ACNANO Cite This:ACS Nano 2017,11,12318-1232 www.acsnano.org Core/Shell Microstructure Induced Synergistic Effect for Efficient Water-Droplet Formation and Cloud-Seeding Application Yanlong Tai,Haoran LiangAbdelali Zaki,Nabil El Hadri,Ali M.Abshaev,Buzgigit M.Huchunaev, Steve Griffiths,'Mustapha Jouiad,and Linda Zou* Department of Civil Infrastructure and Environment Engineering,Masdar Institute,Khalifa University of Science and Technology, Abu Dhabi,United Arab Emirates SHigh Mountain Geophysical Institute of Russian Federal Hydrometeorological Service,Nalchik City,Kabardino-Balkarian Republic, Russian Federation Supporting Information ABSTRACT:Cloud-seeding materials as a promising water-augmentation technology have drawn ypfNac(CSNT ipaic attention recently.we designed and synthesized a cessfully adsorhed me ater vapo (~295 times at low relative humidity,20% RH)than that of pure NaCl,deliquesced at a lower environmental RH of 62-66% remained as a crystal at the same conditions.The enhanced performance was attributed to thes turned it into a liquid faster.Moreover,the critical particle size of the CSNT particles (0.4-10 m)as cloud seeding m aterials was redicted via the classical Kelvin equation based on their surface hydrophilicity.Finally,the benefits of CSNT particles for cloud-seeding applicatior s were determined visually through in itu observation under an envire ntal scanning electron micros on the macroscale,re ectively.These KEYWORDS:core/shell microstructure,synergistic effect,hydrophilic surface,water-droplet formation,cloud-seeding materials Sodium chloride (NaCl)has been used as a typical water -clond-seedir nly materials as cloud condensation nuclei (CCN)is an effective nucleating materials used in a cold cloud,e dry ice (solid method to accelerate the formation of water droplets and then carbon dioxide,CO2),which is associated with high cost and harvest the water vapor in the atmosphere via rain oiootocnootAehhh ugmentation 08》 materials puposes,ind(the precipitation in a desert or arid areas to resolve the serious water-shortage etc.)experience low uptake of water or high-energy issues in the world;(i)regulating the amount or type of consumption for water release.NaCl-based doud-seeding precipitation in different areas,and (ii)suppressing hal materials can adsorb water vapor in warm clouds and ultimately weakening hurricanes, Received:August 28,2017 extreme weather patterns Accepted:November 17,2017 Published:November 17,2017 ACS Publications Amerkcan chm 12318 AC000721923992
Core/Shell Microstructure Induced Synergistic Effect for Efficient Water-Droplet Formation and Cloud-Seeding Application Yanlong Tai,† Haoran Liang,† Abdelali Zaki,† Nabil El Hadri,‡ Ali M. Abshaev,§ Buzgigit M. Huchunaev,§ Steve Griffiths,† Mustapha Jouiad,‡ and Linda Zou*,† † Department of Civil Infrastructure and Environment Engineering, Masdar Institute, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates ‡ Department of Mechanical & Material Science and Engineering, Masdar Institute, Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates § High Mountain Geophysical Institute of Russian Federal Hydrometeorological Service, Nalchik City, Kabardino-Balkarian Republic, Russian Federation *S Supporting Information 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) particle with controlled particle size, which successfully adsorbed more water vapor (∼295 times at low relative humidity, 20% RH) than that of pure NaCl, deliquesced at a lower environmental RH of 62−66% than the hygroscopic point (hg.p., 75% RH) of NaCl, and formed larger water droplets ∼6−10 times its original measured size area, whereas the pure NaCl still remained as a crystal at the same conditions. The enhanced performance was attributed to the synergistic effect of the hydrophilic TiO2 shell and hygroscopic NaCl core microstructure, which attracted a large amount of water vapor and turned it into a liquid faster. Moreover, the critical particle size of the CSNT particles (0.4−10 μm) as cloud-seeding materials was predicted via the classical Kelvin equation based on their surface hydrophilicity. Finally, the benefits of CSNT particles for cloud-seeding applications were determined visually through in situ observation under an environmental scanning electron microscope on the microscale and cloud chamber experiments on the macroscale, respectively. These excellent and consistent performances positively confirmed that CSNT particles could be 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) increasing the precipitation in a desert or arid areas to resolve the serious water-shortage issues in the world;6,7 (ii) regulating the amount or type of precipitation in different areas,8,9 and (iii) suppressing hailstorms, 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 a typical watersoluble hygroscopic warm-cloud-seeding materials for several decades,14−16 which is more commonly employed than icenucleating materials used in a cold cloud, e.g., dry ice (solid carbon dioxide, CO2), which is associated with high cost and the greenhouse effect,17 and silver iodide (AgI), which is associated with controversial environmental risk.18 Other warm-cloud-seeding materials such as porous materials (zeolites,19 silica gels,20 metal−organic frameworks (MOFs),21 etc.) experience low uptake of water or high-energy consumption for water release. NaCl-based cloud-seeding materials can adsorb water vapor in warm clouds and ultimately Received: August 28, 2017 Accepted: November 17, 2017 Published: November 17, 2017 Article Cite This: ACS Nano 2017, 11, 12318−12325 www.acsnano.org © 2017 American Chemical Society 12318 DOI: 10.1021/acsnano.7b06114 ACS Nano 2017, 11, 12318−12325
ACS Nand transform into vater drople 1 th midity (RH,25 the Nacl y symthesis of the CSNT Par Accordingly,using I does not sh we The betwee applica st we copic of the ing RH poin d the ad ing the ，)o water-v apo of Ti w also tally alter the hygroscopic behavior of NaCl and lead to only bed ab 8 M er.the bene pa were n synthe a ind Na (E-SEM)in the details of an orehelNaC/tanitmdioade(Tio,)microstructrewitha respectively. RESULTS AND DISCUSSION we emp P Mechanism of Enhanced Water-Droplet Formation ur Strategy.s nygroscopic point()The ules are in c nolecules and crea layer with state and ultir to th RH value tha he air and effect to fo opic por ompletely absent in the case of pure Nac Cl crystal (P)and the pur the ds.be (~75%RHt25 the Nac crysta does not ow itse b 0)b sed ystal is the Therefore,to,odense,and his per (o ess,water density,etc mechanis the 1.Sche illustration of the hy and particles t oint of the Nac wate ccurred pare mig path of wate delique
transform into water droplets completely.22,23 The formed water droplets continuously grow by a 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 °C), the NaCl crystal does not show any evident water-vapor adsorption performance, especially no detectable water-vapor adsorption at all below 25% RH,24,25 which sets limitations on 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 water-adsorbing capability (polyacrylamide, sodium polyacrylate, etc.).28,29 However, these mechanical-mixing methods cannot fundamentally alter the hygroscopic behavior of NaCl and lead to only minimal improvement. Such limitations motivated us to rationally design on the mico/nano scale and synthesize a kind of NaCl-based cloudseeding material that could outperform the existing materials. We describe below the details of a patented process 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 watervapor 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 a 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 the 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 cloudseeding materials, and then it was used as guidance for the 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. 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 a 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 applications were further determined visually through in situ observation under an environmental scanning electron microscope (E-SEM) in the microscale and cloud chamber experiments in the macroscale, respectively. RESULTS AND DISCUSSION Mechanism of Enhanced Water-Droplet Formation. Explanation of Our Strategy. As described above, NaCl is a commonly used hygroscopic cloud-seeding material. Specifi- cally, when water-vapor molecules are in contact with the NaCl crystal, some of them will condense, hence changing from a gaseous state to a liquid state and ultimately leading to the deliquescence of the NaCl crystal. Note that this is a reversible process, where a dynamic equilibrium exists. The threshold point is defined as the 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 eq 1. 25 In other words, below its hg.p. (∼75% RH at 25 °C), the NaCl crystal does not adsorb and condense water at all. The watervapor pressure in the warm cloud is always saturated (PH2O‑air, ≥100% RH), above the hg.p. of the NaCl crystal, which is the theoretical reason that NaCl can be used as a cloud-seeding material.30 = × ⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ h P P g.p. 100% salt H O2 (1) Therefore, to efficiently adsorb, condense, and form a water droplet, a type of CSNT particle was designed, which is expected to adsorb and then condense more water vapor below its hg.p. to subsequently grow during the collision−coalescence process into a larger drop size and form rainfall more efficiently. This performance could make the seeding materials less selective to the cloud conditions (cloud thickness, watervapor density, etc.). In this mechanism, the core/shell microstructure not only causes deliquescence to occur at lower hg.p. but also contributes to the larger water-droplet formation: (i) This core/shell microstructure improved the interaction between the water-vapor molecule and NaCl crystal, as seen in Figure 1. Specifically, as for the pure NaCl crystal, the adsorption, condensation of water-vapor molecules, and deliquescence of NaCl crystals occurred at the same time and the same hygroscopic interface between NaCl and air, and its deliquescence was totally dependent on the environmental RH Figure 1. Schematic illustration of the hygroscopic mechanism of CSNT particles to promote water-vapor adsorption and waterdroplet formation below the hygroscopic point of the NaCl crystal, compared with that of a commercial pure NaCl crystal. Note the migration path of water molecules is indicated by arrows. ACS Nano Article DOI: 10.1021/acsnano.7b06114 ACS Nano 2017, 11, 12318−12325 12319
ACS Nano a) 6 e 1 2030 ume (ml/cm) 400600800100 man shift(cm) /e ()SEM Nacl:(i)0. ,(w) and 10 EDX.S rdes(14+0.3 ,this process the iO2 shell and the This ed hya int ald mak apor only re loud or fog wate s on the cles than tha of pr sure crysta d a hig ropletg macroscopic drople As for the layer than that in the t area (o)of hich tact angle vstals could g ate nd 1.Based on th elatio Figur Thus stic effec owth as we ce (Nac pton (C 17.60 aller size and wo an The size of the cloud-seedin erials also plays an im an nd ole in the of the rain T part btain C SNT rials is depe thei surface hve rophobi th ydrophili,the *= (eg ding m n.kT In(P/P) (2) ater microdroplets,it is ther can be calculated from the contact angle. 12320
conditions. As for the CSNT particles, this process took place separately. At first, the adsorption of water vapor happened at the interface between the TiO2 shell and air, and then deliquescence of NaCl crystals 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 nanoparticles than that of pure NaCl crystals.31−33 As a result, the accumulated water molecules caused a higher local RH value in the coated TiO2 layer than that in the air, like a moisturizing layer, leading to faster deliquescence of NaCl crystals. (ii) The partial dissolution of NaCl crystals could generate a vapor-pressure gradient (PH2O gradient), 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 complete deliquescence. More details can be seen in Figure 1. Critical Particle Size Range for Cloud-Seeding Materials. The size of the 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 the cloud-seeding materials is too small, it is impossible to surpass the Gibbs free energy barrier (ΔG) for condensation, so water vapor only remains as cloud or fog. According to the Kelvin equation (eq 2), there is an inverse relationship between the saturation ratio of water vapor pressure (P/P0) and the critical radius (r*) of the water microdroplet. Only when its radius exceeds r* can the water droplet grow and become a 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. σ r* = n kT P P 2 ln( / ) L 0 (2) in which n is the number of molecules per unit volume in water, k is the Boltzmann constant, T is the absolute temperature, and σ can be calculated from the contact angle. 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) 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 the NaCl crystal particle. The scale bars are 200, 50, 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. ACS Nano Article DOI: 10.1021/acsnano.7b06114 ACS Nano 2017, 11, 12318−12325 12320
ACS Nano 675%Rn Figure3、Hygro of CSNT pa cles.(a ar b)Wa ption iso rea in (a) in (h) the d)E-SEM B pl CSNT d a 67.5 re 3 um.a-1.a-2. and a-3 in (d)and (e)are cho deliqu e nr of hoth s Synthesis and Characterization of CSNT Particles.As ell as Figu S4.The coated TiO,layer the NaCl ta related to its perty This mean thei r the pre sence of TiO,on the Cl cryst 1-6 the the c SEM lable sie thes d chara cterization of CSNT partidles are hin ayer on the NaCl EDX e 2a Figure S5. 1 the CSNT with size distributi XRD)onC SNT,pure NaCl,and pure TiO Bes by heating nt nd th tw n the mL 6 This due ery lo 3 es of SN phous ch res ere lso co th op of the size of CSN rticle is sumr in Fig of CSNT pl f sh ng ta sim ilar pa that affected the iz as Tio. shell on the NaCl core and the the We hav the drying ed the th size range as des he larg the the for ct ent m Is in the follc the an dand the ine the omple apof C (Figure 3a). ed by tr scopy(TEM), adsorption to water vapor before reaching itsh(75%RH)
Synthesis and Characterization of CSNT Particles. As described above, the critical size of the hygroscopic cloudseeding materials is strongly related to its surface hydrophilic property. This 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 a pure NaCl crystal and synthesized CSNT particles, respectively. The different sizes of the CSNT particle with uniform size distributions were obtained by heating different volumes of TiO2 sol/NaCl/ solvent mixtures under the same conditions (80 °C 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 the mixture and the size of CSNT particle is summarized in Figure 2b. As for the crystal growth by the 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. Here, when the drying conditions remained the same, the difference in mixture volume led to the different crystallization time; that is, 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 completely evaporated. 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 the 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 the NaCl crystal was further confirmed and characterized by energy dispersive X-ray (EDX) spectroscopy on the SEM instrument (Figure 2d) with the elemental mapping images of Na, Cl, Ti, and O.40 The welldefined core/shell microstructure with homogeneous distribution of a thin TiO2 layer on the NaCl crystal surface was successfully confirmed. The corresponding EDX spectrum plot can be seen in 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 a similar pattern to the one observed for pure TiO2, indicating the presence of a TiO2 shell on the NaCl core. 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. Their enhanced water-droplet formation was evaluated through different methods in the following sections.25,41 First, a water-vapor adsorption isotherm analysis was conducted to quantitatively determine the water-vapor adsorption capacity of CSNT particles (Figure 3a). It was found that pure NaCl (NaCl-1) did not show an evident adsorption to water vapor before reaching its hg.p. (∼75% RH). Figure 3. Hygroscopic performance of CSNT particles. (a and b) Water-vapor adsorption isotherm tests to characterize the water-vaporadsorption capacity and hygroscopic property of CSNT particles with different thicknesses of the 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) is highlighted in (b). (c) Relationship between the general thickness of the TiO2 shell on the NaCl crystal and the TiO2/NaCl mixture mass ratios (0.196, 0.314, 0.785, respectively) during the 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 a pure NaCl crystal, respectively. Note S0 and S are the sample-area sizes before (50% RH) and after deliquescence (67.5% or 75% RH), respectively. All 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. ACS Nano Article DOI: 10.1021/acsnano.7b06114 ACS Nano 2017, 11, 12318−12325 12321
ACS Nano This e ted onto the its hye vas impro point for ad (the it pic part of the Tio cm/g compa 7220 D red with RH of NaCl. the nic noint of CSNT particles is Tio、shell al ed Nacl orb more wate potentially teto the sy effect of the hye 1 mi and NaCl.wh not alter the d for i rvation ckground (no 3d,e. pd f.The CSNT particles the both the ter-dropl that CSNT ed to 4.c-1 to c.3 t100 RH.the % al to the han t by Na 15.5 s highe nge in nt with the hyg orma re bo the he of pure erials SNT particles were a type of ）increased to han CONCLUSIONS 10 and s2 The designed and synthesized CSNT ssfull firmed the low rel d ah par ng mat -is the RH of 62 tha a th e p( RH sed and d much l at lo red s the ld create Experin ud These th 12 ten the effects caused oscale and cloud 12322
This hygroscopic performance was not improved after the size of the pure NaCl crystals was 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 by three aspects: (i) the starting point for water-vapor adsorption; (ii) the adsorbed water-vapor volume below the 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 a much higher humidity value of 16% RH, which is highlighted in Figure 3b. Second, as for CSNT-1 (the thickness of the TiO2 shell is ∼24 nm), the adsorbed watervapor volume is 44.24 cm3 /g compared with that (0.15 cm3 /g) of NaCl-1 at 20% RH, which is around a 295 times increase. As for CSNT-2 and CSNT-3, this increase can be up to 196 times and 121 times, respectively, due to the different thicknesses of the 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 the ∼75% RH of NaCl-1. The systematic experimental investigation of the effects of the TiO2-shell thickness on the hygroscopic point of CSNT particles is presented in Figure S8. It can be concluded that the TiO2 shell allowed NaCl crystals to adsorb more water vapor, which potentially contributed to form larger water droplets 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 Figures 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, and the results are summarized in Figure 3d, e, and f. 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 the watervapor 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 their area-size ratio (S/ S0) increased to ∼6−10 times bigger than their original size (the particle size at 50% RH). For more details refer to Figures S10 and S11 and Videos 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 droplets at lower RH conditions, which can translate to a more efficient cloudseeding effect and could create rainfall more easily. Cloud Chamber Experiments. Cloud chamber experiments provide 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 are 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 of 10−25 μm (which is very crucial to the rainfall)42 caused by the CSNT particles was up to 290% higher than that by NaCl and 15.5 times higher than that of the 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 a lower environmental RH of 62−66% than the hg.p. (75% RH) of pure NaCl, and formed larger water droplets ∼6−10 times their original measured size area, whereas the pure NaCl still remained as crystals at the same conditions. 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 the microscale and cloud 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 microscopy comparing the water-droplet size seeded by pure NaCl crystals and CSNT particles in a defined area; all 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 °C and 100% RH, which is very close to real cloud conditions. 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 replicates. ACS Nano Article DOI: 10.1021/acsnano.7b06114 ACS Nano 2017, 11, 12318−12325 12322