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Article

Fabrication and Application of Ag, Black TiO2 and Nitrogen-Doped 3D Reduced Graphene Oxide (3D Black TiO2/Ag/N@rGO) Evaporator for Efficient Steam Generation

by
Fisseha A. Bezza
,
Samuel A. Iwarere
,
Shepherd M. Tichapondwa
and
Evans M. N. Chirwa
*
Water Utilization and Environmental Engineering Division, Department of Chemical Engineering, University of Pretoria, Pretoria 0002, South Africa
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(3), 514; https://doi.org/10.3390/catal13030514
Submission received: 10 January 2023 / Revised: 20 February 2023 / Accepted: 25 February 2023 / Published: 2 March 2023
(This article belongs to the Special Issue Catalytic Elimination of Toxic Environmental Pollutants and Toxic Recalcitrant Organic Compounds from Aqueous Solution)
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Abstract

:
The scarcity of fresh water, which is aggravated by rapid economic development and population growth, is a major threat to the modern world. Solar-driven interfacial desalination and steam generation is a promising strategy that localizes heat at the air-water interface through appropriate thermal management and demonstrates efficient photothermal performance. In the current study, Ag, black TiO2, and nitrogen-doped 3D reduced graphene oxide (3D black TiO2/Ag/N@rGO) hierarchical evaporator was fabricated, and its morphology, elemental composition, porosity, broadband solar absorption potential, photothermal performance, and interfacial desalination potential were assessed. The 3D solar evaporator showed efficient solar absorption over the entire broadband UV-visible near-infrared (UV-Vis NIR) region and demonstrated 99% photothermal conversion efficiency and potential freshwater generation of 1.43 kg·m−2 h−1. The specific surface area and porosity analyses demonstrated an ultrahigh specific surface area, high pore volume, and a mesoporous structure, with a predominant pore diameter of 4 nm. The strong photothermal performance can be attributed to the nitrogen doping of the rGO, which boosted the electrocatalytic and photothermal activity of the graphene through the activation of the excess free-flowing π electrons of the sp2 configuration of the graphene; the broadband solar absorption potential of the black TiO2; and the localized surface plasmon resonance effect of the AgNPs, which induced hot electron generation and enhanced photothermal conversion. Hence, the high photothermal conversion efficiency attained can be attributed to the synergistic photothermal performances of the individual components and the high interfacial surface area, abundant heat, and mass transfer microcavities of the 3D hierarchical porous solar absorber, offering multiple reflections of light and enhanced solar absorption. The study highlights the promising potential of the 3D evaporator for real-word interfacial desalination of seawater, helping to solve the water shortage problem sustainably.

Graphical Abstract

1. Introduction

Freshwater is a scarce resource and represents a global challenge for the 21st century, one which is exacerbated by rapid economic and population growth, lifestyle changes, industrialization, and urbanization [1]. Solar-driven desalination is a sustainable solution for global water shortage challenges, making it possible to desalinate the abundant seawater covering three quarters of the Earth’s surface [2]. However, conventional solar-driven desalination technology, which utilizes volumetric heating to desalinate sea and hypersaline water, suffers from low efficiency due to the wastage of solar energy when heating the non-evaporative bulk water [3]. Recently, innovative interfacial desalination technology has been developed that localizes heat to the water–air interfacial area, avoiding the dissipation of energy to the non-evaporative bulk water and enabling highly enhanced solar-to-heat conversion efficiency [4,5]. The solar-driven interfacial evaporation system consists of solar absorber that has efficient broadband solar absorption potential, appropriate thermal insulation system that avoids heat loss to the bulk water and localizes the solar to thermal conversion process at the liquid water interface as well as a floating structure that can concurrently maximize the evaporation rate and transfer of liquid to the evaporative surface [6]. In the interfacial solar-driven desalination process, photothermal materials are the core components and play an essential role in converting light to heat. To facilitate the real-world application of the interfacial evaporation system, the development of high-performance photothermal materials with high evaporation efficiency and a rational design for the structure of the evaporators, ensuring optimal energy management during solar steam generation, are prerequisites [7]. The ideal photothermal material for water evaporation should exhibit broadband solar absorptance (250–2500 nm) and low thermal emittance [8]. In addition to the broadband solar absorption property, the ability for rapid water-molecule transportation and a highly porous structure are imperative to facilitate water transfer to the active vapour generation site and enhance the vapor-escaping rate [8].
Photothermal materials can be broadly classified into carbon-based materials, such as black carbon, carbon nanotubes, graphite, and graphene; plasmonic metals, such as Ag and Au; and semiconductors, such as Cu2S, CuFeS2, and rutile or anatase TiO2 [9,10]. Black carbon materials are naturally efficient broadband solar absorbers owing to the excitation and subsequent relaxation of electrons [6]. Specifically, graphene and graphene-based composites have demonstrated great potential for photothermal applications thanks to their intriguing physicochemical and mechanical properties [11]. Graphene is a single-atom-thick 2D carbon material with prominent features, including high mechanical strength, extremely high electrical and thermal conductivities, and large specific surface area (2675 m2 g−1), making it an ideal material for various applications in diverse areas, such as energy storage [12], sensors [13,14,15], and solar desalination [16,17,18]. On account of the resonant oscillation of surface plasmons and free electrons in the incident light of plasmonic metal nanoparticles such as AgNPs—which is called the localized surface plasmon resonance (LSPR) effect—plasmonic nanoparticles have very promising photothermal conversion potential [8]. The LSPR effect induces three sequential phenomena: near-field enhancement, hot electron generation, and photothermal conversion [19]. However, the extremely narrow absorption spectra of plasmonic metals fundamentally hinder their application for broadband wavelength light absorption. Hybridization of the plasmonic nanoparticles with porous and hierarchical carbon materials has been reported to enhance the evaporation efficiency through the synergistic plasmonic effect of the plasmonic nanoparticles and broadband absorbance of the carbonaceous material [20,21].
Black titanium dioxide (bTiO2) is a new, narrow-band-gap photocatalyst that exploits the full solar spectrum and has applications in sustainable environmental remediation and energy harvesting [22]. TiO2 has remained the photocatalyst of choice for many applications due to its low cost, low toxicity, high stability, and wide availability. However, TiO2 has significant drawbacks, most notably its wide band-gap (3.0 and 3.2 eV for rutile and anatase, respectively), limiting the optical absorption of TiO2 to the ultraviolet (UV) region, which only accounts for less than 5% of solar energy, leading to insufficient utilization of solar energy [23]. The TiO2 band-gap problem can be effectively addressed by introducing Ti3+ defects and oxygen vacancies into the titanium oxide lattice via hydrogenation. Hybridization of reduced graphene oxide (rGO) with black TiO2 has been reported to enhance photothermal conversion efficiency synergistically [24]. Furthermore, nitrogen doping of rGO has been reported to increase its photothermal conversion and thermal absorption efficiency owing to the introduction of electrocatalytic active sites, as well as increasing the electrical conductivity and surface hydrophilicity [25]. Self-assembly of 2D graphene nanosheets into 3D graphene-based hierarchical interwoven monoliths imparts excellent photothermal conversion efficiency and interfacial desalination potential owing to the avoidance of the aggregation of 2D nanosheets, the provision of more heat and mass transfer channels, and a longer light scattering distance while maintaining graphene’s high thermal and electrical conductivity [26]. Remarkable steam generation potential for 3D grapheme-based solar absorbers has been recently reported in a number of studies [27,28,29,30,31]. The high steam generation and photothermal efficiency of the 3D evaporators, even beyond the theoretical light-to-vapor conversion limit, is due to the recycling of the latent heat from the condensation of the vapour generated [30]. Furthermore, hybridization of materials with various composites, including bimetallic materials, metal–organic materials, and metal–semiconductor composites, has been reported to lead to enhanced photothermal conversion efficiency owing to the synergistic merits of each component [19,32]. In the current study, a highly porous 3D black TiO2, Ag, and nitrogen-doped rGO (3D black TiO2/Ag/N@rGO) composite hierarchical structure was fabricated via hydrothermal reduction of a GO suspension, and its potential application for solar-driven interfacial desalination of hypersaline water was investigated.

2. Results

2.1. Structural and Morphological Analyses

The 3D black TiO2/Ag/N@rGO shown in Figure 1b, was synthesized through hydrothermal reduction of GO along with the black TiO2, Ag nanoparticles and Nitrogen precursor. The graphene oxide was produced following the oxidation of the graphite. GO has various oxygen-containing functional groups, such as hydroxyl, epoxy, carbonyl, and carboxyl functional groups. The presence of oxygen-containing functional groups makes graphene hydrophilic and enables its functionalization with diverse composites [33]. When the Ag nanoparticles and black TiO2 powder were added and hydrothermally treated, the Ag and TiO2 particles and nitrogen atoms bonded with the oxygen-containing functional groups of GO and formed a 3D monolithic network.
The 3D monolithic structure was placed on the cellulose sponge support material. The cellulose sponge supported the 3D hierarchical monolithic structure and served as thermal insulation, helping to avoid the dissipation of heat into the bulk water. The top surface of the interfacial desalination setup was covered with transparent plastic to prevent radiative and convective heat loss into the surrounding environment. Figure 1 displays the graphite used for the synthesis of GO (a), the as-synthesised 3D black TiO2/Ag/N@rGO (b), the cellulose sponge support and thermal insulator (c) and the interfacial desalination setup with the solar absorber enclosed in a transparent plastic cover to shield the radiative and convective heat losses, illuminated with 1-sun solar irradiation (d). Figure 2 presents the schematic diagram of the overall interfacial desalination setup.
A field-emission scanning electron microscopy (FESEM) study was carried out to examine the morphology and microstructure of the 3D nanocomposite. As displayed in Figure 3, highly porous, wrinkled, and fluffy morphologies were observed for both the pristine graphene (Figure 3a,b) and 3D black TiO2/Ag/N@rGO (Figure 3c,d) samples at different magnifications. A closer look at the SEM images revealed a ridged morphology with more surface defects, irregular structures, and pores on the Ag, black TiO2, and nitrogen-doped rGO samples compared to the pristine graphene. This can be attributed to the defects introduced on the rGO surface due to the dopants and the loading of oxygen functional groups in the graphene sheets [34,35].
Energy-dispersive X-ray spectroscopy (EDS) analysis revealed the elemental composition of the 3D black TiO2/Ag/N@rGO sample, as shown in Figure 4. EDS elemental mapping demonstrated the uniform distribution of the dopants on the surface of the composite, as shown in Figure 5.

2.1.1. BET Surface Area and Pore Size Distribution of the 3D Solar Absorber

Specific surface area and porosity are critical parameters for efficient capillary force-driven water transport and effective solar-to-thermal conversion. The N2 adsorption–desorption isotherm of the 3D absorber at 77K and the corresponding pore size distribution curves are displayed in Figure 6a,b. The isotherm displayed a shape closest to a type IV adsorption isotherm, with a hysteresis, as defined by the IUPAC, demonstrating the presence of abundant mesopores and the predominantly mesoporous structure of the 3D solar absorber [36,37]. The BET surface area of 191.5 m2/g and pore volume of 0.48 cm3/g were attained based on the nitrogen adsorption–desorption isotherm. The pore size distribution obtained from the nitrogen desorption data was analysed using the density functional theory (DFT) method. As presented in Figure 6b, the particle size distribution indicated mesopores with a predominant radius of 1.73 nm. The isotherm exhibited a hysteresis from 0.4 < P/Po < 0.9, which is common in mesoporous materials owing to the nitrogen condensation in the pores [38]. On the other hand, the BET specific surface area of the 3D solar absorber synthesized in the absence of KOH activation was determined to be 22.37 m2/g based on the nitrogen adsorption–desorption isotherm (Supplementary Materials, Figure S1). The ultra-high specific surface area and the highly porous structure were attributed to the alkaline (KOH) activation of the 3D absorber after freeze drying [39]. KOH activation of carbon materials has been reported to generate abundant micro- and mesopores owing to a series of reactions between carbon and KOH involving the formation and decomposition of K2CO3 and the gasification of other K compounds [40]. The high specific surface area provided the benefit of increasing the areal exposure of the evaporative surface and the additional advantage of increasing the microchannel for effective water transfer to the evaporative surface of the solar absorber. The high specific surface area of the 3D porous structure ensured an adequate interface between the water at the evaporative surface and the localized heat, favouring the rapid release of the vapour generated and solar–thermal energy conversion.

2.1.2. Fourier Transform Infrared (FTIR) Spectroscopy Analysis of the 3D Absorber

The FTIR study was carried out to examine the degree of oxidation of the graphene and to determine the oxygen-containing functional groups in the oxidized graphene compared to the pristine graphene. Oxidation of graphene imparts hydroxy (C-OH) and epoxy (C-O-C) functional groups on the basal plane and carbonyl (C=O) and carboxyl (−COOH) functional groups on the lateral plane of the graphene [41]. As depicted in Figure 7, strong peaks were observed that were ascribed to carbonyls (C=O, ~1736 cm−1) and C=C from unoxidized graphite (1618 cm−1) hydroxyls (−OH, ~1421 cm−1 and a broad peak at ~3048 cm−1) [41,42].
The peak at ~1025 cm−1 was ascribed to the alkoxy C−O−C [43]. Following the reduction of the 3D absorber, there was a significant reduction in the intensity of the carbonyls (C=O, ~1736 cm−1), hydroxyls (−OH, ~1421 cm−1 and a broad peak at ~3048 cm−1), and alkoxy C-O-C (~1025 cm−1), confirming the reduction of GO to rGO and the restoration of sp2 conjugated carbon networks. The residual oxygen in the rGO facilitated the covalent bonding of the graphene’s basal and lateral plates and imparted wettability to the graphene-based composite. Oxidation of graphene disrupts the normal sp2-sp2 conjugated carbon network of the basal plane through a transition to tetrahedral sp3 hybridization to accommodate the extra bonding for oxidation [41]. However, following the reduction, as observed in the FTIR spectra shown in Figure 7, the oxygen content was significantly reduced, and this would have significantly restored the sp2 carbon–carbon network and conductivity of the graphene-based material. The residual oxygen functional groups on the reduced graphene oxide imparted hydrophilicity to the 3D absorber, enabling efficient water transfer to the evaporative area following capillary forces [40].

2.2. Solar Absorption Potential and Optical Absorption Property

Good broadband solar absorption performance is an important condition for efficient photothermal conversion and solar-driven water evaporation [44]. As presented in Figure 8, the 3D black TiO2/Ag/N@rGO sample showed excellent broadband solar absorption over the UV–Vis NIR region compared to the GO.
Owing to the disruption of the sp2 hybridized carbon network after the intercalation of the oxygen-containing functional groups, the GO functioned as a thermal insulator and showed almost no solar absorption in the visible and IR range [45]. However, following the hydrothermal reduction, the sp2 hybridized conducting network was restored. The composite showed efficient solar absorption over the whole range of broadband UV-Vis NIR regions, as presented in Figure 8. The 3D black TiO2/Ag/N@rGO demonstrated strong broadband solar absorption over the entirety of the UV-Vis NIR region monitored compared to GO. Such enhanced solar absorption in the broadband solar spectra is an essential condition for efficient photothermal conversion. In the UV-Vis NIR spectra, the absorption maximum at around 229 nm represented the π-π* bond of the aromatic C-C bond, and the shoulder at ~300 nm represented the n-π* of the (C=O) bond [46]. Following the hydrothermal reduction, the peak at 300 nm disappeared, and the π-π* bond of the aromatic C-C bond redshifted to ~296 nm, indicating the reduction of GO and the restoration of the π-conjugation network within the rGO [47]. However, it should be noted that the GO was not completely reduced, and the oxygen functional groups remaining after hydrothermal reduction played an important role in tailoring the hydrophilicity of the rGO, which is an important factor for efficient photothermal conversion.
The crystallinity and phase composition of the composite material were analysed using XRD, and the results are displayed in Figure 9. The peaks demonstrated the good crystallinity of the sample. The characteristic XRD peaks at the 2θ value of 25.6° assigned to the (002) plane of the hexagonal graphene structure with an interlayer d-spacing of 3.36 Å corresponded to the rGO [48]. The diffraction peak at 43.35° with the corresponding d-spacing of 2.1 Å in the XRD pattern was assigned to the disordered turbostaic band of the rGO carbon material [49,50]. The XRD peaks at the 2θ values of 25.44°, 38.04°. 48.02°, 53.9°,55.01°, 62.41°, 68.6°, 70.01°, and 74.55° corresponded to the (101), (004), (200), (105), (211), (204), (116), (220), and (215) planes of anatase TiO2 (JCPDS card no. 21-1272) [51]. The X-ray diffractograms of the sample demonstrated the presence of the anatase TiO2 phase only. No diffraction peaks for Ag were identified in any of the patterns, which could have been due to the low concentration of Ag, as it was dispersed on the surface with an undetectable low concentration [52]. Nevertheless, the EDS analysis and elemental mapping confirmed the homogenous distribution of AgNPs over the entire body of the 3D rGO.
The optical performance and corresponding band gap of the black TiO2 were determined and compared with the pristine TiO2. As displayed in Figure 10a, the black TiO2 demonstrated broadband solar absorption across the UV-Vis NIR region monitored, demonstrating its strong potential for the synergistic enhancement of the overall photothermal performance of the 3D black TiO2/Ag/N@rGO solar absorber.
The band-gap energy of a semiconductor, which demonstrates the energy required to excite an electron from the conduction band to the valence band, can be determined using a Tauc plot generated from the relationship shown in Equation (1) [53].
α h ν ϒ = B h ν − E g
where α is the absorption coefficient, h is the Planck constant, ν is the photon’s frequency, E g   is the band-gap energy, and B is a constant. The γ factor depends on the nature of the electron transition and is assigned values of 2 or 0.5 for direct and indirect band gaps, respectively. Figure 10b shows the Tauc plot for the direct band gap generated from the UV–Vis NIR data.
As shown in Figure 10b, the black TiO2 had a band gap of ~1.54 eV, while the pristine white TiO2 had a band gap of ~3.3 eV, endowing the former with broadband light absorption potential across the UV-Vis NIR region [22]. Black TiO2 contains charge carrier-trapping sites within the disordered surface layer, which prevent premature charge carrier recombinations and improve its photocatalytic activity [23]. The enhanced photocatalytic performance of the black TiO2 was related to the increased light absorption resulting from the introduction of lattice disorder and H-doping, which consequently narrowed the optical band gap of the black TiO2 [54]. Black TiO2 rGO composites have been reported to synergistically enhance photocatalytic activity through the reduction in charge carrier recombinations in black TiO2 and the unique thermal and electrical conductivity properties of rGO [55]. Black titania has been reported to exhibit broadband solar absorption and excellent solar steam-generation performance [56,57].

2.3. Photothermal Conversion and Solar-Driven Water Evaporation Potential

The photothermal conversion efficiency and the solar-driven interfacial water vapour generation performance of the 3D absorber were explored under 1-sun illumination (1 kW/m2). The photothermal performance of the 3D black TiO2/Ag/N@rGO absorber was evaluated by determining the mass of the water vapour generated and monitoring the surface temperature of the absorber periodically. The interfacial saline water desalination performance and the stability of the evaporator were investigated for a total time duration of 21 h in three rounds, each consisting of 7 h. The solar-driven water evaporation flux was determined by measuring the changes in the mass of the saline water in the vessel periodically using a digital balance.
The solar-driven water evaporation rate and photothermal conversion efficiency are two crucial parameters for evaluating interfacial desalination performance [58]. The evaporation rate—the quantity of vapour generated in a unit area per unit time with a unit of kg·m−2 h−1—was determined using Equation (2).
m ˙ = Δ m S × t  
where ∆m is the change in mass due to evaporation (kg), S is the effective absorption area (m2), and t refers to the solar irradiation time (h). The water vapour generated (as displayed in Figure 11) was determined to be 1.43 kg·m−2 h−1, while the water vapour generated in the control experiment carried out under the same conditions with no evaporator was determined to be 0.21 kg·m−2 h−1. The temperature of the 3D absorber was monitored using an infrared (IR) thermal imaging camera every 20 min. As observed in Figure 12, the surface temperature of the absorber increased significantly from 27.1 °C to 65.7 °C in 60 min. The 3D interconnected nanoporous structure showed remarkably good vapour generation performance compared to the control and exhibited effective salt rejection, with no salt precipitation on the surface of the evaporator throughout the entire duration of the desalination test carried out in three rounds. The effective salt rejection of the evaporator can be attributed to the nanoporous structure of the evaporator, which had a predominant pore width of 1.73 nm, functionalized with anionic oxygen-containing functional groups and the narrow interlayer spacing of the interconnected rGO nanosheets. The reduced interlayer spacing in the rGO, determined to be 3.36 Å (i.e., <7 Å), was ideal for salt rejection as it provided the potential to sieve hydrated Na+ and Cl− ions [59,60]. Controlled reduction, surface functionalization, and crosslinking of GO have been observed to improve the mechanical stability and salt rejection capacity of reduced graphene oxide-based evaporators [60]. The hydroxyl, epoxy, carbonyl, and carboxyl anionic oxygen functional groups in the bases and edges of the reduced graphene oxide nanosheets reject chloride ion permeation, thereby blocking salt formation on the evaporator surface [61,62,63]. In addition, the nitrogen doping of the graphene nanosheets enhanced the negative charge density, which enhanced the Cl anion rejection capacity of the 3D evaporator surface, preventing salt accumulation [60,64].
The photothermal conversion efficiency η was determined using Equation (3) [65].
η =   m ˙ . L v + Q P i
where ṁ refers to the mass flux (evaporation rate) of water; L v refers to the latent heat of the water vaporization (kJ kg−1), which depends on the vapour temperature; Q refers to the difference in sensible heat between the initial surface temperature T 1 and the final surface temperature T 2 ; and P i   is the incident solar power density, the value of which is1 kW⋅m−2.
Q = C∆T
where C is the specific heat capacity of water (4.2 J g−1 K−1), ∆T refers to the increase in the temperature of the water, and L v is the vaporization enthalpy at the surface temperature.
The latent heat of water L v is 2343.6 kJ kg−1 at 65.7 °C [66]. The photothermal conversion efficiency η was determined to be 99%. The high photothermal conversion efficiency observed can be compared to the efficacies reported in related studies [67,68,69]. To assess the synergistic effects of the Ag and black TiO2 dopants on the overall photothermal performance of the 3D black TiO2/Ag/N@rGO solar absorber, the photothermal performance of the 3D black TiO2/Ag/N@rGO was compared with the photothermal performances of 3D rGO and 3D Ag@rGO obtained in our previous experiments. The studies demonstrated that, while there were no significant changes in the broadband solar absorption performance over the UV–Vis NIR region, there were considerable increases in surface temperature and steam generation potential. While the surface temperatures of the 3D rGO and 3D Ag@rGO increased from 27.1 °C to 51.6 °C and 60.1 °C, respectively, the surface temperature of the 3D black TiO2/Ag/N@rGO increased from 27.1 °C to 65.7 °C. Similarly, the steam generation increased from 1.3 kg·m−2 h−1 to 1.41 kg·m−2 h−1 for the 3D rGO and 3D Ag@rGO solar absorbers and to 1.43 kg·m−2 h−1 for the 3D black TiO2/Ag/N@rGO solar absorber, demonstrating the greater role of the dopants in enhancing the performance of the hybrid 3D solar absorbers.

3. Discussion

The 3D rGO-based interconnected monolithic network demonstrated remarkable photothermal performance and outstanding interfacial heating, which were ascribed to its potential for broadband solar light absorption over the full solar spectrum range, inherent porous features, large surface area, and stable mechanical strength [26]. The broadband multidirectional solar absorption of the 3D rGO hybrid with porous microstructures resulted in multiple internal reflections of light, boosting the solar absorption potential [26]. The high photothermal conversion efficiency was attributed to the synergistic photothermal performance of the AgNPs, the nitrogen doping, and the black TiO2 composites used for the 3D rGO-based evaporator. Hybridization of materials, including bimetallic materials, metal–organic materials, and metal–semiconductor composites, has been reported to lead to enhanced photothermal conversion efficiency through the combination of the merits of the individual materials’ optical properties [19]. Hybridization of reduced graphene oxide with plasmonic AgNPs has been reported to enhance photothermal conversion efficiency owing to the synergistic localized surface plasmon resonance effect of the AgNPs and the broadband solar absorption capacity of the rGO structure resulting from its π electrons and high porosity [21]. Furthermore, addition of AgNPs to TiO2 can form a Ag/TiO2 Schottky heterojunction that inhibits the recombination of electron–hole pairs, enhancing the photothermal performance of 3D solar absorbers [51,70].
Oxygen-deficient black TiO2 particles can boost the photothermal performance and electrical conductivity of 3D black TiO2/Ag/N@rGO composite materials owing to their structural, electronic, and optical properties [54]. Black TiO2 and rGO composites have been reported to synergistically enhance photocatalytic activity due to the reduction in charge carrier recombination provided by the black TiO2 and the unique thermal and electrical conductivity properties of rGO [55]. The hybridization of rGO with black TiO2 has been reported to synergistically enhance the photothermal conversion efficiency of the composite [24]. The improved photothermal performance of the 3D absorber was also ascribed to the synergistic effect of the nitrogen doping of the rGO. As it has a comparable atomic size to carbon and five valence electrons for bonding with carbon, nitrogen has been widely used for bonding with carbon in various photothermal applications [71]. Nitrogen doping of graphene has been reported to modulate the electronic configuration, increase the conductivity, and boost the electrocatalytic and photothermal activities of graphene derivates owing to the activation of the excess free-flowing π electrons of the sp2 configuration of graphene [72]. Heteroatom doping of graphene with heteroatoms such as N, B, and P has been reported to increase the hydrophilicity of the grapheme, introduce additional electrocatalytically active sites, and increase conductivity. Hence, nitrogen activation plays a significant role in creating a defective morphology and enhancing the photothermal performance of the composite [25].
In addition to the remarkable photothermal performance of the absorber, the efficient thermal management resulted in efficient thermal insulation for the conductive heat loss to the bulk water and the radiative and convective heat losses to the external environment. The floating cellulose sponge not only served as a support material for the 3D absorber but also effectively shielded the conductive heat loss to the bulk water. The thermally insulating plastic cover on the top and lateral sides of the absorber shielded the radiative and convective heat losses. The use of a porous matrix material meant that a steady source of water could be obtained through the capillary force on the interface. The localized thermal energy was directly used to heat the water on the porous absorber surface that was transferred through the cellulose sponge via the capillary force, minimizing the heat loss to bulk water and resulting in high vapour generation and photothermal conversion efficiency. Thus, the overall remarkable solar-to-heat conversion of the 3D black TiO2/Ag/N@rGO absorber can be attributed to the synergistic effects of each component and efficient thermal management. Furthermore, the efficient salt rejection of the 3D hybrid evaporator, which is the main problem in many conventional desalination systems, significantly contributed to the stable and efficient performance of the evaporator.
Table 1 presents a comparison of evaporation flux values from various interfacial evaporation studies carried out under different solar intensities.

4. Materials and Methods

4.1. Materials

Graphite flakes, potassium permanganate (KMnO4 ≥99%), hydrochloric acid (HCl, 30 wt.%), sulphuric acid (H2SO4 > 98%), ortho-phosphoric acid (H3PO4 > 97%), hydrogen peroxide (H2O2, 30 wt.%), silver nitrate (AgNO3 > 99%), Titanium oxide (Degussa P-25 > 98%), sodium boron hydride (NaBH4 > 98%), and Ethylenediaminetetraacetic acid (EDTA > 98%) were purchased from Sigma Aldrich-Merck (Darmstadt, Germany).

4.2. Synthesis of Black TiO2, Ag, and Nitrogen-Doped 3D Reduced Graphene Oxide (3D Black TiO2/Ag/N@rGO)

4.2.1. Synthesis of GO

Graphene oxide was synthesized using Tour’s method, as described by Habte and Ayele [73]. Briefly, a mixture of 360 mL of concentrated sulphuric acid (H2SO4) and 40 mL of concentrated ortho-phosphoric acid (H3PO4) (9:1 (v/v) ratio) was prepared and poured into a mixture of 3.0 g graphite powder and 18 g potassium permanganate (KMnO4), resulting in 40–45 °C temperature changes. The mixture was heated to 50 °C using a temperature-controlled water bath and stirred for 24 h. As the reaction continued, the mixture became pasty and muddy. After 24 h, 400 mL of ice water was added to the mixture, then 30 mL of 30 wt.% H2O2 was added to reduce manganese ions to soluble manganese sulphate and manganese oxides and stop the reaction. When the 30 wt.% H2O2 was added, a bright yellow colour was observed, indicating a high oxidation level. The resulting mixture was centrifuged (10 min at 8000 rpm), the supernatant was discarded, and the solid was washed repeatedly with HCl (30 wt.%, 200 mL) solution to remove the sulphate ions completely.

4.2.2. Synthesis of Black TiO2

Black TiO2 was prepared using sodium boron hydride (NaBH4, 98%) reduction of commercial TiO2 (P25, with an anatase to rutile ratio of 80:20), with a slight modification of the previously described method [74]. In this method, 5 g of white TiO2 nanoparticles was thoroughly mixed with 4 g of NaBH4. Then, the mixture was placed in a porcelain boat at 500 °C for 3 h in a N2 environment. The obtained product was cooled down and washed with deionized water several times, dried, powdered, and used to synthesize the 3D black TiO2/Ag/N@rGO.

4.2.3. Synthesis of 3D Black TiO2/Ag/N@rGO

Citrate-stabilized and monodispersed Ag nanoparticles were synthesized through the reduction of AgNO3 using NaBH4 in the presence of trisodium citrate, as previously described by Haber and Sokolov [75]. Ethylenediaminetetraacetic acid (EDTA) was used as a nitrogen source for the nitrogen doping of the rGO. Subsequently, for the synthesis of 3D Ag, black TiO2, nitrogen-doped rGO, 5 g of AgNPs, 10 g of black TiO2 powder, and 20 g of EDTA components were added into a 250 mL Erlenmeyer flask containing 100 mL of 10 mg/mL GO aqueous dispersion and ultrasonicated vigorously for 15 min. The solution was then transferred to a 300 mL Teflon-lined autoclave, sealed, and heated at 180 °C (24 h) for the hydrothermal reduction of the GO suspension containing the composites. The final 3D product was freeze dried for 72 h and used for the interfacial desalination study. rGO can be considered as chemically derived graphene, the structure of which can be varied from one layer to multiple layers, with incomplete removal of oxygen atoms from the carbon structure [76]. The KOH activation process was carried out by mixing 30 mL of 5 mol/L KOH solution with the GO suspension consisting of the constituents mentioned above. Subsequently, a 3D black TiO2/Ag/N@rGO monolith was obtained by freeze drying followed by thermal reduction under N2, as previously described by Wang et al. [77].

4.3. Characterization

High-field-emission scanning electron microscopy (FESEM) coupled with energy-dispersive X-ray (EDX) analysis (EDS, JEOL-7800F, JEOL Ltd., Tokio, Japan) was carried out to study the morphology and elemental composition of the as-synthesized samples. The broadband-diffused solar absorption performance of the GO and rGO samples was assessed over a broad wavelength range of 200–2500 nm using a UV–Vis NIR spectrophotometer (UV3600, Shimadzu Scientific Instruments, Columbia, MD, USA). For the UV–Vis NIR spectroscopy analysis, samples were prepared by dispersing the same amounts of GO and rGO in ethylene glycol with a concentration of 0.5 mg mL−1 through 30 min of ultrasonication. The specific surface areas of the samples were measured using the dynamic Brunauer–Emmett–Teller (BET) method, and pore size distribution was analysed using density functional theory (DFT) with a N2 adsorption isotherm at 180 °C collected with a Micromeritics ASAP 2000 system (Norcross, GA, USA). A Fourier Transform Infrared (FTIR) spectrometer (Perkin Elmer Spectrum 100, Waltham, MA, USA) with a spectral range of 4000–400 cm−1, an increase of 32 scans, and a resolution of 0.2 cm−1 was employed to detect functional groups and other molecular vibrations in the prepared samples. X-ray diffraction (XRD) analysis was carried out using a Rigaku Ultima IV X-ray diffractometer (Tokyo, Japan). with Cu-Kα radiation (γ = 0.154056 nm) at 40 kV and 30 mA over the 2θ range of 0 to 80°. The spacing between the reduced graphene oxide sheets (d) was determined using the Bragg equation: 2d sinθ = nλ [78], where λ is the X-ray beam wavelength (0.154 nm), d is the interlayer distance, and θ is the diffraction angle.

4.4. Solar Steam Generation/Desalination

The solar steam generation/desalination study was performed with the help of a xenon-arc lamp-based solar simulator (SS-X, China) under 1-sun illumination (1 kW m−2). A cellulose sponge with a very low thermal conductivity of 0.043 W m−1 K−1, high hydrophilicity, and a highly porous morphology, which is beneficial for thermal insulation and rapid transfer of water [79], was procured from a local shop and used to support the 3D solar absorber. The 3D black TiO2/Ag/N@rGO aerogel (7.0 × 4.5 cm) was placed on the cellulose sponge, which was floating on a saline water body within a glass basin and served as both a thermal insulating medium to shield the transfer of thermal energy to the bulk non-evaporative water and as a water propagation channel for a continuous water supply. Hence, placing the cellulose foam between the solar absorber and the bulk water, as portrayed in Figure 1, made it possible to avoid direct contact between the solar absorber and the bulk water and ensured there was a sufficient continuous water supply for the solar absorber. The whole evaporator was enclosed with a transparent polymeric cover on the top to reduce heat loss and a polypropene freshwater collector at the bottom. The condensed water flowed into the polypropene collector, and the mass change in the water in a beaker was determined using an analytical balance. Saline water with 35 wt.% NaCl was used in the desalination study to simulate high-salinity seawater. The mass of evaporated water was measured with an analytical balance (MS205DU, Mettler Toledo, Greifensee, Switzerland). An infrared (IR) camera (TG165, FLIR Systems, Portland, OR, USA) was used to monitor the surface temperature of the evaporator.

5. Conclusions

In contrast to conventional solar-driven water evaporation systems, which generate stream through volumetric heating of the bulk water body, the interfacial solar-driven desalination approach focusing on localization of heat at the water-air interface has demonstrated excellent photothermal conversion potential and evaporation rates. In the current work, we synthesized 3D black TiO2/Ag/N@rGO, which demonstrated broadband solar absorption across the UV-Vis NIR region and good photothermal conversion performance. The absorber attained 99% photothermal conversion efficiency and 1.43 kg·m−2 h−1 water vapour generation. The excellent photothermal performance can be attributed to the synergistic effects of the 3D black TiO2/Ag/N@rGO composite, which combined the merits of each component. The effective thermal management of the heat generated was equally vital for the observed photothermal conversion performance and vapour generation. The thermal management and efficient water transfer adjustments using the cellulose sponge played important roles in the achievement of the observed photothermal performance. The current solar-driven interfacial desalination study demonstrates the promising potential of the 3D hybrid composite for real-world applications, making it possible to utilize inexhaustible solar energy to generate fresh water from abundant hypersaline seawater and other saline water sources and overcome the water shortage challenge facing the modern world.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal13030514/s1, Figure S1: The nitrogen adsorption-desorption isotherms for the non-KOH-activated 3D black TiO2/Ag/N@rGO solar absorber.

Author Contributions

Conceptualization, F.A.B. and E.M.N.C.; methodology, F.A.B. and S.A.I.; formal analysis, F.A.B., S.A.I., S.M.T. and E.M.N.C.; investigation, F.A.B., S.A.I. and S.M.T.; resources, S.M.T. and E.M.N.C.; data curation, F.A.B. and S.A.I.; writing—original draft preparation, F.A.B.; writing—review and editing, F.A.B., S.A.I., S.M.T. and E.M.N.C.; visualization, F.A.B. and E.M.N.C.; supervision, E.M.N.C.; project administration, E.M.N.C.; funding acquisition, F.A.B. and E.M.N.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Water Research Commission (WRC) of the Republic of South Africa.

Data Availability Statement

Not applicable.

Acknowledgments

We acknowledge the support given by the Solar Energy Research and Carbon Based Nano Materials Research Groups of the Department of Physics at the University of Pretoria for providing us the solar simulator and freeze dryer facilities.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 00514 g001 550
Figure 1. Digital images of pristine graphite (a); 3D black TiO2/Ag/N@rGO (b); the cellulose sponge support and thermal insulator (c); and the interfacial desalination setup with the solar absorber enclosed in a transparent plastic cover to shield radiative and convective heat losses, illuminated with 1-sun solar irradiation (d).
Figure 1. Digital images of pristine graphite (a); 3D black TiO2/Ag/N@rGO (b); the cellulose sponge support and thermal insulator (c); and the interfacial desalination setup with the solar absorber enclosed in a transparent plastic cover to shield radiative and convective heat losses, illuminated with 1-sun solar irradiation (d).
Catalysts 13 00514 g001
Catalysts 13 00514 g002 550
Figure 2. Schematic showing the solar-driven interfacial desalination setup.
Figure 2. Schematic showing the solar-driven interfacial desalination setup.
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Catalysts 13 00514 g003 550
Figure 3. Field-emission scanning electron microscopy (FESEM) images of pristine graphene (a,b) and black TiO2/Ag/N@rGO with numerous surface defects, irregular structures, and pores (c,d).
Figure 3. Field-emission scanning electron microscopy (FESEM) images of pristine graphene (a,b) and black TiO2/Ag/N@rGO with numerous surface defects, irregular structures, and pores (c,d).
Catalysts 13 00514 g003
Catalysts 13 00514 g004 550
Figure 4. EDS elemental composition of the as-synthesized porous 3D black TiO2/Ag/N@rGO sample.
Figure 4. EDS elemental composition of the as-synthesized porous 3D black TiO2/Ag/N@rGO sample.
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Catalysts 13 00514 g005 550
Figure 5. EDS elemental mapping of the as-synthesized porous 3D black TiO2/Ag/N@rGO sample.
Figure 5. EDS elemental mapping of the as-synthesized porous 3D black TiO2/Ag/N@rGO sample.
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Catalysts 13 00514 g006 550
Figure 6. N2 adsorption–desorption isotherm (a) and pore size distribution (b) for the 3D black TiO2/Ag/N@rGO material at 77K.
Figure 6. N2 adsorption–desorption isotherm (a) and pore size distribution (b) for the 3D black TiO2/Ag/N@rGO material at 77K.
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Catalysts 13 00514 g007 550
Figure 7. FTIR spectra of graphene oxide (GO, red), reduced graphene oxide (rGO, blue), and pristine graphene (black).
Figure 7. FTIR spectra of graphene oxide (GO, red), reduced graphene oxide (rGO, blue), and pristine graphene (black).
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Catalysts 13 00514 g008 550
Figure 8. Broadband UV-Vis NIR absorption spectra for 3D black TiO2/Ag/N@rGO (orange colour) and GO (green colour) samples.
Figure 8. Broadband UV-Vis NIR absorption spectra for 3D black TiO2/Ag/N@rGO (orange colour) and GO (green colour) samples.
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Catalysts 13 00514 g009 550
Figure 9. XRD pattern for the 3D black TiO2/Ag/N@rGO solar absorber.
Figure 9. XRD pattern for the 3D black TiO2/Ag/N@rGO solar absorber.
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Catalysts 13 00514 g010 550
Figure 10. UV–Vis NIR absorption spectra (a) and band-gap energy (b) of the black TiO2 and pristine TiO2 estimated from their corresponding Tauc plots.
Figure 10. UV–Vis NIR absorption spectra (a) and band-gap energy (b) of the black TiO2 and pristine TiO2 estimated from their corresponding Tauc plots.
Catalysts 13 00514 g010
Catalysts 13 00514 g011 550
Figure 11. (a) Digital photos of the solar-driven interfacial desalination tests at the beginning of the experiment and after 30 min, respectively; (b) the average evaporation rate over a seven-hour experiment run in triplicate under 1-sun solar illumination.
Figure 11. (a) Digital photos of the solar-driven interfacial desalination tests at the beginning of the experiment and after 30 min, respectively; (b) the average evaporation rate over a seven-hour experiment run in triplicate under 1-sun solar illumination.
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Catalysts 13 00514 g012 550
Figure 12. Infrared photos of the surface temperature of the 3D black TiO2/Ag/N@rGO absorber under 1-sun solar illumination taken every 20 min (a); average surface temperature of the evaporator monitored over the entire 480 min interfacial desalination test (b).
Figure 12. Infrared photos of the surface temperature of the 3D black TiO2/Ag/N@rGO absorber under 1-sun solar illumination taken every 20 min (a); average surface temperature of the evaporator monitored over the entire 480 min interfacial desalination test (b).
Catalysts 13 00514 g012
Table
Table 1. Comparison of various evaporation fluxes from various interfacial evaporation studies carried out under different solar intensities.
Table 1. Comparison of various evaporation fluxes from various interfacial evaporation studies carried out under different solar intensities.
ReferenceEvaporation Flux (kg·m−2 h−1)
[16]1.31 under 1 sun
[17]1.44 under 1.2 suns
[18]1.36 under 1 sun
[27]From 1.42 to 1.49 under 1 sun
[28]1.37, 1.85, and 2.25 under 1 sun
[21]1.28, 2.76, 4.01, and 5.43 kg under 1, 2, 3, and 4 suns, respectively
[30]1.6 to 3.71 under 1 sun
[31]1.57 under 1 sun
[67]1.27 kg/m2 h under 1 sun
[67]6.70 kg/m2 h under 5 suns
[68]1, 1.1, 1.3, 1.4, and up to 2.6 under 1 sun
[69]1.62 under 1 sun
Current study 1.43 under 1 sun
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MDPI and ACS Style

Bezza, F.A.; Iwarere, S.A.; Tichapondwa, S.M.; Chirwa, E.M.N. Fabrication and Application of Ag, Black TiO2 and Nitrogen-Doped 3D Reduced Graphene Oxide (3D Black TiO2/Ag/N@rGO) Evaporator for Efficient Steam Generation. Catalysts 2023, 13, 514. https://doi.org/10.3390/catal13030514

AMA Style

Bezza FA, Iwarere SA, Tichapondwa SM, Chirwa EMN. Fabrication and Application of Ag, Black TiO2 and Nitrogen-Doped 3D Reduced Graphene Oxide (3D Black TiO2/Ag/N@rGO) Evaporator for Efficient Steam Generation. Catalysts. 2023; 13(3):514. https://doi.org/10.3390/catal13030514

Chicago/Turabian Style

Bezza, Fisseha A., Samuel A. Iwarere, Shepherd M. Tichapondwa, and Evans M. N. Chirwa. 2023. "Fabrication and Application of Ag, Black TiO2 and Nitrogen-Doped 3D Reduced Graphene Oxide (3D Black TiO2/Ag/N@rGO) Evaporator for Efficient Steam Generation" Catalysts 13, no. 3: 514. https://doi.org/10.3390/catal13030514

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