Air–water interface solar heating using titanium gauze coated with reduced TiO2 nanotubes
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Using method of electrochemical anodization and subsequent reduction, titanium gauze with reduced TiO2 nanotubes on the surface (reduced TiO2 nanotubes/Ti gauze) was prepared and used for air–water interface solar heating. The electrochemical reduction method can generate Ti3+ and causes the narrowing of optical band gap of TiO2 (ca. 2.91 eV). Combining with the nanotubular structure, reduced TiO2 nanotubes/Ti gauze demonstrated higher absorption ability of visible light than other types of titanium gauzes (reduced P25 TiO2 nanoparticles/Ti gauze, TiO2 nanotubes/Ti gauze and P25 TiO2 nanoparticles/Ti gauze). For evaluating the property of air–water interface solar heating, solar water evaporation test was conducted. The results demonstrated that the reduced TiO2 nanotubes/Ti gauze can efficiently accelerate water evaporation. The water evaporation rate and solar thermal conversion efficiency were 1.41 kg m−2 h−1 and 44.2%, respectively, under solar light irradiation with intensity of 2 kW m−2, which are higher than that of reduced P25 TiO2 nanoparticles/Ti gauze, TiO2 nanotubes/Ti gauze, P25 TiO2 nanoparticles/Ti gauze and pristine Ti gauze. It was further found that the solar thermal conversion efficiency of reduced TiO2 nanotubes/Ti gauze attained 84.2% when solar light intensity increased to 5.6 kW m−2. This work may provide a new route to design more advanced photothermal materials for industrial applications such as waste water treatment, salt production and solar desalination.
Because of the population growth, climate change, industrial expansion and water pollution around the world, many countries, especially in Africa and the Middle East, are suffering from serious fresh water scarcity. In response to this issue, solar water evaporation is one of the most common solar energy-harvesting technologies that can be developed to satisfy the increasing global requirements on fresh water . Naturally, solar water evaporation can provide 90% of moisture in the atmosphere, produce rain, and attain global water cycling . This approach of water evaporation has also been utilized for seawater desalination using solar energy to mitigate the fresh water shortage [3, 4]. In general, water evaporation is a surface process, and only the water located in surface region can change from liquid phase to vapor phase. However, conventional solar water evaporation is bulk water heat behavior, which employs volumetric solar energy absorbers to heat the bulk water directly. Therefore, a significant portion of non-evaporative water beneath surface region will be concurrently heated, and optical and systematic heat loss will occur and cause a low solar thermal conversion efficiency . In view of this shortcoming for conventional solar water evaporation, a new and rational concept, named “Air–Water Interface Solar Heating,” has attracted increasing interest for the minimized heat loss from heating spot to bulk water.
The concept of “Air–Water Interface Solar Heating” is enhancing only the local temperature of surface water using floating and photothermal materials. With photothermal materials floating on top surface, solar energy can be maximally gathered and utilized in surface water region, generating a localized high temperature. The localized high temperature of surface water can thus diminish the heat loss caused by the non-evaporative lower part of bulk water [6, 7]. Recent efforts on the “Air–Water Interface Solar Heating” technology have focused on exploring materials with high photothermal conversion efficiency. The criteria for designing ideal photothermal materials include three aspects: (1) ability to effectively capture or concentrate solar irradiance at the air–water interface; (2) high efficiency of converting solar energy to heat energy; (3) inexpensive and technological demanding for large-scale expansion [8, 9]. Based on these criteria, to date, carbon materials, semiconductor materials and plasmonic metal nanoparticulate materials have all been reported to function as efficient solar energy absorbers, and high photothermal performance can be achieved [10, 11, 12]. Combining with meticulous membrane-construction technique, these materials can be efficiently used for air–water interface solar water evaporation. For example, Liu et al.  coated carbon black on cotton gauzes through dip-coating method, and achieved water evaporation rate 2–3 times higher than that of traditional process for solar water evaporation application. Through pulsed-laser deposition method, Yao et al.  deposited SnSe onto nickel foam for solar-driven water evaporation, and the maximum water evaporation rate was 0.85 kg m−2 h−1 under solar light irradiation at 100 mW cm−2.
Since black TiO2 nanocrystals were firstly prepared through hydrogenation method in 2011, colorful reduced TiO2 nanomaterials have got continual research interests . Very recently, reduced titanium oxide (TiO2) has stimulated further interests in the area of solar light utilization due to the narrow band gap. For the narrow band gap, the typical character of reduced TiO2 compared to pristine TiO2 is the optical response from ultraviolet light to visible and/or infrared light region . It has been extensively reported that, compared to pristine TiO2, reduced TiO2 possess improved photocatalytic properties under solar light for the enhanced solar light absorption property [17, 18]. So reduced TiO2 has the potential to find efficient applications in solar thermal conversion system through meticulous research on its photothermic capability. Until now, some reduced TiO2 materials, which include TiO x (x < 2) nanoparticles, Ti2O3 nanoparticles and black TiO2 nanocages, have been prepared and used for the light to heat conversion system [19, 20, 21]. However, most of the reported materials need to be firstly synthesized and then integrated into porous supports (such as meshes, gauzes and films) for excellent self-floating ability. This fabrication process is always complex, low level of control and high cost. The porous supports are often breakable air-laid paper and polymeric membrane, which cannot bear severe external environment during water evaporation [22, 23]. It is therefore desirable to explore new avenue for the fabrication of photothermal materials with not only high solar thermal conversion efficiency but also simple manufacturing process, porous supports with more robust architecture are also in demand.
Titanium is an abundant element on the earth; many titanium products, such as titanium sheet, titanium foil, titanium mesh, titanium gauze and titanium foam, can be manufactured with high purity and various sizes. These titanium products can work as important raw material for the fabrication of titanium oxides. In 1999, Zwilling et al.  firstly reported the preparation of TiO2 nanotubes on the surface of titanium foil through anodization in fluoride-containing solution. The anodized TiO2 nanotubes have find various applications in the area of photocatalysis, fuel cells and biomedical coating . However, to the best of our knowledge, the usage of reduced TiO2 nanotubes as photothermal material for the “Air–Water Interface Solar Heating” technology has yet to be investigated. Therefore, in this paper, we reported for the first time that titanium gauze with reduced TiO2 nanotubes on the surface can be used for the “Air–Water Interface solar Heating” technology. The reduced TiO2 nanotubes were formed through direct anodization of titanium gauze and subsequent electrochemical reduction method. The reduced TiO2 nanotubes showed enhanced total absorption of solar light due to the co-doping of Ti3+ and nanotubular structure. Under solar light irradiation with light intensity of 2 kW m−2, titanium gauze with reduced TiO2 nanotubes delivered water evaporation rate of 1.41 kg m−2 h−1 and solar thermal conversion efficiency of 44.3%, which is higher than that of titanium gauzes with TiO2 nanotubes, P25 TiO2 nanoparticles and reduced P25 TiO2 nanoparticles. It is also demonstrated that the solar thermal conversion efficiency can be as high as 81.3% when light intensity reaches 5.6 kW m−2.
Titanium gauze with TiO2 nanotubes
Anodization method was chosen to prepare TiO2 nanotubes on the surface of titanium gauze. Before anodization, titanium gauze (purity: 99.6%; 100 meshes) was firstly cut into small circular pieces with diameter of 20 mm, and then ultrasonic washed with acetone, ethanol and purified water for about 3 min, respectively. The anodization experiment was conducted in a two electrodes system with titanium gauze and platinum foil as working and counter electrodes, respectively, following a two-step process. In the first step, the electrolyte was composed of 57.7 g ethylene glycol, 1.2 g H2O and 0.18 g NH4F. Anodization voltage was 50 V. Time was 25 min. After anodization, the as-anodized titanium gauze was ultrasonic treated in purified water to remove the anodic TiO2 nanotubes on the surface. In the second step, the titanium gauze obtained in the first step was anodized again in the electrolyte containing 57.9 g glycerol, 1.8 g H2O and 0.3 g NH4F. Anodization voltage was 60 V. Time was 1 h. After anodization, titanium gauze was washed with purified water for several times and dried under ambient condition. The calcination of obtained titanium gauzes was conducted in air condition at 150 °C for 3 h, then up to 450 °C for 5 h with a heating and cooling rate of 5 °C min−1.
Titanium gauze with P25 TiO2 nanoparticles
Dip-coating method was chosen to prepare titanium gauze with P25 TiO2 nanoparticles [26, 27]. 5 wt% P25 TiO2 aqueous slurry with pH value of 6.14 was prepared for dip coating. Since surface charge of titanium gauze is negative, the positive charged P25 TiO2 nanoparticles can be deposited on the surface of titanium gauze for the electrostatic attraction [28, 29]. After preparation of TiO2 aqueous slurry, the as-obtained titanium gauze was dipped in the slurry for 1 min and then dried in air dry oven at 70 °C for 30 min. Deposition of P25 TiO2 nanoparticles on the surface of titanium gauze can be achieved for different surface charges. This process was repeated eight times to get 3.5 µm thick P25 TiO2 nanoparticulate film coated on titanium gauze.
Electrochemical reduction was carried out in a two electrodes system. Titanium gauzes with TiO2 nanotubes or P25 TiO2 nanoparticles on the surface were employed as cathode electrode. Platinum foil was employed as anode electrode. The electrolyte was 0.5 mol L−1 Na2SO4 aqueous solution. The applied voltage was 5 V. Time was 90 s. After electrochemical reduction experiment, the cathode electrode of titanium gauze was taken out, washed with purified water and dried under air condition for further use and characterization. Here, titanium gauzes with reduced TiO2 nanotubes and reduced P25 TiO2 nanoparticles are denoted as RTNT-Ti gauze and RP25-Ti gauze, respectively. Titanium gauzes with TiO2 nanotubes and P25 TiO2 nanoparticles are denoted as TNT-Ti gauze and P25-Ti gauze, respectively.
Titanium gauzes were immersed in methanolic solution containing 1.0 wt% 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane (CF3(CF2)5CH2CH2 Si(OCH2CH3)3, PTES, Aladdin) for hydrophobization. The solvent of methanolic solution is a mixture of pure water and methanol (molar ratio: 3:1). After immersion in methanolic solution for 1 h, the samples were taken out, washed with methanol, and subsequently heated at 140 °C for 1 h.
The morphological details of samples were characterized using scanning electron microscopy (SEM, JSM-7001F) and transmission electron microscopy (TEM, JEM-2100F, 200 kV). The crystal structure of samples was examined on an X-ray diffractometer (XRD, DX2700B) at scan rate of 0.08° s−1. Chemical composition of samples was measured on an X-ray photoelectron spectroscopy with ESCALAB 250Xi XPS system using Al anode. Optical characterization of samples with circular diameter of 2 cm was conducted using a UV–Vis spectrometer (UV-2550) equipped with an integrating sphere; the reflected beam including both specular and diffuse reflections from the gauze was scattered and collected in the integrating sphere and measured using a photomultiplier tube detector, and fine BaSO4 powder was used as reference material. The contact angle was measured using an OCAH-230 contact angle measuring device (Dataphysics, Germany). Electron paramagnetic resonance spectra (EPR) were collected at 93 K on a Bruker EMX-8/EPR spectrometer. The concentrations of ions in the water were measured by inductively coupled plasma mass spectroscopy (PerkinElmer Nexion 300, USA).
Results and discussion
According to Eqs. 1 and 3, doping of Ti3+ can also generate larger amount of –OH groups. This is confirmed in Fig. 3b, where a more defined –OH peak emerges for RTNT-Ti gauze. The presence of Ti3+ defects was further verified by EPR measurement. In EPR spectra of Fig. 3c, there are three features: signals from Ti3+, oxygen vacancy and O2−. The signals at g = 1.926, g = 1.956, g = 1.981 correspond to bulk Ti3+, the signal at g = 2.002 corresponds to oxygen vacancy, and the signal at g = 2.035 corresponds to O2− [37, 38]. By comparison, EPR signals belonging to Ti3+ and oxygen vacancy are more intense and discernable for RTNT-Ti gauze, indicating the doping of Ti3+ in bulk TiO2 through electrochemical reduction method. Moreover, Ti3+ may also exist on the surface of TiO2, reduces the adsorbed atmospheric O2, and produces O2− . Therefore, Ti3+ could also increase the intensity of EPR signal of O2−. This is confirmed in Fig. 3c, where EPR signal of O2− increased sharply after electrochemical reduction. In a word, the XPS and EPR spectra confirmed the co-doping of Ti3+ inside TiO2 nanotubes through electrochemical reduction method.
The temperature test was also conducted before water evaporation experiment. Under solar light irradiation, the air–water interface region could be heated up to a maximum temperature when the heat generated by gauzes equals the heat dissipated to surrounding. Figures 5b and S4 demonstrate the time dependent curve of temperature difference between top and bottom water region for various samples with irradiation time of 10 min. Within illumination of 7 min, the temperature difference for all samples reached steady state. After irradiation of 9 min, the temperature difference reached to only ~ 1.8 °C for water only system. However, when titanium gauzes are floating on water surface, the temperature difference increased significantly. For RTNT-Ti gauze, the temperature difference can be as high as 14.1 °C after 9 min irradiation. This sufficiently justifies our strategy of air–water interface heating using titanium gauzes, which is different from conventional bulk heating nature of water only system. With titanium gauzes floating on the surface, its internal pores with length of side of 180–200 nm can not only work as evaporation chamber but also help to capture the incident light, so the air–water interface heating scheme emerge .
With different types of titanium gauzes floating on the surface, the water also demonstrated various temperature differences under the same illumination condition. As shown in Figs. 5b and S4, within 7 min, the temperature difference rises to 14.1 and 11.2 °C, respectively, for RTNT-Ti gauze and RP25-Ti gauze. This is significantly higher than that of TNT-Ti gauze, P25-Ti gauze and Ti gauze, for which the temperature differences are only 7.7, 6.9 and 6.8 °C, respectively. This indicates that the localized heating property has been increased for electrochemical reduced samples. We reason that the improved localized heating effect was induced by the co-doping of Ti3+ inside TiO2. From above discussion, since Ti3+ was generated inside, band gap of TiO2 was narrowed. For the narrowed band gap, a larger portion of photographs from solar light will bring more above band gap electron–hole pairs, which will cause thermalization process through relaxing to band edges . The narrowed band gap also caused improved visible light absorption capacity (Fig. 4). It has also been reported that excitations of electrons in shallow trap states could be enhanced for the co-doping of Ti3+ inside TiO2, improved light absorption in the range of 800–2500 nm can also be expected [46, 47, 48]. Therefore, electrochemical reduced samples of RTNT-Ti gauze and RP25-Ti gauze can more effectively harvest solar energy and generate heat; the surface water temperature increases faster than the underneath water temperature. This is also confirmed in the infrared thermal images shown in Figs. 5c and S4. In the presence of TNT-Ti gauze, P25-Ti gauze and Ti gauze, the top surface temperature reached to 31.3, 30.2 and 29.5 °C, respectively. In contrast, the top surface temperature for RTNT-Ti gauze and RP25-Ti gauze rose to 37.5 and 33.6 °C, indicating improved photothermal performance of TiO2 nanotubes and P25 TiO2 nanoparticles by the electrochemical reduction method.
Herein, it is worth to note that the temperature difference for RTNT-Ti gauze is higher than that for RP25-Ti gauze. This could be ascribed to the convolution of several factors, which include size of nanocrystals, grain boundary contacts of nanocrystals and structural geometry. On the one hand, different from the P25 TiO2 nanoparticulate film (Fig. S1), the anodic TiO2 nanotubular film is integrated with many “crevasses” inside. The integrated structure can effectively inhibit the formation of interface boundaries between nanocrystals, where the trapping and direct recombination of photogenerated electron–hole pairs is available . It has also been reported that the “crevasses” structure is benefit for broadband absorption in the visible to near infrared light region due to the nanofocusing effect . On the other hand, the nanotubular structure may also increase the total light absorption capacity due to the light trapping effect of nanotubes . In Fig. 1g, it can also be observed that “knots” were formed on tube wall. The “knots” on tube wall could enhance the light scattering inside nanotube array, and further improve the light absorption capacity [52, 53, 54]. For above reasons, anodic TiO2 nanotubes could be typically an optimal material for solar thermal conversion applications when compared to P25 TiO2 nanoparticles. The photothermal conversion efficiency can be efficiently improved through the anodic fabrication of TiO2 nanotubes on titanium gauze and subsequent electrochemical reduction method.
In summary, we have fabricated titanium gauze with reduced TiO2 nanotubes on the surface through anodization and subsequent electrochemical reduction method. The electrochemical reduction process caused the visible light absorption capacity due to the narrowed band gap for the formation of Ti3+ inside TiO2. Compared with P25 TiO2 nanoparticulate film, TiO2 nanotubular film can further enhance the solar light utilization efficiency for the unique features of nanotube and “crevasses.” After hydrophobization by PTES modification, titanium gauzes can spontaneously float on water surface. For water evaporation tests, air–water interface solar heating can be achieved efficiently for titanium gauze with reduced TiO2 nanotubes. The water evaporation rate and solar thermal conversion efficiency could reach to 1.41 kg m−2 h−1 and 44.3% under solar light intensity of 2 kW m−2, which are higher than that of P25 TiO2 nanoparticles modified titanium gauze and bare titanium gauze. The solar thermal conversion efficiency can be as high as 81.3% when solar light intensity achieves 5.6 kW m−2. Considering the great demand of solar energy usage, we believe that our work can provide an attractive alternative for the design of advanced solar evaporation materials in addressing increasing serious world water and energy issues.
This work was supported by the National Natural Science Foundation of China (Grant No. 51602292), Shanxi Province Science Foundation for Youths (Grant No. 201701D221087) and the starting fund for scientific research of North University of China (Grant No. 130082).
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