Asghar
Ali
ab,
Muhammad
Qasim
*ac,
Piotr A.
Piatkowski
ad,
Ganjaboy
Boltaev
bd,
Andra N. K.
Reddy
bd and
Ali S.
Alnaser
*abd
aMaterials Science and Engineering Program, College of Arts and Sciences, American University of Sharjah, Sharjah 26666, United Arab Emirates. E-mail: aalnaser@aus.edu; mqasim@aus.edu
bDepartment of Physics, American University of Sharjah, Sharjah 26666, United Arab Emirates
cDepartment of Chemical and Biological Engineering, American University of Sharjah, Sharjah 26666, United Arab Emirates
dMaterials Research Center, American University of Sharjah, Sharjah 26666, United Arab Emirates
First published on 19th May 2025
Interfacial solar desalination relies on enhanced optical absorbance, heat localization at the air/water interface, and effective water management on photothermal evaporators. However, its commercialization is hindered by marginal vaporization rates, processing challenges, and unacceptable stability. This study presents a novel substoichiometric molybdenum oxide (MoOx) solar absorber with a unique nanochannel-on-microchannel architecture, designed to enhance broadband absorbance, concentrate heat within thin layers of water, and promote superwicking. For the first time, a tightly focused, non-diffracting Bessel laser beam is employed to create nanochannels layered over hierarchically designed open microchannels. The nanochannels promote cluster evaporation by distributing water in very thin layers, while the hierarchical morphology and rough oxide microchannels contribute to strong broadband absorbance and generate capillary forces that enable superwicking on the surfaces at any angle. Outdoor tests demonstrated exceptional performance, with evaporation rates of 4.21 kg m−2 h−1 under 1 sun and 19.3 kg m−2 h−1 under 3 suns, outperforming existing evaporators. Comparison of these rates with indoor rates under controlled lab conditions suggests that ∼50% of the total evaporation rate was contributed by wind and ambient temperature. Moreover, the impact of water salinity on interfacial evaporation is revealed by performing experiments and comparing results from both saline and deionized water. Salt ions that are specifically adsorbed at the solution/MoOx interface are found to inhibit direct contact between MoOx and the secondary water, thereby enhancing evaporation by lowering the adsorption energy. A comprehensive analysis of hydrogen bonding states, the electrical double layer, temperature measurements, vaporization enthalpy, and efficiency calculations corroborates the performance improvements. Our findings demonstrate significant potential for large-scale solar desalination and provide new possibilities for advancing interfacial solar desalination.
A variety of photothermal materials are used in interfacial solar evaporation,12 including carbon-based materials,13–15 noble metals,16–18 nanoparticles,19 semiconductors,20 organic coatings,21 and polymers.22 These are generally categorized based on the photothermal conversion mechanism: thermal vibration of molecules, non-radiative relaxation of semiconductors, and localized surface plasmon resonance (LSPR).8,10,23 Carbon-based materials convert solar energy to heat through thermal vibrations of carbon molecules, offering advantages such as low cost, availability, and scalability.24–26 However, they may suffer from low mechanical strength and difficulties in achieving stable powder deposition on substrates.9 Examples of low cost carbon-based photothermal materials include carbon monoliths from carbonized waste polyethylene terephthalate (PET) bottles,24 carbon black,27,28 and carbonized natural materials.25,26,29 Structurally stronger carbon materials such as nanotubes,15 graphene,14 and their derivatives,13 offer improved strength but involve complex processing and often reduced wettability. Semiconductor absorbers such as CuS,30 NiO,31 SnSe,32 and MoS233 convert solar light to heat through electron–hole pair creation and their subsequent non-radiative relaxation. These typically require narrow band gaps for efficient conversion, which are often difficult to synthesize and may degrade under intense light and heat.34
Solar evaporators utilizing the LSPR effect are based on metallic nanoparticles that, when exposed to light at their resonance wavelengths, generate hot electrons that dissipate energy non-radiatively to increase the surface temperature.8,10,35 Typically, the plasmonic solar evaporators based on individual nanoparticles are narrowband, requiring the use of nanoparticles of a wider size distribution to allow for broadband absorption.36 Also, chemical stability and particle agglomeration remain challenges. In this regard, ultrafast lasers can play a significant role in fabricating efficient solar evaporators. These lasers can structure surfaces by delivering very high peak powers instantaneously, quickly ablating the surface and forming hierarchical morphologies with varying size features.37,38 This process imparts broadband light absorption to the laser-treated metal surface through the combined LSPR effects of particles of different sizes. Additionally, laser structuring can simultaneously bestow superhydrophilic and superwicking properties by surface oxidation during structuring in an oxygenated environment,39,40 which are essential for efficient water transport during interfacial solar evaporation. Only few studies have explored the use of laser-treated metals for this purpose. Yin et al.41 used femtosecond lasers on titanium (Ti) foam, achieving broadband solar absorption of >97% and an evaporation rate of 1.79 kg m−2 h−1 under 1 sun irradiance. However, the proposed evaporator design was not solar-trackable since it employed a floating evaporator that must always float on the water surface. Also, the porous solar evaporator was inherently susceptible to clogging when used with saline water feeds. The cost of Ti foam poses another limitation. Chen et al.42 used a picosecond laser to inscribe microchannels on the surface of an aluminum (Al) sheet. An optical absorbance of 47–85% was achieved in the VIS-NIR region and a modest evaporation rate of 1.24 kg m−2 h−1 was reported under 1 sun illumination, highlighting the need for improved and optimized laser structured solar evaporators. Another study by Singh et al.11 demonstrated a femtosecond laser-structured Al sheet in a similar vertical configuration for solar absorption. A broadband optical absorbance of 97% and an evaporation rate of 0.9 kg m−2 h−1 under 1 sun illumination were achieved. Under 3-sun irradiation, the highest reported evaporation rate was approximately 5 kg m−2 h−1. However, this study did not assess high-salinity or outdoor performance with natural sunlight. While these studies highlight the promise of laser-treated solar evaporators in boosting interfacial evaporation, further research is needed to boost the performance further to levels suitable for industrial deployment. Moreover, comprehensive evaluation under real-world conditions—such as outdoor environments—and systematic investigation of critical factors like salinity, wind, and concentrated solar irradiation are essential for practical implementation.
In this study, we fabricate a novel and highly-efficient molybdenum (Mo) oxide-based solar absorber for interfacial solar evaporation consisting of engineered nanochannels on top of hierarchically structured microchannels. Mo was chosen for its broadband optical absorbance and excellent photothermal conversion efficiency in both metallic and oxidized states,43,44 structural robustness,45 excellent ambient stability,46 thermal and chemical stability of its oxides,47 more localized laser ablation due to its refractory nature,48,49 and its role as an essential micronutrient.50 In addition, MoOx structures exhibit superhydrophilicity which is essential for high wettability and water transport during interfacial solar evaporation. Owing to these favorable attributes, MoOx-based solar absorbers and evaporators have been previously explored; however, their performance has generally remained sub-optimal. For instance, Wang and co-workers44 developed a solar evaporator based on Mo nanoparticles embedded within an amorphous MoOx matrix (Mo@MoOx). Although the reported solar absorptivity was 91%, the solar evaporator was not employed in solar evaporation. In another study, Wang and co-workers51 fabricated nano-multilayered MoOx-based solar evaporator coatings (SSACs) with a solar absorptivity of 93%, without evaluating its interfacial solar evaporation. Also, Wang and co-workers52 fabricated bi-layered MoOx cermet-based SSACs. Although the solar absorptivity was 91%, the interfacial evaporation performance was not reported. A study by Lu et al.53 reported a MoOx-based solar evaporator, developing an oxygen-deficient MoOx-based evaporator in the form of ultrathin nanosheets loaded onto a PTFE membrane. The reported evaporation rate with seawater was 1.26 kg m−2 h−1 under 1 sun, which is lower than that of the MoOx-based evaporator reported in this study. Also, the MoOx/PTFE membrane proposed by Lu et al.53 was prepared by an extraction filtration method and the MoOx layer is perceived to exhibit limited physical stability. While solar evaporators based on MoOx have been reported, their performance in interfacial solar evaporation remains largely underexplored. As demonstrated in this study, judicious engineering of the MoOx absorber holds significant promise for substantially increasing the evaporation rate.
For the first time, we demonstrate the use of a tightly focused, non-diffracting femtosecond laser Bessel beam to inscribe substoichiometric MoOx nanochannels, uniquely integrated with open microchannels, for significant enhancements in interfacial solar evaporation. Compared to the commonly-used Gaussian beams, non-diffracting Bessel beams are more intense and tightly focusable, allowing for more precise, and narrower channels with fewer ablation effects on the surrounding material. To show the effect of morphology on interfacial evaporation, we compare the open microchannel morphology developed with a Gaussian beam to the nanochannel-superimposed microchannel morphology created by the Bessel beam. To illustrate the role of Mo delocalized d-orbital electrons in enhancing interfacial evaporation, we compare amorphous Al oxide (AlOx) evaporators produced with Bessel and Gaussian beams to the MoOx evaporators. Moreover, the impact of water salinity on interfacial evaporation is revealed by performing experiments and comparing results from both saline and deionized (DI) water. To show that interfacial evaporation under concentrated solar irradiation favors MoOx over AlOx, desalination experiments were performed under 1, 2, and 3 suns irradiance. Furthermore, Raman analysis of hydrogen bonding provides valuable information about changes in cluster evaporation in response to the electronic nature of the evaporator, its morphology, and adsorbed salt ions. Equilibrium potentials and Tafel plots provided insights into how salinity influences the evaporation rate by modifying the electrical double layer at the evaporator/water interface during interfacial evaporation. Finally, we show results from our outdoor experiments under real field conditions to demonstrate compatibility with solar tracking and concentrated solar irradiance.
The 2 and 3 sun experiments were performed outdoors under the sun at the American University of Sharjah, University City, Sharjah, United Arab Emirates (25°18′28.7′′N, 55°29′27.5′′E). Solar light was concentrated using a Fresnel lens to achieve the desired irradiance measured with a power meter (Ophir Optronics Solutions, Nova II) and irradiated onto the absorber that was open to the atmosphere. To demonstrate solar tracking, the evaporator inclination angle was adjusted according to the solar zenith angle, ensuring that light fell perpendicularly on the evaporator at various points of time throughout the day. Each measurement was performed over 5 minutes, and the sample orientation was readjusted to match the solar zenith angle before the next measurement. Multiple readings were taken throughout the day, and the average values are reported here. During these experiments, the maximum ambient temperature was 42 °C, whereas the average humidity and wind speed were 30% and 14 km h−1, respectively. The schematic diagram and a photograph of the outdoors experimental setup are shown in Fig. 2(b) and (c), respectively.
The SEM images of Mo-Gaussian and Mo-Bessel–Gauss are shown in Fig. 3(e and g). Compared to Al solar evaporators, Mo solar evaporators showed a higher content of nanoparticles and a reduced degree of sintering effect, irrespective of the laser beam used. This is because Mo is an intrinsically high melting point material (2623 °C) and could not coalesce as easily as Al (melting point: 660.3 °C) during laser structuring. As in the case of Al, the microchannels in the Mo-Bessel–Gauss samples are thinner compared to those in the Mo-Gaussian sample due to differences in the focal spot size and distinct intensity profiles of the Gaussian and Bessel beams (Fig. 1). The Bessel beam concentrates most of its energy tightly at the center, causing more localized ablation. A footprint of this energy localization can be observed in the Mo-Bessel sample (Fig. 3(g and h)), with the ablation debris accumulated in the microchannels, thus constituting networks of nanochannels atop the microchannels. This effect was selectively evident in the Mo-Bessel sample because the tightly focused beam does not provide sufficient energy to significantly reablate the ablation debris (from previous scans), which settle in the vicinity of a scanned line. This morphology is important because it does not significantly compromise the optical absorption or superwicking properties. Instead, it provides nanochannels for enhanced interfacial evaporation while simultaneously mitigating heat losses to bulk water by filling the microchannels with nanochannel networks. Such filled microchannels could not be observed in the Al-Bessel sample (Fig. 3(c and d)) because Al has a much lower melting point compared to Mo, making it easily ablatable at lower fluences. The same rationale applies to both Al-Gaussian (Fig. 3(a and b)) and Mo-Gaussian samples (Fig. 3(e and f)), where the Gaussian beam due to its wider energy distribution significantly reablates the neighboring ablation debris.
Surface chemistry imparts superwicking characteristics to these solar evaporators (refer to ESI Video 1† for freshly prepared AlOx and ESI Video 2† for 50 days old MoOx solar evaporators). The presence of considerable O amount on the surface makes the solar evaporators hydrophilic, whereas the hierarchical structures augment the hydrophilicity by rendering the surface superhydrophilic. The superhydrophilic micro-/nanocapillaries, therefore, impart superwicking properties to the evaporators. It was also observed that all the solar evaporators exhibited superwicking behavior irrespective of their orientation, implying compatibility with solar tracking and concentrated solar exposure throughout the day.
IR spectroscopy (ESI Note 5†) of Al-Gaussian and Al-Bessel samples reveals the presence of amorphous aluminum oxides and hydroxides in these samples. Similarly, the IR spectra of Mo-Gaussian and Mo-Bessel samples reveal the existence of mixed Mo oxide phases in these samples. Further insights into the structure and chemical composition of the surface were acquired with Raman analysis (ESI Note 6†). The surface of both Al-Gaussian and Al-Bessel-Gauss samples is primarily composed of amorphous Al oxides (AlOx). Similarly, substoichiometric Mo oxides (MoOx, x < 3) are found to dominate the surface of both Mo-Gaussian and Mo-Bessel samples. These findings about the surface chemical composition were further confirmed with high resolution XPS (Fig. 5) analysis. The peaks at 74.8 eV and 75.9 eV (Fig. 5(a)) correspond to the different oxidation states of Al. The peak at 74.8 eV corresponds to a mixed oxidation state, AlOx, with an oxidation state less than that of stoichiometric Al2O3.61 AlOx, which we previously introduced as amorphous Al oxide constitutes the major portion on the surface. Furthermore, the relatively smaller peak at 75.9 eV is assigned to Al(III) in Al2O3.61,62 Depth profiling (ESI Note 7†) reveals that the AlOx component increases as we move down the surface, which is anticipated due to the reduced exposure of the sub-surface to oxygen during and after the laser treatment. The above findings indicate that amorphous AlOx is the major constituent of our Al-based evaporators. Similarly, in the case of Mo-based evaporators, several XPS peaks corresponding to mixed oxidation states were identified (Fig. 5(b)). A minor fraction of metallic Mo was also detected, as confirmed by the presence of the Mo(0) 3d5/2 peak at 227.9 eV. Substoichiometric Mo(IV) 3d5/2 could be located at 229.3 eV. The more dominant 3d5/2 and 3d3/2 doublets of Mo(V) and Mo(VI) were identified at 231.5 eV & 234.6 eV, and 232.5 eV & 235.8 eV, respectively.63–65 Below the top surface, the relative content of lower oxidation states, such as Mo(0) and Mo(IV), increases with depth (ESI Note 7†). This is attributed to the reduced exposure of the underlying material to oxygen during and after the laser treatment.
Mo(IV), present as MoO2, is a highly substoichiometric form of MoOx (x < 3) that features delocalized d-orbital electrons. It has Mo–Mo metallic bonds and exhibits semimetallic characteristics.66 Moreover, the Fermi level in MoO2 occupies the same band as the d-orbitals.67 These delocalized d-orbital electrons are expected to influence the adsorption of water and salt ions on hydrophilic MoOx in the desalination experiments.68,69 Additionally, the d-orbital electrons are responsible for the surface plasmon resonance effect, which enhances the optical absorption of MoO2 in the visible to NIR region.44
The absorbance recorded for the laser-structured solar absorbers is shown in Fig. 6(a). All the samples depicted considerable broadband absorbance over the spectral range from 250–2000 nm. Fig. 6(b) provides the numerical interpretation of Fig. 6(a) in terms of the average and maximum absorbance recorded. Al-Gaussian depicted the least average absorbance of 87.1%, followed by Al-Bessel with 91.3% average absorbance. On the other hand, Mo-Gaussian depicted the highest average absorbance of 96%, followed by Mo-Bessel with 93.1% average absorbance.
The significant enhancement in optical absorbance arises due to the highly rough surface morphology achieved with laser treatment. The microgrooves decorated with hierarchical structures (Fig. 3) are responsible for causing high diffuse reflectance, multiple reflections, and subsequent absorption of a large fraction of the incident rays by the absorber. The reason for the improved absorbance of Al-Bessel relative to Al-Gaussian is the denser, thinner, and deeper grooves inscribed by the Bessel beam. On the other hand, the MoOx samples exhibit higher absorption than AlOx primarily due to the availability of a narrow and dense d-band overlapping with the s–p conduction band in MoOx.70
When considering the surface morphology consisting of microchannels, it can be said that MoOx holds DI water more strongly at the evaporator/water interface than AlOx does. This is attributed to the much stronger adsorption of water on MoOx than on AlOx,71,72 which has important implications for interfacial evaporation where the morphology of the water channels is more critical than the optical absorbance for evaporators attaining ∼87% or above absorbance under 1 sun illumination. Channels with smaller free volumes hold fewer water bodies and are more efficient evaporators than channels with larger free volumes. Observations supporting this claim are the higher evaporation rates observed for Al-Gaussian (with shallower channels, partially filled with nanoparticles) compared to Al-Bessel (with deeper and unfilled channels) (Fig. 3(a–d)). Also, the unique morphology of Mo-Bessel consisting of nanochannels superimposing microchannels resulted in a high evaporation rate compared to Mo-Gaussian with wider and unfilled microchannels (Fig. 3(e–h)). This again highlights the importance of morphology and the interactions at the evaporator/water interface in dictating the evaporation rate. These interactions are further elucidated in later sections.
When experiments were performed with saline water under controlled laboratory conditions under 1 sun irradiance, the evaporation rates on Al-Gaussian, Mo-Gaussian, Al-Bessel, and Mo-Bessel solar evaporators were 1.89, 1.23, 1.79, and 1.92 kg m−2 h−1, respectively. This indicated that, regardless of the evaporator, the evaporation rate of saline water is always lower than that of DI water at 1 sun. This behavior was anticipated and can be associated with boiling point elevation and interface modification due to salinity, which is discussed in detail in the section on the electrochemical analysis of the interfaces. Moreover, MoOx evaporators can attain higher evaporation rates, partly due to the higher photothermal temperatures (ESI Note 9†) attained due to their relatively higher optical absorption (Fig. 6). Fig. 7(b) compares the average evaporation rates achieved with 3.5 wt% saline water on the MoOx and AlOx evaporators under real field conditions at 1, 2, and 3 suns. Fig. 7(b) also indicates that regardless of the evaporator, the evaporation rate changes non-linearly with a linear increase in solar irradiance from 1 to 3 suns. The change in the evaporation rate is more pronounced when solar irradiance increases from 1 to 2 suns, while it is less significant between 2 and 3 suns. Moreover, Mo-Bessel depicts the highest evaporation rate with saline water at 1, 2, and 3 suns. Mo-Gaussian surpasses both Al-Gaussian and Al-Bessel evaporators at 2 and 3 suns. It is evident that both salinity and solar concentration selectively favor evaporation on MoOx. The origin of the rapid and non-linear increase in the total evaporation rate on MoOx is the stronger plasmonic effect of delocalized electrons in MoOx under concentrated solar irradiation, activating stronger interfacial evaporation. This has been discussed in more detail in the section on the electrochemical analysis of the interfaces. In contrast, the less rapid non-linear increment from 2 to 3 suns is attributed to excessive salt accumulation hindering efficient optical absorption and amplifying optical reflection from the accumulated NaCl crystals. Efficient evaporators such as Mo-Bessel require regular surface cleaning to remove accumulated salt. Luckily, rinsing with water easily washes off most of the accumulated salt from the evaporator.
In outdoor experiments, multiple factors influence the evaporation rate, especially the wind speed, ambient temperature, and humidity. Under controlled laboratory conditions at 1 sun irradiance, the evaporation rates on Al-Gaussian, Mo-Gaussian, Al-Bessel, and Mo-Bessel solar evaporators were 1.89, 1.23, 1.79, and 1.92 kg m−2 h−1, respectively. Comparing these results with the outdoor results (Fig. 7(b)), around ∼42–54% of the evaporation rate is attributable to wind and ambient conditions under 1 sun. It is therefore imperative that approximately half of the total evaporation rates recorded in Fig. 7(b) under 1, 2, and 3 suns are attributable to wind and higher outdoor temperature. Since wind speed fluctuates continuously, the evaporation rate varies accordingly.
The enthalpy of vaporization (HLV) of saline water over AlOx and MoOx can be computed by assuming that bulk water is an ideal device operating at 100% energy conversion efficiency in the dark. If the respective dark evaporation rates (ṁ) of the ideal bulk solution device (ṁbulk) and the evaporators (i.e., ṁAlOx and ṁMoOx) are known, then the latent heat of vaporization of saline water over AlOx and MoOx
can be computed using the following relation.73
The energy conversion efficiency (η) of a typical solar evaporator (under controlled laboratory conditions) may be computed using the following expression.53,73,75
In general, for a water molecule, the hydrogen bonding network formed can be characterized by its interactions with the neighboring molecules acting either as a proton donor (D), a proton acceptor (A), or both.77 This way five types of hydrogen bonded motifs have been reported, namely, donor acceptor–acceptor (DAA-OH), donor–donor acceptor (DDA-OH), donor acceptor (DA-OH) donor–donor acceptor–acceptor (DDAA-OH), and free water molecules (free-OH).76,77 In the OH stretching vibration region of the Raman spectrum, these appear as five sub-bands centered around 3005, 3226, 3434, 3573, and 3640 cm−1, respectively.11 In our case, DA-OH (chain or ring) and DDAA-OH (tetrahedral) species showed the highest contributions (57.3% and 30.4%, respectively) in bulk water and represented the primary types of hydrogen bonding states that were present. This is consistent with the results from previous studies.11,77–79 For DI water confined in the channels of solar evaporator, we observed alterations in the hydrogen bonding networks, and there was a significant increase in the DDA-OH networks in the case of Al-Gaussian, Al-Bessel–Gauss, and Mo-Bessel–Gauss samples. For DI water in the channels of the solar evaporators, the percentage of DDA-OH hydrogen bonding networks was higher compared to bulk DI water (only 6.4%). Al-Gaussian and Mo-Bessel–Gauss showed comparable percentages of the DDA-OH hydrogen bonding networks (>30%) and produced similar evaporation rates with DI water. In the case of Al-Bessel–Gauss, the percentage of DAA-OH hydrogen bonding networks was less compared to Al-Gaussian and Mo-Bessel–Gauss. As a result, the evaporation rate was lower. Mo-Gaussian produced the lowest evaporation rate with DI water. This was consistent with the observation that, with Mo-Gaussian, DI water possessed the smallest percentage of DAA-OH networks. It is known that DDA-OH networks, which are clusters of four water molecules, can bind more water molecules and have the tendency to form large-sized water clusters. The formation of water clusters reduces the enthalpy of vaporization compared to the conventional latent heat. This is because in large-sized water clusters, only the hydrogen bonds formed between the clusters and the evaporator surface need to be broken. Consequently, large-sized water clusters tend to escape collectively at a lower energy per unit mass compared to monomeric water molecules that require all individual hydrogen bonds to be broken.11,80 In the case of Al-Bessel–Gauss, the population of water clusters was not as high, which implied higher enthalpy of vaporization for DI water on the Al-Bessel–Gauss surface and, consequently, a lower evaporation rate compared to Al-Gaussian and Mo-Bessel–Gauss. For Mo-Gaussian, DI water possessed the smallest percentage of DAA-OH networks and, therefore, the lowest population of water clusters. This resulted in Mo-Gaussian producing the lowest evaporation rate with DI water. With the established DI water evaporation trends for different evaporators, we can now use these data as a reference to further explore the evaporation behavior of saline water on these evaporators.
Photothermal heating, morphology of the evaporator channels, and the salt water/evaporator interface are important from the design perspective and have been discussed below.81,82 The superior performance of MoOx evaporators with 3.5 wt% saline water can be attributed to the changes in the hydrogen bonding states on the solar evaporator surface in the presence of salt ions. The deconvoluted Raman spectra of the OH stretching vibrations in saline water (ESI Note 11†) suggest that the percentage of DDA-OH hydrogen bonding networks is much higher for water confined in MoOx evaporators (>75%) compared to AlOx evaporators (∼15%). This distinction in the hydrogen bonding between the MoOx and AlOx evaporators is attributed to the delocalized d-orbital electrons on MoOx responsible for strongly adsorbing the salt ions. We discuss this fact in more detail in the next section on electrochemical probing of the evaporator/water interface.
Fig. 9 contrasts the deconvoluted Raman spectra of OH stretching for the best AlOx, i.e., Al-Gaussian (DAA-OH percentage: 34%), and the best MoOx, i.e., Mo-Bessel–Gauss (DAA-OH percentage: 76.3%) in saline water. These results imply a relative abundance of water clusters responsible for a reduction in the vaporization enthalpy, and a consequent increase in the evaporation rate for Mo-Bessel–Gauss in saline water. It is evident that besides the nanochannel superimposed microchannel morphology, the salinity is another factor favourably affecting the evaporation rate on MoOx compared to AlOx. Due to the delocalized d-orbital electrons, MoOx is anticipated to form an electrical double layer (EDL) different from that on AlOx in saline water. The EDL is essentially an array of charged species, which comprise salt ions and water dipoles in our case, that align themselves along the (conducting) evaporator surface when immersed in saline water. The EDL typically comprises two layers of charged species (discussed in the next section), with one of the layers specifically adsorbing on the evaporator.83 The presence of the EDL is due to adsorption; therefore, it is imperative to investigate the EDL in saline water. In the following section, we electrochemically investigate the nature of the EDL that develops along the MoOx and AlOx evaporators.
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Fig. 9 Deconvoluted OH stretching vibration in the Raman spectra for (a) Al-Gaussian and (b) Mo-Bessel–Gauss (with DAA-OH percentage zero) in 3.5 wt% NaCl solution. |
Salt addition elevates the boiling point of bulk water, such that 3.5 wt% NaCl has a boiling point elevation (BPE) of ∼0.33 °C relative to pure water at sea level at 25 °C (ESI Note 12†). The BPE is caused by additional interactions associated with the hydration of Na+ and Cl− ions. The changes in the interfacial evaporation rate with salinity do not correlate linearly with the BPE of 0.33 °C (Fig. 7(a) and (b)). This is due to differences between bulk and interfacial evaporation mechanisms, where interfacial evaporation depends on the nature of the evaporator interface. Salt ions tend to adsorb strongly on the solid evaporators.84 A stable EDL appears due to a dynamic equilibrium of charge transfer between the evaporator and the adsorbed salt ions/water dipoles. Depending on the equilibrium position, net charges can accumulate on the electrode and in the solution phase, such that a potential appears across the EDL. Under equilibrium, no net current passes across the EDL, and therefore, this equilibrium potential is referred to as open circuit potential (OCP).85 The stronger the adsorption interaction, the easier the charge exchange between the evaporator and saline solution. Thus, a change in OCP can indicate changes in the EDL, corresponding to alterations in the specifically adsorbed species.
The OCP of AlOx and MoOx in DI water (ESI Note 13†) and saline water (Fig. 10(a)) show interface modification with salt ion adsorption on the interface. For AlOx, the difference in OCP is −0.35 V, whereas for MoOx, the difference is −0.85 V (Fig. 10(b)). For MoOx, this difference in OCP is more than twice that of AlOx and can be attributed to the difference in EDL emerging at the evaporator/solution interface (ESI Note 13†).
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Fig. 10 Comparison of the (a) Tafel plots in saline water and (b) the corresponding rest potentials (in saline as well as DI water) and exchange current densities of structured AlOx and MoOx samples. |
Here, it is important to consider the type and extent of adsorption interactions at the evaporator/electrolyte interface. In the case of AlOx in DI water, there are two dominant modes of interaction between AlOx and the electrolyte. One is the strong hydrogen bonding due to water dipoles interaction with AlOx. The second dominant interaction results from the partial negative oxygen (O(δ−)) of H2O interacting with the positive Al centers of AlOx. In the case of saline water, salt ions also compete for specific adsorption on the surface. The −0.35 V shift from −0.35 V in DI water to −0.7 V in saline water shows that salt ions have specifically adsorbed on AlOx. As suggested in the Bockris–Devanathan–Müller model, which is detailed in ref. 83 the adsorbed layer along the Helmholtz plane comprises adsorbed water dipoles and salt ions. This well-organized monolayer screens the secondary water and ions in the outer Gouy layers from directly interacting with the AlOx. Although this layer reduces the average direct contact between H2O and AlOx, it also introduces a concentration gradient with more salt ions near the AlOx interface. This higher salt ion concentration is responsible for the BPE beyond ∼0.33 °C in the interfacial region and can be the primary factor contributing to the reduced evaporation rate of AlOx in a saline environment.
In the case of MoOx, besides the interactions mentioned above, strong d-band interactions also exist between the positively charged metallic Mo centers and the adsorbates. Due to the d-band interactions, the adsorbates can adsorb more strongly on MoOx, resulting in higher heats of adsorption.84 This is the primary reason for the relatively lower evaporation rates observed on MoOx in DI water. In saline water, the large OCP shift of −0.85 V and the higher exchange current density of 90 μA cm−2 indicate stronger salt ions adsorption on the interface. Due to the higher competitive specific adsorption of salt ions (as evidenced by the higher potential shift), the Helmholtz layer has a better screening effect and strongly limits the direct water contact with the MoOx. This releases more water molecules from direct d-band interaction with MoOx. In this case, the water favoring rapid evaporation occupies the Gouy layer.
Along with the screening effect at the interface, the excess local salt concentration promotes BPE to retard the evaporation rate as well. The interfacial evaporation rate thus represents a balance between d-band screening and BPEs. We observed that the evaporation rate of Mo-Gaussian was slightly lower than that of Al-Gaussian at 1 sun but surpassed Al-Gaussian at 2 and 3 suns. Mo-Bessel outperformed all the samples at 1, 2, and 3 suns in saline water. The origin of the significantly improved MoOx performance at multiple suns is the provision of sufficient energy to overcome the d-band interactions of the remaining specifically adsorbed water along the Helmholtz plane. The favorably modified interface, the enhanced optical absorbance by the MoOx delocalized d-band electrons due to a stronger plasmonic effect under concentrated solar irradiation, and preferred interface geometry (e.g., Mo-Bessel), resulted in evaporators with the highest evaporation rates reported to date. Lastly, pertinent to corrosion resistance, the MoOx type evaporators depict geometric corrosion current densities within an acceptable limit of 100 μA cm−2 (Fig. 10).84
Table 1 compares the performance of solar evaporators reported in this study with other interfacial evaporators reported elsewhere. The MoOx solar evaporator produced with a femtosecond Bessel beam laser exhibited superior saline water evaporation rates, especially under concentrated solar. Under 1 sun (outdoor conditions), the observed evaporation rate was 4.21 kg m−2 h−1, exceeding most of the reported carbon, semiconductor, and plasmonic-based solar evaporators. Under 3 suns, the evaporation rate was remarkably high (19.3 kg m−2 h−1) compared to the carbonized wood reported by Liu et al.86 and the laser-treated Al reported by Singh et al.11 The present study, therefore, reports an efficient, physically stable, and easy-to-fabricate MoOx solar evaporator that holds great promise for use in interfacial solar evaporation and offers new directions for future research in solar thermal water desalination.
Evaporator material | Experimental conditions | Enthalpy of evaporation (J g−1) | Optical absorbance (%) | Evaporation rate (kg m−2 h−1) | Efficiency (%) | Reference |
---|---|---|---|---|---|---|
Carbon monoliths PET wastes | Intensity: 1 sun, feed: 3.5 wt% NaCl | — | 92 | 0.99 | 63.5 | 24 |
Carbon sponge | Intensity: 1 sun, feed: 3.5 wt% NaCl | — | 95–97 | 1.31 | 90.0 | 87 |
Multilayer carbon-fiber fabric | Intensity: 1 sun, feed: seawater | 1094.28 | 97 | 3.39 | 96.69 | 88 |
Carbonized wood | Intensity: 3 suns, feed: 3.5 wt% NaCl | — | 97 | 4.03 | 91.3 | 86 |
Carbon black | Intensity: 1 sun, feed: 3.5 wt% NaCl | — | 96.5 | 2.31 | — | 89 |
SnSe on Ni foam | Intensity: 1 sun, feed: artificial seawater | — | 89 | 0.85 | — | 90 |
MoOx nanostructures | Intensity: 1 sun, feed: seawater | — | 90 | 1.26 | 85.6 | 53 |
Au nanofilm | Intensity: 1 sun, feed: 150 g L−1 NaCl | — | 80 | 0.88 | 49.5 | 91 |
Femtosecond laser-treated Al | Intensity: 3 suns, feed: pure water | 1220 | 97 | 5.5 | — | 11 |
Femtosecond laser-treated Ti foam | Intensity: 1 sun, feed: pure water | — | 97 | 1.79 | 90.0 | 41 |
Picosecond laser-treated Al | Intensity: 1 sun, feed: artificial seawater | 541.25 | 85 | 1.24 | 67.0 | 42 |
Femtosecond laser (Gaussian beam)-treated Al | Intensity: 1 sun, feed: 3.5 wt% NaCl (aq) | 1420 | 87.1 | 3.64 | 75 | This work |
Femtosecond laser (Gaussian beam)-treated Al | Intensity: 2 suns, feed: 3.5 wt% NaCl (aq) | — | 87.1 | 8.71 | — | This work |
Femtosecond laser (Gaussian beam)-treated Al | Intensity: 3 suns, feed: 3.5 wt% NaCl (aq) | — | 87.1 | 11.49 | — | This work |
Femtosecond laser (Bessel beam)-treated Mo | Intensity: 1 sun, feed: 3.5 wt% NaCl (aq) | 1234 | 93.1 | 4.21 | 66 | This work |
Femtosecond laser (Bessel beam)-treated Mo | Intensity: 2 suns, feed: 3.5 wt% NaCl (aq) | — | 93.1 | 15.28 | — | This work |
Femtosecond laser (Bessel beam)-treated Mo | Intensity: 3 suns, feed: 3.5 wt% NaCl (aq) | — | 93.1 | 19.3 | — | This work |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5na00249d |
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