In situ fabrication and infrared emissivity properties of oriented LDHs films on Al substrates

Abstract

The fabrication of film materials with controlled structure and desired morphology offers the ability to control the infrared radiation properties. Here we report the controlled fabrication of oriented layered double hydroxides (LDHs) films on Al substrates and examine their infrared radiation (8–14 μm) characteristics. A series of LDHs films with gradients in morphologies were prepared by tuning the hydrothermal temperatures and times. Morphological and structural evolution of the as-prepared LDHs films was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results show that the LDHs films exhibit rough surfaces composed of platelet-like LDHs microcrystals. It is revealed that the longer reaction times are favorable for the formation of regular LDHs sheets. The infrared emissivity properties of the LDHs films are studied, and the results demonstrated that the surface structures of the films strongly influence its infrared radiation properties. The infrared emissivity of LDHs films can be controlled by controlling the hydrothermal time. Because of the controlled morphological and tunable structure of LDHs sheets, the as-fabricated LDHs films possess morphology-dependent infrared emissivity properties, which have promising application in low infrared emissivity materials and thermal control materials.

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1. Introduction

Infrared materials with controllable emissivity have attracted much research interest due to their civil and military applications such as thermal control in industry, camouflaging military equipments, protecting vehicles from infrared detection.^1–4 In the last decade, many research efforts have been devoted to developing organic–inorganic composite materials to decrease infrared emissivity.^5–7 However, poor mechanical properties and insufficient thermal stabilities of these composites have long been the major obstacles for practical applications. The inorganic films have excellent mechanical properties and thermal stabilities, however, the potential use of these inorganic films as infrared materials is hampered by the limited tunability of infrared emissivity values. According to Kirchoff's law, the infrared emissivity values, determined by surface properties (such as microstructures, surface functional groups, hydrogen bonds, etc.) of materials, are equivalent to infrared absorbance and relates infrared reflectivity at any specified temperature and frequency.^8–10 The film materials with smooth surface morphology may have potential applications in the field of low infrared emissivity materials. To meet the compatibility with microwave-absorbing materials, the infrared emissivity of materials should be controlled within a certain range for versatile applications. Hence, it is necessary to find a membrane material with controllable surface structures and morphologies.

LDHs are one of the most useful classes of inorganic layered compounds that have attracted great attention in the last decades due to their potential applications in catalysts,^11,12 separations^13–15 and optics.^9,16 The LDHs based materials may have potential applications in infrared emissivity control due to the tunability of interlayer spacing, metal cations in the host layer and guest anions in the interlayer space. The multilayer LDHs films and collagen/LDHs hybrids have demonstrated as low infrared emissivity materials.^7,17 Utilizing the exfoliated LDHs nanosheets as building blocks, the Ni–In LDHs multilayer ultrathin films have been fabricated by restack of LDHs nanoplatelets on quartz substrate, as reported in our previous work.¹⁶ The dense orderly structure and smooth surface of LDHs films can lead to the decrease the infrared emissivity values from 0.70 to 0.41. Although infrared emissivity of LDHs films can be controlled by number of deposited layers, the mechanical and thermal stability of the biohybrid films cannot meet the requirements of future optical applications. In addition, the fabrication processes of films are somewhat more complex and uncontrollable, involving the preparation, exfoliation and deposition of LDHs, which are time consuming and involves toxic chemicals. Hence, it should be interesting to fabricate LDHs films with controlled morphological and tunable structure by a one-step method, instead of multiple-step methods.

Aluminum metals are known as low infrared emissivity materials due to the high reflectivity and low absorptivity. Aluminum metal sheets are usually used as substrate materials for infrared emissivity testing due to their low infrared radiation properties. This provides opportunities to control infrared radiation properties by tuning of film morphology on aluminum substrates. Several studies suggested that the oriented LDHs films could be fabricated on substrate materials by in situ growth methods.^18–20 Zhang et al.²¹ fabricated an oriented LDHs films with micro-nanometer scale by in situ crystallization on aluminum substrate at constant pH. Lin et al.^22,23 reported the corrosion resistance of LDHs films on Mg alloy using the two-step method. Drewien et al.²⁴ prepared the Li–Al LDHs coating by immersion aluminum alloy panels in LDHs sols. Although several successful synthetic routes exist for LDHs films, the production of LDHs films with controlled structure by a simple method is still challenging. Most of the reported LDHs films on aluminum substrate have focused on the anticorrosion applications. To our knowledge, there has been no report yet of the infrared radiation properties investigation of in situ fabricated LDHs films.

In this work, we take advantage of the infrared radiation properties of aluminum substrate to fabricate the LDHs films with controllable morphology and infrared radiation properties. The high-density in situ-grown LDHs platelets are formed on aluminum sheets such that each stood almost perpendicularly to the substrate surface. The morphologies and structures of the LDHs films are controlled by varying hydrothermal time and temperature during LDHs nanocrystal growth procedure. The crystal structure, chemical composition and infrared emissivity properties of the LDHs films were characterized in detail.

2. Materials and methods

2.1. Materials

Al sheets (>99% purity) with thickness of 0.1 mm was purchased from Sinopharm Chemical Reagent Co., Ltd. After being washed several times with distilled water, the Al sheets were cleaned in absolute ethanol under ultrasonic treatment for 30 min. Other reagents (analytical grade) were used as received from commercial suppliers without any further purification. Hexamethylenetetramine (HMT, C₆H₁₂N₄) was provided by Jiangsu Yonghua Fine Chemical Co., Ltd. zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) was obtained from Sinopharm Chemical Reagent Co., Ltd. All the reagents used in this synthesis were commercially available and double-distilled water was employed in all experiments.

2.2. Fabrication of LDHs films

The fabrication of hierarchically structured LDHs films was performed by a hydrothermal method using HMT as precipitant for crystal growth. In a typical preparation, 0.297 g of Zn(NO₃)₂·6H₂O and 0.280 g of HMT were dissolved in 80 ml of distilled water in a 100 ml autoclave Teflon vessel. Then, 0.540 g of cleaned Al sheets was immersed in the above mixed solution. The vessel was maintained at a temperature of 110 °C for 10 h to obtain the final conversion film. After that, the as-prepared products were taken out of the vessel, washed with distilled water several times, and dried in air at 80 °C. In order to obtain the LDHs films with gradient in morphologies, effects of reaction times (1–10 h) and hydrothermal temperatures (80–170 °C) on the morphologies and structures of the films were investigated in detail.

2.3. Characterization

The general structure of the LDHs films was investigated using an X-ray diffraction measurement on a Bruker-AXS D8 X-ray diffractometer system with Cu Kα radiation at 40 kV and 40 mA. The morphologies and microstructures of prepared samples were characterized by obtaining scanning electron microscope (SEM) images using a Hitachi S-3400N SEM at an acceleration voltage of 20 kV. An Agilent 5500 atomic force microscope (AFM) instrument with a Si tip from Budget Sensors was employed to characterize the surface morphology and roughness of LDHs films. The mean infrared emissivity values of the LDHs films were measured by an IR-2 Double Band Infrared Emissometer at the waveband of 8–14 μm. All values were obtained by averaging over the data measured from six different regions of each coating (the maximum and minimum data were removed before averaging).

3. Results and discussion

3.1. Characterization of LDH films

It has been reported that the LDHs platelets prepared by in situ growth are perpendicular to the substrate materials. Fig. 1 presents the SEM images of the pretreated Al sheets and LDHs films prepared at hydrothermal temperature of 110 °C. As can be seen in Fig. 1A and B, Al sheets, which were used as substrate materials for in situ growth of LDHs films, had smooth surfaces. It should be noted that small amounts of cracks can be found on surfaces of Al sheets, which may arise from the surface polishing. The relative smooth surface of Al sheets would be beneficial to enhance the infrared reflection and reduce the infrared absorption. However, the high reflectivity of films is unfavorable for radar stealth. Fig. 1C clearly reveals the resulting LDHs films exhibit rough surfaces composed of platelet-like LDHs microcrystals, which can induce additional diffuse reflection and change infrared radiation. Hence, the infrared emissivity values of films can be controlled by varying the morphologies of LDHs films. The magnified SEM image of the film surface is displayed in Fig. 1D. It can be seen that LDHs platelets have a thickness of less than 300 nm and a diameter of several of micrometers, with a relative smooth surface.

Fig. 1 SEM images of (A and B) Al sheets and (C and D) LDH films (preparation conditions: hydrothermal temperature, 110 °C and hydrothermal time, 5 h).

Surface roughness was assessed by AFM as part of films characterization and also for further influence the infrared radiation performance of films. Fig. 2 shows the tapping-mode AFM images (10 × 10 μm²) of pretreated Al sheets and LDHs films, which indicates the surfaces of Al sheets changed greatly after in situ growth of LDHs platelets. It is evident from the Fig. 2A that the surface of Al sheets is fairly smooth. Only a small amount of irregular nanoparticles exist on the smooth Al sheets surfaces. As can be seen in Fig. 2B, the surface LDHs films possess very rough surfaces that will enhance infrared absorption and control infrared reflectance. The height profile (Fig. 2C) revealed that the terrace of the Al sheets was rather flat with little wrinkling and an average thickness of around 70 nm, which also agreed well with that detected by SEM. As expected, the height profile of LDHs films that the films had a fairly rough terrace with an average thickness of 480 nm, approaching the sizes of the LDHs platelets growth on the surface of Al₂O₃ fibers as reported in our previous work.

Fig. 2 (A and B) AFM images of Al sheets and LDH films and (C and D) height profile corresponding to the line marked on the image A and B.

3.2. Structure evolution

In the present system, the hydrothermal temperatures not only determine the microstructure of obtaining LDHs films, but also control the composition of materials. HMT was used to gradually raise the pH value of the reaction mixture because the HMT can be decomposed into formaldehyde and ammonia above hydrothermal temperature of 60 °C.¹⁸ The XRD patterns of LDHs films prepared at various hydrothermal temperatures are displayed in Fig. 3A. As can be seen in Fig. 3A, the XRD patterns of films exhibit the coexistence of two crystalline phases: a relatively crystalline LDHs and a highly crystalline Al substrates. It can be seen from Fig. 3A the peak intensities of LDHs are enhanced by increasing hydrothermal temperature from 80 °C to 140 °C, implying that the higher hydrothermal temperatures are favorable for the crystal growth of LDHs. This film obtained at hydrothermal temperature of 140 °C, however, contains a new impurity phase, best assigned as zinc carbonate hydroxide hydrate (Zn₄CO₃(OH)·6H₂O JCPDS 11-0287) in addition to the main phase of LDHs still present. However, the characteristic peaks of LDHs completely disappeared and the impurity phase appear at hydrothermal temperature of 170 °C. It should be noted that the XRD patterns of sample prepared at hydrothermal temperature of 80 °C exhibit two diffraction peaks at the angle (2 theta) of 9.97°and 11.4°, which are possibly associated the d₀₀₃ reflections. Partial oxidation of HMT may introduce CO₃²⁻ into interlayer of LDHs at hydrothermal temperature of 80 °C. Hence, the appearance of two different crystal phases of LDHs may be ascribed to two different interlayer anions (NO₃⁻ and CO₃²⁻). These results are in agreement with the FT-IR analysis (ESI Fig. S1†), which also indicates the changes in the character of the interlayer anions.

Fig. 3 (A) XRD patterns of LDH films prepared at various hydrothermal temperatures (preparation conditions: hydrothermal time, 5 h; hydrothermal temperature, (a) 80 °C, (b) 110 °C, (c) 140 °C and (c) 170 °C). (B) XRD patterns of LDH films prepared at various hydrothermal times (preparation conditions: hydrothermal temperature, 110 °C; hydrothermal time, (a) 0 h, (b) 1 h, (c) 3 h, (d) 5 h and (e) 10 h).

The hydrothermal time may play a critical role in the LDHs nanoplatelets growth and further determines the structure of the LDHs films. The effect of hydrothermal time on the film structures was studied by control the hydrothermal time from 1 h to 10 h at the hydrothermal temperature of 110 °C. As the hydrothermal time increase, the structure transformation of the prepared LDHs films is shown in Fig. 3B. It can be seen from Fig. 3B that the peak intensity of LDHs phase is enhanced with the increasing hydrothermal time and no impurity phase reflections were observed, implying that the structure of the LDHs films can be controlled by controlling the hydrothermal time. The XRD patterns of metal substrates (Fig. 3B(a)) only exhibit the characteristic peaks of Al. The broad diffraction peaks centered at 10° be ascribed to the d₀₀₃ reflection of LDHs (Fig. 3B(b)), indicating the disordered structure of the LDHs films. It should be noted that Fig. 3B(c) shows the characteristic diffraction peaks at the angle (2 theta) of 10.0° and 11.3° are ascribed to the d₀₀₃ reflection for LDHs-NO₃ and LDH-CO₃, respectively. In this case, CO₃²⁻ was produced by slow oxidation of the HMT, resulting in the change of the interlayer anions. Hence, two characteristic diffraction peaks of d₀₀₃ are appeared at hydrothermal time of 3 h and the characteristic diffraction peaks of d₀₀₃ are slightly shifted to higher angle with increasing hydrothermal time. In addition, the increasing intensity of the LDHs reflections with increasing hydrothermal time indicates the formation of ordered structure of LDHs.

3.3. Morphology evolution

To investigate the influencing factors and morphology evolution of as-obtained LDHs films, samples subjected to different hydrothermal temperatures were investigated by SEM. Fig. S2† illustrates the morphology evolution of the LDHs films prepared at various reaction temperatures. Fig. S2A and B† show SEM images of the LDHs films obtained at hydrothermal temperature of 80 °C. As can be seen, the surfaces of films are composed of many ultrathin LDHs platelets with irregular shape. The macropore structures with submicron sizes are formed by the disorderly LDHs nanoplatelets, which can produce interesting surface effects from micro- to nanostructures. The morphology of samples prepared at hydrothermal temperature of 140 °C are exhibited in Fig. S2C and D.† It can be seen in Fig. S2C† that the surfaces of the films are covered by a layer of LDHs sheets, and small amounts of irregular fragments are present above the surfaces of LDHs films. The fragments may be derived from impurity phase of samples confirmed by XRD analysis in Fig. 3A. The magnified SEM image of the LDHs sheets is displayed in Fig. S2D.† It should be noted that the LDHs sheets become more regular and the sheet thickness is increased with the increase in hydrothermal temperatures. The SEM results of samples prepared at a hydrothermal temperature of 170 °C are shown in ESI Fig S3.† Only irregular sheets and particles were found in film surfaces.

The morphological evolution with hydrothermal time clearly illustrated that the LDHs films formed according to a stepwise growth mechanism. Fig. S4† displays the morphology transformation of the samples prepared at various reaction times. Fig. S4A and B† show the morphology of LDHs films prepared with a hydrothermal treatment for 1 h, and indicate that the obtained films exhibited homogeneous morphology. The magnified surfaces of the LDHs films exhibited a dendritic structure with interconnected tendrils branching in various directions. The typical SEM images of the LDHs films prepared with a hydrothermal time of 10 h are shown in Fig. S4C and D.† Compared with the LDHs sheets prepared at shorter hydrothermal time (Fig. 1D and S4B†), the structure of LDHs sheets becomes more ordered and dense packing, the thickness of LDHs sheets was increased, mainly due to the varying the interlayer anions as mentioned in XRD analysis. It should be noted in Fig. S4C† that homogeneous morphology of LDHs films was changed by overlaying LDHs particles onto film surfaces. The thickness of LDHs sheets and interlayer anions are the most influential parameters of LDHs films that determine the surface structure, composition and infrared radiation performance of films.

On the basis of the above analysis, it can be seen that the hydrothermal temperature and time have a prominent influence on the structure and morphology of LDHs films. In the present system, the ultrathin LDHs platelets can be synthesized at lower hydrothermal temperatures. The appropriate hydrothermal temperatures and reaction times might facilitate the formation of ordered microstructure of LDHs sheets and homogeneous morphology of films. However, the higher hydrothermal temperatures can lead to the appearance of impurity phase and the longer reaction times can result in the collapse of the layered structure. Considering the structure and phase purity of LDHs films, the hydrothermal temperature of 110 °C and hydrothermal time of 5 h may be a good selection for LDHs films fabrication.

3.4. Infrared radiation properties

The infrared radiation properties of LDHs films prepared at various hydrothermal temperatures and times are shown in Fig. 4, confirming that the infrared radiation properties of LDHs films can be controlled by controlling LDHs microcrystals growth conditions. It is clearly seen from Fig. 4A that the hydrothermal temperatures of LDHs films have significant effect on infrared radiation. The Al sheets possess very low infrared emissivity due to their high reflectivity and low infrared absorption derived from the smooth surfaces. The slight increase of infrared emissivity observed for samples prepared at hydrothermal temperature of 80 °C may originate from homogeneous morphology of films and ultrathin nature of LDHs platelets, since previous studies have shown that flatness of the flat LDHs films can decrease the performance of infrared radiation. However, the dramatically increased infrared emissivity for the films prepared at the higher hydrothermal temperatures indicates the changed surfaces of LDHs films. From the analysis of the SEM images, the films prepared at hydrothermal temperature of 80 °C exhibit homogeneous morphology composed of many irregular LDHs sheets. Although the homogeneous surfaces of films are beneficial to reduce the infrared emissivity, the hierarchically porous structure of samples may have significant impact on infrared radiation. The dramatically increased infrared emissivity may originate from the increased sizes and thickness of LDHs sheets, as the hydrothermal temperatures increase from 80 °C to 110 °C. It should be noticed that infrared emissivity values for samples prepared at higher hydrothermal temperatures are close to the maximum infrared emissivity value, indicating that the impurity phases are less effective in infrared radiation at the waveband of 8–14 μm. Fig. 4B presents the relationship between hydrothermal times and infrared emissivity of LDHs films. It is clear that the infrared emissivity of the films increased gradually with the increasing of hydrothermal times, which may be attributed to the increase of crystallite size and the thickness of LDHs sheets as the hydrothermal times increase. It can be seen from Fig. 4B that the infrared emissivity values increased from 0.146 to 0.726 as the hydrothermal times increased from 1 h to 10 h, indicating that the infrared emissivity of LDHs films can be controlled by controlling hydrothermal time. In addition, the rough surfaces of LDHs may result the increased infrared emissivity values.

Fig. 4 The relationship of infrared emissivity of the LDH films with hydrothermal temperatures (A) and times (B).

4. Conclusions

In summary, the fabrication of hierarchically structured LDHs films by in situ growth on surface Al substrates was performed in this work, and the application of the resulting LDHs films in infrared emissivity control was demonstrated. The hydrothermally synthesized LDHs films exhibit rough surfaces composed of platelet-like LDHs microcrystals that will enhance infrared absorption and control infrared emissivity. The advantage might stem from the tunability of the structure and morphology of LDHs platelets. The experimental results show that the longer reaction times are favorable for the formation of regular LDHs sheets. However, the impurity phase was observed at higher hydrothermal temperatures. The role of the microstructure of LDHs films for the control of infrared emissivity is investigated. The infrared emissivity value of the films increased gradually with the increasing of hydrothermal times, indicating that the infrared emissivity of LDHs films can be controlled by controlling hydrothermal time. Therefore, this work presents a successful paradigm for in situ fabrication of LDHs films with controlled morphological and tunable structure. It is anticipated that the obtained LDHs films are promising candidates for low infrared emissivity materials and thermal control materials.

Acknowledgements

The authors are grateful for the financial support from the Natural Science Foundation of China (No. 51077013), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (No. BA2011086) and Fundamental Research Funds for the Central Universities (CXZZ13-0091, CXLX13-107 and CXLX12-0107).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15962h

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