Research Article www.acsami.org
Constructing Black Titania with Unique Nanocage Structure for Solar Desalination Guilian Zhu,†,∥ Jijian Xu,†,‡,∥ Wenli Zhao,† and Fuqiang Huang*,†,‡,§ †
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China ‡ State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China § State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China S Supporting Information *
ABSTRACT: Solar desalination driven by solar radiation as heat source is freely available, however, hindered by low efficiency. Herein, we first design and synthesize black titania with a unique nanocage structure simultaneously with light trapping effect to enhance light harvesting, well-crystallized interconnected nanograins to accelerate the heat transfer from titania to water and with opening mesopores (4−10 nm) to facilitate the permeation of water vapor. Furthermore, the coated self-floating black titania nanocages film localizes the temperature increase at the water−air interface rather than uniformly heating the bulk of the water, which ultimately results in a solar−thermal conversion efficiency as high as 70.9% under a simulated solar light with an intensity of 1 kW m−2 (1 sun). This finding should inspire new black materials with rationally designed structure for superior solar desalination performance. KEYWORDS: black titania, nanocage, light trapping, solar desalination, mesoporous, photothermal
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nontoxicity, good chemical stability, and low cost.17 To the best of our knowledge, black titania applied in solar desalination as a solar−thermal material has yet to be investigated. Black titania with nanocage structure to further enhance the total absorption due to the internal light trapping effect and mesopores to facilitate the permeation of water vapor is designed. However, the preparation methods of black titania such as hydrogen thermal treatment,18 reduction methods (Al,19 Zn20), and oxidation methods21 reported so far are high-pressure- or hightemperature-involved, which increases costs and has limited capability for mass production. What is more is that a hightemperature process easily leads to structure cracks, preventing careful nanostructure design for better performance. Apart from novel approaches having been developed based on Kirkendall effect,22 chemical etching,23 and templating process24 to obtain specific architectures, the design and synthesis of high-quality nanostructured black titania remain a great challenge. Herein, we first report black titania with unique nanocage structure designed and synthesized for solar desalination. The resulted nanocage structure increases the total absorption due to the light trapping effect. Additionally, nanocages with interconnected nanograins which are well-crystallized accelerate
INTRODUCTION Water scarcity is one of the most serious global challenges and will be increasingly important in the future due to population expansion, industrial growth, and environmental pollution.1,2 Various technological solutions have been developed to meet the increasing demand for fresh water, among which desalination is the only method to increase water supply beyond the existing water resources.3 Solar desalination evaporation is used by nature to produce rain with unlimited energy supply and minimum environmental impacts, however, limited by low energy efficiency.4,5 Different from the conventional bulk heating of water, a new concept named “air−water interface solar heating” has been developed to gather solar radiation only at the water−air interface and generate a localized high temperature of the interfacial water.6,7 To enable efficient solar desalination, various black materials including hollow carbon beads,8 carbon nanoparticles,9 polypyrrole,6 Au nanoparticles,10−12 and Al nanoparticles13 have been explored as solar−thermal absorbers. Broadband light harvesting is the essential first step for efficient solar desalination. Motivated by our recent works in which black titania shows high solar absorption,14−16 black titania is thus expected to present promising solar desalination performance. In addition, black titania has merits over other reported solar−thermal absorbers due to its superior light-to heat conversion rate, © 2016 American Chemical Society
Received: September 9, 2016 Accepted: November 1, 2016 Published: November 1, 2016 31716
DOI: 10.1021/acsami.6b11466 ACS Appl. Mater. Interfaces 2016, 8, 31716−31721
Research Article
ACS Applied Materials & Interfaces
partially attributed to the effective reduction converting Ti4+ into Ti3+ by introducing oxygen vacancies.25 All the colored titania show enhanced absorption compared with the conventional black titania (H-TiO2) synthesized via hydrogen reduction, which is attributed to the unique nanocage structure. The salt species in the molten phase act as a solvent to reassemble the titania into a mesostructure.26,27 The opening mesopores could effectively enhance solar energy harvesting attributed to the light trapping effect, which multiply scatters and extends the path length and residence time of light within the nanocages structure, thereby increasing the total light absorption. Raman scattering (Figure 2a) of the samples was recorded to further understand the nature of the structure changes after
the heat transfer from titania to water. The opening mesopores (4−10 nm) facilitate the permeation of water vapor which helps with the light-to heat conversion. The coated self-floating black titania nanocages film localizes the temperature increasing at the water−air interface rather than uniformly heating the bulk of the water. Based on the above features, black titania nanocages deliver remarkable solar desalination performance as well as cycle stability.
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RESULTS AND DISCUSSION Via a facile and easily scaled up AlCl3−NaCl molten-saltassisted Al reduction, the initial white anatase nanoparticles (commercial) were converted into the blue, gray, and black ones after being reacted at 160, 180, and 210 °C, respectively. The conventional black titania synthesized by hydrogen reduction was also prepared for comparison, which is denoted as H-TiO2. Detailed synthetic procedures are described in Preparation of Black Titania Nanocages. The phase structure of the as prepared samples has been examined by X-ray diffraction (XRD). The diffraction patterns of the colored titania (Figure 1b) and initial anatase nanoparticles are very similar and exhibit
Figure 2. (a) Raman spectra of, (b) low-temperature EPR spectra of, and (c, d) XPS spectra of the black titania nanocages synthesized at 210 °C and initial anatase nanoparticles.
reduction. There are Raman peaks at 136, 390, 509, and 633 cm−1 assigned as four Raman active modes (Eg, B1g, A1g or B1g, and Eg) for anatase in accord with previous work.28 Obvious broadening and a blue shift for the Eg peak centered at 136 cm−1 of the black titania nanocages (210 °C) can be observed compared with pristine anatase nanoparticles. As is well-known that Raman scattering is sensitive to crystallinity and microstructure, the blue shift of Raman peaks of the black titania nanocages (210 °C) indicates that the reduction has broken down the original symmetry of the titania lattice. This result coincides with the amorphous phase observed under TEM. Electron paramagnetic resonance (EPR) is employed to detect paramagnetic species containing unpaired electrons, which has been widely used to characterize the existence of Ti3+ and oxygen vacancies. As shown in Figure 2b, the black titania nanocages (210 °C) present a small EPR signal at g = 2.03 and a rather strong signal at g = 1.93, while no evident resonance peak in initial anatase nanoparticles is observed. The signal with g = 2.03 arises from the superoxide anions, which are formed by reaction of surface Ti3+ with atmospheric oxygen (Ti4+-O2− species).29 The superoxide anions species play a key role in both the reductive and oxidative processes in the TiO2 photocatalytic reactions. The signal with g = 1.93 can be attributed to the lattice Ti3+ introduced during the reduction process. Furthermore, X-ray photoelectron spectroscopy (XPS) was used to investigate the elemental composition and surface
Figure 1. (a) Digital photographs, (b) XRD patterns, (c) UV−vis absorption spectra of initial anatase nanoparticles, colored titania synthesized at 160, 180, and 210 °C, and conventional black titania synthesized by hydrogen reduction, respectively. Inset: standard XRD pattern of anatase TiO2 (JCPDS No. 21-1272).
the characteristic peaks belonging to anatase TiO2 (JCPDS No. 21-1272). It is clearly noticed that an increasing of reaction temperature is accompanied by a decreasing of the XRD peaks’ intensity of the obtained colored titania, indicating that the initial anatase nanoparticles undergo more effective reduction at higher temperature. The broadened peaks of colored titania compared with the initial anatase nanoparticles are associated with the amorphous phase after reduction, which is consistence with the following TEM observation. Moreover, the observed pure anatase phase indicates the homogeneous reduction. The different colors from blue to black of colored titania suggest excellent visible light absorption. Figure 1c displays the diffuse reflectance spectra of the colored titania and initial anatase nanoparticles for comparison. All show a steep increase in absorption at wavelengths shorter than 400 nm induced by the intrinsic bandgap absorption of titania. The colored titania possess a significant increase of the visible and infrared light absorption at wavelengths longer than 400 nm, much superior to the initial anatase nanoparticles. The wide-spectrum absorption of colored titania increases with the increasing of synthesis temperature. The enhanced visible light absorption is 31717
DOI: 10.1021/acsami.6b11466 ACS Appl. Mater. Interfaces 2016, 8, 31716−31721
Research Article
ACS Applied Materials & Interfaces chemical status of the black titania nanocages (210 °C) and initial anatase nanoparticles for comparison. Figure 2c displays Ti 2p XPS spectra from 454 to 468 eV. As seen, the symmetric Ti 2p peaks of initial anatase nanoparticles centered at 458.7 and 464.6 eV are the typical pattern of Ti4+−O bonding, indicating an unchanged Ti4+ transition state with a low concentration of defects. For the black titania nanocages, a negative shifting indicating lesser electron density around the Ti atom is obviously observed compared with the Ti 3d spectrum of the initial anatase nanoparticles, demonstrating that a reduction reaction has happened.30 It is reasonable to find out that the negative shifting peak could not be deconvoluted into two peaks centered at 458.7 (Ti4+) and 457.9 eV (Ti3+) due to the surface Ti3+ tending to be oxidized, considering that the XPS technique can only detect the surface information about 5−10 nm deep. The O 1s peak at 529.4 eV in initial anatase nanoparticles is typically assigned to Ti−O bonds. The slightly broader O 1s peak in the black titania nanocages (210 °C) is due to a small peak at about 531.8 eV from Ti−OH bonds, similar to the reported results.31 These results confirm the formation of a hydroxyl group on the surface of black titania nanocages (210 °C). The morphology of the black titania nanocages was observed by scanning electron microscope (SEM) and transmission electron microscope (TEM) as shown in Figure 3, from which
microstructure change after the molten-salt-assisted selfassemble route, the black titania nanocages (210 °C) and initial anatase nanoparticles were examined by high-resolution transmission electron microscopy (HRTEM). As shown in Figure 3f, the black titania nanocages (210 °C) display an obvious amorphous phase, which is attributed to the Al reduction. While no amorphous phase was observed for the initial anatase nanoparticles, as shown in Figure S1b. Similar to the reported hydrothermal treatment of P25 to selectively dissolve the amorphous TiO2 component and deposit crystalline TiO2 between the partially interconnecting particles, the interconnected nanograins show well-crystallized planes.33 The measured interplanar spacing of 0.353 nm is in agreement with the d-spacing of the (101) lattice plane of the anatase phase. As is well-known, the high crystallinity accelerates the heat transfer from solid to liquid for solid materials. Moreover, opening mesopores (4−10 nm) which are benificial for harvesting the solar energy and facilitating the permeation of water vapor could be clearly observed. The detailed pore structures and pore size distribution, as well as the surface areas, were also measured using nitrogen adsorption−desorption. The results of nitrogen adsorption−desorption data confirm the mesoporous structure of the black titania nanocages (210 °C). The isotherms spectra of the black titania nanocages display type IV isotherms with a hysteresis in the desorption branch from 0.6 to 0.8 of relative partial pressure (P/P0), as shown in Figure 4a. The average BET surface areas using the
Figure 4. (a) N2 adsorption−desorption isotherm and (b) pore size distribution curves of the black titania nanocages synthesized at 210 °C and initial anatase nanoparticles.
desorption branch are 255 and 85 m2/g for the black titania nanocages (210 °C) and initial anatase nanoparticles, respectively. Such a value of surface area is rather high for titania considering its high bulk density of 3.78 g cm−3. The pore size distribution derived using nonlocal density functional theory is given in Figure 4b; two peaks centered at 3.8 and 9.5 nm are observed for the black titania nanocages (210 °C). The peak around 3.8 nm is originated from the amorphous phase dissolved in the molten salt, and the peak around 9.5 nm is caused by the self-assembling into nanocages accompanied by stacking holes. Both types of porosities could be clearly observed in TEM. It is believed that the enhanced surface area augments the air−water interface area and the opening mesoporous characteristic accelerates the transport of water which helps to facilitate the light-to heat conversion. Therefore, the nanocages structure not only can generate more heat as photothermal effect for water desalination but also can provide the paths for water steam through opening mesopores. The floatable hydrophobic colored titania films were fabricated through coating the active materials powder mixed with poly(vinylidene fluoride) (PVDF) as the binder on a glass
Figure 3. (a) Schematic of the formation of black titania nancages. (b, c) Characteristic SEM and (d−f) TEM image of the black titania nanocages synthesized at 210 °C showing a uniform particle size of about 70 nm with interconnected nanograins of 15 nm and mesopores (4−10 nm), and corresponding HRTEM image (g).
we can see a uniform dispersion of nanocages about 70 nm with interconnected nanograins of 15 nm and mesopores (4−10 nm), while the initial anatase nanoparticles have a particle size of about 20 nm, as seen from the SEM image in Figure S1a. The salt species in the molten phase act as a solvent to dissolve the starting anatase nanoparticles and a mineralizer to reassemble the titania into a mesostructure which is analogous to the well-known microemulsion as a powerful tool to synthesize a uniform size of nanoparticles.32 To verify the 31718
DOI: 10.1021/acsami.6b11466 ACS Appl. Mater. Interfaces 2016, 8, 31716−31721
Research Article
ACS Applied Materials & Interfaces
temperature increase at the water−air interface rather than uniformly heating the bulk of the water. Thus, the light-to-heat conversion could be largely enhanced, resulting in a more efficient solar desalination.6 On the basis of the air−water interface solar heating concept, the facile evaporation performances of colored titania films were examined in comparison with the initial anatase film and pure water under a simulated solar illumination with an intensity of 1 kW m−2 (1 sun), to demonstrate the potential application for solar desalination. Figure S5 presents the schematic setup for the water desalination measurement. The mass loading of all the films was fixed at the optimized 5 mg/ cm2 for comparasion. Figure 6b presents the mass of the
plate. Digital image of the initial anatase nanoparticles and black titania nanocages (210 °C) films is shown in Figure S2a, which demonstrates the uniform film could be straightforwardly obtained. As it can be seen, the diameter of the titania films is controlled at 5 cm. In addition, the UV−vis spectra of the films are almost the same as the colored titania nanoparticles (Figure S3), further indicating the uniform coating. SEM was conducted to explore the surface of the black titania nanocages (210 °C) film (as shown in Figure S2b). A rough surface structure with titania nanoparticles embeded in the matrix is clearly observed, confirming the homogeneous coating of the titania nanoparticles. It is worth pointing out that the coated films are hydrophobic themselves without any posthydrophobic modification and possess self-floating property. As shown in Figure S2c,d, both of the initial anatase nanoparticles and black titania nanocages (210 °C) films present hydrophobic surfaces with contact angles around 109°. The hydrophobic property enables them to float on the surface of the water (Figure 5a), which is beneficial for the application of solar desalination.
Figure 5. (a) Optical image of the black titania nanocages synthesized at 210 °C and initial anatase nanoparticles films floating on the water. The surface temperature distribution after 10s (b), 30s (c), and 60s (d) solar illumination at the power density of 1 kW m−2, which was monitored by IR camera.
The photothermal properties of the black titania nanocages (210 °C) and initial anatase films were explored. An infrared (IR) camera was employed to probe the temperature of the film under a simulated solar light with an intensity of 1 kW m−2, verifying the “air−water interface solar heating” concept in the solar desalination process. Figure S4 shows that both films exhibited almost the same uniform surface temperature distribution around 30 °C before light irradiation. Panels b−d of Figure 5 present the water surface temperature of two containers as a function of the irradiation time for comparison. Upon light illumination, the interfacial water temperature with the black titania nanocages (210 °C) film increased immediately. In the presence of the black titania nanocages (210 °C) film, interfacial water temperature becomes higher when increasing the light irradiation time. A steady-state temperature as high as 80 °C was reached at the water−air interface after 120 s irradiation or even longer, while no obvious temperature change is observed for the container with the initial anatase film, ascribed to the failure of solar energy harvesting of the initial white anatase. The black titania nanocages (210 °C) film is capable of efficiently absorbing the solar irradiation, converting it to heat energy, localizing the
Figure 6. (a)Schematic of the solar desalination. (b) Mass of the evaporated water as a function of the irradiation time, and (c) corresponding solar efficiency in the absence of the titania film and in the presence of anatase film, colored titania synthesized at 160, 180, and 210 °C films, and conventional black titania synthesized by hydrogen reduction. The error bars indicate the standard deviation of the measurements. (d) Measured concentrations of four primary ions in an actual seawater sample before and after desalination. (e) Cycle performance of the black titania nanocages synthesized at 210 °C film.
evaporated water as a function of the solar illumination time. The solar energy to thermal water evaporation conversion efficiency (η) of the colored titania films can be estimated by6 η = Q e/Q s
(1) −2
where Qs is the incidence light power (1 kW m ) and Qe is the power of evaporation of the water, which can be estimated by Qe = 31719
dm He = vHe dt
(2) DOI: 10.1021/acsami.6b11466 ACS Appl. Mater. Interfaces 2016, 8, 31716−31721
Research Article
ACS Applied Materials & Interfaces where m is the mass of evaporated water, t is the time, ν is the evaporation rate of water, and He is the heat of evaporation of water (≈2260 kJ kg−1). The water evaporation rate (ν) was calculated by linear fitting as shown in Figure 6b, and the conversion efficiency (η) was then caculated; the results are presented in Figure 6c. Detailed results are summarized in Table S1. The conversion efficiency of the colored titania films are all higher than that of the initial anatase and the water itself under illumination, indicating the light harvesting is the crucial first step for solar desalination. The colored titania with a darker color exhibited a higher conversion efficiency. The black titania nanocages (210 °C) have the fastest water evaporation rate of 1.13 kg m−2h−1 and the highest conversion efficiency of 70.9%, which are 1.9 times higher than that of water itself and among the highest of current reported results.34 For solar desalination, a real seawater sample (from the East Sea, China) was used for desalination by the black nanocages (210 °C). The concentrations of the five ions (Na+, K+, Mg2+, Ca2+, and B3+) in the seawater are all largely reduced after desalination (Figure 6d), including that the concentrations of Mg2+ and B3+ are below the detection limit of inductively coupled plasma spectroscopy (ICP-OES). Durability is another important factor for practical application. The cycle performance of the black titania nanocages (210 °C) was conducted under a simulated solar light with an intensity of 1 kW m−2 (1 sun), as shown in Figure 6e. It is found that the performance is maintained for more than 10 cycles, with each cycle being over 5 h. The black titania nanocages (210 °C) have merits over other reported solar−thermal materials ascribed to the nontoxicity and chemical stability.
remove the Al residue. Lastly, the black titania was separated by centrifugation and washed with deionized water for several times. After being dried in an oven at 80 °C for 12 h, the black titania sample was prepared eventually. The conventional black titania synthesized by hydrogen reduction was synthesized according to the literature, which is denoted as H-TiO2. The colored titania films were made of active materials powder (50 wt %) and PVDF binder (50 wt %) homogeneously mixed in N-methylpyrrolidinone (NMP) solvent and then coated uniformly on a glass plate. Sample Characterization. Raman spectra were collected on a thermal dispersive spectrometer using a laser with an excitation wavelength of 532 nm at laser power of 10 mW. XPS experiments were carried out on a RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Mg Kα radiation (hν = 1253.6 eV). The EPR spectra were collected using a Bruker EMX-8 spectrometer at 9.44 GHz at 300 K. XRD patterns were obtained with a Bruker D8 advance diffractometer operating with Cu Kα radiation. The morphologies of the samples were observed on a JEOL-JEM 2100F transmission electron microscope (100 kV) and a Hitachi S-4800 field emission scanning electron microscope (FE-SEM; 5 kV). The Brunauer− Emmett−Teller (BET) specific surface area of the composites was measured by nitrogen adsorption at 77 K on a surface area and porosity analyzer (Micrometrics ASAP 2020). Before each measurement, 0.10 g of sample was degassed at 300 °C for 4 h. A pore size distribution plot was derived from the adsorption branch of the isotherm based on density functional theory (DFT). Diffuse reflectance spectra (DRS) and UV−visible absorption spectra were measured using a Hitachi U-4100 spectrometer with an integrating sphere accessory, using BaSO4 as the reference material. The real-time temperatures of the samples were measured by an IR thermograph, and a 300 W Xe lamp was used to simulate the full spectrum of solar light. Solar Desalination. Solar desalination by titania films was tested in a cylinder container with an internal diameter of 5 cm and a depth of 10 cm. The cylinder container was wrapped with a thermal insulation layer and stored on an electronic balance to measure the weight of the evaporated water. For each run, the container was filled with 150 mL of distilled water, and the titania films floated on the water surface. A 120 W xenon lamp was used with an AM 1.5 filter to simulate the solar irradiance, and the aperture diameter was adjusted to be the same as the film. The light intensity is ∼1 kW/m2, which was measured by a Newport 91150 V calibrated reference cell and meter. The spectral output was measured with a StellarNetInc. spectrophotometer. After certain time intervals, the weights of water in the container were recorded.
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CONCLUSIONS In summary, as designed and prepared black titania nanocages was demonstrated as a promising candidate for the application of solar desalination. The nanocage structure increases the total absorption due to the light trapping effect which contributes to the enhancement of water evaporation. What’s more, nanocages with interconnected nanograins which are well-crystallized accelerate the heat transfer from titania to water, and opening mesopores (4−10 nm) facilitate the permeation of water vapor which help the light-to-heat conversion. The self-floating black titania nanocages (210 °C) film is capable of efficiently absorbing the solar irradiation, converting it to heat energy, localizing the temperature increase at the water−air interface rather than uniformly heating the bulk of the water. Thus, the light-to-heat conversion could be largely enhanced as high as 70.9% under a simulated solar light with an intensity of 1 kW m−2, resulting in a more efficient solar desalination. This finding should inspire new black materials with rationally designed structure for superior solar desalination performance.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11466. Comparison of water evaporation rates and conversion efficiency, SEM and HRTEM images of initial anatase nanoparticles, digital image of anatase nanoparticles and black titania nanocages, SEM image of the film, contact angles of nanoparticles and nanocages, UV−vis absorption spectra, surface temperature distribution before solar illumination, and schematic of water evaporation measurement setup, (PDF)
METHODS
Preparation of Black Titania Nanocages. The black titania nanocages were prepared by a NaCl-AlCl3 molten-salt-assisted method. In a typical reaction, 2 g of anatase TiO2 and 1 g of aluminum powder were mixed with a eutectic composition of NaClAlCl3 (5.6−14.3 g), and then the powder was homogenized by grinding. Afterward, the powder mixture was transferred into a 50 mL autoclave which was subsequently sealed and kept in an oven at 210 °C (160 or 180 °C) for 20 h. After naturally cooling to room temperature, the product was immersed in deionized water to remove the salts, and after ultrasound dispersion, most of the unreacted Al sediments were discarded. Subsequently, the suspension was added with diluted hydrochloric acid solution and stirred mildly for 5 h to
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions ∥
G.Z. and J.X. contributed equally to this work.
Notes
The authors declare no competing financial interest. 31720
DOI: 10.1021/acsami.6b11466 ACS Appl. Mater. Interfaces 2016, 8, 31716−31721
Research Article
ACS Applied Materials & Interfaces
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ACKNOWLEDGMENTS This work was financially supported from the National Key Research and Development Program (Grant No. 2016YFB0901600), NSF of China (Grant Nos. 61376056, 51502331, and 51402334), and the Science and Technology Commission of Shanghai (Grant Nos. 13JC1405700 and 14520722000).
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DOI: 10.1021/acsami.6b11466 ACS Appl. Mater. Interfaces 2016, 8, 31716−31721
