Shell Microstructure Induced Synergistic Effect for Efficient Water

Subscriber access provided by READING UNIV

Article

Core/Shell Microstructure Induced Synergistic Effect for Efficient Water-Droplet Formation and Cloud-Seeding Application Yanlong Tai, Haoran Liang, Nabil El Hadri, Ali M. Abshaev, Buzgigit M. Huchinaev, Steve Griffiths, Mustapha Jouiad, and Linda Zou ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06114 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 19, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Nano is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

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

ACS Nano

Core/Shell Microstructure Induced Synergistic Effect for Efficient Water-Droplet Formation and Cloud-Seeding Application Yanlong Tai1, Haoran Liang1, Nabil El Hadri2, Ali M. Abshaev3, Buzgigit M. Huchinaev3, Steve Griffiths1, Mustapha Jouiad2, Linda Zou1* 1

Department of Civil Infrastructure and Environment Engineering, Masdar Institute, Khalifa

University of Science and Technology, Abu Dhabi, United Arab Emirates 2

Department of Mechanical & Material Science and Engineering, Masdar Institute, Khalifa

University of Science and Technology, Abu Dhabi, United Arab Emirates 3

High Mountain Geophysical Institute of Russian Federal Hydrometeorological Service, Nalchik

City, Kabardino-Balkarian, Republic, Russian Federation

ABSTRACT

Cloud-seeding materials as a promising water-augmentation technology have drawn more attention recently. We designed and synthesized a type of core/shell NaCl/TiO2 (CSNT) particles with controlled particle size, which successfully adsorbed more water vapor (～ 295 times at low

ACS Paragon Plus Environment

1

ACS Nano

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

Page 2 of 28

relative humidity, 20 % RH) than that of pure NaCl, deliquesced at lower environmental RH of 62 - 66 % than the hygroscopic point (hg.p., 75 % RH) of NaCl, and formed larger water droplets ～ 6 - 10 times of its original measured size area, whereas the pure NaCl still remained as crystal at the same condition. The enhanced performance was attributed to the synergistic effect of the hydrophilic TiO2 shell and hygroscopic NaCl core microstructure, which attracted large amount of water vapor and turned it into liquid faster. Moreover, the critical particle size of CSNT particles (0.4 - 10 µm) as cloud-seeding materials was predicted via classical Kelvin equation based on their surface hydrophilicity. Finally, the benefits of CSNT particles for cloud-seeding application were determined visually through in-situ observation under Environmental - Scanning Electron Microscope (E-SEM) in microscale and cloud chamber experiments in macroscale, respectively. These excellent and consistent performances positively confirmed that CSNT particles could be the promising cloud-seeding materials.

KEYWORDS Core/shell microstructure, Synergistic effect, Hydrophilic surface, Water-droplet formation, Cloud-seeding materials.

Water vapor in the atmosphere is a natural resource equivalent to about 10 % of all fresh water from rivers and lakes on Earth.1,2 Using cloud-seeding materials as cloud condensation nuclei (CCN) is an effective method to accelerate the formation of water droplets, and then harvest the water vapor in the atmosphere via rain precipitation.3-5 Hence, it is considered as the most promising water-augmentation technology to serve several weather modification purposes, including i) increase the precipitation in the desert


2

Page 3 of 28

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

ACS Nano

or arid areas to resolve the serious water-shortage issues in the world;6,7 ii) regulate the amount or type of precipitation in different areas,8,9 and iii) suppress hailstorms or weakening hurricanes, etc..10,11 It is crucial to understand and master these processes and capabilities to combat the current frequent occurrence of the extreme weather patterns around the world.12,13 Sodium chloride (NaCl) has been used as the typical water-soluble hygroscopic warmcloud-seeding materials for several decades,14-16 which is more commonly employed than ice nucleating materials used in cold cloud, e.g. dry ice (solid carbon-dioxide, CO2) that is associated with high cost and greenhouse effect,17 silver iodide (AgI) that is associated with controversial environmental risk.18 Other warm-cloud seeding materials such as porous materials (zeolites,19 silica gels,20 metal-organic framework (MOF),21 etc.) experience low uptake of water or high energy consumption for water release. NaClbased cloud seeding materials can adsorb water vapor in warm clouds, and ultimately transform into water droplets completely.22,23 The formed water droplets continuously grow by collision/coalescence process, until the droplets become large enough to fall as rain. However, below its hygroscopic point (hg.p.) at ~ 75 % relative humidity (RH, 25 ℃ ), NaCl crystal doesn’t show any evident water-vapor adsorption performance, especially no detectable water-vapor adsorption at all below 25 % RH,24,25 which sets limitations to the cloud seeding application. Therefore, several strategies have been used to improve the hygroscopic capability of NaCl, such as mechanical mixing with other hygroscopic salts (CaCl2, NaNO3, CaSO4, etc.)26,27 or with other polymer particles with strong wateradsorbent capability (poly acrylamide, sodium polyacrylate, etc.).28,29 However, these


3

ACS Nano

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

Page 4 of 28

mechanical-mixing methods cannot fundamentally alter the hygroscopic behavior of NaCl, only lead to minimal improvement. Such limitations motivated us to rationally design in mico/nano scale and synthesize a kind of NaCl-based cloud seeding materials that could outperform the existing materials. We describe below the details of a patented process that using this core/shell NaCl/Titanium dioxide (TiO2) microstructure with a critical particle size range. First, we employed a different kind of hygroscopic mechanism through a core/shell NaCl/TiO2 (CSNT) microstructure that aimed to adsorb water vapor below the NaCl’s hygroscopic point (hg.p.). The coated TiO2 nanoparticles formed a hydrophilic shell to i) adsorb and condense water-vapor molecules and create a moisturizing layer with higher local RH value than that of the air, and ii) contribute to the subsequent more rapid deliquescence. This core/shell microstructure fostered a synergistic effect to form much larger water droplets at lower vapor-pressure range (Figure 1.). However, the benefit of such an additional moisturizing layer interface is completely absent in the case of pure NaCl. Second, we explored the relationship between critical water-vapor-saturation ratio and CSNT-particle-size range based on their hydrophilicity and condensation-nucleation theories to predict the critical particle size for specific cloud seeding materials, and then it was used as guidance for synthesis of the CSNT particles. Accordingly, using the evaporation-crystallization technique, we synthesized the CSNT particles with controlled particle size. The interaction between the generally uniform shell of TiO2 and NaCl crystal core was also addressed.


4

Page 5 of 28

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

ACS Nano

Third, we evaluated the hygroscopic behaviors of the synthesized CSNT particles at different RHs, including the starting RH point for water-vapor adsorption, the adsorbed water-vapor volume below the hg.p. and the practical hg.p., comparing with that of commercial pure NaCl crystal. The influence of the shell thickness of TiO2 was also specifically investigated to confirm this strategy and maximize the synergistic effect described above. Moreover, the benefits of CSNT particles for cloud-seeding application were further determined visually through in-situ observation under Environmental - Scanning Electron Microscope (E-SEM) in microscale and cloud chamber experiments in macroscale, respectively.


5

ACS Nano

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

Page 6 of 28

Figure 1. Schematic illustration of the hygroscopic mechanism of CSNT particles to promote water-vapor adsorption and water-droplet formation below the hygroscopic point of NaCl crystal, compared with that of commercial pure NaCl crystal. Note the migration path of water molecules is indicated by arrows.

RESULTS AND DISCUSSION The Mechanism of Enhanced Water Droplet Formation The explanation of our strategy As described above, NaCl is a commonly used hygroscopic cloud-seeding material. In specific, when water vapor molecules are in contact with the NaCl crystal, some of them will condense, hence changing from gaseous state to liquid state and ultimately leading to the deliquescence of NaCl crystal. Note that this is a reversible process, where a dynamic equilibrium exists. The threshold point is defined as hygroscopic point (hg.p.), which is an intrinsic attribute of NaCl, related to the equilibrium vapor pressure above the NaCl crystal (Psalt), and the pure water-vapor pressure (PH2O) at the same temperature, as seen equation 1.25 In other words, below its hg.p. (~ 75 % RH at 25 ℃), NaCl crystal does not adsorb and condense water at all. The water-vapor pressure in the warm cloud is always saturated (PH2O-air, ≥ 100 % RH), above hg.p. of NaCl crystal, which is the theoretical reason that NaCl can be used as the cloud-seeding materials.30


6

Page 7 of 28

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

ACS Nano

ℎ.. =

× 100%

(1)

Therefore, to efficiently adsorb, condense and form water droplet, a type of CSNT particles was designed, which is expected to adsorb and then condense more water vapor below its hg.p. to subsequently grow during collision-coalescence process into larger drop size and form rainfall more efficiently. This performance could make the seeding materials less selective to the cloud conditions (such as cloud thickness, water-vapor density, etc.). In mechanism, the core/shell microstructure not only cause deliquescence to occur at lower hg.p., but also contribute to the larger water droplet formation,

i) this core/shell microstructure improved the interaction between water-vapor molecule and NaCl crystal, as seen in Figure 1. Specifically, as for pure NaCl crystal, the adsorption, condensation of water-vapor molecules and deliquescence of NaCl crystal occurred at the same time and the same hygroscopic interface between NaCl and air, and its deliquescence was totally dependent on the environment RH conditions. As for the CSNT particles, this process took place separately, at first, the adsorption of water vapor happened at the interface between TiO2 shell and air, and then deliquescence of NaCl crystal occurred at the core/shell interface. This enhanced hygroscopic interface could make CSNT particles attract water molecules from air more easily at low water-vapor pressure because of the abundant hydroxyl groups on the surface of TiO2 nanoparticle, than that of pure NaCl crystal.31-33 As a result, the accumulated water molecules caused a


7

ACS Nano

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

Page 8 of 28

higher local RH value in the coated TiO2 layer than that in the air, like a moisturizing layer, leading to faster deliquescence of NaCl crystal. ii) the partial dissolution of NaCl crystal could generate vapor-pressure gradient (PH2OGradient) which would benefit the continuous accumulation of water molecules, eventually forming larger water droplets.34 Thus, this synergistic effect between hydrophilic adsorption (TiO2 shell) and hygroscopic deliquescence (NaCl core) would continue until the complete deliquescence. More details can be seen in Figure 1. Critical particle size range for cloud seeding materials The size of cloud seeding materials also plays an important role in the effectiveness of the rain enhancement.35,36 According to the report, the critical size of the cloud seeding materials is dependent on their surface hydrophobic/hydrophilic property: if the particle surface is more hydrophobic, the critical size will be larger, and if the particle surface is more hydrophilic, the critical size will be smaller.30,37 However, if the amount of cloud seeding materials per unit volume of air mass is too small (e.g., oversized particles), based on the theory of collision-coalescence of water microdroplets, it is then impossible to trigger the chain reaction effectively to form rainfall. In addition, there is also a minimum critical dimension of cloud seeding particles, which needs to be at least bigger than 200 nm (0.2 µm) in diameter. This is because if the diameter of cloud seeding materials is too small, it is impossible to surpass the Gibbs free energy barrier (∆G) for condensation, so water vapor only remains as cloud or fog. According to the Kelvin equation (Equation (2)), there is an inverse relationship between the saturation ratio of water vapor pressure (P/P0) and the critical radius (r*) of water


8

Page 9 of 28

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

ACS Nano

microdroplet. Only when its radius exceeds r* can the water droplet grow and become macroscopic droplet. As for the same nuclei materials, they have a constant surface free energy per unit area (σ) of water, which is contact angle dependent.37 As for our CSNT particles, the water contact angle was determined at around 17.6 °. Based on the critical water-vapor saturation ratio and particle size relationship in Figure S2, hydrophilic particles can reduce the required supersaturation levels for water droplet growth as well as lower the critical particle size. So the very low contact angle of CSNT particles (17.6 °) can allow them to have smaller size and work effectively at lower supersaturation levels, both of which are plausible improvements for the cloud seeding process. With a broad critical size range of CSNT particles around 0.4 - 10 µm found in S-I, S-II, and Figure S3, we decided to control the synthesis conditions to obtain CSNT particles with the size range of 1.4 ± 0.3 µm. Their efficiency in water-vapor capturing will be further verified in hygroscopic performance experiments.

∗ =

(⁄ )

(2)

in which n is the number of molecules per unit volume in water, k is the Boltzmann constant, T is the absolute temperature, σ can be calculated from contact angle.


9

ACS Nano

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

Page 10 of 28

Figure 2. Synthesis and characterization of CSNT particles. a) SEM images of CSNT particles via different volumes: i) pure NaCl; ii) 0.5 ml/cm2; iii) 0.25 ml/cm2, iv) 0.025 ml/cm2; The scale bars are 10 µm, 10 µm, 5 µm and 1 µm, and 100 nm inset in iv, respectively; b) The summarized relationship between the volume (ml/cm2) and size of CSNT particles; c) TEM images d) EDX-SEM mapping images to show the TiO2 shell on NaCl crystal particle; All the scale bars are 200 nm, 50 nm, and 20 nm in c) and 1 µm in d), respectively; e) XRD patterns and f) Raman spectra of pure NaCl crystals, pure TiO2 nanoparticles, and CSNT particles, respectively. Note that CSNT particles (1.4 ± 0.3 µm) were used as default particle size in the following unless otherwise stated.


10

Page 11 of 28

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

ACS Nano

Synthesis and characterization of CSNT particles As described above, the critical size of hygroscopic cloud seeding materials is strongly related to its surface hydrophilic property. It means that different materials and their structures used to modify the surface of NaCl will have an impact on the critical size of the cloud-seeding materials. So, it is very necessary to develop a facile route of synthesizing hydrophilic modified NaCl particles with controllable sizes. Here, the synthesis and characterization of CSNT particles are shown as an example.38,39 Figure 2a shows the SEM images of pure NaCl crystal and synthesized CSNT particles, respectively. The different sizes of CSNT particle with uniform size distributions were obtained by heating different volumes of TiO2 sol/NaCl/solvent mixtures under the same conditions (80 ℃ for 3 h). It can be found that when the mixture volumes were 1 ml (0.025 ml/cm2), 10 ml (0.25 ml/cm2) and 20 ml (0.5 ml/cm2), the resultant sizes of CSNT particles were around 150 ± 20 nm, 1.4 ± 0.3 µm and 6 ± 1 µm, respectively. This proportional relationship between the volume of mixture and the size of CSNT particle was summarized in Figure 2b. As for the crystal growth by evaporation technique, there were many factors that affected the final crystal size, such as the supersaturation degree of ions, the diffusion rate of the ions, the solution temperature and the crystallization time, etc. Here, when the drying conditions remained the same, the difference in mixture volume led to the different crystallization time, i.e. the larger the mixture volume, the longer the time for crystal growth. This was because the crystals could only grow in the liquid phase where the diffusion of ions occurred, and the growth stopped after the liquid was completed evaporated.


11

ACS Nano

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

Page 12 of 28

Figure 2c further presents the microstructure of CSNT particles observed by Transmission electron microscopy (TEM), as well as Figure S4. The coated TiO2 layer on NaCl crystal with a generally uniform thickness around 23 ± 3 nm can be observed, as well as the stacked layer-by-layer nanostructures. Moreover, the presence of TiO2 on NaCl crystal was further confirmed and characterized by Energy Dispersive X-ray (EDX) spectroscopy on SEM instrument (Figure 2d) with the elemental mapping images of Na, Cl, Ti and O.40 The well-defined core/shell microstructure with homogenous distribution of a thin TiO2 layer on the NaCl crystal surface was successfully confirmed. The corresponding EDX spectrum plot can be seen Figure S5. Accordingly, Figure 2e shows results of X-ray diffraction (XRD) on CSNT, pure NaCl and pure TiO2 particles. Besides the diffraction peaks from crystalline NaCl which can be observed clearly, only a weak peak at approximately 25.5 ° might be assignable to TiO2. This is due to the very low intensity of the XRD pattern of pure TiO2, which indicated that TiO2 is likely to have partial crystalline or amorphous characteristics. Characterization experiments were also conducted by using Raman spectroscopy at the same samples. The Raman spectrum of CSNT plotted in Figure 2f shows similar pattern to the one observed for pure TiO2, indicating the presence of TiO2 shell on the NaCl core.


12

Page 13 of 28

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

ACS Nano

Figure 3. Hygroscopic performance of CSNT particles. a) and b) Water-vapor adsorption isotherm tests to characterize the water-vapor-adsorption capacity and hygroscopic property of CSNT particles with different thicknesses of TiO2 shell (CSNT-1(~ 24 nm); CSNT-2 (~ 18 nm); CSNT-3(~ 9 nm)), NaCl crystal (NaCl-1(before grind); NaCl-2 (after grind)); respectively; Note the blue area in a) was highlighted in b). c) Relationship between the general thickness of TiO2 shell on NaCl crystal and the TiO2/NaCl mixture mass ratios (0.196, 0.314, 0.785, respectively) during synthesis process; Insets are the corresponding TEM images of CSNT particles and all scale bars are 50 nm. d), e) and f) E-SEM experiments to observe and compare the hygroscopic capability between CSNT particles and pure NaCl crystal, respectively; Note S0 and S are the sample-area sizes before (50 % RH) and after deliquescence (67.5 % RH or 75 % RH), respectively. All the


13

ACS Nano

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

Page 14 of 28

scale bars are 3 µm. a-1, a-2 and a-3 in d) and e) are chosen to show the typical deliquescence processes of both samples.

Hygroscopic performance of CSNT particles We have successfully obtained the CSNT particles with the chosen size of 1.4 ± 0.3 µm within the critical-size range as described above. Its enhanced water-droplet formation was evaluated through different methods in the following sections.25,41 First of all, water-vapor adsorption isotherm analysis were conducted to quantitatively determine the water-vapor adsorption capacity of CSNT particles (Figure 3a). It was found that pure NaCl (NaCl-1) didn’t show an evident adsorption to water vapor before reaching its hg.p. (~ 75 % RH). This hygroscopic performance was not improved after the size of pure NaCl crystals reduced from 8 ± 1 µm to 3 ± 0.5 µm by grinding (NaCl-2), as seen in Figure S6. However, when an ultrathin TiO2 shell was coated onto the NaCl core, its hygroscopic performance was improved greatly (Figure 3a), which was reflected in three aspects: i) the starting point for water-vapor adsorption; ii) the adsorbed water-vapor volume below hygroscopic point; iii) the deliquescence. More specifically, first, it can be found that CSNT particles started to adsorb water vapor molecules at a very low relative humidity (1 % RH), whereas pure NaCl (NaCl-1) started at much higher humidity value of 16 % RH, which was highlighted in Figure 3b. Second, as for CSNT-1 (the thickness of TiO2 shell, ~ 24 nm), the adsorbed water-vapor volume is 44.24 cm3/g compared with that (0.15 cm3/g) of NaCl-1 at 20 % RH, which is around 295-time increase. As for CSNT-2 and


14

Page 15 of 28

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

ACS Nano

CSNT-3, this increase can be up to 196-times and 121-times, respectively, due to the different thicknesses of TiO2 shell, as seen in Figure 3c and Figure S7. Third, the hygroscopic points of all CSNT samples with different TiO2-shell thicknesses shifted to lower values of ~ 62 - 66 % RH, compared with ~ 75 % RH of NaCl-1. The systematic experimental investigation of the effects of TiO2-shell thickness on the hygroscopic point of CSNT particles was presented in Figure S8. It can be concluded that the TiO2 shell allowed NaCl crystal to adsorb more water vapor which potentially contributed to form larger water droplet at its deliquescence. This feature was attributed to the synergistic effect of the hydrophilic shell and hygroscopic core, which has been explained in the previous mechanism section. Moreover, this enhanced performance was further confirmed through pure TiO2 nanoparticles which did not have deliquescent behaviors (i.e., no hg.p.), and the mechanical mixing of TiO2 and NaCl which did not alter the original hg.p. of NaCl, as seen in Figure S6 and S9. Moreover, E-SEM experiments were also employed for in-situ observation of the enhancement on condensation and deliquescence of the CSNT particles, the results were summarized in Figure 3d, 3e, and 3f. The parallel experiments were conducted under the same environmental parameters. Results show that CSNT particles started to deliquesce below 67.5 % RH, which is consistent with the results via water-vapor adsorption isotherm test. Accordingly, the CSNT particles underwent both phase change and significant size growth. Whereas the time-sequential images revealed that the NaCl crystals did not show any evident change in morphology. When the hg.p. of pure NaCl (75 % RH) was reached, CSNT particles were completely dissolved, and its area-size ratio


15

ACS Nano

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

Page 16 of 28

(S/S0) increased to ～ 6 - 10 times bigger than its original size (the particle size at 50 % RH). More details can refer to Figure S10 and S11, Video S1 and S2. Generally, this performance strongly confirmed the theoretical prediction described above. These improvements enabled CSNT particles to be a better cloud-seeding material, as they not only adsorbed more water vapor from the air, but also condensed and formed much larger water droplet at lower RH condition, which can translate to the more efficient cloud-seeding effect and could create rainfall more easily.


16

Page 17 of 28

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

ACS Nano

Figure 4. Cloud chamber experiments to characterize the actual efficiency of CSNT particles as cloud-seeding materials. a) Illustration of the rain-enhancement performance with different cloud-seeding materials: a-1, background (no seeding materials); a-2, NaCl crystals; a-3, CSNT particles; respectively. b) Optical microscopes to compare the waterdroplet size seeded by pure NaCl crystals and CSNT particles in a defined area; all the scale bars are 25 µm. c) Spectra of the concentration of water droplets in different size ranges in the chamber: c-1, 10 - 25 µm; c-2, 5 - 10 µm; c-3, 1 - 5 µm; respectively. Note the chamber conditions were controlled at 5 ℃ and 100 % RH, which is more close to the real cloud condition. The default loading of NaCl crystals or CSNT particles was 0.05 g for each experiment, and the above data are the average results for three times.

Cloud chamber experiments Cloud chamber experiments provided a scientific approach in which the cloud-seeding materials can be evaluated inside a three-dimensional environment when all conditions were controlled.2 It is an essential step employed to validate the rain enhancement effects caused by the cloud-seeding materials before the airborne seeding operations. The results were presented in Figure 4 and Figure S12 through the comparison with background (no seeding materials), NaCl crystals, and CSNT particles. After adding CSNT particles into the chamber, both the water-droplet concentration and the droplet size increased greatly across all size ranges (Figure 4 c-1 to c-3). Especially, at 100 % RH, the concentration of water - droplet size between 10 - 25 µm (which is very crucial to the rainfall) 42 caused by CSNT particles was up to 290% higher than that by NaCl, and 15.5 times higher than that


17

ACS Nano

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

Page 18 of 28

of background. These excellent results were highly consistent with the hygroscopic performance investigated above, and positively confirmed that CSNT particles were a type of effective cloud-seeding materials.

CONCLUSIONS The designed and synthesized CSNT particles successfully adsorbed more water vapor (～ 295 times at low relative humidity, 20 % RH) than that of pure NaCl, deliquesced at lower environmental RH of 62 - 66 % than the hg.p. (75 % RH) of pure NaCl, and formed larger water droplets ～ 6 - 10 times of its original measured size area, whereas the pure NaCl still remained as crystal at the same condition. These excellent performances showed that CSNT particles were efficient as cloud-seeding materials. These behaviors were explained through the synergistic effect of the hydrophilic TiO2 shell and hygroscopic NaCl core microstructure, and confirmed through the real-time E-SEM monitoring of water droplet growth under different RH profiles in microscale and cloud chamber experiments in macroscale. This is a useful strategy to design nano/microstructured materials for rain enhancement and broad water augmentation technology. In the future, this core/shell microstructured seeding material and other seeding materials with similar concept will be investigated for more rain enhancement applications.

METHODS


18

Page 19 of 28

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

ACS Nano

Materials. Sodium chloride (≥ 99.8 %, NaCl), isopropyl alcohol (≥ 99.8 %, IPA), ethanol (≥ 99.8 %, C2H6O), titanium (IV) butoxide reagent grade, (≥ 97 %, TBT), and nitric acid (≥ 65 %, HNO3) were all purchased from Sigma Aldrich. Deionized (DI) water was used in all experimental processes. Synthesis of TiO2 sol. TiO2 sol was prepared through the hydrolysis of the titanium butoxide solution.43 First, solution-A was prepared by dispersing titanium butoxide (10 ml) in ethanol (40 ml) under mild stirring (300 rpm). After, solution B was prepared by mixing deionized water (100 ml) with nitric acid. Then, solution-B was added dropwise into solution A under vigorous stirring (500 rpm) until the formation of semi-transparent TiO2 sol. The pH was the most important parameter controlling the TiO2 particles size. When the pH value was less than 2, particles smaller than 10 nm were obtained. Accordingly, the synthesized TiO2 sol (pH = 1 ± 0.1, 1.52 wt. %) was stored at room temperature for the next step. The typical synthesis process can be seen in Figure S13a. Synthesis of CSNT particles. First, commercial NaCl crystals (0.2 g) was added into isopropyl alcohol (50 ml, IPA) with magnetic stirring (320 rpm) for 30 minutes at room temperature. Second, the above prepared TiO2 sol (10 ml) was added dropwise with the color changing from semitransparency to cloudy. During this drop casting process, NaCl crystal started to dissolve due to the water brought by TiO2 sol. After stirred for another 60 minutes, the mixture was evenly divided into 6 500-ml beakers (10 ml for each beaker), and dried at 80 °C for 3 hours to recrystallize NaCl, and calcined in air at 250 °C for 3 hours to remove extra water molecules, changing TiO2 sol to TiO2 nanoparticles. Finally, the synthesized CSNT particles were obtained and kept in a dryer, defined as CSNT-1. The typical synthesis process was shown in Figure S13b. In addition, when the


19

ACS Nano

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

Page 20 of 28

loading of NaCl is 0.5 g, 0.8 g, the corresponding CSNT particles were defined CSNT-2 and CSNT-3, respectively.

Characterization and measurements. The synthesized CSNT particles were characterized through Scanning Electron Microscopy (SEM, Quanta 250, FEI Company) with Energy Dispersive X-ray spectroscopy (EDX) for element mapping test; Transmission Electron Microscopy (TEM, Tecnai from FEI™ Company operating at 200 KV); X-ray diffraction (XRD, Empyrean, PANalytical) using Cu Kα X-rays (λ = 0.154 nm) at 45 kV and 40 mA with a step size of 0.002 ˚ and a scan speed of 0.04 ˚/s; Raman Spectroscopy instrument (Witek alpha 300) using a 473-nm laser with the beam energy of 75 mW; Water staticcontact-angle Measurements (Kyowa DM-701) elaborated with an interface Measurement & Analyses System and the droplets of 0.8 µl. The hygroscopic performance of pure NaCl crystals and CSNT particles was investigated quantitatively via water-vapor adsorption isotherm test (Belsorb Max, MicrotracBEL Corp. Japan). Note that the samples should be evacuated at 200 ℃ for 3 hours under a pressure < 10-4 Pa before commencing the analysis and the whole test required 24 hours. E-SEM was used to observe the water-vapor-condensation performance of pure NaCl crystals and CSNT particles with the same size of 1.4 ± 0.3 µm in microscale. Meanwhile, cloud-chamber test was also used to confirm the actual efficiency of CSNT particles as cloud-seeding materials. Cloud chamber experiments were conducted in a facility designed and manufactured in a house with dimension of 1.8 m × 1.8 m × 2 m, the volume of the chamber is 6.48 m3


20

Page 21 of 28

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

ACS Nano

(Figure S14). An aerosol particle counter device (Lasair III Models 310B, Particle Measuring Systems, Inc., USA) with a sampling apparatus (nozzle) was installed inside the cloud chamber. The dispersion of the seeding particles in chamber was controlled via bursting of a thin resin ball containing seeding particle agents, which was about 30 cm in diameter and was inflated by air compressor in the chamber. After bursting of ball and releasing the particles, an aerosol particle counter took a measurement every 20 seconds, where 10 seconds was for sampling and 10 seconds for processing the obtained data. Particulate spectrum information was logged automatically by the aerosol particle counter device. The same operational procedure was used in all three experiments: the background experiment (only aerosol particles), pure NaCl crystals and CSNT particles, respectively. Note the default loading of seeding materials samples is 0.05 g for each test. The room humidity is 45.3 % RH.

ASSOCIATED CONTENT Supporting Information. The water-droplet growth videos via E-SEM, the mechanism analysis, and more characterization and experiment information. The Supporting Information is available free

of

charge

on

the

ACS

Publications

website

at

http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Prof. Linda Zou) Author Contributions


21

ACS Nano

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

Page 22 of 28

Dr. Yanlong Tai, Mr. Haoran Liang, Prof. Steve Griffiths, and Prof. Linda Zou have contributed on synthesis, optimization, characterization of the cloud seeding materials and drafting of the manuscript. Dr. Nabil El Hadri and Dr. Mustapha Jouiad have contributed on E-SEM and TEM analysis and cloud seeding materials size optimization processes.

The cloud chamber

experiments were conducted by Prof. Ali M. Abshaev and Prof. Buzgigit M. Huchinaev. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This material is based on work supported by the National Center of Meteorology & Seismology, Abu Dhabi, UAE under the UAE Research Program for Rain Enhancement Science. The authors acknowledge the financial support of UAE Research Program for Rain Enhancement Science and Khalifa University of Science and Technology. The support provided by Prof Daniel Cziczo of Massachusetts Institute of Technology on this work is acknowledged.

DISCLAIMER Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Center of Meteorology & Seismology, Abu Dhabi, UAE, funder of the research.

REFERENCES


22

Page 23 of 28

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

ACS Nano

(1) Kim, H.; Yang, S.; Rao, S. R.; Narayanan, S.; Kapustin, E. A.; Furukawa, H.; Umans, A. S.; Yaghi, O. M.; Wang, E. N., Water Harvesting from Air with Metal-Organic Frameworks Powered by Natural Sunlight. Science 2017, 356, 430-434. (2) Rohwetter, P.; Kasparian, J.; Stelmaszczyk, K.; Hao, Z.; Henin, S.; Lascoux, N.; Nakaema, W. M.; Petit, Y.; Queißer, M.; Salamé, R., Laser-Induced Water Condensation in Air. Nat. Photon. 2010, 4, 451-456. (3) Kiselev, A.; Bachmann, F.; Pedevilla, P.; Cox, S. J.; Michaelides, A.; Gerthsen, D.; Leisner, T., Active Sites in Heterogeneous Ice Nucleation—the Example of K-Rich Feldspars. Science 2017, 355, 367-371. (4) Murray, B.; O'sullivan, D.; Atkinson, J.; Webb, M., Ice Nucleation by Particles Immersed in Supercooled Cloud Droplets. Chem. Soc. Rev. 2012, 41, 6519-6554. (5) Zhu, H.; Li, X.; Yang, R. In A Novel Catalyst of Warm-Cloud Seeding to Enhance Precipitation, IOP Conference Series: Materials Science and Engineering, IOP Publishing: 2016; pp 012002. (6) Langmuir, I., The Growth of Particles in Smokes and Clouds and the Production of Snow from Supercooled Clouds. Proc. Am. Philos. Soc. 1948, 92, 167-185. (7) Axisa, D.; DeFelice, T. P., Modern and Prospective Technologies for Weather Modification Activities: A Look at Integrating Unmanned Aircraft Systems. Atmos. Res. 2016, 178, 114-124. (8) Rosenfeld, D.; Andreae, M. O.; Asmi, A.; Chin, M.; Leeuw, G.; Donovan, D. P.; Kahn, R.; Kinne, S.; Kivekäs, N.; Kulmala, M., Global Observations of Aerosol‐Cloud‐Precipitation‐ Climate Interactions. Rev. Geophys. 2014, 52, 750-808.


23

ACS Nano

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

Page 24 of 28

(9) Creamean, J. M.; Suski, K. J.; Rosenfeld, D.; Cazorla, A.; DeMott, P. J.; Sullivan, R. C.; White, A. B.; Ralph, F. M.; Minnis, P.; Comstock, J. M., Dust and Biological Aerosols from the Sahara and Asia Influence Precipitation in the Western US. Science 2013, 339, 1572-1578. (10) Fajardo, C.; Costa, G.; Ortiz, L.; Nande, M.; Rodríguez-Membibre, M.; Martín, M.; Sánchez-Fortún, S., Potential Risk of Acute Toxicity Induced by AgI Cloud Seeding on Soil and Freshwater Biota. Ecotox. Environ. Saf. 2016, 133, 433-441. (11) Rosenfeld, D., New Insights to Cloud Seeding for Enhancing Precipitation and for Hail Suppression. The Journal of Weather Modification 2007, 39, 61-69. (12) Qiu, J.; Cressey, D., Meteorology: Taming the Sky. Nature News 2008, 453, 970-974. (13) Lv, J.; Song, Y.; Jiang, L.; Wang, J., Bio-Inspired Strategies for Anti-Icing. ACS nano 2014, 8, 3152-3169. (14) Farmer, D. K.; Cappa, C. D.; Kreidenweis, S. M., Atmospheric Processes and their Controlling Influence on Cloud Condensation Nuclei Activity. Chem. Rev. 2015, 115, 41994217. (15) Kuba, N.; Murakami, M., Effect of Hygroscopic Seeding on Warm Rain Clouds– Numerical Study Using a Hybrid Cloud Microphysical Model. Atmos. Chem. Phys. 2010, 10, 3335-3351. (16) Woods III, E.; Hull, A. W.; Tigue, M. L., Uptake of Pyrene onto Fatty Acid Coated NaCl Aerosol Particles. J. Phys. Chem. A 2016, 120, 10160-10168. (17) Mitrofanov, I.; Zuber, M.; Litvak, M.; Boynton, W. V.; Smith, D.; Drake, D.; Hamara, D.; Kozyrev, A.; Sanin, A.; Shinohara, C., CO2 Snow Depth and Subsurface Water-Ice Abundance in the Northern Hemisphere of Mars. Science 2003, 300, 2081-2084.


24

Page 25 of 28

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

ACS Nano

(18) Carrasco, J.; Michaelides, A.; Forster, M.; Haq, S.; Raval, R.; Hodgson, A., A OneDimensional Ice Structure Built from Pentagons. Nat. Mater. 2009, 8, 427-431. (19) Stein, A., Advances in Microporous and Mesoporous Solids—Highlights of Recent Progress. Adv. Mater. 2003, 15, 763-775. (20) Seo, Y. K.; Yoon, J. W.; Lee, J. S.; Hwang, Y. K.; Jun, C. H.; Chang, J. S.; Wuttke, S.; Bazin, P.; Vimont, A.; Daturi, M., Energy‐ Efficient Dehumidification over Hierachically Porous Metal–Organic Frameworks as Advanced Water Adsorbents. Adv. Mater. 2012, 24, 806810. (21) Wang, C.; Liu, X.; Demir, N. K.; Chen, J. P.; Li, K., Applications of Water Stable Metal– Organic Frameworks. Chem. Soc. Rev. 2016, 45, 5107-5134. (22) Pöschl, U.; Martin, S.; Sinha, B.; Chen, Q.; Gunthe, S.; Huffman, J.; Borrmann, S.; Farmer, D.; Garland, R.; Helas, G., Rainforest Aerosols as Biogenic Nuclei of Clouds and Precipitation in the Amazon. Science 2010, 329, 1513-1516. (23) Ghorai, S.; Wang, B.; Tivanski, A.; Laskin, A., Hygroscopic Properties of Internally Mixed Particles Composed of NaCl and Water-Soluble Organic Acids. Environ. Sci. Technol. 2014, 48, 2234-2241. (24) Weis, D. D.; Ewing, G. E., Water Content and Morphology of Sodium Chloride Aerosol Particles. Geophys. Res. Atmos. 1999, 104, 21275-21285. (25) Tereshchenko, A. G., Deliquescence: Hygroscopicity of Water ‐ Soluble Crystalline Solids. J. Pharm. Sci. 2015, 104, 3639-3652. (26) Gupta, D.; Kim, H.; Park, G.; Li, X.; Eom, H.-J.; Ro, C.-U., Hygroscopic Properties of NaCl and NaNO3 Mixture Particles as Reacted Inorganic Sea-Salt Aerosol Surrogates. Atmos. Chem. Phys. 2015, 15, 3379-3393.


25

ACS Nano

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

Page 26 of 28

(27) Gysel, M.; Weingartner, E.; Baltensperger, U., Hygroscopicity of Aerosol Particles at Low Temperatures. 2. Theoretical and Experimental Hygroscopic Properties of Laboratory Generated Aerosols. Environ. Sci. Technol. 2002, 36, 63-68. (28) Tai, Y.; Lubineau, G., Heating-Rate-Triggered Carbon-Nanotube-based 3-Dimensional Conducting Networks for a Highly Sensitive Noncontact Sensing Device. Sci. Rep. 2016, 6, 19632-19639. (29) Alshawa, A.; Dopfer, O.; Harmon, C. W.; Nizkorodov, S. A.; Underwood, J. S., Hygroscopic Growth and Deliquescence of NaCl Nanoparticles Coated with Surfactant AOT. J. Phys. Chem. A 2009, 113, 7678-7686. (30) Amujuri, M. S., Rain Formation in Warm Clouds: General Systems Theory. Springer: 2015. (31) Anand, S.; Paxson, A. T.; Dhiman, R.; Smith, J. D.; Varanasi, K. K., Enhanced Condensation on Lubricant-Impregnated Nanotextured Surfaces. ACS Nano 2012, 6, 1012210129. (32) Rykaczewski, K.; Scott, J. H. J., Methodology for Imaging Nano-to-Microscale Water Condensation Dynamics on Complex Nanostructures. ACS Nano 2011, 5, 5962-5968. (33) Setvin, M.; Aschauer, U.; Hulva, J.; Simschitz, T.; Daniel, B.; Schmid, M.; Selloni, A.; Diebold, U., Following the Reduction of Oxygen on TiO2 Anatase (101) Step by Step. J. Am. Chem. Soc. 2016, 138, 9565-9571. (34) Hou, Y.; Yu, M.; Chen, X.; Wang, Z.; Yao, S., Recurrent Filmwise and Dropwise Condensation on a Beetle Mimetic Surface. ACS nano 2014, 9, 71-81. (35) Patil, S., A Study of Artificial Rainfall. International Journal of Multifaceted and Multilingual Studies. 2016, 3.


26

Page 27 of 28

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

ACS Nano

(36) Garvey, D. M., Testing of Cloud Seeding Materials at the Cloud Simulation and Aerosol Laboratory, 1971–1973. J. Appl. Meteorol. 1975, 14, 883-890. (37) Fletcher, N. H., The Physics of Rainclouds. Cambridge University Press: 2011. (38) Liu, G.; Yan, X.; Chen, Z.; Wang, X.; Wang, L.; Lu, G. Q.; Cheng, H.-M., Synthesis of Rutile–Anatase Core–Shell Structured TiO2 for Photocatalysis. J. Mater. Chem. 2009, 19, 65906596. (39) Ma, C.; Pang, G.; He, G.; Li, Y.; He, C.; Hao, Z., Layered Sphere-Shaped TiO2 Capped with Gold Nanoparticles on Structural Defects and their Catalysis of Formaldehyde Oxidation. J. Environ. Sci. 2016, 39, 77-85. (40) Yang, J.; Wang, Y.; Li, W.; Wang, L.; Fan, Y.; Jiang, W.; Luo, W.; Wang, Y.; Kong, B.; Selomulya, C., Amorphous TiO2 Shells: A Vital Elastic Buffering Layer on Silicon Nanoparticles for High‐Performance and Safe Lithium Storage. Adv. Mater. 2017, 17005231700529. (41) Harmon, C. W.; Grimm, R. L.; McIntire, T. M.; Peterson, M. D.; Njegic, B.; Angel, V. M.; Alshawa, A.; Underwood, J. S.; Tobias, D. J.; Gerber, R. B., Hygroscopic Growth and Deliquescence of NaCl Nanoparticles Mixed with Surfactant SDS. J. Phys. Chem. B 2010, 114, 2435-2449. (42) O'dowd, C. D.; Aalto, P.; Hmeri, K.; Kulmala, M.; Hoffmann, T., Aerosol Formation: Atmospheric Particles from Organic Vapours. Nature 2002, 416, 497-498. (43) Hartmann, P.; Lee, D. K.; Smarsly, B. M.; Janek, J., Mesoporous TiO2: Comparison of Classical Sol−Gel and Nanoparticle Based Photoelectrodes for the Water Splitting Reaction. ACS Nano 2010, 4, 3147-3154.


27

ACS Nano

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

Page 28 of 28

TOC image


28

Shell Microstructure Induced Synergistic Effect for Efficient Water

Recommend Documents