Introduction

The investigation and application of film materials with different physical properties are a very important research field that is widely studied. Both thermal and electrically conductive properties of different nano filler composites have been reviewed in the literature1,2. Most researchers are focused on developing thermal and electronic conductive film application of different assembly methodologies with either conductive organic or inorganic metal oxide nanoparticles that yield various properties and applications3,4,5,6,7,8,9. Our research goal is to develop electronic conductive polymer films with thermal insulation properties with potential applications especially for automobiles, buildings and tents.

Residential homes are a dominant source of energy consumption globally. Thus, energy-efficient buildings are critically important to reduce energy consumption and hence to reduce carbon dioxide (CO2) greenhouse gas emissions. Heat leakage is one of the critical assessment factors of whether a building is energy efficient10,11,12.

Composite inorganic (ceramic) nanomaterial polymeric films have many advantages e.g., low cost, easy preparation and ease of tuning the compositions as well as the ability to upgrade the performance properties13,14. Nowadays polyethersulfone (PES) has become a commercially available hydrophobic material that has many desirable characteristics such as good heat-aging resistance and environmental strength, as well as easy processing. However, the natural hydrophobicity of the PES polymer makes it resistant to water self-cleaning15,16. If it is possible to add novel conductive properties to hydrophobic the PES polymer film, it will be more widely applicable. Therefore, efforts have focused on improving the film electronic conductive properties of PES by chemical or physical modifications17,18. There are a couple methods to obtain higher conductive PES films; for example, addition of organic polymer and conductive nanomaterial to improve conductivity and insulation properties19,20,21.

There has been great interest in recent years in investigations of the synthesis of nano-sized materials due to their unique properties compared with those for raw materials, such as magnetic, optical, electrical, mechanical, chemical, and biological characteristics22,23,24. The preparation of novel inorganic-organic hybrid nano-composite films has been of considerable interest over the last two decades. Moreover, the films modified by incorporating nanoparticles inside the polymer matrices can combine the natural properties of both inorganic and organic materials and provide advantages with respect to thermal insulation and water resistance, and suitability to applications in harsh environments25,26.

There are a few conductive inorganic nano materials, such as indium doped tin oxide (ITO) and antimony doped tin oxide (ATO). Recently, the synthesis and stabilization of these nanoparticles, which makes it possible to assemble different nano-composite material films incorporating these homogeneously dispersed solution nanoparticles in solution, have been widely explored27,28,29,30,31,32,33,34,35. ATO is much cheaper than ITO and it also has a wide range of applications including catalysts for the oxidation of phenol36, conductive transparent optical thin films37and heat shields38.

Regarding the research of electron conductive films by incorporating conductive nanoparticles, there are very few studies available in literature. Very recently, a transparent electrically conductive window glass film was prepared by surface coating a dispersion polymer and a nano conductive ITO nanoparticle solution21. This film has shown good conductivity and excellent radiation insulation properties39,40,41, prompting us to conduct a study on the inexpensisve electrically conductive ATO-PES film by incorporation of conductive inorganic ATO nanoparticles to the PES organic polymer matrices. This inorganic/organic hybrid film supposes the presence of unusual properties and performance as a thermal insulation application.

The main objective of this work is to assemble a conductive ATO nanoparticle nano-composite film and to investigate the film performance, conductivity and thermal insulation properties. As is well-known, electrically conductive materials are either metallic or crystalline semiconductor films. Assembling conductive nano-composite films by wrapping the semiconductive ATO nanoparticles into the polymer matrices is a great challenge. Because the polymer is not only a support medium but also an insulator, it can prevent each individual nanoparticle from acting as a separate medium and eventually reduce the electrical conductivity of the prepared film. There are a couple of critical factors in the preparation: first one must synthesize good quality conductive ATO nanoparticles, and second the assembly conditions, procedure and affective factors must be optimized to generate good conductive films. This work was conducted systematically: (1) synthesis and characterization of ATO nanoparticles; (2) preparation and characterization of ATO-PES nano-composite films; (3) thermal insulation performance evaluation of the conductive nano-composite films.

The essential step for the incorporation of the ATO nanoparticles to PES polymer matrices is to assemble homogeneous films to disperse the particles into an organic solution, like the NMP polar organic solvent popularly used to dissolve the PES polymer. A typical selective method is to pick molecules containing both functional groups and hydrophobic tails which can bind to the nanoparticle surface by forming chemical bonds; meanwhile the hydrophobic carbon tail has the capability to stabilize particle suspensions in organic media42,43.

In this study, first, we conducted the synthesis of homogenous uniformed solid ATO nanoparticles that hold excellent electronic conductivity. The synthesized ATO nanoparticles were fully characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and conductivity measurements. Second, the synthesized ATO nanoparticles were dispersed to organic solvents through surface modification and surface functionalization. Third, ATO nanoparticles were introduced to polymer PES in an N-methyl-2-pyrrolidone (NMP) solution and through it the nano-composite ATO-PES film was prepared by casting this solution onto a glass surface and then solidifying it via a phase-separation process. Finally, the generated nano-composite film morphology, conductive properties and insulation performance were fully tested and characterized.

Experimental section

Materials

Samples were obtained from commercial suppliers: tin(IV) chloride pentahydrate (SnCl4 5H2O, Strem Chemicals, 98%), antimony (III) chloride (SbCl3, Riedel-de Haën, 99%), ammonium hydroxide solution (ACS reagent, 28.0–30.0%, Sigma-Aldrich), polyether sulfone (PES Ultrason E6020P with MW = 58,000 g/mol, BASF Company), methanol and N-methyl-2-pyrrolidone (NMP, Sigma-Aldrich) and oleic acid, 90% (C18H36O2, Sigma-Aldrich).

Synthesis of ATO nanoparticles

The antimony-doped tin oxide (ATO) was synthesized using a modified literature procedure31. 43.82 g SnCl4⋅5H2O (0.125 mol) was first dissolved in a 500 ml methanol solution, then 2.58 g SbCl3 (0.0113 mol) was added to the solution. This solution was stirred for 10 min, 70 ml of ammonium hydroxide solution was added under fast stirring and the solution was kept stirring for 2 h. The final pH of the reaction solution was measured to be 7 to 8 using pH indicator paper. Centrifugal force (3000 rpm, 20 min) was applied to allow the resulting precipitation to be separated. The separated white precipitate was collected into two 200 ml bottles and washed with methanol three times with the cycle of addition of 200 ml methanol/10 min, stirring/20 min centrifugation (3000 rpm). The generated white paste (about 81.0 g) was dried in a vacuum oven at 70 °C overnight. After drying, the white soft powder (24.3 g) had converted into dark blue crystalline ATO nanoparticles (19.45 g at a yield of 95.0%) by calcination at 700 °C for 3 h.

Characterization of synthesized solid ATO nanoparticles

The size and crystalline phase of ATO nanoparticles were determined by field emission scanning electron microscopy (FESEM) and X-ray powder diffraction (XRD) using a Hitachi S4800 high resolution scanning electron microscope and an INEL XRG 3000 X-ray diffractometer equipped with a Cu Kα radiation source (λ = 1.54 Å). A JEOL 2010 TEM was used with a LaB6 electron gun and a computer-driven CCD camera. Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR): Varian FTS7000 FTIR Image System using a 25 mW diode laser (780 nm). Powder samples were stuck to a transparent window sample holder. Each spectrum was acquired in reflectance mode at a spectral resolution of 8 cm × 1 cm with 64 scans coadded. The conductivity of ATO nanoparticles was measured using a Potentiostat/Galvanostat Model 263 A, Princeton Applied Research. The electrical volume resistance of the ATO nanoparticles and the nanocomposite films was evaluated by determining their current-voltage (I-V) characteristic. Two electrodes were made using 1 cm by 1 cm copper tape. A potentiostat (Model 263 A) was employed to apply a voltage with a ramp of 1 mV/s over the sample, and the electrical current was recorded. The resistance (R) of the sample was obtained from the reciprocal of the slope of the I-V curve. The resistance and resistivity (ρ) can be interconverted using the formula ρ=(R×A)/L (A: cross-sectional area and L: length). The solid ATO sample characterization procedure is typically used for our synthesized dark blue color ATO powder to differentiate the sample holders according to the selected instrument. All measurements were conducted at 22 °C with low humidity (20%).

Preparation of ATO/PES blended films

1.25 g oleic acid and 2.5 g ATO blue powder were mixed with 20.0 ml NMP in a stainless-steel container. The solution was sonicated for 40 min at 40% amplitude using a big probe Sonicator in an underwater bath generating the homogenous oleic acid capped ATO solution. The solution was divided into five parts using a balance with each part containing 0.5 g ATO and 0.25 g oleic acid. 0.73, 0.97, 1.8, 4.3 and 9.3 g PES polymer powders with proportional NMP solvents were added to these five aliquots respectively yielded five ATO-PES blended homogenous solutions. Afterwards, the casting solutions were kept at room temperature for at least 12 h to release the air bubbles from the solution. The generated polymer ATO-PES solutions were scattered on a 250 mm thick glass plate using a TQC automatic film-applicator. The glass plate was dipped in a non-solvent water bath (distilled water at 25 ± 0.1 °C). After primary phase separation and film formation, the film was stored in fresh distilled water for at least 24 h to guarantee complete phase separation and stored in distilled water until use. The nominal composition of the five ATO-PES and oleic acid blended solutions was 5.0, 9.9, 19.6, 29.0 and 33.8% ATO, respectively, but solid ATO analysis showed these casted final five individual dried films contain 4.5, 9.8, 18.8, 28.7 and 32.2% ATO, respectively.

In a similar procedure a low molar ratio of oleic acid and ATO PES nano-composite film was prepared. Typically, a 5.0 g ATO blue nano powder and 0.942 g oleic acid was added to a 30 ml NMP solvent, then sonicated for 40 min and finally 7.6 g PES powder was added. The ATO-PES nano-composite film was cast using this homogenous blended solution. Nominally, this solution contains 37.2% ATO nanoparticles but analysis of the dried film showed 35.8% weight% ATO particles in the final generated film.

Characterization of the prepared films

The prepared ATO-PES film was characterized by scanning electron microscopy, atomic force microscopy, and thermogravimetric analysis (TGA). Conductivity measurements were undertaken using a Potentiostat/Galvanostat Model 263 A, Princeton Applied Research, combined with a home-designed device (two conductive copper tapes separated by 0.5 cm, stuck to the glass surface); the measured material fine powder covered the surface of the copper taped glass. Then a second piece of glass was put on and tightly held together (see Results section, Fig. 3).

Thermal stability evaluation

The thermal stability of the synthesized films was measured using TGA, which measures the weight and rate (velocity) of change in the mass of a sample as a function of temperature or time in a controlled atmosphere. 10 mg of film samples were put in a sample holder and loaded into the TGA Q500 (TA Instruments). The temperature was increased to 1000 °C with a ramp of 10 °C/min while the weight reduction was recorded. All measurements were conducted under a nitrogen atmosphere.

Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM)

Field emission scanning electron microscopy (FESEM) provides images of sample surfaces, yielding qualitative information on the dispersion and distribution of nanoparticles in the polymer matrices. Prior to the analysis, the films were coated with a thin film of gold. Surface images of the films were obtained using a JEOL 6301 F FESEM model. Films were imaged at magnifications of up to 70,000 times.

AFM surface scanning and current mapping of ATO-PES nano-composite conductive films: AFM topography and current mapping were performed using a Bruker Icon system (Santa Barbara, CA) operating in the PF-TUNA mode. A commercially available Pt-Ir coated conductive probe, ANSCM-PT (AppNano, CA) with a typical spring constant of 3 N/m, resonance frequency of 60 kHz and a radius of curvature of 30 nm was used to perform the topography as well as the current measurements. The current sensitivity was calibrated using a standard resistance and other parameters optimized while performing the current measurements. Nanoscope analysis software V.1.40 was used to process the AFM data, to remove noise, and to calculate the root mean square roughness Rq.

Thermal radiation insulation test

The ATO polymer film thermal insulation performance assessment was conducted using home-built test devices as shown in Scheme 1. In principle, it is a comparison of performance results under the same tested conditions. IR heater lamps (250 W with the IR wavelength ranging from 750 nm to 1 mm) radiated on top of the film while the temperature was monitored with a digital thermometer at the bottom, as shown in Scheme 1.

Scheme 1
scheme 1

Home-built film thermal insulation performance test device.

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Results and discussions

Synthesis and characterization of ATO nanoparticles

The synthesis of high-conductivity ATO nanoparticles is a critical step for the success of this work. As seen from the simulated hydrolysis in aqueous solutions (Fig. S1 in Supporting Materials), it is difficult to control the synthesis of a homogeneous antimony-doped tin hydroxide phase. The reaction precursors of Sn and Sb only exist in a very concentrated acidic solution and the Sb and Sn hydrolysis pH range is quite different (Supporting Materials). ATO nanoparticles were synthesized in an organic solvent methanol medium using a modified procedure31. Precursors of SnCl4⋅5H2O and SbCl3 were selected, and the co-precipitation reaction was carried out in methanol with a one-step addition of a 28.0% NH4OH aqueous solution as described in the experimental section. The white precipitate was isolated by centrifugal force and washed three times with a methanol solvent. The dried white powder was processed at 700 °C to convert to the crystalline phase and yielded a dark blue ATO powder.

Fig. 1
figure 1

XRD patterns of synthesized solid blue ATO nanoparticles (JCPDS SnO241-1445)31.

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The final ATO nanoparticles were characterized by XRD (Fig. 1). The diffraction patterns of the synthesized ATO agree well with the cassiterite SnO2 (JCPDS card number SnO2: 41-1445)31; see also additional data in Fig. S2 in Supporting Materials. No other phases were detected, indicating that all antimony ions come into the crystal lattice of SnO2 to substitute for tin ions. All diffraction peak lines are broadened, indicating the presence of nanosized crystalline particles in the samples. The average nanoparticle diameter of ATO calculated from the XRD data using the Scherrer equation is 15.00 ± 0.3 nm.

Characterization of ATO nanoparticles

The ATO dark blue powder was added to methanol and the mixed solution was sonicated for 10 min. The generated suspension solution was drop-coated to a silicon wafer stuck to the FESEM sample holder. It was kept in air for 2–3 days to completely evaporate the methanol solvent. HR-FESEM images were taken at three different magnifications as shown in Fig. 2a–c. It can be seen from these images that the ATO nanoparticles are small and uniform. The size distribution of the primary particles, as shown in Fig. 2d, is homogenous and the measured histogram of the diameters obtained from FESEM using the ImageJ software is 14.91 ± 1.82 nm (Fig. 2d), which agrees with the average size obtained from the XRD diffraction data. Characterization of the ATO nanoparticles was also conducted by TEM; the TEM image and selected area electron diffraction (SAED) data are shown in Fig. 2e, f. Uniform ATO formed around 15 nm that agrees with the FESEM results. The SAED data clearly show a crystal diffraction pattern identical to that expected for the ATO crystal phase and also agrees with the XRD data indicating that ATO has successful been synthesized.

The ATO properties, for example conductive performance, are largely dependent on the degree of antimony doping. For example, the level of Sb doping in the SnO2 phase, is an important factor, then we must also consider whether the final doped phase is homogenous or not. These two parameters of composition and phase homogeneity are easily determined by energy dispersive X-ray (EDX) analysis. Different spot regions of the specimens were selected and analyzed by EDX. The identical results shown in the EDX spectra (Supporting Materials Fig. S3) for all the selected spots indicate that these two parameters are constant for the prepared ATO sample. The atomic composition of Sn/Sb molar ratio of the sample is 90.3%/9.7% (EDX) and 90.2%/9.77% (ICP), which are very close to the initial mixed molar ratio of the precursors SnCl4/SbCl3, 91.7% Sn/8.3% Sb (Data see Supporting Materials Fig. S4) indicating that hydrolysis in organic solvent is a promising approach for the formation of a good doped ATO phase. The final yield is 95.0% and shows the percentage of tin and antimony changed slightly during the preparation, purification and isolation processes. FESEM results show that the nanoparticles obtained using the modified hydrolysis are smaller and more uniform than the previous report data31.

Fig. 2
figure 2

High resolution FESEM and TEM images of synthesized conductive (a): FESEM 100k, (b): FESEM 300k, (c): ATO FESEM after surface modification, (d): ImageJ-measured histogram of blue ATO nanoparticles, (e) TEM and (f) selected area electron diffraction (SAED) pattern taken from the TEM picture.

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The ATO nanoparticle conductivity is a critical factor for the assembly of high conductivity ATO-PES films. We took the effort to optimize the preparation procedure to synthesize the highest possible conductive ATO nanoparticles. The conductivities of our samples were measured using our home-built sample holder (see Fig. 3 and description in "Experimental section"). Conductivity plots (potential vs. current) are shown in Fig. 3. The resistances (R) of the sample can be obtained from the slope of the I–V curve; R is equal to 350 Ω. Under the same conditions, the conductivity of commercial ATO nanoparticles were measured using the same device; that for our ATO nanoparticles has increased by a factor of 138 compared to that for the commercial ATO nanoparticles (R = 48100 Ω). This is attributed to both the efficient homogenous dope reaction in methanol and the high-temperature crystallization process.

Fig. 3
figure 3

Conductivity measurements of synthesized ATO nanoparticles and a commercial ATO sample measured under the same conditions. Top graph describes the sample measurement conditions.

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Conductivity characterization of synthesized ATO nanoparticles

As is well-known, conductivity is closely related to the surface radiation reflection property20,21. That for the synthesized ATO nanoparticles were assessed using an ATR-FTIR spectrometer and compared with the conductive indium tin oxide (ITO) under the same conditions (Supporting Materials Fig. S5). These results indicate that ITO is showing better conductivity than ATO, but that the surface reflection property of ATO is similar to that for ITO. Our ATR-FTIR spectra indicate that ATO is very conductive, consistent with the conductivity measurement results.

Dispersion of ATO nanoparticles to organic media upon surface modifications

A couple of dispersion reagents were selected to perform the test, such as commercialized organic phase small copolymer reagents BYK-106, BYK-107 and some small molecule solvents such as oleic acid (C18H34O2) and oleylamine (C18H37N). It has been determined that nanoparticle conductivities are sensitive to two parameters, size of the capping molecule and concentration of capping molecule. For example, copolymer agents of BYK-106 and 107 as ATO nanoparticle surface capping molecules were not suitable for the assembly of conductive films even if the modified particles were very stable in an NMP solvent. We realized it is because the large capping molecule completely isolates the conductive nanoparticles by a non-conductive organic phase. Therefore, more of our attention was focused on small molecules. The 6-, 12- and 18-carbon-chain amines of hexylamine, dodecylamine and oleylamine were tested and the results clearly show that short carbon-chain amines are more conductive than long carbon chains in ATO-PES blended solutions, but the same trend does not appear for the films. After analysis of the final remaining solid nanoparticles inside the dry films following the casting and phase separation processes, the results show that the lower nanoparticle weight% remains in the dry films using shorter-chain capping agents. That means the shorter ligand surface modified nanoparticles have less stability. It is sufficient to maintain more nanoparticles in films by selecting long carbon chains such as dodecylamine (12 carbons), oleylamine (18 carbons) and oleic acid (18 carbons).

Oleic acid is a popular capping ligand in hydrophobic organic phases and is commercially available at low cost, so it was selected as a surface modified reagent for ATO nanoparticles44,45. The optimized weight ratio of oleic acid to ATO nanoparticles was explored. As shown in Fig. 4, five solutions with the same amount of ATO (1 g) in 20 ml NMP with different amounts of oleic acid, 0, 0.25, 0.50, 0.75 and 1.00 g respectively, were investigated. The conductivity measurements were performed under the same conditions for five different solutions after 40 min of sonication for each individual solution. For each solution, the current (I) vs. potential (V) graph was measured and the plot fit to obtain the solution resistance (W) from the slope (= 1/R). The plot of R vs. oleic acid/ATO weight ratio is shown in Fig. 4 and it shows that the lowest R is at the ratio 0.5; R is much higher for ratios smaller than 0.5 and increases slightly for ratios larger than 0.5. This indicates that the weight ratio of oleic acid/ATO at 0.5 is the optimum condition for the highest conductivity or lowest resistance of oleic acid surface modified ATO nanoparticles in solution. FESEM images were taken after surface modification with oleic acid (Fig. 2c and more images in Fig. S10 in Supporting Materials) and it is clearly showing some distance between each individual particle compared to that without surface modification (Fig. 2c).

Oleic acid (cis-9-octadecenoic acid) has been widely used for the surface modification of inorganic nanoparticles, because the carboxylic group of oleic acid interacts with the nanoparticle surface and the long alkyl chain acts as a dispersion drive force to form a homogeneous phase with hydrophobic polymers by the principle of similarity dissolves interaction force46. In this system, the oleic acid binds to the ATO nanoparticle surface through the carboxylic group and the long alkyl chain interacts with the PES polymer chain to disperse the binding nanoparticles into the organic polymer matrices as described in the molecular model (Supporting Materials Fig. S6)47.

Fig. 4
figure 4

Optimized maximum conductivity of oleic acid surface modified ATO nanoparticles in an NMP solution.

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Preparation of ATO and PES blended films

The ATO-PES nano-composite films were prepared using the procedure described in the experimental section. First the ATO powder was added to a steel-metal beaker, then the oleic acid dispersion agent and finally the NMP solvent were added. Applying only magnetic stirring did not yield a mixed solution indicating that a strong force such as ultrasound wave is needed to break the softly agglomerated nanoparticles. Upon ultrasound through a probe sonicator for 40 min, a homogenous dark blue solution that is stable for a couple hours without phase separation was generated. The PES polymer was added immediately to the ATO-oleic acid NMP solution after sonication. This solution was kept under magnetic stirring at 60 °C overnight; the PES powder dissolved completely and a high viscosity dark blue solution that was stable for a long time with no phase separation was formed, indicating that the oleic acid-capped ATO nanoparticles are being incorporated into the PES polymer matrices and forming homogenous ATO-PES nano-composite blended solutions. This generated solution was used to cast the films as described in the experimental section. The liquid films quickly solidified after immersion in distilled water; however, it takes at least 24 h to finish the phase separation to eliminate the NMP solvent from the water medium.

The five different ATO compositions of PES and oleic acid blended solutions containing 5.0, 9.9, 19.6, 29.0 and 33.8% ATO, respectively, were prepared following the procedure in the "Experimental section". These five solutions were used to cast the films; they were immersed in pure water for 24 h for phase separation. After the water process, the films were collected and dried overnight at 70 °C inside a vacuum oven. The solid ATO remaining in the films was analyzed using thermal decomposition at 700 °C in a furnace for 2 h; by weight, about 75.0 mg of each dried film was placed in quartz boats. The final analysis data show the solid ATO compositions of the five films were 4.5, 9.8, 18.8, 28.7 and 32.2%, respectively, indicating a loss of ATO during the film preparation process.

Conductive characterization of ATO-PES blended nano-composite films

Five samples (4.5, 9.8, 18.8, 28.7 and 32.2% of solid ATO) ATO-PES nano-composite films prepared using the solution of oleic acid capping ATO nanoparticles (oleic acid/ATO ratio at 0.5) were obtained under the same conditions; the film thicknesses are all around 0.22 mm. Conductivity measurements show that only the 32.2% ATO film has slight conductivity while the others are not conductive (Fig. 5a). However, bottom conductivity measurements of the film bottom show that both the 28.7 and 32.2% ATO ATO-PES films are conductive, although that for the latter is much higher, and the top films do not show any conductivity (Fig. 5b). These results demonstrate that to have a conductive ATO-PES film the ATO nanoparticles inside the film must be at least 32.2%. The conductivity of the film is much lower than that measured from the ATO-oleic acid solution. That indicates that the PES polymer also plays a role in insulating the ATO nanoparticles from contact and further reduces the film conductivity. It is notable that the film conductivity is not homogenous, with high conductivity at the bottom and no conductivity at the top. The capping reagent of oleic acid is a surfactant which may cause ATO nanoparticles to form gradients from top to bottom during the phase separation process as it was added in the liquid cast solution. The conductivity results prove that this occurs after removing the solvent to become solid films. To maintain high and homogenous conductive films, it seems necessary to further reduce the capping reagent in the film. Therefore, the ratio of oleic acid to ATO was reduced to 0.19 from 0.5; the film using the former ratio shows much higher conductivity on the top side than does the latter. Thus, the ratio 0.19 was used for all the film preparations.

Fig. 5
figure 5

Conductivity measurement results of different percentages of ATO nanoparticles inside the PES polymer films under the same conditions. a Conductive measurement through film thickness 0.22 mm. b Conductive measurement through bottom of film for a 0.5 cm distance.

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Fig. 6
figure 6

Conductivity of optimized ATO-PES nano-composite films (a): conductivity of top, bottom and middle; b conductivity at different distances, and (c): correlation between resistance and measured film distances.

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A good conductivity film was prepared by casting an NMP solution which contains 5.0 g ATO nanoparticles, 0.942 g oleic acid and 7.5 g PES polymer. The nominal ATO nanoparticle concentration inside the dry film was 37.2%, but analysis on the final dry film indicated it was actually 35.8%. The prepared ATO-PES nano-composite conductive film (oleic acid/ATO ratio 0.19) containing 35.8% ATO nanoparticles was measured using the home-built device described above. The thickness of the dry film was 0.25 mm; the conductivity of film at the top, bottom and middle was studied. The measured current vs. potential graphs are shown in Fig. 6a. These show that all are conductive, but that the middle is more conductive than either the top or bottom because the distance between the two copper tapes is only 0.25 mm, much shorter than the 0.5 cm used for the top and bottom measurements. The bottom is slightly more conductive than the top indicating that the densities of ATO nanoparticles vary slightly following phase solidification process; perhaps it loses slightly more nanoparticles at the top than at the bottom.

The homogeneity of the conductive film and particularly its top side is also an important contribution to the film performance. We investigated the conductivity of the film top to see whether it is homogenous or not. First the conductivity of different regions of the top surface was measured and it is identical to the I vs. V graph obtained. Second, the conductivity was measured with varying distances of the two copper tapes. All graphs obtained with different distances are shown in Fig. 6b, and the plot of resistances obtained from each slope vs. distance (d) is a linear correlation Fig. 6c. This demonstrates that the resistance shows a linear increase with distance, and it proved that the film conductivity is homogenous on the top surface.

Compared to the conductive results of 0.19 and 0.5 (oleic acid/ATO ratio), it is clear after reducing the oleic acid, the film becomes more homogenous based on its conductivity measurements.

TGA measurements to assess the film’s thermal stabilities

TGA measurements are used primarily to determine thermal and/or oxidative stabilities of materials as well as their compositional properties. It is especially useful for the study of polymeric materials, including films and fibers. The TGA analysis results of the prepared ATO-PES nano-composite films including those with different percentages of ATO nanoparticles are given in Fig. 7. The casting solutions were prepared by mixing a calculated amount of oleic acid/ATO solution with a calculated amount of PES polymer, so the high percentage of these nano ATO PES films should also contain a relative high percentage of oleic acid (Inset: percentage of ATO as listed in Fig. 7). The initial weight loss at temperatures between 200 and 400 °C is proportional to the percentage of added organic dispersion agent oleic acid in the casting solutions. Hence, this part is very much related to the decomposition of oleic acid inside the films, which is in the expected range. The high percentage of added ATO is in proportion to oleic acid, so it shows more mass loss for high ATO films (Fig. 7). The second part of the weight loss occurs in the 415 to 600 °C range, which is close to the decomposition temperature of PES film40,41. The pure PES film started decomposing at 415 °C; addition of ATO to PES films increased the decomposition temperature by 24 to 51 °C. The third part of the weight loss in the high-temperature range of 600 to 1000 °C was relatively slow with the temperature increase.

The overall weight loss is much more modest for PES/ATO film in comparison with bare PES film, which implies that the incorporation of the ATO nanoparticles within the matrix of PES polymers makes it more thermally stable than the original PES film. This observation can be attributed to the fact that the presence of the inorganic nanoparticles within the polymer matrix decreases the chain mobility of the host polymer and thus enhances its resistance to thermal decomposition.

The calculated DTA data from TGA is shown in Fig. S7 (Supporting Materials). These results indicate that the ATO-PES nanocomposite films behave as an endotherm in the overall period (Fig. S7a), but the small peaks indicate exotherm behavior (Fig. S7b) relative to the pure PES film. The ΔT and endotherm range is likely proportional to the film weight% of ATO nanoparticles, indicating ATO-PES nanocomposite films are more thermally stable than PES film.

Fig. 7
figure 7

TGA of ATO-PES nano-composite films with different percentages of ATO nanoparticles in the films. Measurements were conducted under a nitrogen atmosphere.

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The fact that the nanocomposite ATO-PES films have higher decomposition temperatures and greater thermal stability may be attributed to the induced IR reflectance and thermal conductive properties of ATO nanoparticles. The 32.2% ATO-PES film is measured to have very high conductivity and reflectance in a broad range of near IR (NIR) and IR wavelengths. This attests to the original hypothesis that use of ATO nanoparticles can deflect heat from the core of the film, effectively keeping the interior of the polymer film relatively shielded from external heat, increasing the longevity of a polymeric films under high-temperature conditions.

FESEM and AFM characterization of ATO and PES blended films

A homogenous conductive ATO-PES nano-composite film was prepared by casting a 37.2% ATO PES polymer NMP solution and ultimately 35.8% ATO nanoparticles were present within the dry film. This prepared ATO-PES nano-composite film was fully characterized by FESEM and AFM.

Fig. 8
figure 8

FESEM images of optimized ATO-PES nano-composite flims, cross-section (a) and topography at different magnifications (b–f).

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It is important to investigate the film morphology through images of both the top and the cross-section. The FESEM images of the cross-section and the surface morphologies of the ATO-PES nano-composite blended films are shown in Fig. 8a–d; the ATO nanoparticles are clearly seen both from the surface and the interior. The FESEM images show that most of the ATO nanoparticles are hidden in the polymer matrices, but some remain on the surface. The ATO nanoparticles are less homogenous in the polymer matrices; some agglomerated nanoparticle block/slug or clusters also present on the top surface (Fig. 8e, f).

The film surface details were also investigated using AFM. The surface roughness scanning and current mapping was performed using a Bruker Icon system (Santa Barbara, CA) operating in the PF-TUNA mode. The topography image shows agglomerates of densely packed ATO nanoparticles embedded into the polymer matrices inside the film. The root mean square roughness (Rq) of the surface, generated from AFM topography image data, is around 40 ± 2.5 nm (Fig. 9a), and the corresponding current map shows that current is only present in the vicinity of the nanoparticles (Fig. 9b). The map indicates that the current is nearly homogenous on the film surface and that the current areas somewhat overlap. That is critically important and allows the film to be conductive through the surface, thickness and bottom. The observed AFM current map agrees with the conductivity measurement results from the potential-current spectrometer discussed above.

Fig. 9
figure 9

AFM topography and current mapping of the film surface containing 35.8% ATO nanoparticles inside the film. a Film topography image and (b) film current mapping image.

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The percolation threshold is around the weight% of 33.8% ATO nanoparticles inside the dried film, based on the conductivity measurements (Supporting Materials Fig. S8). As the weight% reached 33.8%, the film conductivity increased greatly at or over this composition point. The film conductivity is a combination of dispersion of ATO nanoparticles and aggregation. The suspension of oleic acid binding ATO nanoparticles in liquid solution showed the lowest resistance at an oleic acid/ATO weight ratio at 0.5, however as the ratio was reduced to 0.19, the same ATO weight in the film and using less dispersion agent of oleic acid, its conductivity increased dramatically, indicating that less dispersion agent with more aggregation of ATO nanoparticles in the film is favorable to its conductivity. This agrees with the film AFM current mapping results and FESEM images of the top film, which indicated that aggregation ATO nanoparticles as a current bridge allows the current to pass through the whole film. This is a good point to assemble the more conductive performance film. So, this assembled film is more like a homogenous ATO distribution on a large scale (micrometer) and more of an agglomeration distribution on a small scale (nano scale). Our results show better conductivity with nano scale agglomeration of ATO because it enhances the opportunity to bridge different individual ATO nanoparticles within the film to allow current to pass.

ATO-PES nano-composite film thermal insulation test results

After a few steps the ATO-PES conductive film was assembled successfully. The surface reflection property for its final homogenous optimization was measured by ART-FTIR spectroscopy in comparison with the no-ATO PES film (Supporting Materials Fig. S9). The ATO-PES film clearly showed the reflection performance in the far-IR region indicating this ATO-PES nano composited film has thermal insulation properties.

The ideal application of this thin film is as roofing or as tent material in tropical regions, where many streets and beaches are covered with various types of tents to avoid direct solar radiation. To explore the possible application of organic ATO nano composite film material as a thermally stable solar block to increase the thermal insulation, the performance of the assembled ATO-PES thin film was tested under similar conditions as required for applications such as tents, as described in the experimental section (Scheme 1). The temperature difference of two films with or without ATO was assessed and the correlation of temperatures versus radiation time is shown in Fig. 10. The temperature on the film reached 106 °C in 10 min, then the temperature below the films slowly increased. However, the temperature below the ATO film increased more slowly than that without ATO; after 10 min, the temperature difference was about 3.2 °C. This clearly shows that ATO films present better thermal insulation properties, as expected for a conductive film.

In three steps, the organic-inorganic PES-ATO nano composite film was successfully assembled. First step, very conductive ATO nanoparticles that showed much better conductivity than commercial ATO nanoparticles were synthesized. Secondly, by varying the ratio of OA/ATO and studying the correlation of this ratio and the conductivity, an optimized OA/ATO ratio was found. Then, by assessing a series of five different ATO loading PES-ATO films, the minimum ATO loading percentage was determined to assemble a conductive film. Final, by combining the tuning of both OA and ATO loading percentages as well as PES loading weight, the optimized homogeneous conductive film preparation composition was finalized, and a good quality of conductive film was assembled.

Fig. 10
figure 10

Temperature versus radiation time for the 22 mm ATO-PES films.

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The overall weight loss for the PES/ATO film is different from that for the bare PES, implying that the incorporation of the ATO NPs within the matrices of PES polymers makes them more thermally stable (see Fig. 7). This behavior can be attributed to the presence of inorganic NPs within the polymer matrices which decreases the chain mobility of the host polymer and thus enhances its resistance against thermal decomposition48,49,50,51.

From the TGA weight loss results of four different percentages of ATO NPs/PES films (Fig. 7), one can clearly see that the lowest percentage ATO NPs/PES film, 4.5%, is the most thermally stable, since the decomposition temperature decreases with increasing ATO weight%. However, once the ATO weight% reaches 32.2% and the film becomes conductive, then its decomposition temperature increases again. This can be attributed to the conductive film allowing induced IR reflectance and thermal conductive properties which enhance its thermal resistance20,47.

Conclusion

Conductive crystalline uniform ATO nanoparticles were synthesized and fully characterized, focusing on their conductivity properties. ATO nanoparticles were dispersed into an NMP solution using oleic acid as a surface modified reagent. ATO-PES nano-composite conductive films have been assembled through this blended ATO-PES solution. Five different ATO composited films (4.5, 9.8, 18.8, 28.7 and 32.2%) were prepared and studied systematically. The combination of compositions of PES, ATO and oleic acid ratios was optimized. All our presented results demonstrate that a conductive ATO-PES nano-composite film has successfully been assembled. Optimized conditions to assemble a homogenous high conductivity film were an oleic acid/ATO weight ratio of 0.19 with 35.8% ATO remaining in the dry film. The current map shows that the current is near homogenously distributed and overlaps on the film surface, which is consistent with the conductivity measurements. The conductivity of the film was investigated at both large (cm) and micro (nm) scales to observe the current flow and micro current mapping. The optimized ATO-PES film shows surface reflection properties in the far-IR region. Our assessment indicates that this organic-inorganic nano composite film performed as a good solar radiation insulation blocker and has several potential uses, including as building material and as tent radiation insulation material.