Thermally-Induced Evolution of 'Ge(OH)2': Controlling the

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C: Physical Processes in Nanomaterials and Nanostructures 2

Thermally-Induced Evolution of ‘Ge(OH)’: Controlling the Formation of Oxide-Embedded Ge Nanocrystals Morteza Javadi, Vladimir K. Michaelis, and Jonathan G. C. Veinot J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04640 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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The Journal of Physical Chemistry

Thermally-induced Evolution of ‘Ge(OH)2’: Controlling the Formation of Oxide-Embedded Ge Nanocrystals Morteza Javadi, Vladimir K. Michaelis, and Jonathan G. C. Veinot* Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, Alberta, Canada. *e-mail: [email protected]; Fax: +1-780-492-8231; Tel: +1-780-492-7206

Abstract Germanium nanocrystals (GeNCs) hold great promise as active materials in many applications including solar cells, biological imaging, Bragg reflectors, light-emitting diodes, and non-volatile memory devices. However, because of the size-, shape-, and compositionallydependent nature of their properties, it is essential that methods affording high purity, welldefined GeNCs be developed. Herein, we report a systematic investigation of a series of ‘Ge(OH)2’ precursors using XRD, XPS, Raman, and solid-state 73Ge NMR, and have correlated their properties with the GeNCs obtained from their thermal processing. This study provides important insight into the role of the ‘Ge(OH)2’ internal structure and will allow for future rational design and practical application of GeNCs.

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Introduction As silicon’s heavier cousin, germanium is often overlooked. While some similarities exist between these two periodic congeners (e.g., both are in-direct bandgap semiconductors, chemical reactivity, crystal structure, etc.), key differences make germanium an attractive functional material for numerous applications. In light of its unique (bulk) properties including a small band gap (0.67 eV at 300 K),1 large Bohr-exciton radius (24.3 nm),2,3 and high electron and hole mobility (~3900 cm2/V·s),4 as well as large capacity for and diffusivity of ions (e.g., Li+), germanium has the potential to impact many far-reaching applications in semiconductor industries.5 Preparing Ge at the nanoscale adds an additional degree of freedom that offers further property tailoring because of the impacts of surface area and quantum confinement.6,7 To date, numerous prototype applications of germanium quantum dots (or nanocrystals; GeNCs) have appeared including: solar cells,8 biological imaging,9,10 Bragg reflectors,8 light-emitting diodes, and non-volatile memory devices.11–15 If these, and heretofore unknown applications of GeNCs are to be realized, methods for the preparing well-defined NCs must be established and their material properties must be understood. To date, GeNCs have been prepared using a variety of methods including top-down procedures such as laser ablation16 and ball milling,17 as well as bottom-up techniques like plasma pyrolysis of GeH4,18 metathesis of Ge Zintl salts,6,7,19,20 solution-phase reduction of Ge(II) and Ge(IV) compounds,21–23 and thermal decomposition of organogermane precursors.24– 26

While these methods provide access to GeNCs, each method brings with it its own unique

challenges which may include limited product yield, size polydispersity, ill-defined shape, material purity, and complex surface chemistry.


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A series of sub-stoichiometric metal oxides (RMOx, 0 < x < 2, M: Si or Ge and R: phenyl, ethyl, n-butyl, allyl, benzyl, carboxyethyl, and t-butyl) have been prepared via sol-gel processing of appropriate precursors.” Thermal processing of these sol-gel derived products provides a convenient method for preparing oxide-embedded NCs that can be used directly as a NC/metal oxide composite, or liberated and employed as freestanding NCs.27–31 This general approach is particularly appealing because the metal oxide precursor often provides a high purity source of the constituents of the target nanomaterial (e.g., Si, Ge, etc.) as well as a matrix (‘solvent’) in which the NCs form and grow – this combination provides a system that allows effective separation of NC nucleation and growth leading to narrow size distributions, as well as controlled particle size. We and others have exploited metal sub-oxide thermal processing extensively to prepare size-tunable SiNCs with narrow size distributions.27,28,32,33 While thermal processing of organic functionalized germanium rich oxides [(RGeO1.5)n] has provided a means of obtaining oxide- and oxycarbide-embedded GeNCs,28,30,31 these materials were not ideal because of the possibility of contamination, as well as issues with NC liberation. Until recently, a suitable precursor containing only germanium, oxygen and hydrogen had not been reported. However, independently our group and the Ozin team published procedures in which oxide embedded GeNCs were prepared via thermal processing ‘Ge(OH)2’.34,35 Despite promising results, the NCs reported in these early reports are not ideal – they often exhibited ill-defined shapes, wide polydispersities, and even sintered. In this context, we have turned our attention to investigating the nature of ‘Ge(OH)2’, which has a controversial structure and composition, to gain greater insight into the processes leading to GeNC formation.35 Herein, we report a systematic study of three ‘Ge(OH)2’ variants (i.e., yellow-, orange- and brown-Ge(OH)2 obtained from established literature procedures that employed X-ray diffraction (XRD), X-ray 3 ACS Paragon Plus Environment


photoelectron spectroscopy (XPS), vibrational spectroscopy (Infrared and Raman), and

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solid-state nuclear magnetic resonance (NMR) spectroscopy to elucidate the ‘Ge(OH)2’ composition and internal structure. Once identified, the ‘Ge(OH)2’ variants were each evaluated as GeNC precursors and a mechanism for NC formation was proposed that will facilitate further advancement of GeNC design and application. Experimental Reagents and Materials. Germanium dioxide powder (GeO2, 99.9%) was purchased from Eagle-Picher. Hypophosphorous acid solution (50 wt. % in H2O), sodium hydroxide pellets, 1-dodecene (97%), as well as reagent grade methanol, toluene, and ethanol were purchased from Sigma-Aldrich. Electronics grade hydrofluoric acid (HF, 49 % aqueous solution) was purchased from J. T. Baker. Hydrochloric acid (36.5 - 38%) and ammonium hydroxide (28 - 30%) were purchased from Caledon Lab. Ultrapure H2O (18.2 MΩ/cm) purified in a Barnstead Nanopure Diamond purification system was used in all reactions. Synthesis of germanium (II) hydroxide (‘Ge(OH)2’). ‘Ge(OH)2’ was prepared using a modified literature procedure.35–41 Commercial germanium dioxide powder was dissolved in freshly prepared aqueous NaOH (~17 M, 7.0 ml). Subsequently, aqueous HCl (6 M; 24 mL) was added slowly to cause a white precipitate to form and then redissolved to provide a solution of Ge (IV) ions to which an aqueous solution of H3PO2 (50 %, 7.5 mL) was added. This colorless solution was refluxed under static Ar for 5.5 h, then cooled to room temperature, and then in an ice-salt (NaCl) bath. After cooling to below 5 °C concentrated aqueous NH4OH solution (10 mL) was added drop-wise to yield a yellow precipitate of ‘Ge(OH)2’. (Caution: This reaction is extremely exothermic; the temperature must 4 ACS Paragon Plus Environment

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be maintained at ca. 5 °C.) The yellow precipitate was recovered by vacuum filtration and washed with deionized/deoxygenated water (3 × 100 mL), dried in vacuo for 12 hours and stored in an argon filled glovebox for future use and analysis. Orange and brown versions of ‘Ge(OH)2’ (referred to as ‘orange-Ge(OH)2’ and ‘brownGe(OH)2’, respectively) were prepared by maintaining the temperature of the reaction mixture at 50 and 100 °C, respectively during the addition of aqueous NH4OH. The orange- and brownGe(OH)2 were isolated, purified and stored as described above for yellow-Ge(OH)2. Thermal processing of ‘Ge(OH)2’. Thermal processing of four varieties of ‘Ge(OH)2’ was performed using a Lindberg/Blue tube furnace in flowing argon atmosphere (15 mL/min). The ‘Ge(OH)2’ (ca. 0.5 g) of choice was placed in a quartz boat housed in the furnace and heated to the desired processing temperature (i.e., 300, 350, 400, 450, or 500 °C) at 20 °C/min where it remained for 1 h. The product (i.e., a germanium oxide composite of GeNCs) obtained from the thermal processing was cooled to room temperature, mechanically ground using an agate mortar and pestle, and stored under ambient atmosphere for characterization. Typical yields were >0.4 g. Isolation of hydride-terminated GeNCs (H-GeNCs). Hydride-terminated GeNCs (H-GeNCs) were liberated from the composites noted above

upon etching in alcoholic HF solutions using a variant of a procedure developed in our laboratories.35 Briefly, the finely ground thermally processed product of choice (~50 mg) was magnetically stirred in 1.0 mL of a 1:1 solution of ethanol and water for ca. 10 min in a PET beaker. Aqueous HF (49 %; 0.5 mL) was added drop-wise (Caution: HF must be handled with care and in accordance with local guidelines.). The mixture was stirred for 15 min. Subsequently, toluene (3×5 ml) was added to extract the GeNCs. The cloudy toluene layer was removed via 5 ACS Paragon Plus Environment


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pipette and a black precipitate was isolated upon centrifuging at 3000 rpm. This precipitate was dispersed in toluene containing activated molecular sieves (ca. 1 g, 4 Å) and agitated for approximately one minute. The suspension was transferred to centrifuge tubes and the black precipitate (i.e., H-GeNCs) was recovered upon centrifuging at 3000 rpm and immediately functionalized as outlined below. The typical yield of H-GeNCs was ca. 10 mg (Note: This yield is dependent upon the nature of the ‘Ge(OH)2’ precursor used to prepare the GeNC/GeO2 composite). Preparation of dodecyl-GeNC. The black product (i.e., H-GeNCs) obtained from the etching procedure was dispersed in dodecene (5.0 mL), deoxygenated using an argon charged double manifold via three freezepump-thaw cycles. Subsequently, the mixture was heated to 190 °C with stirring in an argon atmosphere for 12 h. The resulting transparent orange-brown suspensions of crude dodecylterminated GeNCs (dodecyl-GeNCs) were purified via precipitation/resuspension cycles as described previously.35 The resulting purified dodecyl-GeNCs were dispersed in ca. 5 mL of dry toluene and filtered through a 0.45 µm PTFE syringe filter to yield a clear brown suspension. A typical yield for dodecyl functionalized GeNCs was ca. 15 mg from 50 mg of GeNC/GeO2 composite. The dodecyl-GeNCs were stored as a toluene suspension until needed for material characterization. Material Characterization and Instrumentation. Fourier-transform infrared spectroscopy was performed using a Nicolet Magna 750 IR spectrophotometer. Samples were drop-cast from a toluene suspension containing GeNCs. X-ray powder diffraction was performed using an INEL XRG 3000 X-ray diffractometer equipped with a Cu-Kα radiation source (λ = 1.54 Å). The crystallinity of all samples was evaluated for finely 6 ACS Paragon Plus Environment

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ground powders mounted on a low-intensity background Si (100) holder. Bright-field transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analyses were performed using a JEOL-2010 (LaB6 filament) electron microscope with an accelerating voltage of 200 kV. High-resolution TEM (HRTEM) imaging was performed on JEOL-2200FS TEM instrument with an accelerating voltage of 200 kV. TEM samples of GeNCs were prepared by drop-coating 1-3 drops of toluene suspension (ca. 1 mg/mL) containing NCs of choice onto a holey carbon coated copper grid (300 mesh, Electron Microscopy Science) and the solvent was removed under vacuum. TEM and HRTEM images were processed using ImageJ software (version 1.48 v). Particle size distributions (PSD) were determined by measuring ca. 300 NCs. Raman spectroscopy was performed using a Renishaw inVia Raman microscope equipped with a 514 nm diode laser operating at a power of 3.98 mW on the sample. Samples were prepared by mounting the suspension on the gold-coated glass. X-ray photoelectron spectra were acquired in energy spectrum mode at 210 W, using a Kratos Axis Ultra X-ray photoelectron spectrometer. X-ray source was Al (Mono) Kα line (1486.6 eV) with a probing area of 1 x 2 mm2. Samples were prepared as films drop-cast from a toluene solution/suspension onto a copper foil substrate. Binding energies were calibrated using the C 1s emission as an internal reference (284.8 eV). CasaXPS Version 2.3.5 software was used to accomplish the Shirley-type background subtraction. The high-resolution Ge 3d region of spectra was collected for all samples and was fit to Ge 3d3/2/Ge 3d5/2 partner lines, with spin-orbit splitting fixed at 0.6 eV, and the Ge 3d3/2/Ge 3d5/2 intensity ratio was set to 0.67.31 Solid-state Nuclear Magnetic Resonance Spectroscopy Germanium-73 NMR spectra were acquired at 21.1 T on a Bruker Avance II 900 NMR spectrometer. Non-spinning

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Ge NMR spectra were collected using a home-built 7 mm H/X 7 ACS Paragon Plus Environment


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probe with a two-coil design. Magic-angle spinning (MAS) experiments were acquired using a specially designed low-ringing 7 mm single resonance ultralow-gamma Bruker probe. The magic-angle was set using the Hall sensor method42 and acquired with a spinning frequency (ωr/2π) of 3 kHz. The air-sensitive samples were handled in an Ar-filled glove-box, packed into thin-walled 7 mm (o.d., 400 µl fill volume) ZrO2 Bruker rotors and Vespel® caps equipped with O-ring seals. All spectra were acquired using either a Bloch pulse (MAS) or solid-echo (nonspinning) with short interpulse delays of 20 µs and

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Ge (ƔB1/2π) of 10 kHz (solution, I=9/2;

solid π/2 = 4 µs). All spectra were acquired with a recycle delay of 5 s, and between 512 and 5,632 co-added transients. All 73

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Ge spectra were referenced externally to GeCl4 by setting the

Ge peak to 30.9 ppm as a secondary reference with respect to GeH4 (73Ge, ߜiso = 0.0 ppm).43

All experiments were acquired at room temperature and under dry nitrogen purge gas. All spectra were processed using Topspin 3.5 Bruker software with between 25 and 100 Hz exponential apodization. Sensitivity enhancing experimental methods including CPMG and WURST-CPMG and longer recycle delays were also attempted during the characterization of these solids. We note the

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Ge NMR nutation behavior of microcrystalline Ge metal (Fm-3m),

δiso = -73.8 ppm, is nearly identical to the liquid reference (GeCl4, δiso = 30.9 ppm).


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Results and Discussion Germanium (II) hydroxide, commonly referred to as hydrous germanium (II) oxide, has an uncertain stoichiometry; it is typically represented by formulae such as Ge(OH)2, Ge(OH)2·xH2O, GeO·xH2O, etc.37 Existing literature is conflicted regarding the nature of ‘Ge(OH)2’ and this solid product can vary widely in physical appearance; it can be yellow, orange, or brown (Figure 1a-c) depending upon seemingly minor variations in the conditions under which it was prepared.37–39,41 For example, a yellow precipitate forms if ammonium hydroxide is added when the reaction mixture is chilled to below 5 °C, while orange and brown precipitates form if the reaction mixture is 50 and 100 °C, respectively.



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Figure 1. Photographs of (a) yellow-, (b) orange-, and (c) brown-Ge(OH)2. (d) XRD patterns obtained from (i) yellow-, (ii) orange-, and (iii) brown-Ge(OH)2. Standard reflections of bulk Ge (PDF#04-0545) and rutile GeO2 are provided for comparison (e) Raman spectra obtained from (i) yellow-, (ii) orange-, and (iii) brown-Ge(OH)2.

Not surprisingly, the differences exhibited by these materials reach beyond straightforward physical appearance and are evident when using more detailed characterization techniques. X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) provide complementary information regarding the underlying structure of yellow-, orange-, and brown-Ge(OH)2. In agreement with literature reports, none of the ‘Ge(OH)2’ variants show well-defined reflections expected for crystalline materials.37,41 In all cases the XRD patterns show broad features at ca. 2θ = 25-35° and 45-50° (Figure 1d). While similar features have been attributed to the presence of amorphous Ge,22 the complexity of the present systems makes any definitive assignment premature. Sharp (albeit weak) reflections are also noted at 2θ = 28.7, 37.4, 56.7, 59.3, and 72.3° that are confidently assigned to rutile GeO2 (P42/mnm (136)).44 These rutile GeO2 features are more intense for the orange- and brownGe(OH)2 and may be related to these materials being precipitated from hot Ge(II) reaction mixtures. Raman spectroscopy allows detection/identification of Ge-Ge bonds within the ‘Ge(OH)2’ variants. Ge-Ge bonding could exist in Ge inclusions (e.g., crystalline, noncrystalline, and/or clusters) that could reasonably form as a result of thermally induced disproportionation reactions occurring during precipitation from Ge (II) reaction mixtures. If present, these Ge inclusions could influence GeNC formation and growth by acting as heterogeneous nucleation sites and/or sources of elemental Ge and impact the shape, as well as size uniformity, of the resulting NCs (vide infra). Raman data (Figure 1e) acquired for the 10 ACS Paragon Plus Environment

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yellow-, orange-, and brown-Ge(OH)2 show a general increase in the intensity of the Ge-Ge optical phonon feature at ca. 280 cm-1 with the temperature of the ‘Ge(OH)2’ precipitation. The lack of features associated with Ge-Ge in the Raman spectrum of yellow-Ge(OH)2 indicates no detectable Ge-Ge bonding and suggests broad reflections in the XRD pattern obtained for this compound arise from an amorphous Ge(OH)X structure. In contrast, the appearance of Ge-Ge bonding in the orange- and brown-Ge(OH)2 qualitatively support the proposal that amorphous Ge nanodomains are present. Survey XP spectra of the yellow-, orange-, and brown-Ge(OH)2 (Figure S1) indicate that they contain only germanium and oxygen at the sensitivity of the method. Previous reports indicate orange- and brown-Ge(OH)2 possess the empirical formula Ge(OH)2.37–39,41 Based upon the experimental XPS atomic percentages (Figures S1) yellow-, orange-, and brown-Ge(OH)2 possess empirical formulae of GeO1.80, GeO1.64 and GeO1.65. (Note, XPS cannot detect hydrogen) Clearly all materials are “germanium rich” oxides. Ge LMM signals are also observed at binding energies of 300-600 eV.45,46 Comparing the deconvolution of the high-resolution XP spectra Ge 3d region, it becomes clear that yellow-Ge(OH)2 (Figure S2a) is dominated by a Ge(II) emission centered at ca. 31 eV. More intense features related to intermediate oxides as well as Ge (0) and Ge (IV) centered at ca. 29 and 33 eV, respectively appear in the spectra of orange- and brownGe(OH)2 (Figure S2a). These observations are consistent with Raman and XRD data (vide supra) and suggest disproportionation reactions are occurring during the synthesis of orange- and brown-Ge(OH)2 and that these reactions are inducing the formation of low valent Ge and rutile GeO2. 73

Ge NMR can provide valuable insight into the internal structure of the present

‘Ge(OH)2’ species, however it is extremely challenging as a result of the characteristically 11 ACS Paragon Plus Environment


unfavorable NMR properties of

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Ge which include, low natural abundance (7.7 %), low

gyromagnetic ratio (γ = -0.936 × 107 rad s−1 T−1) characterizing it as an ultralow-gamma NMR nucleus with a low resonance frequency (࢜L=31.4 MHz at 21.1 T), and large quadrupolar moment (-19.6 fm2).47 Unfortunately, evaluation of yellow-, orange-, and brown-, Ge(OH)2 using solid-state 73Ge NMR did not reveal any signals even after exhaustive attempts using a variety of specialized pulse sequences including WURST-CPMG, CPMG and echo. Complimentary

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Ge

NMR spectra of Ge metal, composite and hexagonal GeO2 can be found in Figure S2b. Considering the ‘Ge(OH)2’ precursors have not been heat treated (vide infra) it is reasonable they contain a highly disordered Ge chemical environment(s) that suffers from a sizable quadrupolar coupling constant. This phenomenon is illustrated well when considering that progressing from cubic Ge metal to rutile GeO2 provides nearly a 13,000-fold increase in linewidth; in this context, even minor deviations from cubic symmetry can produce substantial and significant inhomogeneous broadening of the central transition and has been reported in 73Ge NMR studies of oxides, halides and organogermanium solids.43,48–52 For example, quadrupole coupling constants for hexagonal, rutile, and amorphous GeO2 vary from 9 to 19.3 MHz (900 to 4000 ppm broad at 21.1 T).49 Combining the complementary data obtained from XRD, Raman, XPS, and solid-state 73

Ge NMR analyses we conclude that none of the presented ‘Ge(OH)2’ variants contain

nanocrystalline domains, however, all possess low oxidation state germanium (i.e., Ge(II) and Ge(III)). Furthermore, the evidence of Ge-Ge bonding in the orange- and brown-Ge(OH)2 is consistent with the presence of non-crystalline Ge inclusions. To gain insight into the evolution of the ‘Ge(OH)2’ precursors upon thermal processing, the discussion will now shift to the structural interrogation of the products obtained from solid phase heating of the yellow-, orange-, 12 ACS Paragon Plus Environment

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and brown-Ge(OH)2 at predefined temperatures in Ar. In all cases, thermal processing produces dark-brown/black powders. XRD, XPS, and Raman show these dark-brown/black powders contain elemental germanium (i.e., Ge(0)) as well as germanium dioxide (i.e., Ge(IV)), regardless of the processing temperature. The general trends in material properties evaluated using these spectroscopic techniques are consistent across the series of yellow-, orange-, and brown-Ge(OH)2 (Figures S3 and Table S1 for yellow-Ge(OH)2 and Figure S4 for orangeGe(OH)2). For convenience, the following discussion will focus on brown-Ge(OH)2 because, of the present ‘Ge(OH)2’ species, it provided the most well-defined GeNCs (vide infra). Figure 2a shows the evolution of the XRD pattern obtained for brown-Ge(OH)2 upon solid-state thermal processing for 1 h in an argon atmosphere at the indicated temperatures (i.e., 300, 350, 400, 450, and 500 °C). As noted above, brown-Ge(OH)2 shows broad, ill-defined reflections that we attribute to an amorphous GeOx species. Raman and XPS data indicate Ge-Ge bonding present that may provide heterogeneous nucleation sites for GeNC formation and a source of elemental Ge that could supply NC growth. After heating, the ill-defined features of the XRD evolve into broadened reflections centered at 27.3, 45.3, 53.7, 66.1, 72.9, and 83.8° characteristic of cubic Ge (PDF#04-0545) and consistent with the formation of crystalline nanodomains (i.e., GeNCs).53



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Figure 2. (a) XRD patterns, (b) High-resolution XP spectra of the Ge 3d region, and (c) Raman spectra obtained for the brown-Ge(OH)2 before and after thermal processing at indicated temperatures (for 1 h under Ar). Reflections of Ge (PDF#04-0545) and hexagonal GeO2 are provided for comparison.

The breadth of the XRD reflections narrows with increasing processing temperature, suggesting growth of these domains. Debye-Scherrer analyses of XRD peak broadening (assuming the dimensionless shape factor is 0.94, calculations are provided in Table S2) indicates that raising the processing temperature through the series of 400, 450, 500 °C induces nucleation and growth of the nanocrystal Ge domains with estimated dimensions of ca. 6.1, 8.0, 9.4 nm, respectively. In addition, higher thermal processing temperatures (i.e., 500) are sufficient to also induce crystallization of the GeOx matrix to produce hexagonal GeO2, P3121 (152).44,54 These observations support the proposal that disproportionation is occurring and brown-Ge(OH)2 is transforming into two thermodynamically favored crystalline materials (i.e., GeNCs and GeO2 matrix). Survey XP spectra of the GeNCs/GeO2 composites obtained from thermal processing of brown-Ge(OH)2 1 h under Ar are shown in Figure S5. In all cases the composites are 56 atomic % germanium and 43 atomic % oxygen. Evolution of the Ge 3d spectral region of the high14 ACS Paragon Plus Environment

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resolution XP spectrum (Figure 2b) is consistent with a general disproportionation reaction mechanism. Brown-Ge(OH)2 shows a broad range of binding energies from ca. 34 to 28 eV. These emissions broaden and the maximum initially shifts to higher binding energy upon solidstate heating to as high as 300 °C. Consistent with XRD analyses (vide supra) that show the appearance of nanodomains of Ge upon thermal processing at 350 °C, two emissions arising from Ge(0) (ca. 29 eV) and Ge (IV) (ca. 33 eV) remain upon heating to higher temperatures.30 Raman spectroscopy further supports the thermally induced disproportionation proposal. The Raman spectrum of the parent brown-Ge(OH)2 shows some evidence of Ge-Ge bonding. Upon thermal processing at 300 - 350 °C the spectrum of the resulting composite shows an asymmetric peak at ca. 298 cm-1 associated with the Ge-Ge optical phonon (i.e., Figure 2c). In addition to confirming the presence of Ge-Ge bonds, the shape and position of the peak are useful for understanding the domain size and crystallinity.31,55,56 The Ge-Ge optical phonon at 298 cm-1 intensifies and sharpens with increased processing temperature (i.e., from 350 to 500 °C; Figure 2c) qualitatively indicating growth and increasing crystallinity of the nanodomains, which support the present XRD and XPS observations. Ex-situ thermal processing of ‘Ge(OH)2’ annealing followed by evaluation using

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Ge

NMR further illuminates the formation mechanism of crystalline Ge nano-domains (i.e., GeNCs). Brown-Ge(OH)2 yields the most well-defined GeNC morphology (vide infra), hence, it was chosen for the present study. Figure 3 shows the

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Ge NMR spectra of brown-Ge(OH)2

before and after thermal processing at the indicated temperatures (for 1 h under Ar); the

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Ge

NMR spectrum of bulk Ge metal is provided for comparison. There is no detectable 73Ge NMR signal of the brown-Ge(OH)2 precursor suggesting the absence of crystalline cubic Ge (i.e., the electric field gradient (EFG) about Ge is sizeable inhibiting the ability to observe a

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Ge NMR 15

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signal). After thermal processing at 400 oC signals arising from GeNCs begin to emerge; these findings agree with XRD, XPS and Raman spectroscopy data, vide supra further supporting our analysis. As the processing temperature is increased from 400 to 600 oC, the particles increase in size, from 4 to 10 nm according to our diffraction results (see above); as a result, the resonance (ca. -78 ppm) associated with the Ge atoms within the NCs sharpens. The broad resonance observed within the nanocrystalline Ge solids is tentatively assigned to the high surface area of the Ge nanoparticles (i.e., disordered Ge on the surface with a non-zero EFG). As the particles grow and the core size increases (i.e., surface area decreases), the linewidth will shrink as the interruption of the periodic crystal potential decreases. On the micron scale (Ge metal) the isotropic shift for Ge is 10 Hz or 0.3 ppm broad. This effect in NMR has been reported in the past for many materials including glass,

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Na or

207

Pb NMR of Na or Pb metal confined in porous

195

Pt NMR of small particles, or 69Cu NMR of silica-supported colloidal copper particles,

among others.57–62 Further analysis of this broader 73Ge component to lower frequency (Figure 4, 400 oC, attributed to a non-cubic Ge chemical environment) is limited due to the long 73Ge NMR experimental times (~48 hrs).


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Figure 3. Germanium-73 MAS NMR spectra of brown ‘Ge(OH)2’ before and after ex situ thermal processing treatment indicated at specific temperatures (for 1 h under Ar) compared to Ge metal.

Direct evaluation of the morphology of GeNCs within oxide matrices formed during the thermal processing of the presented ‘Ge(OH)2’ variants using electron microscopy (i.e., TEM) is non-trivial. It is necessary to liberate freestanding GeNCs from the GeNCs/GeO2 composites; this is achieved via alcoholic HF etching. This procedure provides freestanding hydrideterminated H-GeNCs. While the NCs are freed from the oxide matrix that occludes imaging, the H-GeNCs are also challenging to image due to limitations associated with effective sample preparation. Hence, H-GeNCs were surface modified with dodecyl moieties and rendered solution compatible using thermally induced hydrogermylation.35 Spectroscopic characterization (e.g., FTIR, XPS, EDX) of these surface functionalized GeNCs is shown in Figures S6. A 17 ACS Paragon Plus Environment


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detailed discussion of these analyses is beyond the scope of the present contribution and has been reported elsewhere.35 Insight into the formation and growth of oxide-embedded GeNCs can be inferred from the bright-field TEM imaging of the functionalized NCs. While the surface functionalization and limited purification procedures used herein may impact the final size distributions of the NCs slightly because the NCs obtained from the different precursors were treated identically, it is reasonable to expect the present analysis to be representative. Bright-field electron micrographs of dodecyl-GeNCs (Figure 4) qualitatively show that the NC morphology depends upon the precursor employed. While all the NCs are pseudospherical, the uniformity of particle shape qualitatively improves through the series of yellow- to orange- to brown-Ge(OH)2. This observation may result from the impact of seeded growth that is possible in orange- to brownGe(OH)2 which contain Ge inclusions (Raman and XPS analysis, vide supra). Consistent with our proposal of seeded growth occurring in the orange- and brown-Ge(OH)2, we note that the NC size distribution narrows dramatically through yellow- to orange- to brown-Ge(OH)2 and the average particle size decreases (See histograms in Figure 4).


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Figure 4. Bright-field TEM images and associated particle size distributions of dodecyl-GeNCs obtained from thermal processing of (a) yellow-, (b) orange, and (c) brown-Ge(OH)2. Inset: photographs showing colors and physical appearances for each ‘Ge(OH)2’ material.

Bright field TEM of dodecyl-GeNCs liberated from Ge/GeO2 composites obtained from the thermal processing of brown-Ge(OH)2 at predefined temperatures provide additional information regarding the particle nucleation and growth processes. Figure 5 shows representative bright field TEM images of dodecyl-functionalized GeNCs obtained from thermal processing brown ‘Ge(OH)2’ at 350, 400, 450, and 500 °C; consistent with XRD and 73Ge NMR analyses of oxide embedded GeNCs (vide supra), the average particle sizes of the GeNCs are 5.9 ± 1.2, 7.2 ± 0.9, 8.3 ± 1.1, and 10.1 ± 4.3 nm, respectively. This trend in average particle size combined with broadening of the size distribution at higher processing temperatures is consistent



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with an Ostwald ripening model in which Ge atom diffusion is occurring at higher temperatures.63

Figure 5. Brown-Ge(OH)2 and its thermal processing. (a-d) TEM images and particle size distribution of dodecylGeNCs synthesized from thermally processing of brown-Ge(OH)2 for 1 h under Ar at 350 °C (a), 400 °C (b), 450 °C (c), and 500 °C (d).

Conclusions The present investigation provides important insight into the nature of ‘Ge(OH)2’ and its of yellow, orange and brown variants. Despite maintaining the empirical formula of ‘Ge(OH)2’, 20 ACS Paragon Plus Environment

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it is clear the internal structure depends upon the preparation conditions and more specifically the temperature at which the ‘Ge(OH)2’ is prepared. The products precipitated at higher temperatures contain higher concentrations of the Ge-Ge band. In addition, the present data are consistent with disproportionation reactions occurring upon thermal processing of ‘Ge(OH)2’ and that the low valent inclusions of Ge clearly impact the formation and growth of oxide embedded GeNCs. Supporting Information Survey XP spectra of various ‘Ge(OH)2’ precursors before and after thermal annealing. Full characterization of yellow and orange-Ge(OH)2. FT-IR, Raman and EDS of dodecyl-GeNCs. Acknowledgements The Natural Sciences and Engineering Research Council (NSERC) of Canada Discovery Grants Program and the University of Alberta are acknowledged for generous research support. The ATUMS training program supported by NSERC CREATE is thanked for the continued generous financial support. The authors would like to thank Dr. Victor Terskikh for scientific discussions and assistance at the National Ultrahigh-Field NMR Facility for Solids. Access to the 21.1 T NMR spectrometer was provided by the National Ultrahigh- Field NMR Facility for Solids (Ottawa, Canada), a national research facility funded by a consortium of Canadian Universities, and by an NSERC RTI grant and supported by the National Research Council of Canada and Bruker BioSpin, and managed by the University of Ottawa (http://nmr900.ca). G. Popowich and W. Moffat are thanked for assistance. Michelle Ha, Dr. Y. Khaniani and the members of the Veinot research team are also thanked for useful discussions.



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849x474mm (110 x 149 DPI)


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Figure 1. Photographs of (a) yellow-, (b) orange-, and (c) brown-Ge(OH)2. (d) XRD patterns obtained from (i) yellow-, (ii) orange-, and (iii) brown-Ge(OH)2. Standard reflections of bulk Ge (PDF#04-0545) and rutile GeO2 are provided for comparison (e) Raman spectra obtained from (i) yellow-, (ii) orange-, and (iii) brownGe(OH)2. 1165x1088mm (96 x 96 DPI)



Figure 2. (a) XRD patterns, (b) High-resolution XP spectra of the Ge 3d region, and (c) Raman spectra obtained for the brown-Ge(OH)2 before and after thermal processing at indicated temperatures (for 1 h under Ar). Reflections of Ge (PDF#04-0545) and hexagonal GeO2 are provided for comparison. 1569x671mm (150 x 150 DPI)


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Figure 3. Germanium-73 MAS NMR spectra of brown ‘Ge(OH)2’ before and after ex situ thermal processing treatment indicated at specific temperatures (for 1 h under Ar) compared to Ge metal. 106x151mm (300 x 300 DPI)



Figure 4. Bright-field TEM images and associated particle size distributions of dodecyl-GeNCs obtained from thermal processing of (a) yellow-, (b) orange, and (c) brown-Ge(OH)2. Inset: photographs showing colors and physical appearances for each ‘Ge(OH)2’ material. 1246x754mm (96 x 96 DPI)


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Figure 5. Brown-Ge(OH)2 and its thermal processing. (a-d) TEM images and particle size distribution of dodecyl-GeNCs synthesized from thermally processing of brown-Ge(OH)2 for 1 h under Ar at 350 °C (a), 400 °C (b), 450 °C (c), and 500 °C (d). 1096x1092mm (96 x 96 DPI)


Thermally-Induced Evolution of 'Ge(OH)2': Controlling the

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