This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Sacrificial Silver Nanoparticles: Reducing GeI2 To Form Hollow Germanium Nanoparticles by Electroless Deposition Bradley M. Nolan,† Eric K. Chan,† Xinming Zhang,‡ Elayaraja Muthuswamy,† Klaus van Benthem,‡ and Susan M. Kauzlarich*,† †
Department of Chemistry and ‡Department of Chemical Engineering and Materials Science, University of California, One Shields Avenue, Davis, California 95616, United States S Supporting Information *
ABSTRACT: Herein we report the electroless deposition of Ge onto sacrificial Ag nanoparticle (NP) templates to form hollow Ge NPs. The formation of AgI is a necessary component for this reaction. Through a systematic study of surface passivating ligands, we determined that tri-n-octylphosphine is necessary to facilitate the formation of hollow Ge NPs by acting as a transport agent for GeI2 and the oxidized Ag+ cation (i.e., AgI product). Annular darkfield (ADF) scanning transmission electron microscopy (STEM) imaging of incomplete reactions revealed Ag/Ge core/shell NPs; in contrast, completed reactions displayed hollow Ge NPs with pinholes which is consistent with the known method for dissolution of the nanotemplate. Characterization of the hollow Ge NPs was performed by transmission electron microscopy, ADF-STEM, energy-dispersive X-ray spectroscopy, UV−vis spectrophotometry, and Raman spectroscopy. The galvanic replacement reaction of Ag with GeI2 offers a versatile method for controlling the structure of Ge nanomaterials. KEYWORDS: germanium nanoparticles, hollow, electroless deposition, galvanic replacement, silver nanoparticles
I
difficulty in developing colloidal syntheses that allow for size and structure/morphology control. Examples demonstrating size control of nanoscale germanium are limited in comparison to the Groups II−VI and IV− VI nanomaterials.22−24 When reducing GeI2, particle sizes between 3 and 11 nm can be obtained by varying the concentration of the GeI2 precursor.17,18 Narrower size distributions with similar average sizes have recently been reported through the use of mixed Ge4+/Ge2+ halides.2 Besides the reduction of mixed valence germanium halides, the role of ligands on the size of Ge NCs has also been studied. Hanrath and co-workers found that poorly passivating ligands such as tri-n-octylphosphine (TOP) produce larger diameter Ge NCs due to the coalescence of nascent particles.25 Controlling the structure and morphology of nanoscale Ge is another synthetic endeavor within the scientific community for a variety of applications, as mentioned above. Recently, Vela and co-workers reported the synthesis of core/shell Ge/CdS and Ge/ZnS NCs that exhibit near-infrared photoluminescence.26 Besides spherical Ge NCs, one-dimensional Ge nanowires have been synthesized by supersaturated metal (Au, Ag, Bi)−germanium alloys using vapor−liquid−solid,
n recent years the development of Group IV nanoparticles (Si, Ge) has progressed rapidly as interest in applying nanoscale Ge in optoelectronics,1,2 solar energy conversion,3 batteries,4−8 and bioimaging9,10 has steadily increased. Bulk germanium has a band gap of 0.67 eV at 300 K that can be increased and tuned through precise nanoparticle (NP) size control due to quantum confinement effects.11 As a battery anode material, Ge possesses a high theoretical capacity (1600 mAhg−) and Li-ion diffusion coefficient; however, the volume change of Ge during the Li−Ge alloy/dealloy process compromises the material’s structure and consequently its cyclability. To alleviate volume strain, nanoscale Ge anodes have been investigated in the form of Ge nanowires, hollow Ge nanotubes, mesoporous Ge particles, and encapsulated Ge NPs.7,12−15 Besides mitigating volume strain, Ge nanocrystals (NCs) have been demonstrated to exhibit photoluminescence in the visible and near-infrared region.2,16,17 Several synthetic methods exist to prepare colloidal Ge NCs, including the reduction of germanium halides, decomposition of organogermane molecular species, reduction of GeO2, and the metathesis reaction of Zintl germanides.16,18−21 The reduction of germanium salts often necessitates the use of high temperatures and mild to strong reducing agents, including but not limited to oleylamine (OAM), n-butyllitihium, and metal hydrides. These prerequisites of high temperatures and strong reducing agents to form Ge NCs contribute to the © XXXX American Chemical Society
Received: March 5, 2016 Accepted: April 20, 2016
A
DOI: 10.1021/acsnano.6b01604 ACS Nano XXXX, XXX, XXX−XXX
Article
www.acsnano.org
Article
ACS Nano
average NP size of 8.56 ± 1.69 nm from the analysis of 250 individual NPs (Figure 1A,B). The reproducibility of the Ag NP
solution−liquid−solid, supercritical fluid−liquid−solid, and vapor−solid−solid methods. Cho and co-workers expanded upon the chemistry of one-dimensional Ge nanowires by using the Kirkendall effect to generate hollow Ge nanotubes from Ge/Sb core/shell nanowires.7 Other morphologies previously synthesized include germanium nanocubes, tetrahedra, and triangles.27,28 For further information, a comprehensive review on the current state of synthesis, properties, and applications of colloidal Ge has recently been published.29 Herein, we report the application of electroless deposition, also known as a galvanic replacement reaction, to synthesize hollow Ge NPs. Xia’s description of a galvanic replacement reaction directly agrees with Djokic and Cavallotti’s description of one of the two main types of electroless deposition known as displacement deposition or galvanic plating, and as such, we use the terms electroless deposition and galvanic replacement reaction interchangeably.30,31 The electroless deposition occurs through the displacement deposition of Ge onto a sacrificial Ag NP template via the oxidation of the Ag NP to AgI. The oxidized Ag cations are dissolved into the solution and stabilized by TOP. While the use of Ag NPs as a sacrificial template to form hollow noble metal nanostructures via electroless deposition of metal salts is widely known and studied, there are relatively few examples non-noble metal hollow nanostructures synthesized via electroless deposition.32,33 Electroless deposition through galvanic replacement reactions dates back more than two decades when it was first reported to form noble metal Pd and Ag thin films on Cu.34 It was later shown that noble metal NP films and nanoinukshuks could be formed on the surface of Ge by controlled electroless deposition.35,36 Galvanic replacement reactions have also been extensively studied as a method for producing hollow and porous metal nanostructures.30,37 Typical reactions involve the formation of noble metal shells (e.g., Au, Pd, and Pt) on the surface of a sacrificial template. In 2002, Sun et al. first reported the reduction of HAuCl4 (aq) to form elemental Au on the surface of Ag NPs.38 The oxidized Ag+ diffused away from the NP’s surface without forming AgCl deposits due to the elevated reaction temperature which increased the Ksp of AgCl. The result was a hollow Au nanostructure. In the present work, we demonstrate that Ag NPs reduce GeI2 to form hollow Ge NPs via a galvanic exchange reaction, along with the formation of AgI. Additionally, we show that TOP is necessary for the formation of the hollow Ge NPs due to its role as a transport agent of the Ge2+ ion and the oxidized Ag+ cation.39,40 The oxidation of Ag to AgI is considered an important step. We characterized structure and morphology of the hollow Ge NPs by transmission electron microscopy (TEM) and annular dark-field (ADF) imaging using aberrationcorrected scanning transmission electron microscopy (STEM), the chemical composition by energy-dispersive spectroscopy (EDS), powder X-ray diffraction (PXRD), and Raman spectroscopy, the surface passivating ligands by Fourier transform infrared spectroscopy (FTIR), and the optical properties by UV−vis spectrophotometry.
Figure 1. (A) TEM image of spherical, OAM-stabilized Ag NPs. (B) Histogram data of the average size and standard deviation from 250 Ag NPs.
syntheses was confirmed with UV−vis spectroscopy. The spectra displayed the expected surface plasmon resonance band with an absorbance maximum at ∼416 nm, consistent with previous reports for the diameter of the Ag NP (Figure S1).41,42 EDS and PXRD of the Ag NPs confirmed the chemical composition of the NPs (Figure S2). Initial control experiments were performed to confirm the role of the Ag NPs as reducing agents in the reaction. The Ag NPs were first reacted with GeI2 at either 250 or 350 °C in DOE. Then, in a second step, the reaction vessel was heated at 100 °C for 1 h to functionalize the surface of the Ge NPs with DDT. The bulk product of the reaction heated at 250 °C was AgI confirming the reduction of GeI2 by the Ag NPs. Heating at 350 °C produced AgI and bulk crystalline Ge (Figure S3). Table 1 displays the reduction potentials of Ag+ and Ge2+. The reduction potentials presented are for aqueous solutions at 25 °C, but the relative values can be used to explain the unexpected reduction of Ge2+ by Ag. Comparing the reduction potential of Ag+ (0.7993 V) to Ge2+ (0.1 V), Ag would not be Table 1. Reduction Potentials of Ag and Ge Relative to the Standard Hydrogen Electrode (SHE)43
RESULTS AND DISCUSSION Ag NPs were synthesized by modifying a previously reported procedure.41 The Ag NPs were surface stabilized by OAM ligands and could be stored in the glovebox as a pellet for several days without losing the ability to be dispersed into dioctyl ether (DOE). Characterization by TEM showed that the Ag NPs were spherical and polydisperse. We determined an
reduction reaction
Eo (V vs SHE)a
Ag+(aq) + e− → Ag(s) AgI(s) + e− → Ag(s) + I−(aq) Ge2+ + 2e− → Ge(s)
0.7993 −0.152 0.1
a
Reduction potentials given are for reactions in aqueous solutions at 25 °C.
B
DOI: 10.1021/acsnano.6b01604 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano expected to spontaneously reduce Ge2+ (Table 1). In fact, Ge should reduce Ag+. It is from this conclusion that Buriak and co-workers empirically demonstrated the deposition of Ag on to a Ge substrate to form Ag nanoinukshuks.36 However, solving for the cell voltage using the reduction potentials of AgI and Ge2+, where the concentrations of Ge2+, Ag+, and I− are 0.04, 0.08, and 0.08 M, respectively, gives a positive voltage of 0.146 V (see SI for calculation). Because the voltage is positive, the net reaction for the reduction of Ge2+ is expected to be spontaneous. Thus, when considering Ag as the reducing agent for the reduction of Ge2+, the formation of AgI is a necessary driving force. Additionally, it was found that the presence of TOP is required to obtain the hollow Ge NPs. The optimized synthesis of hollow Ge NPs was developed through a systematic exploration of reaction temperatures, heating times, and passivating ligands. Several surface passivating ligands were explored, including TOP, DDT, OAM, and oleic acid (Table 2). From the investigation of the individual ligands, TOP was
determined to be necessary to both stabilize the hollow Ge NPs as well as to facilitate the reaction. Mixtures of ligands containing 3 mL TOP and 2 mL of DDT, OAM, or oleic acid were also investigated. TOP is a necessary ligand for the production of these hollow Ge NPs as they could not be prepared from any reactions containing DDT, OAM, or oleic acid in the absence of TOP. The role of TOP in facilitating the exchange of Ag+ away from the deposited Ge can be understood through hard−soft acid−base chemistry. Previously, Zhang et al. reported tributylphosphine acting as a phasetransfer agent in the cation exchange of Ag+ with Cd2+ in the transformation of Au/Ag2S core/shell NPs to monocrystalline Au/CdS core/shell NPs.40 In this example, tributylphosphine was a soft base that facilitated the exchange of the soft acid, Ag+ (acid softness = +3.99), with the Cd2+ metal cation. Similarly, we propose that the soft base TOP transports GeI2 to the surface of the Ag NP where Ge2+ is reduced to Ge0 and Ag0 is oxidized to Ag+. The Ag+ cation coordinates with the TOP and is removed from the surface of the NP. As silver halide phosphines (e.g., RAg(PPh3)n (R = Cl, Br and n = 1 or 3)) are known to exist,44 the formation of a TOP/AgI complex is highly likely. The formation of a TOP/AgI complex would explain why the precipitation of bulk AgI is not observed upon the addition of TOP, allowing for isolation of the hollow Ge NPs via antisolvent precipitation methods. TEM characterization of the TOP-capped Ge NPs displayed semispherical hollow NPs with an average diameter of 9.80 ± 1.87 nm as determined by the analysis of 307 individual NPs (Figures 2A and S4). The larger average Ge NP diameter is consistent with the hypothesis that the Ge atoms are being
Table 2. Surface Passivating Ligands Investigated To Prepare Hollow Ge NPsa reaction ligand (3 mL) ligands (3 mL/2 mL)
1 TOP −
2 DDT 5 TOP/DDT
3 OAM 6 TOP/OAM
4 oleic acid 7 TOP/oleic acid
a
Tri-n-octylphosphine (TOP); dodecanethiol (DDT); oleylamine (OAM).
Figure 2. (A) Bright-field TEM image of TOP-capped hollow Ge NPs. (B) Raman spectrum of the hollow Ge NPs. (C) ADF-STEM image displaying “pinholes” in the hollow Ge NPs. (D) EDS spectrum acquired from the area displayed in (A). C
DOI: 10.1021/acsnano.6b01604 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano deposited on the surface of the Ag NPs. The Raman spectrum of the hollow Ge NPs is shown in Figure 2B as a broad, asymmetric phonon mode centered at ∼278 cm−1. This result is in agreement with our XRD results showing a featureless diffraction pattern and the literature value of amorphous Ge, which describes a broad band with a maximum between 275 and 280 cm−1.45 Several factors such as excitation power, bond strain, and Fano interference may influence the Raman spectra, in particular those of nanomaterials.46−48 Ou et al. demonstrated that increasing the excitation power from 1 to 25 mW shifted, broadened, and attenuated the Raman signal of Ge NCs embedded in a SiO2 matrix toward lower wavenumbers.46 Under the conditions in which the Raman spectra were acquired, the power directly under the objective lens was 40%, TCI America), dioctyl ether, DOE (99%, Sigma-Aldrich), dodecanethiol (98%, Sigma-Aldrich) were degassed at 100 °C prior to use. Tri-n-octylphosphine, TOP (min. 97%, Strem Chemicals), was used as received. Methanol and toluene were purchased from Fisher Scientific, purified using a solvent purification system, and stored under inert atmosphere conditions in an argon-filled glovebox. Synthesis of Silver Nanoparticles. Ag NPs were synthesized by modifying a previously reported procedure.41 In an argon-filled glovebox, 0.9 mmol of AgNO3 was added to 10 mL of OAM in a two-neck round-bottom flask equipped with a stopcock adapter and rubber septum. The flask was connected to a Schlenk line, and the system was heated at 155 °C for 30 min while under an Ar flow. The reaction was initially colorless, but as the AgNO3 was reduced to form OAM stabilized Ag NPs, the reaction turned dark brown. Upon cooling to room temperature, the Ag NPs were isolated using a solvent/antisolvent system under ambient conditions. The solvent/ antisolvent mixture was comprised of the Ag NPs dispersed in 10 mL of OAM along with 5 mL of toluene and 25 mL of methanol. The solvent/antisolvent mixture was centrifuged at 8500 rpm for ∼15 min, which resulted in the precipitation of Ag NPs. The supernatant was discarded and the Ag NP pellet was transferred into the glovebox. Synthesis of Hollow Germanium Nanoparticles. Initial exploratory experiments were performed in a microwave reactor and later adapted to the Schlenk line (see SI and Figure S7). The optimized synthesis was performed as follows: Under inert atmosphere conditions, 5 mL of DOE was added to a centrifuge tube containing 0.9 mmol Ag in the form of a Ag NP pellet. An excess of 0.1 mmol Ag was used to account for sample loss in the synthesis and transfer of the Ag NPs. The mixture was then sonicated or manually agitated to disperse the Ag NPs. The Ag NP dispersion was transferred to a twoneck round-bottom flask in an argon-filled glovebox. An additional 5 mL of DOE was used to rinse the centrifuge tube wall to ensure the complete transfer of Ag NPs. 0.4 mmol of GeI2 was added to the system along with 3 mL of TOP. The reaction was connected to a Schlenk line and while under argon flow and medium stirring heated to 200 °C for 5 h. For the investigation of surface passivating ligands, individual experiments were performed following the above stated procedure, except in place of 3 mL of TOP, 3 mL of either dodecanethiol, OAM, or oleic acid was used. Additionally, mixed ligand reactions were performed using a 3:2 mL ratio where TOP was held constant at 3 and 2 mL of either dodecanethiol, OAM, or oleic acid was added. After the reaction was completed and had cooled to room temperature, the round-bottom glassware was placed under vacuum while connected to the Schlenk line and then pumped into an argonfilled glovebox. The hollow Ge NPs were purified and isolated under inert atmosphere conditions using a solvent/antisolvent system in the following manner. The hollow Ge NPs dispersed in 10 mL DOE, 3 mL TOP, and 2 mL dodecanethiol, e.g., were transferred into a centrifuge tube. The glassware was rinsed with 5 mL of toluene which was subsequently added to the centrifuge tube containing the hollow Ge NPs. The 20 mL solution was then separated into two centrifuge tubes with each containing 10 mL. To both centrifuge tubes, 7.5 mL of toluene was added along with 22.5 mL of methanol to give a final volume of 40 mL in each centrifuge tube. The centrifuge tubes were closed and pumped out of the box where the solvent/antisolvent mixture was centrifuged at 8500 rpm for ∼15 min, which resulted in the precipitation of the hollow Ge NPs. After centrifugation, the two centrifuge tubes were pumped back into the glovebox. The supernatant was discarded, and the hollow Ge NPs were dispersed in toluene and stored in a glass vial within the glovebox.
Figure 5. UV−vis spectrum displaying the indirect band gap absorbance of TOP-capped hollow Ge NPs. Inset displays the Tauc plot used to determine the indirect band gap energy.
process of forming the hollow Ge NPs, no absorbance from the Ag NPs was observed. From the UV−vis spectrum, a Tauc plot was prepared to determine the band gap energy.54,55 A Tauc plot is a common way to determine the band gap of both direct and indirect semiconductor materials. We compared the Tauc plot of (Ahν)n vs Ahν with n = 0.5 for indirect band gap materials to n = 2 for direct band gap materials. By extrapolating a tangential line from the linear portion of the Tauc plot to the abscissa, an approximate band gap was determined. The indirect band gap was determined to be 1.53 eV, which is in agreement with the absorbance onset at 800 nm. The direct band gap was >3 eV and is inconsistent with our UV−vis data. The hollow Ge NP’s band gap of 1.53 eV is nearly 0.5 eV larger than that of bulk, amorphous germanium (Eg = 1.05 eV).56
CONCLUSIONS We have presented an application of electroless deposition to synthesize hollow Ge NPs using Ag NPs as sacrificial templates. The reaction requires the formation of AgI and the presence of TOP as a transporting agent. The as-synthesized hollow Ge NPs were amorphous with diameters defined by the diameter of the Ag NP template. ADF-STEM imaging verified the galvanic replacement mechanism of Ag diffusion through pinholes in the Ge NP. Electrochemically, the formation of AgI affects the reduction potential of Ag+ such that the reduction of Ge2+ is preferential. In the reaction, TOP acts as a soft base which transports the soft acid Ag+ and is necessary to be able to isolate the hollow Ge NPs. Reactions containing TOP along with another stabilizing ligand (e.g., dodecanethiol, oleic acid, OAM) were explored to passivate the surface of the NP. Future investigation into other Ag templates (e.g., nanorods, nanocubes, etc.) may provide hollow Ge nanostructures with a variety of morphologies and potential applications for energy storage or conversion. Employing electroless deposition to synthesize hollow Ge nanostructures opens up E
DOI: 10.1021/acsnano.6b01604 ACS Nano XXXX, XXX, XXX−XXX
Article
ACS Nano Characterization. PXRD patterns were collected on a Bruker D8 Advance diffractometer using Cu radiation (Cu-Kα: λ = 1.541 Å). The X-ray diffraction patterns were collected for 2θ values ranging between 20 and 75° with a step size of 0.02°. The background observed for the PXRD is due to the quartz substrate. Solid samples, e.g., AgI, were characterized on a quartz substrate. Dispersions of hollow Ge NPs were drop casted onto a quartz substrate and dried under a gentle air stream to form a dry thin film. Jade 5.0 software was used to analyze the XRD patterns and compare the experimental patterns to a powder diffraction file from the International Center of Diffraction Data (ICDD) database. TEM samples were prepared by drop casting the Ge NP dispersion onto holey carbon grids (300 mesh, SPI) and drying under a lamp overnight to reduce contamination during electron beam irradiation. Ge NPs were imaged using a JEOL JEM 2500SE TEM and an aberration-corrected JEOL JEM-2100F/Cs STEM equipped with a Gatan ADF detector. Both microscopes are operated at 200 kV. For STEM imaging the electron probe convergence semi-angle was approximately 23 mrad and the ADF inner detector semi-angle was 33 mrad. Under such imaging conditions, coherent scattering contributions cannot be neglected. However, the Ge NPs were amorphous, hence enabling the identification of Ag by relative bright contrast in ADF micrographs.58 Histograms of particle size distributions were created by using ImageJ software after analyzing more than 200 individual NPs in a variety of different images acquired from different areas on the sample grid. Energy-dispersive X-ray spectra were collected on a Thermo Corporation EDS spectrometer attached to the JEOL JEM 2500SE. In every EDS spectra acquired, even on blank grids, we observe C, Cu, O, and Si signals which is ascribed to the manufacturing process SPI uses to make their holey carbon TEM grids. Raman spectra were acquired using a Reinshaw RM1000 laser Raman microscope (514 nm) with a motorized stage. The Raman laser has an output excitation power of 25 mW that is attenuated by a 10% neutral density filter. The spectrum was acquired through a 50× objective piece. Subsequently, the excitation power that probes the Ge NPs was measured to be
