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. 2026 Apr 20;16:18283. doi: 10.1038/s41598-026-44712-0

Nano-scaled In:SnO2 TCOs as a novel technology for minimizing water evaporation

D Kpeglo 1,2, N Botha 1,2, M Maaza 1,2,✉
PMCID: PMC13260849  PMID: 42009875

Abstract

A novel technique for lowering surface water evaporation is validated by this investigation. In the latter scenario, a nanoscale Transparent Conducting Oxide (In:SnO2) layer is coated onto a flexible polymeric substrate. It has been demonstrated that the intrinsic properties of the In:SnO2 TCO coating allow for the relative blockage of infrared solar radiation (> 60% average) while permitting the transmission of ultraviolet and visible solar energy (up to a maximum of 72.4%). Consequently, the bottom surface of the In:SnO2 TCO/PET and in volume reservoir temperatures are kept at low values throughout its corresponding solar day cycle. Evaporation is significantly low as a direct result. TUP-Down (the temperature difference between the In:SnO2 TCO/PET surface and underneath) and ΔTUP-Bottom (the temperature difference between the In:SnO2 TCO/PET surface and the reservoir bottom) are significant thermal differences in relation to the surface. More specifically, TUP-Down ranges from 5.01 °C (early artificial solar exposure) to 10.11 °C (azimuthal artificial sun exposure). In addition to validating the In:SnO2 TCO/PET coatings’ efficacy as a possible technique for reducing water evaporation, this method offers the following benefits over other conventional physical, chemical, and bio-based methods. (i) the advantage of optical selectivity; (ii) buoyant as a cover (hydrophobic surface); (iii) the anti-biofouling response of both coated and untreated In:SnO2 TCO-coated surfaces; and (iv) weightless as a cover, which may be economical.

Keywords: Water evaporation, Evaporation minimization Technology, Transparent Conducting oxides, Indium tin-oxide, PET substrate, Optical selectivity, Solar Infrared radiations

Subject terms: Climate sciences, Hydrology, Materials science, Nanoscience and technology

Introduction

Only 1% of the water on Earth is known to be fit for human consumption, yet making up approximately 70% of the planet1–3. The planet’s water supply is also under unprecedented pressure due to a number of issues, including but not limited to population expansion, industrial pollution, and global warming in particular.

As a result, it is expected that water prices would continue to climb substantially in many parts of the world, especially in dry or semi-arid climates like those found in many African countries. According to projections from a number of international organisations, including the United Nations and the World Water Council, most African countries will, in fact, fall under the vulnerability or stress thresholds, if not the shortage one (Fig. 1a)4–6. Water evaporation poses a significant threat to the effectiveness of water storage and supply globally since it is the primary environmental activity that can reduce the amount and quality of water available for home, agricultural, and industrial purposes5,6.

Fig. 1.

Fig. 1

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(a) Water availability in Africa in 1990 &2025 as projected by several UN agencies including United Nations Economic Commission for Africa (UNECA), (b) Normalized solar intensity distribution within the UV–VIS-IR spectral range.

These evaporative losses are mostly caused by increased Direct Normal Incidence (DNI) solar radiation. These latter are brought on by the high levels of infrared sun spectrum radiation, which in Africa surpass 200 W/m2 per year. As seen in Fig. 1b, the solar spectrum is dominated by the infrared radiation spectral region (52%) rather than the visible (43%) and ultraviolet (5%). Furthermore, it is anticipated that these evaporative losses will grow dramatically with any global temperature rise brought on by climate change.

In an attempt to lower such evaporation losses from water reservoirs (natural or manufactured), monolayer molecular films were first used as an impermeable barrier for water surfaces as early as the 1960s6. To improve control over evaporation losses, a number of physical, chemical, and biological techniques have recently been developed7–11. This includes, but is not limited to, suspended or floating covers. Furthermore, it seems that using compressed air thermal mixing has a lot of potential for lowering evaporation in deep reservoirs12. Aquatic plants, wind barriers, and biological palm fronds are other forms of covers that can considerably lower the amount of water reservoir evaporation13–19.

This new pathway backs an additional novel approach to minimizing water evaporation technologies. More accurately and within the latter, a nanoscale coating of transparent conducting oxide (TCO) is applied on a flexible polymeric substrate. Apart from being lightweight, the TCO coating has a significant optical transmission in the UV–VIS solar spectral range conjugated to a significant optical opacity in the infrared (NIR& IR). In fact, as schematically shown in Fig. 2a, the inherent characteristics of the TCO’s covering coating10–15 allow the solar IR radiation to be reflected back while the UV and VIS radiations are transmitted. Because the water’s surface beneath the TCO-coated polymeric substrate is kept at a lower temperature and is not heated, there would be little to no evaporation.

Fig. 2.

Fig. 2

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(a)Schematic configuration of the proposed In:SnO2 TCO cover for selective solar protection and minimazition of surface water evaporation, (b) Standard diurnal day cycle of the solar intensity; (——) Diurnal day cycle earth surface temperature, (——) Diurnal day cycle earth outgoing radiations, (—…—…—) Expected diurnal day cycle surface temperature beneath the TCO cover at the interface of Water & TCO cover.

Given the standard distribution of the solar angle, intensity, and average temperature shown in Fig. 2b, which represents the standard diurnal daily cycle, it is reasonable to assume that applying the recommended flexible TCO/PET sheets to any natural or artificial water reservoir would reduce surface water evaporation by keeping the water-TCO/PET coating’s interface at a lower temperature than the air-TCO/PET interface. In other words, if one looks at the schematic representation of a typical diurnal cycle in Fig. 2b, the surface temperature beneath the TCO coating (blue dashed curve) will be lower than the regular average daily cycle of surface temperature (red continuous curve).

As will be validated, the proposed new pathway has the following benefits over conventional physical, chemical, and biological methodologies28, in addition to confirming the efficacy of In:SnO2 TCO coatings as a promising technology for water evaporation reduction:

  • (i)

    The benefit of optical selectivity;

  • (ii)

    Potential buoyancy as a cover (hydrophobic surface);

  • (iii)

    In:SnO2 TCO-coated and uncoated surfaces’ anti-biofouling response; and,

  • (iv)

    weightlessness as a cover;

Materials & methods

The utilisation of an In:SnO2 TCO-coated flexible polymeric substrate constructed of Polyethylene Terephthalate (PET) with a total surface area of 210 × 297 mm2 and a thickness of 0.5 mm is, as previously pointed to, the most novel aspect of this contribution (if validated). The thin layer of In:SnO2 that comprises the TCO coating has three different thicknesses: 135 nm, 260 nm, and 500 nm. This particular contribution focused on the 260 nm thickness. It has a surface resistance of 60 Ω/square and an optical transmittance of ~ 73% at 550 nm. The commercially available In:SnO2 TCO/PET set was acquired from Sigma Aldrich. The specific selection of the 260 nm In:SnO2 TCO/PET is due to its highest optical transmission as suggested by its technical specificities.

A halogen lamp equivalent to a 1.5 Sun flux source was used to mimic or reproduce the sun and was placed at various distances L (10, 20, and 30 cm separation distance), as shown graphically in Fig. 3. The temperature readings were obtained using both pyrometry and traditional thermometers. By taking temperature readings from various interfacial locations at various times, the equivalent of diurnal daily light cycle was replicated. The experiments began early in the day in order to replicate the natural cycle. However, throughout the article, any reference to time corresponds to exposure time.

Fig. 3.

Fig. 3

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Schematic representation of the design used to evaluate the effectiveness of the proposed novel technology for the reduction of water evaporation. The artificial sun consisted of a 1.5 sun artificial lamp positioned at various distances “L” from the surface of the In:SnO2/PET coated water surface of the reservoir.

More precisely, the temperature was recorded at the following locations: The interface is located at position -1: air-In:SnO2 TCO coating/PET cover (marked as TUp); position -2 is the interface or the space directly beneath the cover. The In:SnO2 TCO coating/PET cover-Water surface (designated as TDown) and the reservoir’s bottom (designated as TBottom), which is roughly 150 mm from the top surface, are (iii) locations. The various temperature readings of TUP, TDown, and TBottom were collected 10 times during the daily hours of 8:00–13:00 and correctly averaged for each time of the day.

Results & discussion

Morphological Surface investigations

Atomic force microscopy of the untreated (a) and coated (b) surfaces of the 260 nm In:SnO2/PET sample is shown in Fig. 4. The surface morphology of both coated and untreated sides is noticeably rough, with average mean root square roughness (σrms) of around ~ 191 Å and ~ 25 Å, respectively. The columnar growth of the In: SnO2 thin film coating onto the PET polymeric substrate, as seen in Fig. 4, is probably connected to the comparatively high roughness value of the 260 nm of the In:SnO2 TCO coating (Fig. 4b).

Fig. 4.

Fig. 4

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Atomic Force Microscopy 2-D scan of the surface of the (a) uncoated surface of the In:SnO2/PET, (b) coated surface of the In:SnO2/PET, (c) Fourier Transform spectrum of the AFM image (a).

Such a rough surface’s Fourier transform (Fig. 4c) appears to indicate that there is no periodicity in this type of columnar structure, as supported by earlier research15. However, according to surface tension studies covered in the next part, such a columnar arrangement would potentially favour the hydrophobicity of the In: SnO2 TCO coated PET substrate. This roughness plays a crucial role in the hydrophobicity of the In: SnO2 TCO coated PET substrate and, consequently, in its capacity to float on the water’s surface, as will be discussed in the sections that follow. A priori, its floatability guarantees that the TCO cover won’t sink.

Surface tension hydrophobicity investigations

The surface tension images on the virgin untreated (a) and coated (b) surfaces of the In: SnO2 TCO/PET sample are shown in Fig. 5. The relative hydrophilicity of the virgin/uncoated virgin surface and the relative hydrophobicity of the In: SnO2 TCO coated surface were measured by the corresponding contact angles, which are around ~ 82.2 ± 0.5° and ~ 109.7 ± 0.5°, respectively. According to several theoretical models, this later hydrophobicity is anticipated given the corrugated shape of the In: SnO2 TCO surface. Among these latter hypotheses, models by Cassie-Baxter, Wenzel, Young, or Quere et al. should be mentioned20–25. In fact, if a smooth surface is hydrophilic by nature, it would show significant hydrophobicity after it became rough (greater roughness, higher contact angle), as schematically shown in Fig. 5c. This roughness-induced hydrophobicity has been validated in a number of instances, particularly in the case of other TCOs, such as ZnO26,27. A priori, this hydrophobicity would support the TCO/PET coverage’s floatability. It should be noted, therefore, that the TCO/PET’s potential surface tension-driven floatability may only be restricted in the presence of steady, unwavering surface water. Therefore, the TCO/PET cover would need to be attached to a possible floating system in the case of a natural dam where the surface water moves often.

Fig. 5.

Fig. 5

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Surface tension images on both surfaces of the In: SnO2 TCO /PET sample on the virgin uncoated (a) & coated (b) parts, (c) Roughness effect on contact angle as per theoretical models.

Optical investigations

The optical transmission of the 260 nm In: SnO2 TCO/PET sample within the 200–2500 nm spectral range (Fig. 6a) and the corresponding zooms in Region-1, the UV-Blue-VIS solar spectral range (200–1000 nm) (Fig. 6b), and Region-2, the infrared Sect. (1000–2500 nm) (Fig. 6b).

Fig. 6.

Fig. 6

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(a) Optical transmission of the 260 nm In: SnO2 TCO/PET substrate sample within the spectral range of 200–2500 nm, (b) corresponding zoom within Region-1; the UV-Bleu-VIS solar spectral range (200-1000 nm), and (c) corresponding zoom within Region-2; i.e. the Infrared Section (1000–2500 nm), and (d) optical diffuse reflectance.

With a cut-off wavelength of about 319 nm and an optical transmission that varies between 10 and 30% in the 319–400 nm spectral range, region-1’s optical transmission is comparatively large and broad, with a maximum peaking at about 489 nm and an average optical transmission above 62.8%. The optical transmission steadily declines from the NIR-IR spectral range, or region-2 (1000–2500 nm) (Fig. 5c), with a few dips at 1657.8, 1901.5, 2124.6, 2164.8, 2339.2, and 2446.2 nm, respectively. These resonances in the IR are likely to be associated to the IR active modes of the various teraphtalate & ethylene groups, including but not limited to C–H, C = C, CH2 = CH2 & CH2—CH2 among others of the PET substrate. These infrared resonances are probably connected to the infrared active modes of the several teraphtalate and ethylene groups of the PET substrate, such as C–H, C = C, CH2 = CH2, and CH2–CH2, among others. The In: SnO2 TCO/PET substrate’s diffuse reflectivity is shown in Fig. 6d. In the VIS spectral range, reflectivity is the quasi-conjugate of optical transmission, reaching almost 0% reflection at about 318 nm. However, in the NIR spectral region, reflectivity increases significantly above 80%, followed by a likely abrupt further increase above 1057 nm (i.e., the IR spectral region). Therefore, as a preliminary conclusion, the goals of this contribution are in line with the relatively large and satisfactory optical transmission in the VIS (62.8%) coupled to an appreciable optical transmission in the UV-Bleu (10–30%) and conjugated to a relatively high reflectivity in the infrared region (> 80%).

Thermal investigations

This study’s primary focus is the crucial collection of thermal investigations discussed in this section. The experimental setup used to finish this thermal analysis is shown in Fig. 3. The proportions of the reservoir were planned to fit the In: SnO2 TCO sheet. As a result, the reservoir measured 298 mm in length, 211 mm in width, and 110 mm in depth. As previously stated, in order to mimic or recreate the daily solar cycle, temperature measurements were conducted at many interfacial locations at different times. More specifically, the temperature readings at different exposure durations were recorded at the following interfacial locations:

  • (i)

    Interface air/In: SnO2 TCO coating/PET cover (designated as TUp) (location -1);

  • (ii)

    Interface In: SnO2 TCO coating/PET cover-Water surface (designated as TDown) beneath the cover; and,

  • (iii)

    the reservoir’s bottom (designated as TBottom).

The average for each exposure duration was calculated based on ten readings of the various temperature values for TUP, T Down, and TBottom. The temperature readings obtained with the 260 nm In: SnO2 TCO coating/PET substrate covering the water reservoir with the artificial sun source at roughly 10 cm from the reservoir surface are shown in Fig. 7. TUp, TDown, and TBottom rise with exposure time within the corresponding half-cycle, as shown in Fig. 7a. The primary goal of this investigation is validated by the fact that both TDown and TBottom are much lower than TUp. The heat difference between TUP-Down and TUP-Bottom in relation to the surface is significant. More specifically, TUP-Down ranges from 5.01 °C (early hours) to 10.11 °C (Azimuthal time exposure). TUP-Bottom fluctuates between 6.451 °C (early hours) and 20.1 °C (Azimuthal time exposure) along a similar pattern. These are significant improvements in lowering the temperature. The temperature readings obtained with the 260 nm In: SnO2 TCO coating/PET substrate covering the water reservoir with the artificial sun source at roughly 10 cm from the reservoir surface are shown in Fig. 7. TUp, TDown, and TBottom rise with exposure time within the corresponding half-cycle, as shown in Fig. 7a. The primary goal of this investigation is validated by the fact that both TDown and TBottom are much lower than TUp. The heat difference between TUP-Down and TUP-Bottom in relation to the surface is significant. More specifically, ΔTUP-Down ranges from 5.01 °C (early hours) to 10.11 °C (Azimuthal time exposure). ΔTUP-Bottom fluctuates between 6.451 °C (early hours) and 20.1 °C (Azimuthal time exposure) along a similar pattern. These are significant improvements in lowering the temperature. The temperature readings obtained with the 260 nm In: SnO2 TCO coating/PET substrate covering the water reservoir with the artificial sun source at roughly 10 cm from the reservoir surface are shown in Fig. 7. TUp, TDown, and TBottom rise with exposure time within the corresponding half-cycle, as shown in Fig. 7a. The primary goal of this investigation is validated by the fact that both TDown and TBottom are much lower than TUp. The heat difference between ΔTUP-Down and TUP-Bottom in relation to the surface is significant. More specifically, ΔTUP-Down ranges from 5.01 °C (early hours) to 10.11 °C (Azimuthal time exposure). ΔTUP-Bottom fluctuates between 6.451 °C (early hours) and 20.1 °C (Azimuthal time exposure) along a similar pattern. These are significant improvements in lowering the temperature.

Fig. 7.

Fig. 7

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(a)Variation of the Up (TUp), Down (TDown) & Bottom (TBottom) temperatures versus exposure time, (b) Variation of the thermal difference relatively to the surface ΔTUP-Down & ΔTUP-Bottom temperatures versus exposure time.

Figure 8 displays the variation of the level of surface water within the reservoir with time of exposure at various time of the day at various Artificial sun-surface water distances L; (a) 100 mm, (b) 200 mm & (c) 300 mm. Figure 8c displays the considered configuration of the set-up to simulate sun-surface water evaporation without the In: SnO2 TCO/PET coating. As one can notice, there is a net decrease of the water level versus the exposure time for the various 3 considered artificial Sun-reservoir’s surface distances. Within the bar error, it seems, a priori within such a time range & artificial sun irradiance, the rate of evaporation is quasi linear. More precisely, the rate of evaporation is about 0.78, 0.56 and 0.45 mm/h for the Artificial sun-surface water distances L of 100, 200 & 300 mm respectively. Within the current configuration, the corresponding volume evaporation rate is of the order of 51.1, 36.6, and 29.5 cm3/h. This values are significant & worth to be minimized using the proposed In: SnO2 TCO coatings as a protective coating. To further sustain this evaporation minimization effectiveness, the reservoir water level was investigated with & without the In: SnO2 TCO/PET coverage. Figure 8e displays the variation of the level of the surface of the water in the reservoir with and without the In: SnO2 TCO/PET coverage with the artificial sun placed at 300 mm from the reservoir surface at various exposure time. This latter supports once again the efficacy of the proposed In:SnO2 TCO coverage in minimizing the surface water evaporation (Table 1).

Fig. 8.

Fig. 8

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Variation of the level of surface water within the reservoir with time of exposure at various time of the day at various Artificial sun-surface water distances L (a) L = 100 mm, (b)200 mm& (c) 300 mm, (c)Configuration of the set-up to simulate sun-surface water evaporation, (d) set up configuration, and (e) Variation of the level of the surface of the water in the reservoir with and without the In: SnO2 TCO/PET coverage with the artificial sun placed at 300 mm from the reservoir surface at various exposure time.

Table 1.

Change in temperature ΔTUP-Down & ΔTUP-Bottom as well as the estimated volumetric reflected back heat (Q) for the mounted solar distance of L = 10 cm.

Exposure time (hour) ΔTUP-Down(°C) ΔTUP-Bottom (°C) Volumetric Q(UP-Down) (J/l) Massic Q(UP-Bottom) (J/l)
8:30 5.0 6.1 5641 103 5664 103
9:00 5.1 8.2 5641 103 5740 103
9:30 5.1 10.1 5641 103 5750 103
10:00 6.2 12.3 5664 103 5764 103
10:30 7.1 14.1 5687 103 5823 103
11:00 7.2 14.3 5687 103 5823 103
11:30 5.8 14.8 5687 103 5843 103
12:00 8.1 17.2 5740 103 5893 103
12:30 9.1 18.1 5731 103 5923 103
1:00 10.1 19.9 5742.5 103 5923 103
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It is however worth pointing to the UV instability of PET and henceforth the lifetime of the In: SnO2 TCO/PET cover. Indeed it is established that PET degrades under UV radiations inducing its yellow coloration & hence its optical transmission39. This latter is caused by spontaneous chain scission by oxidation and consequently shortening of the polymer chains. Accordingly, for a longer lifetime In: SnO2 TCO/ transparent polymer substrates, it would be ideal using UV resistant & optically transparent polymeric substrates including but not limited to Polycarbonate40. Besides the relatively UV resistance, PC substrate have an additional advantage, their decomposition temperature is within the 75–95 °C41 compared to PET (65–85 °C).

In view of sustaining the effectiveness of the TCO coverage, additional experiments were carried. More precisely; experiments of investigating the change of the volume of the reservoir level versus:

  • (i)

    Nature of the illuminating artificial sun sources: U-VIS–NIR (190–110 nm) & NIR-IR-FIR (0.75–10 µm),

  • (ii)

    Varying the distance from the illuminating source to the surface of the reservoir (10,20 & 30 cm),

  • (iii)

    Carrying out the experiments without any cover on the reservoir,and with the reservoir coated with PET cover only and then with the TCO coated PET.

Figure 9 reports the variation of the level of the surface of the water in the reservoir under illumination with a white UV–VIS-NIR artificial sun source at various source-reservoir surface distance without the In: SnO2 TCO/PET coverage (Open reservoir surface), with the PET coverage, and with the In: SnO2 TCO/PET coverage (a) artificial sun-reservoir surface distance of 100 mm, (b) artificial sun-reservoir surface distance of 200 mm, and (c) artificial sun-reservoir surface distance of 300 mm,

Fig. 9.

Fig. 9

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Variation of the level of the surface of the water in the reservoir versus the exposure time under illumination with a white UV–VIS-NIR artificial sunsource at various source-reservoir surface distance without the In: SnO2 TCO/PET coverage (Open reservoir surface), with the PET coverage, and with the In: SnO2 TCO/PET coverage (a) artificial sun-reservoir surface distance of 100 mm, (b) artificial sun-reservoir surface distance of 200 mm, and (c) artificial sun-reservoir surface distance of 300 mm,

As one observes, and within the bar error, the level of the water surface in the reservoir is constant and seems not changing in the case of full coverage with the In: SnO2 TCO/PET coverage. However with the PET coverage, the water level decreases quasi linearly with the exposure time. Likewise, a similar variation but a significantly higher decrease of the water level is observed without any cover (Open surface). More precisely, and as one could expect, the surface water level reaches its lowest level with the shortest artificial sun source-reservoir distance of 100 mm.

A crystal clear similar trend is observed in the case of the NIR-IR-FIR illuminating source. Indeed, Fig. 10 displays the corresponding variation of the level of the surface of the water in the reservoir versus the exposure time under illumination with a NIR-IR-FIR artificial sun source at various source-reservoir surface distance without the In: SnO2 TCO/PET coverage (Open reservoir surface), with the PET coverage, and with the In: SnO2 TCO/PET coverage (a) artificial sun-reservoir surface distance of 100 mm, (b) artificial sun-reservoir surface distance of 200 mm, and (c) artificial sun-reservoir surface distance of 300 mm.

Fig. 10.

Fig. 10

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Variation of the level of the surface of the water in the reservoir versus the exposure time under illumination with a NIR-IR-FIR artificial sun source at various source-reservoir surface distance without the In: SnO2 TCO/PET coverage (Open reservoir surface), with the PET coverage, and with the In: SnO2 TCO/PET coverage (a) artificial sun-reservoir surface distance of 100 mm, (b) artificial sun-reservoir surface distance of 200 mm, and (c) artificial sun-reservoir surface distance of 300 mm,

The major difference between the usage of UV–VIS-NIR and the NIR-IR-FIR illuminating artificial source is the rate of evaporation. This latter is significantly higher with the NIR-IR-FIR illuminating artificial source.

As a pre-conclusion especially in relation to this last section, one can safely conclude on the effectiveness of the In:SnO2 TCO coated PET coatings as an efficient approach for minimization of water in dams, swimming pools and agri-pools among others.

Hence, it might be sound to summarize the advantages offered by the proposed validated pathway; among which one should mention:

  • (i)

    The advantage of optical selectivity;

  • (ii)

    Buoyancy as a cover (hydrophobic surface);

  • (iii)

    The reaction of coated and untreated In:SnO2 TCO-coated surfaces to biofouling; and

  • (iv)

    Light weight as a cover.

  • (v)

    Apart from the aforementioned benefits, the In:SnO2 TCO coating’s optical selectivity would not impact the live plants and animals in the artificial or natural water reservoir (VIS solar radiation transmission) and may have antimicrobial properties. due to the UV radiations transmitted (event not elevated so far),

  • (vi)

    Last but not least, the additional advantage of using the above mentioned TCO based technology coverage is the compatibility towards fauna & flora living within the natural or artificial water reservoir. While the slight UV optical transmission within 319–350 nm spectral range would likely act as an antibacterial source, the relatively significant VIS–NIR optical transmission within the spectral range of 400–1000 nm would be necessary to the photosynthesis processes as such range fits with the optical activity of chlorophylls’ & related complexes such as carotenoids such as β- carotene and lutein which absorb in such spectral range of 400–500 (blue) and 650–680 (red)29,30.

Last but not least, for practical use, TCO materials must be safe and affordable. Because indium is expensive and scarce, it is desirable to use substitutes; impurity-doped ZnO, such as Al: ZnO, and Ga: ZnO or even pure SnO2 in addition Zn2SnO4 are one such substitute that shows great promise. To produce these impurity-doped ZnO films on large-area substrates with high deposition rates, more advancements in deposition processes are necessary & currently intensively researched31–41. As a follow up of this study, other coatings will be explored including a series of TCO oxides both n & p-type. This would be necessary to generalize this approach to a broader TCO coatings beyond the In:SnO2 TCO 42–50 especially those which do not necessitate the usage of the expensive Indium (Fig. 11).

Fig. 11.

Fig. 11

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Variation of the level of the surface of the water in the reservoir versus the exposure time under illumination with aNIR-IR artificial sun source at various source-reservoir surface distance without the In: SnO2 TCO/PET coverage (Open reservoir surface), with the PET coverage, and with the In: SnO2 TCO/PET coverage (a) artificial sun-reservoir surface distance of 100 mm, (b) artificial sun-reservoir surface distance of 200 mm, and (c) artificial sun-reservoir surface distance of 300 mm.

Conclusion

In the field of water evaporation technology minimization, this contribution validated a novel technology. The latter involves applying a transparent conducting oxide (TCO) nanoscale layer on a flexible polymeric substrate. In addition to being lightweight, the In:SnO2 TCO coating has a notable optical transmission in the UV–VIS solar spectral region and a high opacity in the infrared (IR). Such a coating (In:SnO2) allowed the transmission of UV and VIS radiations while reflecting back solar IR radiations. As a consequence, there is a net decrease of both surface & in volume temperature inducing no water evaporation during a typical diurnal cycle. Yet, this pathway was validated using a specific In:SnO2 TCO coating, it should, a priori, work for any other passive or active TCO.

Acknowledgements

We are grateful to the various supporting institutions including, The German Deutscher Akademischer Austauschdienst (DAAD, German Academic Exchange Service), the University of South Africa (UNISA), iThemba LABS (ITLABS), the National Research Foundation of South Africa (NRF), the African Laser Centre (ALC), the international Organization of Women in Science (OWSD), the Abdus Salam International Centre for Theoretical Physics (The Abdus Salam-ICTP), the United Nations Education, Sciences and Culture Organization (UNESCO) as well as the Centre for High Performance Computation (CHPC) is acknowledged.

Author contributions

D.K.: Synthesis & various characterizations, Data Analysis N.B.: Optical characterization studies, M. M.: Concept, Data Analysis & Manuscript Writing.

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author upon request.

Declarations

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Associated Data

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Data Availability Statement

The datasets used and/or analysed during the current study are available from the corresponding author upon request.

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