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Rheological and microstructural characterization of aqueous suspensions of carbon black and reduced graphene oxide
Colloids and Surfaces A 592 (2020) 124591
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Rheological and microstructural characterization of aqueous suspensions of carbon black and reduced graphene oxide
T
Yuzi Zhang1, Joseph P. Sullivan1, Arijit Bose* Department of Chemical Engineering, University of Rhode Island, Kingston, Rhode Island, 02881, United States
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Carbon black Reduced graphene oxide Suspensions Microstructure Rheology
Carbon black is often used as a conductivity enhancer in battery electrodes. Reduced graphene oxide is another conducting material, with a high aspect ratio sheet-like morphology, and offers the possibility of generating a conducting network at loadings that are small compared to carbon black. Suspensions of conducting carbon, active materials and binders are typically coated on current collectors during the formation of electrodes. The rheology of these suspensions is an important indicator of the connectivity of particles in the suspension and impacts the electrical conductivity as well as the thickness of the coated layer. In this work, the rheology and microstructures of aqueous suspensions of para-aminobenzoic acid terminated carbon black (CB) with and without reduced graphene oxide (RGO) were investigated. The CB loading varied between 0.05 wt.% -10 wt.%, while the samples containing RGO had an RGO loading of 0.05 wt.%. The pH of the suspension was varied from 7.5 to 3. The carboxyl groups on the CB surface are deprotonated at pH 7.5, making the particles hydrophilic. Protonation of the carboxylate groups at pH 3 makes the particles partially hydrophobic. Suspensions of CB showed Newtonian behavior at all loadings for pH 7.5, and at loadings below 1.5 wt.% for pH 3. They are shear thinning at loadings between 1.5–10 wt.%. Interestingly, the low shear viscosity is non-monotonic with CB loading, rising up to a CB loading of 7.5 wt.%, and then dropping at 10 wt.%. Cryogenic scanning electron microscopy (SEM) showed CB particles forming aggregates at pH 3 for CB loadings greater than 1.5 wt.%. At pH 3, addition of 0.05 wt.% RGO resulted in a viscosity increase for suspensions containing 0.5 wt.% CB, as the RGO formed connections with CB particles that enhanced network formation. Adding RGO had little effect on suspensions that had 1.5 wt.% CB. The combination of SEM imaging and suspension rheology provides insight into the particle connectivity for the carbon-based materials studied here. These insights will be useful in determining coating conditions for slurries used in rechargeable battery electrodes.
Abbreviations: CB, Carbon black; LVR, Linear viscoelastic region; PABA, Para-aminobenzoic acid; RGO, Reduced graphene oxide; SEM, Scanning electron microscope, scanning electron microscopy ⁎ Corresponding author. E-mail address: [email protected] (A. Bose). 1 Equal contributions to this work. https://doi.org/10.1016/j.colsurfa.2020.124591 Received 10 November 2019; Received in revised form 2 February 2020; Accepted 12 February 2020 Available online 17 February 2020 0927-7757/ © 2020 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 592 (2020) 124591
Y. Zhang, et al.
1. Introduction
unperturbed suspensions using cryogenic scanning electron microscopy (cryo-SEM) [33,34].
Carbon black (CB) is commonly used in many commercial products including batteries [1,2], tires [3–5], ink and paints [6–8], and plastics [9–11] as a reinforcing, coloring, or conductive agent. Because of its high electrical conductivity, electronic equipment and portable electronic devices have incorporated CB-polymer composites as sensors [12,13]. For applications where conductivity is a priority, CB must reach the percolation threshold, creating a continuous network [14,15]. Reduced graphene oxide (RGO) exhibits adequate electrical conductivity for many of the same applications as CB, particularly in electronics and energy storage [16–21]. RGO is obtained by partial reduction of graphene oxide (GO), and is prepared by a modified Hummers method [22–24]. The ratio of the lateral dimension to thickness for RGO is of the order of 103-104, leading to a low percolation threshold; this feature has been exploited for the reduction of the total carbon content in Si-based battery anodes [25,26]. CB is a fractal particle whereas RGO has a sheet-like morphology with a very high aspect ratio and is flexible. These geometric features make a study of the rheology and microstructure of suspensions containing these particles distinct from other suspensions containing particles of less complex geometries. Microstructures in CB suspensions can be tuned by alteration of the suspending media [27,28], and differs with CB loading [15,29]. Previous studies used para-aminobenzoic acid-terminated CB for emulsion templated processing of Si battery anodes [26,30–32]. The loading and pH dependence of para-aminobenzoic acid (PABA)-terminated CB on the viscosity of these suspensions are reported here. The changes induced by the addition of a small loading of RGO are then investigated. We complement rheological characterization with direct images of
2. Experimental procedures A 15 wt.% aqueous suspension of PABA-terminated carbon black, at pH 7.5, was provided by Cabot Corporation. The CB particles have a fractal structure with a nominal dimension of 150 nm. At pH 7.5, the carboxyl groups on CB are deprotonated. These particles are highly hydrophilic, and form a stable suspension in water [30]. Reduced graphene oxide was purchased from Graphene Supermarket. These particles have a specific surface area of 833 m2/g and a carbon to oxygen ratio of 10:1. The average lateral dimension of the RGO is 4 μm and thickness is 3.5 nm. The specific surface area and thickness imply that the RGO has ∼ 8–10 layers. Hydrochloric acid (HCl, 37 wt.%) was purchased from Sigma Aldrich. Deionized water was obtained from a Millipore Milli Q system. Aqueous suspensions containing 0.05 wt.% 10 wt.% of the PABA-functionalized CB were examined, as well as suspensions of CB with an RGO loading of 0.05 wt.%. A Gatan Alto 2500 cryo system was used to prepare samples for cryogenic scanning electron microscopy (cryo-SEM). Approximately 5 μL of the sample was placed on a holder, and then plunged into liquid nitrogen. The rapidly frozen specimen was fractured using a cold flatedge knife in a chamber cooled to −130 °C. The sample temperature was then raised to −95 °C for 5 min, to enhance surface topological details by sublimation of the water. The sample was cooled back to −130 °C, sputtered with gold, and moved from the preparation chamber to the imaging stage. Samples were imaged on a Zeiss Sigma VP field emission scanning electron microscope. A TA Instruments, AR2000 EX rheometer with a cone and plate
Fig. 1. (A-B) Viscosity versus shear rate for suspensions of CB at different loadings (A) pH 7.5 and (B) pH 3. (C-D) Viscosity versus shear stress for CB suspensions at (C) pH 7.5 and (D) pH 3. 2
Colloids and Surfaces A 592 (2020) 124591
Y. Zhang, et al.
geometry was employed for rheological characterization at 25 °C. The cone has a diameter of 40 mm, cone angle of 2 °, and truncation gap of 51 μm. The minimum gap width was 50 μm, much larger than the lateral dimensions of the RGO. All samples were vortexed at 3000 rpm for 30 s, and immediately loaded onto the rheometer. Samples were then pre-sheared at 0.1 s−1 for 60 s before characterization. Suspension viscosity was recorded over a shear rate range of 3-2000 s-1.
1 Pa and 5 Pa respectively. The sample at 7.5 wt.% CB has a large low shear viscosity indicating a strong network with a yield stress of ∼ 5 Pa. CB particles have surface carboxylate groups that are deprotonated at neutral pH, producing a negative surface charge. Addition of HCl and reducing the pH to 3.0 protonates some of these groups and induces partial hydrophobicity to the particles. These CB particles, exposed to water, then begin connecting into a network, increasing the suspension viscosity [35]. These partially hydrophobic particles also start aggregating. Network formation dominates at CB loadings up to 7.5 wt.% while aggregation dominates at higher loadings. The suspensions that showed shear thinning behavior at pH 3 were probed using small amplitude oscillatory shear experiments. We did not observe a linear viscoelastic region for any of these samples. Thus, no storage and loss moduli are reported for these suspensions. Additional insight into this rheological behavior can be obtained by examining cryo-SEM images of suspensions. Fig. 2 shows images of CB suspensions at pH 3. At CB concentrations of 0.05 wt.% and 0.5 wt.%, a few discrete CB aggregates were distributed in water (Figs. 2A, 2B). At 1.5 wt.% CB, the aggregates are connected (Fig. 2C). At 4 wt.% CB, a partially connected network was formed (Fig. 2D), correlating with an increase of the viscosity of the suspension. At 7.5 wt.% CB, a more complete network was observed (Fig. 2E). This network formation was consistent with the increased low shear viscosity and shear thinning behavior presented in Fig. 1B. The aggregates observed at 10 wt.% CB are large, and do not form a network (Fig. 2F). Thus the CB structures
3. Results and discussion Fig. 1 shows the pH dependence of the viscosity of CB particle suspensions. At pH 7.5, the viscosity of the samples remained a constant at essentially the viscosity of water, indicating little interparticle interaction within these suspensions (Fig. 1A). At pH 3 the viscosity of the samples rose significantly, and shear thinning was observed in CB suspensions at all loadings (Fig. 1B). Interestingly, the low-shear viscosity shows a non-monotonic behavior with an increase in CB loading. The low-shear viscosity of the 1.5 wt.% and 4 wt.% samples at pH 3 is approximately 0.1 Pa s, whereas the 7.5 wt.% sample exhibits a lowshear viscosity of approximately 0.9 Pa s. At 10 wt.%, the low-shear viscosity decreases to approximately 0.1 Pa s. Figs. 1C and 1D display the viscosity versus stress in these samples for pH values of 7.5 and 3, respectively. The suspension viscosity remains constant at pH 7.5 (Fig. 1C). At pH 3, samples with 4 wt.% and 10 wt.% CB have a finite low shear viscosity and display a yield stress of
Fig. 2. Cryo-SEM images of carbon black suspensions at pH 3 at different loadings. (A) 0.05 wt.% CB, (B) 0.5 wt.% CB, (C) 1.5 wt.% CB, (D) 4 wt.% CB, (E) 7.5 wt.% CB, (F) 10 wt.% CB. Arrows indicate CB aggregates. Scale bars = 10 μm. 3
Colloids and Surfaces A 592 (2020) 124591
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show a pattern with increasing particle loading – small disconnected aggregates at low loading, a network at 7.5 wt.%, and large disconnected aggregates at 10 wt.%. These microstructures are responsible for the observed rheology of the samples. We examined if the addition of RGO influenced the rheological behavior and microstructure of the mixed CB-RGO suspensions at pH 3. Because the RGO has a carbon to oxygen ratio of 10:1, it is partially hydrophobic. At a CB concentration of 1.5 wt.%, addition of 0.05 wt.% RGO did not cause a change in the viscosity of the suspension (Fig. 3A). The viscosity was dominated by the CB. However, for a suspension of 0.5 wt.% CB, there was an increase in the low shear viscosity (Fig. 3B), indicating interaction between CB particles and RGO sheets. Further information about these systems was gathered from cryoSEM images (Figs. 3C, 3D). For a loading of 1.5 wt.% CB (Fig. 3C), the RGO sheets do not perturb the existing CB network (red arrow). For 0.5 wt.% CB (Fig. 3D), the RGO sheets connect the CB particles (red arrow). In turn, this network formation is marked by a viscosity increase in the mixed suspensions. Higher magnification cryo-SEM images, taken in the regions marked by the dashed rectangles in Figs. 3C and 3D, are shown in Fig. 4. For the 1.5 wt.% CB sample, CB aggregates are seen and the RGO is either incorporated within these aggregates or wrapped over a CB network. At 0.5 wt.% CB, the RGO sheets form the primary network, connecting the well-distributed CB particles. These images are consistent with the viscosity data.
4. Conclusions We studied the rheological behavior and microstructures of aqueous suspensions of para-aminobenzoic acid-terminated CB and mixtures of this CB and RGO. Both the pH and particle loading had significant effects on the observed viscosities and microstructures of the suspensions. The samples were Newtonian at all CB loadings and pH 7.5. At pH 3 and CB loadings beyond 0.5 wt.%, the suspensions showed a large increase in the low shear viscosity, and demonstrated shear-thinning behavior. Corresponding cryo-SEM images showed the microstructural evolution of CB suspensions from aggregates to a network as the loading was increased. The zero shear viscosity for these samples showed a nonmonotonic behavior with CB loading, resulting from a balance between aggregate formation and connectivity. Tuning of a CB network has been shown previously [27,28]. However, a non-monotonic relationship between CB loading and viscosity has not been previously reported [15,29,36,37]. Addition of 0.05 wt.% RGO in suspensions containing 0.5 wt.% CB at pH 3 led to a low shear viscosity increase, with the RGO providing connections between CB particles. This low shear viscosity increase was not present in suspensions containing 1.5 wt.% CB at the same pH and RGO loading because the CB dominated network formation. This work has consequences on electrodes being made for lithium ion batteries. The purpose of CB addition to these suspensions is to enhance the electrical conductivity of the electrode. CB does not add to the capacity of the electrode. To increase the specific capacity of the electrode, carbon loading must be minimized. Addition of a small
Fig. 3. Plots of viscosity versus shear rate (A, B) and cryo-SEM images (C, D) for CB suspensions with and without RGO at pH 3. (A) 1.5 wt.% CB, (B) 0.5 wt.% CB, (C) 1.5 wt.% CB + 0.05 wt.% RGO, (D) 0.5 wt.% CB + 0.05 wt.% RGO. In C and D, RGO sheets marked by red arrows form connections with CB particles. Scale bars are 10 μm (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
4
Colloids and Surfaces A 592 (2020) 124591
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Fig. 4. Cryo-SEM images of aqueous suspensions at pH 3 of (A) 1.5 wt.% CB and 0.05 wt.% RGO (B) 0.5 wt.% CB and 0.05 wt.% RGO. The images are from the regions marked by the dashed rectangles in Figs. 3C and 3D. Scale bars = 1 μm.
quantity of RGO to the CB allows networks to form at low carbon content, potentially resulting in high electrical conductivity in the samples.
[15]
CRediT authorship contribution statement
[16]
Yuzi Zhang: Data curation, Writing - original draft, Writing - review & editing. Joseph P. Sullivan: Data curation, Writing - original draft, Writing - review & editing. Arijit Bose: Supervision, Writing - original draft, Writing - review & editing.
[17] [18]
Declaration of Competing Interest
[19]
The authors declare that they have no financial interests in this work.
[20] [21]
Acknowledgements
[22]
We gratefully acknowledge funding from Department of Energy, Office of Basic Energy Sciences, EPSCoR Implementation award DESC0007074. The cryo-SEM images were acquired at the RI Consortium for Nanoscience and Nanotechnology, a URI College of Engineering core facility partially funded by the National Science Foundation EPSCoR, Cooperative Agreement #OIA-1655221.
[23] [24] [25]
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Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Rheological and microstructural characterization of aqueous suspensions of carbon black and reduced graphene oxide
T
Yuzi Zhang1, Joseph P. Sullivan1, Arijit Bose* Department of Chemical Engineering, University of Rhode Island, Kingston, Rhode Island, 02881, United States
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Carbon black Reduced graphene oxide Suspensions Microstructure Rheology
Carbon black is often used as a conductivity enhancer in battery electrodes. Reduced graphene oxide is another conducting material, with a high aspect ratio sheet-like morphology, and offers the possibility of generating a conducting network at loadings that are small compared to carbon black. Suspensions of conducting carbon, active materials and binders are typically coated on current collectors during the formation of electrodes. The rheology of these suspensions is an important indicator of the connectivity of particles in the suspension and impacts the electrical conductivity as well as the thickness of the coated layer. In this work, the rheology and microstructures of aqueous suspensions of para-aminobenzoic acid terminated carbon black (CB) with and without reduced graphene oxide (RGO) were investigated. The CB loading varied between 0.05 wt.% -10 wt.%, while the samples containing RGO had an RGO loading of 0.05 wt.%. The pH of the suspension was varied from 7.5 to 3. The carboxyl groups on the CB surface are deprotonated at pH 7.5, making the particles hydrophilic. Protonation of the carboxylate groups at pH 3 makes the particles partially hydrophobic. Suspensions of CB showed Newtonian behavior at all loadings for pH 7.5, and at loadings below 1.5 wt.% for pH 3. They are shear thinning at loadings between 1.5–10 wt.%. Interestingly, the low shear viscosity is non-monotonic with CB loading, rising up to a CB loading of 7.5 wt.%, and then dropping at 10 wt.%. Cryogenic scanning electron microscopy (SEM) showed CB particles forming aggregates at pH 3 for CB loadings greater than 1.5 wt.%. At pH 3, addition of 0.05 wt.% RGO resulted in a viscosity increase for suspensions containing 0.5 wt.% CB, as the RGO formed connections with CB particles that enhanced network formation. Adding RGO had little effect on suspensions that had 1.5 wt.% CB. The combination of SEM imaging and suspension rheology provides insight into the particle connectivity for the carbon-based materials studied here. These insights will be useful in determining coating conditions for slurries used in rechargeable battery electrodes.
Abbreviations: CB, Carbon black; LVR, Linear viscoelastic region; PABA, Para-aminobenzoic acid; RGO, Reduced graphene oxide; SEM, Scanning electron microscope, scanning electron microscopy ⁎ Corresponding author. E-mail address: [email protected] (A. Bose). 1 Equal contributions to this work. https://doi.org/10.1016/j.colsurfa.2020.124591 Received 10 November 2019; Received in revised form 2 February 2020; Accepted 12 February 2020 Available online 17 February 2020 0927-7757/ © 2020 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 592 (2020) 124591
Y. Zhang, et al.
1. Introduction
unperturbed suspensions using cryogenic scanning electron microscopy (cryo-SEM) [33,34].
Carbon black (CB) is commonly used in many commercial products including batteries [1,2], tires [3–5], ink and paints [6–8], and plastics [9–11] as a reinforcing, coloring, or conductive agent. Because of its high electrical conductivity, electronic equipment and portable electronic devices have incorporated CB-polymer composites as sensors [12,13]. For applications where conductivity is a priority, CB must reach the percolation threshold, creating a continuous network [14,15]. Reduced graphene oxide (RGO) exhibits adequate electrical conductivity for many of the same applications as CB, particularly in electronics and energy storage [16–21]. RGO is obtained by partial reduction of graphene oxide (GO), and is prepared by a modified Hummers method [22–24]. The ratio of the lateral dimension to thickness for RGO is of the order of 103-104, leading to a low percolation threshold; this feature has been exploited for the reduction of the total carbon content in Si-based battery anodes [25,26]. CB is a fractal particle whereas RGO has a sheet-like morphology with a very high aspect ratio and is flexible. These geometric features make a study of the rheology and microstructure of suspensions containing these particles distinct from other suspensions containing particles of less complex geometries. Microstructures in CB suspensions can be tuned by alteration of the suspending media [27,28], and differs with CB loading [15,29]. Previous studies used para-aminobenzoic acid-terminated CB for emulsion templated processing of Si battery anodes [26,30–32]. The loading and pH dependence of para-aminobenzoic acid (PABA)-terminated CB on the viscosity of these suspensions are reported here. The changes induced by the addition of a small loading of RGO are then investigated. We complement rheological characterization with direct images of
2. Experimental procedures A 15 wt.% aqueous suspension of PABA-terminated carbon black, at pH 7.5, was provided by Cabot Corporation. The CB particles have a fractal structure with a nominal dimension of 150 nm. At pH 7.5, the carboxyl groups on CB are deprotonated. These particles are highly hydrophilic, and form a stable suspension in water [30]. Reduced graphene oxide was purchased from Graphene Supermarket. These particles have a specific surface area of 833 m2/g and a carbon to oxygen ratio of 10:1. The average lateral dimension of the RGO is 4 μm and thickness is 3.5 nm. The specific surface area and thickness imply that the RGO has ∼ 8–10 layers. Hydrochloric acid (HCl, 37 wt.%) was purchased from Sigma Aldrich. Deionized water was obtained from a Millipore Milli Q system. Aqueous suspensions containing 0.05 wt.% 10 wt.% of the PABA-functionalized CB were examined, as well as suspensions of CB with an RGO loading of 0.05 wt.%. A Gatan Alto 2500 cryo system was used to prepare samples for cryogenic scanning electron microscopy (cryo-SEM). Approximately 5 μL of the sample was placed on a holder, and then plunged into liquid nitrogen. The rapidly frozen specimen was fractured using a cold flatedge knife in a chamber cooled to −130 °C. The sample temperature was then raised to −95 °C for 5 min, to enhance surface topological details by sublimation of the water. The sample was cooled back to −130 °C, sputtered with gold, and moved from the preparation chamber to the imaging stage. Samples were imaged on a Zeiss Sigma VP field emission scanning electron microscope. A TA Instruments, AR2000 EX rheometer with a cone and plate
Fig. 1. (A-B) Viscosity versus shear rate for suspensions of CB at different loadings (A) pH 7.5 and (B) pH 3. (C-D) Viscosity versus shear stress for CB suspensions at (C) pH 7.5 and (D) pH 3. 2
Colloids and Surfaces A 592 (2020) 124591
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geometry was employed for rheological characterization at 25 °C. The cone has a diameter of 40 mm, cone angle of 2 °, and truncation gap of 51 μm. The minimum gap width was 50 μm, much larger than the lateral dimensions of the RGO. All samples were vortexed at 3000 rpm for 30 s, and immediately loaded onto the rheometer. Samples were then pre-sheared at 0.1 s−1 for 60 s before characterization. Suspension viscosity was recorded over a shear rate range of 3-2000 s-1.
1 Pa and 5 Pa respectively. The sample at 7.5 wt.% CB has a large low shear viscosity indicating a strong network with a yield stress of ∼ 5 Pa. CB particles have surface carboxylate groups that are deprotonated at neutral pH, producing a negative surface charge. Addition of HCl and reducing the pH to 3.0 protonates some of these groups and induces partial hydrophobicity to the particles. These CB particles, exposed to water, then begin connecting into a network, increasing the suspension viscosity [35]. These partially hydrophobic particles also start aggregating. Network formation dominates at CB loadings up to 7.5 wt.% while aggregation dominates at higher loadings. The suspensions that showed shear thinning behavior at pH 3 were probed using small amplitude oscillatory shear experiments. We did not observe a linear viscoelastic region for any of these samples. Thus, no storage and loss moduli are reported for these suspensions. Additional insight into this rheological behavior can be obtained by examining cryo-SEM images of suspensions. Fig. 2 shows images of CB suspensions at pH 3. At CB concentrations of 0.05 wt.% and 0.5 wt.%, a few discrete CB aggregates were distributed in water (Figs. 2A, 2B). At 1.5 wt.% CB, the aggregates are connected (Fig. 2C). At 4 wt.% CB, a partially connected network was formed (Fig. 2D), correlating with an increase of the viscosity of the suspension. At 7.5 wt.% CB, a more complete network was observed (Fig. 2E). This network formation was consistent with the increased low shear viscosity and shear thinning behavior presented in Fig. 1B. The aggregates observed at 10 wt.% CB are large, and do not form a network (Fig. 2F). Thus the CB structures
3. Results and discussion Fig. 1 shows the pH dependence of the viscosity of CB particle suspensions. At pH 7.5, the viscosity of the samples remained a constant at essentially the viscosity of water, indicating little interparticle interaction within these suspensions (Fig. 1A). At pH 3 the viscosity of the samples rose significantly, and shear thinning was observed in CB suspensions at all loadings (Fig. 1B). Interestingly, the low-shear viscosity shows a non-monotonic behavior with an increase in CB loading. The low-shear viscosity of the 1.5 wt.% and 4 wt.% samples at pH 3 is approximately 0.1 Pa s, whereas the 7.5 wt.% sample exhibits a lowshear viscosity of approximately 0.9 Pa s. At 10 wt.%, the low-shear viscosity decreases to approximately 0.1 Pa s. Figs. 1C and 1D display the viscosity versus stress in these samples for pH values of 7.5 and 3, respectively. The suspension viscosity remains constant at pH 7.5 (Fig. 1C). At pH 3, samples with 4 wt.% and 10 wt.% CB have a finite low shear viscosity and display a yield stress of
Fig. 2. Cryo-SEM images of carbon black suspensions at pH 3 at different loadings. (A) 0.05 wt.% CB, (B) 0.5 wt.% CB, (C) 1.5 wt.% CB, (D) 4 wt.% CB, (E) 7.5 wt.% CB, (F) 10 wt.% CB. Arrows indicate CB aggregates. Scale bars = 10 μm. 3
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show a pattern with increasing particle loading – small disconnected aggregates at low loading, a network at 7.5 wt.%, and large disconnected aggregates at 10 wt.%. These microstructures are responsible for the observed rheology of the samples. We examined if the addition of RGO influenced the rheological behavior and microstructure of the mixed CB-RGO suspensions at pH 3. Because the RGO has a carbon to oxygen ratio of 10:1, it is partially hydrophobic. At a CB concentration of 1.5 wt.%, addition of 0.05 wt.% RGO did not cause a change in the viscosity of the suspension (Fig. 3A). The viscosity was dominated by the CB. However, for a suspension of 0.5 wt.% CB, there was an increase in the low shear viscosity (Fig. 3B), indicating interaction between CB particles and RGO sheets. Further information about these systems was gathered from cryoSEM images (Figs. 3C, 3D). For a loading of 1.5 wt.% CB (Fig. 3C), the RGO sheets do not perturb the existing CB network (red arrow). For 0.5 wt.% CB (Fig. 3D), the RGO sheets connect the CB particles (red arrow). In turn, this network formation is marked by a viscosity increase in the mixed suspensions. Higher magnification cryo-SEM images, taken in the regions marked by the dashed rectangles in Figs. 3C and 3D, are shown in Fig. 4. For the 1.5 wt.% CB sample, CB aggregates are seen and the RGO is either incorporated within these aggregates or wrapped over a CB network. At 0.5 wt.% CB, the RGO sheets form the primary network, connecting the well-distributed CB particles. These images are consistent with the viscosity data.
4. Conclusions We studied the rheological behavior and microstructures of aqueous suspensions of para-aminobenzoic acid-terminated CB and mixtures of this CB and RGO. Both the pH and particle loading had significant effects on the observed viscosities and microstructures of the suspensions. The samples were Newtonian at all CB loadings and pH 7.5. At pH 3 and CB loadings beyond 0.5 wt.%, the suspensions showed a large increase in the low shear viscosity, and demonstrated shear-thinning behavior. Corresponding cryo-SEM images showed the microstructural evolution of CB suspensions from aggregates to a network as the loading was increased. The zero shear viscosity for these samples showed a nonmonotonic behavior with CB loading, resulting from a balance between aggregate formation and connectivity. Tuning of a CB network has been shown previously [27,28]. However, a non-monotonic relationship between CB loading and viscosity has not been previously reported [15,29,36,37]. Addition of 0.05 wt.% RGO in suspensions containing 0.5 wt.% CB at pH 3 led to a low shear viscosity increase, with the RGO providing connections between CB particles. This low shear viscosity increase was not present in suspensions containing 1.5 wt.% CB at the same pH and RGO loading because the CB dominated network formation. This work has consequences on electrodes being made for lithium ion batteries. The purpose of CB addition to these suspensions is to enhance the electrical conductivity of the electrode. CB does not add to the capacity of the electrode. To increase the specific capacity of the electrode, carbon loading must be minimized. Addition of a small
Fig. 3. Plots of viscosity versus shear rate (A, B) and cryo-SEM images (C, D) for CB suspensions with and without RGO at pH 3. (A) 1.5 wt.% CB, (B) 0.5 wt.% CB, (C) 1.5 wt.% CB + 0.05 wt.% RGO, (D) 0.5 wt.% CB + 0.05 wt.% RGO. In C and D, RGO sheets marked by red arrows form connections with CB particles. Scale bars are 10 μm (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
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Fig. 4. Cryo-SEM images of aqueous suspensions at pH 3 of (A) 1.5 wt.% CB and 0.05 wt.% RGO (B) 0.5 wt.% CB and 0.05 wt.% RGO. The images are from the regions marked by the dashed rectangles in Figs. 3C and 3D. Scale bars = 1 μm.
quantity of RGO to the CB allows networks to form at low carbon content, potentially resulting in high electrical conductivity in the samples.
[15]
CRediT authorship contribution statement
[16]
Yuzi Zhang: Data curation, Writing - original draft, Writing - review & editing. Joseph P. Sullivan: Data curation, Writing - original draft, Writing - review & editing. Arijit Bose: Supervision, Writing - original draft, Writing - review & editing.
[17] [18]
Declaration of Competing Interest
[19]
The authors declare that they have no financial interests in this work.
[20] [21]
Acknowledgements
[22]
We gratefully acknowledge funding from Department of Energy, Office of Basic Energy Sciences, EPSCoR Implementation award DESC0007074. The cryo-SEM images were acquired at the RI Consortium for Nanoscience and Nanotechnology, a URI College of Engineering core facility partially funded by the National Science Foundation EPSCoR, Cooperative Agreement #OIA-1655221.
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