Colorful Wall-Bricks with Superhydrophobic Surfaces for Enhanced Smart Indoor Humidity Control.

Humidity-control materials have attracted increasing attention because of energy savings and smart regulation of indoor comforts. The current research is a successive work to face challenges, such as poor performance, limitations for large-scale production, and surface contamination. Here, we report a smart humidity-control wall-brick manufactured from sepiolite using CaCl2 as an additive. Low-temperature sintering generated a super hygroscopic interior structure, and further silane modification produced bricks with superhydrophobic surfaces. These superhydrophobic surfaces can promote the moisture storage and prevent the CaCl2 solution from leaking even after the surface is wiped 100 times. Meanwhile, the superhydrophobic surfaces make the wall-bricks easy to clean; also, these materials possess antifouling and antifungal properties. The 24 h and saturated moisture adsorption-desorption contents reached 630 and 1700 g·m-2, respectively. Furthermore, a test was performed using model houses in a real environment, which indicates that the wall-bricks can narrow the daily indoor humidity fluctuations by more than 20% in both wet and dry seasons. The white wall-brick can also be dyed with different colors and thus shows promise for applications in interior decorations of houses.

Introduction

In nature, creatures have the capability to construct and exist in a comfortable living environment. For example, ants build a porous nest for temperature and humidity regulation.[1,2] For mankind, indoor comfort has always been a pursuit that has been achieved by controlling the temperature, humidity, air quality, etc.[3−5] Among these comforts, the relative humidity (RH, where the preferable range is 40–70%) of indoor air has attracted increasing concerns because it relates to long-term health.

Humidity-control materials (HCMs) can save energy by regulating the air humidity intelligently. Recently, metal–organic frameworks,[6,7] porous organic polymers,[8,9] hydrogels,[10] and natural inorganic porous materials (NPMs)[11−14] have been developed to fabricate a variety of HCMs. Among these materials, NPMs have aroused widespread interest because they are durable and inexpensive and can be used in large-scale applications in buildings.

However, the ability of NPMs to control humidity is usually limited and incomparable with the above-mentioned synthetic materials. In general, the moisture adsorption–desorption content of NPM-based materials is only approximately 2–10% of their own weight,[13−15] which is much lower than that of the Y-shp-MOF5[9] (30–40%) and Zn–O hydrogels (230%).[10]

We recently demonstrated that CaCl2 could improve the ability of an NPM to control humidity by enhancing the capillary condensation in mesopores.[16] However, excessive CaCl2 leaks when the NPM is exposed to moist environments for long periods restricts our ability to further enhance the moisture adsorption–desorption contents of the HCM. In addition, particulate matter pollution has become a serious problem in some developing countries.[17−19] The particulates may reduce the ability of a material to control humidity and contaminate the HCM surface by blocking pores with floating dust. Because HCMs are porous, determining how to clean HCMs is another key issue of their long-term use.

Antiwetting can prevent liquid permeation; thus, an antiwetting surface is easy to clean, as dirt can be removed by running water droplets.[20−23] Antiwetting surface treatments are a promising technology for solving the problem of CaCl2 leakage and can be used to clean the HCM surfaces; as a result, these HCMs with antiwetting surfaces have excellent ability to control humidity and have a long service life. However, the breathability of this kind of materials was less of a priority. The in situ preparation of the surface texture provides a perfect solution for achieving hydrophobic materials that are both breathable and durable.[24]

Herein, we present the preparation of a wall-brick with superhydrophobic surfaces via a common sintering process and facile fluoride silane decoration. The mesopores and CaCl2 in the brick can automatically absorb or desorb moisture by responding to changes in the environmental humidity, while macropores store moisture in wet seasons and act as diffusion channels in dry seasons. The 24 h moisture adsorption–desorption content and the maximum moisture storage content reach 630 and 1700 g·m–2, representing an increase of 15 and 90% compared to our previous study, respectively.[16] The superhydrophobic surfaces also make the wall-brick an easy-to-clean, antifouling, and antifungal material. The wall-brick possesses an excessive 90% whiteness and can be dyed to be colorful. Thus, this wall-brick has promise for application as a functional interior decoration in houses.

Results and Discussion

Structural Features and Chemical Composition

Figure A shows the scheme used to fabricate the humidity-control wall-brick containing sepiolite, a kind of NPM (Figure S1), where CaCl2 was used as an additive. The hierarchical porous and micro/nanosized rough-surface structures were constructed using a common sintering process. The scanning electron microscopy (SEM) images reveal a hierarchical structure and confirm that adding CaCl2 changes the morphology from flat (Figure B) to rod-shaped sepiolite crystals[25,26] (Figure C,D). Two endothermic peaks are present in the DSC curves (Figure E) and are attributed to two weight-loss events: the event occurring below 200 °C corresponds to the adsorbed water and some crystallized water in CaCl2·6H2O and CaCl2·2H2O, while the event occurring between 500 and 800 °C corresponds to the decomposition of slaked lime and volatilization of CaCl2 (8.23% weight loss; Figure S2, Table S1). The alkali activation and volatilization weaken the sintering effect of the sepiolite fibers, giving rise to the previously mentioned developed rod-shaped nanostructure.[27] After sintering, CaCl2 remained in the brick, as shown in the XRD results. The elemental mappings (Figure F) show that Ca and Cl do not segregate and are instead homogeneously distributed in the interfiber mesopores or on the surface of the sepiolite fibers.

Figure 1

(A) Schematic illustration of the wall-brick fabrication. (B, C) SEM images of the wall-bricks prepared without or with CaCl2, respectively. (B′, C′) Magnified SEM images of (B) and (C). (D) XRD pattern of the wall-brick prepared with CaCl2; note, the XRD pattern was obtained at 150 °C. (E) DSC curves are denoted by solid lines and TG curves are denoted by dash lines. (F) TEM image and the EDS mappings results.

The pore size distributions and pore volume are directly correlated to the capillary condensation and the influence of the moisture adsorption–desorption content of the material.[28−30]Table shows that the mesopore volume and specific surface area decreased by adding CaCl2. Further measurements (Figure S3) show that the number of mesopores with diameters less than 15 nm decreased, which is possibly caused by CaCl2 occupying the mesopores (Figure F). Although the nitrogen-adsorption capacity is weakened, once moisture is absorbed, CaCl2 can deliquesce, forming a CaCl2 solution that exists in the mesopores. Thus, the moisture adsorption–desorption content increases because of the enhanced capillary condensation in the pores and the inherent physisorption capacity of the material.[16,31,32] The porosity and median macropore diameter are 36.6% and 402 nm, respectively.

Table 1

Physical Properties of the Wall-Brick

wall-bricks	9 wt % CaCl2	0 wt % CaCl2
BET (m2 g–1)	19.2	43.1
mesopore volume (cm3 g–1)	0.086	0.117
CA (deg)	155 ± 2	140 ± 2
SA (deg)	8 ± 3	60 ± 5
density (g cm–3)	1.51	1.53
porosity (%)	36.6
median macropore diameter (nm)	402

Superhydrophobic and Antileakage Property

The surface micro/nanostructures on the wall-brick surface were constructed in situ during sintering; thus, the liquid on the surface of the wall-brick may exist in the Cassie–Baxter state. Stable air pockets may exist in the rough interstices between the droplets and the surfaces, leading to a composite liquid–vapor–solid interface. A superhydrophobic surface can be obtained facilely by spraying the surface with hydrolytic PFOTS. In the high-resolution XPS patterns (Figure S4), the peaks attributed to F, −CF2, and −CF3 groups were detected, meaning that the brick surface was successfully coated with PFOTS.[33,34] The CA is 155°, which agrees well with the value calculated using the Cassie–Baxter model, as shown in Figure S5. It is an obvious enhancement compared to the flat surface (without CaCl2; Table ). The sliding angle (SA) decreases as the density decreases because the surface roughness is strongly affected by the density of the wall-brick (Figure S6). Nevertheless, the wall-brick must be mechanically robust for practical applications. A suitable density of wall-brick is approximately 1.5 g·cm–3, and the flexural strength is in excess of 10 MPa. The corresponding sliding angle is less than 10°. The dynamic antiwetting behavior of the wall-brick against water droplets can be further observed in the movies presented in the Supporting Information (Movies S1 and S2).

Indeed, to achieve a long service life, the durability of the hydrophobic layer is a critical issue. Hence, nonwoven fabrics were used to wipe the brick surface 100 times (Figure B). Figure A shows that the water/oil droplets maintained their spherical shape on the surface of the wall-brick, and the surface morphologies observed using SEM showed that they maintained their micro/nanostructures (Figure S7). The water contact angles of the treated surface are somewhat decreased; meanwhile, the sliding angles increased to approximately 65° (Figure B). Thanks to the in situ generated, rough, hierarchical microstructure constructed by sintering, the wall-brick retained its hydrophobicity even after 100 wipes. Regarding the low-surface-tension oil (Figure Aa–c), we observed that the treated wall-brick exhibited much slower impregnation dynamics than the pristine hydrophilic wall-brick. Thus, sufficient interval was provided for cleaning to occur (Figure S8).

Figure 2

(A) Cooking oil and blue-dyed water on the wall-brick surfaces: (a) pristine surface, (b) hydrophobic, and (c) hydrophobic after 100 wipes. The photos shown in (a–c) were taken 2 min after the initial deposition of a 20 μL liquid drop. (B) Evolution of CA (for a 5 μL water droplet) and SA (for a 10 μL water droplet) during wiping. The inset photo shows a 100 g weight wrapped with a nonwoven fabric that is used to wipe the wall-brick surface. (C) Digital images of the high-humidity leakage test used to characterize the superhydrophobic character of the wall-brick. (D) Schematic diagram illustrating high-humidity leakage, where the top row shows a superhydrophobic material (green coating) and the bottom row shows a superhydrophilic material.

Superhydrophobic surfaces allow the CaCl2 solution to be held inside the wall-brick because of the Laplace pressure.[35,36] The CaCl2 solution has a larger surface tension than that of pure water, generating a greater force between the gas–liquid interfaces (Table S2). A leakage test shows that, in contrast to pristine hydrophilic surface (Figure S9), hardly any leakage appeared in the superhydrophobic specimen even after 100 wipes (Figure C). Figure D reveals a schematic diagram depicting the ability of the superhydrophobic surface to prevent the CaCl2 solution from leaking and to promote moisture storage. Moisture can be alternatively adsorbed or desorbed on the wall-brick depending on the changes in the environmental humidity, and the macropores work as accelerated channels for moisture diffusion. When the wall-brick is exposed to moist environments for long periods, the adsorption contents exceed the carrying capacity of the mesopores in the brick. The CaCl2 solution leakage occurred for the pristine hydrophilic specimen. In contrast, the superhydrophobic layer can prevent the excess CaCl2 solution from leaking outside and instead stores this excess CaCl2 solution in the macropores.

Humidity-Control Property

The wall-brick prepared in this study has an excellent ability to control humidity owing to the synergy of the hierarchical porous structure and CaCl2 in the wall-brick. Since PFOTS was graft directly to the superficial rod-shaped crystals, the initial breathability of the wall-brick was maintained. Using 9 wt % CaCl2 as an additive, the 24 h moisture adsorption–desorption content reached 630 g·m–2 at RH of 92–33%, which is almost equal to that of the pristine hydrophilic specimen (Figure A, S9). Figure B shows steady moisture adsorption/desorption cycles, and the wall-brick is shown to maintain its working capacity in a moisture content in the range of 280–320 g·m–2 every 8 h with a contact angle (CA) of ≥150°. When the wall-brick was exposed to abundant moisture (4 days in an environment with RH of 92%), the moisture adsorption–desorption content was nearly saturated and reached 1700 g·m–2 (Figure C), and was maintained at that level even after 9 cycles (Figure S10). This means that the macropores were completely used to store moisture. In contrast, a leakage occurred for the pristine hydrophilic specimen, which is denoted by a dash-dotted circle in Figure C, and only 150 g·m–2 was observed for the specimen prepared without CaCl2.

Figure 3

(A) Twenty-four hour moisture adsorption–desorption contents of the hydrophobic wall-brick before and after 9 saturated adsorption–desorption cycles. (B) Eight hour moisture adsorption–desorption cycles and the corresponding CA values. (C) Saturated moisture adsorption–desorption contents. (D) Photographs of the two model houses with a magnified view of the wall-bricks and humidity sensor probe. (E) Simulation test result. The left of the axis break shows the RH fluctuations of the two model houses, in which the RHs differ by approximately 2%. To the right of the axis break is the RH records. The dotted lines indicate the temperature variations in the two houses.

With a high moisture adsorption–desorption content, the practical humidity-control property of the wall-brick was investigated. For a wall-brick (50 mm × 20 mm × 4.5 mm) in a closed space (1 L), the RH was nearly constant at both low and high temperatures (Figure S11). Furthermore, a simulation test was performed using two model houses (Figure D), which were placed outdoors at the Shanghai University. The wall-bricks (with total area = 0.12 m2) were put into the test house (0.45 m3 volume), and the other house was kept empty and used as a control. The tests were performed from October 14 to November 15, 2018 (wet season) and from February 26 to March 22, 2019 (dry season). In Figure E, the temperature variation of the two model houses was almost the same. Therefore, we believe that the difference in the “indoor humidity” between the two model houses mainly resulted from the ability of the wall-bricks to control humidity. The daily humidity in the test house did not fluctuate as much as it did in the control house, and the daily humidity fluctuation was narrowed by more than 20% RH. The wall-bricks work well in both wet and dry seasons, which is compelling for practical applications in humidity control.

Ease of Cleaning and Antifouling Property

The superhydrophobic surface of the wall-brick prevents the inside CaCl2 solution from leaking outside; meanwhile, the wall-brick surface can resist foreign dirt. Figure A,B shows the wall-bricks treated to perform dirt removal and antifouling tests, where half of the surface was treated with superhydrophobic modification (Movies S3 and S4). The superhydrophobic side can be cleaned by removing dirt (MnO2) with water droplets, while the pristine hydrophilic side is still dirty. The superhydrophobic surface can also protect the wall-brick against potential daily exposure to contaminants including water-based dyes, coffee, soy sauce, and cooking oils.

Figure 4

(A) Dirt removal test, where water was used to clean the brick. (B) A water-based dye, coffee, and soy sauce were used in the antifouling test. (C) Antifungal activity test: the top specimen is the control filter paper, the bottom specimen is the wall-brick prepared without CaCl2, the right specimen is the pristine hydrophilic wall-brick prepared with 9% CaCl2, and the left specimen is the superhydrophobic wall-brick prepared with 9% CaCl2. The magnified images of the specimens on the left, right, and bottom after 14 days are shown in (a), (b), and (c), respectively.

Aspergillus niger, a common fungus present on moist walls, was used to assess the antifungal activity of the wall-brick. Figure C shows photographs of the cultivating fungi. After 14 days at an RH of 85%, the culture medium and the control filter paper were covered with black fungi. Hardly any fungus was observed on the wall-brick prepared with CaCl2 (Figure Ca,b). However, the CaCl2 solution leaked from the pristine hydrophilic wall-brick and led to a sterile area, as depicted by the red dashed line. In contrast, nearly 30% of the surface of the brick prepared without CaCl2 was colonized by Aspergillus niger. The hierarchical structures trap air into the nanoscale interstices of the superhydrophobic surfaces; as a result, only a small interaction area exists between the spore liquid and the substrate,[37,38] which restricts spore growth. In addition, the antifungal activity of the CaCl2 solution[39] can further dehydrate the few spores that contact the substrate; thus, the spores die because of the high concentration (approximately 23 wt %; Table S2) at a 85% RH. The dual effects make the wall-brick a perfect antifungal material.

It is highly desirable that superhydrophobic wall-bricks capable of humidity control be processed into various colors and patterns for interior decoration in houses. Basic white bricks were obtained after sintering, and the reflectance spectrum in the visible-light region was obtained for the white bricks, as shown in Figure A. The whiteness of the wall-brick was in excess of 90%. Correspondingly, Figure B shows the whiteness of the wall-brick. By spraying commercial dyes on the surface before hydrophobic modification, wall-bricks with different colors, such as pink, orange, yellow, green, blue, and purple, can be fabricated (Figure C). Additionally, colorful patterns can be easily drawn on the humidity-control wall-bricks by spraying or using other commercial printing methods (Figure D).

Figure 5

(A) Diffuse reflectance spectroscopy results of the wall-brick in the visible-light region and the whiteness of the wall-brick. (B) Image of a white specimen on a newspaper. (C) Superhydrophobic wall-bricks dyed with different colors. (D) Various colorful patterns printed on wall-bricks using commercial dyes.

Conclusions

We prepared a wall-brick that can achieve indoor humidity control without consuming energy. A hierarchical structure was generated in situ during sintering. CaCl2 was added to the wall-brick material to enhance the ability of the resulting material to control humidity. The daily adsorption–desorption content of the wall-brick reached 450 and 630 g·m–2 at RH of 33–75 and 33–92%, respectively. A superhydrophobic surface prevents the CaCl2 solution from leaking during long-term exposure to high-humidity environments and ensures that the material achieves great humidity control. Meanwhile, these superhydrophobic surfaces endow the wall-bricks with antifouling and antifungal properties and make the wall-bricks easy to clean. The flexural strength is in excess of 10 MPa. Therefore, these wall-bricks are promising for use as functional interior decorations in houses.

Experimental Section

Materials and Chemicals

Sepiolite was obtained from Henan Province (China). 1H,1H,2H,2H-Perfluorooctyltrimethoxysilane (PFOTS) was purchased from Weng Jiang Reagent Co., Ltd., (China). All other chemicals used in this experiment were of analytical grade and used without further purification.

Fabrication of the Wall-Brick

The pristine hydrophilic wall-brick was fabricated by a common procedure reported in the literature.[16] In short, sepiolite was activated by HCl (3 mol·L–1). Subsequently, CaCl2 and 20 wt % low-melting point glass powder (the compositions are shown in Table S1) were added to the activated sepiolite. The mixture was adjusted to a pH of 10 using a NaOH solution (5 mol·L–1) and separated by filtering. The precursor powders were dried and pressed at a pressure of 8 MPa, and then the pressed powders were sintered at 700 °C for 40 min in a common electric-heated furnace. The thickness of the specimen was approximately 4.5 mm. The CaCl2 contents in the specimen were estimated from the XRF data, and are shown in Table S1.

Superhydrophobic Modification

PFOTS, 0.5 g, was dissolved in 20 g of alcohol. Then, 0.1 g of acetic acid (30% aqueous solution) was added to the above-mentioned mixture. After stirring for 30 min at 30 °C, the hydrolytic PFOTS was sprayed directly onto the surface of the specimen using compressed air.

Dyeing of the Wall-Brick

After sintering, the wall-brick was dyed by repeatedly spraying commercial dyes (rose bengal, methyl orange, brilliant yellow, brilliant green, methyl blue, and crystal violet) on the brick surface and maintaining the brick at 80 °C for 3 days.

Moisture Adsorption–Desorption and Simulation Test

The moisture adsorption–desorption properties of the wall-brick were measured by a humidity response apparatus according to Japanese Industrial Standards (JIS A1470-1, 2008). The specimens were covered with aluminum foil so that the moisture could only exit through the top surface. During the test process, the specimen was placed in a homothermal bottle and weighed every 10 min. A hygrothermograph (TH12R-EX, Miao xin Co., Ltd, China) was used to monitor the RH in the bottle. Two model houses with 0.45 m3 of space were used for a simulation test. The variations of the temperature and relative humidity inside the two model houses were recorded every 20 min.

CaCl2 Solution Leakage Test

The wall-brick was put on alkaline litmus papers (Φ = 70 mm) and placed in an airtight vessel. At 25 °C, an environment with RH of 92% was constructed by a saturated KNO3 solution. After 4 days and appearance of a leakage, the CaCl2 solution (pH 5–6) touched the litmus paper, whose color then changed from blue to red.

Antifouling Test

The antifouling test was carried out according to China’s Nation Standard (GB/T 1741-2007). The wall-bricks were placed in a potato-agar culture medium at 28 °C and RH of 85%. Aspergillus niger spore liquid was sprayed on the surface and allowed to cultivate for 14 days.

Characterization of the Samples and Instruments

The crystal structures and compositions were characterized by X-ray diffraction (3kw-D/MAX2500V, Rigaku) using a Cu Kα radiation (λ = 1.5406 Å), where the data were collected at a scanning rate of 4°/min, and X-ray fluorescence spectroscopy (XRF-1800, Shimadzu). TG-DSC measurements were performed using a NETZSCH thermal analyzer, where the samples were heated at 5 °C/min in air. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo ESCALAB 250XI equipped with a monochromatic Al Kα X-ray source. The microstructure and surface morphology were examined by scanning electron microscopy (using an FEI JFM-7500F scanning electron microscope) and an optical profiler (Contour GT-K, Bruker). The surface area and mesopore analyses were performed using a nitrogen gas sorption porosity analyzer (Autosorb-IQ2, Quantachrome). The porosity and macropore size distribution were obtained by mercury intrusion porosimetry (MIP, AutoPore Iv 9510, Micromeritics). The CA and SA were measured with water droplets at room temperature using a JCY-2 instrument (Fangrui, China). The three-point flexural strengths of the sintered samples were measured with a strength testing machine at a loading rate of 2 N/s (Instron-5566, 10 kN).