Tuning Electrical Conductivity of Inorganic Minerals with Carbon

Nov 6, 2015 - Conductive powders based on Barite or calcium carbonate with chemically converted graphene (CCG) were successfully synthesized by adsorp...
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Tuning Electrical Conductivity of Inorganic Minerals with Carbon Nanomaterials Anton A. Kovalchuk† and James M. Tour*,†,‡,§ †

Department of Chemistry, ‡Department of Materials Science and NanoEngineering, §Smalley Institute for Nanoscale Science and Technology, Rice University, 6100 Main Street, Houston, Texas 77005, United States S Supporting Information *

ABSTRACT: Conductive powders based on Barite or calcium carbonate with chemically converted graphene (CCG) were successfully synthesized by adsorption of graphene oxide (GO) or graphene oxide nanoribbons (GONRs) onto the mineral surfaces and subsequent chemical reduction with hydrazine. The efficient adsorption of GO or GONRs on the surface of Barite and calcium carbonate-based mineral particles results in graphene-wrapped hybrid materials that demonstrate a concentration dependent electrical conductivity that increases with the GO or GONR loading. KEYWORDS: conductive powders, Barite, graphene, inorganic hybrids, drilling fluids, graphene oxide, graphene nanoribbons

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INTRODUCTION The synthesis of materials with tunable physical properties (e.g., electrical, magnetic, or thermal) is of importance for various industries. For example, transforming common inorganic, nonconductive minerals such as carbonates, sulfates, hydroxides, or aluminosilicates into conductive materials could result in hybrid structures that are useful as fillers for otherwise insulating polymer composites. The conductive-coated minerals could also be used as components for oil-based drilling fluids in the oil and gas industry, providing the capability of logging while drilling. Intuitively, electrical conductivity can be conferred to nonconducting particles by creating conductive networks on their surfaces. But, one must achieve a significant deposition of conductive layers on the mineral surface while maintaining sufficient interaction strength. Chemically converted graphene (CCG)1−3 is considered a good conductive coating material due to its high conductivity (for a nonmetal containing material) and the possibility of using solution-based chemistry for its deposition on mineral particles. Highly watersoluble graphene oxide (GO)4−9 is widely used as a precursor for CCG. It is a product of graphite oxidation and contains large amounts of oxygenated functional groups (e.g., carboxyl, carbonyl, epoxy and hydroxyl) that can be reduced into CCG by a variety of chemical reducing agents or thermal treatment. GO can be obtained on a large scale by the chemical oxidation of natural graphite, making it a feasible material for industrial applications.10 It is also known that GO has excellent sorption of a wide variety of metal ions11−13 that can be used in water purification. Because of this feature, we predicted that GO nanosheets would be adsorbed on the surface of various mineral particles by the interaction of GO with the surfacemetal cations, resulting in the formation of GO-coated structures. The GO shell could then be chemically reduced © XXXX American Chemical Society

into conductive CCG while atop the mineral particles. Another related material that we studied was graphene oxide nanoribbons (GONRs) that can likewise be reduced to graphene nanoribbons.14,15 Compared to GO sheets, GONRs have a much higher aspect ratio and they might form a more efficient conductive network on the surface of mineral particles at lower loadings. There are previous publications reporting the synthesis of inorganic/CCG hybrids that were primarily used for energy storage as electrode materials for Li-ion batteries (LIBs).16−19 In these studies, the reduced GO served as a conductive layer and structural support improving stability of the active material under cycling. In a recent report20 we employed reduced GONRs as a platform for growing tin oxide, thus making a SnO2/CCG hybrid to achieve excellent electrochemical performance of LIBs. However, the conductivity of this class of materials has not been the focus of the previous research. The focus of the present work is the electrical conductivity of inorganic/CCG materials and their syntheses. Here we demonstrate that both GO and GONRs can be successfully immobilized on the surface of Barite and calcium carbonate particles (CaCO3) by adsorption from aqueous solutions resulting in coated conductive mineral structures. Subsequent chemical reduction of GO or GONR in the liquid phase leads to the formation of electrically conductive Barite/ CCG and CaCO3/CCG hybrids. For this study, we focused on low concentrations of CCG (0.25−2.0 wt %), in consideration of the cost-constraints of the oil and gas industry. The morphology and electrical conductivity of these CCG-coated Received: July 29, 2015 Accepted: November 6, 2015

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DOI: 10.1021/acsami.5b06941 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Process for the formation of conductive hybrid particles from inorganic minerals.

Figure 2. Photographs of GO and CaCO3 during the adsorption process. (a) Original aqueous GO solution (1 mg/mL). (b) CaCO3 dispersed in 10 mL of water by stirring. (c) Freshly mixed GO and CaCO3 solution. (d) CaCO3/1 wt % GO composite precipitated after stirring for 1 h. (e) CaCO3/1 wt % reduced GO composite after 1 h of chemical reduction with hydrazine hydrate. Finally, the resultant Barite/CCG and CaCO3/CCG composites were extracted by filtration, washing with water and acetone and the solid was then dried under vacuum (∼10−2 Torr) at 60 °C for 24 h. Characterization. Scanning electron microscopy (SEM) of the materials was performed using a JEOL 6500 field emission scanning electron microscope at 15 kV voltage; no surface coating was required for the imaging because the sample was already conductive. The average size of mineral particles was evaluated from SEM images using ImageJ software; 100 measurements were performed for each material. Transmission electron microscopy (TEM) was conducted using a JEOL1230 high contrast transmission electron microscope at 120 kV. Thermogravimetric analysis (TGA) was performed using a TGA Q50 (TA Instruments) at a heating rate of 10 K/min, from room temperature to 800 °C. The experiments were conducted under an air atmosphere at a flow rate of 50 mL/min. Raman spectra were recorded with an InVia Renishaw microscope using a 514.5 nm Ar laser. Electrical conductivity measurements of the compressed pelletized powders of Barite/CCG and CaCO3/CCG were performed using a Keithley 195A digital multimeter connected with a custom-built 4-point probe (Figure S1). Details of the experimental scheme for measuring conductivity are provided in the Supporting Information. Four independent measurements of each sample were made in order to ensure reproducibility. The detailed spectroscopy, current vs voltage vs temperature (IVT) and transport properties of both rGO and rGONRs have been previously published from our laboratory.14,15,21,22

mineral powders was studied. This process is simple, costeffective, and feasible for large scale industrial synthesis, and it could provide a wide array of new materials with tailored properties.

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EXPERIMENTAL SECTION

Materials. Barite (86.9 wt % BaSO4, M-I SWACO, A Schlumberger Company, M-I WATE 20140853-1), calcium carbonate (CaCO3, Fischer Scientific), concentrated sulfuric acid (95−98 wt %, SigmaAldrich), phosphorous acid (85 wt %, Sigma-Aldrich), potassium permanganate (J.T. Baker), hydrazine hydrate (64 wt %, Acros Organics), hydrogen peroxide (30 wt %, EMD Chemical, Inc.), ammonium hydroxide (29 wt %, Fisher Scientific), graphite flakes (Sigma-Aldrich, batch # 13802EH), and multiwalled carbon nanotubes (MWCNTs) with an average length of 1 mm and a diameter of 40−80 nm (NanoTechLabs, Inc.) were used as received. Synthesis. GO was synthesized from graphite by following the oxidation procedure reported by Marcano et al.10 GONRs were produced from MWCNTs using the protocol described by Higginbotham et al.15 (reaction 11). Preparation of graphene-wrapped minerals involved using a one-pot process where Barite or CaCO3 was first dispersed in water and mixed with an aqueous solution of GO or GONR and then reduced with hydrazine hydrate. Depending upon the target material composition, the required amount of Barite or CaCO3 (980 to 997.5 mg) was dispersed in 50 mL of water using a magnetic stir bar and then GO or GONR solution (2.5 to 20 mg in 50 mL of water) was slowly added and left stirring for 1 h at room temperature. Next, hydrazine hydrate (50 μL) and ammonium hydroxide (100 μL) were added to the solution and the reaction mixture was kept at 90 °C for 60 min.

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RESULTS AND DISCUSSION A scheme of the one-pot conductive coating process is shown in Figure 1, where inorganic mineral particles (Barite or B

DOI: 10.1021/acsami.5b06941 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

verified by TGA (Figure 3) that show that weight loss occurs primarily in the temperature range between 250 and 600 °C apparently due to the CCG decomposition. This weight loss corresponds to the calculated CCG content (1 and 2 wt %). The Raman spectra of the Barite, CaCO3 and their composites with the reduced GO and reduced GONRs are shown in Figure 4. They exhibit strong D and G peaks at ∼1350 and ∼1550 cm−1, characteristic of CCG. Thus, these results show the presence of disordered graphene on the mineral hybrids. The morphological character of the materials was assessed using TEM and SEM. The typical sheet structure of the original GO is revealed in Figure 5a where the GO flakes are up to several micrometers wide, which usually only consists of a few layers. The ribbon-like morphology of the GONRs shown in Figure 5b are several layers thick and are 250 to 400 nm wide, with a length of 10 to 25 μm. Because these two CCG precursors have quite different geometries, they are expected to provide different electrical properties in a reduced state when heterogenized on the surface of the mineral particles. The average size of the mineral particles was determined from the SEM images. The Barite particles have an average size of 1.92 μm and the average size of the CaCO3 particles was 4.66 μm. The size distribution histograms for the mineral particles are shown in Figure S3. SEM images of the mineral-based composites with 1 wt % loading of reduced GO and GONRs are shown in Figure 6. These materials have a distinct morphology showing a heterogeneous interface formed on the surface of the mineral particles. The electrical conductivity of the mineral/CCG hybrids was measured using a 4-point probe conductivity method for pelletized powder samples (see the Supporting Information). The electrical conductivity of the mineral/CCG hybrids vs CCG content was plotted in Figure 7, and the corresponding electrical resistance values are shown in Figure S4. The electrical conductivity of both Barite-based and CaCO3-based systems clearly demonstrates increasing conductivity with an increase in rGO or rGONR loading. Resistance of the composites with the lowest GO or GONR concentration (0.25 wt %) was above the instrumentation limit (≥10 MΩ) and therefore could not be measured. However, materials with a CCG concentration of ≥0.5 wt % additive showed conductivity that could be measured with the instrument. As expected, the mineral hybrids with reduced GONRs demon-

Figure 3. TGA composite analysis. TGA plots for the Barite and its composites with reduced GO and GONR. rGO is reduced GO; rGONR is reduced GONR.

CaCO3) serve as a template for the deposition of GO or GONRs in the aqueous phase. Figure 2 show photographs of aqueous GO and CaCO3 at each stage of the process. Mixing the brown GO solution (Figure 2a) with the white CaCO3 suspension (Figure 2b) forms a light brown suspension (Figure 2c) that precipitates after 1 h of mixing (Figure 2d). Figure S2 shows that Barite also exhibits the same behavior upon adsorption of GO onto the particles. Figure 2e and Figure S2e show that after chemical reduction with hydrazine hydrate, both CaCO3/GO and Barite/GO powders become dark gray, which is a simple visual indication of graphene formation on the surface of the mineral particles. An identical behavior was observed for the GONR to GNR-coated minerals. Because of the fact that the solvent phase becomes colorless after 1 h of mixing of GO or GONR solutions and subsequent settling for 15 min with the Barite or CaCO3, we assume that GO or GONRs becomes adsorbed on the surface of the mineral particles. Even small concentrations of the GO or GONR in water (0.025 mg/mL, the smallest concentration used in this work to obtain mineral composites with 0.25 wt % CCG loading) form brown-colored solutions that can be easily visually identified. Accordingly, the GO or GONR weight contents in the synthesized composites were calculated assuming that all of the GO or GONRs was completely adsorbed on the surface of the inorganic particles. This was

Figure 4. Raman analysis of composites. Panel a: (1) Raman spectra of Barite, (2) Barite hybrids containing 1 wt % of rGO and (3) 1 wt % of rGONR. Panel b: (1) Raman spectra of CaCO3, (2) CaCO3 hybrids with 1 wt % of rGO and (3) 1 wt % of rGONR. C

DOI: 10.1021/acsami.5b06941 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. TEM characterization. (a) GO sheet, (b) GONRs, (c) Barite/2 wt % rGO composite.

Figure 6. SEM characterization. SEM images of (a) the Barite particles, (b) CaCO3 particles, (c,d) Barite/1 wt % rGO composite, (e,f) CaCO3/1 wt % rGO composite, (g,h) Barite/1 wt % rGONR composite and (i) CaCO3/1 wt % rGONR composite.

the concentration-dependent conductivity; however, their conductivity values were nearly 2 orders of magnitude lower that the Barite/CCG systems. Moreover, the CaCO3/rGO system exhibited a measurable conductive behavior starting at 1.5 wt % rGO loading. Note, at lower rGO content, the material’s resistance was above the instrumentation limit and could not be measured. The highest conductivity for the CaCO3/CCG composites (at 2 wt % CCG loading) is 4.04 × 10−6 ± 1.34 × 10−6 S/cm for the rGO and 8.70 × 10−4 ± 2.77 × 10−4 S/cm for the rGONR. The lower values for the CaCO3based systems vs the Barite-based systems can be explained by

strated the highest electrical conductivity compared to reduced GO-containing materials, apparently due to a higher aspect ratio of the GONRs and formation of more efficient conductive pathways on the surface of the mineral particles. The highest conductivity values were achieved in Barite/CCG hybrids at the highest CCG concentration (2 wt %): 2.7 × 10−4 ± 0.52 × 10−4 S/cm for the rGO and 4.3 × 10−2 ± 1.72 × 10−2 S/cm for the rGONR. At the same time, CaCO3/CCG composites exhibited a lower conductivity in comparison with the Barite-based materials. They also demonstrated the same trend in terms of D

DOI: 10.1021/acsami.5b06941 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 7. Electrical conductivity of the (a) Barite/CCG hybrids and (b) CaCO3/CCG hybrids.

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ACKNOWLEDGMENTS This work has been supported by MI-SWACO, a Schlumberger Company.

the different geometry of the mineral particles. Barite particles have an irregular shape and a smaller size enabling more efficient packing in the compressed state. At the same time, the particles of CaCO3 have a cubic shape and a much larger size, and therefore should not be able to fill the same volume as densely as the Barite particles. As a result, in the compressed state CaCO3 particles are expected to have more gaps between them and accordingly form less efficient conductive pathways when having CCG on their surface.

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CONCLUSION A series of hybrid materials based on inorganic minerals (Barite and CaCO3) and CCG have been successfully synthesized by a simple and efficient solution blending method in the aqueous phase. Near complete adsorption of the CCG precursors (GO or GONRs) on the surface of the mineral particles takes place during the mixing process. The obtained mineral/CCG hybrids demonstrate an increase in the electrical conductivity with the CCG content. This method opens up the possibility of making a wide range of conductive materials based on different minerals (clays, carbonates, sulfates, oxides, etc.) and tuning their conductivity by controlling the amount of CCG precursor. These conductive mineral hybrids can find applications as components in building materials, fillers in polymer-based composite materials, and in conductive drilling fluids. This technology is easily scalable and environmentally friendly, especially with the use of low-toxicity reducing agents such as sodium bisulfite,23,24 because GO and its reduction products25 have been shown to be environmentally benign.26

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b06941. Scheme for measuring the electrical conductivity of the powder samples, size distribution histograms for the mineral particles, electrical resistance of the materials (PDF).

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*J. M. Tour. E-mail: [email protected]. Notes

The authors declare no competing financial interest. E

DOI: 10.1021/acsami.5b06941 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.5b06941 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX