Author’s Accepted Manuscript Molybdenum doped TiO 2 nanocomposite coatings: visible light driven photocatalytic self-cleaning of mineral substrates B. Miljević, J.M. van der Bergh, S. Vučetić, D. Lazar, J. Ranogajec www.elsevier.com/locate/ceri PII: DOI: Reference: S0272-8842(17)30531-X http://dx.doi.org/10.1016/j.ceramint.2017.03.149 CERI14918 To appear in: Ceramics International Received date: 23 February 2017 Revised date: 23 March 2017 Accepted date: 23 March 2017 Cite this article as: B. Miljević, J.M. van der Bergh, S. Vučetić, D. Lazar and J. Ranogajec, Molybdenum doped TiO 2 nanocomposite coatings: visible light driven photocatalytic self-cleaning of mineral substrates, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.03.149 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Molybdenum doped TiO2 nanocomposite coatings: visible light driven photocatalytic self-cleaning of mineral substrates B. Miljevića,1 , J.M. van der Bergha, S. Vučetića, D. Lazarb, J. Ranogajeca a University of Novi Sad, Faculty of Technology, Bul. cara Lazara 1, 21000 Novi Sad, Serbia b University of Novi Sad, Faculty of Sciences, Department of Physics, Trg Dositeja Obradovića 4, 21000 Novi Sad, Serbia Abstract The molybdenum doped TiO2 nanocomposite layer double hydroxide (LDH) suspensions, Mo:TiO2-LDHs, were synthesized by a wet impregnation method in order to enhance the pure TiO2 (water suspension) photocatalytic activity and consequently its self-cleaning efficiency under exposure to visible light. The aim was to produce nanocomposites by a simple, energy saving and cost beneficial synthesis. The mass ratio Mo/Ti was systematically varied (0.03, 0.06, 0.09, 0.12). The obtained nanocomposite Mo:TiO2-LDH suspensions were first characterized by UV-Vis spectroscopy (band-gap energies), Zeta-sizer (particle size distribution and stability) and X-ray diffraction (XRD) (structure) and then applied onto the model mineral substrates, brick and stone. The photocatalytic activity of the obtained coating was determined based on the degradation kinetics of the Rhodamine B (RhB) under artificial visible light irradiation (white LED). The obtained results were compared to the ones of the unmodified TiO2-LDH suspension. The obtained results also showed that all prepared  Corresponding author, tel.: +381 21 4853623; fax: +381 21 450413. E-mail address: miljevic@uns.ac.rs (Bojan Miljević) 1 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ nanocomposites have good photocatalytic activity, particularly the suspension Mo:TiO2-LDH with the Mo/Ti 0.03 mass ratio which possesses the best value. In addition, as regards the visible light driven self-cleaning effect, this suspension has proven to be a good protective functional coating for porous mineral substrates (bricks and stones). Keywords: B. Nanocomposites; C. Photocatalytic activity; D. TiO2; E. Functional applications 1. Introduction Recently, the attention of the scientific community dealing with environmental pollution issues has been drawn by self-cleaning coatings and their commercial applications [1]. The researchers are mostly focused on hydrophilic photocatalytic coatings prepared with anatase TiO2 which is the most used and the most efficient representative [1–5]. TiO2 is non-toxic and it has a high chemical stability, wide availability and relatively low cost [2]. Pure anatase TiO2 has a wide band-gap energy of 3.2 eV [6–8] and it needs UV irradiation in order to induce photocatalytic and hydrophilic phenomena and thus, self-cleaning effect. Since UV fraction of the spectrum makes less than 5% of the available solar light intensity, it is of a great importance to produce a visible light driven photocatalyst with an enhanced photocatalytic activity. This would considerably extend the range of TiO2’s possible industrial applications, not just in the field of environmental contaminants degradation but also in the field of solar energy conversion. One of the possible ways for reducing the TiO2’s band-gap energy is to produce a nanocomposite semiconductor photocatalyst by doping of TiO2 with cationic 2 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ transition metals or metal-oxides. A promising element among them is molybdenum, the dopant proposed by Grätzel and Howe [9] and subsequently used in the research of numerous authors [2,3,7,10–22]. It is known that doping a material using metals with a higher oxidation state (as Mo5+ and Mo6+) than that of the parental atom (Ti4+) increases the photocatalytic activity. This is due to improved transfer and separation of photogenerated electrons and holes [13]. Grätzel reported that Mo6+ ion occupies interstitial sites of the TiO2 matrix and behaves as an irreversible electron trap, while Mo5+ acts as a reversible hole trap substitutional in the TiO2 matrix. The Mo6+ was chosen as a dopant not only for its higher oxidation state than the parental Ti4+, but also because it is not toxic [23–25] (especially in the form of a suspension). Additionally it possesses good solubility, high availability and it is reasonably inexpensive. The improvement of the photocatalytic activity seemed to be dependent on the concentration of the doping cations [11]. However, there is a problem of balancing between the improved photocatalytic activity and unwanted enhanced recombination of the charge carriers which might occur due to higher absorption of light [26]. Although this fact has been dealt with in literature, there is a huge discrepancy between different optimal molybdenum contents reported. A group of researchers quotes the small values of the optimal Mo concentration (e.g. 0.5-3%1 [7,9,10,13,17–21,26–28] ) and reports on the limited solubility. Devi et al. reported on even smaller Mo concentrations, below 0.1% [14,15]. On the other hand, there are reports by Kubacka et al. on relatively high Mo concentrations (8-26% [16,22]) with >12% of solubility limit into the anatase structure. What all the methods already used for synthesis of Mo-doped TiO2 nanocomposites have in common, is a necessary energy input for heating or even cooling [19]. Most of 1 Authors refer on molybdenum content non-consistently, in three different ways: atomic percentage, weight percentage and in molar percentage. 3 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ the synthesis methods of the Mo-doped TiO2 reported in the literature used sol-gel procedure with consequent calcination. This paper reports a rather simple and energy saving synthesis of Mo-doped TiO2. It is later directly intercalated into the ZnAl-LDH structure enabling its low cost commercial production and use for the coating of mineral substrates. In order to provide the best possible compatibility with the mineral substrates, the obtained photocatalytically active molybdenum doped TiO2 described in this paper was intercalated in the ZnAl-layered double hydroxide which possesses a structure similar to clay minerals. Beside their unique nano-sheet structure, layered double hydroxides (LDH) possess a very good compositional flexibility and controllable particle size. Moreover, their low cost and easy manufacturing [29] make these materials convenient for various engineering applications. Also, LDH structures themselves have been proven as good redox catalysts [30,31]. In this study the obtained nanocomposite Mo:TiO2-LDHs were applied on two model mineral substrates, bricks and stones, in order to inspect its applicability on different types of materials, natural (stone) and artificial (brick). Moreover, as these two substrate types have different pore size distributions, the compatibility of the obtained Mo:TiO2-LDHs was studied in the correlation with the substrate porosity. The developing of self-cleaning properties of newly synthesized suspensions was monitored by the change of the contact angle [1] with an increase of visible light irradiation exposure time. 2. Experimental procedures 2.1 Materials and synthesis 4 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ A newly synthesized nanomaterial based on LDH (layered double hydroxides) and photocatalytically active TiO2 particles altered with paramolybdate was obtained using a modified low super saturation co-precipitation method [32,33]. The modified TiO2 particles were obtained by dissolving appropriate amounts of ammoniumparamolybdate in 10 mass% commercial TiO2 suspension (Evonik VP Disp. W 2730 X) giving series of solutions with different Mo/Ti mass ratios: 0.03, 0.06, 0.09 and 0.12. These solutions were added simultaneously into Zn and Al salts (4 ml min-1) maintaining a constant pH value (using a mixture of NaOH and (NH4)2CO3) necessary for LDH synthesis. The stabilization of the newly produced nanocomposite (Mo:TiO2-LDH) was achieved by dilution and addition of a polyelectrolyte stabilizer (ammonium-polyacrylate) [34]. The amount of the nanocomposite in diluted suspension was 1 mass%. The synthesis was performed under the atmospheric pressure and at room temperature. The obtained nanocomposites (Mo:TiO2-LDH with different Mo/Ti mass ratios) were firstly applied on the glass samples, and the suspension with the best photocatalytic activity was further used for the application on the chosen brick and stone mineral substrates. The nanocomposite suspensions were applied by spray technique directly onto the glass samples/model substrates in 3 deposited layers with the period of t = 5 min between two applications. The chosen mineral substrates were bulk samples of natural stone and brick samples prepared in laboratory conditions. The used stone samples are known as Ptujska Gora sandstone from Jelovice quarry, in eastern Slovenia. The brick samples were prepared by hand in a traditional way, using non-carbonate raw material mixture. The dried brick models were fired in a laboratory kiln (2 hours at Tmax= 980 °C). 5 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ 2.2 Characterization of nanocomposites and mineral substrates 2.2.1 Particle size and colloid stability The particle size distribution (PSD) and the colloid stability measurements of zetapotential were performed using the Malvern Instruments Zeta-nanoseries, model: Nano ZS under the following conditions: refraction index of the investigated suspensions, n= 2.61, refraction index of the dispersant (demineralized water) n0= 1.33, light absorption = 0.3, suspension pH= 9.5. The PSD and the mean zeta potential results were obtained as the average value after 12 scans had been performed. 2.2.2 Structural and compositional characterization by X-ray diffraction The X-ray diffraction (XRD) measurements of the dried and powdered nanocomposite suspensions and of the powdered mineral substrates were carried out using a Philips PW 1710 instrument, with Cu Kα1,2 radiation, and a step scan mode of 0.02° in the angular range 2θ = (5–60)°. The exposition time at each point was 2 s. Bragg-Brentano parafocusing geometry was used to increase the intensity and angular resolution. The evaluation of the XRD patterns and their Rietveld refinement analysis were performed using the PANalytical “HighScore Plus” software. 2.2.3 UV-Vis spectroscopy measurements The absorbance  of the newly synthesized Mo:TiO2-LDH nanocomposite suspensions was measured by UV-VIS spectrophotometer Evolution 600, Thermo Scientific in the range between 240 nm and 840 nm with the step of 1 nm and speed of 10 nm min-1. Demineralized water was used as reference. 6 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ The colour change of the substrates (non-carbonate brick and sandstone samples) after the application of the chosen nanocomposite suspension (Mo/Ti mass ratios: 0.03) was studied using the same UV-VIS spectrophotometer with the addition of the DRA-EV-600 diffuse reflectance integrating sphere accessory. The evaluation of the coated substrates optical parameters was done using softer “VL COLOR CALC”. 2.2.4 Measurements of photocatalytic activity The photocatalytic behaviour was investigated in liquid phase [35] by the deposition of the Mo:TiO2-LDH suspensions with the Mo/Ti mass ratios: 0, 0.03, 0.06, 0.09 and 0.12 onto the glass samples which were chosen as non-porous substrate models. The suspension with the best value of the photocatalytic activity measurements on the glass samples was further used for the application onto the mineral substrates (non-carbonate brick and sand-stone samples). According to the procedure adjusted to a porous substrate [35], the photocatalytic activity of the coated mineral substrates was monitored using a Rhodamine B (RhB) concentration change [4] under visible light irradiation. The samples were irradiated for (30, 90, 150 and 210) min. and 24 h with a BRILIGHT white LED visible light source (irradiance of 25 W·m-2). White LED source was used in order to assure visible light irradiation without any UV light. The irradiance was measured with Solar Light PMA2100 instrument using probe heads PMA2130 and PMA2110 for visible and UV-A light, respectively. The same UV/VIS spectrophotometer which was used for the colour change and absorbance  measurements, was also used now in order to carry out the monitoring of the RhB concentration change at the major absorption peak at λ= 554 nm. 7 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ 2.2.5 Measurements of roughness and micro-hardness In order to assess the influence of the chosen suspension (Mo:TiO2-LDH with the Mo/Ti mass ratios 0.03) on the surface properties, the measurements of the roughness and micro-hardness of the model substrates with and without coating were performed. The surface roughness of the coated and non-coated mineral substrates was determined with a precision device (Surtronic 25, Taylor Hobson) and the obtained data were calculated applying ISO 4287 standard. The surface roughness was evaluated based on the Ra parameter which represents the average roughness value of the five measurements. The mechanical properties were determined by measuring the Vickers microhardness (HV) with a microhardness tester model HVS 1000A, ZZV Precision Tool Supply. 2.2.6 Substrate porosity measurements Pore size distribution was measured with a mercury porosimeter (AutoPore IV 9500, Micromeritics). Maximal intrusion pressure used was 228 MPa. 2.2.7 Contact angle measurements The Surface Energy Evaluation System, Advex Instruments was used for the measurements of the contact angle between the experimental fluid (glycerol) and the coated surfaces of the investigated mineral substrates. Liquid drops, of about 5 μl in volume, were gently deposited on the substrate surface by using a micro syringe. The measurements of the initial contact angle (ci, after 1 s) were performed at five different 8 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ points for each of the three specimens of the examined substrates. Each droplet deposited onto the surface was measured five times. The contact angle values of the coated and non-coated mineral substrates were monitored after visible light source irradiation (under the same experimental conditions as in the case of photocatalytic activity measurements). 3. Results and discussion 3.1 Colloid stability and particle size distribution Figure 1. shows particle size distribution of the newly synthesized Mo:TiO2-LDH nanocomposite suspensions. The sample with the Mo/Ti ratio of 0.09 has the most uniform particle size diameter. All samples with molybdenum have narrower particle size distributions than the sample without molybdenum (Fig. 1 - black). With the increase of the Mo/Ti ratio (up to 0.09), a decreasing of the mean value of particle size diameter was identified except in the case of the suspension with Mo/Ti ratio 0.12, due to the agglomeration, where an increased value was obtained. 9 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Figure 1. The particle size distribution of Mo:TiO2-LDH nanocomposite suspensions The zeta-potential  was around -45 mV for all synthesized nanocomposite suspensions, both for those with molybdenum as dopant and for the one without molybdenum content which indicates a good colloid stability and a uniform distribution of the particles within the colloid volume. 3.2 Structure and morphology characterization of nanocomposites Figure 2. shows a comparison of the XRD patterns of the Mo:TiO2-LDH powders. The intensity in arbitrary units was shifted up for clarification. Figure 2. The powder X-ray diffraction patterns of the Mo:TiO2-LDHs systems The identified crystal phases within the XRD pattern were the following: anatase-TiO2 (JCPDS No. 21-1272), rutile-TiO2 (JCPDS No. 89-4920), MoO3 (JCPDS No. 65-2421), ZnAl-LDH (JCPDS No. 38-0486). All patterns show clearly unchanged positions of 10 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ anatase (A) and rutile (R). MoO3 peaks are present in the case of the samples with larger Mo content (Mo/Ti 0.09 and Mo/Ti 0.12) while they are absent in the case of the samples with lower Mo content (Mo/Ti 0.03 and Mo/Ti 0.06) and the one without molybdenum (Mo/Ti 0). Thin films and multilayers exhibit thickness oscillations (so-called Kiessig fringes [36]) in XRD spectra, especially in the region of low 2 values, close to the reciprocal space origin and this phenomenon has been made use of in several methods for investigation of layered type of materials [37–39]. Similar phenomenon was observed in the samples with lower molybdenum content (Mo/Ti 0.03 and Mo/Ti 0.06) and without molybdenum (Mo/Ti 0) which have broad features with 2 maxima at 5.81°, 12.4° and 18.99° (figure 2). These features appear periodically at a regular distance of (2) = 6.59 corresponding to the length of d = 1.35 nm in the real space (equation 1). (1) where Q denotes the difference of the scattering vectors Q in the reciprocal space. That depends on the X-ray source wavelength , and on the Bragg’ angle  as:  . (2) The determined value of d (1.35 nm) might be a nanosheet interlayer distance of the Mo:TiO2-ZnAl-LDH systems. A similar value (d= 1.2 nm), determined with a high resolution transmission electron microscopy was obtained for the TiO2-ZnCr-LDH structure [29]. Moreover, a comparable value was also obtained by X-ray reflectivity 11 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ (XRD  -2 at low angles) in the case of a LDH structure with intercalated large molecules in the existing interlayers [33]. Broad features, which are visible for low molybdenum content samples, are absent in the cases of high molybdenum content samples. However, MoO3 pure oxides are formed only in the samples with higher molybdenum content, which might indicate that smaller quantity of molybdenum has gone through the intercalation. In the case of lower Mo content, the pure oxide peaks are absent, but the interlayer thickness oscillations are visible indicating that molybdenum was intercalated between the ZnAl-LDH nanosheets before the oxidation could have occurred. 3.3 Band gap energies Band gap energy Eg of the nanocomposite samples was determined using the Tauc’s plot [40]. The method is based on the fact that the absorption of the light is dependent on the band gap energy of the absorbing material (Kubelka-Munk theory) [41,42]. A relation between the absorbance  and the band gap energy Eg is given by [43]: C( ) (3) with the h being a Planck’s constant,  the frequency (linked to the measured wavelength by  = c/, c is the speed of light in vacuum) and C an energy-independent constant. The factor n depends on the transition type, direct allowed (n=2) or indirect allowed (n=1/2). According to the eq. 3, the band gap energy can be determined from a plot of the modified absorbance vs. the energy hν by extrapolating the linear fit of the straight section to the =0 intercept of the energy coordinate. Figure 3.(a) 12 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ shows the measured absorbance of the obtained nanocomposite Mo:TiO2-LDHs with different molybdenum content, while figure 3.(b) presents a plot of the modified absorption against the energy, according to the eq. 3. There is a reference of the measurements of the pure commercial TiO2 in both figures. The band gap energy Eg was calculated assuming the indirect transition (n=1/2) [8] for the prepared Mo:TiO2-LDH nanocomposites and for pure titania as well. (a) (b) Figure 3. The absorbance vs. wavelength (a) and the modified absorbance plotted against the energy (b) measured for the Mo:TiO2-LDHs and for the pure TiO2. The results presented in the table 1. show that the band gap energy of pure TiO2 is 3.22(6) eV, which is in a good agreement with the results reported in the literature for anatase phase pure TiO2 (3.20 eV) [8]. The results determined for the Mo:TiO2-LDHs show that the band energy decreases with the increase of the molybdenum content and varies between 2.70 eV and 2.24 eV. This corresponds to the light emission wavelengths between 458 nm and 554 nm covering a wide part of the visible fraction of the electromagnetic spectrum. 13 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Table 1. Band gap energies of the pure titania and the synthetized Mo:TiO2-LDHs with the corresponding wavelengths Sample pure TiO2 Mo/Ti 0 Mo/Ti 0.03 Mo/Ti 0.06 Mo/Ti 0.09 Mo/Ti 0.12 Eg (eV) 3.22(6) 2.66(3) 2.70(4) 2.56(2) 2.66(3) 2.24(1)  (nm) 385 466 458 485 466 554 Although TiO2-LDH (without any molybdenum) also exhibits the band gap energy belonging to visible light (2.66 eV, 466 nm), it has to be considered with reservation concerning the photocatalytic activity. This is clearly shown by our results (Fig. 4) in the next section, where it is discussed further. 3.4 Photocatalytic activity The photocatalytic activity Aeff was evaluated based on the efficiency of the RhB degradation at the given absorption peak expressed by the equation as follows: e (4) where C is the RhB concentration after a certain exposition time and C0 is the initial RhB concentration. The results of the photocatalytic activity of the glass samples coated with the suspensions of different Mo/Ti mass ratio are presented in the figure 4. The obtained results confirm that all of the prepared suspensions have satisfying photocatalytic activity (20% and above after 24 hours, Fig. 4). 14 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Figure 4. The photocatalytic activity of Mo:TiO2-LDH coatings on the glass substrates using visible light as a source measured at different time. The Mo:TiO2-LDH suspension with the Mo/Ti mass ratio 0.03 showed the best photocatalytic activity (about 35% after 24h). For the suspensions with the larger molybdenum content (Mo/Ti mass ratios 0.06, 0.0.9 and 0.12) the photocatalytic activity values are lower than for the TiO2-LDH suspension. What is significant here is the fact that not only does the band gap energy decrease with the increase of molybdenum content (Tab. 1), but this also enables more intense electron-hole recombination and thus the suppression of the photocatalytic activity [44]. The mechanism of recombination in heterogeneous photocatalysis has not yet been completely understood [45]. Despite significant improvement of visible light absorption properties, only a small increase of visible light activity has been observed [44,46] in attempts to produce more efficient TiO2 nanowires. The interplay between visible light photo-induced generation of electron-hole pairs and their recombination, in 15 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ our case study, revealed the sample with Mo/Ti mass ratio of 0.03 as the most photocatalytically active one. A possible explanation why Mo:TiO2-LDH 0.03 properties prevailed in comparison to the one of TiO2-LDH could be that the indirect recombination in the system with molybdenum is slower. This is due to the donor level which is formed in the band gap of TiO2 between conduction band and Fermi level [45] by doping with molybdenum. Based on the fact that the glass samples coated with Mo:TiO2-LDH 0.03 possess the highest values of the photocatalytic activity, this suspension was further used for the application on the mineral substrates (brick and stone samples). 3.5 Properties of mineral substrates 3.5.1. Phase composition and porosity of the mineral substrates The XRD patterns of the measured substrates are shown in figure 5. The intensity of the diffraction measured on the stone sample was shifted to the higher values in arbitrary units for better clarity. In the case of stone sample (black line and dots), quartz (Q), calcite (C), dolomite (D), mica (M), feldspar (F), but also hematite (H) and chamosite (Ch) were identified. For the brick sample only quartz, feldspar and hematite were present confirming a typical composition (Fig. 5, red line and dots) for bricks produced from clayey materials free of carbonates. 16 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Figure 5. The powder XRD patterns of the measured mineral substrates, stone (black) and brick (red) with the identified mineral phases The obtained results of the textural analysis (Fig. 6) reveal the stone models as samples with relatively low total porosity, estimated around 13%. This value is 25% in the case of the brick models, which is approximately two times higher. Considering the fact that the brick samples are non-carbonate systems, higher values of the total porosity is the consequence of their production (traditional, handmade). Moreover, the difference in the origin of the two kinds of materials (natural stone and artificial brick) is also reflected on the pore size distribution. In the case of the stone samples, the dominant pore radius is in the range of (1-2) m, while in the case of the brick samples (Fig. 7) the dominant pore radius is in the range of (0.5-1) m. 17 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Pore voulme fraction (%) 25.00 Stone 20.00 15.00 10.00 5.00 0.00 Pore diameter (μm) Figure 6. Pore size distribution of the stone mineral substrate Pore volume fraction (%) 50.00 Brick 40.00 30.00 20.00 10.00 0.00 Pore diameter (μm) Figure 7. Pore size distribution of the brick mineral substrate 3.6 Functional properties of the coated mineral substrates 18 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ In order to obtain the effect of the addition of molybdenum, during the synthesis, on the functional properties of the mineral substrates, the following groups of samples were examined: mineral substrates without coating, mineral substrates coated with the TiO2-LDH (without addition of molybdenum) and the mineral substrates coated with Mo:TiO2-LDH (Mo/Ti mass ratio of 0.03). 3.6.1 Surface properties of mineral substrates The results of the surface characterization performed by measuring the Vickers microhardness values (HV) and the roughness parameters (Ra) before and after the application of the chosen suspensions remain almost the same. This suggested no significant influence of the suspension application on the surface characteristics. Considering the possible industrial application, one of the most important characteristics of the coating applied on mineral substrates is the surface colour change, which should be as low as possible. Calculations of colour differences were performed using CIE (Commission international de l`Eclairage) L*a*b* relative to a white background. The L* parameter corresponds to the degree of lightness and darkness, whereas a* and b* coordinates correspond to red or green (+a* = red, a* = green) and yellow or blue (+b* = yellow, b* = blue), respectively. The spectrophotometer was calibrated with a standard white plate made o “Spectralon” material. Total colour differences before and after the application of the nanocomposite suspensions were calculated as follows: (5) 19 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ where ∆L*, ∆a* and ∆b* are the di erences in the respective values be ore and after the application of coatings [47]. Table 2. The total colour di erence (ΔE) of the coated mineral substrates Coating TiO2-LDH Mo (0.03)TiO2-LDH Stone 0.83 0.98 Brick 0.95 0.94 Sample The values of the total colour difference (Tab. 2) after the application of the photocatalytic suspensions by spray technique, are minor and are identified as very low (∆E*<1). The obtained values of colour difference indicate a low risk of chromatic incompatibility before and after the application of the photocatalytic suspension. Evidently, the existence of high level of compatibility of the newly designed suspension with the used mineral substrates is proved. 3.6.2. Self-cleaning effect of the coated mineral substrates The photocatalytic activity of the coated mineral substrates as the function of visible light irradiation time is given in figure 8. Considering the presented results, good photocatalytic properties of the coated mineral substrates are obtained. In the case of brick mineral substrates the chosen suspension has stronger impact on the photocatalytic activity value compared with that on the stone samples. This could be the consequence of higher porosity which might initiate an increase of the development of the pore network of active sites at the surface of the brick samples (Fig. 6 and Fig. 7). 20 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Figure 8. Photocatalytic activity of the coated mineral substrates as the function of visible light irradiation time (a) (b) Figure 9. Contact angle (θci) of the coated and non-coated mineral substrates: (a) brick and (b) stone as a function of visible light irradiation time 21 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ The results of the contact angle measurements (Fig. 9) indicate that the application of the developed suspension (Mo (0.03)-TiO2-LDH) leads to the decreasing of the contact angle. For the brick samples (Fig. 9 (a)), the hydrophobic surface of non-coated mineral substrate (θci  105°) switched into the hydrophilic surface for the coated surface (θci  87°) after 24h of visible light irradiation (Fig. 10). In the case of the stone sample (Fig. 9 (b)), the non-coated hydrophilic surface (θci  58°) became more hydrophilic (θci  36°) after coating. These results proved an improvement of the photocatalytic properties after the application of the developed suspension. Moreover, with the increased irradiation time (visible light source) in the case of all performed measurements, the contact angle values constantly decreased. These results directly prove that by the application of the newly developed suspension good self-cleaning properties of the coated mineral substrates under the visible light irradiation are obtained. Figure 10. Contact angle (θci) measurements of the non-coated and coated mineral substrates, stone (top) and brick (bottom) after 0h and 24h of visible light irradiation 22 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Conclusion The inorganic-inorganic nanocomposites, based on layered double hydroxides (LDHs) associated with the photocatalytically active molybdenum doped TiO2 were synthesized and characterized. In order to investigate possible applicability, the composite with the best properties considering functionality was applied on mineral substrates: natural stone and artificial non-carbonate brick samples. To the best of the authors’ knowledge, it was the first time that molybdenum as a dopant for TiO2, intercalated in layer double hydroxide, was used with the aim to improve its visible light driven photocatalytic activity and, consequently, to enhance its self-cleaning efficiency. The newly synthesized nanocomposite suspensions (with the Mo/Ti mass ratio 0, 0.03, 0.06, 0.09, 0.12) were characterized as stable nano-suspensions (zeta potential  -45 mV) with the uniform particle size distribution. The powder XRD study revealed that the photocatalytically active Mo:TiO2 is intercalated into ZnAl-LDHs for the lower molybdenum content (Mo/Ti 0, 0.03, 0.06) whereas for the content higher than 9 mass%, a molybdenum oxide is formed. This indicates a possibility that the intercalation is less successful for higher concentrations. The existence of broad XRD features with the systematic periodical appearance indicates that the intercalation of Mo:TiO2 into the LDH structure was successful. In the experiment, the band gap energy decreased with the molybdenum content for all of the prepared nanocomposites, in comparison with the pure TiO2. Moreover, the obtained results of the band gap energies correspond to the light emission wavelengths covering a wide part of the visible fraction of electromagnetic spectrum 23 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ (458 nm-554 nm) giving the basis for the expectation of the visible light driven photocatalytic properties of the prepared nanocomposites. The measurements of the photocatalytic properties of the glass samples coated with the prepared nanocomposites proved good visible light photocatalytic activity (more than 20% after 24h) for all of the suspensions. The same measurements indicated that the nanocomposite with the Mo/Ti 0.03 mass ratio possesses the highest value of the photocatalytic activity and it was further used for the application on the mineral substrates. Good visible light driven photocatalytic activity of the mineral substrates (both, brick and stone) coated with the nanocomposite (Mo/Ti 0.03 mass ratio LDH) was also confirmed. Additionally, the contact angle values of the coated mineral substrates decreased with the irradiation time and the self-cleaning properties of the developed nanocomposite were proved. As regards the aesthetical appearance, the newly developed nanocomposite (Mo/Ti mass ratio 0.03) shows very good transparency properties without significant influence on the surface colour which is a necessary factor for general application. In view of all the presented results it could be concluded that the developed product proved noticeably promising regarding the simplicity of the energetically beneficial production protocol, the aesthetical appearance, visible driven photocatalytic effect and self-cleaning properties. Regarding long-term photocatalytic applications of this product on porous building materials, a further durability assessment is needed. Acknowledgments The authors are grateful for the support of the Ministry of Education, Science and Technological Development of the Republic of Serbia (Project number: III 45008). 24 © 2017. 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