(164r) Development of Energy Efficient, Photocatalytic and Eco-Friendly Roof Tiles

1. Introduction

Given the great significance of a roof in the energy efficiency and heat conditions of a room, solutions for the design of the so-called "cool roof" are systematically developed. A cool roof is defined as a roof covering system of a building that is capable of reflecting as much sunlight as possible and absorbing as little heat as possible. As a result, the roof remains cool and the amount of heat transferred to the building decreases, thus maintaining a stable and cool temperature inside [1].

This can be achieved by adding a thin film coating over the roof tiles. Using this technique, we maximize heat emission and minimize the absorption of solar radiation while also maintaining a steady and cool temperature inside the building. Titanium dioxide (TiO₂) is a chemical known for its photocatalyst advantage, its zero toxicity, and its ability to degrade organic pollutants [4]. Its largely available, low cost and stimulated either directly by ultraviolet radiation or by visible mechanism of photosensitivity. It occurs as two important polymorphs, the stable rutile and metastable anatase. These polymorphs exhibit different properties and consequently different photocatalytic performances. Anatase has high photoreactivity, can reflect long wave ultraviolet (UVA) and visible light. Rutile is a more stable structure than anatase. Rutile nanoparticles absorb violet visible light [5].

This research is being conducted towards the research project entitled “Green Tile development-KERAMI” in collaboration with “KEBE-Northern Greece Ceramics”, aiming to develop a new bioclimatic product which will have photoreflective and photocatalytic properties for use in cool roofs.

2. Materials and Methods

The clay roofing tiles were manufactured by the “KEBE-Northern Greece Ceramics”. Two types of TiO2 materials (Titanium (IV) oxide, anatase nanopowder and Titanium (IV) oxide, mixture of rutile and anatase nanoparticles in colloidal dispersion) were purchased from Sigma-Aldrich. Polyethylene glycol (PEG) (MW: 600) was purchased from Baker, Germany. All other chemicals used in this study were purchased from Sigma-Aldrich (USA) and used without further purification.

Aqueous suspensions of TiO2 nanoparticles were developed along with the addition of PEG, as a surface-active material. The TiO₂ suspensions were prepared by mixing deionized water with 2.5 mass % of the two types of TiO2 and 0.208 mass % of polyethylene glycol (PEG) (MW: 600) in ultrasonic bath (40 min). Samples with and without PEG were dried in an industrial furnace at 100°C and 290°C. A total number of 8 samples was prepared. Subsequently, all samples were sprayed in the form of a photocatalytic active suspension onto the surface of the fired clay roofing tiles.

Characterization techniques were carried out, determination of the phases compositions of the samples were examined by XRD Analysis. XRD measurements of dried and thermally treated samples were performed on Brucker D8 Advance diffractometer operating in the reflection mode with Cu Ka radiation in the 2θ interval of 10-80֯. Diffraction patterns of the powders were compared with the reference in the ICDD database.

The particle size distribution of the suspensions (2.5% TiO2 water suspension and the newly formed photocatalytic active suspension) was measured by a laser diffraction technique using a Malvern Instruments, zeta-nano series, Nano ZS. Water was used to disperse both suspensions in the sample cell. Approximately 0.1 ml of these suspensions was dispersed in 30ml of the dispersant (water). The refractive index (for TiO2 n=2.5) of each solvent was used as a preference index for statistical calculation with the particle sizing program Dispersion Technology Softver (DTS). To examine the variation in particle size distribution due to ultrasonic action, prior to the measurements, the samples were exposed to water ultrasonic action for 10 min.

The surface area and pore size distribution were obtained by low temperature nitrogen sorption using Micromeritics TriStar instrument. All samples were degassed before isotherm measurements. Surface area of the samples was determined using the Brunauer-Emmet-Teller (BET) method, whereas pore size distribution was defined using the Barret-Joyner-Halenda (BJH) equation.

The hydrophilicity of the surfaces of all samples was determined by contact angle measurements, monitoring the change in the glycerine contact angle with UV-irradiation time. All samples were UV-irradiated for 90, 150 and 210 min. The light intensity measured using a commercial UV radiometer at the sample position. The distance between the UV lamp and the samples was 35 cm. Each contact angle was measured at least 10 times in ambient air, with glycerine droplet volume 2 ml, and the average value of the contact angle used as a final result.

The photocatalytic activity of the samples was evaluated by monitoring the decomposition of NO and VOCs. The experimental set-up was developed by the research team and was composed by the reactor, an ultra-violet source, a target gas pollutant (NO and VOCs) supply and flow rate valves. NO gas with synthetic air or VOCs with synthetic air were inserted into the layout, into the chamber where the prepared coated tiles were placed and subsequently irradiated with ultraviolet light. The decrease in concentration of pollutants was recorded at predetermined time intervals.

3. Results and Discussion

The diffraction patterns exhibited strong diffraction peaks corresponding to anatase and rutile phases. The contents of anatase and rutile phases in the powders were determined using X-Ray diffraction meter. The quantitative analysis for these four specimens gave as anatase and rutile analogues. For samples prepared from titanium (IV) oxide, anatase nanopowder, the analogue was approximately 90% for anatase and 10% for rutile. For samples prepared from Titanium (IV) oxide, mixture of rutile and anatase nanoparticles in colloidal dispersion, the analogue was approximately 80% for anatase and 20% for rutile.

Results obtained from particle size distribution analysis exhibited that particle sizes of all samples range within the fraction of 70 to 110 nm, with the sizes of PEGylated nanoparticles exhibiting slightly greater sizes.

Textural characteristics (BET Analysis) of the TiO2 suspensions indicated that temperature and PEG addition can change the surface area. Textural characteristics of the TiO₂ (Titanium (IV) oxide, anatase nanopowder) suspensions (dried at 100Ö¯C and 290Ö¯C) indicated that addition of PEG increased the surface area from 45 m²/g to 79.18 m²/g while for the suspensions without PEG the increase of temperature, from 100Ö¯C to 290Ö¯C, decreased the surface area from 46.72 m²/g to 44.56 m²/g. For the TiO₂ (titanium (IV) oxide, mixture of rutile and anatase nanoparticles in colloidal dispersion) suspensions (dried at 100Ö¯C and 290Ö¯C) addition of PEG decreased the surface area from 36 m²/g to 7.49 m²/g while for the suspensions without PEG the increase of the temperature increased the surface area from 31.09 m2/g to 42.36 m2/g.

Results obtained from hydrophilicity and photocatalytic experimental procedures exhibited high hydrophilicity and good photocatalytic activity in the decomposition of methylene blue.

4. Conclusions

As yet, our results indicate that clay roofing tiles with a TiO₂ photocatalytic coating, exhibit high hydrophilicity and good photocatalytic activity.

Cool and eco-friendly roof tiles prove to be ideal for the enhancement of environmental protection, reduction of air pollution and greenhouse gas emissions, while also promoting the decrease of energy consumption, due to reduced use of air conditioning units, improving thermal comfort for buildings’ occupants and reducing the urban-heat island phenomena.

5. Funding sources

This research study was funded by the Operational Programme Competitiveness, Entrepreneurship and Innovation 2014-2020 (EPAnEK) of the Ministry of Economy and Development, aiming to develop a new bioclimatic product which will have photoreflective and photocatalytic properties for use in cool roofs.

References

[1] (Yun, Cho, & Cho, 2018) Yun, Y., Cho, D., & Cho, K. (2018). Performance evaluation on cool roofs for green remodeling. Paper presented at the AIP Conference Proceedings.

[2] U.S. Department of Energy, Guidelines for selecting cool roofs. 2010.

[3] Yang, J., Bou-Zeid, E., 2019. Scale dependence of the benefits and efficiency of green and cool roofs. Landscape and Urban Planning. 185, 127-140.

[4] Bahnemann, D., 1999. Environmental Photochemistry, The Handbook of Environmental Chemistry. Springer.

[5] Simons, P. Y., Dachille, F., 1967. The structure of TiO2II, a high-pressure phase of TiO2. Acta Crystallographica. 23, 334-336.