Thermophysical & photothermal characteristics of surfactant free graphene oxide,functionalized graphene oxide and reduced graphene oxide nanofluids

Abstract

Graphene nanostructures-based dispersions, owing to their improved thermophysical and optical characteristics are being actively explored for use in thermal energy conversion and management. Here, thermophysical and photothermal characteristics of graphene oxide (GO), functionalized graphene oxide (f-GO), and reduced graphene oxide (rGO) based nanofluids (without surfactants) of various concentrations (0.05–0.23 wt.%) are evaluated. The structural, morphology and stability of the nanofluids are determined by XRD analysis, Transmission electron microscopy, UV- visible spectroscopy and zeta potential analysis. Subsequently, thermal conductivity and contact angle measurements are performed. Photothermal behaviour is investigated experimentally. Overall, it is found that among the prepared nanofluids, the f-GO nanofluids performed better compared to GO and rGO nanofluids. Maximum stability of f-GO nanofluids is obtained at 0.05 wt.% while the thermal conductivity and photothermal response is best obtained at 0.23 wt.% concentration.

Keywords

nanofluids photothermal behaviour graphene oxide functionalized graphene oxide reduced graphene oxide

Introduction

As per the International Electronic Manufacturing Initiative (iNEMI) technology assessment, control of electrical device's temperature is the major bottleneck in realising compact and high-performance electronic devices.¹ The main challenges are irregular heat dissipation and high heat flux. There is an inability to remove high heat flux due to insufficient heat transfer characteristics of the coolants. Furthermore, in the realm of solar thermal systems, efficient photo-thermal energy conversion and its transportation have been the two major long-standing issues. Therefore, developing low-viscosity thermally conducting heat transfer fluids (HTFs) capable of transferring heat at high rates is required. At the same time, these should be non-flammable, non-toxic, non-corrosive, and thermally (and chemically) stable to be deployed under real-world conditions. Recently, ‘nanofluids’ (stable dispersions of nanoparticles in base fluid), have been shown to possess improved thermo-physical and optical properties. For solar thermal applications, particularly in direct volumetric solar thermal platforms, the nanofluids have proven to be quite useful for photothermal energy conversion.²

Nanoparticles may be metallic, non-metallic, metal oxide, metalloid, etc., and the base fluid may be any liquid such as water, ethylene/propylene glycol, engine oil, etc. To enhance the stability of nanofluids, researchers have employed various types of surfactants. In this direction, the dispersion behaviour of metal based nanofluids AuNPs-water, Ag-oil, Cu-water, CNT-water had been explored^3–6 but the prepared nanofluids were reported to have poor stability. In another study, metal oxide-based ZnO-EG, TiO₂-water^7,8 and hydride-based Al₂O₃-MWCNT-water, Cu-Zn-water^9,10 nanofluids also showed poor stability which hampered the performance of nanofluids.

Several factors influence the nanofluids stability, viz., nature of nanoparticles, particle size, shape, concentrations and pH value, Brownian motion etc. Smaller size and spherical shaped nanoparticles reduce the potential of nanoparticles to aggregate.¹¹ Conversely, when the particle concentration is lower, then the separation distance of particles will be higher, which lowers Van der wall forces of attraction - resulting in stabilisation of nanofluids. The electrical charge density on particle surface is governed by the pH of the nanofluids and affects the nanofluids stability.

Because of highest thermal conductivity of two-dimensional graphene (approx. 5300 W/m K), the graphene based nanofluids are being actively studied. In literature, a variety of nanofluid of GO based variants have been reported but these mostly involve use of surfactants.^12–14 However, at high temperatures, the surfactant molecules start to detach from nanoparticle surface, hence rendering the nanofluid unstable.^15,16 A study¹⁷ reported graphene - distilled water based nanofluid but due to hydrophobic nature of graphene, the prepared nanofluid was stable only upto 24 h. Also, stability of graphene nanofluids was improved by adding surfactant^18,19 but prepared nanofluids were stable for short interval . Further graphene nano-platelets (GNPs) and functionalised graphene nano-platelets (f-GNPs) based nanofluids have been reported to be stable for 15 days and 240 h respectively.^20,21 Also, these nanofluids were shown to have enhancements in thermal conductivity values relative to the base fluids (enhancements on the order of 21%, 16%, and 56% for GNPs/EG, GNPs/H₂O, and f-GNPs/H₂O nanofluids, respectively).

Although graphene based nanofluids show promising results, further investigations are warranted to truly understand and quantify these especially for photothermal energy applications. Graphene variants have been used for energy storage applications.^22–25 In particular, it is imperative critically investigate the thermophysical and photothermal behaviour of graphene based nanofluids. Specifically, in the present work, nanofluids of graphene variants viz., graphene oxide (GO), functionalized graphene oxide (f-GO) and reduced graphene oxide (rGO) are synthesised and compared for the thermophysical and photothermal properties.

Methods

Materials

For the synthesis, Graphite powder (99.99% purity), Hydrogen peroxide, Sodium nitrate (NaNO₃), Sulphuric acid - 98% (H₂SO₄), L-Ascorbic acid (C₆H₈O₆) (99%), and Potassium per magnetic (H₂SO₄ were used as obtained from Sigma - Aldrich Ltd Further, Otto Chemie Pvt. Ltd supplied 2-Aminoterpthalic acid (C₈H₇NO₄).

Synthesis of GO, f-GO and rGO nanoparticles

Modified Hummer's method²⁶ was adopted to synthesise GO. Briefly to synthesise graphene oxide (GO), the procedure begins with dispersing 6 g of graphite powder and 3 g of NaNO₃ in 130 ml of sulphuric acid (H₂SO₄), followed by cooling in an ice bath for one hour. Then, 18 g of KMnO₄ is gradually added while stirring for 45 min. After removing the ice bath, the temperature is increased to 35 °C, and the mixture is stirred continuously for two hours. To reduce heat dissipation, 250 ml of deionised water (D.I.) is slowly added, and stirring continues for an additional hour. Next, 20 ml of H₂O₂ is introduced to the mixture and stirred for 15 min. The mixture is then washed with distilled water and centrifuged at 3000 rpm. The resulting filtrate is washed three to four times with D.I. water and once with ethanol, yielding graphene oxide (GO) powder.

The f-GO nanoparticles were synthesised by using 2-Aminoterpthalic acid.²⁷ In short, take 30 mg GO and 15 mg 2-Aminoterpthalic acid in 20 ml D.I. water and stir both the mixture for 30 min. Then mix them together and raise the temperature upto 50 °C for 60 min. Wash the precipitate (3–4 times) with D.I. water and remove unwanted impurities by centrifugation at 10000 rpm. Dry the above pellets overnight at 60 °C. Powder form of functionalised graphene oxide (f-GO) was obtained.

rGO was synthesised by the reducing GO (1 mg/ml) with suitable reducing agent i.e., ascorbic acid (250 mg in 50 ml D.I. water).²⁸ Both solutions were put on stirrer separately for one hour and before mixing them together. The resulting suspension was raised to 95 °C on continuous stirring for one hour. The reaction is allowed to stand for 12 h at room temperature. Filtrates were washed multiple times and then finally dried at 70 °C to obtain powdered rGO. Synthesis steps of GO, f-GO and rGO nanoparticles are shown in Figure 1.

Preparation of GO, f-GO and rGO nanofluids

Probe sonication (LABMAN, PRO-650, having 650 W output power and 50 Hz frequency power supply) was employed for dispersion of GO, f-GO and rGO nanoparticles in D.I. water. Sonication duration is 4 h with 5 s ON and 3 s OFF sonication cycle. Further the sonication is done at maximum temperature limit of 40 °C. Initial weight percent's of the nanoparticles were chosen as 0.05, 0.15 and 0.25 wt.% for GO, f-GO and rGO respectively. For the observation of agglomeration, all the nanofluids were kept undisturbed for 5 days because after that no further agglomeration was seen. On the end of 5^th day, any visible agglomerated volumes were separated out from the top & bottom of the tubes and working suspensions were obtained as nanofluids for further investigations. Final concentrations of the GO, f-GO and rGO nanofluids were 0.05, 0.14 and 0.23 wt.%, respectively.

Characterisation analysis

The synthesised GO, rGO and f-GO nanoparticles were characterised by different analytical techniques. For colloidal stability, suspended nanoparticles in D.I. water were analysed using UV- visible spectrometer (LABINDIA UV-3200). Structural analysis of all the nanoparticles (GO, rGO, f-GO) was done by using XRD (D8 Advance, Bruker, Germany) at different angles from 5° to 50°. For the analysis of functional groups, fourier transform infrarred spectroscopy was performed for the nanoparticle samples (PerkinElmer L1600400 Spectrum TWO DTGS). Stability (zeta potential) of the nanofluids was determined by ZETASIZER (Nano–ZS90, Malvern Panalytical, United Kingdom). Geometric structure and bonding within the molecule were studied by Raman spectroscopy (RENISHAW) using monochromatic light of 532 nm. Transmission electron microscopy was used to record morphology and size of nanoparticles (JEOL, JEM-2100). By using thermal gravimetric analyser (TGA), the thermal stability of nanoparticles was analysed with heating range @10 °C min⁻¹. Contact angles of nanofluids with different surfaces were measured by drop size analyser (KRUSS, DSA100A).

KD2 Pro thermal property analyser was used for thermal conductivity measurements recorded at room temperature with Decagon Devices, USA with single needle (KS1) of diameter 1.3 mm and 6 cm length. In this equipment a metallic conducting wire is used as both heat source and temperature sensor.

Experimental set up for assessing photo-thermal response

To truly assess the candidature of the as-prepared nanofluids particularly in solar thermal applications, it is imperative to investigate their photo-thermal behaviour. In essence, photo-thermal energy conversion involves direct interaction of the incident radiant energy with the absorbing/scattering medium (here the as-prepared nanofluids) via absorption and scattering mechanisms – leading to thermal energy gain of the medium (which is manifested in terms of the magnitude of the temperature increase). Figure 2 shows the experimental setup employed to investigate the photo-thermal behaviour of GO, f-GO, and rGO nanofluids of 0.05, 0.14, 0.23 wt. % respectively. Herein, a 150 W halogen lamp (Philips-3400 K) was employed as the light source . A data acquisition system (DAQ, model number - cDAQ-9174 - National Instrument) was attached to a sample holding polypropylene container with a set of five K-type thermocouples (TCs) located at 0.2, 0.4, 0.6, 0.8, and 1 cm heights from bottom of the container. The temperature of surroundings was detected by the sixth thermocouple. All the experiments were performed at normal ambient temperature. To prevent evaporation, the container was covered with a 0.4 mm thick glass slide. Temperatures were recorded for 60 s of lamp ON and then 30 s of lamp OFF conditions. It may be noted that in the present work, the experimental setups were robust enough to accommodate the effect of nanoparticles concentration as well direction of irradiation i.e., irradiation from bottom and top.

Results and discussion

Different parameters i.e.,- the nanofluid stability, thermal conductivity, colloidal stability, morphology, chemical structure, crystal structure, functional groups etc. of GO, f-GO and rGO nanofluids were characterised by TGA, KD2 pro, Zeta potential, FT-IR, Raman spectrophotometry, TEM, UV-visible spectroscopy, XRD, drop size analyser. In addition, the pH was measured for GO, f-GO and rGO nanofluids were 7.5, 6.8 and 7.9, respectively. This section covers the associated outcomes in depth.

Structural analysis

Figure 3(a) illustrates the structural change in GO, f-GO and rGO. Oxidation, functionalizarion and reduction of gaphite is observed by the shifting of the peaks for these nanoparticles. Due to water molecules and oxygen functional group present in between carbon layer structure, a peak at ∼10° is observed in GO, f-GO and rGO nanoparticles.²⁹ Incomplete oxidation of graphite powder in GO, f-GO and rGO nanoparticles is indicated by a peak at ∼42°.³⁰ A peak in f-GO nanoparticles at 15.06° indicates the functional groups attached on GO surface³¹ and another peak at 27.12° is for natural graphite. A broad peak at 24.28° in rGO is observed.^32,33

As seen in Figure 3(b), the functional groups in GO, f-GO and rGO have been detected. Here, GO exhibits, FTIR spectral peaks at 3378 cm⁻¹ and 1720cm⁻¹ are detected for carboxylic (-COOH) group corresponding to the O-H and C = O stretching frequencies^34–36 respectively. A very sharp band at 1619 cm⁻¹ is owing to C = C bending.^37,38 Additional peaks at 1224 cm⁻¹ and 1052 cm⁻¹ indicates the stretching of the C-O (phenolic) and C-O-C (epoxy) chains respectively.^39,40

Further, for f-GO, N-H stretching (1° amine) produces two distinct low intensity peaks at 3497 cm⁻¹ and 3389 cm⁻¹, which shows that GO has been functionalization successfully.⁴¹ Absorption band at 928 cm⁻¹ and 760 cm⁻¹.⁴² correspond to N-H wag formation.

When GO is reduced to rGO, hydroxyl (O-H) group produces peak at 3432 cm⁻¹,⁴³ while C = O and C = C groups with lower intensities produce peaks at 1712cm⁻¹ and 1536 cm⁻¹ respectively, and C-OH group⁴⁴ shows steep rise at 1183 cm⁻¹.

Figure 4 shows the morphology and size of the synthesised nanoparticles and the corresponding sizes for GO, f-GO and rGO nanoparticles are 86 nm, 78 nm and 128 nm respectively.

Stability of the nanofluids

Stability of the GO, f-GO, and rGO-based nanofluids is investigated by UV- visible spectrophotometer at various time intervals. Figure 5 shows that the stability of the nanofluids is quantified at specific time intervals for 30 days at an interval of 10 days each. In GO nanofluids, the absorption peak at 230 nm⁴⁵ is obtained due to the transition of C = C bonds electron from π-π*. Also, a slight hump at wavelength ∼300 nm⁴⁶ is found because of the transition of electrons from n-π* of C = O bonds. Similarly, in f-GO nanofluids, the π-π* and n-π* transitions of electrons in C = C and C = O bonds shifted the peaks at 224 nm²⁷ and ∼320 nm, respectively because after functionalization, π-conjugation decreases resulting in shift the peak wavelength. Whereas after reduction of GO to rGO, peak shifted to higher wavelength i.e., 267 nm⁴⁵ because after reduction π-conjugation Increases, which require less energy for the transition of electron from π-π* in C = C bonds.⁴⁷ All the above observations (see Figure 5) confirm the formation & stability of GO, f-GO and rGO.

It is understood from Figure 5(a) that absorption maxima (magnitude of peak absorption) of f-GO nanofluids (0.05 wt. %) remains almost same from the 1st to the 30th day, conforming the nanofluid's stability. There is slight reduction in absorption maxima for f-GO nanofluids at 0.14 wt. % and 0.23 wt. % after the 20th day (Figure 5(b-c)) because brownian motion of nanoparticles becomes less pronounced with time and at higher concentration.

In contrast, as indicated in Figure 5(d-f) the absorption maxima of GO nanofluid at 0.05 wt.% begins to decline after the 20th day, at 0.14 wt.% after the 10th day, and at 0.23 wt.%, it starts to decline after the first day. However, rGO-based nanofluids (shown in Figure 5(g-i)) exhibit a bit higher reduction in absorption maxima (apart from 0.05 wt.%, a reduction in maxima after 10 days).

It is seen from Figure 5, that all the nanofluids (GO, f-GO and rGO) exhibit remarkable stability at 0.05 wt. %, while higher wt. % results in lowering of absorption magnitude in general for all the nanofluids especially 20^th day onwards. Here, f-GO nanofluid performs better than GO and rGO nanofluid. For further clarity, Figure 6 shows the overall nanofluids’ stability during 30 days by closely observing the magnitude of peak absorbance at corresponding peak wavelengths of GO-230 nm, f-GO-225 nm, and rGO-265 nm.

It is re-confirmed from Figure 6 that f-GO nanofluids exhibit better stability for all 30 days as compared to GO and rGO nanofluids which are more stable upto 20 days. Similar experimental investigation has been done and rGO based nanofluids were stable for about 15 days.⁴⁸ Also, the stability of surfactant free GO nanofluids is reported upto 7 days.⁴⁹ Here, our synthesised surfactant-free nanofluids have got the stability of about 30 days. Moreover, these surfactant-free nanofluids have been shown to possess enhanced optical and thermo-physical characteristics. Zeta potential of nanofluids containing GO, f-GO and rGO at varying concentrations is shown in Figure 7.

Overall, it is found that f-GO nanofluids are more stable than those of GO and rGO nanofluids. There are different factors behind it's stability. It is seen from Figure 7, that f-GO nanofluids with lower concentration i.e., 0.05 wt.% have higher zeta potential (−42.4 mV) compared to rest of the nanofluids. Values of zeta potential with standard deviation (measured 3 times for each set) for the prepared nanofluids are shown in Table 1. DLVO theory also claims that if the nanofluids have zeta potential ≥ ± 30 mV, then high electrostatic potential lower the Van der wall interaction among the particles of the nanofluid leading to stability of the nanofluid suspension. The stability of f-GO nanofluids is also due to it's controlled pH and nanoparticle size. Measured pH of f-GO nanofluids is 6.8 and this pH changes the interaction behaviour and particle's surface charges. Also, smaller and spherically shaped nanoparticles improve the stability of f-GO nanofluids. Overall, the stability of f-GO nanofluids (without use of any surfactants) is due to the hydrophilic nature of amine groups, these groups create steric hindrance and electrostatic repulsion between the graphene oxide nanoparticles, results in preventing restacking and forming hydrogen bonding with water molecules.

Table 1.

Average zeta potential values (mV) of prepared nanofluids.

Concentration (wt. %)	GO nanofluids	f-GO nanofluids	rGO nanofluids
0.05	−40.5 ± 0.4	−42.4 ± 0.3	−34.9 ± 0.4
0.14	−37.6 ± 0.3	−38.4 ± 0.2	−32.6 ± 0.1
0.23	−33.5 ± 0.2	−36.3 ± 0.1	−30.8 ± 0.2

Thermal stability of the synthesised nanoparticles

Figure 8(a) shows that most of the weight loss within 205 °C to 258 °C is seen in GO nanoparticles. It is due to the degradation of the functional groups containing carbon and oxygen. Below 900 °C, the weight loss of GO nanoparticles is 58.5% (see Figure 8(a)). The thermal decomposition of f-GO changed compared to GO, and the weight loss of f-GO is 60.1% at temperature below 900 °C. It is due to the presence of an amine group present in f-GO. On the other hand, rGO nanoparticles show lesser weight loss as compared to GO and f-GO. It is because of the fewer oxygen functional groups in rGO nanoparticles,⁵⁰ and its weight loss is 39.5% below 900 °C. Furthermore, less weight loss in rGO shows higher thermal stability than in GO and f-GO which is in agreement with reports.⁵¹

Raman spectra of GO, f-GO and rGO is measured at a laser wavelength of 532 nm and has been shown in Figure 8(b). Here, D band peaks of GO, f-GO and rGO appear at ∼1351 cm⁻¹, 1353 cm⁻¹ and 1354 cm⁻¹ respectively shows the defect in the materials. Because of sp² hybridised carbon,⁵² G band appears at 1594 cm⁻¹, 1590 cm⁻¹ and 1591 cm⁻¹. Intensity corresponding to D and G bands for f-GO and rGO reduces in comparison to GO, demonstrating the successful functionalization and reduction of GO. I_d/I_g ration for GO, f-GO and rGO are 0.8, 0.9 and 0.9 respectively. This intensity ratio changes shows that rGO lowered the average sp² domain size and restoration of sp² carbon.

Thermal conductivity of the prepared nanofluids

The temperature at which thermal conductivity of prepared nanofluids is calculated is room temperature corresponding to 18 °C. Figure 9 shows the mean value of observed thermal conductivity of GO, f-GO, and rGO nanofluids at wt.% of 0.05, 0.14, and 0.23. It has been observed that when concentration of the particle increases (0.05 wt. % to 0.23 wt. %), nanofluids’ thermal conductivity also rises. At the same temperature deionised water is found to have thermal conductivity 0.563 ± 0.001 W/m-K. Using a formula, thermal conductivity enhancement in percentage (k_enh%) is determined by -

k_{e n h %} = (\frac{k - k_{b f}}{k_{b f}}) \times 100

Where, symbols k and k_bf denote thermal conductivity of nanofluids and base fluid respectively.

Maximum thermal conductivity values of GO, f-GO and rGO (at 0.23 wt.%) are 0.65 W/m-K, 0.69 W/m-K and 0.63 W/m-K (average value of thermal conductivity with standard deviation is shown in Figure 9) respectively. Also, percentage enhancement in thermal conductivity w.r.t. deionised water for GO, f-GO and rGO at 0.23 wt.% are 15.3%, 21.8% and 11.9% respectively. Similar, GO nanofluids with concentration 0.25 wt.% achieved 23% enhanced thermal conductivity at 20 °C.⁴⁶ Here, it can be see that f-GO based nanofluids possess maximum thermal conductivity enhancement because due to covalent and H- bonding, increase the interlayer coupling of the f-GO nanofluid as compared to GO and rGO based nanofluids. It is to be noted that f-GO nanofluid also exhibit better stability.

Analysis of contact angle

In general, contact angle determine wettability factor and this wettability is the property of liquid in contact with solid surface wherein surface forces govern the surface wettability. When a liquid drop spreads over solid surface and spread area increases then its contact angle is decreased. At 25°C temperature, the mean contact angle of GO, f-GO and rGO nanofluids (0.05- wt.% with drop size 10 μl) at different flat surfaces (glass, stainless steel, copper and solar selective surface) is shown in Figure 10. Mean contact angle along with the standard deviation is presented in Table 2 for all the surfaces.

Table 2.

Contact angles of the fluids at different surfaces.

	Mean contact angle with standard deviation
Type of fluids	Glass surface	Stainless steel surface	Copper surface	Solar selective surface
D. I.	73.6° ± 0.6°	77.4° ± 0.4°	71.9° ± 0.4°	114.1° ± 0.6°
GO	48.4° ± 0.7°	68.9° ± 2.6°	137.6° ± 2.5°	98.5° ± 0.6°
f-GO	45.4° ± 1.1°	70.3° ± 0.3°	48.8 °± 3.5°	86.5° ± 1.6°
rGO	64.1° ± 0.2°	66.5° ± 0.8°	106.7° ± 1.5°	96.4° ± 1.0°

Table 2, shows that contact angle of deionised water at different surfaces is larger than GO, f-GO and rGO based nanofluids. Improvement in the wettability factor of GO, f-GO and rGO nanofluids causes decrease in contact angle⁵³ because nanofluids completely spread on the solid surface. On glass and stainless steel surfaces all of the nanofluids (GO, f-GO, and rGO) exhibit hydrophilicity, while on copper and solar selective surfaces, GO and rGO shows hydrophobicity. However, f-GO exhibits hydrophilicity on every surface (Figure 10), indicating the high wettability and the strong interaction with the considered solid surfaces.

Photothermal behaviour of the nanofluids

As mentioned earlier, the purpose of carrying out photo-thermal experiments is to investigate the effects of nanoparticles concentration as well the direction of irradiation on the spatial-temporal temperature distribution and hence the photothermal behaviour.

The spatiotemporal temperature distribution for GO, f-GO and rGO nanofluids, at various nanoparticles concentrations i.e., 0.05, 0.14 and 0.23 wt. %, irradiated from top and bottom are shown in Figure 11 and Figure 12, respectively.

A non-uniform temperature distribution within the nanofluids is observed during top heating due to poor convection within the nanofluids. For GO, f-GO and rGO nanofluids, the maximum temperature of T.C.-1 at 60 s at concentration of 0.23 wt.% is 49.18 °C, 44.87 °C and 49.80 °C respectively. While, at lower concentration i.e., 0.05 wt.%, the temperature rise of T.C.-1 at 60 s is 37.98 °C, 38.65 °C and 40.13 °C for GO, f-GO and rGO respectively. Figure 11 shows the mean temperature with standard deviation (three measurements of each) at all the thermocouple locations.

For the top heating, at higher concentrations i.e., 0.14 wt.% and 0.23 wt.%, a high non-uniform temperature distribution is observed as compared to lower concentration i.e., 0.05 wt. % because at higher concentration, penetration depth of incident radiation decreases and therefore most fraction of the incident radiation is absorbed by the upper layer of the nanofluids and lesser fraction of incident radiation is available at deeper locations. Therefore, at deeper location (from T.C.-2 to T.C.-6) absorption of the light by the nanofluid is lesser leading to low temperature rise. So, T.C.-1 shows higher temperature distribution compared to other T.C's.

Hence, the thermocouples (T.C.-1 to T.C.-5) within the nanofluids have significant temperature differences (Figure 11(a-i)). In GO and rGO-based nanofluids (at 0.23 wt.%), this temperature differential (non-uniform temperature distribution) is 30.51°C and 31.58°C, respectively shown in Figure 11(c) & 11(i), while in f-GO nanofluids, it is 25.55°C, as represented in Figure 11(f). This reveals that f-GO provides most uniform temperature distribution among GO, fGO and rGO due to comparatively higher thermal conductivity of f-GO.

During top heating, topmost thermocouple T.C.-1 shows (Figure 11(a-i)) instant rise in temperature due to direct heating while other thermocouples take time to show rise in temperature as heat takes times to move from top to bottom of the container. Further, Figure 11(a-i) also revealed that T.C.-1 shows instantaneous fall in temperature while T.C.-2 to T.C.-5 show rise in temperature even after light source has been switched off which occurs due to conduction of heat from high temperature i.e., T.C.- 1 to lower temperature region (T.C.- 2 to T.C.- 5).

As seen from Figure 12, during bottom heating, however, all the thermocouples (for all concentrations of GO, f-GO and rGO) show a uniform temperature distribution, and this temperature distribution is within ∼2.5 °C (between T.C.-1 and T.C.-5). Here, temperature difference between T.C.-1 and T.C.-5 for GO, f-GO and rGO nanofluids at concentration of 0.23 wt.% is 1.65 °C, 1.01 °C and 1.5 °C respectively. At this concentration, the temperature difference in f-GO nanofluid is lesser than other nanofluids. The f-GO nanofluids show comparatively more uniform temperature which is due to higher thermal conductivity of f-GO. However, convection currents develop within the nanofluids during bottom heating. This process helps in mixing and hence uniform heating of the nanofluids (Figure 12(a-i)). Furthermore, due heat dissipation T.C.-5 shows no heating than other after radiation is off.

The uncertainty in terms of standard deviation is included for all concentrations of GO, f-GO and rGO nanofluids for experimental determinations of thermal conductivity, wettability/contact angle and photothermal properties). The values (standard deviation range) for best outcome i.e., f-GO are ±0.002–0.007 for thermal conductivity at 0.23 wt.% concentration, ± 0.3–3.5 for wettability/contact angle at different surfaces and ±0.004–0.66 for photothermal response at 0.23 wt.%.

In the realm of nanofluid synthesis, surfactants have been inherently used to render stability to nanoparticles so that agglomeration could be avoided. Major Surfactants used in the preparation of nanofluids are cetyl trimethyl ammonium bromide (CTAB), Sodium Dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), TritonX-100 (TX-100).^54,55 Although usage of surfactants does increase the stability of nanofluids, but this approach is effective only at low temperatures. At elevated temperatures, detachment of surfactant molecules from the nanoparticle's surface starts leading to agglomeration of nanoparticles. Moreover, surfactants have been shown to have dramatic implications with regard to environment, in particular harmful for aquatic ecosystem. These surfactants are harmful and may cause the aquatic ecosystem disruption, water and soil contamination risk.^56,57

In the present work, we have been able to synthesise surfactant-free nanofluids which do not involve any kind of surfactants for its synthesis. Moreover, these surfactant-free nanofluids have been shown to possess good stability and at the same time possess enhanced optical and thermo-physical characteristics.

Conclusions

This work reports the surfactant free synthesis and characterisation of GO, f-GO and rGO based nanofluids at various concentrations (within wt.% of 0.05 to 0.23) with focus on their stability and photo-thermal behaviour. Using two step method, these nanofluids are prepared. Further, the thermal conductivity of nanofluids is compared at room temperature. Here, absorption spectroscopy, zeta potential and TEM measurement data over the days of storage conform the structural stability of f-GO based nanofluids upto 30 days (0.05 wt.% highly stable as compared to 0.14 and 0.23 wt.%) as compared to GO and rGO nanofluids. Also, the thermal stability of f-GO nanoparticles is confirmed by TGA at high temperature range till 800–900 °C. It is evident that f-GO nanofluids possess high thermal conductivity compared to GO and rGO nanofluids at room temperature, resulting in them being more effective in transferring heat. At different surfaces (Glass, Stainless steel, Copper and Solar selective), f-GO nanofluids shows the hydrophilicity (polarity). Regarding photo-thermal behaviour, it is found that photothermal temperatures fluctuate in the range of 25-32 °C for top heating while 0.5–2.2 °C for bottom heating at all concentration of GO, f-GO and rGO. So, it can be concluded that among the surfactant free prepared nanofluids, the f-GO based nanofluid has remarkable stability, higher thermal conductivity, high wettability and photothermal activity, making it appropriate for applications involving photothermal energy conversion and heat transfer.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethical approval

Not applicable.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

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