ZrO2 incorporated TiO2 based solar reflective nanocomposite coatings on glass to be used as energy saving building components
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Single layer design ZrO2 incorporated TiO2 based transparent hard reflecting nanocomposite coatings on glass substrates were developed by sol–gel dip-coating technique using zirconium (IV) n-propoxide (ZP) and titanium (IV) isopropoxide (TTIP) followed by heat treatment at 500 °C for 1 h. The above nanocomposite based TiO2–ZrO2 coated glass could be used as component for efficient energy saving building materials with esthetic beauty. The TZ85:15 coated glasses (dimension: 200 × 200 mm2) with coating thickness 90 ± 10 nm having refractive index value of 1.985 ± 0.002 were found to be flawless with good hardness ~ 5H and adhesion properties. GIXRD and Raman analysis of specimens revealed the crystalline nature of the heat-treated coatings, whereas the formation of Ti–O–Zr network was observed by XPS and FTIR. Coated glass showed > 28% of average reflection (within wavelength range of 350–2500 nm) and ~ 31% (within wavelength range of 400–800 nm). It also revealed the golden yellow reflected color under tube-light exhibiting enhanced aesthetic beauty making suitable candidate for heat reflecting component.
KeywordsNanocomposite TiO2–ZrO2 Reflective coating Sol–gel GIXRD
Solar energy gives a lot of benefit to the earth, but its radiation also causes temperature rise inside houses and buildings [1, 2, 3]. In the solar spectrum approx 50% energy is in the UV–visible region and the rest of the total solar energy is in the infrared radiation region which is absorbed by the earth surface. Hence, absorption of this rays is responsible for the heating up of the surfaces [1, 3] leading to rise in temperature and increases the demand of cooling within the conditioned buildings. As a consequence, development of effective energy storage system, especially prevention of loss of stored energy, particularly those are harvested from renewable sources are most important and critical in terms of commercial applications . Several R&D are carried out to arrest the energy loss, caused by several means [5, 6, 7, 8]. Here, the application of reflective coatings on the surface of building components could be one of the preventive measurements. Such component with reflective coating could minimize the temperature on the surface as well as inside the room of the building by enhancing reflectance of near IR radiation.
Thus, reflective coating becomes a key interest of work for the researchers to promote energy efficient technologies along with energy conservation in buildings . In respect to this, materials with high solar reflectance and high thermal emittance are mostly desirable. Transparent heat reflecting (THR) coating shows a wide opportunity in this field, specially when they possess high reflectance at the near infrared (IR) radiation and a high transmittance at the visible region which is a great alternative in solving the purpose [9, 10]. For this purpose different metal/metal oxides like SiO2 , ZrO2 [1, 6, 9], Al2O3 , Cu2O , MgO , Fe , Ni , ZnO [1, 14] are generally used as dopants in TiO2 matrix for high reflecting material to enhance IR radiation for various applications. In addition, thin film of silver, gold and copper used as the metal in dielectric/metal/dielectric structures are studied as a potential material to reduce surface or inside room temperature by enhancing reflectance at the IR radiation solar spectrum for encourage energy saving building applications [8, 9, 10]. But most of these THR coatings are reported multilayer structure mainly by using highly expensive coating deposition technique, as a consequence not cost effective.
Metal oxide, specifically ZrO2 and TiO2 thin films, particularly a combination of TiO2–ZrO2 has been used to develop such reflecting coatings because TiO2 has unique UV-resistant and self-cleaning property while ZrO2 has high mechanical strength with chemical resistant property [2, 15]. Moreover, properties of the composite can be easily tailored by a simple control on the composition of the systems. Beside this, reflective coatings made from TiO2–ZrO2 composites are also widely investigated for various optical applications such as filters, lenses, waveguides, optical adhesives and anti-reflective coatings etc. [16, 17, 18, 19, 20]. So, metal oxide, specifically using TiO2–ZrO2 composite sol for the formation of great quality reflective coatings is quite a practiced topic in this field. But best of our knowledge from literature, the formation of single layer TiO2–ZrO2 composite based coating for the development of heat reflecting in NIR region is not yet reported.
Keeping the above views in mind, main motivation of this work is to investigate the sol–gel deposited (dip coating technique) TiO2–ZrO2 single layer coating on glass substrate for the development of low cost THR windows. In this paper we have described the development of stable TiO2–ZrO2 nanocomposite sol compositions capable of producing highly transparent, protective and reflective hard-coatings (single/one layer) on glass substrates. The detailed synthetic process for the formation of TiO2–ZrO2 nanocomposite coating is discussed as a function of sol processing and thermal heating (500 °C) steps supported by systematic analysis of Fourier-transform infrared spectroscopy (FTIR), Raman, X-ray photoelectron spectroscopy (XPS), optical measurement and Grazing Incidence X-ray Diffraction (GIXRD) studies. The adhesion (coating material to the substrate), hardness, reflection and chemical endurance characteristics of the coatings have also been investigated.
All chemicals were used as received. Titanium (IV) iso-propoxide (TTIP), Zirconium (IV) n-propoxide (70% in 1-propanol) (ZP), were supplied by Sigma-Aldrich. While, acetylacetone (acac),1-propanol and HNO3 (~ 71%) were supplied by MERCK Specialties Pvt. Ltd. Mili-Q (Millipore) water (18.2 MΩ) was used throughout the study.
2.2 Preparation of coating solution (sol)
2.3 Preparation of coatings
Prior to coatings deposition, glass substrates were cleaned with neutral detergent followed by washing with tap water and rinsing with distilled water and ethanol. The coatings were prepared using the dipping technique (Dip-master 200, Chemat Corporation) with withdrawal speed in the range of 4–6 inches/min. The as-prepared films were first dried at 60 °C in an air oven for 1 h followed by heat treated at 500 °C (ramp 2 °C/min) for 1 h. Similar coatings were deposited on silicon wafers (both side polished, intrinsic, IR transparent) as well as single side polished silicon wafers and soda-lime glass substrates for the FTIR studies, RI (refractive index) with thickness measurements, and GIXRD, XPS and Raman analysis, respectively.
2.4 Characterization of the coatings
Cross cut and adhesive tape test following ASTM D 3359: Using a cutting device such as a razor blade, six parallel cuts 1.5 mm ± 0.5 mm apart and approximately 15 to 20 mm in length are made in the coating. Another six parallel cuts 1.5 mm ± 0.5 mm apart are made in the coating perpendicular to the first set. This forms a cross-hatched pattern of squares over which tape is applied, such as Birla 3 M Scotch Magic Tape #810. The tape then is pulled rapidly as close to an angle of 180° as possible, and the percent adhesion is quantified by the amount of coating removed from the squares in the cross-hatched pattern. If no coating material is peeled off from the substrate, it is quantified as ASTM Class 5B (highest standard).
Abrasion test using pencil hardness tester following ASTM D 3363: Pencil hardness of the coated surface was evaluated following ASTM D 3363 specifications using a pencil hardness tester (BYK Gardner instrument). The pencil hardness value is given according to grade of pencil such as 9B–9H. For testing the sample, first pencil is inserted into the machine then it must touch the test surface, and is tighten the lamping screw. Then pencil is moved over the surface about 6–12 mm under a fixed load of 750 g and a fixed angle of 45 degrees. The test is repeated using successive grade pencils where one does not scratch and next one does scratch. The pencil grade for which it does not scratch the sample is the value of hardness.
Boiling salt water test: This boiling in salt solution test evaluates the ability of a hard-coat to adhere to a substrate and the susceptibility of the coating to crazing. A coated glass is subjected to five to ten cycles of thermal shock by submersing the coated substrate for 2 min in a boiling salt water solution which comprises 3.5 L of deionized water, 157.5 g of sodium chloride, and 29.2 g of sodium dihydrogen orthophosphate, followed by submersing the coated glass for 1 min in water at 24 ± 2 °C Coating performance is quantified by whether or not coating layer detachment or complete delamination from the substrate occurs, and by whether or not crazing of the coating occurs.
3 Results and discussion
3.1 Properties of sols and its stability
In this work the different nanocomposite sols were prepared by varying TiO2 and ZrO2 mol ratio. Viscosity of the as prepared sols was in the range of 5–6 cPs. For example, initial viscosity of the TZ85:15 coating sol was of about 6 cPs having pH 3–3.5. Stability of the sol (usable condition for coating deposition) at (RT) (25 ± 2 °C) was in the range 40–50 days and 170–180 days when stored at RT (25 ± 2 °C) and refrigerator (4 ± 1 °C), respectively.
3.2 FTIR spectra of sol and coatings
3.2.1 FTIR study
3.3 Raman and XRD studies
3.4 XPS analysis
3.5 Physiochemical properties of the coatings
Variation of refractive index (RI) values of the heat treated coatings having different titanium (IV) iso-propoxide (TTIP) and zirconium (IV) n-propoxide (ZP) content
RI value of the heat treated coating (measured at 633 nm)
2.192 ± 0.002
1.992 ± 0.002
1.985. ± 0.002
1.965 ± 0.002
1.920 ± 0.002
1.855 ± 0.002
Evaluation of the heat treated TiO2–ZrO2 (TZ85:15) coatings deposited on glass substrates having coating thickness 90 ± 10 nm
Name of the test
Optically clear with a characteristic golden yellow reflection colour hue
90 ± 10 nm (thickness increases with withdrawal speed)
Adhesion (coating material to the substrate)
DIN 53151 or ASTM D 3359
ASTM class 5B (highest standard)
ASTM D 3363
≥ 5H (TZ85:15); hardness decreases with increasing of Ti-content; ~ 2H (TZ100:0) and ≥ 7H (TZ0:100)
Boiling salt water
Chemical endurance test
Coating can resist ~ 8 cycles; with increasing of Ti-content resistance decreases
80 °C/6 h in an air oven
No cracking/crazing of the coating
Kept 40 h in isopropanol
Coating remains unaffected
In this work, ZrO2 incorporated TiO2 based transparent hard nanocomposite coatings (single/one layer) on glass substrates were developed by sol–gel dip-coating technique using ZP and TTIP followed by heat treatment at 500 °C. TZ85:15 coating composition was found to be optimum in terms of mechanical and optical properties. Large area such coating was developed on float glass substrate up to a dimension of 200 × 200 mm2 in view of commercial application. The resultant coatings of 90 ± 10 nm in thickness having refractive index value of 1.985 ± 0.002 were found to be flawless, uniform, and showed good hardness ~ 5H (ASTM D 3363) and adhesion (ASTM class 5B; ASTM D 3359 specification). Formation of Ti–O–Zr network of the heat-treated coatings was confirmed by FTIR and XPS analysis whereas crystalline nature was revealed by GIXRD and Raman. Evaluation of optical properties showed > 28% of average reflection (in the wavelength region of 350–2500 nm) and ~ 31% (in the wavelength region 400-800 nm). In addition, coating also showed the golden yellow reflected color which could enhance aesthetic beauty from reflected color hue of the coating. So, the above nanocomposite based coated glass could be useful as suitable component for heat reflecting window for building materials with aesthetic beauty. Moreover, coating with crystalline anatase phase (titania) could be very used for self cleaning application due to its photocatalytic property.
DST, Govt. of India is thankfully acknowledged for financial support (Sanctioned No. IUSSTF/JCERDC-IBEE/2016-17; dated 19/01/2017).
Compliance with ethical standards
Conflict of interest
The authors declare no competing financial interest.
- 4.Anand Y, Gupta A, Maini A, Gupta A, Sharma A, Khajuria A, Tyagi SK (2014) Comparative thermal analysis of different cool roof materials for minimizing building energy consumption. J Eng Res, Article ID 685640-685648Google Scholar
- 18.Prasad K, Goyal A, Gohil K, Jagyasi I (2018) Highly reflective coatings. Int J Appl Eng Res 13(22):15773–15782Google Scholar
- 26.Juma A, Oja Acik I, Oluwabi AT, Mere A, Mikli V, Danilson M, Krunks M (2016) Zirconium doped TiO2 thin films deposited by chemical spray pyrolysis. Appl Surf Sci 38:7539–7545Google Scholar