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Ultrahigh Self-Sensing Performance of Geopolymer Nanocomposites via Unique Interface Engineering Shuguang Bi, Ming Liu, Jingjing Shen, Xiao Matthew Hu, and Liying Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00419 • Publication Date (Web): 23 Mar 2017 Downloaded from http://pubs.acs.org on March 26, 2017

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ACS Applied Materials & Interfaces

Ultrahigh Geopolymer

Self-Sensing

Performance

Nanocomposites

via

of

Unique

Interface Engineering Shuguang Bi,1 Ming Liu,1 Jingjing Shen,2 Xiao Matthew Hu,2* and Liying Zhang1* 1

Temasek Laboratories, Nanyang Technological University, 50 Nanyang Drive, 637553, Singapore

2

School of Material Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue,

639798, Singapore

KEYWORDS: geopolymer nanocomposite, surface coating, carbon nanotubes, interface, self-sensing performance

ABSTRACT: Monitoring and assessment of the health of a civil structural material are the critical requirements to ensure its safety and durability. In this work, a coating strategy on carbon nanotubes (CNTs) was employed for the dispersion of CNTs in geopolymer matrix. The geopolymer nanocomposites prepared exhibited ultrahigh self-sensing performance based on the unique behaviors of SiO2 coating on CNTs in the geopolymer matrix. The SiO2 layer on CNTs was partially or fully removed during the fabrication process to restore the conductive nature of CNTs, facilitating the dispersion of CNTs and forming well-connected 3D electrical conductive networks. The gauge factor (GF) of geopolymer nanocomposites reached up to 663.3 and 724.6, under compressive and flexural loading, respectively, with the addition of only 0.25 vol% of SiO2 coated CNTs (SiO2-CNTs). The values were at least twice higher than those recently reported self-sensing structural materials containing

1 ACS Paragon Plus Environment


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different types of carbon-based fillers. The underlying mechanisms on the electrical signal change with respect to ionic conduction and electronic conduction were explored and correlated to the self-sensing performance. Additionally, the uniform dispersion of CNTs and good interaction between CNTs and geopolymer matrix contributed to the improvement in flexural and compressive strengths.

INTRODUCTION During long-term service, the civil structure may suffer deterioration in its performance or be damaged to some extent when subjected to natural disasters such as earthquake, tsunami and typhoon. The aging or post-disaster structure can lead to safety hazards which may not be apparent to the naked eye. In the worst-case scenario, people’s life may be threatened. Monitoring and assessment of such potential threats are the important requirements for a civil structure to ensure the safety, durability and sustainability. Sensing devices, including resistance strain gauges, fibre optic sensors, untrasonic sensors and piezoelectric sensors,1-3 provide effective ways to monitor the health of the structural materials.4 However, a separate sensor device is required and the limitations such as high cost and compatibility issue with the structural materials restricted the usage of external sensor in real applications. For example, ultrasonic flaw detection is the versatile non-destructive testing method for the detection of hidden cracks, voids, porosity, and other internal discontinuities in structural materials based on the propagation of ultrasonic waves in the object or material tested. One of the limitations of ultrasonic flaw detection is that a medium is needed for ultrasonic sensors and it cannot work in vacuum environment such as outer space. The disadvantages of such external sensor drive us to develop a self-sensing structural material which can monitor itself in real time without the need of embedded, attached


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or remote sensors. Geopolymer is a new class of structural materials being considered as an alternative material to ordinary Portland cement (OPC) for construction applications owing to advantages including low cost,5 higher early compressive strength,6 improved acid resistance,7-8 fire resistance,9-10 and reduced greenhouse gas emission during production.11 However, geopolymer is inherently brittle12-13 due to the cross-linked structure. The structural health monitoring of geopolymeric structures are deemed essential. Recently, geopolymer was demonstrated to possess direct piezoelectric effect originating from the migration of mobile hydrated cations in the pores of the geopolymeric structure under loading.14 This unique effect in geopolymer makes it possible to be explored as a self-sensing material. However, the sensing capability was unsatisfactory and an effective strategy is needed to realize the actual application. The incorporation of conductive fillers to establish a conductive network can greatly improve the self-sensing capability of a structural material.15 Recently, graphene16 was introduced into geopolymer matrix to enhance the self-sensing capability of geopolymer. The reported gauge factors (GF) value of 43.9, a parameter for quantitatively characterizing the self-sensing performance, was inferior compared to the traditional self-sensing structural materials (GF: 300-350)17-18 which may be due to the poor dispersion of graphene. The dispersion issue in an inorganic matrix was even more pronounced and posed huge difficulty in the fabrication of a highly sensitive inorganic matrix based self-sensor. Physical and chemical dispersion were two commonly used methods to promote the dispersion of nanofillers. The physical methods, such as ball milling and ultra-sonication, caused the fracture of the nanofillers and the reduction of the aspect ratio due to the high energy applied despite being demonstrated to be effective in



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achieving a uniform dispersion.19-20 The chemical methods including using surfactants and functionalization of the nanofillers were employed to decrease the hydrophobicity of the surfaces to achieve better dispersion in the water containing inorganic structural materials, such as cement and concrete. D'Alessandro et al21-22 used plasticizers as surfactants to disperse CNTs in cement to improve the self-sensing performance. However, the dispersion is largely dependent on the degree of physical interaction between the surfactants and the CNTs. The surfactants may also cause bubbles in the composite, reducing the strength of the material.23-24 Wen et al17-18 used ozone treatment to improve the wettability of the carbon fiber (CF) and obtained a better dispersion in cement. However, this method only enhanced the hydrophilicity of the fillers without taking the matrix-filler chemical interaction into consideration. We believe that a surface treatment method which is able to address both aspects will greatly enhance the dispersity of the filler and hence improve the self-sensing performance. In this work, we specially designed a SiO2 coating on CNTs to achieve ultrahigh sensitive geopolymer based self-sensor. A uniform dispersion of CNTs in an inorganic matrix was obtained for the first time through the chemical reactions between SiO2 coating and geopolymer. Since geopolymer is an inorganic polymer synthesized by alkali activation of aluminosilicates, coating a chemically compatible SiO2 layer on CNTs is a feasible way to promote the dispersion of CNTs in geopolymer matrix. However, it is likely that the existence of the SiO2 layer could hinder the electrons movement due to its insulating nature, which is undesirable for the self-sensing performance. What is interesting in this work is that the unique SiO2 layer was simultaneously removed in the alkaline medium used for geopolymer fabrication to restore the electrical conductivity of CNTs while promoting the


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dispersion. A well-connected electrical conducting network was formed resulting in the enhancement in the self-sensing performance. Additionally, the well-dispersed CNTs and the interfacial interaction occurred between the SiO2 coating and geopolymer matrix significantly improved the mechanical properties of the geopolymer nanocomposites. The demonstrated self-sensing performance of the geopolymer nanocomposites was superior to other self-sensing structural materials ever reported till date.

EXPERIMENTAL SECTION Materials. Metakaolin (MK, Metamax®) with the composition of SiO2 (~ 53.0 wt%), Al2O3 (~ 43.8 wt%) and other (~ 3.2 wt%) was provided by BASF, USA. Raw multi-walled carbon nanotubes (r-CNTs) with an average diameter of 10 nm and length of 1.5 µm were purchased from Nanocyl, Belgium. Sulfuric acid (H2SO4) (95%) and nitric acid (HNO3) (60%) were provided by Thermo Fisher Scientific Inc. Tetraethyl orthosilicate (TEOS), ammonium hydroxide solution (NH4OH) (28.0-30.0% NH3

basis),

sodium

hydroxide

(NaOH),

and

sodium

silicate

solution

(Na2O·(SiO2)x·xH2O, composition: Na2O, ~ 10.6 wt%; SiO2, ~ 26.5 wt%; H2O, ~ 62.9 wt%) were purchased from Sigma-Aldrich. All the materials and chemicals were used in the as-received condition. Deionized (DI) water was used during the sample treatment. Synthesis of SiO2 coated CNTs (SiO2-CNTs). The SiO2 coating process was described in previous publications.25-26 Figure S1A, Supporting Information, shows the schematic illustration of the preparation of SiO2-CNTs. The slightly modified procedure was briefly described as below. The r-CNTs were firstly stirred in the mixed solution of H2SO4 and HNO3 (Vsulfuric : Vnitric = 3:1) for 3 h at 60 °C. The



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mixture was then repeatedly washed with DI water until the pH value was close to neutral. After dried by vacuum freezing-dryer, the acid-treated CNTs (a-CNTs) were ultrasonically dispersed into a fixed volume mixture of ethanol, DI water and NH4OH for 30 min, followed by vigorous mechanical agitation for another 0.5 h to obtain a stable and homogeneous suspension. Immediately afterward, an appropriate amount of TEOS was added and kept for 12 h under stirring at room temperature. After the reaction, the mixture was separated by a centrifuge at 4000 rpm for 30 min. The resultant sediment was ultrasonically re-dispersed in water for 15 min, followed by the vacuum filtering to fully remove the free silica particles. The dispersion in water and the centrifuging procedure was repeated at least four cycles. Finally, the darkish product was obtained by filtration through a nylon membrane and vacuum freezing dried, yielding SiO2-CNTs. Preparation of geopolymer nanocomposites. Geopolymer was synthesized by activating MK with the alkaline silicate solution. Certain amounts of NaOH and sodium silicate solution were firstly manually mixed followed by ultra-sonication for 0.5 h. An appropriate amount of MK powder was then added. The molar ratio of Si/Al was fixed at 1.9:1 since it demonstrated highest mechanical properties.27-28 The mixture was put into THINKY ARE 310 Mixer for 10 min to form slurry. The slurry was then poured into Teflon mold with two copper wire (Kynar insulated 30awg wire) pre-embedded in a proper position. In order to minimize water loss during curing, the molds were sealed and followed by progressive curing at 40 °C for 2 h, 60°C for 24 h and room temperature for 6 days. For preparing geopolymer nanocomposites, 0.1, 0.25 and 0.5 vol% of a-CNTs or SiO2-CNTs were ultrasonically dispersed into the mixture of NaOH and sodium silicate solution for 0.5 h in an ice water bath, followed by blending with MK powder and curing. The procedures were the same as mentioned


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above. It is noted that the same volume fraction of the practical CNTs was used to compare the effects of a-CNTs and SiO2-CNTs (see Table S1, Supporting Information) Characterization. Bruker VERTEX-70 Fourier transform infrared (FTIR) was used to identify the chemical composition of r-CNTs, a-CNTs, and SiO2-CNTs in transmission mode over the wavenumber range of 650-2000 cm-1. X-ray diffraction (XRD) was performed on a Bruker D8 Advanced XRD using Cu Kα radiation (λ = 1.5406Å) in the scattering range (2θ) of 10-50° with a scan rate of 0.02° sec-1 and slit width of 0.1mm. Thermogravimetric analysis (TGA) was performed to determine the content of SiO2 coating using TGA Q500 (TA instrument) under air atmosphere. The heating rate is 10 °C min-1 from 50-800 °C. Transmission electron microscopy (TEM) was performed using Carl Zeiss LIBRA 120 Plus operated at 120 kV. Field emission scanning electron microscope (FE-SEM) was carried out on the Jeol JSM 7600F microscope with an acceleration voltage of 5 kV. The compression behavior of pure geopolymer and the nanocomposites were carried out using Instron Tester (Model 8516) with 100 kN load cell at a cross-head speed of 0.5 mm min-1. The dimension of specimens was 25.0 × 25.0 × 12.5 mm3. The matching pressure-sensing characterization was measured using the electrochemical workstation (ZIVE SP2). The experimental setup is shown in Figure 1. Two pieces of wire perpendicular to the compressive stress were fixing on two sides of the square specimen as electrodes. The embedded length of the wire was 12.5 mm and the distance between the two load points was 6.5 mm. Each electrode was connected with the electrochemical workstation by copper wires. The steel loading plates were electrically isolated by Teflon foils of 70 µm thick. When the compressive load was applied to the specimen, the current changing under 2 V was recorded by the



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electrochemical workstation. The data was averaged by measuring at least 5 times for each sample. The

flexure

behavior

of

pure

geopolymer

and

the

corresponding

nanocomposites were carried out using Instron Tester (Model 5567) with 500 N load cell in 3-points bending at a cross-head speed of 0.02 mm min-1. The dimension of specimens was 127 × 12.7 × 3 mm3. The span between the two loading points was kept at 48 mm. Two pieces of wire perpendicular to the flexural stress were embedded in the rectangular specimen served as electrodes. The embedded length of the thin wire was 6.3 mm and the distance between the two load points was 11 mm, as shown in Figure 1. The matching pressure-sensing characterization was the same as mentioned above.

RESULTS AND DISCUSSION The synthesis scheme of SiO2-CNTs is illustrated in Figure S1A, Supporting Information. The preparation procedure of SiO2-CNTs and the corresponding detailed characterizations were described in the Experimental Section and Supporting Information, respectively. The preparation of geopolymer nanocomposites is schematically illustrated in Figure S2, Supporting Information. The a-CNTs or SiO2-CNTs were dispersed in the alkaline silicate solution in an ice water bath by sonication, followed by the addition of MK powder. The obtained mixture was centrifugal mixed and finally poured into a mold for curing. To compare the effects of a-CNTs and SiO2-CNTs, same content of CNTs was used throughout the work. For example (see Table S1, Supporting Information), 0.25 vol% of a-CNTs corresponds to 0.25 vol% of CNTs loading; while 0.85 vol% of SiO2-CNTs corresponds to 0.25 vol% of CNTs loading since approximately 70% weight increase was obtained after


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SiO2 coating (see Figure S1B, Supporting Information). In this paper, the geopolymer nanocomposite samples were defined as geop/a-CNTs X or geop/SiO2-CNTs X. X refers to the volume fraction of CNTs. For example, geop/SiO2-CNTs 0.25 refers to geopolymer nanocomposite containing 0.25 vol% of SiO2-CNTs. Figure 2A shows the digital pictures of the alkaline silicate solution containing the same volume content of a-CNTs and SiO2-CNTs after sonication. It is obvious that SiO2-CNTs achieved a better dispersion in the alkaline silicate solution. The a-CNTs and SiO2-CNTs taken out from the solution, labeled as a-CNTs-Na and SiO2-CNTs-Na, respectively, were analyzed and compared with the freshly prepared a-CNTs and SiO2-CNTs using FTIR spectroscopy, XRD spectroscopy, TGA and TEM. In Figure 2B, the peak at 1387 cm-1 corresponding to C-OH stretching remained unchanged. The peak at 1640 cm-1, corresponding to O-H vibration of the carboxylic group of a-CNTs (dotted red line) shifted to 1660 cm-1, corresponding to C=C of a-CNTs-Na (solid red line),29 indicating the removal of hydrophilic oxygen-containing groups, which caused the relatively poor dispersion of a-CNTs in the alkaline silicate solution as shown in Figure 2A. For SiO2-CNTs-Na (Figure 2B solid blue line), the peak at 1640 cm-1 was more obvious than the same peak observed in SiO2-CNTs (dotted blue line) indicating that the coated SiO2 layer on CNTs was removed in the alkaline solution by a certain extent. Additionally, the Si-O-Si and Si-OH stretching shifted from 1100 and 950 cm-1 of SiO2-CNTs (dotted blue line) to 1020 and 880 cm-1 (solid blue line), respectively. The observation implied that there was a chemical reaction between the coated SiO2 layer and the alkaline solution which facilitated the dispersion of SiO2-CNTs in the solution. XRD spectra of a-CNTs-Na and SiO2-CNTs-Na in Figure 2C confirmed the structural transformation after the immersion in the alkaline silicate solution.



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Compared with a-CNTs (dotted red line), the graphitic peaks (002 and 100) of a-CNTs-Na (solid red line) were not sharp because of the XRD signal interruption by the hydrolysis and condensation of sodium silicate on CNTs surfaces.30-31 After coated with amorphous SiO2, since the coating is much thicker (~25 nm) compared with the diameter of r-CNTs (~ 10nm), the diffraction graphitic peaks for SiO2-CNTs become insignificance. The similar observations were also reported in some published papers.32-33 In comparison, the re-appearance of graphitic peaks was clearly visible from SiO2-CNTs-Na (solid blue line) although they are not as sharp as those appeared in a-CNTs (dotted red line), which provided indirect evidence for the partial removal of the SiO2 layer in the alkaline solution. The weight change obtained by TGA (Figure 2D) was used to quantify the SiO2 content coated on the CNTs surfaces. The hydrolysis and condensation of sodium silicate led to the adsorption of silicate anions onto the CNTs surfaces as evidenced by the residual weight increase from 8.8% of a-CNTs (dotted red line) to 44.9% a-CNTs-Na (solid red line) at 800 °C. On the contrary, a slight drop in the residual weight from 78.9% (dotted blue line) to 62.5% (solid blue line) was obtained for SiO2-CNTs-Na at 800 °C since an amount of SiO2 coating was removed in the alkaline environment. Figures 2E-G show the TEM images of a-CNTs, SiO2-CNTs, and SiO2-CNT-Na. A smooth surface with an average diameter ~ 10 nm of a-CNTs can be observed in Figure 2E. After coated with the SiO2 layer, a core-shell structure appeared along the axial direction of the nanotubes. The diameter of SiO2-CNTs increased to ~ 60 nm, about ~ 25 nm thick SiO2 layer on the CNTs (see Figure 2F). Three types of morphological appearances of the SiO2-CNTs-Na were observed as a result of different removal extent as shown in Figure 2G and were schematically illustrated,


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i.e., (1) bare CNTs with complete removal of the SiO2 layer, (2) CNTs with discontinuous and thinner SiO2 layer (diameter reduced to ~ 10 nm), and (3) CNTs with continuous and thinner SiO2 layer (diameter reduced to ~ 10 nm). Although random in nature, the CNTs with distinct morphologies benefited the final nanocomposite in a couple of aspects. The SiO2 layer facilitated the dispersion of CNTs in the geopolymer matrix at the initial mixing stage due to the chemical compatibility. The partial and complete removal of the SiO2 layer upon exposure to the alkaline environment helped to restore the conducting nature of the CNTs which is essential for the self-sensing performance. It is worthwhile to investigate the occurrence of the unique behavior of SiO2 coating in a lower alkalinity such as cement system. Researchers have reported that SiO2 nanoparticles can react with systems of calcium-silicate-hydrate (C-S-H) resulting in higher densification with improved strength and durability.34 Therefore, our SiO2-CNTs can, in principle, form excellent interfacial bonding with Portland cement too. A preliminary experiment was carried out using calcium hydroxide instead of sodium hydroxide with the same molar concentration. Figure S3, Supporting Information, shows the TEM image of SiO2-CNTs-Ca (The SiO2-CNTs taken out from calcium hydroxide solution were labeled as SiO2-CNTs-Ca). It is observed that the SiO2 layer dissolved better under stronger alkalinity while partial removal of SiO2 layer was observed under lower alkalinity. The observation indicated that the unique interface design methodology is universal for alkaline systems, but shown different extent of SiO2 removal depending on the alkalinity. Here, we need to point out that with partial removal of the SiO2 coating in this case, the sensitivity of the sensing capability might be compromised. However, it can be overcome through tuning the thickness of SiO2 coating on the CNTs. A lower alkaline medium might be able to achieve more complete removal of



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SiO2 coating if the costing thickness is reduced. It was reported that the amorphous SiO2 favored the polycondensation reaction of MK to form the consolidated material.35-36 The dissolution of MK by alkaline hydrolysis produced Al and Si species and subsequently polymerized into an alumina and silica tetrahedral joined at the corners with oxygen and associated with alkali cations.27, 37-38 As SiO2-CNTs are comprised of high amorphous SiO2 content, the diffusion and transportation of the dissolved Si complexes occurred during geopolymerization formed the strong interfacial interaction between CNTs and geopolymer matrix (illustrated in Figure 3). The dispersion behaviors of a-CNTs and SiO2-CNTs in geopolymer matrix were examined using FE-SEM as shown in Figure 4. It can be seen that both a-CNTs and SiO2-CNTs were dispersed homogeneously in the geopolymer matrix at 0.1 vol% of CNTs (Figures 4A and 4D). At 0.25 vol% of CNTs (Figures 4B and 4E), the uniform dispersion of SiO2-CNTs maintained and formed a well-connected network throughout the matrix, whereas agglomerations of a-CNTs were observed. The agglomerates of the CNTs can be treated as conductive islands that are randomly distributed within the matrix. Further increase the content of CNTs to 0.5 vol% caused agglomerations for both cases, with the larger size of agglomerates being observed from a-CNTs (Figures 4C and 4F). The more uniform dispersion of SiO2-CNTs was attributed to the unique behavior of SiO2 layers as discussed above. The self-sensing property arises from the change of the electrical signal inside the sample under external force or deformation. In this work, samples were subjected to axial compression and flexure tests for self-sensing performance characterization. The experimental setup for the characterization shown in Figure 1 was designed by combining a mechanical tester with an electrochemical workstation. The details were


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described in the Experimental Section. The digital pictures of the series of geop/a-CNTs and geop/SiO2-CNTs with different CNTs contents are shown in Figure S4, Supporting Information. It can be clearly seen that the color of the geop/SiO2-CNTs was darker than that of geop/a-CNTs at the same CNTs loading indicating that the SiO2 coating promoted the dispersion of CNTs in the matrix. The GF is used to quantify the self-sensing performance (piezoresistive response) of the geopolymer nanocomposites and is defined as the slope of ∆R/R0-strain curve.39-40 ࢾ(

ࡳࡲ =

∆ࡾ ) ࡾ૙

ࢾࢿ

(1)

where R0 is the initial electrical resistance, ∆R is the change in the electrical resistance and ɛ is the applied strain. In general, higher filler loading tends to form a more electrically conductive network, which is beneficial to the enhancement of the signal to noise ratio and desirable in sensing measurements.41 Meanwhile, high filler concentration usually causes agglomeration and is detrimental to the mechanical properties which in turn restrict the practical applications of the self-sensing material. In this work, as discussed above, agglomerations were found in both geop/a-CNTs 0.50 and geop/SiO2-CNTs 0.50 (Figures 4C and 4F), causing the decrease in flexural and compressive strengths. (The mechanical properties of geop/a-CNTs and geop/SiO2-CNTs with different CNTs contents were shown in Figures S5, Supporting Information and were discussed in detail.) As a structural material, high mechanical properties to ensure the safety and durability during real applications should be the primary concern. The self-sensing capability serves as an additional bonus to new generation structural materials. Hence, the geopolymer nanocomposite containing 0.25 vol% of CNTs showed the highest mechanical properties was selected for self-sensing performance investigation. 13 ACS Paragon Plus Environment


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Figures 5A and 5B show the relative change in resistance (∆R/R0) versus the applied compressive and flexural strain, respectively. ∆R/R0 increased continuously with increasing applied strain till an abrupt drop or rise was detected. The observation in pure geopolymer (black line in Figures 5A and 5B) can be explained by Figure S6, Supporting Information without considering the effects of CNTs. Geopolymer is generally described as a three-dimensional (3D) amorphous network consisting of tetrahedral SiO4 and AlO4 joined at the corners by oxygen atoms.38 Na+ present in the framework balances the negative framework charge carried by the tetrahedral Al (III). A substantial amount of water was introduced into the geopolymer structure during the fabrication to provide the medium needed for the dissolution of MK particles and the transfer of Na+. After the geopolymer was solidified, a certain amount of the water remained in the framework which facilitated the migration of Na+ within the framework during the mechanical loading, resulting in a certain degree of ionic conduction. Since the ionic conduction is associated with the movement of hydrated Na+, the formation of the cracks with compressive or flexural strain may hinder the migration process, leading to the continuous increase of ∆R/R0. A sudden break caused the wires embedded to touch each other under compressive loading, i.e. short circuit, thus abruptly decreased the electrical resistance. On the contrary, the sudden break under flexural loading due to broken circuit caused the sharp increase in the electrical resistance. A higher sensitivity was observed from geop/a-CNTs 0.25 (red line in Figures 5A and 5B) as shown by the doubled GF compared to that of the pure geopolymer. The increase was attributed to the electronic conduction coming from the interconnected CNTs besides the ionic conduction originating from the movement of the hydrated Na+ in pure geopolymer. Two factors may contribute to the electronic


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conduction, i.e., the intrinsic resistance of CNTs and the contact resistance at CNTs junctions. However, the first factor is expected to be negligible because the physical damage of CNTs occurred during mechanical loading was extremely small.42-43 The second factor becomes important and depends on the effective physical contact among the CNTs within the network. In the case of geop/a-CNTs, the cracks hindered the hydrated Na+ migration and broke the contacts between the adjacent CNTs under the applied load. As a result, ∆R/R0 increased and larger GF values were obtained (see Figure 5C with the effects of CNTs). A more pronounced change in GF was observed for geop/SiO2-CNTs 0.25 (blue line in Figures 5A and 5B). The GF of geop/SiO2-CNTs 0.25 under compressive and flexural loading reached up to 663.3 and 724.6, respectively, which were 4.8 and 1.9 times higher than that of the corresponding samples geop/a-CNTs 0.25. The uniform dispersion of CNTs in geopolymer matrix (see Figure 4E) achieved through SiO2 coating facilitated the formation of a well-connected CNTs network throughout the matrix by eliminating large CNTs agglomerates. The unique coating behavior ensured the restoration of the intrinsic conductive nature of CNTs by partial or full removal of the SiO2 layers during the fabrication process. A well-connected electrical conductive network is more effective in detecting the ∆R/R0 caused by the randomly formed cracks under an applied load. In comparison, the localized conductive islands of a-CNTs agglomerates are only sensitive when the cracks are formed within the agglomerates. ∆R/R0 for regions without the conductive islands is solely dependent on the ionic conduction, which gives a much smaller GF. Here, it is worth noting that the GF is only responsible for the detection of the formation of cracks. It is almost impossible to quantify or locate the exact position of the cracks. In summary, CNTs were purposefully coated with amorphous SiO2 layer in the



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consideration of the chemical compatibility with geopolymer matrix. The partial and complete removal of the SiO2 layer upon exposure to the alkaline environment facilitated the dispersion of CNTs in the geopolymer matrix. At the same time, as SiO2-CNTs are comprised of high amorphous SiO2 content, the diffusion and transportation of the dissolved Si complexes occurred during geopolymerization formed strong interfacial interaction between CNTs and geopolymer matrix. The better dispersion of CNTs and strong interface between CNTs and geopolymer matrix resulted in the improvement of the mechanical properties. Furthermore, the removal of the SiO2 coating on the CNTs restored the conducting nature of the CNTs, contributing to ultrahigh self-sensing performance of the geopolymer composites. As a result, both structural properties and self-sensing functions of geopolymer nanocomposites were achieved through this unique interface engineering. Table 1 summaries and compares the GF obtained from the compression tests in this work with the recently reported self-sensing structural materials containing different types of carbon-based fillers. GF obtained from the flexural tests was omitted since the value was not reported in most cases16 and a fair comparison cannot be established. Comparing with the recently reported work, geop/SiO2-CNTs 0.25 produced the highest GF value of 663.3 at significantly lower filler content of 0.26 wt% SiO2-CNTs. The superior self-sensing performance was credited to the conductive network established by the SiO2 coating assisted CNTs dispersion. In comparison, for example, at 15 wt% carbon black (CB), the intrinsically low electrical conductivity and the challenge in forming a connected network due to geometry restriction led to a relatively low GF.44 The well dispersed electrically conductive CF showed higher GF such as 35017 and 32718 in the cement at 0.5 wt%. By doubling the CF content did not prove to be effective in further increase the GF18 and replacing the filler with


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conducting nanofillers such as CNTs provided a solution. The GF of the cement with 1.0 wt% of CNTs was only 130,21 which can be attributed to the improper dispersion of CNTs in the cement matrix. D'Alessandro et al22 improved the dispersion of CNTs by using plasticizer as surfactants and achieved GF of 484 at 2.0 wt% of CNTs. Recently, Saafi et al16 used reduced graphene oxide (rGO) as a nanofiller to enhance the self-sensing performance of geopolymer. Although the rGO with 2D structure has a larger surface area and is easier to form the conductive network at lower content, the GF was only 43.9 at 0.35 wt% of rGO which may be due to the poor dispersion of rGO. These results highlighted the importance of the conductive filler and effective filler dispersion in the fabrication of self-sensing structural materials. With the unique behaviors of SiO2 coating, geopolymer/SiO2-CNTs performed much better as a structural health monitoring self-sensing material.

CONCLUSIONS In this work, the unique behavior of the SiO2 coating on CNTs allowed for the fabrication of geopolymer nanocomposites with ultrahigh self-sensing performance. The SiO2 coating facilitated the dispersion of CNTs in geopolymer matrix at the initial fabrication stage owing to the good chemical compatibility between each other. The SiO2 layer on CNTs was subsequently partially or fully removed to restore the conductive nature of CNTs, resulting in the formation of well-connected electrical conductive networks. The underlying mechanisms on the electrical signal change with respect to ionic conduction and electronic conduction were explored and correlated to the self-sensing performance. The GF achieved reached up to 663.3 and 724.6, under compressive and flexural loading, respectively, with the addition of only 0.25 vol% of SiO2-CNTs, which were at least twice higher than those recently reported self-sensing



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structural materials containing different types of carbon-based filler. Additionally, uniform dispersion of CNTs and strong interfacial bonding between CNTs and geopolymer matrix resulted in ~181.2% and ~21.7% enhancement in flexural and compressive strength, respectively. It is believed that the geopolymer nanocomposite with ultrahigh sensitivity can be used as an ideal self-sensing structural material.

ASSOCIATED CONTENT Supporting Information. Schematic illustration of the synthesis of SiO2-CNTs, TGA and FTIR of r-CNTs, a-CNTs and SiO2-CNTs; schematic illustration of the fabrication

of

geopolymer/SiO2-CNTs

nanocomposite;

TEM

images

of

SiO2-CNTs-Ca (The SiO2-CNTs taken out from calcium hydroxide solution were labeled as SiO2-CNTs-Ca); digital pictures of geopolymer nanocomposites for self-sensing characterization: compression tests and flexural tests; flexural and compressive strain-stress curves of pure geopolymer and geopolymer nanocomposites, flexural strength and compressive strength of pure geopolymer and geopolymer nanocomposites, FE-SEM images of the fracture surface of geop/a-CNTs 0.25 and geop/SiO2-CNTs 0.25. Schematic illustration of the piezoresistive response in pure geopolymer This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] [email protected]


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Acknowledgements The authors would like to acknowledge the funding supported by Nanyang Technological University (NTU) with the grant number M4061124 and the support from School of Materials Science and Engineering at NTU on the present work. The authors also express appreciation to Dr. Yen Nan Liang for the TEM characterization.

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FIGURES

Figure 1. Schematic illustration of sensor performance measuring system: (A) compressive mode and (B) flexural mode. (Series digital pictures of the samples with

Transmittance % (a.u.)

difference formulations were shown in Figure S4, Supporting Information)

a-CNTs-Na

B

SiO2-CNTs-Na

a-CNTs 1640

SiO2-CNTs 1020 880

1387

950

800

1100

1660

2000 1800 1600 1400 1200 1000 -1

800

Wavenumber (cm )

a-CNTs-Na

SiO2-CNTs-Na

a-CNTs

SiO2-CNTs

(002)

10

100

(100)

20

30 2θ

40

Weight (%)

C Intensity (a.u.)

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


50

D 78.9%

80

62.5%

60 40

a-CNTs-Na a-CNTs SiO2-CNTs-Na

20

SiO2-CNTs

0

44.9%

8.8%

200

400

600

Temperature (°C)


800


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

Figure 2. (A) Digital pictures of a-CNTs and SiO2-CNTs with the same CNTs content dispersed in an alkaline silicate solution after sonication; (B) FTIR, (C) XRD and (D) TGA of a-CNTs a-CNTs-Na, SiO2-CNTs and SiO2-CNTs-Na; TEM images of (E) a-CNTs, (F) SiO2-CNTs and (G) SiO2-CNTs-Na. (Schematic illustration shows the three morphological appearances of SiO2-CNTs-Na)


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Figure 3. Schematic representation of the interface between SiO2-CNTs and geopolymer matrix.

Figure 4. FE-SEM images of the cross-sectional area of the fracture surface of geop/a-CNTs 0.10 (A), 0.25 (B) and 0.50 (C); geop/SiO2-CNTs 0.10 (D), 0.25 (E) and 0.50 (F). (Red cycles indicate the agglomerations of CNTs)



A

B

Geop Geop/a-CNTs 0.25 Geop/SiO2-CNTs 0.25

4000

3

3000 3.

2000 =

66

Short circuit

0 0

4

GF

=

4 72

= GF

1 137. GF = = 62.7 GF

2

40

20

GF

1000

Geop Geop/a-CNTs 0.25 Geop/SiO2-CNTs 0.25

60

∆R/R0 (%)

5000

∆ R/R0 (%)

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

6

8

0 0.00

10

Compressive strain (%)

Page 28 of 30

.6

387

.4

G F = 18

0.02

Broken circuit 9.8

0.04

0.06

0.08

0.10

Flexural strain (%)

Figure 5. Fractional change in electrical resistance of pure geopolymer and geopolymer nanocomposites with 0.25 vol% a-CNT or SiO2-CNTs content versus applied (A) compressive strain and (B) flexural strain (Geopolymer is abbreviated to Geop in figures), and (C) schematic representation of the piezoresistive response detected in geopolymer nanocomposites.


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TABLES Table 1. Comparison of GFs (obtained from compression tests) of structural materials containing different types of carbon-based fillers Sample

Gauge Factor

Ref

Cement/15 wt% CB

55.3

44

Cement/ CF and CB (ratios not mentioned)

227

45

Cement/0.5wt% CB and 0.5 wt% CF

17.3

46

Cement/0.5 wt% CF

350

17

Cement/0.5 wt% CF

327

18

Cement/1.0 wt% CF

332

18

Cement/1.0 wt% CNTs

130

21

Cement/2.0 wt% CNTs

484.8

22

Geopolymer (fly ash)/0.35 wt% rGO

43.9

16

Geopolymer (metakaolin)/0.26 wt%* SiO2-CNTs

663.3

This work

CB: carbon black; CF: carbon fiber; CNTs: carbon nanotubes; rGO: reduced graphene oxide *CNTs loading (see Table S1, Supporting Information)



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Table of Contents Graphic


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