Iron Pyrite Nanocrystal Inks: Solvothermal ... - ACS Publications

Subscriber access provided by Otterbein University

Article

Iron Pyrite Nanocrystal Inks: Solvothermal Synthesis, Digestive Ripening, and Reaction Mechanism Tara S. Yoder, Jacqueline E. Cloud, G. Jeremy Leong, Doreen F. Molk, Matthew Tussing, Jonathan Miorelli, Chilan Ngo, Suneel Kodambaka, Mark E Eberhart, Ryan M. Richards, and Yongan Yang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5030553 • Publication Date (Web): 17 Nov 2014 Downloaded from http://pubs.acs.org on November 24, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Iron Pyrite Nanocrystal Inks: Solvothermal Synthesis, Digestive Ripening, and Reaction Mechanism Tara S. Yoder,† Jacqueline E. Cloud,† G. Jeremy Leong,† Doreen F. Molk,† Matthew Tussing,† Jonathan Miorelli,† Chilan Ngo,‡ Suneel Kodambaka,‡ Mark E. Eberhart,† Ryan M. Richards,† and Yongan Yang*,† † Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colorado 80401 ‡ Department of Materials Science and Engineering, University of California Los Angeles, Los Angeles, California 90095 Supplemental Information ABSTRACT: Colloidal iron pyrite nanocrystals (or FeS2 NC inks) are desirable as active materials in lithium ion batteries and photovoltaics, and are particularly suitable for large-scale, roll-to-roll deposition or ink-jet printing. However, to date, FeS2 NC inks have only been synthesized using the hot-injection technique, which requires air-free conditions and may not be desirable at an industrial scale. Here, we report the synthesis of monodisperse, colloidal, spherical, and phase-pure FeS2 NCs of 5.5 ± 0.3 nm in diameter via a scalable solvothermal method using iron diethyldithiocarbamate as the precursor, combined with a post digestive ripening process. The phase purity and crystallinity are determined using X-ray diffraction, transmission electron microscopy, far-infrared spectroscopy, and Raman spectroscopy techniques. Through this study, a hypothesis has been verified that solvothermal syntheses can also produce FeS2 NC inks by incorporating three experimental conditions: high solubility of the precursor, efficient mass transport, and sufficient stabilizing ligands. The addition of ligands and stirring decrease the NC size and led to a narrow size distribution. Moreover, using density functional theory calculations, we have identified an acid-mediated decomposition of the precursor as the initial and critical step in the synthesis of FeS2 from iron diethyldithiocarbamate.

INTRODUCTION Iron pyrite (FeS2) is a widely studied and utilized functional material, particularly for two energy-related applications: 1-4 5-8 lithium ion batteries and photovoltaics. The battery application is based on its activity of electrochemical 2, 9, 10 lithiation. Since the 1980s, FeS2 has been utilized as a cathode material in batteries, including rechargeable (secondary) lithium ion batteries at high temperatures (400 o C) and non-rechargeable (primary) lithium batteries at o 1-4, 9-19 ambient temperatures (-40 to 60 C). The photovoltaic application of FeS2 is due to its semiconductor properties, particularly because its band gap (Eg = 0.95 eV) is in the 20 ideal range for single-junction photovoltaic devices. Other 21-23 properties, such as non-toxicity, environmental benignity, cost-effectiveness, and the natural abundance of iron and 21-23 make FeS2 very attractive in the endeavor of sulfur, establishing a clean and renewable energy-based society. The potential impacts of using synthetic FeS2 nanostructures in these two applications have intensified in the past decade.

In 2002, Shao-Horn et al. reported that natural FeS2 nanocrystals (NCs, ~50 nm) were superior to microcrystals 3 (~10 µm) in battery reaction kinetics. In 2013, Yersak et al. demonstrated for the first time that synthetic FeS2 microcrystals (~2.5 µm) could be charged and discharged at full capacity (four Li-ions per FeS2 unit) for at least 20 cycles o 4 at 60 C. These studies imply that synthetic FeS2 NCs might present enhanced cathode performance, as observed with 24-26 many other electrode materials. To be compatible with the large-scale electrode fabrication processes, such as roll8, 27, 28 to-roll deposition and ink-jet printing, colloidal NC 4 solutions (NC inks) are highly desired. Similarly, FeS2 NC inks for photovoltaics have also been a hot topic in recent 20, 23, 29, 30 years. Table 1 summarizes the representative publications of three wet-chemistry methods that hold the promise for synthesizing FeS2 NC inks: 1) hot injection (injecting one or all precursors into a pre-heated flask under a water-free and

1 ACS Paragon Plus Environment

Chemistry of Materials 7, 8, 31-43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

air-free environment); 2) microwave (heating the 4, 44-46 reaction flask with microwave power); and 3) solvothermal (heating a sealed autoclave vessel in a furnace 47-53 The key findings in these studies are as or oven). follows: 1) the most important factors for controlling the phase purity are precursor identities, molar ratios between 4, 7, 8, 31Fe and S, reaction temperatures, solvents, and ligands; 53 2) the morphological evolution is further affected by reaction duractions and concentrations of reactants, in 8, 43, 51 3) the NC formation is addition to the factors above; 41, 43 governed by both thermodynamics and kinetics; and 4) 48, H2S seems to be a critical intermediate of the S precursor. 52, 54 To date, hot-injection is the only technique among the three to have successfully synthesized FeS2 NC inks. In 2011, Puthussery et al. pioneered a method for the synthesis of highly-dispersed, spherical FeS2 NCs (~12 nm in diameter) 8 that were passivated by octadecylamine; their work subsequently motivated a series of studies on protocol 31-33, 35, 38, 39 modification. Some protocols succeeded in 31, 32, 38, 39, 42, 43 synthesizing non-spherical NCs. For instance, Gong et al. synthesized nanocubes and nanosheets by tuning 31 the injection temperature; Lucas et al. obtained 80-nm 38 nanocubes by adding hexadecanesulfonate as the ligand; and Bai et al. produced nanospheres, nanowires, and nanosheets by varying reaction durations and the molar 39 ratios of reactants.

Page 2 of 11

This hot-injection method, as shown in Table 1, is popular and effective. However, it requires stringent air-free conditions, which may increase the cost of materials synthesis and limit its scalability. In addition, while FeS2 NCs have been synthesized many times in air-compatible 4, 44-47, 49, 50 microwave and solvothermal reactors, the resulting NCs are heavily agglomerated and not suitable for making homogeneous inks. This motivated us to examine if and how one could succeed in synthesizing high-quality FeS2 NC inks using the air-compatible solvothermal technique. 8, 31-33, 35, 38, 39, 47-53, 55 After a literature review, we hypothesized that the solvothermal synthesis of FeS2 NC inks would require efficient dissolution of the precursors in the chosen solvents, good mass transport (such as mechanical stirring) during the reaction, and the aid of an appropriate stabilizing ligand. This article has verified this hypothesis by screening four solvents, testing several ligands, and studying both stirred and non-stirred reactions. The as-synthesized NCs are polydisperse FeS2 NCs. After a post-synthesis digestive 56-61 ripening treatment, monodisperse colloidal FeS2 nanospheres are obtained. Furthermore, density functional theory (DFT) computations provide quantum mechanical insights into the decomposition mechanism of the precursor at the molecular level for the first time.

Table 1. Reported wet-chemistry methods for the synthesis FeS2 NCs* o

Method

Ref.

Precursor

T/ C

Solvents

Ligands

Size/nm

Shape

Ink ?

Air?

Hot

7,

FeCl2 + 6S

220

ODA + DPE

ODA

12

spherical

Yes

No

Injection

31, 32

40 - 80

cubes

Yes

No

60 - 200

cubes

Yes

No

30 - 50

cubes

Yes

No

150,

cubes,

Yes

No

Microwave

8,

41

FeCl2 + 8S

120 - 220

ODA + DPE

ODA

33, 34

FeCl2 + 6S

220

OLA + TOPO

TOPO

35

FeCl2 + 6S

200 - 250

HDA + OLA

HDA OLA

36

FeCl2 + 6S

220

OLA

OLA

10

dendrites

37

FeCl2 + 12S

240

ODE

OA, OLA

12 - 18

spheres

Yes

No

38

FeCl3 + 6S

220

OLA

HDSA

80

cubes

Yes

No

39

FeCl2 4.5Na2S2O3

139

DMSO + H2O

TGA

2 - 5,

spheres,

80 - 120,

wires,

Yes

No

100 - 200

sheets

40

FeZ3

42

FeCl2•4H2O 7.5S

43

FeCl2•4H2O + 6S

+

+

+

200

H2O

CTAB

200

polyhedra

No

Yes

230

OLA

OLA

60

cube

Yes

No

220

OLA ODA) DPE

Yes

No

(or and

TOPO, Diol

10

rod

200

quasi-cube

32

Fe(CO)5 + S

120 – 240

OLA

OLA

200 - 500

nanoplates

Yes

No

44

FeCl3 + NaHS

N/A

H2O

N/A

≥ 100

spherical

No

Yes

4, 45

FeCl2 + 9S

190

H2O

PVP

2000 - 3000

cubes

No

Yes

46

FeSO4 + 2S

200

EG

PVP

2700

spheres

No

Yes


Page 3 of 11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Solvothermal

47

FeCl2 + S

200

H2O

Gelatin

≥ 500

polyhedra

No

Yes

48

FeE3

180

H2O

N/A

500

cubes

No

Yes

49

FeCl2 + 9S

180

EG

TX

250,

cubes

No

Yes

PVP

150

octahedra

50

FeSO4 + 4Na2S2O3

200

Ethanol

N/A

≥ 200

irregular

No

Yes

51

FeCl2 + 6S

180

OLA + Diol

OLA

80 - 120

cubes

No

Yes

52

Fe(acac)3 2Na2S2O3•H2O 5S

220

Ethanol

OLA

10 - 35

cubes

No

Yes

53

Fe2O3 + 20S

290

OA + OLA

CTAB

100

cubes

No

Yes

+ +

*ODA: octadecylamine; DPE: diphenyl ether; OLA: oleylamine; TOPO: trioctylphosphine oxide; HDA: hexadecylamine; ODE: 1-octadecence; OA: oleic acid; HDSA: 1-hexadecanesulfonic acid; DMSO: dimethyl sulfoxide; TGA: thioglycolic acid; FeZ3: iron diethyldithiophosphate; CTAB: cetyltrimethylammonium bromide; N/A: not available; Diol: 1,2-hexadecanediol; EG: ethylene glycol; PVP: polyvinylpyrrolidone; FeE3: iron diethyldithiocarbamate. procedures involving other solvents, ligands, ligand concentrations, and non-stirred reactions, please refer to the Supplemental Information (SI).

EXPERIMENTAL SECTION Chemicals All chemicals were used as received. Iron chloride ACS grade) and hexahydrate (FeCl36H2O, polyvinylpyrrolidone (PVP, avg MW 58,000) were purchased from Alfa Aesar. Sodium diethyldithiocarbamate trihydrate (denoted as NaE∙3H2O, E = -S2CN(CH2CH3)2, >99.0%), octadecylamine (ODA, 97.0%), dodecylamine (DDA, %), oleylamine (OLA, technical grade, 70%), trioctylphosphine oxide (TOPO, technical grade, 90%), dodecanethiol (DDT, >90%) and sulfur (99.998%) were purchased from Sigma Aldrich. Methanol (ACS grade, Pharmco), ethanol (ACS grade, Pharmco), dimethylformamide (DMF, 99.8%, Macron), chloroform (CHCl3, ChromAR grade, Macron), and carbon disulfide (CS2, 99.95%) were purchased from Fisher. Toluene (99.5%) and acetone (99.9%) were purchased from VWR. Octadecylxanthate (ODX) was synthesized by 8 following a literature method. The nanopure water (18.3 -1 MΩcm ) was from a Barnstead water purification system.

Preparation of the Iron Diethyldithiocarbamate (FeE3) Precursor The procedure is a modified version of a method reported in 48 the literature. First, two stock solutions were prepared by dissolving 1 mmol of FeCl36H2O in 40 mL of methanol and 3 mmol of NaE∙3H2O in 10 mL of methanol. Then the NaE solution was added drop-by-drop to the stirred solution of FeCl3, producing a black precipitate. The obtained product was filtered with a Buchner funnel and washed once with water and three times with methanol. Finally, the collected crystalline powder was dried in a desiccator at room temperature overnight. The typical yield of FeE3 was 400-500 mg.

Solvothermal Synthesis of Iron Pyrite (FeS2) NCs In a typical synthesis, 0.2 mmol (100 mg) of FeE3, 2 mmol (0.5 M) of ligand, 1.2 mmol (38 mg) of sulfur powder (FeE3:S ratio of 1:6, denoted as 6S), and 4 mL of DMF were added, along with a magnetic stir bar, to a Teflon-lined, stainless steel autoclave (Parr Instr., model 4749, 23 mL capacity). The o reactor was sealed and heated at 190 C for 3 – 22 h in a sand bath set on a stir plate (700 rpm). Finally, the reactor was allowed to cool to room temperature; the resulting product solution was a viscous, dark solution. For more detailed

The resulting product was separated and purified via typical precipitation protocols. First, the crystals and excess ligand were separated using a relative centrifugal force (RCF) of 1228 G (3100 rpm on a RCF-fixed Medilite centrifuge) for five minutes. The supernatant was decanted and the large pellet was dissolved in chloroform. Second, the NCs were precipitated out with acetone (5:1 acetone to CHCl3) and left undisturbed for 12 h; this sedimentation process was repeated three to four times. Third, carbon disulfide was used (5:1 CS2 to CHCl3) to wash away any unreacted sulfur. Finally, the purified NCs were obtained as a colloidal solution in chloroform, after dispersing the collected NCs in chloroform with sonication and further removing all aggregates through a gentle centrifugation at RCF = 9 G (200 rpm on Fisher Scientific Marathon 22KBR centrifuge). The critical steps in this protocol were the undisturbed sedimentation and the low-force centrifugation, as higher speed centrifugation steps aggregated the crystals irreversibly.

Digestive Ripening of Synthesized FeS2 NCs

the

Solvothermally-

The digestive ripening treatment is a widely used technique for producing monodisperse nanoparticles of metals and metal alloys through refluxing a solution of the premade 56-61 polydisperse particles with an excess amount of ligands. The typical procedure for inducing digestive ripening of FeS2 in this work was as follows. First, 50 mL of DMF solution containing 0.9 mmol (29 mg) of sulfur powder and 0.01 - 0.75 o M of DDA solution was brought to a reflux (~150 C) in air. Next, a DMF solution containing ~18 mg of FeS2 (as determined by the theoretical yield) from a typical DMF reaction system was injected into the refluxing system and further heated for 24 ± 1 h. After cooling naturally to room temperature, the products were extracted for further analysis. The production yield was determined to be 62%.

Characterization Powder X-ray diffraction (XRD) patterns were obtained using a Philips X’Pert X-ray diffractometer with Cu Kα radiation (λ = 0.154054 nm). The XRD samples were prepared by drop-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

casting the FeS2 inks onto glass slides to form thin films. Scanning electron microscopy (SEM) images were acquired in a field emission SEM (JEOL JSM-7000F FESEM). The SEM samples were prepared by drop-casting the FeS2 inks onto silicon wafers to form thin films. Transmission electron microscopy (TEM) images were obtained using a FEI Titan S/TEM operated at 300 kV (in the main text) and a Philips CM200 TEM (in the Supporting Information), equipped with a Princeton Gamma-Tech Prism energy dispersive X-ray spectrometer (EDX). The TEM samples were prepared by dipping ultrathin carbon-coated copper grids (Ted Pella 01824) in FeS2 reaction solutions and then drying under o vacuum (20 mTorr) in a tube furnace at 100 C for 15 minutes. Fourier transform infrared (FTIR) spectra in the far-IR range were measured using the attenuated total reflection (ATR) mode on powder samples with a Thermo Scientific Nicolet -1 iS50 spectrometer at a spectral resolution of 4 cm . UVvisible near infrared absorption (UV-Vis-NIR) spectra were collected on a single-beam spectrometer (Varian Cary 5G) at a spectral resolution of 1 nm. These samples were chloroform solutions in quartz cuvettes. Raman spectra were + collected using a WiTec confocal Raman spectrometer (Ar -1 laser, λ = 532 nm) at a spectral resolution of 5 cm . The Raman samples were prepared by pressing FeS2 NCs (5 wt%) with KBr powders (95 wt%), in order to avoid laser burning. The sizes and size distributions of the synthesized NCs in the TEM images were analyzed using ImageJ (National Institute 62 of Health, version 1.45s). The NC diameters were calculated from their two-dimensional projections. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) was conducted on an ICP-AES spectrometer (Perkin Elmer 5300 DV), by measuring dilution factor controlled aqueous solutions of digested FeS2 NCs. The digestion was performed by adding concentrated HNO3 to the FeS2 sample.

Page 4 of 11

magnification TEM image in Figure 1B shows large spherical crystals of 350 ± 110 nm in diameter, while the highresolution TEM (HRTEM) image in Figure 1C reveals that the large crystals are actually aggregates of smaller (~ 10 nm) crystals. The selected area electron diffraction pattern with ring features (the inset in Figure 1B) matches the lattice spacings for FeS2 (JCPDS 01-071-0053) and reveals that the particle examined is polycrystalline. The XRD spectra in Figure 1D indicate that the products from both water and DMF are phase-pure face-centered cubic (fcc) FeS2. The XRD peaks obtained for the DMF product are broader due to 8 smaller crystal sizes, as expected. However, while the crystal size is smaller, the DMF mediated synthesis of FeS2 did not yield highly dispersed NCs. In the following sections, we will demonstrate two other factors that influence crystal dispersity.

Computations The charge density was calculated using Amsterdam density 63, 64 functional release 2013.01. The reported results were 65 computed using a TZ2P basis set and local density 66 approximation (LDA) exchange/correlation potential. Results were also confirmed using the generalized gradient approximation exchange and correlation functional 67 developed by Perdew, Burke, and Ernzerhof (PBE). The topology of the charge density was analyzed using Quantum 68 Theory of Atoms in Molecules (QTAIM).

RESULTS AND DISCUSSION

Figure 1. Characterization of the products obtained using water (A and D) and DMF (B-D) as the solvent. (A) SEM image; (B) TEM image and the selected area electron diffraction pattern of the crystals (inset); (C) HRTEM image of one crystal in (B), with arrows indicating grain boundaries; and (D) XRD profiles of the products from water (red) and DMF (blue), with stick patterns denoting the FeS2 reference (JCPDS 01-071-0053).

Solvothermal Synthesis of FeS2 NCs The effect of precursor solubility: Following the existing protocol for synthesizing FeS2 using FeE3 through the 48 hydrothermal method, a water suspension of the FeE3 o precursor was heated in an autoclave reactor at 180 C for 18 h, resulting in a mixture of cubes (200 – 1000 nm) and irregular shapes (Figure 1A). These results are comparable 48 with the previously reported results. To test our hypothesis that efficient dissolution of FeE3 is necessary to make smaller, well-dispersed NCs, we examined three organic solvents – ODA, toluene, and DMF (SI-Figures 1-3). It was found that excess sulfur was necessary in all organic solvents to form phase-pure FeS2 and avoid impurities (SI-Figures 13). The best solvent for NC synthesis was DMF (at 190 °C and for 22 h); the results are shown in Figures 1B-D. The low

The effects of mass transport and ligand concentration: Two additional factors were hypothesized to be critical for producing highly dispersed NC inks: efficient mass transport during the reaction and ligand concentration. Efficient mass transport is necessary to promote a homogenous distribution of precursors and thus facilitate simultaneous nucleation of particles across the solution followed by growth at nearly the 69 same rate. Since aggregation of NCs is driven by the need to minimize the total surface energy, adsorption of ligands on NC surfaces can lower their total free energies and hence stabilize their sizes. Therefore, ligands are important to 69 prevent aggregation. Ligands and efficient mass transport (such as mechanical stirring) have been widely used in the 8, 31-33, 35, 38, 39 hot-injection method. However, in all the


Page 5 of 11

Chemistry of Materials 47-50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

literature on solvothermal synthesis of FeS2, the autoclave vessels were not stirred, due to the limitations of the heating style in either an oven or a muffle furnace. To overcome this limitation, we replaced the muffle furnace with a sand bath, which allowed for the reaction system to be stirred. ODA was used as the ligand, since it as a solvent produced phase-pure FeS2 and was widely used in the hot 8, 31, 32 injection technique for producing FeS2 NC inks. The optimal conditions were found to be 0.5 M ODA and stirring for 22 h, resulting in FeS2 NC inks (8 ± 2 nm) (Figure 2A) with noticeable spacing between the crystals (inset). When the reaction duration was decreased to 3 h, although highly dispersed NCs (6 ± 1.2 nm) were observed (Figure 2B), many small aggregates were also evident (inset). The better dispersion resulting from a longer reaction duration could be due to improved ODA passivation of the NCs during 8, 31 growth. If the reaction system was not stirred (Figure 2C), a mixture of colloidal NCs (5.4 ± 1 nm, indicated by arrows), larger crystals (11 ± 9 nm), and aggregates (276 ± 83 nm, not shown here) were obtained. If the ODA concentration was further reduced to 0.1 M (Figure 2D), the obtained crystals were similar to those synthesized without ODA (Figure 1B), although the aggregates are significantly smaller (120 ± 30 nm in diameter). Stirring the reaction mixture containing 0.1 M ODA resulted in a mixture of dispersed crystals (6.4 ± 1 nm) and aggregates (SI-Figure 4). For these three experiments (Figure 2B-D), all other parameters (temperature, reaction time, and precursor concentration) are the same. Thus, Figures 2C and 2D indicate that ligand concentration and stirring are both important for producing FeS2 NC inks. To understand the effect of ligand identity, other ligands were also studied, including ODX, DDA, OLA, and TOPO; none were found to produce well-dispersed NCs and some failed entirely to form phase-pure FeS2 (SI-Figure 5). The results in Figures 1 and 2 together verify our hypothesis that efficient mass transport and surface passivation, which are missing in hydrothermal methods, are necessary for the synthesis of FeS2 NC inks.

NC dispersion. TEM images of the NCs obtained from the o DMF system at 190 C with 0.5 M ODA and stirring for 22 h(A), 0.5 M ODA and stirring for 3 h (B), 0.5 M ODA without stirring for 3 h (C), and 0.1 M ODA without stirring for 3 h (D). The insets in (A) and (B) are higher magnification images of a few individual NCs. The arrows in (C) indicate small (5.4 ± 1 nm) NCs.

Comprehensive Characterization Next, the obtained FeS2 NC inks were systematically characterized. The TEM image in Figure 3A was obtained at a higher magnification than that in Figure 2D allowing us to visualize the crystal shapes more clearly and to measure the crystal sizes more accurately. Despite the variation in shapes (from sphere-like to rod-like), all the particles are of similar sizes (7.4 ± 1.4 nm), as shown by the histogram of size distribution in Figure 3B. The XRD profile in Figure 3C 38 confirms the phase purity of face-centered cubic (fcc) FeS2. The representative HRTEM image in Figure 3D shows highly resolved lattices of a single-crystalline FeS2 crystal. Lattice modeling with the Crystal Maker® software indicated that the image was taken along the [111] zone axis from the rows of iron atoms, with lattice spacings of 0.42 nm and 0.24 nm 70 from the (011) and (211) planes, respectively. The sulfur 2atoms (S2 pairs) were not observed, likely due to the limited resolution of the TEM used and their low contrast on a carbon film, resulting from sulfur’s low atomic number 71, 72 compared with that of iron.

Figure 2. The effects of mass transport and ligands on the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 11

Table 2. Elemental analysis from the EDX spectrum in Figure 4A when different elements are considered. Element

Carbon

Oxygen

Iron

Sulfur

32.38

67.62

56.65

14.03

29.31

1.61

0.77

1.29

Atomic % Atomic % Atomic %

96.33

The Raman spectra of the FeS2 NCs (red) and mineral (blue) 74, 75 in Figure 4C is consistent with literature results. Such shifts in nanomaterials versus the bulk counterparts have 73, 76, 77 -1 been widely reported. The peaks centered at 344 cm , -1 -1 385 cm , and 424 cm are assigned to the eg (S-S libration), ag (S-S stretching) and tg (coupled libration and stretching) 74, 75 modes, respectively. These spectra are substantially different from other Fe-S compounds, such as marcasite, which has the same chemical formula as pyrite but a different crystal structure (orthorhombic vs. cubic for 78-80 pyrite).

Figure 3. Product synthesized using 0.5 M ODA/DMF and o stirring for 22 h at 190 C. (A) TEM image and (B) histogram showing the size distribution of 100 NCs. (C) XRD spectrum. (D) HRTEM image of an individual FeS2 nanocrystal, showing plane spacing that matches the (011) and (211) spacings of FeS2. Figure 4 shows the characterization results from EDX, FarIR, Raman, and UV-Vis-NIR. The molar ratio of S/Fe extracted from the EDX spectrum (Figure 4A and Table 2) is 2.08, reasonably consistent with the expected 73 stoichiometry. The C signal can be assigned to the carbon film on the grid as well as the ligands. The O and Cu signals are ascribed to the surface-oxidized copper grid. The Far-IR spectrum (Figure 4B) of the FeS2 NC ink shows three -1 -1 characteristic absorption peaks at 404 cm , 344 cm , and 286 -1 8 cm , consistent with the values reported in the literature. As expected, the comparison spectrum of FeS2 mineral also shows three peaks around those positions. However, another -1 peak at 438 cm was additionally observed for the mineral. This fourth peak is likely due to the larger crystal size, as 8 observed for the annealed FeS2 NCs in the literature. Surprisingly, all absorption peaks in the mineral sample were abnormally negative. In addition, TOPO-passivated NCs and ligand-free NCs also showed positive absorption peaks and negative peaks, respectively (SI-Figure 6). Hence, it was inferred that the unusually negative absorption peaks are likely a consequence of changes in surface properties. Further elucidation was beyond the scope of this article and is currently under investigation.

The UV-Vis-NIR spectrum of the FeS2 NC inks (Figure 4D), showing the absorption edge at ~1430 nm and a shoulder 8, 35 peak at ~520 nm, also agrees with the reported values. The optical image in the right side of the inset demonstrates the solubility of the FeS2 NC inks. The left side optical image shows a drop-cast thin film using the obtained FeS2 NC inks, which can be useful for applications in photovoltaics and lithium ion batteries.

Figure 4. Characterizations of FeS2 NC inks synthesized o using 0.5 M ODA/DMF and stirring for 22 h at 190 C. (A) EDX spectrum of FeS2 NC inks, (B) Far-IR spectra of FeS2 NC inks (red) and FeS2 mineral (blue). (C) Raman spectra of FeS2 NC inks (red) and FeS2 mineral (blue) with green, cyan, and brown lines corresponding to fitted peaks. (D) UV-VisNIR spectrum of FeS2 NC inks, with insets showing optical images of the ink solution (right) and a drop-cast thin film on a glass substrate (left).


Page 7 of 11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Digestive Ripening of Synthesized FeS2 NCs

Chemistry of Materials the

Solvothermally-

While FeS2 NC inks were successfully synthesized by verifying the hypothesis, our ultimate goal of producing monodisperse (in both size and shape) colloidal FeS2 NCs was not yet achieved. Since the dispersion was moderately improved by extending the reaction duration from 3 h to 22 h, we conducted a post-synthesis digestive ripening to pursue this goal. Digestive ripening is a process to generate monodisperse NCs from a polydisperse material in the 56-61 The presence of ligands when refluxing the solvent. mechanism involves the proliferation of small NCs at the 58 cost of dissolving the larger ones, as opposed to conventional Ostwald ripening, which promotes the growth 58 of large NCs at the expense of smaller ones. In an actual 81 synthesis, these two processes are coupled. The digestive 57-59, 61 ripening treatment has been previously used for metals, 56 82 alloys, and metal sulfides, and was adapted in this work for FeS2. The initial digestive ripening experiments were conducted by using purified FeS2 NCs as the starting material, toluene as the solvent, and four different chemicals (ODA, OLA, DDA, and DDT) as the ligand (SI-Figure 7). Only the DDA system (SI-Figure 7C) produced a small number of individual particles, in addition to a large quantity of aggregates as in all other three systems. We ascribed this to the instability of the purified FeS2 NCs in toluene and the lack of excess sulfur. Thus, subsequently, we changed the starting material to the unpurified FeS2 solution directly out of the solvothermal reaction, added six portions of sulfur versus FeS2, and used DMF (the original solvent during the solvothermal reaction) instead of toluene as the solvent, but only for ODA (the original ligand) and DDA systems. The use of ODA resulted in a poorer morphology (SI-Figure 8A). In contrast, the DDA system produced monodisperse colloidal FeS2 NCs (Figure 5), whereas the product without using excess sulfur was significantly inferior (SI-Figure 8B). The monodispersity of the successful sample is demonstrated by a self-assembled monolayer of FeS2 NCs on the TEM grid (Figure 5A) and a 5% standard deviation in the corresponding size histogram (5.5 ± 0.3 nm, Figure 5B). Additional TEM images of larger and different areas are shown in SI-Figure 9. As in Figures 3C and 3D, the XRD data (Figure 5C) and HRTEM image (Figure 5D) also confirm the phase purity and single-crystallinity of the product.

Figure 5. Characterization of the digestive ripening product from polydisperse FeS2 NCs in a refluxing DMF solution with 0.75 M DDA for 23 h. (A) TEM image and (B) histogram showing the size distribution of 100 NCs. (C) XRD spectrum. (D) HRTEM image of an individual FeS2 nanocrystal, showing lattice fringes that match the (011) and (211) spacings of FeS2.

A Computational Understanding of the Reaction Mechanism 48

While FeE3 has been used before to synthesize FeS2, the decomposition mechanism of FeE3 molecules, which is the first and the most critical step for FeS2 formation, has not been well understood. The structure of the FeE3 molecule, as shown in Figure 6A, is featured with an octahedral 3+ coordination of Fe surrounded with six sulfur atoms from 48 three E groups, due to the delocalized -CS2 moieties. In 2+ contrast, in the fcc-structured FeS2, each Fe ion is coordinated with six sulfur atoms and each sulfur atom (in 22+ the form of S2 pairs) is coordinated with three Fe ions and 71, 72 one sulfur atom. Thus, two key questions arise. First, 2how do some of the Fe-S bonds break? Second, how do S2 pairs form? The Quantum Theory of Atoms in Molecules 83 (QTAIM), developed by Bader and coworkers, and 84 extensions of the theory by Knoerr and Eberhart, were utilized to address these questions. QTAIM is a topological theory that exploits the extrema of the charge density, referred to as critical points or CPs (maxima, minima, and saddle points), to analyze bonding and provide an estimate



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

of structural stability. Ring and bond CPs, both examples of saddle points, are indicated in Figure 6 by green and red spheres, respectively. Our computations suggest an acid-mediated mechanism. Protonation can occur either on the carbon atom or one of the complexed sulfur atoms in the dithiocarbamate moiety (– S2CNEt2). As illustrated in Figure 6, we propose one possible scenario. First, carbon protonation (Figure 6B) weakens both C-S bonds, as shown by the decrease in charge density (SI-Table 1 and SI-Figure 11). Second, a nucleophile (HO or HS ) attacks the previously protonated carbamate carbon (the electrophilic center, Figure 6C), causing cleavage of the C-S’ bond. Next, protonation at a sulfur site (S”) results in degradation of the Fe-S” bond, liberating the ligand and producing an FeE2S’ intermediate state (Figure 6D). Followed by dimerization, as shown in Figure 6E, two liberated sulfur atoms are bridged between monomers to 22form an S-S bond, i.e., the oxidation of S to S2 . It is believed that the protonation, nucleophilic attack, and dimerization would occur at other ligand sites and lead to the ultimate formation of FeS2 seeds and NCs.

Page 8 of 11

A combination of these two acid catalyzed steps (via protonation of carbon and sulfur in sequence) appears to be responsible for the decomposition of the ligands on the complex. The protonation steps could happen simultaneously or in reverse order. The proton can be from water in the case of a hydrothermal reaction or from H2S that is produced in situ through the reaction between sulfur and 54 ODA. While a similar mechanism has been proposed for 48 the hydrothermal synthesis, our computation provides, for the first time, quantum mechanically based insight into the reaction mechanism for both hydrothermal and solvothermal reactions. The suggested mechanism may also shed light on the reactions of other metal sulfides from metal 48, 85, 86 diethyldithiocarbamates (MEx).

CONCLUSIONS This article reports the synthesis of highly dispersed, colloidal, single-crystalline FeS2 nanocrystals (FeS2 NC inks), which has been a challenge for the hydrothermal technique. Many factors were found to play critical roles, including solvent, precursors, ligand, temperature, time, and stirring. The success was a result of verifying our hypothesis that solvothermal processes can makeFeS2 NC inks by using three experimental conditions: high solubility of the precursor, enough stabilizing ligands, and efficient mass transport. The findings here can be applied for synthesizing other types of NC inks. The addition of ligands and stirring decreased the NC size and led to a narrow size distribution. FeS2 NCs directly out of the autoclave reactor were polydisperse, irregular crystals of 7.4 ± 1.4 nm in diameter. A subsequent digestive-ripening treatment converted them to monodisperse, spherical (5.5 ± 0.3 nm) NCs. These materials can find important applications in lithium ion batteries and photovoltaics, and are suitable for large-scale manufacturing processes. Furthermore, density functional theory computations indicated that the initial and critical step during the solvothermal synthesis was an acid mediated degradation of the precursor, iron diethyldithiocarbamate, gaining quantum mechanical insight into the decomposition mechanism of iron diethyldithiocarbamate at the molecular level for the first time.

ASSOCIATED CONTENT Supporting Information Available

Figure 6. The reaction mechanism for the first step of FeS2 formation, proposed by the DFT computation. (A) The left side is the structure of the FeE3 precursor; the right side is a ball-and-stick rendering of the highlighted region shown on the left. The red spheres correspond to bond CPs and the green sphere represents the ring CP for Fe-S-C-S-Fe. (B) Protonation of the central carbon. (C) Nucleophilic attack of the central carbon by HO or HS . (D) Protonation of one of the sulfur atoms in one E moiety. (E) Dimerization through bridging the free sulfurs (circled in red).

Additional experimental details, TEM images, XRD profiles, Far-IR spectra of the as-synthesized and digestively-ripened FeS2 under various conditions, and additional computational details and results. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Address: Department of Chemistry and Geochemistry, th Colorado School of Mines, 1012 14 Street, Golden, CO, 80401, U.S. Fax:1-303-273-3629; Tel: 1-303-384-2389; E-mail: [email protected] (Researcher ID: C-2688-2011).

Author Contributions


Page 9 of 11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The manuscript was written through contributions of all authors.

Funding Sources This work is supported by the ACS-PRF grant (#52877DNI10), the Startup Fund for YY from Colorado School of Mines, and Seed Grants from the Renewable Energy Materials Research Science and Engineering Center under Grant No. DMR0820518.

ACKNOWLEDGMENTS SK acknowledges NSF support (CMMI-1200547); ME and JM acknowledge support from the Office of Naval Research (N00014-14-1-0739); RR and GJL acknowledge support from NSF Chemical Catalysis (1214068) and the National Renewable Energy Laboratory (UGA-0-41025-05).

Notes The authors declare no competing financial interest.

REFERENCES (1) Shao-Horn, Y.; Horn, Q. C., Electrochim. Acta 2001, 46, 2613-2621. (2) Fong, R.; Dahn, J. R.; Jones, C. H. W., J. Electrochem. Soc. 1989, 136, 3206-3210. (3) Shao-Horn, Y.; Osmialowski, S.; Horn, Q. C., J. Electrochem. Soc. 2002, 149, A1499-A1502. (4) Yersak, T. A.; Macpherson, H. A.; Kim, S. C.; Le, V. D.; Kang, C. S.; Son, S. B.; Kim, Y. H.; Trevey, J. E.; Oh, K. H.; Stoldt, C.; Lee, S. H., Adv. Energy Mater. 2013, 3, 120-127. (5) Altermatt, P. P.; Kiesewetter, T.; Ellmer, K.; Tributsch, H., Sol. Energy Mater. Sol. Cells 2002, 71, 181-195. (6) Richardson, B. J.; Zhu, L. Z.; Yu, Q. M., Sol. Energy Mater. Sol. Cells 2013, 116, 252-261. (7) Steinhagen, C.; Harvey, T. B.; Stolle, C. J.; Harris, J.; Korgel, B. A., J. Phys. Chem. Lett. 2012, 3, 2352-2356. (8) Puthussery, J.; Seefeld, S.; Berry, N.; Gibbs, M.; Law, M., J. Am. Chem. Soc. 2011, 133, 716–719. (9) Pemsler, J. P.; Litchfield, J. K., Pyrite cathode material for a thermal battery. Patent Application Department of the Navy, Washington, DC.: 1991. (10) Reisner, D. E.; Xiao, T. D.; Ye, H.; Dai, J.; Guidotti, R. A.; Reinhardt, F. W., J. New Mater. Electrochem. Syst. 1999, 2, 279282. (11) Guidotti, R. A.; Reinhardt, F. W.; Dai, J.; Reisner, D. E., J. Power Sources 2006, 160, 1456-1464. (12) Henriksen, G. L.; Vissers, D. R., J. Power Sources 1994, 51, 115128. (13) Guidotti, R. A.; Reinhardt, F. W.; Dai, J. X.; Roth, J.; Reisner, D. E., J. New Mater. Electrochem. Syst. 2002, 5, 273-279. (14) Takeuchi, T.; Kageyama, H.; Nakanishi, K.; Inada, Y.; Katayama, M.; Ohta, T.; Senoh, H.; Sakaebe, H.; Sakai, T.; Tatsumi, K.; Kobayashi, H., J. Electrochem. Soc. 2012, 159, A75-A84. (15) Montoro, L. A.; Rosolen, J. M.; Shin, J. H.; Passerini, S., Electrochim. Acta 2004, 49, 3419-3427. (16) Montoro, L. A.; Rosolen, J. M., Solid State Ionics 2003, 159, 233240. (17) Golodnitsky, D.; Peled, E., Electrochim. Acta 1999, 45, 335-350. (18) Shao-Horn, Y.; Osmialowski, S.; Horn, Q. C., J. Electrochem. Soc. 2002, 149, A1547-A1555. (19) Li, L.; Caban-Acevedo, M.; Girard, S. N.; Jin, S., Nanoscale 2014, 6, 2112-2118. (20) Ennaoui, A.; Fiechter, S.; Pettenkofer, C.; Alonsovante, N.; Buker, K.; Bronold, M.; Hopfner, C.; Tributsch, H., Sol. Energy Mater. Sol. Cells 1993, 29, 289-370. (21) Wadia, C.; Alivisatos, A. P.; Kammen, D. M., Environ. Sci. Technol. 2009, 43, 2072-2077. (22) Lide, D. R., CRC Handbook of Chemistry and Physics. 88th Edition ed.; CRC Press: Taylor & Francis Group, NY: 2007-2008. (23) Jaegermann, W.; Tributsch, H., J. Appl. Electrochem. 1983, 13, 743-750. (24) Wu, H.; Yu, G.; Pan, L.; Liu, N.; McDowell, M. T.; Bao, Z.; Cui, Y., Nat. Commun. 2013, 4, Article No. 1943. (25) Cui, G. L.; Gu, L.; Zhi, L. J.; Kaskhedikar, N.; van Aken, P. A.; Mullen, K.; Maier, J., Adv. Mater. 2008, 20, 3079-3083.

(26) McDowell, M. T.; Lee, S. W.; Ryu, I.; Wu, H.; Nix, W. D.; Choi, J. W.; Cui, Y., Nano Lett. 2011, 11, 4018-4025. (27) Akhavan, V. A.; Goodfellow, B. W.; Panthani, M. G.; Reid, D. K.; Hellebusch, D. J.; Adachi, T.; Korgel, B. A., Energy Environ. Sci. 2010, 3, 1600-1606. (28) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P., Science 2005, 310, 462-465. (29) Ogawa, S., J. Appl. Phys. 1979, 50, 2308-2311. (30) Sato, K., Prog. Cryst. Growth Charact. Mater. 1985, 11, 109-154. (31) Gong, M. G.; Kirkeminde, A.; Ren, S. Q., Sci. Rep. 2013, 3, Article No. 2092. (32) Kirkeminde, A.; Ruzicka, B. A.; Wang, R.; Puna, S.; Zhao, H.; Ren, S. Q., ACS Appl. Mater. Interfaces 2012, 4, 1174-1177. (33) Bi, Y.; Yuan, Y. B.; Exstrom, C. L.; Darveau, S. A.; Huang, J. S., Nano Lett. 2011, 11, 4953-4957. (34) Mangham, S. C.; Khan, M. A.; Benamara, M.; Manasreh, M. O., Mater. Lett. 2013, 97, 144-147. (35) Macpherson, H. A.; Stoldt, C. R., ACS Nano 2012, 6, 8940-8949. (36) Li, W.; Doblinger, M.; Vaneski, A.; Rogach, A. L.; Jackel, F.; Feldmann, J., J. Mater. Chem. 2011, 21, 17946-17952. (37) Wang, D. Y.; Jiang, Y. T.; Lin, C. C.; Li, S. S.; Wang, Y. T.; Chen, C. C.; Chen, C. W., Adv. Mater. 2012, 24, 3415-3420. (38) Lucas, J. M.; Tuan, C. C.; Lounis, S. D.; Britt, D. K.; Qiao, R. M.; Yang, W.; Lanzara, A.; Alivisatos, A. P., Chem. Mater. 2013, 25, 1615-1620. (39) Bai, Y. X.; Yeom, J.; Yang, M.; Cha, S. H.; Sun, K.; Kotov, N. A., J. Phys. Chem. C 2013, 117, 2567-2573. (40) Wadia, C.; Wu, Y.; Gul, S.; Volkman, S. K.; Guo, J. H.; Alivisatos, A. P., Chem. Mater. 2009, 21, 2568-2570. (41) Kirkeminde, A.; Ren, S. Q., J. Mater. Chem. A 2013, 1, 49-54. (42) Alam Khan, M.; Sarker, J. C.; Lee, S.; Mangham, S. C.; Manasreh, M. O., Mater. Chem. Phys. 2014, 148, 1022–1028. (43) Zhu, L.; Richardson, B. J.; Yu, Q., Nanoscale 2014, 6, 1029-1037. (44) Kim, E. J.; Batchelor, B., Mater. Res. Bull. 2009, 44, 1553-1558. (45) Wang, D. W.; Wang, Q. H.; Wang, T. M., CrystEngComm 2011, 12, 755-761. (46) Li, M. L.; Yao, Q. Z.; Zhou, G. T.; Qu, X. F.; Mu, C. F.; Fu, S. Q., CrystEngComm 2011, 13, 5936-5942. (47) Golsheikh, A. M.; Huang, N. M.; Lim, H. N.; Chia, C. H.; Harrison, I.; Muhamad, M. R., Chem. Eng. J. 2013, 218, 276-284. (48) Chen, X. Y.; Wang, Z. H.; Wang, X.; Wan, J. X.; Liu, J. W.; Qian, Y. T., Inorg. Chem. 2005, 44, 951-954. (49) Hsiao, S. C.; Hsu, C. M.; Chen, S. Y.; Perng, Y. H.; Chueh, Y. L.; Chen, L. J.; Chou, L. H., Mater. Lett. 2012, 75, 152-154. (50) E'Jazi, N.; Aghaziarati, M., Adv. Powder Technol. 2012, 23, 352357. (51) Liu, W. L.; Rui, X. H.; Tan, H. T.; Xu, C.; Yan, Q. Y.; Hng, H. H., RSC Adv. 2014, 4, 48770-48776. (52) Xia, J.; Jiao, J. Q.; Dai, B. L.; Qiu, W. D.; He, S. X.; Qiu, W. T.; Shen, P. K.; Chen, L. P., RSC Adv. 2013, 3, 6132-6140. (53) Yuan, B. X.; Luan, W. L.; Tu, S. T., Dalton Trans. 2012, 41, 772776. (54) Thomson, J. W.; Nagashima, K.; Macdonald, P. M.; Ozin, G. A., J. Am. Chem. Soc. 2011, 133, 5036-5041.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 11

(55) Pan, D. C.; Zhao, N. N.; Wang, Q.; Jiang, S. C.; Ji, X. L.; An, L. J., Adv. Mater. 2005, 17, 1991-1995. (56) Kalidindi, S. B.; Jagirdar, B. R., Inorg. Chem. 2009, 48, 4524-4529. (57) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J., Langmuir 2002, 18, 7515-7520. (58) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J., Chem. Mater. 2003, 15, 935-942. (59) Lin, X. M.; Sorensen, C. M.; Klabunde, K. J., J. Nanopart. Res. 2000, 2, 157-164. (60) Smetana, A. B.; Klabunde, K. J.; Sorensen, C. M.; Ponce, A. A.; Mwale, B., J. Phys. Chem. B 2006, 110, 2155-2158. (61) Smetana, A. B.; Klabunde, K. J.; Sorensen, C. M., J. Colloid Interface Sci. 2005, 284, 521-526. (62) Cloud, J. E.; McCann, K.; Perera, P. A. K.; Yang, Y., Small 2013, 9, 2532-2536. (63) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.; Van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T., J. Comput. Chem. 2001, 22, 931-967. (64) Guerra, C. F.; Snijders, J. G.; te Velde, G.; Baerends, E. J., Theor. Chem. Acc. 1998, 99, 391-403. (65) Van Lenthe, E.; Baerends, E. J., J. Comput. Chem. 2003, 24, 11421156. (66) Vosko, S. H.; Wilk, L.; Nusair, M., Can. J. Phys. 1980, 58, 12001211. (67) Perdew, J. P.; Burke, K.; Ernzerhof, M., Phys. Rev. Lett. 1996, 77, 3865-3868. (68) Bader, R. F. W., Atoms in Molecules: A Quantum Theory. Oxford University Press: 1994. (69) Yang, Y. A.; Wu, H. M.; Williams, K. R.; Cao, Y. C., Angew. Chem. Intl. Ed. 2005, 44, 6712-6715. (70) Edington, J. W., Practical electron microscopy in materials science. 1976. (71) Eyert, V.; Hock, K. H.; Fiechter, S.; Tributsch, H., Phys. Rev. B 1998, 57, 6350-6359. (72) Murphy, R.; Strongin, D. R., Surf. Sci. Rep. 2009, 64, 1-45. (73) Berry, N.; Cheng, M.; Perkins, C. L.; Limpinsel, M.; Hemminger, J. C.; Law, M., Adv. Energy Mater. 2012, 2, 1124-1135. (74) Kleppe, A. K.; Jephcoat, A. P., Mineral. Mag. 2004, 68, 433-441. (75) Vogt, H.; Chattopadhyay, T.; Stolz, H. J., J. Phys. Chem. Solids 1983, 44, 869-873. (76) Yang, Y. A.; Cao, Y. W.; Loo, B. H.; Yao, J. N., J. Phys. Chem. B 1998, 102, 9392-9396. (77) Ahlawat, A.; Sathe, V. G., J. Raman Spectrosc. 2011, 42, 10871094. (78) Sourisseau, C.; Cavagnat, R.; Fouassier, M., J. Phys. Chem. Solids 1991, 52, 537-544. (79) Mernagh, T. P.; Trudu, A. G., Chem. Geol. 1993, 103, 113-127. (80) Anthony, J. W.; Bideaux, R. A.; Bladh, K. W.; Nichols, M. C., Handbook of Mineralogy, Mineral Data Publishing. Mineralogical Society of America: Tucson Arizona, USA, 1990. (81) Xin, H. L. L.; Zheng, H. M., Nano Lett. 2012, 12, 1470-1474. (82) Heroux, D.; Ponce, A.; Cingarapu, S.; Klabunde, K. J., Adv. Funct. Mater. 2007, 17, 3562-3568. (83) Bader, R. F. W., Atoms in Molecules: A Quantum Theory. Oxford University Press: 1994. (84) Knoerr, E. H.; Eberhart, M. E., J. Phys. Chem. A 2001, 105, 880884. (85) Xie, G.; Qiao, Z.-P.; Zeng, M.-H.; Chen, X.-M.; Gao, S.-L., Cryst. Growth Des. 2004, 4, 513-516. (86) Barrelet, C. J.; Wu, Y.; Bell, D. C.; Lieber, C. M., J. Am. Chem. Soc. 2003, 125, 11498-11499.


Page 11 of 11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials TOC Figure


Iron Pyrite Nanocrystal Inks: Solvothermal ... - ACS Publications

Recommend Documents