Transparent and anti-fogging AlPO4-5 films constructed by oblique oriented nano-flake crystals

Fei Tong,Jie Gong,Liang Yu,Ming Li,Lixiong Zhang,*

1 School of Chemistry and Environmental Engineering,Jiangsu University of Technology,Changzhou 213001,China

2 State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemical Engineering,Nanjing Tech University,Nanjing 211816,China

3 Chemical Technology,Lule? University of Technology,SE-971 87 Lule?,Sweden

Keywords:AlPO4-5 thin film Oriented film Transparent Superhydrophilicity Anti-fogging

ABSTRACT In the present work,transparent and anti-fogging AlPO4-5 films were prepared on glass substrates using a novel developed process.The process entails a simple in-situ sol–gel followed by vapor phase transport.The in-situ sol–gel process was implemented by coating the precursor sols for the synthesis of AlPO4-5 on the glass substrates successively using the spin-coating method.The films and powders scribed from the films were characterized by X-Ray diffraction (XRD),Fourier transform infrared spectroscopy (FT-IR),scanning electron microscope (SEM),atomic force microscope (AFM),X-ray photoelectron spectroscopy and transmission electron microscope(TEM).The unique films were composed of oblique oriented nanoflake AlPO4-5 crystals with the thickness of about 20 nm.The formation of nano-flake crystals can be ascribed to the high concentration of the precursors,resulting in the formation of a supersaturation system.The obtained films showed high antifogging performance due to the superhydrophilicity with a water contact angle of lower than 1.0°.The silicone oil contact angle was also low about 8.2°.In addition,heteroatom-substituted AlPO4-5 films showing different colors can be obtained easily by simply adding transition metal ions in the phosphate acid solution during the preparation that can extend the application of the method for different coating demand.

1.Introduction

Functional coatings on substrates with anti-fogging,antifouling and anti-reflective properties have attracted much attention in the last decades[1–5].To achieve the goal,tuning the wetting ability of the coating layer is an effective approach [6].Both superhydrophilic and superhydrophobic coatings can lead to the anti-fogging surfaces.Superhydrophilic surfaces cause the coalescence of water droplets to form a pseudofilm[7],which minimizes the scattering of incident light caused by water droplets,while the superhydrophobic surfaces follow the lotus system [8].However,most of the superhydrophobic surfaces were achieved by interacted with bio-molecules and organic substances,which limited their applications [9,10].Nowadays,superhydrophilic coating are commonly fabricated on substrates by depositing nano-sized metal oxide particles,such as TiO2,ZnO[11,12].However,the dormant of the superhydrophilic pure TiO2or ZnO coatings before exposure to strong ultraviolet(UV)irradiation is an issue.The SiO2-TiO2coating could address the issue mentioned above;however,the multiple-step sol–gel deposition method for the preparation of the high-quality composite coating is complex.

Zeolites are porous crystalline materials with a threedimensional network consisting of tetrahedra linked together at the corners.The corner atom can be Si,P or Aletc.that bonded to four oxygen atoms.Due to the difference of the charge between atoms,the negatively charged framework is obtained when Al substitutes Si or P.The negative framework is compensated by cations,such as Na+and K+.Therefore,zeolites have high hydrophilicity,especially for zeolites with high alumimium content or heteroatom-substituted zeolites.Zeolites have been widely used as water adsorbents due to the hydrophilicity [13].Zeolites also display high thermal and chemical stability compared to many other oxides due to the well-defined three-dimensional oxides framework.Due to the hydrophobicity and stability,zeolite films are promising candidates for anti-fogging coatings.It is worth noting that as anti-fogging coatings,zeolite films must display superhydrophilicity,i.e.with the contact angle less than 5°.MFI suspension with the particles size of about 100 nm has been coated on glass for anti-fogging.However,the stability is probably low,i.e.MFI nanoparticle could drop from the glass due to the weak interaction between each other.Apart from anti-fogging coating,zeolite films are promising materials at new emerging areas such as insulating layers in microprocessors,light-harvesting devices,antireflection coating in display industry,anti-corrosion coatings for metals [13–15].Generally,most of the zeolite films are composed of randomly or partially oriented zeolite crystals supported on substrates with micro size,resulting in opaque coatings,which limited their use in optical fields.Also,the films are probably not superhydrophilic due to the larger crystal size and lower surface activation energy.The way to solve this problem is by preparing zeolite thin films composed of nano-sized crystals.The obtained film will be transparent due to the short pathway from the film surface to the substrate [16,17].In addition,the nano-sized crystals will be metastable structure that can generate high surface activation energy plus with the hydrophilicity of zeolites,the obtained film could display superhydrophilicity.So far,however,transparent and superhydrophilic zeolite coating has been rarely reported.

In the present work,we report the preparation of transparent and superhydrophilic zeolitic AlPO4-5 films constructed by oblique oriented nano-flake crystals on glass slides by anin-situsol–gel combined with a vapor phase transport (VPT) technique.The spin-coating method was employed to coat the alumina sol and phosphate acid solution on glass slides successively and consequently,thein-situsol–gel occurred.Subsequently,AlPO4-5 films composed of nano-flake crystals (thickness<20 nm) were obtained on the glass slides after the VPT crystallization in a triethylamine-water atmosphere.To the best of our knowledge,AlPO4-5 films composed of such small nano-flake crystals have not been reported.The obtained AlPO4-5 films were explored deeply using SEM,TEM,AFMetc.techniques.The hydrophobicity and transparency were determined for the film prepared at different conditions.In addition,heteroatom-substituted AlPO4-5 films showing various colors were also prepared that extend the applicability of the developed method.

2.Experimental

2.1.Materials and methods

Substrate:The glass substrates (Cat.No.7101 25.4×38.0 mm,Sail Brand) were treated in a piranha solution (concentrated H2SO4and 33 % H2O2in a volume ratio of 3:1) at 95–100°C for 1 h to remove organic residues from the surface.Then sonicated in deionized water for 30 min and dried at 50 °C.

Film preparation:A spin coater(WS-400BZ-6NPP,Laurell,USA)was employed for coating the synthesis sols on the glass substrates.The alumina sol (10% and 20% (mass) Al2O3in water,Jiangsu Jiangyin Xiagang Chemical Factory,China) was first spun onto the substrates at 600 r?min-1for 10 s with an acceleration of 136 r?min-2,then increased the speed to 3500 r?min-1with an acceleration of 1360 r?min-2and kept for 40 s.Subsequently,the phosphate acid solution(85%(mass),Sinopharm Chemical Reagent Co.,Ltd.,China),which contains a certain amount of cobalt acetate(99.5%(mass),Sinopharm Chemical Reagent Co.,Ltd.),cupric chloride (99% (mass),ShangHai Xinbao Fine Chemical Factory,China),manganese acetate(99%(mass),Shantou Xilong Chemical Factory,China),or fluorescein (Sinopharm Chemical Reagent Co.,Ltd.)when necessary,was spun onto the alumina sol-coated substrates at 6500 r?min-1for 40 s with an acceleration of 1360 r?min-2.Afterwards,the coated glass slide was placed in a Teflon autoclave for crystallization using a VPT method.The TEA(99%(mass),Sinopharm Chemical Reagent Co.,Ltd.) aqueous solution (0.2 ml TEA and 20 ml H2O) was poured into the bottom of the autoclave in advance.The autoclave was kept in a 180°C oven for 24 h.Finally,the obtained samples were washed with deionized water,dried at 50 °C overnight,and calcined at 550 °C for 3 h when necessary.Table 1 summarizes the obtained films and the corresponding preparation conditions.The samples were denoted as SP-n.Deionized water was all used in the synthesis.All chemicals were used as received without further purification.

2.2.Characterization

X-ray diffraction(XRD)patterns were recorded by a Bruker D8-Advance (Germany) powder diffractometer with Ni-filtered Cu Kα radiation source at 40 kV and 40 mA and a Braun position sensitive detector at a scan rate of 5(°)?min-1and step size of 0.05°.The XRD patterns were indexed according to the standard AlPO4-5 diffraction patterns listed by the International Zeolite Association.The X-ray photoelectron spectroscopy (XPS,PHI550,PerkinElmer,USA) analysis was performed using an AlKα radiation (1486.6 eV),and scans of Al 2p,P 2p,and O 1s were recorded.The binding energies were calibrated using O1s peak value of AlPO4-5 at 532.6 eV.The size and morphology of crystals on the surface of glass were observed with scanning electron microscope(SEM,1530,LEO,Germany and S4800,Hitachi,Japan)and transmission electron microscope(TEM,JEL-200CX,Engelsmann,Germany).Surface roughness of the films was analyzed by atomic force microscopy(AFM,Autoprobe CP-Research,Veeco,USA).The Fourier transform infrared(FT-IR) spectra were obtained on a Nexus 870 FT-IR spectrometer(ThermoFisher,USA) using the powders collected by scraping the surfaces of the films.The powders were prepared to be pellets by mixing with KBr with the mass ration of 1:10 for FTIR measurement.The measurements were performed in the wavenumber range of 400–2000 cm-1.The powders were also used for thermo gravimetry analysis,differential scanning calorimetry (TG-DSC,STA 409,Netzsch,Germany).The samples were heated under the air atmosphere to 800°C with a heating rate of 10°C?min-1.Transmittance of visible light was evaluated by a UV–Vis scanning spectrophotometer (Lambda 950,PerkinElmer,USA) using air as reference.The glass slide was placed perpendicularly to the beam to maintain the same positioning during each measurement.Contact angles were measured by a contact-angle system (DSA100,KRüSS,Germany) using 5 μl water droplets at ambient temperature.

3.Results and Discussion

3.1.Preparation of the AlPO4-5 films

Table 1 lists preparation conditions for AlPO4-5 films prepared in this work.All the samples are examined by XRD and are confirmed to be AlPO4-5,as shown in Fig.1(a).The high diffraction intensity at 2θ=7.43°,12.89° and 14.89° indicated that the films are preferentially (100),(110),and (200) oriented,i.e.a &boriented for samples SP-1 to SP-5.Similar orientation zeolite membranes have been reported for MFI membranes and denoted as oblique orientation [15].All diffraction peaks of AFI type zeolite were displayed in the XRD pattern of sample SP-6 that indicated a randomly oriented film.

Table 1 Preparation conditions and corresponding properties of the AlPO4-5 films

Fig.1(b) shows the Fourier transform infrared spectra of the zeolite powders that were collected by scraping the surfaces of the films.The absorption bands at 1051,741,and 467 cm-1are assigned to P–O,Al–O,and Al–O–P vibrations.The presence of the bands at 562 and 631 cm-1related to the specific lattice vibration of aluminophosphate zeolite framework.All the results confirmed that the prepared sample was aluminophosphate film rather than isostructural with α-cristobalite [16].

Fig.1.XRD patterns (a) and FT-IR spectra (b) of samples SP-1 to SP-6.

Fig.2.TG-DSC curves (a) and XPS spectrum (b) of sample SP-3.

Fig.2(a) shows the TG-DSC curves of the sample SP-3.The first(endothermic) stage of weight loss started from 100°C is associated with the loss of water.The mass loss is about 13%on average which means lots of water in the as-synthesized sample.The mass loss and an endothermic peak at 400°C resulting from the decomposition of triethylamine (TEA) template in AlPO4-5 framework.It has previously been reported that TEA+cations are occluded and stabilized by interacting with the AFI framework by forming Al–O–TEA+bonds[17].The interaction between TEA+cations and aluminum species are strong that starts decomposing at 400°C.This is consistent with the results of our TG analysis.In addition,the water used in the autoclave is excess that not only can keep the saturated vapor pressure in the autoclave at the crystallization conditions but also provide water to saturate the dry gel on the glass substrate that is important for crystallization.The FT-IR and TG-DSC results are in accordance with the reported AlPO4-5 crystals synthesized by conventional hydrothermal synthesis [17].The X-ray photoelectron spectroscopy (XPS) result of sample SP-3 is shown in Fig.2(b).The binding energy of the phosphorus 2p transition at 134.7 eV is the same as that of in AFI structure [18].Also,the binding energy of aluminum 2p transition at 75.2 eV is consistent with those reported for the SAPO-5 and AlPO4-5 [19].Similar TG-DSC curves and XPS spectra (not shown) were also obtained from other samples.

The orientation of the zeolite crystals on the glass substrate can be further confirmed from SEM images of the samples (Fig.3).It can be clearly observed that the zeolite films are composed of thin flakes that oriented aligned on the substrate.Fig.3(a)shows a large number of voids between the crystals that probably resulted from the low density of gel due to the lower concentration of H3PO4solutions.More compact films could be observed with the increase of the H3PO4concentration when 10%(mass)alumina sol used(see Fig.3(a)to(e)).However,the thickness of the nano-flakes is almost the same about 20 nm and the hexagonal size is 1.5 to 2 μm.Tsapatsiset al.reported AlPO4-5 multilayer crystals composed of nanoflakes [20],however,the morphology of the single nano-flake is similar to our results.In addition,the abundant voids among the nano-flakes could generate capillary condensation that can probably enhance superhydrophilicity by quickly absorption and spreading water.

Fig.3.SEM images of the AlPO4-5 films for samples SP-1 (a),SP-2 (b),SP-3(c),SP-4(d),and SP-5(e)prepared with a 10%(mass) Al2O3 alumina sol and 4,6,8,10,and 12%(mass) H3PO4 solutions and the samples SP-6 (f) prepared with a 20% (mass) Al2O3 alumina sol and 8% (mass) H3PO4 solution.

Similar films were also obtained when the mass concentration of alumina sol increased from 10% to 20% (mass)at an H3PO4concentration of 8%(mass)(sample SP-6,Fig.3(f))that indicated high flexibility of the preparation method.Figs.4(a) and (b) show the TEM images of powder scrapped from sample SP-3.The nanoflakes exhibit a nice hexagonal shape with a thickness of about 20 nm.The hexagonal shape is the typical structure of AlPO4-5[21].The electron diffraction of the sample (The inset in Fig.4(a))demonstrates that the nano-flakes are monocrystals with high crystallinity.The cross-section SEM image shows a film thickness of SP-3 about 2 μm (Fig.4(c)) that is close to the twofold size of the nano-flakes(see Fig.3(c)).In addition,the top-view SEM image of SP-6 shows that morec-out-of-plane growth of the nano-flakes.This is consistent with the XRD results that showed stronger diffraction from (002).This probably resulted from the higher Al2O3/P2O5molar ratio since the low molar ratio favors preferential growth along with thea-andb-axis of the crystal[22].In Fig.4(c),no clear boundary can be seen between the zeolite layer and the glass substrate.This result suggest that the zeolite layer attach to the glass tightly,leading to a good stability of the AlPO4-5 film.This phenomenon was contributed to the piranha solution pretreatment.The piranha solution can enrich the substrates with-OH groups[23].It can be inferred from Fig.S1(in Supplementary Material ),the water contact angle of piranha solution treated glass was 23.1°,while the water contact angle of un-treated glass was 67.6°.These results suggest that the hydroxy groups on glass increased.After the pre-treatment,the aluminophosphate gel layer was formed on glass by the spin-coating method,and the hydrogen bond was emerged between the gel and the glass.Then the AlPO4-5 film with good stability was synthesized through the vapor phase transport technique.

3.2.Transparency,anti-fogging,and amphiphilic properties

The AlPO4-5 films for samples SP-1 to SP-6 are transparent as shown in the optical pictures in Fig.S2.Fig.5 shows the transmittance of the glass substrate and films in the spectral range of 350–850 nm.The glass substrate has a transmittance of 92% over the whole spectral range.The transmittance levels were slightly lower after coating AlPO4-5 films on the substrates.The transmittances for samples SP-2 and SP-3 were similar about 86%–88%,however,the transmittances decreased to around 80% for samples SP-1,SP-4,SP-5,and SP-6,probably resulting from the change in microstructure and surface roughness of the films prepared with different H3PO4and Al2O3concentrations[24].The change also can be observed from XRD and SEM results as discussed above.

Fig.4.TEM images (a and b) and SEM image (c) of cross-section of the AlPO4-5 film (sample SP-3).The inset of the image is the electron diffraction of sample SP-3.

Fig.5.Transmittance of bare glass and the AlPO4-5 films on glass substrates.

All the AlPO4-5 films were superhydrophilic as determined by water contact angle (WCA) measurements as shown in Table 2.Fig.6 shows the photographic image of WCA measurement for sample SP-3.The WCA is 0.5° and the time for a water droplet to spread evenly is less than 0.5 s.The time required for water droplets to spread on other samples are listed in Table 2.It is interesting to note that the silicone oil contact angle of SP3 is about 8.2°as shown in Fig.6(b).The results indicated that the prepared AlPO4-5 films are amphiphilic.The high oil affinity can be ascribed to the metastable surfaces of the films because the films are composed of 20 nm crystals planes that resulted from the oblique orientation of the nano-flakes crystals.In contrast,the water and silicone oil contact angles of the piranha-treated bare glass are 22.5° and 73.2°.

Table 2 Hydrophilic,oleophilic and surface roughness property of the AlPO4-5 films

The anti-fogging properties of bare glass substrates and sample SP-3 are shown in Fig.7.Both the samples were cooled at–4°C for 0.5 h followed by exposure to steam from boiling water.It can be seen that the AlPO4-5 film shows excellent anti-fogging capability while the glass substrate is fogged as soon as it was exposed to the steam.The anti-fogging capability of AlPO4-5 film coated glass was attributed to the superhydrophilicity.Meanwhile,the AlPO4-5 film was calcined after crystallization to remove the template TPAOH.Under the circumstance,the unique AlPO4-5 pore structure of was formed.And the pore structure of AlPO4-5 had a short pathway due to its nanometer size.The porous structure of AlPO4-5 could also adsorb water that can further prevent the formation of water droplets on the glass.

Since the surface roughness and void fraction of the films would affect the wettability significantly [25],the surface structures of samples SP-2,SP-3,and SP-4 were examined by AFM.Fig.8 shows the AFM images.The surface roughness is about 49,120,and 215 nm for sample SP-2 and SP-3,respectively.The superhydrophilic surface is much smoother compared to the other reported superhydrophilic zeolite coating [26,27].On a rough sur-face,Wenzel roughness factor [28,29] could describe the relation between surface roughness and contact angle.The apparent Wenzel contact angle,θw,on a rough surface is defined as

Fig.6.Photographs of water (a) and silicone oil (b) contact angle on SP-3 film.

Fig.7.Fogging behavior of AlPO4-5 film SP-3(left)and bare glass substrate(right).Both glasses were exposed to steam after being cooled at–4 °C for 0.5 h.

Fig.8.AFM images of AlPO4-5 films SP-2 (a),SP-3 (b) and SP-4(c).

Fig.9.IR spectra of the AlPO4-5 film (SP-3) at varying crystallization times:(a) 4,(b) 8,(c) 12,(d) 16 and (e) 20 h.

Fig.10.SEM images of varying crystallization times of the AlPO4-5 film (SP-3):(a) 4,(b) 8,(c) 12,(d) 16 and (e) 20 h.

whereris the roughness factor and is defined as the ratio of the actual surface area to the projected surface area and θ0is the equilibrium contact angle on a flat surface.The θ0is 22.5° in our case.Therefore,the roughness factor was about 1.08 for our AlPO4-5 films that also indicates a smooth surface.

3.3.Crystallization kinetics

Fig.9 shows the FTIR spectra of the samples SP-3 at different crystallization times.At the initial 4–12 h,only asymmetric TO4vibration bands at 1115 cm-1that resulted from the sol could be observed from all of the three spectra.At the 16–20 h,there is also a symmetric stretching band at around 748 cm-1.In addition,two bands at around 562 and 634 cm-1are observed which can be attributed to the specific lattice vibration of AlPO4-5.After 16 h of crystallization,the FTIR spectrum for the synthesized sample exhibited typical characteristics of AlPO4-5.

Fig.11.Water contact-angle as a function of crystallization time for AlPO4-5 film(SP-3).

Fig.10 demonstrates the transformation of morphology on the films.Fig.11 shows the WCA of the sample at different crystallization times.The film density increased with the increase of crystallization time.The films obtained after 4 h of steaming contain a layer of gel covered on the glass and few flake shaped crystal aggregates grown between the gel layer and glass (see Fig.10(a)).The nucleation may occur at the interface of the gel layer and the glass.The thick and viscous gel layer can create a supersaturated system,which may promote nucleation and preferentially led to the formation ofa-andb-oriented crystals [30].Since the gel layer has not fully crystallized,the WCA of the film was as high as 52.7° (Fig.11).After steaming for 8 h,more nanoflake crystals with a size of 4–5 μm could be observed (see Fig.10(b))and crystals composed of 3–5 pieces of nano-flakes were generated.Meanwhile,cplanes of some crystals were observed clearly.The morphology is the same as the reported results from Tsapatsiset al.[22].The corresponding WCA was decreased to 42°.

More crystals clusters composed of nano-flake crystals were formed when the crystallization time increased to 12 h (see Fig.10(c)) andcplanes were disappeared probably due to the higher density of the nano-flakes crystals.The WCA of the sample decreased to 15.2° that demonstrated better hydrophilicity.At the crystallization time of 16 h,the glass was covered completely by nano-flakes with the size of 4–8 μm (see Fig.10(d)).The corresponding WCA decreased to 9°.In the end of 20 h,the glass was covered by more compact nano-flake crystals (see Fig.10(e)) and the corresponding WCA of the sample decreased to as low as 1.3°which meant superhydrophilicity.

3.4.Easy-tuned property

To investigate the easy-tuned property of the films,heteroatom-substituted AlPO4-5 films on the glass substrate were also prepared by adding a certain amount of transition metal ions,such as Co2+,Cu2+,Mn3+,in the phosphate acid solution.The content of cobalt acetate,cupric chloride,and manganese acetate was 0.01% (mass) in the phosphate acid solution,respectively.The other synthesis conditions were maintained the same as the above.The resultant heteroatom-substituted AlPO4-5 films were not only constructed by oriented nano-flake crystals,transparent,superhydrophilic (Fig.S3),but also show different colors,depending on the transition metal ions.For example,CoAPO4-5,CuAPO4-5,and MnAPO4-5 films show colors of lavenderblush,light green,and mauve,respectively (see Fig.S4).The results indicated that the applicability of the method and the prepared transparent and superhydrophilic films with different colors can probably be useful in optics.

4.Conclusions

In summary,we have demonstrated that a novelin-situsol–gel combined with a VPT technique enables the synthesis of AlPO4-5 films constructed by oriented nano-flake crystals on glass slides.The resulting films show transparent,superhydrophilic and antifogging properties.The formation of nano-flake crystals can be ascribed to the high concentration of the aluminosilicate precursors,resulting in the formation of a supersaturation system.By simply adding transition metal ions in the phosphate acid solution,heteroatom-substituted AlPO4-5 zeolitic films showing different colors can be obtained.In addition,due to its simplicity,versatility,and controllability,thein-situsol–gel process in combination with spin coating and VPT methods could be an applicable approach for the preparation of various zeolite films with unique hierarchical and functional structures.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We are grateful for financial support from the Key University Science Research Project of Jiangsu Province(16KJA430007),Opening Topic of Key Laboratory of Attapulgite Resources Utilization in Jiangsu Province (HPK201804),Opening Topic of National Local Joint Engineering Research Center for Deep Utilization of Mineral and Salt Resources (SF201804).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.02.004.