/or the influence of the halogen incorporation in precursor played a crucial role in determining the morphology. In addition, literature reports reveal that the role of halide ions, as a ligand, controlling the crystal growth of semiconductor metal selenide nanocrystals is widely studied. One of the most ideal hosts studied for structural change is CdSe.121−124,126 Saruyama et al. reported the Cl− ion induced Ostwald ripening process in the cadmium chalcogenide (CdS, CdSe, and CdTe) nanocrystals system.121 Shin and co-workers122,123 reported the morphology and size controlling of cadmium chalcogenide nanocrystals by chemical and photochemical etching process in the presence of chloride in solution. Adsorption of Cl− onto the nanocrystals surface remained the key in both cases for shape change of the

3. SUMMARY AND OUTLOOK This review illustrated the crystal shape and/or phase modification in the presence of selective inorganic impurity 5232


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(5) In several cases, impurity ionsboth cations and anionsreduce the monomer concentration by forming complexes.113,115,125,127 Hence, growth rate is slowed down, which always favors anisotropic shape formation. This suggests that, with suitable impurity ions, the growth rate can be significantly controlled and so can the morphology. (6) It is also established that dopants/impurities can affect strongly the oriented attachments102,105,109 and Oswald ripening112,113,121 and can trigger Kirkendall effects,104 which are very basic phenomena in nanocrystal growth and typically governed by several other factors. Overall the above results demonstrate that impurities and dopants, regardless of whether they remain in or out of the crystal lattice, influence the crystal growth and shape/phase change of different semiconductor and metal oxide nanocrystals. This behavior has also been widely observed in the synthesis of metal nanoparticles,93−99 which are out of the scope of this review. Hence, it can be concluded that the composition, size, shape, and crystallographic phase of the nanocrystals can be varied by relevant foreign species with appropriate additional reaction parameters. An increase in the number of published reports and better understanding of the role of these foreign species will enable easier fabrication of functional nanocrystals.

species. These foreign ions or atoms control the crystal growth either by entering the crystal lattice of the host or simply by guiding the same while remaining in the bulk solution. Beyond organic ligands and several other controlling factors, these impurities also displayed their versatility in the wet chemical synthesis of nanocrystals. Since both nucleation and growth of nanocrystals were found to be altered significantly, the presence of impurity ions can be considered as one of the parameters that regulate crystal phase, size, and morphology. This approach opens up a new era of research to engineer the morphology and architectures of nanocrystals. In addition, it is also expected to strengthen our understanding of the fundamental aspects of dopants/impurities that are present inside/outside of hosts for possible shape and phase conversions. Some of the key prospects in this research are summarized below: (1) Reversible thermal incorporation of Mn in WZ ZnS and the corresponding change in phase/shape100 suggests that doping is not necessarily controlled by the nature of hosts; rather, dopants can change the surroundings to occupy an energetically stable environment even by altering the lattice arrangement of the hosts. This capability of dopants generates significant fundamental interest in finding suitable guest−host pairs and reaction conditions for their accommodation in a single nanocrystal matrix. This can be extended for many other systems, and it would help in solving the query, “Why is doping undoable in some cases?” (2) The change in the crystal phase of CdSe caused by halide ions that have not even entered into the crystal suggests that impurities can change the atomic arrangement without being doped. This promotes the shape architecture method because one does not have to manipulate the nucleation of nanocrystals. In synthesizing CdSe tetrapods, the addition of halide ions destabilizes some facets of the embryonic ZB CdSe nanocrystals by removing the organic ligands, and the WZ arms are able to grow.124 After detailed investigations, it is very likely that several more examples might be discovered where, in the presence of foreign impurity ions, atomic rearrangement commences inside a nanocrystal. This work also provides some new fundamental aspects of “why and how”, but more indepth investigations are needed to completely resolve these mysteries. (3) Anionic effects are observed to be a dominant protocol for site-specific control of crystal growth. Typically, organic ligands having different binding strengths are the most favorable specified tools used, to date, for directional growth, but anions like Cl− were observed to be excellent for assisting the temporary binding to the crystal facets and controlling the growth.127 This has been used for several studies, but finding the appropriate host and also manipulating the reaction conditions can expand this anion-assisted morphology control to several other nanocrystals. (4) The valence of cations also strongly affects the facetdependent crystal growth. The complexes formed by those cations are the temporary binders, and thus, they change the dimensions of the nanostructures during growth without entering the crystals. This was observed for ZnO, where the aspect ratios were varied by bivalent and trivalent cations.114 This suggests that there are many more such elements present in the periodic table that can assist the shape evolution of nanostructure.

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AUTHOR INFORMATION

Corresponding Author

*(N.P.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS DST of India (Project DST/SJF/CSA-01/557 2010-2011) is acknowledged for funding. N.P. acknowledges DST Swarnajayanti Project for fellowship.

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REFERENCES

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