(205e) Development of Green Roofing Tiles with the Use of Novel Photocatalytic and Reflective TiO2 Nanoparticles

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 very important to be developed. Cool roofs are capable of enhancing the reflectance properties of a building, thus absorb a minimum amount of heat. This way, the roof is able to remain cool and maintain the building’s internal temperature stable, by reflecting the greatest percentage of the sunlight [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 largely available and low-cost chemical which exhibits high photocatalytic and photoreflective activity, while being non-hazardous to human and environmental health. It is also well-known for its ability to degrade organic pollutants [4]. It occurs as two important polymorphs, rutile and anatase. Both polymorphs demonstrate differentations among their properties, including their photoreflective and photocatalytic activity. Anatase is metastable, exhibiting high photoreactivity and high photoreflectance of the long wave ultraviolet (UVA) and visible light. On the contrary, rutile is more stable and absorbs violet visible light [5].

This research study was conducted towards the research project entitled “Green Tile evelopment-KERAMI” in collaboration with “KEBE-Northern Greece Ceramics”, aiming to develop a novel “green” roof tile with photoreflective and photocatalytic properties.

2. Materials and Methods

The clay roofing tiles were manufactured by the “KEBE-Northern Greece Ceramics”. Three types of TiO₂ materials were purchased from Sigma-Aldrich, including anatase, rutile and a mixture of rutile and anatase nanoparticles. 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.

Νovel TiO₂ nanoparticles were developed along with the addition of polyethylene glycol (PEG). Being an amphiphilic copolymer, PEG is able to self-organize in aqueous solutions and thus encapsulate the hydrophobic nanoparticles of TiO₂. For this reason, deionized water was mixed with 2.5 % w/v of one of each of the three types of TiO₂ and 0.208 mass % of PEG (MW: 600), while inserted into an ultrasonic bath (50 min). For the characterization of the nanoparticles, all prepared samples were dried at 100°C and 290°C. TiO₂ nanoparticles without PEG have also been developed as control samples, following the same procedure. In total, 12 samples with and without PEG have been developed. For the preparation of the final “green” clay roofing tiles, aqueous suspensions of all nanoparticles were sprayed onto the surface of the roof tiles and formed a coating with photocatalytic and photoreflective properties.

XRD analysis was performed in order to determine the phase compositions of the purchased nanoparticles. Measurements were conducted on a Brucker D8 Advance diffractometer operating in the reflection mode with Cu Ka radiation in the 2θ interval of 10-80 ^oC.

The particle size distribution of the nanoparticles was estimated by a laser diffraction technique using a Malvern Instruments, zeta-nano series, Nano ZS. In order to obtain clear results in particle size distribution analysis, the aqueous suspensions of all examined samples underwent ultrasonic action for 10 minutes, prior to the measurements.

Brunauer-Emmet-Teller (BET) method was used for the estimation of the surface area and pore size distribution of the nanoparticles. Experiments were conducted using a Micromeritics TriStar instrument, by low temperature nitrogen sorption. Pore size distribution was defined using the Barret-Joyner-Halenda (BJH) equation.

The photocatalytic activity of the samples was evaluated by monitoring the decomposition of NO gas. 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) 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.

The reflectance of the samples was measured by spectrometer IFS-113V made from Bruker Physik AG, Silberstreifen, 7512 Karlsruhe-Rheinstettent/B.R. Deutschland. During measurements, an integrated sphere was used for the collection of both specular and diffuse reflectance. The spectrum of the studied wavelength of reflectance ranged from 200 to 900 nm.

3. Results and Discussion

The diffraction patterns exhibited strong diffraction peaks corresponding to anatase and rutile phases. The percentage of anatase and rutile phases in the purchased TiO₂ materials was determined by an X-Ray diffraction meter. The quantitative analysis for these four specimens provided anatase and rutile analogues. For samples prepared from TiO₂ anatase, the analogue was approximately 90% for anatase and 10% for rutile. For samples prepared from the mixture of anatase and rutile TiO₂, 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.

According to the results obtained by BET Analysis, different temperatures applied during the samples’ preparation and the potential addition of PEG may change the surface area of the samples. Textural characteristics of the TiO₂ anatase suspensions, dried at 100 ^oC, indicated that TiO₂/PEG nanoparticles exhibit a greater surface area (91.5 m²/g) than the TiO₂ nanoparticles without PEG (77.7 m²/g). TiO₂/PEG and TiO₂ anatase nanoparticles dried at 290 ^oC reported almost equal surface areas (77.1 and 76.5 m²/g, respectively). For the mixture of rutile and anatase TiO₂/PEGsuspensions, dried at 100 ^oC, the surface area was 58.9 m²/g while for the same suspensions dried at 290 m²/g, the respective surface area was 49.9 m²/g. TiO₂ nanoparticles based on the mixture of rutile and anatase polymorphs, with and without the addition of PEG and dried at 290 ^oC, reported again almost equal surface areas (50.0 for TiO₂/PEG and 49.0 m²/g for TiO₂). The same trend was also shown by rutile TiO₂/PEG and TiO₂nanoparticles. More specifically, rutile TiO₂/PEG (36.6 m²/g) nanoparticles exhibited a greater surface area comparing to rutile TiO₂ without PEG (31.0 m²/g), when dried at 100 ^oC. For the TiO₂/PEG and TiO₂ nanoparticles dried at 290 ^oC, the difference among the surface areas was decreased to 3.1 m²/g (33.6 and 30.5 m²/g, respectively).

In all cases, the addition of PEG lead to an increase in the surface area of the samples when dried at 100 ^oC. On the contrary TiO₂/PEG and TiO₂ nanoparticles reported almost the same surface areas when dried at 290 ^oC due to the degradation of PEG.

Reflectance measurements demonstrated great results with the reported total reflectance (UV-Vis, visible light and Infrared) exceeding 70%. Additionally, results obtained from the experimental procedure for the estimation of the photocatalytic activity of the samples, exhibited the great photocatalytic activity of the novel nanoparticles in the decomposition of NO gas.

4. Conclusions

The results reported in this study indicate that the novel TiO₂ nanoparticles, used as a photocatalytic and photoreflective coating onto the surface of roof tiles, exhibit high reflectivity and good photocatalytic activity.

“Green” roof tiles prove to be ideal for the enhancement of environmental protection and reduction of air pollution by decomposing pollutants, while also promoting the decrease of energy consumption, due to reduced use of air conditioning systems, mainting cool temperatures in the interior of buildings occupants and reducing the urban-heat island phenomena.

1. 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.