Environmental performance of nano-structured Ca(OH)2/TiO2 photocatalytic coatings for buildings

This paper describes the environmental performance of a mixed phase coating (photocatalytic nanolime) manufactured from a colloidal nano-structured calcium hydroxide in alcohol (nanolime) combined with titanium dioxide. While nanolime is used as a consolidant in the field of cultural heritage and titanium dioxide is used as a photocatalytic material for self-cleaning coatings and environmental pollution control within the construction industry both materials are often used separately. We report in this paper an approach to combine both materials for the development of a photocatalytic nanolime coating. The photocatalytic effect of titanium dioxide on the carbonation of nanolime and its influence on the degradation processes in polluted environments is explored. A suspension of 25g/l of nanolime in ethanol and 7.4% wt/vol titanium dioxide was applied to specimens of Bath stone. For comparison, additional specimens were treated only with the nanolime. The specimens were exposed to oxides of nitrogen and sulphur under 30% RH for 120 hours. Exposure was carried out under both, UV and daylight. After exposure, the effect of titanium dioxide on the carbonation of nanolime and on the degradation processes was investigated using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS) and X-ray photo electron spectroscopy (XPS). Results were evaluated considering the dissolution processes of the two oxides in water, and modelled using PHREEQC. Results show that the nanolime and the photocatalytic nanolime coatings promote the reaction of SO2. In addition, the photocatalytic effect of the anatase form of titanium dioxide promotes the formation of a surface layer of calcium sulphate and inhibits carbonation of calcium hydroxide. That was attributed to the oxidation of sulphur dioxide to sulphur trioxide by hydroxyl radicals which led to the formation of sulphuric acid.


Introduction
Since the industrial revolution concentrations of the acid forming pollutants sulphur dioxide (SO 2 ) and nitrogen oxides (NO x ) have increased. This has resulted in the accelerated degradation of new and historic buildings in city centres and industrial areas. The introduction of more resistant materials and coatings to reduce these undesirable effects represent a challenge for the modern construction industry.
The processes of chemical decay of calcium carbonate (CaCO 3 ), which is one of the main constituents of limestone, has been the focus of numerous studies. The most aggressive chemical attack is mainly attributed to the oxides of sulphur and nitrogen. When SO 2 dissolves in water sulphurous acid (H 2 SO 3 ) is formed whereas dissolution of SO 3 in water generates sulphuric acid (H 2 SO 4 ). Both acids are strong enough to dissolve CaCO 3 contained in the stones or renders that are used as outer skins of buildings. Equations 1-7 show the proposed mechanism for sulfonation and the role of NO 2 in the oxidation of SO 2 [1], [2]: Sulfonation, however, is not the only process that can lead to chemical degradation of CaCO 3 . This, in fact, can also be degraded by the products formed by the dissolution of NO 2 in water, as shown in equations 8-9 [3], [4]:  (9) In this work nanolime is examined as the calcium carbonate based material. Nanolime is as a colloidal suspension of nano-sized particles of calcium hydroxide (Ca(OH) 2 ) in an alcohol such as ethanol and isopropanol. It is commonly used as a consolidant in the field of cultural heritage [5]. The material acts as a binding agent of degraded surfaces because after application the Ca(OH) 2 reacts with the atmospheric carbon dioxide (CO 2 ) to produce CaCO 3 . In contrast to building lime which is formed by burning a source of calcium carbonate, nanolime is produced by precipitation. Nano-sized Ca(OH) 2 crystals are precipitated following mixing of supersaturated aqueous solutions of calcium chloride (CaCl 2 ) and sodium hydroxide (NaOH) under carefully controlled conditions. Following precipitation, water is substituted with alcohol to extend the long term stability of the nano sized Ca(OH) 2 crystals [6]. Precipitated crystals are plate-like with hexagonal shape and an aspect ratio of approximately 10:1. The average particle size is 150 nm (IBZ-Freiberg n.d.) although crystals can range from tens to several hundred nanometres [7]. The product was developed at the beginning of the new millennium as a consolidant for frescos [8], [9] and as de-acidification treatment for paper and canvas [10]. Later, however, its use extended to stone consolidation [6], [11] and as a nano-coating.
When mixed with other materials such as titanium dioxide (TiO 2 ) the nanolime acts as a binding agent.
Titanium dioxide is the natural oxide of Ti +IV which is a semiconductor with three main crystallographic structures. Anatase is one of the polymorphs and is widely studied for its photocatalytic properties when exposed to UV radiation. Previous researchers have investigated the use of titanium dioxide and its photocatalytic reactions in a wide range of applications such as water treatment [12]- [14], air purification [15]- [18] self-cleaning surfaces [19]- [24] or for the development of new super hydrophilic surfaces [20].
Photocatalysis is a chemical phenomenon observed in certain compounds that are able to absorb UV photons which in turn promote the migration of electrons from the conduction to the valence band, This paper investigates the effect of a TiO 2 on the carbonation and sulphation of nanolime in SO 2 and NO 2 rich environments. Surfaces consisting of nano-particles of anatase mixed with nanolime were applied to specimens of Bath stone: a porous building limestone that has been used for centuries in the South West of the United Kingdom.

Experimental Method
All specimens used in this test were cut from a single piece of newly quarried Bath stone extracted from the Combe Down mines in Bath and North East Somerset, UK. The stone was cut into test specimens 5 x 10 x 10 mm where all surfaces were freshly cut (un-weathered). Each piece was individually cleaned with acetone and distilled water to remove impurities prior to being dried in an oven at 60ºC for 12 hours.
Three different specimens were used in each test: 1) a control specimen made of uncoated sound stone; 2) a specimen coated with nanolime and 3) a specimens coated with a mixture of nanolime and Nanolime suspension was supplied by IBZ-Salzchemie GmbH & Co. KG (Germany) under the trade name CaLoSiL®. A product with a concentration of 25g/l in ethanol was used in all cases. Anatase in the form of Aeroxide® P25 was supplied by Sigma Aldrich (≥99.5% trace metals basis, average particle size 21 nm, surface area from 35m 2 /g to 65m 2 /g). In order to obtain a stable colloidal suspension, 7.4% wt/vol Anatase was dispersed in CaLoSiL® just before the application and then sonicated for about 20 seconds.
The coating was applied by immersing the specimens in the suspensions for approximately 10 seconds. This allowed five sides of each specimen (main face of 10 x 10 mm and four sides of 5 x 10 mm) to be evenly covered. After immersion, the specimens remained in air at room temperature for several seconds to allow evaporation of the ethanol before being placed in the reactor at 25ºC.
Photocatalytic activity was evaluated for the specimens exposed to SO 2 under both UV and daylight radiation. UV radiation was obtained from a 4x4 array of 16 individual GaN UV-LEDs with a wavelength range of 376-387 nm and total intensity of 8 W/m 2 . All other specimens were only exposed to daylight simulating external environmental conditions. The power of the light in the UVA range was 3 mW/m 2 .
All specimens were exposed for 120 hours to atmospheres produced with: 1) artificial air containing 7 ppm SO 2 ; 2) artificial air containing 7 ppm NO 2 ; 3) artificial air containing 3.5 ppm SO 2 and 3.5 ppm NO 2 . The concentration of pollutants was set to 7ppm in all the experiments; previous research showed optimum results after exposing to 5-10ppm pollutants [2], [29] as well RH remained constant at 7ppm exposure and 30% respectively. Table 1 shows details of the experimental conditions.
The cylindrical 3mm thick soda glass reactor with a diameter of 78mm used as a reactor in the tests is shown in Figure 1. The reactor contained two gas inlets, a gas outlet and a lid to facilitate introduction and removal of the specimens. The gas system that allowed production of specific atmospheres was designed in order to be able to introduce all the three gases at the same time. The desired gas compositions for the experiments were produced by introducing the following gases into the cell: being passed into the cell. The other gas inlet of the cell was used to introduce the dry gas from the other two cylinders that were mixed using a three-way valve.

PHREEQC modelling
To evaluate the experimental results, dissolution of SO 2 , NO 2 and CO 2 in water was modelled using PHREEQC [38]. The program is based on equilibrium chemistry of aqueous solutions interacting with several phases, including gases. PHREEQC is routinely used in disciplines such as geochemistry, hydrology, environmental pollution, water treatment and the construction industry. PHREEQC has been used to simulate the ion-ion and the ion-solid interactions in the multi-ionic transport in concrete [30], [31], chemical degradation of concrete [32]- [34], dissolution processes of calciumsilicate-hydrate [35], [36], and phase and morphology evolution of CaCO 3 precipitated by carbonation of hydrated lime [37].
Simulations applied in this work were based on previously reported PHREEQC protocols [38]. Initial conditions consisted of 1 kg of pure water at 25 °C with pH=7 (this was set to be as an adjustable parameter to achieve charge balance). The equilibrium phases included two species: gas and solid. CO 2 , SO 2 and NO 2 were considered as gas with the respective concentrations calculated according to

Chemical and physical characterisation
Micro-morphologies of the Bath stone and coatings used in the tests were evaluated using a JEOL JSM64802V scanning electron microscope (SEM) with a working distance of 10 mm, an accelerating voltage of 15 kV and a spot size of 30 nm. Specimens were coated with a 20 nm thick layer of gold using an Edward Sputter S150B Coater to prevent surface charging. Energy Dispersive X-ray Spectroscopy (EDS) was performed on uncoated specimens using a working distance of 10 mm, accelerating Voltage of 25 kV and a spot size of 51 nm in high vacuum (25 MPa). Due to its penetration depth (1-2 µm), this technique was used to evaluate the degradation processes carried within the bulk of the coating.
The surface layers (within 1-10 nm) were characterized using X-ray Photoelectron Spectroscopy (XPS) using a Thermo Scientific Theta Probe with an AlK α (1486 eV) X-ray source, operated at 100 W (Thermo Fisher Scientific Inc., Waltham, MA.) using a flood gun in an operating vacuum of 10 -8 mbar. The adventitious hydrocarbon C1s peak at 284.8eV was used to correct the spectra for specimen charging. The software CASAXPS 2.3.16 RP 1.6 (Casa Software Ltd., Teingmouth, Devon, UK) was used for the peak fitting. The atomic percentage of each element detected was calculated from the peak areas, assuming a Shirley background and fitted using a Gaussian/Lorentzian ratio and asymmetry factors to give the best fit to the peaks. All the fitted peaks were normalized excluding the area of the adventitious carbon peak.
Wide scan survey spectra were obtained between 0 and 1400eV binding energy (BE) with pass energy of 200 eV and dwell time of 50 ms. In addition, scans of higher resolution were taken at different BE ranges with pass energy of 40 eV and dwell time of 100ms for C1s, N1s, O1s, S2p, Ca2p and Ti2p regions.      Figure 2 shows the surfaces of the control specimen (a), the specimen coated with nanolime (b) and the specimen coated with the nanolime-titania coating (c) after exposure to SO 2 . Figure 2a shows the structure of Bath stone. A distributed set of clusters composed by Oolites makes the stone's structure very porous. Bath stone was previously characterized by mercury intrusion porosimetry (MIP) using a Micrometrics AutoPore III [39]. The mayority of the pores were around 10 and 0.1µm in diameter, and the macro pores were over 100µm and micropores (between oolites) which are between 10 and 1µm in diameter. Figure 2b illustrates the even distribution of nanolime on the surface and within the surface pores of the stone, covering the Oolites and making them invisible. Figure 2c shows Bath stone covered with a photocatalytic mixed phase nanolime coating. The presence of TiO 2 in the photocatalytic nanolime coating is not visible in the SEM images due to the limited magnification.

Energy dispersive X-ray spectroscopy (EDS)
Following exposure to the conditions in Table 1, quantitative analysis of the specimens was carried out using EDS, the results of which are shown in table 5. The data in the table is an average calculated from the results of five individual analyses, each carried out at a randomly selected representative location of the specimen surface. EDS analysis did not detect the presence of nitrogen based molecules in the surface of specimens exposed to 7 ppm of NO 2 . Between 4.4 and 7.12 atomic percentage titanium was identified on the specimens coated with TiO 2 whereas no appreciable amount was detected on the other specimens.
However in comparison to the specimen exposed to 7 ppm SO 2 , results indicated the presence of sulphur, with higher values in the specimens coated with nanolime and photocatalytic nanolime.

X-ray photelectron spectroscopy results
The nature of the surface layer of the specimens after five days exposure was evaluated using XPS analysis, a survey spectra is shown in Figure 3. Binding energies and elemental ratios for carbon, nitrogen, oxygen, sulphur, calcium and titanium, calculated from higher resolution spectra in specific regions, are shown in Table 6.
The adventitious carbon contamination peak at 284.8eV was identified on all the specimens examined and used to calibrate the spectra for surface charging, however an additional peak corresponding to CaCO 3 was also identified. The binding energy of this peak ranged between 288.6 and 289.2eV which is in agreement with studies by Kang [40] and Demri [41] who report CaCO 3 binding energies of 288.6 and 289.2eV. The binding energy for the Ca 2p peak ranged between 346.3 and 347.5eV which is in accordance with previous studies by Ming [42] and Stipp [43] who reported CaCO 3 binding energies of 346.5 and 347.7eV. The specimens exposed to 7 ppm NO 2 diluted in air did not show the presence of sulphur, indicating that no significant amount of sulphur was present in the specimen stone before testing, and that any sulphur detected on the surface of the specimens exposed to SO 2 originated from a surface reaction.
Peaks corresponding to N 1s were identified between binding energies of 399.1 to 400.6eV, related to the presence of a carbon-nitrogen bond within an organic matrix [44]. In the spectral region corresponding to nitrate (N 1s 407.5eV) [45] no signal was detected suggesting that Ca(NO 3 ) 2 was not formed.
Binding energies for Ti 2p peaks varied between 458.1 and 459.0, and for O 1s the observed peaks are within the range of 528.8-529.5eV. These results agreed with studies carried by Dementjev [46] and Bedri [47], which identified Ti 2p binding energies from 458.0 to 459.4ev and for O 1s from 529.4 to 530.6eV. These were attributed to the TiO 2 in the coatings.
The XPS spectra of specimens exposed to 3.5 ppm NO 2 and 3.5 ppm SO 2 also showed S 2p 3/2 peaks at binding energies between 167.8 and 168.3 eV which corresponded to CaSO 3 . Calculated atomic percentages of 0.5, 0.3 and 0.1 for the untreated, nanolime coating, and photocatalytic nanolimes were less than that observed on the specimens exposed to 7 ppm SO 2 . These smaller concentrations may be attributed to the lower concentration of SO 2 .
The XPS spectra from specimens exposed to 7 ppm SO 2 diluted in air at a relative humidity of 33% with daylight cycles all showed peaks corresponding to sulphur. The binding energies between 166.6 and 168.6 eV are in agreement with studies by Siriwardane and Hong [48], [49] and characteristic of calcium sulphite (SO 3

2-
). Compositions of 1.0, 3.2 and 4.6 atomic % for the untreated (stone surface), nanolime coating, and photocatalytic nanolime indicated that the nanolime is more reactive when compared to the stone substrate. This may be attributed to the higher solubility of Ca(OH) 2 compared to CaCO 3 and the higher surface area of the nanolime compared to stone.
The XPS spectra from specimens exposed to 7 ppm SO 2 diluted in air at 33% RH under UV light also showed peaks corresponding to sulphur. The binding energies between 168.4 and 168.6 eV observed for the nanolime and untreated stone are in agreement with studies by Siriwardane and Hong [48], [49] and characteristic of calcium sulphite (SO 3

2-
). The atomic percentages of 2.2 and 4.0 are slightly higher, but still comparable with the equivalent specimens exposed to daylight cycles.
The specimen containing photocatalytic nanolime exposed to UVA had a Ca 2p binding energy of 348.2eV which was higher than values between 346.3 and 347.5 eV which was detected for all the other specimens. This is comparable to a Ca 2p binding energy of 348eV observed by Siriwarde [48] for calcium sulphate. A higher binding energy for S 2p 3/2 of 169.9eV, compared to values ranging between 167.8 and 168.6 eV, is also in agreement with previous research by Moulder et al. [44] who observed a binding energy of 169.4eV for sulphate. The difference in behaviour of the photocatalytic nanolime under UV irradiation is illustrated in Figure 4 which shows the spectra over the S 2p range for the nanolime coated specimen and photocatalytic nanolime. A shift of approximately 2eV is observed between the S 2p peaks of nanolime coating and photocatalytic coating. On nanolime CaSO 3 was observed whereas for the photocatalytic coating CaSO 4 was generated.

Discussion
The combined results given by EDS and XPS reveal a difference in the elemental concentration between the bulk of the coating (1-2 µm) and the surface layer (maximum 10 nm). The concentration of CO 2 in the gas mixture was approximately 55 times higher than that of the NO 2 and SO 2 , however the concentration of sulphur compounds detected on the surface was similar to the CaCO 3 detected.
Simulations using PHREEQ suggest that, because of the very low activity of the NO 2 ion in aqueous solution only the solid phases produced by the interaction of SO 2 with the system should be detected at the end of the experiments.
For the experiment that exposed specimens to 7 ppm of NO 2 , the exposure did not affect the composition within the bulk; it is possible that the gas concentration or the activity of nitrogen ions was too low to produce phases detectable by EDS. In addition, XPS showed a small amount of organic matrix in the outermost layer.
The formation of CaSO 4 on the surface of carbonate containing materials historically used in construction, such as limestone, is widely accepted. However previous research, under atmospheric conditions, by Chin [50] describes the formation of CaSO 4 as a two stage process involving the initial formation of CaSO 3 (IV) which is followed by further oxidation to sulphate (VI) [50]. The rate of these two reactions is dependent on the local water and oxygen concentrations.
Analysis of the S 2p 3/2 binding energies obtained for the XPS spectra from the specimens indicated the presence of SO 3 2-in the form of calcium sulphite. This phase was identified on the uncoated limestone surface in addition to nanolime and photocatalytic nanolime surfaces. The concentration of sulphur on surfaces coated with nanolime and photocatalytic nanolime was three times higher than that on the stone surface due to the presence of more reactive Ca(OH) 2 and the associated higher surface area. EDS indicated a six-fold increase, the higher value compared to XPS, was attributed to the greater penetration depth of the analysis into the specimen surface.
The addition of anatase P25 to the nanolime, increased the surface area and the absorption of SO 2 and NO x onto the outermost layer of material. When the photocatalytic nanolime coating was exposed to

Conclusions
This study provides an insight into the effect of photocatalysis in the carbonation process of nanolime and in its degradation process when exposed to SO 2 .
There was no evidence of nitration under exposure to 7 ppm NO 2 . XPS data indicated the binding energies of the N1s peak were characteristic of N-C bonds in an organic matrix. The concentration of nitrogen deposited was increased when the gas composition consisted of 3.5 ppm of SO 2 and 3.5 ppm of NO 2 .
In the case of the specimens coated with nanolime the results show a preferential reaction of Ca(OH) 2 with SO 2 relative to CO 2 , which is further enhanced in presence of anatase and UV light. Under these conditions the majority of the Ca(OH) 2 reacts with SO 2 to form calcium sulphate, and only a small amount reacted with CO 3 2to form CaCO 3 . Results show the presence of anatase increased the deposition of sulphur and nitrogen when the specimens were exposed to 7 ppm of SO 2 and UV light, and the CaSO 4 generated reduced carbonation. This was attributed to the oxidation of SO 2 to SO 3 by hydroxyl radicals generated on the surface of nano-anatase thus forming sulphuric acid opposed to sulphurous acid. Results from the photocatalytic nanolime formulation suggest they could be used to reduce atmospheric pollutant levels through creation of a sacrificial layer. This would prevent further damage to the limestone façade and could be applied cities where pollution is detrimental to historic buildings.

Acknowledgement
The authors acknowledge research funding from a University of Bath Research Studentship and the Engineering and Physical Sciences Research Council (EPSRC) through project EP/K025597/1.
Thanks are also due to the National EPSRC XPS Users' Service, hosted by nanoLAB at the University of Newcastle, UK, in particular Dr Anders Barlow. The authors are grateful to Professor W. N. Wang for specifying the UV-LED source and to Dr John Mitchels and Ursula Potter for assistance with EDS spectroscopy and SEM imaging.

Appendix
Algorithm 1 -PHREEQC model used to simulate the dissolution of CO 2 , NO 2 and SO 2 and water with and without solid phases. The code shows the model used to simulate the dissolution of 7 ppm of SO 2 and 385 ppm of CO 2 in pure water and in absence of solid phases as initial conditions (hashtags inform the software to ignore the code lines where these are located). For each condition described in