Bifunctional, Moth-Eye-Like Nanostructured Black Titania

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Energy, Environmental, and Catalysis Applications

Bifunctional, Moth-Eye-Like Nanostructured Black Titania Nanocomposites for Solar-Driven Clean Water Generation Xinghang Liu, Haiyan Cheng, Zhenzhen Guo, Qian Zhang, Jingwen Qian, and Xianbao Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13374 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 28, 2018

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ACS Applied Materials & Interfaces

Bifunctional,

Moth-Eye-Like

Nanostructured

Black

Titania

Nanocomposites for Solar-Driven Clean Water Generation

Xinghang Liu, Haiyan Cheng, Zhenzhen Guo, Qian Zhan, Jingwen Qian, Xianbao Wang* Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials (Hubei University), School of Materials Science and Engineering, Hubei University. Wuhan 430062, P. R. China. * Corresponding author. Tel.: +86 27 88661729; fax: +86 27 88661729. E-mail addresses: [email protected] (Xianbao Wang)

Keywords: Moth-eye-like, Bifunctional, Black titania, Solar steam generation, Photocatalytic degradation

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Solar steam generation and photocatalytic degradation have been regarded as the most promising techniques to address clean water scarcity issues. Although enormous efforts have been committed to exploring high-efficiency clean water generation, many challenges still remain in aspects of single decontamination function, relatively low efficiency and inability to practical application. Herein we first report the bioinspired fabrication of black titania (BT) nanocomposites with moth-eye-like nanostructures on carbon cloth for solar-driven clean water generation through solar steam generation and photocatalytic degradation. The moth-eye-like BT nanoarrays can largely prolong the effective propagation path of absorbing light and enhance the scattering of light, thereby exhibiting outstanding light absorption of 96% in the full spectrum. Such hierarchical-nanostructured BT nanocomposites not only impressively achieve solar steam efficiency of 94% under a simulated light of 1 kW m-2 but also show the prominent performance of desalination and steam generation in real life condition. In addition, 96% of Rhodamine B is degraded using BT nanocomposites as a photocatalyst in 100 min. The moth-eye-like bioinspired designing concept and bifunctional applications in this study may open up a new strategy for maximizing solar energy utilization and clean water generation.

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INTRODUCTION Clean water is the cornerstone of the living and development of human being.1 However, with the rapid population growth, industrial expansion and the high-speed depletion of fossil fuel, the problem of environmental pollution has become increasingly serious, especially water pollution.2 It’s predicted that nearly two-thirds of the world’s population will suffer clean water shortage in the next seven years.3 Therefore, it is extremely urgent to look for energy-saving and high-efficiency water purification methods. As an inexhaustible source of clean energy, solar energy can not only solve the energy crisis but also reduce the pollution of fossil fuels.4-6 Due to the outstanding characteristic of solar energy, the utilization of solar energy for clean water generation has drawn great attention.7-10 One widely studied and efficient way to use solar energy for water purification is solar evaporation process (Table S1, Supporting Information). Solar steam is generated by utilizing solar energy to convert liquid water into steam water without heating bulk water.11, 12 In particular, the use of surface plasmon resonance effects for efficient solar steam generation has attracted widespread attention.13-15 In order to further improve thermal management and solar steam efficiency, a new concept named “Air-Water Interface Solar Heating” has been used for solar steam generation.16-19 Up to now, based on the concept of localized heating, various excellent photothermal materials have been widely studied, such as black gold membrane,20 graphene oxide film,21, 22 flexible wood

23, 24

and carbonized melamine

foam.25 Black titania (BT), exhibiting exceptional light absorption performance, is 3



expected to present excellent solar steam performance.26-28 In this work, we prepared the moth-eye-like BT nanorod arrays on the carbon cloth (BTCC) by three-step method, and combined it with two-dimensional water channels for solar steam generation. This moth-eye-like hierarchical nanostructures cause light to be trapped in the nanorod gaps, resulting in multiple internal reflections until completely absorbed.29, 30 It not only achieves excellent light absorption performance but also reduces heat loss. Another promising way for solar-driven water purification is photocatalytic degradation.31, 32 Photocatalytic degradation, that is, the catalysts in the light produce highly active hydroxyl radical (·OH), superoxide radical (·O2-) and other active substances, which can directly oxidize various organic pollutants to CO2, H2O and other inorganic small molecules.33 Various photocatalysts have been widely studied, especially titania.34-37 However, it is still faced with the problems of rapid recombination of photogenerated electron-hole pairs, and the limitation of photocatalytic activity to special wavelengths.38, 39 Many efforts have been tried to improve the photocatalytic efficiency of titania, such as metal ion doping,40 the loading of nanotitania photocatalyst,41 the complex of semiconductors,42 synergistic catalysis.43 These methods are not only complicated but also not obvious improvement. The moth-eye-like BTCC nanocomposites introducing oxygen vacancies and surface disorder show excellent light absorption in the full solar spectrum range, and the nanorod arrays provide rich active sites for photocatalytic reaction and reduce rapid recombination of photogenerated electrons and holes. In 4


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addition, carbon cloth (CC) is conductive to the transfer of electrons, thereby further retarding the recombination of photogenerated electron-hole pairs. In this paper, we designed and prepared bifunctional nanocomposites system that combined solar vaporization with photocatalytic degradation for the production of clean water. This bifunctional nanocomposites system is composed of moth-eye-like hierarchical

nanostructured

BT

grown

on

a

CC.

The

moth-eye-like

hierarchical-nanostructured BTCC nanocomposites provide excellent full spectrum absorption for solar vapor and photocatalytic processes by extending the effective path of light absorption and introducing oxygen vacancies and surface disorder. In the solar steam generation process, high solar spectrum absorption, sufficient water supply, and low heat loss are achieved by combining BTCC nanocomposites with two-dimensional (2D) water channels. This steam generation system exhibits outstanding light-to-heat conversion efficiency and stability when tested in both pure water and seawater under simulated and actual conditions. The interaction between BT introducing oxygen vacancies and nanorod arrays greatly improve the photocatalytic efficiency. Solar steam generation and photocatalytic degradation are integrated into single nanocomposites for the production of clean water, which not only maximize the use of solar energy but also achieve bifunctional efficient water purification.

EXPERIMENTAL SECTION Materials 5



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Carbon cloth (CC, HCP 330) was obtained from Shanghai Chuxi Industrial Co., Ltd. Titanium tetrachloride (TiCl4, 99%) and Tetrabutyl titanate (TBT, 98%) were purchased from Shanghai McLean Biochemical Technology Co., Ltd. Nitric acid (HNO3), Hydrochloric acid (HCl), Ethanol (99.7%) and Sodium borohydride (NaBH4, 96%) were provided from Sinopharm Chemical Reagent Co., Ltd. Rhodamine B (RhB) was supplied from Shanghai Aladdin Biochemical Technology Co., Ltd. Airlaid paper was available from Dongguan filexair cleanroom products Co., Ltd. All chemicals were used as original materials without further purification. Deionized (DI) water was used for all experiments. Preparation

of

moth-eye-like

hierarchical-nanostructured

BTCC

nanocomposites The BTCC nanocomposites were prepared by infiltration, hydrothermal reaction and NaBH4 reduction three-step method. First of all, the CC was treated with concentrated HNO3 to obtain a hydrophilic structure. In the first step, the dried CC was immersed into TiCl4 solution and then dried. This step was repeated 4 times to ensure that the CC had an effective coating. Afterward, the CC with TiCl4 coating was placed into muffle furnace for annealing. In the second step, the carbon with TiCl4 coating was placed into the mixed solution of TBT, concentrated HCl and DI water, and subjected to hydrothermal reaction at 160 oC for 16 h. In the third step, the nanocomposites of white titania nanorods grown on CC (TCC) were placed into a magnetic boat with NaBH4 and transferred into tube furnace, then calcined at 300, 400, 500, 550 and 600 o

C

for

5

h

under

an

Ar

atmosphere

respectively.

6


The

moth-eye-like

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hierarchical-nanostructured BTCC nanocomposites were obtained (Refer to the Supporting Information for more details). Solar steam generation To systematically explore the solar steam properties of the BTCC nanocomposites, the solar steam generation ability of various contrast samples including pure water, CC, TCC and BTCC nanocomposites were compared under simulated light (the xenon lamp with light intensity of 1 kW/m2 was used to simulate solar-irradiance). Each sample was illuminated for 40 min and the mass change of the sample solution was recorded by an electronic balance (three-decimal precision). Solar steam experiments were cycled for 14 times under simulated irradiation to demonstrate the stability and reusability of BTCC nanocomposites. We placed BTCC nanocomposites on polyethylene (PE) foam (1.5-cm-thick) wrapped with airlaid paper to reduce heat loss. And the real-time temperature distributions were monitored by IR camera and thermocouple during the experimental process. In order to further explore the practical application of BTCC nanocomposites, the steam generation and desalination experiments were performed under natural sunlight respectively. Photocatalytic performance measurement The photocatalytic property of the BTCC nanocomposites was evaluated by the degradation of the organic dye RhB in aqueous media under simulated solar illumination. 20 ml of the aqueous RhB solution (4 mg/L) was transferred to a 25 ml beaker, the sample was kept floating above the RhB solution using a PE foam ring. And the photocatalytic system was kept in a dark environment at room temperature 7



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for 1 h to achieve adsorption-desorption equilibrium. After the pretreatment, the beaker with sample and RhB solution was placed under Xenon lamp with light intensity of 1 kW/m2 to start photocatalytic test. The aqueous RhB solution was kept magnetic stirring during testing. The part of the sample solution was taken to the quartz cuvette every 20 min for absorbance spectra testing. After the test was completed, the sample solution was poured back into the beaker. The absorbance at 554 nm was used to determinate the concentration of RhB. The efficiency of photocatalytic degradation (De) was calculated by the following equation: ஼

‫ܦ‬௘ = ቀ1 − ቁ × 100% ஼బ

(1)

Where C0 is the initial concentration of RhB, and C is the concentration of RhB at illumination time t. In order to evaluate the photocatalytic stability of the BTCC nanocomposites, several cycles testing were conducted. Before performing the cycle testing, the samples were rinsed and dried without being affected by the previous testing. Due to the presence of steam generation in the photocatalytic process, the concentration of RhB was regulated based on the mass loss of the sample solution. Characterization The morphologies and sizes of samples were observed with scanning electron microscopy (SEM, JEOL JSM 7100F, 15KV) and high resolution transmission electron microscopy (TEM, JEOL JEM 2100F, 200 kV). X-ray diffraction patterns of CC, TCC and BTCC nanocomposites were obtained by X-ray diffraction (XRD, Bruker D8 phaser) analyses with Cu Kɑ radiation. Raman spectra were carried out on confocal Raman microscope (Renishaw, Britain) with excitation wavelength of 532 8


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nm at laser power of 120 mW. XPS experiments of TCC and BTCC nanocomposites were conducted on THERMO FISHERSICENTIFIC Escalab 250Xi. The electron paramagnetic resonance (EPR) spectra were obtained by using Bruker A200 spectrometer at 9.8 GHz at 17 mW. The optical absorption spectra were measured by Shimadzu UV-3600 spectrometer (using intergrating sphere). The real-time temperature distributions were conducted on IR thermograph. Ions concentrations were measured by inductively coupled plasma optical emission spectrometer (ICP-OES, EP Optimal 8000). The xenon lamp (CELHXF300, Education Au-light Co., China) are used to simulate the generation of natural sunlight.

RESULTS AND DISCUSSION BT nanocomposites with moth-eye-like hierarchical nanostrucrure were prepared by three-step method (Figure 1a). The CC treated with concentrated HNO3 (Figure S2) was infiltrated in a TiCl4 solution and calcined to obtain a titania seed layer, and then white TCC nanocomposites were obtained by a hydrothermal reaction. In order to prepare TCC nanocomposites with excellent optical properties, we investigated the temperature (Figure S3) and time (Figure S4) of the hydrothermal reaction. The TCC nanocomposites prepared for 16 h at 160 oC have regular moth-eye-like nanorod arrays, which are more conducive to prolonging the optical path and preparing for high spectral absorption of the BTCC nanocomposites. As shown in Figure 1, the TCC nanocomposites exhibit relatively regular nanorod arrays and flexible. The nanorod exhibits a square-column shape with an average length of ca. 1.93 µm and 9



diameter of ca. 118 nm. The XRD patterns in Figure S5a indicate that TCC nanocomposites have seven diffraction peaks assigned to rutile phase titania (21-1276, JCPDS). The D (1343 cm-1) and G (1594 cm-1) Raman peaks of CC are disappeared from the TCC nanocomposites, indicating that the CC surface is completely covered with titania nanorods (Figure S5b), which are further demonstrated by the elemental mapping images of Ti, C and O (Figure S6). The XPS was conducted to analyze the chemical composition of TCC nanocomposites (Figure S5c and S5d). The two peaks at 459.2 eV and 464.9 eV, respectively assigned to titania of Ti4+ at 2p3/2 and 2p1/2, indicate that the chemical state of the Ti atoms is the same in the sample. The spectra of O 1s reveal that the two peaks formed at 532.04 eV and 530.48 eV are attributed to adsorbed water and lattice oxygen in titania.44

Figure 1. (a) Schematic illustration of the preparation of the BTCC nanocomposites; (b-c) Optical images and (d-h) SEM images of the TCC nanocomposites; (i) SEM image of CC. 10


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The BTCC nanocomposites with different colors were successfully synthesized by NaBH4 reduction reactions under different temperature (Figure S7). The BTCC nanocomposites treated at 550 oC are the blackest according to the color contrast, indicating that it has the best light absorption in the visible region. As shown in Figure 2j, the TCC nanocomposites and BTCC nanocomposites (300 oC-550 oC) have strong light absorption at wavelengths shorter than 400 nm due to intrinsic absorption of titania. When the temperature rises to 600 oC, the moth-eye-like hierarchical nanostructures collapsed, and the bare carbon fiber caused the disappearance of the intrinsic band gap absorption of titania. With the gradual increase of temperature, the photoresponse range of BTCC nanocomposites extends from the ultraviolet region to the visible region and the near-infrared region. In particular, the BTCC nanocomposites reduced at 550 oC have excellent full spectrum absorption. The enhanced spectral absorption is attributed not only to the introduction of oxygen vacancies and surface disorder by effective reduction reaction, but also to the extension of the light absorption path due to the design of moth-eye-like hierarchical nanostructures (Figure S8). As shown in Figure 2i, the XRD patterns of the BTCC nanocomposites show clear diffraction peaks of the rutile phase titania (21-1276, JCPDS). The increase of the processing temperature was accompanied by the decrease of diffraction peak's intensity and the broadening of line-width, combined with the significant color changes of the nanocomposites. These results clearly reveal the structural stability of 11



BTCC nanocomposites and the successful surface disordering through reduction reactions. The morphology and crystal structure of the BTCC nanocomposites (550 oC) were systematically investigated by SEM and HRTEM. As shown in Figure 2, the BTCC nanocomposites are flexible and dark black, indicating that it has more excellent light absorption comparing with CC and TCC nanocomposites. The SEM and HRTEM images (Figure 2C-2h) further confirm that the morphology of the BTCC nanocomposites after the reduction treatment has no obvious change, and still shows nanorod arrays with average length of ca. 2 µm and width of ca. 120 nm. The HRTEM images (Figure S9) reveal that the BTCC nanocomposites have well-resolve lattice features and clear 0.32 nm lattice fringes, corresponds to the fringe spacing between the (110) lattice planes of rutile titania. The surface disordered layer formed by the reduction treatment is consisted of a thickness of ca. 1-2 nm, and combined with its highly crystalline internal structure, indicating that it exhibited a core-shell structure.45 It is well known that Ti3+ generated by oxygen vacancies causes the white titania to turn black, thereby causing the absorption spectrum to expand from the ultraviolet region to the visible and near-infrared regions.46 The EPR spectroscopy was used to detect the presence of Ti3+ (Figure S10a). A strong EPR signal at g=1.95 was observed, which is attributed to Ti3+.47 It fully demonstrates that Ti3+ exists in the black titnaia nanorods. However, the weak EPR signal at g=2.02 formed by the reaction of surface Ti3+ with atmospheric oxygen (O2-) reveal that most of the Ti3+ is located in the bulk, 12


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which also indicates that the BT has excellent stability. As shown in Figure S10b, the Ti 2p3/2 peak shifts from 458.3 eV to 457.8 eV accompanying with the Ti 2p1/2 shifts from 464.2 eV to 463.7 eV, respectively. The shifts of high binding energy Ti4+-O to low banding energy further indicate that Ti3+ and oxygen vacancies are present in black TiO2 nanorods.48-50

Figure 2. (a-b) The optical images; (c-f) SEM images and (g-h) HRTEM of the BTCC nanocomposites (550 oC); The inset in (h) shows the corresponding SAED image; (i) XRD patterns of BTCC nanocomposites treated at different temperature; (j) The UV-Vis-NIR absorption spectra of CC and BTCC nanocomposites treated at different temperature.

13



We

combined

the

moth-eye-like

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BTCC

nanocomposites (550 oC) with two-dimensional water channels to apply for solar steam generation (Figure 3d). This bioinspired system achieves following criteria needed for an efficient solar steam system: excellent full spectrum absorption; sufficient water supply; effective thermal management (Note S2). To evaluate the light absorption capacity of the nanocomposites, we further measured the light transmittance and reflectivity of the BTCC, TCC nanocomposites and CC at the wavelengths between 250 and 2000 nm (Figure S11). The BTCC nanocomposites exhibit nearly 2% optical reflectivity and transmittance, which are much lower than the TCC nanocomposites and CC. Thus, about 96% of light absorption is achieved by the BTCC nanocomposites in the full spectrum (Figure 3a), which fully illustrates the moth-eye-like hierarchical nanostructures, oxygen vacancies and surface disorder engineering play an important role in light capture. As

mentioned

above,

the

combination

of

BTCC

nanocomposites

and

two-dimensional water channels provides adequate water supply for solar steam generation. As shown in Figure S12, due to the outstanding hydrophilicity of the BT, when the BTCC nanocomposites contact with the airlaid paper, it is quickly wetted. And the wetted area gradually diffuse from one end that firstly contact with the airlaid paper to the periphery until the nanocomposites are completely wetted. In addition, the open and macro pores of the BTCC nanocomposites provide channels for steam leaving.

14


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Figure 3. (a) The absorption spectra of CC, TCC and BTCC nanocomposites (across 250-2000 nm); (b) Temperature distribution of the bulk water, the height of BTCC surface was regard as depth of 0 cm; (c) The surface temperatures changing course of BTCC nanocomposites; (d) The Schematic diagram of the evaporation experiment; (e) IR images of BTCC nanocomposites and pure water.

The effective thermal management is an important requirement for high-efficiency solar steam generation. In this bioinspired system, we utilize PE foam as a thermal insulator to reduce heat loss and concentrate the thermal energy generated by non-radiative relaxation on solar steam generation. The IR thermograph camera was used to record the temperature distribution of the surface of the nanocomposites and the bulk water (Figure 3e and Figure S13). Traditional solar steam utilizes light to heat the entire water so that heat cannot be concentrated, for the reason that severe 15



heat loss results in extremely low steam efficiency. When we placed the nanocomposites on PE foam wrapped with airlaid paper for solar steam generation, heat loss was effectively suppressed. After 30 min of light irradiation, the surface temperatures of the nanocomposites and CC increased dramatically. In particular, the surface temperatures of BTCC nanocomposites increased from 24.8 oC to 43.2 oC, which were much higher than that of TCC nanocomposites and CC. However, the bulk water temperature only increases about 4 oC. After 40 min of light irradiation, the surface temperatures of the BTCC nanocomposites was stable at 48.6 oC, and the bulk water temperature compared with 30 min of light irradiation only increased 0.3 oC. And the dramatic temperature difference between the top and bottom of the steam system also indicates that the system achieved effective thermal management (Figure 3b). Figure 3c shows that when the BTCC nanocomposites received light irradiation, its surface temperatures risen rapidly, indicating that the BTCC nanocomposites have excellent photothermal conversion capability. To systematically evaluate the solar steam generation performance of the BTCC nanocomposites, the multiple groups of experiments were performed under solar illumination of 1 kW m-2. As shown in Figure 4a, the two-dimensional water channels system (without photothermal conversion materials) achieved the mass loss of 0.066g after 40 min of solar illumination, which is 3 times that of the traditional steam generation system. That sufficiently demonstrates that local heating is an efficient choice for solar steam generation. When the BTCC nanocomposites with excellent optical property were combined with two-dimensional water channels, the mass loss 16


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of 0.292g was achieved after 40 min of solar irradiation, which is 11 times, 1.6 times and 1.2 times of pure water, TCC nanocomposites and CC, respectively.

Figure 4. (a) The weight change as function of the illumination time, including pure water (Equivalent to traditional steam generation), 2D water path (without photothermal material), CC, TCC and BTCC nanocomposites; (b) The evaporation efficiencies (left) and rates (right) of different samples; (c) The evaporation efficiency of the BTCC nanocomposites compared with the reported evaporation efficiency, See the Support Information for the order of reference; (d) The mass change of evaporated water per hour and the corresponding actual solar intensity during 8:00-17:00; (e) The ions concentrations change after seawater desalination of BTCC nanocomposites; (f) Cycle performance of the BTCC nanocomposites.

Figure 4b shows the evaporation efficiencies and vaporization rates of the five groups steam experiments. They were calculated based on the following equation: 51 17



ߟ௘ =

௩·௛೗ೡ

(2)

௤೔

Where ‫ ݒ‬stands for the vaporization rate, ℎ௟௩ is the total enthalpy of liquid-gas phase change, including the two parts of sensible heat and phase-change enthalpy, ‫ݍ‬௜ is the incident light intensity (1 kW m-2). The BTCC nanocomposites not only achieve the steam efficiency of 94% but also reach the steam rate of 1.515 kg m-2 h-1 (The natural evaporation of the system is negligible in the dark environment, Figure S14), which is far higher than the steam efficiencies and rates of other different systems and materials reported earlier (Figure 4c). Therefore, excellent photothermal materials and logical structure design play the key to realize efficient solar steam generation. In order to detect the stability of the BTCC nanocomposites, 14 times cycle tests were carried out (Figure 4f). After each test, the BTCC nanocomposites were washed and dried to ensure that the next test was not affected. The results show that the vaporization efficiency remaining around 94%, indicating that the BTCC nanocomposites could be reused. To evaluate the performance of BTCC nanocomposites in real life conditions, the seawater desalination and solar steam generation experiments were performed under natural light respectively. the concentrations of the five basic ions (K+, Ca2+, Na+, Mg2+, B3+) in real seawater (from the south china sea, Shenzhen, China), direct drinking water and desalted water after were measured by inductively coupled plasma optical emission spectrometer (Figure 4e). The ion concentration of desalted water treated by the BTCC nanocomposites is not only lower than the concentration of direct drinking water but also far below the salinity levels prescribed by the World 18


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Health Organization (WTO). The varieties of ions concentrations indicate that the BTCC nanocomposites have great promise in seawater desalination. Considering that the actual light intensity easily changes with the environment, the actual light intensity (April 25, 2018, cloudy to clear, 8:00-17:00, ambient temperature 24 oC, humidity 52%; September 23, 2018, clear, 8:00-17:00, ambient temperature 22 oC, humidity 64%, Wuhan, China) and steam production were measured (Figure 4d and Figure S15). The single BTCC nanocomposites and large area BTCC nanocomposites still produced 1.125g and 21.72g of clean water in the ever-changing actual environment, which indicates that the BTCC nanocomposites still have high performance under actual conditions.

Figure 5. (a) The photocatalytic degradation of RhB; (b) The semilogarithmic plots of CC, TCC and BTCC nanocomposites; (c) The absorption spectra of sample solution after the different degraded time; (d) Cycle performance of the BTCC nanocomposites; (e) The absorption spectra of contaminated water, partial purified water and 19



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condensed pure water, and (f) the corresponding optical image.

Photocatalytic degradation is one of the efficient and promising technologies for solar-driven clean water generation. Figure S17b shows that the photocatalytic degradation of RhB dye was evaluated the photocatalytic property of BTCC nanocomposites.

After

60

min

of

adsorption-desorption

equilibrium,

the

photocatalytic degradation of the different samples were measured under simulated sunlight irradiation (SSI, 1 kW m-2). After 100 min solar illumination, CC only degraded 16% of Rhodamine B (Figure 5a). Such slight degradation was mainly attributed to the thermal effect of photothermal conversion. However, the BTCC nanocomposites exhibited the highest photocatalytic activity with 95% RhB degraded after 100 min solar illumination, which is 6 times and 1.3 times that of CC and TCC nanocomposites, respectively. And the photocatalytic degradation performance of the BTCC nanocomposites is superior to commercial titania at the same quality (Figure S18). The high photocatalytic performance of BTCC nanocomposites is mainly attributed to the following four reasons. First, the BTCC nanocomposites with moth-eye-like hierarchical nanostructure design and the introduction of oxygen vacancies and surface disorder achieve nearly 96% of full-spectrum absorption, which break through the limitations of photocatalysis at specific wavelengths. Second, the BT with the introduction of oxygen vacancies and surface disordered not only achieves high light absorption but also reduces recombination of photogenerated electron-hole pairs (Figure S16), thereby enhancing the photocatalytic activity of the 20


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BTCC nanocomposites. Third, CC as substrate material is beneficial to the transfer of electrons, thereby further delaying the recombination of photoinduced electron-hole pairs. Fourth, the thermal effect generated by photothermal conversion in photocatalytic processes also improves photocatalytic activity. To explore the reaction kinetics of the photocatalytic degradation of RhB using the BTCC nanocomposites (See Supporting Information), we evaluated the rate constants of the three photocatalytic materials, respectively (Figure 5b). The rate constant of the photocatalytic degradation using the BTCC nanocomposites as photocatalyst was calculated as 0.02947 min-1, which is much larger than TCC nanocomposites and CC. Therefore, it shows that the BTCC nanocomposites have excellent performance in photocatalytic degradation. Figure 5d shows that the BTCC nanocomposites still has stable photocatalytic properties after 10 times photocatalytic cycling tests. In addition, the absorption spectra and optical images of contaminated water, partial purified water and condensed pure water are shown (Figure 5e and 5f).

CONCLUSION In conclusion, we have successfully synthesized the BTCC nanocomposites with moth-eye-like nanostructure by three-step method. This bionic light-harvesting structure, together with the introduction of oxygen vacancies and surface disorder, achieve a 96% light absorption in the full spectrum. The BTCC nanocomposites are applied to solar steam generation and photocatalytic degradation for the production of clean water. In the solar steam generation system, we combined the BTCC 21



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nanocomposite with 2D water channel to achieve solar steam efficiency of 94% and evaporation rate of 1.515 kg m-2 h-1 at solar illumination of 1 kW m-2. In addition, we performed seawater desalination and steam collection experiments under natural light respectively. The dramatic change of ions concentration and the steam collection of 1.125 g indicate that the BTCC nanocomposites have great potential for solar-driven clean water generation by solar steam generation under real life condition. In the photocatalytic degradation of RhB, the RhB of 95% was degraded after 100 min of irradiation under solar illumination of 1 kW m-2. That fully demonstrates that the BTCC nanocomposites not only has remarkable solar steam generation properties but also has outstanding photocatalytic properties. The efficient bifunctional applications of the BTCC nanocomposites break the limitation of the single application of previous water purification materials, and provide new strategy for the development of efficient and practical water purification materials.

ASSOCIATED CONTENT Supporting Information Preparation

of

moth-eye-like


black

titania

nanocomposites, discussion on the mechanism of solar steam generation, the kinetics of photocatalytic degradation, the experimental flow chart for preparing the BTCC nanocomposites, the optical image of clean CC, SEM images of clean CC and CC treated with TiCl4, SEM images of TCC nanocomposites prepared at different temperature and time, XRD pattern of CC and TCC nanocomposites, Raman spectra 22


Page 23 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and XPS spectra of TCC nanocomposites, the SEM image of TCC nanocomposites and the corresponding elemental mapping images of C, Ti and O, the optical images of BTCC nanocomposites treated at different temperatures, the light response diagram of moth-eye-like nanorod arrays, the HRTEM images of BTCC nanocomposites (550 o

C), the EPR spectra and XPS spectra of rutile and the BTCC nanocomposites, the

tansmittance and reflectance of CC, TCC and BTCC nanocomposites in the wavelength range of 250-2000 nm, the water supply process of the BTCC nanocomposites, IR images of CC, TCC and BTCC nanocomposites after 0 min, 30 min and 40 min, comparison of solar evaporation of BTCC nanocomposites under dark environment and a light intensity, schematic illustration of the outdoor test using BTCC nanocomposites in large area, the mass change of evaporated water per hour and the corresponding actual solar intensity, photocatalytic mechanism Schematic of black titania, the photograph of the process of solar steam generation and photocatalytic degradation, comparison of photocatalytic degradation properties of the BTCC nanocomposites and commercial titania at the same quality, the comparisons of parameters of solar steam generation system.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] ORCID Xinghang Liu: 0000-0003-3163-456X 23



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Haiyan Cheng: 0000-0002-4450-0440 Zhenzhen Guo: 0000-0001-7282-2378 Xianbao Wang: 0000-0001-7765-4027 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work is financially supported from the Ministry of Science and Technology of China (2016YFA0200200), National Natural Science Foundation of China (51203045 and

21401049)

and

Wuhan

Science

and

Technology

Bureau

of

China

(2018010401011280).

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Bifunctional, Moth-Eye-Like Nanostructured Black Titania

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