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Synthetic Development of Low Dimensional Materials Long Men, Miles A White, Himashi Andaraarachchi, Bryan A. Rosales, and Javier Vela Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02906 • Publication Date (Web): 02 Nov 2016 Downloaded from http://pubs.acs.org on November 3, 2016

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Chemistry of Materials

Synthetic Development of Low Dimensional Materials Long Men,1,2 Miles A. White,1 Himashi Andaaraarachchi,1,2 Bryan A. Rosales,1 Javier Vela*,1,2 1

2

Department of Chemistry, Iowa State University, and Ames Laboratory, Ames IA 500

ABSTRACT: In this invited paper, we highlight some of our most recent work on the synthesis of low dimensional nanomaterials. Current graduate students and members of our group present four specific case systems: Nowotny-Juza phases, nickel phosphides, germanium-based core/shells, and organolead mixed-halide perovskites. Each system is accompanied by commentary from the student involved, which explains our motivation behind our work, as well as by a protocol detailing the key experimental considerations involved in their synthesis. We trust these and similar efforts by others and us will help further advance our understanding of the broader field of synthetic nanomaterials chemistry, while, at the same time, highlighting how important this area is to the development of new materials for technologically relevant applications.

Introduction Nanomaterials continue to captivate and occupy the minds and labor of scientists and engineers across the globe. From a fundamental perspective, the ability to control catalytic or opto-electronic properties by tuning the composition, size and morphology of low dimensional materials is fascinating. From a practical perspective, because of their ability to strongly interact with outside stimuli, such as light through well defined physical processes such as absorption, energy transfer, charge separation and emission, nanomaterials offer unparalleled opportunities in a wide range of applications. As synthetic chemists, our long-term goal has been to design distinct, powerful and widely applicable synthetic strategies that span the continuum between the molecular and nano scales, and that enable effective processing and incorporation of low dimensional materials into innovative energy conversion, catalysis, and biological imaging applications. Our work in this area builds on our background and expertise in synthetic inorganic and materials chemistry, and is guided by some important principles (1-7): (1) First and foremost, we write a chemical equation for each and all of our reactions. If all products are known, a chemical equation can be balanced. If, as is much more common, not all or any of the products are known, or are yet to be confirmed, we clearly state this, and leave the right side of the chemical equation open. (2) We adhere to widely accepted (ACS, IUPAC) and well-known chemistry terminology. We strongly believe that scientific communication is more fluid and greatly facilitated by avoiding unnecessary, imaginary, or otherwise dubious new acronyms and abbreviations. (3) We focus our synthetic efforts on reagents that are commercially available or relatively easy to make. An alternative and complementary approach involves extensive purification of precursors and solvents in order to remove chemical impurities, as these are well

known to affect the outcome of nanocrystal preparations.1-4 This approach is important in advancing our fundamental understanding of the field, however it is less commonly followed by others, especially those without a synthetic background. (4) We purposely avoid using some of the most toxic, expensive, and noxious elements and reagents, for example, those containing As, Tl or Hg. When this is not possible, we clearly state the hazards associated with working with these. (5) If a reaction does not work once or twice, we recognize that simply repeating it is unlikely to change its outcome. This is particularly important for new or inexperienced students and other junior group members, who often spend countless hours trying to ‘fix something they did wrong’. (6) Once we have a ‘hit’, or a reaction that actually works, we optimize it. In other words, we actively search for reagents and conditions that succeed in giving a desired product, as judged by some of the most common optical (absorption and photoluminescence) and structural (X-ray diffraction) techniques. We then optimize such conditions, in order to maximize both chemical yield and purity. (7) Once we have a clean compound, we throw ‘everything at it’, using the full array of analytical and spectroscopic techniques available to us to fully characterize it. More often than not, this leads to unexpected results and new questions to explore. In this invited paper, we highlight some of our most recent work where we apply and continue to develop this approach. Four case systems are presented by current graduate students and members of our group: NowotnyJuza phases by Miles ‘Art‘ White, nickel phosphides by Himashi Andaraarachchi, germanium-based core/shell nanocrystals by Long Men, and organolead mixed-halide perovskites by Bryan Rosales. Each system is accompanied by commentary from the student involved (see Supporting Information), highlighting our motivation or ‘big

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picture‘ behind our work, as well as by a protocol detailing the key experimental considerations involved in each system (see Methods in the Supporting Information). We trust these and similar efforts by others and us will help further advance our mastery and understanding of the broader field of synthetic nanomaterials chemistry, while, at the same time, highlighting how important this area is to the development of new materials for technologically relevant applications.

Discussion Synthesis of Nanocrystalline Nowotny-Juza Phases Nowotny-Juza phases are a class of semiconductors comprising elements from groups I, II, and V (groups 1, 12 and 15, according to IUPAC).5,6 The group II and V elements form a zinc-blende structure with monovalent group I cations inserted into the octahedral holes of the fcc II lattice (Figure 1). The electronic structure of these compounds resembles classic 8 e- semiconductors due to this similar tetrahedral framework. The presence of the closed shell cation makes the band gap of these materials direct due to the interstitial insertion rule.7 Furthermore, the band gap can be tuned by altering the electronegativity difference of the elements comprising the tetrahedral sublattice.8 These properties make Nowotny-Juza phases ideal for thermoelectric devices, solar cells, and anode materials for Li ion batteries.9-12 Synthesis of NowotnyJuza phases via low temperature and solution phase techniques could improve their processability and implementation into practical devices.

the solvent, phosphorus source (above 300 °C), and surface passivating ligand.14-20 These properties, along with the ability to substitute the aliphatic group on the phosphine, make phosphines, and TOP in particular, an ideal starting point as a phosphorus source. Furthermore, many of the heavier pnictide analogs (for example: VPh3, V = Sb, Bi) are also commercially available, making the extension of a phosphine synthesis to heavier pnictides more straightforward. The most frequently reported mechanism for metal phosphide nanoparticle formation progresses through a metal intermediate. Diethyl zinc is known to decompose rapidly to zinc metal at elevated temperatures via reductive elimination, and zinc chloride is easy to reduce to form zinc metal under relatively mild conditions, making these zinc precursors ideal. Further, lithium hydride (LiH) was selected as a lithium source and reducing agent to facilitate the reduction of the phosphorus in TOP from a formal oxidation state of 3+ to 3- and to reduce the zinc precursor (in the case of zinc chloride) to zinc metal. Interestingly, LiH is the thermal decomposition product of organolithium reagents via β-hydride elimination; therefore, these could be decomposed in situ and also serve as lithium sources (Scheme 1 and Movie 1).21 This alternative route provides a way to increase the solubility of the lithium source. LiH is highly ionic and, thus, insoluble in organic solvents. Using an organolithium reagent that releases LiH into solution results in a homogenous and more controllable reaction. Scheme 1. Synthesis of LiZnP showing precursor selection flexibility. 5 LiR + Zn(R')2 + xs TOP R = H, n-Bu, iPr 2N, Ph R' = Et, Cl, stearate

Figure 1. Crystal structure of Nowotny-Juza phases emphasizing the tetrahedral environments of the constituent elements. The first phase that our group attempted to synthesize among this class of compounds was LiZnP.13 The successful synthetic development of nanomaterials often rests on two key criteria: precursor choice and surface passivation. In some instances, these two decisions are one in the same. For instance, numerous metal phosphides have been synthesized utilizing tri-n-octylphosphine (TOP) as

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1) Et 2Zn / 250 °C 2) 330 °C, t

LiZnP unbalanced

Movie 1. Miles White's commentary on Nowotny-Juza phases. In order to optimize this synthesis, absorption spectroscpy and powder X-ray diffraction (XRD) were used in tandem. The first showed a light absorption onset corresponding to a band gap of 2.0 eV, which indicated the formation of LiZnP. XRD showed, first, the formation of a zinc metal intermediate; over the course of 20 min, zinc


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metal disappeared in favor of the formation LiZnP (Figure 2).

or colloidal synthesis methods are becoming increasingly popular.16,17,19,32 One of the major challenges in nanocrystal synthesis lies in the ability to reproducibly and precisely controlling composition and morphology. The most common approach involves altering and optimizing several different reaction conditions, which can be very time-consuming. Alternatively, others and we demonstrated a new way to control the final outcome of nanocrystal synthesis by manipulating the reactivity of chemical precursors, while keeping the rest of the experimental conditions constant.33-36 Using families of phosphine chalcogenide and disubstituted dichalcogenide precursors, our group systematically studied how chemical structure and reactivity affect the composition and morphology of CdS and CdS1xSex nanocrystals. Recently, we expanded and generalized this strategy to the controllable synthesis of metal phosphide nanomaterials such as Ni2P, utilizing relatively unexplored organophosphites as a tunable family of phosphide precursors.25

Figure 2. Time evolution of nanocrystalline LiZnP from LiH, Et2Zn and TOP as shown by powder XRD. To examine whether the choice of precursor achieved the level of flexibility desired during the synthetic design, we re-attempted the synthesis using different phosphorous (triphenylphosphine), lithium (n-butyllithium, phenyllithium, lithiumdiisopropylamine), and zinc precursors (zinc chloride and zinc stearate), all of which successfully produced LiZnP. Similarly, we changed the constituent elements with the substitution of cadmium for zinc using dimethyl cadmium. Overall, these results demonstrate the flexibility and generality of this synthesis for I-II-V semiconductors, which previously had not been made by solution phase techniques. Through careful precursor selection, this method opens a wealth of possibilities in the synthesis of a large family of Nowotny-Juza phases. Because it offers the potential for tuning particle composition, morphology and size, this synthesis could enhance the application of Nowotny-Juza phases in thermoelectric and photovoltaic devices with near optimum performance. Synthesis of Nanocrystalline Nickel Phosphides Nanostructured, first row transition metal phosphides such as Ni2P have attracted a lot of interest because they are excellent catalysts for hydrodesulfurization (HDS), hydrodenitrification (HDN), and hydrogen evolution reactions (HER).22-24 Compared to noble metal catalysts, Ni2P is very appealing because it is made of more abundant elements. Interestingly, the presence of defects in Ni2P can make this otherwise metallic material behave as a semiconductor.25-27 Ni2P has been synthesized by chemical vapor deposition, electrochemical, solid state, hydrothermal and solvothermal methods,28-31 but solution phase

Organophosphites (P(OR)3) are appealing as phosphorus precursors because they are highly reactive, possibly tunable with group (R) group substitution, commercially available and fairly inexpensive. This contrasts with other common molecular precursor alternatives such as P4, which is unsupported and not tunable, and trioctylphosphine, which requires relatively high decomposition temperatures (>320-340 °C).37-41 As shown in Chart 1, readily available organophosphites bear a wide range of aliphatic and aromatic substituents with different electronic effects, and their Tolman cone angles, a measure of their steric profile, vary from 107 to 192°.42 Chart 1. Relative ability of commercially available organophosphites to form nickel phosphide nanocrystals.

O

O P

O

P(OMe) 3

- Increasing precursor reactivity +

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O

O P

O

P(OEt) 3 O

O P

O

P(O nBu) 3

O

O

O P

O P

O

P(OCH 2tBu) 3 O

P(O iPr) 3

O

P O

O

P(OPh) 3


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Our synthetic procedure involves a hot injection of the organophosphite to a reaction mixture of nickel(II) precursor, oleylamine and 1-octadecene at 275 °C (Scheme 2 and Movie 2). Different organophosphite precursors selectively yield metallic Ni or nickel phosphide phases (Ni2P and Ni12P5) that evolve over time through separate mechanistic pathways (Figure 3). Based on our experimental observations, the rate of formation of nickel phosphide increases in the order of P(OMe)3 < P(OEt)3 < P(OnBu)3 < P(OCH2tBu)3 < P(OiPr)3 < P(OPh)3. Formation of nickel rich tetragonal Ni12P5 precedes the formation of hexagonal Ni2P phase, which agrees with prior literature reports. The reactivity of above precursors does not directly correlate with Tolman cone angles or calculated homolytic and heterolytic bond energies. A bulky P(O2,4-tBu2C6H4)3 precursor, with the largest cone angle and weakest P-O bond, fails to form crystalline NixPy phases. This observation suggests that phosphite coordination to the nickel(II) precursor is required in order to produce nickel phosphides. Furthermore, we observe that more reactive phosphite precursors such as P(OPh)3 generate small, solid nanocrystals, whereas less reactive precursors such as P(OMe)3 generate large, hollow nanocrystals. While all the other organophosphites in Chart 1 form nickel phosphides, some yield Ni nanocrystals, at least transiently namely P(OMe)3 and P(OiPr)3. The bulky P(O-2,4-tBu2C6H4)3 precursor consistently forms metallic Ni nanocrystals that never progress into a nickel phosphide. Scheme 2. Synthesis of NixPy and Ni using organophosphite molecular precursors (see Chart 1). Ni xP y (unbalanced) NiX 2 + P(OR) 3 X = Cl, OAc R = Aryl, alkyl

275 °C, t, oleylNH2 ODE / Ar Ni

(unbalanced)

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Figure 3. Time evolution of NixPy and Ni nanocrystals from three representative organophosphite precursors. Using density functional theory (DFT) calculations, we offered two mechanisms for the initial decomposition of the organophosphite precursor into NixPy and Ni nanocrystals. As mentioned above, a mechanism that requires organophosphite coordination to the nickel(II) precursor is consistent with our observations for NixPy formation. For the formation of reduced (metallic) Ni, we considered both inner and outer sphere electron transfer mechanisms. Because the most sterically hindered P(O-2,4t Bu2C6H4)3 forms metallic Ni and these persist over the time, while P(OMe)3 and P(OiPr)3 only form Ni transiently, we rule out an inner sphere electron transfer mechanism involving a five-coordinated intermediate. Thus, outer sphere electron transfer by uncoordinated organophosphite to the nickel(II) precursor is consistent with the formation of zerovalent metallic Ni. Organophosphites are a valuable new family of pnictide precursors. These and similar findings enable a faster and more systematic approach to controlling the composition, size, and shape of pnictide nanomaterials. Synthesis of Germanium-Based Core/shell Nanocrystals

Movie 2. Himashi Andaraarachchi's commentary on nickel phosphide nanocrystals.

A grand challenge in the area of semiconductors is the development of more robust and photostable small band gap materials for energy conversion and biological imaging. Nanostructured germanium is an interesting candidate for such applications, including photovoltaics, nearinfrared biological imaging and telecommunications.43,44 Germanium is a tetrahedral (diamond) semiconductor with a large Bohr radius of 24 nm and a small indirect bandgap of 0.661 eV, which provides for a wide range of emission energies by quantum confinement (Figures 4a and 5a).45,46 Unfortunately, typical Ge nanostructures have low extinction coefficients and weak photoluminescence (PL < 1%), which hampers their study and incorporation into probes and devices. To solve these problems, others have explored doping of Ge nanostructures with tin (Sn).47-49 Our group utilizes IV/II-VI epitaxy to improve


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the PL quantum yield and overall photostability of germanium nanocrystals.44 Epitaxial growth of a surfacepassivating layer of CdS enhances the photoluminescence by one to three orders of magnitude compared to the bare Ge cores (Figure 5b). Epitaxy can also induce interfacial strain and result in germanium nanocrystals with direct bandgap character.45 Further, the CdS shell can be either capped50 or completely replaced with ZnS,44 which may render the final core/shells more biocompatible.

Figure 5. (a) Cubic unit cell of CdS zinc blende. In the diamond unit cell, all positions are filled with Ge. (b) Photoluminescence spectra of Ge and Ge/CdS nanocrystals (normalized by absorbance or optical density at the excitation wavelength, λexc = 450 nm).

Figure 4. (a) Band gap, (b) lattice parameter, and lattice mismatch of common IV and II-VI semiconductors.

II-VI epitaxy has been widely used to enhance the optical properties of quantum dots.51,52 For example, CdSe/CdS core/shell nanocrystals are the best nonblinking quantum dot fluorophores to date.53 To achieve IV/IIVI epitaxy at the nanoscale and successfully make Ge/IIVI core/shell nanocrystals, we carefully selected shell materials to make sure the interfacial boundaries remain mostly defect-free and are continuous. Ge cores have a diamond structure, which is topologically similar to the zinc blende phase of CdS or ZnS, and the lattice mismatch between them is relatively small (3.1% for Ge/CdS and 4.4% for Ge/ZnS, Figure 4b). For these reasons, CdS and ZnS are near ideal shell materials for Ge nanocrystals. Our core/shell synthesis process uses successive ion layer adsorption and reaction (SILAR). Briefly, Ge nanoparticles, freshly synthesized by reduction of GeI2 with nbutyllithium,54 are reacted with M (M = Cd or Zn) and S precursors alternately to form CdS shells (Scheme 3 and Movie 3).55 Syringe pumps are used to inject shell precursor solutions at regular 15 min intervals. To avoid the oxidation of Ge, we remove air by degasing the core and shell precursor solutions for 30 min at 80 °C before beginning shell growth, under a dry argon atmosphere. In this way, we can fabricate Ge/II-VI core/shell nanocrystals with enhanced PL (Figure 5b). We expect these materials will extend the application of similar Ge-based nanostructures for near-IR applications.



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Scheme 3. Synthesis of Ge/CdS core/shell nanocrystals. GeI 2

1) n-BuLi / 200 °C 2) 300 °C, 1 h

Ge

unbalanced

/ n-hexadecylNH 2 Ge

1) S8 / ODE, 230 °C, 15 min 2) Cd(oleate)2 / oleic acid, ODE, (n-octyl) 2NH, 230 °C, 15 min

Ge/nCdS unbalanced

/ ODE, (n-octyl) 2NH (repeat 1, 2 n times)

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ble range and enhanced moisture stability.79,80 Nonetheless, questions remain regarding the degree of alloying and phase segregation in mixed-halide perovskites. Our group uses advanced spectroscopy techniques to probe the effect of synthetic conditions on the true microscopic structuring of organolead mixed-halide perovskites (Scheme 4 and Movie 4), to a level that cannot be achieved by other, more commonly used techniques such as XRD and optical spectroscopy.81 For example, 207Pb ssNMR is an ideal technique to study these materials because the 207Pb nucleus has a spin of ½, 22.6% natural abundance, is highly sensitive to both electronic structure and coordination environment, and its chemical shift (δ) range spans over 10,000 ppm.82-85 207Pb ssNMR is uniquely suited to probe the fundamental question of what is the exact degree of alloying, phase segregation, and lead site speciation, in these materials. Scheme 4. Synthesis of organolead mixed-halide perovskites in solution and solid phase.

Movie 3. Long Men's commentary on germanium-based core/shell nanocrystals. Synthesis and Spectroscopic Characterization of Organolead Halide Perovskites Organolead halide perovskites (CH3NH3PbX3, X = I, Br, Cl) are some of the most promising solar cell materials, having gained attention because of their high power conversion efficiencies (PCE) of near 22+%.56,57 In addition to their low cost, high solution processability, and high structural and optical tunability,58 organolead halide perovskites are composed of cheap, earth abundant materials and benefit from large absorption coefficients, low exciton binding energies, long exciton diffusion lengths, high dielectric constants, intrinsic ferroelectric polarization, photon recycling, and defect tolerance.59-66 In the past, our group has controlled the size and morphology (dots, rods, and plates) of (CH3NH3PbX3, X = I, Br) nanocrystals by varying the solvent(s) and precursor injection rates.67 Other groups have exploited these properties in other applications such as LED’s,68,69 control of nanoparticle morphology,67 or the growth of nanocrystals in mesoporous silica without the use of surface ligands.70 Unfortunately, organolead halide perovskites suffer some disadvantages, notably their instability and decomposition under a variety of conditions including moisture, light and heat.71,56 Therefore, much recent work has gone into improving stability under these conditions.72-76 Currently, the best state-of-the-art perovskite materials, having the highest efficiency and stability are hybrids of two or more phases.56,77,78 Mixed-halide perovskites (CH3NH3PbX3-aX'a, X, X' = I, Br, Cl), for example, help overcome some of the limitations of single-halide perovskites through light absorption tunability across the visi-

(a) Solution phase synthesis 3-a 3-a (1-a)PbX 2 '+ (1-a)CH3NH 3 X (1) polar' '3 '3 solvent a a (2) toluene aPbX' 2 + aCH3NH 3 X' 3 3

CH3NH 3PbX 3-aX a’

(b) Solid phase synthesis 3-a a ( CH3NH 3PbX 3 + CH3NH 3PbX' 3 ∆' CH3NH 3PbX 3-a X a’ '3 3 (X, X' = I, Br, Cl; 3 > a > 0)

Movie 4. Bryan Rosales' commentary on organolead mixed-halide perovskites. Using 207Pb ssNMR, we recently detected the presence of phase-segregated nanodomains caused by spinodal decomposition in organolead mixed-halide perovskites (CH3NH3PbX3-aX'a, X, X' = I, Br, Cl).86 207Pb ssNMR measurements showed that these materials do not form a pure alloy, as initially expected, but instead contain phasesegregated nanodomains composed of nonstoichiometric lead octahedra that are richer in one halide and poor in the other (Figures 6 and 7).81 These phases are persistent upon thermal annealing, and are present regardless of whether the samples are made by solution phase or solid phase synthesis. The extra peaks and the formation of such nanodomains are consistent with spinodal decom-


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position caused by the presence of a miscibility gap in heavily mixed perovskites.86 Our work has added another piece to the puzzle that is perovskite synthetic development, and which will help improve device efficiency and stability. Synthetic control of the phase-segregated nanodomains must be achieved to maximize the performance of perovskite solar cells, especially as the most efficient devices at present are not composed of single-phases but are rather hybrids of multiple perovskites. Outlook and Conclusions We are excited to be part of a new generation of experimental chemists that are using their synthetic and spectroscopic skills to explore the burgeoning field of low dimensional optical materials. The number of chemistry scholars working in this area has finally reached critical mass. This vibrant community is now regularly represented at symposia in yearly American Chemical Society and Materials Research Society meetings, and holds, since 2014, biannual Gordon Research Conferences on Colloidal Semiconductor Nanocrystals. Further growth in this area will ensure that new and established researchers continue to have an avid readership, receive critical feedback, and have access to additional opportunities for funding the synthetic development of new optical nanomaterials and devices. While often guided by computations, and drawing much inspiration from transformative ideas such as the Materials Genome Project, we recognize in our own work that computations alone cannot and should not be an absolute substitute for the synthesis of new compounds with new properties. It is up to the members of this growing community of scholars to actively promote and educate others about the value of fundamental knowledge and, in particular, of synthetic materials and preparative inorganic chemistry.

Figure 6. Comparison of static 207Pb ssNMR spectra (22 °C) of representative organolead mixed-halide perovskites prepared by solution and solid phase syntheses. Multiple 207 Pb resonances indicate the presence of both stoichiometric and non-stoichiometric chemical species (see reference 81 for details).

Figure 7. Illustration showing the spontaneous formation of non-stoichiometric or ‘dopant’ sites in hybrid organolead mixed-halide perovskites by spinodal decomposition.

ASSOCIATED CONTENT Supporting Information. Methods section, including all experimental procedures.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes The authors declare no competing financial interest.



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ACKNOWLEDGMENT Our research on germanium- and perovskite-based optical materials for solar energy is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences through the Ames Laboratory. The Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358. Our research on nickel phosphides and I-II-V semiconductors is supported by the U.S. National Science Foundation's Division of Chemistry, Macromolecular, Supramolecular and Nanochemistry program CAREER grant No. 1253058. We thank Emily Smith, Jake Petrich, Aaron Rossini, Gordie Miller and Levi Stanley for fruitful collaborations, and Brian Marczewski for video assistance.

ABBREVIATIONS ODE = 1-octadecene.

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