Core-shell-embedded Mesoporous Silica Capsules for Atmospheric Water Harvesting

ZHANG Shangsheng,XU Shuman,LEI Ruicheng,PAN Yuliang,MA Tao,ZHANG Zheng,LIU Chunsheng,ZHANG Zengzhi

(School of Mechanical Electronic and Information Engineering, China University of Mining and Technology-Beijing, Beijing 10083,China)

Abstract: A one-step ultrasonic mechanical method was used to synthesize a kind of atmospheric water harvesting material with high water harvesting performance in a wide relative humidity (RH) range,especially at low RH (RH ＜ 40%),namely,mesoporous silica capsule (MSC) with core-shell structure.Transmission electron microscopy (TEM),nitrogen adsorption and other characterization techniques were used to study the formation process of nano-microspheres.A new mechanism of self-adaptive concentration gradient regulation of silicon migration and recombination core-shell structure was proposed to explain the formation of a cavity in the MSC system.The core-shell design can enhance the specific surface area and pore volume while maintaining the monodispersity and mesoporous size.To study the water harvesting performance of MSC,solid silica nanoparticles (SSN) and mesoporous silica nanoparticles (MSN) were prepared.In a small atmospheric water collection test (25 ℃,40% RH),the water vapour adsorption and desorption kinetics of MSC,SSN,MSN and a commercial silica gel (CSG) were compared and analyzed.The results show that the MSC with mesoporous channels and core-shell structure can provide about 0.324 gwater/gadsorbent,79% higher than the CSG(0.181 gwater/gadsorbent).It is 25.1% higher than that of 0.259 gwater/gadsorbent of un-hollowed MSN and 980% higher than that of 0.03 gwater/gadsorbent of un-hollowed SSN.The material has a large specific surface area and pore volume,simple preparation method and low cost,which provides a feasible idea for realising atmospheric water collection in arid and semi-arid regions.

Key words: mesoporous silica nanocapsules;core-shell structure;atmospheric water harvesting;adsorption performance

1 Introduction

Fresh water on Earth is only 2.5 percent of all water resources,about 70 per cent of which are hardto-use glacial water,and only very few shallow surface water and fresh river water can be used directly by humans[1].Therefore,the global water resources crisis has become one of the most severe crises that human beings must face,which significantly threatens the survival of all animals and in some arid and semiarid regions.Seawater plants desalination is one of the essential methods to solve the water crisis at present.However,the cost of desalination is high,which can only be established on the seaside and does not have the advantages of all-terrain.It cannot effectively solve the problem of drinking water shortage in inland arid and semi-arid areas that need water,especially the issue of substantial agricultural water demand.Atmospheric water harvesting (AWH) and processing technology may help us solve this life-threatening problem.Atmospheric water harvesting techniques for direct collection of liquid water from the air by water condensation (e g,fog capture and dew collection)have been extensively studied[2-6],providing more than 50 000 cubic metres of water without time and terrain constraints.However,the traditional adsorptionbased fog collection system is easy to realize the direct extraction of atmospheric water by condensing cold air below the dew point,but it is not feasible in arid environments.It requires the environment to maintain high relative humidity (RH ＞ 90%) and airflow for a long time to promote condensation[7-11].In contrast,collecting water at low relative humidity or even from desert air must depend on the high adsorption performance of the adsorbent.Therefore,the development and design of porous materials with high specific surface area and porosity in the past few decades include zeolite,porous carbon,silica gel,metal-organic frameworks (MOFs),and water adsorbents,have made significant progress[12-16].

Commercial silica gel is widely used in engineering,but its low porosity and disordered pores limit the potential water absorption,and it needs hightemperature dehydration to be reused[17].To solve this problem,the next generation adsorbent should have the following advantages: (i) high specific surface area,(ii) customizable adsorption interface,(iii) adjustable pore size,(IV) high,and (V) excellent adsorption and desorption kinetics.MOFs are expected to meet all the above requirements because they are composed of molecular construction modules.The diversity of molecular structures allows the necessary coordination optimization to achieve the required water absorption performance.Although the adsorption of H2,CH4,CO2and other gases has been extensively studied[18-20],the design principle of MOFs with a high adsorption rate of these gases has been established,the adsorption of water vapour has received little attention.In addition,the limitation of MOFs in materials,production cycle and cost hinders its large-scale application in water collection.In this regard,mesoporous silica nanospheres have good adsorption capacity due to their tunable pore system,high specific surface area and continuous particle skeleton[21-24].

To increase the effective surface area of the adsorbent and the droplet capture site,in this work,a large number of mesoporous channels were inserted at the surface interface of silica nanospheres,and a large cavity was inserted inside to form a ringing core-shell structure and a core-shell MSC was synthesized.We studied the water vapour adsorption kinetics of several mesoporous materials and found that the water vapour adsorption rate and saturated water vapour adsorption capacity of core-shell structure MSC were better than MSN,CSG and SSN.As far as the authors know,this is the first attempt to use mesoporous channels and core-shell structures inserted inside silica for atmospheric water collection at low RH.Field emission scanning electron microscopy (FE-SEM),transmission electron microscopy (TEM) and X-ray diffraction(XRD) were used to study the surface morphology,mesoscopic morphology and skeleton composition of the nanospheres.The results show that the wormlike mesoporous channels in the three-dimensional direction provide many nano-scale capillaries for the interface.With the insertion of a core-shell structure,the droplet capture sites can be further increased.At the same time,compared with the traditional two-dimensional plane water collection device,the self-storage structure has no additional water storage equipment,and the capture,condensation,storage and release of water molecules in the atmosphere can be realized step by step.

In this paper,the authors first proposed an adaptive concentration gradient regulation mechanism for silicon migration and recombination to explain the formation mechanism of MSC core-shell in this system.At low RH (RH ＜ 40%),the dynamic vapour adsorption (DVS) gravimetric analysis was used to study the condensation rate further and saturated vapour adsorption capacity of three hydrophilic silica nanospheres with different structures and an invisible droplet on a CSG surface.The steam adsorption kinetics curve evaluated the influence of mesoporous channels and core-shell structure on the mass transfer performance of silica adsorbent,and it was confirmed that the insertion of mesoporous channels and ringing core-shell form inside the silica could improve the water capture effect.This method has the advantages of simple raw materials,no complex preparation process and high cost-effectiveness,which is expected to become a sustainable AWH alternate method in the future and has reference value for the basic theoretical research of efficient AWH.

2 Experimental

2.1 Materials

Deionized water (laboratory-made),absolute alcohol,tetraethyloxysilane (TEOS),ammonia,cetyltrimethylammonium bromide(CTAB),and tetrapropylammonium hydroxide(TPAOH,40wt%),were directly used when received.

2.2 Method

In this group,we prepared MSC by one-step ultrasonic mechanical method.The synthesis strategy is shown in Fig.1.In this method,TEOS as the precursor,ethanol as the organic solvent,ammonia as TEOS hydrolysis catalyst,CTAB as a mesoporous template.CTAB was dissolved in the mixed solvent of deionized water and ethanol and then stirred until CTAB was wholly dissolved to form a stable waterin-oil microemulsion.Then ammonia and TEOS were added in turn to conduct ultrasonic stirring in a sealed environment.In the microemulsion system,TEOS as oil phase,ethanol and deionized water together form water phase.It is worth noting that CTAB is not only coated inside the silica nanospheres as a mesoporous template but also acts as a cationic surfactant to stabilize the microemulsion so that the formed silica nanospheres are more evenly dispersed.

2.2.1 Preparation of solid silicon nanospheres (SSN)

Firstly,the conical bottle,measuring cylinder,beaker,surface dish,disposable dropper,glass rod,magnetic rotor and constant pressure funnel were put into the ultrasonic cleaning instrument with deionized water in turn.The ultrasonic cleaning was carried out three times,10 minutes each time.After the cleaning was completed,the ultrasonic cleaning was put into the electric hot blast drying box.

100 mL deionized water,40 mL anhydrous ethanol,1 mL ammonia,0.38 g CTAB were added into the conical flask in turn and stirred with a multi-head magnetic heating stirrer.CTAB was dissolved entirely,and 1.5 mL TEOS was added.

After the reaction is completed,the solution is filtered,put into the electric hot blast drying box,80 ℃dryings.The white powder after drying is SSN.

2.2.2 Preparation of mesoporous silica nanospheres(MSN)

The above-dried powder was put into a muffle furnace,calcined at 500 ℃ for three hours,and the heating rate was set to 5 ℃/min.When the mesoporous template is wholly burned,wormlike mesoporous channels are inserted into silica nanospheres,which can be confirmed in Fig.4(e) of Section 3.2.

2.2.3 Preparation of core-shell mesoporous silica capsules (MSC)

The above samples were poured into 600 mL TPAOH solution with a particular molar concentration.

After 8 h,the solution was filtered and then put into the electric hot blast drying box and dried at 80 ℃.

The dried powder was put into a muffle furnace,calcined at 500 ℃ for 3 h,and set the heating rate to 5℃/min.MSC with core-shell structure can be obtained by completely burning TPAOH remaining on the coreshell interface.

2.3 Characterization

The Hitachi SU8000 series FE-SEM observed the morphology of the samples at 10 kV acceleration voltage,and the gold spraying was performed before observation.Microphotographs are taken in high vacuum mode.Tecnai G30S-TWIN TEM was used to analyze the microstructure,and the accelerating voltage was 100 kV.The samples were ultrasonically dispersed in ethanol solvent and dropped on the carbon-coated copper grid for TEM observation after drying.The elemental composition of the piece was determined by energy-dispersive X-ray spectroscopy (EDS).

The X-ray diffraction patterns of 40 kV and 40 mA Cu-Kα radiation (λ=1.541 8 Å) were collected on the German D8ADVANCE X-ray diffractometer,and the scanning rate was 1 °/min.

Micromeritics ASAP 2460 system was used to determine the nitrogen adsorption/desorption isotherms at 77 K.The sample was activated at 473 K for 10 h before the test.Brunauer emmett teller (BET) method was used to calculate the specific surface area.The desorption branch of isotherm was analyzed by Barrett Joyner Holanda (BJH) algorithm,and the pore size distribution curve was obtained.The adsorption amount calculated the total pore volume under the maximum relative pressure (P/P0).

3 Results and discussion

3.1 Elements

EDS was used to detect the chemical composition of MSC.Fig.2 is the element distribution map of the MSC spectrum and the atomic percentage table of element distribution.It can be seen that the atomic percentage of the Si element is 54.62,and the nuclear rate of the O element is 45.38,which proves that we have successfully prepared silica nanospheres.

Fig.2 Element distribution map and atomic percentage of element distribution in MSC spectra

To further clarify the crystallization behaviour of the samples,we carried out the XRD test of MSC,and the XRD spectrum is shown in Fig.3.Fig.3(a) shows the small-angle X-ray diffraction pattern of MSC.As shown in the figure,a peak was observed at a slight angle centred at 2θ=1.5°,reflecting the mesoporous structure of the sample.Compared with the results of other research groups in the preparation of highly ordered mesoporous MCM-41,a peak appeared near 2θ=2.0°,and two additional Bragg reflection peaks appeared[25-31].Fig.3(b) is a high angle XRD pattern.It can be seen that there is a broad and flat diffraction band near 2θ=16°-33°,indicating that the sample is amorphous silica.

Fig.3 (a) Low-angle X-ray diffraction pattern of MSC;(b) High-angle XRD pattern of MSC

3.2 Morphology and microstructure

Fig.4 shows the morphology and microstructure of the samples observed by FE-SEM and TEM.Figs.4(a),4(d) and 4(g) are the FE-SEM images of SSN,MSN and MSC,respectively.Figs.4(b),4(e) and 4(h) are the TEM images of SSN,MSN and MSC,respectively.It can be seen that the silica nanospheres prepared in this work have good spherical morphology,and the particle size is relatively uniform and highly dispersed.The particle size distribution (PSD) was calculated by Image-Pro Plus software.Figs.4(c),4(f),4(i) show that the particle size distribution is concentrated in 100-360 nm.The average particle sizes of SSN,MSN and MSC calculated by measuring 100 spheres are 172,149 and 168 nm,respectively.It is proved that the insertion of mesoporous channels and core-shell structure does not change the particle size.Fig.4(g) shows the fracture morphology of MSC.The core-shell form can be seen,and the shell thickness is about 37 nm.Combined with TEM,it can be seen that MSC has a bright and dark interface.The internal brilliant represents the hollow structure,and the external dark represents the silicon shell with a large density.It further proves that the ringing core-shell structure of nanoparticles has been successfully fabricated.In the TEM image (Fig.4(e)),irregular mesopores with a diameter of about 3 nm can be seen,and the microstructure is worm-like pores.It has also been reported that monodisperse silica microspheres with small particle sizes tend to aggregate to form large particles in the process of template removal[32,33].It is worth noting that the nanocapsules prepared by our simple one-step ultrasonic stirring method have only slight agglomeration after calcination.In the preparation process,the particle size of mesoporous silica nanocapsules can be adjusted by changing the temperature.

Fig.4 FE-SEM,TEM and particle size distribution: (a),(d),(g) FE-SEM images of SSN,MSN and MSC;(b),(e),(h) TEM images of SSN,MSN and MSC;(c),(f),(i) particle size distribution maps of SSN,MSN and MSC

Fig.5 is the N2adsorption and desorption isotherms of MSN and MSC,respectively.In the low vapour pressure region,adsorption is only a function of relative vapour saturation,which corresponds to the accumulation of single-layer and multi-layer nitrogen on the solid wall.At high pressure,a large number of adsorption corresponds to capillary condensation.Before reaching saturation pressure,liquid nitrogen has been filled with porous bodies.This adsorption branch usually shows a substantial lag,and the capillary desorption is condensed at a low vapour pressure ratio.Therefore,the two nanospheres have similar type IV curves and type H4hysteresis loops.The difference is that the hysteresis loop of MSC is more significant than that of MSN(Fig.5(a)),and large cavity may cause in MSC.The pore size distribution curve gives a clear proof (Fig.5(b)).The pore size distribution curve of MSN only showed a peak at 2.8 nm,while MSC showed a further broad rise at more than 80 nm in addition to a peak at 2.8 nm.The peak at 2.8 nm was mesoporous structure,and the broad peak at more than 80 nm was core-shell structure.

Fig.5 (a) N2 adsorption-desorption isotherms of MSC and MSN;(b) Pore size distribution curves measured by the BJH method

Table 1 shows the structural data of the sample.It can be seen that the BET specific surface area of MSC was 942 m2·g-1,which was 179 m2·g-1higher than that of MSN (763 m2·g-1),because the core-shell structure has been successfully inserted.The surface area of MSN is four times larger than that of SSN (151 m2·g-1)due to the insertion of more mesopores on the MSN interface.After the mesoporous silica nanospheres were inserted into the core-shell structure,the mesoporous pore size remained constant.The specific surface area and pore volume increased by 23.5% and 81.3% compared with MSN,respectively.The saturated water vapour adsorption capacity and adsorption rate also improved significantly.The specific data are given in Section 3.5.

Table 1 Specific surface area SBET,total pore volume Vt,pore diameter Dp and average size of the investigated SSN,MSN and MSC

3.3 Thermal stability and reuse stability

Since the sample was dried at 120 ℃ before the adsorption-desorption test,the thermal stability of the model should be examined.In this paper,thermal gravimetric analysis technology was used to study its stability.The sample was heated at a heating rate of 10 Kmin-1in a thermally inert environment (pure nitrogen flow in an Al2O3container),and the sample quality was measured.It can be seen from the TGA curve (Fig.6)that the sample shows a typical one-step weight loss below 120 ℃ (about 5%),which can be attributed to the removal of adsorbed water.

Fig.6 TGA curve of sample MSC

The daily temperature difference in arid areas varies greatly.In summer,the maximum near-surface temperature can reach 50 ℃,but it can be reduced to below 8 ℃ at night.The continuous thermal expansion and cold contraction challenge the structural strength of MSC,and the repeated use stability is an important parameter affecting the functional performance.We put the sample into the artificial climate box to fully simulate the desert environment,kept it at 70 ℃ for 2 hours,and then cooled it naturally.When it drops to room temperature,the refrigeration mode is opened,and the temperature is reduced to 7 ℃,so the cycle is 50 times,and then the water vapour adsorption and desorption cycle is carried out.Fig.7 shows the five cycles of water vapor adsorption and desorption of MSC.It can be seen that the saturated adsorption amount did not decrease significantly,indicating that the thermal stability and reusability of MSC synthesized in this work were good.

Fig.7 Five complete water vapor adsorption-desorption cycles of four adsorbents (Adsorption condition is T=25 ℃,RH=40%,and desorption condition is T=50 ℃)

3.4 Mechanism

Tetraethoxysilane was hydrolyzed under the catalysis of ammonia,and OH-was substituted for ethoxy to generate monosilicate and ethanol.Intermolecular dehydration condensation of monosilicicic acid or dealcoholization condensation between monosilicicic acid and remaining tetraethoxysilane formed oligomers.Long-chain Si-OSi network condensed matter was formed by further polymerization between the oligomers.When the concentration of Si-O-Si network condensed matter reached supersaturation,it suddenly became nucleation under ammonia’s catalysis and created dozens of nanometer-sized low-density polymer silica particles by self-assembly.Then the oligomers continued to polymerize,grow and assemble around the core and finally formed silica nanospheres.

Since CTAB was added as the mesoporous template in the reaction process,the oligomers formed in the second step of forming silica spheres would take CTAB as the core.At this time,CTAB,as a part of the framework of silica spheres,was entirely coated by these oligomers in a self-assembly manner,and then CTAB in the framework was removed by hightemperature calcination to obtain mesoporous silica nanospheres.

A new mechanism of self-adaptive concentration gradient regulating silicon migration and recombination core-shell structure was proposed.The gradient Si migration driven by the concentration effect occurs at different interfaces of the same shell,crossing the homogeneous interface through wormlike mesoporous channels,which have low resistance and promote Si migration and recombination.With the increase of reaction time,the reaction rate decreases with the decrease of reactant concentration.After migration and recombination,the order degree of Si and O atoms increases and the condensation degree increases,resulting in the rise of the density of silica spheres from inside to outside.The internal silicate with low density will be hydrolyzed to form a cavity.We find that two key factors control this unique process,increased order gradient,and reaction rate.This work confirmed that monodisperse silica experienced preferential dissolution in the shell,and the concentration gradient regulated the silicon migration and recombination to form a ringing core-shell structure.The chemical reaction equation is shown in Fig.8.

Fig.8 MSC chemical reaction equation

3.5 Adsorption and desorption properties of water vapour

To determine the AWH performance,we compared water vapour adsorption-desorption properties of MSC,MSN,SSN and CSG,and investigated the feasibility of a practical application.The preparation of these materials is described in Section 2.2.Before each adsorption measurement,the adsorbents were heated to 120 ℃ for complete dehydration,and then they were carefully and rapidly transferred to the artificial climate chamber.In this study,the above adsorbents with the same mass were selected.In the adsorption experiment,the temperature of the artificial climate chamber was maintained at 25 ℃,and the humidity was set to 20%,30% and 40% RH,respectively.The sample weight (the mass data accuracy was 0.001 g) was recorded,and the time interval was 1 h.The sample mass did not change for 2 h and was used as the material’s saturated water vapour adsorption amount.

The water vapour adsorption kinetics curve is shown in Figs.9(a)-(c).We monitored the water absorption at 25 ℃,20%,30% and 40% RH.It can be seen that the performance of core-shell structure MSC is better than MSN and CSG.After inserting mesoporous channels and core-shell structure in solid silica material SSN,the water vapour adsorption rate and saturated adsorption capacity significantly improve.The adsorption rate and saturated adsorption capacity of core-shell structure MSC are the best in all test samples.The results showed that the adsorption rates of the four adsorbents at 20% were SSN,CSG,MSN and MSC,respectively.The adsorption rates also showed the same law at 30% and 40% RH.This is because the insertion of mesoporous channels increases the effective specific surface area of the material so that the steam molecules are quickly adsorbed on the pore wall(Fig.9(f)).The insertion of the core-shell structure further increases the effective specific surface area of the material,thereby increasing the absorption rate.

Fig.9 Adsorption (a-c) and desorption (d) kinetics of four samples under different RH environments were measured.Solid lines represent the experimental data,and the colour of bars represents the type of adsorbent,which is based on the change of water quality of each mass of adsorbent with time: (a) 20% RH;(b) 30% RH;(c) 40% RH.The ambient temperature of all adsorption experiments was 25 ℃.Before adsorption,the adsorbent material was fully dehydrated at 120 ℃;(d) Hydrolysis absorption at 50 ℃.Before desorption,the adsorbent material was saturated at 25 ℃ and 40% RH;(e) The saturated adsorption capacity of 10%-40% RH at 25 ℃;(f) Schematic diagram of MSC adsorption of atmospheric water molecules

Fig.9(e) shows the saturated adsorption capacity of four materials in different RH.It can be seen that the saturated adsorption capacity of four materials increases with the increase of RH.At any RH,the saturated adsorption capacity of MSC is the largest among the four materials,and the order of saturated adsorption capacity from small to large is SSN,CSG,MSN and MSC.Taking the saturated adsorption capacity of MSC and MSN at 25°C and 40% RH as an example,the saturated adsorption capacity of MSC(0.324 g·g-1) was 79% and 980% higher than that of commercial silica gel adsorbent CSG (0.181 g·g-1) and solid silica material SSN (0.03 g·g-1),respectively,while that of mesoporous silica material MSN (0.259 g·g-1) was only 43% and 763% higher than that of commercial silica gel adsorbent CSG and solid silica material SSN,respectively.Since the insertion of mesoporous channels increases the total pore volume of the material,the steam molecules are quickly adsorbed on the pore wall,and the insertion of the coreshell structure further increases the total pore volume of the material.Table 1 shows that the insertion of mesoporous channels increases the pore volume of SSN from 0.12 to 0.47 cm3·g-1,an increase of 292%,while the rise of the core-shell structure further increases the pore volume to 0.87 cm3·g-1,an increase of 625% compared with SSN.The continuous increase of total pore volume provides enough space for the rise of the water vapour saturated adsorption capacity of MSC.

To fully characterize the performance of adsorbent materials in atmospheric water collection,we also measured the desorption kinetics of adsorbent materials at 50 ℃ (Fig.9(d)) using an artificial climate chamber to simulate the climatic conditions of inland arid areas.The time interval of weighing desorption weight change was 10 min,and the data of 30 min continuous mass no change were used as the total desorption mass of water vapour.

At 50 ℃,the dynamic desorption results of MSC,MSN,CSG and SSN showed that the desorption rate of MSC was the fastest among the four materials under the same environmental conditions.The desorption rate and adsorption rate showed an apparent corresponding relationship.That is,the greater the adsorption rate,the greater the desorption rate.The desorption rates from small to large were SSN,CSG,MSN and MSC,respectively.The insertion of mesoporous channels and core-shell structures increases the effective specific surface area and droplet nucleation sites of the material,which accelerates the adsorption rate and the desorption rate of the adsorbent.The use of MSC can significantly improve the cyclic adsorption capacity.

4 Conclusions

This study successfully produced a mesoporous silica capsule (MSC) adsorbent with a core-shell structure.The formation mechanism of the mesoporous and core-shell form was discussed,and a new means of self-adaptive concentration gradient regulation of silicon migration and recombination core-shell structure was proposed.The climatic conditions at low RH in arid areas were simulated,and the atmospheric water was collected.The results showed that using mesoporous channels and core-shell structures in silica nanospheres could significantly increase the water vapour adsorption rate and saturated adsorption capacity.Our strategy can help to overcome the growing water consumption problem and reveal the possibility of collecting atmospheric water in arid areas in the future by increasing the effective surface area and droplet capture sites through texture-rich mesoporous channels and super-large core-shell structures.

Conflict of interest

All authors declare that there are no competing interests.