The aim of this investigation was the synthesis of nanocomposite coatings based on Zn-Al layered double hydroxides (Zn-Al LDH) and TiO2. The Zn-Al LDH material, which acted as the catalyst support of the active TiO2 component (in the content of 3 and 10 wt. %), was synthesized by a low super saturation co-precipitation method. The interaction between the Zn-Al LDH and the active TiO2 component was accomplished by using vacuum evaporation prior to the mechanical activation and only by mechanical activation. The final suspension based on Zn-Al LDH and 10wt. % TiO2, impregnated only by mechanical activation, showed the optimal characteristics from the aspect of particle size distribution and XRD analysis. These properties had a positive effect on the functional properties of the coatings (photocatalytic activity and self-cleaning efficiency) after the water rinsing procedure.
Titanium-dioxide (TiO2) has been extensively studied recently in the field of environmental protection due to its high photocatalytic efficiency in the processes of water and air purification (
In spite of the fact that TiO2 is currently regarded as a nontoxic material, the possibility of TiO2 to be a biohazard has still remained open (
Layer double hydroxides (LDH), also known as hydrotalcite, – present the largest group of materials which can be synthetically produced by using different methods such as sol gel, hydrothermal precipitation (
The Zn/Al LDH materials have been receiving increasingly more attention as the supports of the TiO2 particles forming LDH/TiO2 nanocomposites. The TiO2 nanoparticles associated with the LDH substrate retain their photoactivity and do not endanger either the environment or human health. Moreover, the TiO2 nanoparticles are embedded in the structure of a porous LDH avoiding the formation of macroscopic aggregates that may lead to the diminution of their efficiency. Furthermore, LDHs are stable supports that protect the TiO2 particles from washing or erosion; they are non-toxic and present inexpensive materials (
The main purpose of the present study was the synthesis and characterization of TiO2/ZnAl-LDH active nanocomposites and suspensions. For that purpose, first LDH was synthesized and then the small sized TiO2 particles were embedded into the structure of the LDH by using two different methods of impregnation: vacuum evaporation impregnation prior to mechanical activation and impregnation only by mechanical activation. The synthetized coatings were characterized from physical, mineralogical and morphological aspect. The decomposition efficiency of Rhodamine B was chosen as the test reaction for the investigation of photocatalytic activity of the synthesized powders and suspensions. Also, the functional properties (photoactivity and self-cleaning efficiency expressed through the contact angle measurements) were determined before and after the water rinsing procedure in order the durability of the nanocomposites to be characterized for.
The Zn-Al LDHs were synthesized by a low super saturation co-precipitation method at constant pH value (9–9.5). The synthesis was realized at constant temperature (40oC) by intensive stirring. The chosen precursors Zn(NO3)2·6H2O and Al(NO3)3·9H2O with molar ratio of 0.7 M and 0.3 M, respectively, were continuously added (4 cm3/min). For pH regulation, alkali solution of the Na2CO3 and NaOH with a concentration of 0.67 M and 2.25 M, respectively, were employed. The obtained precipitate was aged at 40oC/24h by constant stirring at 200 rpm, washed with demineralized water until pH 7, and then dried at 100oC/24h prior to the calcination at 500oC/24 (
The active ZnAl-LDH/TiO2 nanocomposite powder was obtained by the impregnation of TiO2 suspension (in contents of 3 and 10 wt.%) into Zn-Al-LDH calcined powder. The commercial TiO2 suspension (80 wt.% anatase and 20 wt.% rutile; grain size < 100 nm; content of dry matter 30.0 ± 1.0 wt. % and pH 7), used for this purpose, was obtained from Degussa company, Germany. The process of impregnation was carried out in an alkali solution (pH= 9–9.5) by vacuum evaporation, prior to the mechanical activation (the first protocol), and only by mechanical activation (the second protocol). The pH value during impregnation was maintained with the same basic solutions (0.67 M Na2CO3 and 2.25 M NaOH) as in the previous process of Zn-Al-LDH synthetizing.
The impregnation by vacuum evaporation was realized in a rotary vacuum evaporator (DEVAROT) at 55oC for 30 min. Further on, the obtained powder was mechanically activated in a planetary mill during 180 min using the speed of 200 rpm, while the material and ball ratio was 1:5. The process of impregnation performed only by mechanical activation was realized in an attritor mill for 90 min with the speed of 1500 rpm, in air atmosphere (material: ball ratio was 1: 5).
According to the way of impregnation and the TiO2 content, the obtained ZnAl-LDH/TiO2 nanocomposite powders were assigned as: A1 and A2 representing ZnAl-LDH/TiO2 nanocomposite powders with 3 and 10wt.% TiO2 content, respectively, impregnated by vacuum evaporation prior to mechanical activation and B1 and B2 representing ZnAl-LDH/TiO2 nanocomposite powders with 3 and 10wt.% TiO2 content, respectively, impregnated only by mechanical activation.
The nanocomposite suspension was formed by suspending ZnAl-LDH/TiO2 powder (in a quantity of 0.1 and 0.5g) into demineralized water (100 ml) by using di-ammonia-hydrogen citrate as a dispersing agent and by a stirring procedure at 300 rpm, for 1hour. Ultrasonic bath was used for 30 min in order to prevent possible agglomeration.
The obtained nanocomposite suspension was applied onto the surface of clay roofing tile samples by using spray technique in the following conditions: spraying pressure was 6.5 MPa, distance of the spray device from the sample was 90 cm, angle of spraying was 45o, and diameter of nozzle was 1.3 mm. The coated roofing tile samples were afterwards dried at RT/24h.
The particle size distribution of the synthetized ZnAl-LDH/TiO2 nanocomposite powder was determined by Malvern Instruments, zeta-nanoseries, NanoZS under the following conditions: refraction index of the investigated suspensions (n = 1.55), light absorption (a = 0.3) and pH value = 9.
The X-ray diffraction (XRD) was performed by Philips PW 1710 device in order to determine the phase composition of the designed nanocomposite powders. The employed conditions for this investigation were: monochromatic CuKα radiation with λ = 1.5418 Ȧ in the 5–55o of 2o range, scan rate 0.02o, 0.5 s per step.
The photocatalytic activity of the obtained nanocomposite powders was determined spectrophotometrically by monitoring the photocatalytic degradation of Rhodamine B (RhB) used as a model pollutant in the photocatalytic tests. The photocatalytic tests were performed on the newly synthetized ZnAl-LDH/TiO2 powders and ZnAl-LDH/TiO2 suspensions. The suspension was sprayed onto the samples (4x4x1 cm) of clay roofing tiles (total porosity: 32.93% roughness evaluated by Ra: cca 3μm). The powder and the coated clay roofing tile samples were UV/VIS irradiated (EVERSUN lamp, intensity of UV-A and Visible light spectra were 0.8 mWcm-2 and 0.3 Wcm-2, respectively). The distance between the UV/VIS lamp and the samples was 35 cm (
The photocatalytic activity tests of the powders were performed in a Pyrex flasks patch-type reactor (
Where:
C0 is the RhB concentration of the sample in the dark at the defined time
C is the RhB concentration of sample under the UV light irradiation at defined time.
In order to enable direct contact between the active surface of the clay tile sample and the dye/RhB solution, a glass tube (d = 3 cm, h = 6 cm) was fixed with silicon on the surface of each tile sample and filled with 12 ml of the dye/RhB solution. Considering that the porosity and roughness of the substrate (clay roofing tiles) is an important parameter that may control photocatalytic activity, a pre-absorption test of RhB solution was also carried out in order to saturate samples before the photocatalyitic test. It was possible to change the color of RhB solution during the examination of photocatalytic activity even without any photocatalytic coating, due to the occurrence of RhB absorption by the substrate. Namely, the tile samples were submerged (24h) in a glass tank filed with the RhB solution with the same concentration (10 ppm dm-3). During the pre-absorption test, the solution of RhB in the glass tubes and in the tank was constantly replaced with a fresh solution until the tiles were saturated and the difference in color concentration between the two consecutive measurements was within the limit of 5%. After the pre-absorption period, the RhB solution was replaced with a new one freshly prepared. At defined time intervals of UV/VIS irradiation and correspondingly the samples kept in the dark, aliquots of glass tube solution were taken for spectrophotometric concentration testing.
The photocatalytic activity of the coatings was estimated by degradation efficiency of the RhB solution by using the same formula as for the powder samples (
The contact angle between the appropriate experimental fluid (glycerol) and the surfaces of the coated samples, before and after water rinsing procedure, was measured by using Surface Energy Evaluation System, Advex Instruments, (Brno, Czech Republic). The liquid droplets of about 5 μl in volume were gently deposited on the coated substrate using a micro syringe. All measurements of the initial contact angle (θci, after 1 s) at room temperature were performed at five different points for each of three specimens of the investigated samples. Each droplet deposited onto surface was measured five times.
Water rinsing durability test was performed in order to define the stability of the coating in the severe conditions (rain rinsing procedure) on the porous clay tile. The rinsing procedure was simulated in laboratory conditions by using the equipment which provides constant tap water flow (0.04 l/s) through a pipe system (nozzle diameter of 0.90) and water fall (height of 50 cm) on the sample (set at an angle of 45°). The duration of the test was 30 min.
The functional properties (photocatalytic activity and self-cleaning efficiency) of the coated tiles were measured before and after the rinsing procedure during UV/VIS irradiation. The RhB (10ppm) was used as the model pollutant for photocatalytic activity assessment while the contact angle measurements for the estimation of self-cleaning properties.
Particle size distribution of the
Particle size distribution of the ZnAl-LDH/TiO2 powders impregnated by vacuum evaporation prior to the mechanically activation (A1 – 3wt.% TiO2, A2 – 10wt.% TiO2) and only mechanically activated (B1 - 3wt.% TiO2, B2 – 10wt.% TiO2).
The diagram presented in
The X-ray diffractograms of the obtained powders are shown in
X-ray diffractograms of the powders A1, A2, B1 and B2.
Generally, regarding the procedure of the TiO2 impregnation, differences in the intensity of the identified peaks can be observed from the XRD diffractograms. This indicates that there are differences in the phase composition (presence and step of crystallinity) of the analyzed powders. Namely, the powders impregnated only by mechanical activation (powders B1 and B2) show significantly higher crystallinity compared to the powders impregnated in a vacuum evaporator prior to the mechanical activation (A1 and A2). The XRD measurements of the samples also revealed that the crystallinity of the samples depends not only on the amount of the impregnated TiO2, but also on the procedure of impregnation.
As for the sample of B1 (3wt.% TiO2), it can be noticed that LDH (JCPDS 38-0486) and ZnO (JCPDS 80-0075) are the only dominant phases. Evidently, the TiO2, anatas crystal phase (JCPDS 84-1286), was dispersed in the intermediate LDH layers. It is an interesting fact that LDH appears mainly as the dominant phase although this carrier of the photocatalytic active component TiO2 was calcinated before the impregnation process. This phenomenon points towards the conclusion that in this sample the memory effect was achieved. Namely, returning of the structure of the mixed oxides into the memory of the original structure of the layered double hydroxides during the process of impregnation was present (
SEM micrographs of ZnAl-LDH/TiO2 powders obtained only by mechanical activation (a. B1 - 3wt.% TiO2, b. B2 – 10wt.% TiO2).
This observation implies the idea that the rigid, smooth plain surface of the LDH particles without cracks are not a suitable matrix for the TiO2 incorporation (
The presence of TiO2 on the LDH particles edges was additionally confirmed by EDS (Energy Dispersive Spectroscopy) analysis (
EDS analysis for the samples B1 and B2
C | O | Al | Ti | Zn | Total | |
---|---|---|---|---|---|---|
B1 spectrum 1 | 19.56 | 37.5 | 7.45 | 2.3 | 33.18 | 100 |
B2 spectrum 1 | 23.06 | 34.75 | 5.62 | 4.45 | 32.13 | 100 |
SEM micrographs of the location of EDS spectra in the samples B1 (a) and B2 (b).
The results of the EDS analysis (
The results from the testing of the photocatalytic activity during a period of 24h irradiation with UV/VIS rays (powder systems A1, A2, B1 and B2 and blank sample of RhB solution) are shown in the
Photocatalytic activity of A1, A2, B1 and B2 powder samples and of the blank sample of RhB solution.
As shown in
Based on the obtained results of the potocatalitic activity, the powders B2 and A2 from groups A and B, were further used for the creation of photocatalytic suspensions which were later deposited by spray technique on the surface of the clay roofing tile samples used as porous substrates,
Photocatalytic activity of the coatings formed with 0.1g A2 /B2 powder and 0.5g B2 powder.
The values of the photocatalytic activities of the coating composed of 0.1g B2 powder, after UV/VIS irradiation of 210 min (3.5 h) and 1440 min (24 h), are 7.61% and 22.28%, respectively, while the coating composed of 0.1g A2 powder, showed insignificant values of the photocalytic activity even after (24 h) of UV/VIS irradiation.
Based on the results above, a new suspension with 0.5g of the powder B2 was prepared in the same conditions as in the case of the previously two B suspensions and deposited onto the ceramic samples (porous substrate). The results from
The water rinsing durability of the coating with the best photocatalytic activity (B2 with 0.5g of the powder) was investigated regarding the two main functional properties: photocatalytic activity and self-cleaning efficiency. The clay tiles coating formed with 0.5g of the B2 suspension were previously exposed to water rinsing procedure (30 min) and then the photocatalytic activity and self-cleaning efficiency were analyzed. The obtained results are presented in
The results of the photocatalytic activity after rinsing procedure showed a small decline in terms of the measurements made before the rinsing procedure,
Photocatalytic activity of the coating formed with 0.5g B2 powder, before and after the water rinsing procedure.
Self-cleaning efficiency assessment of the coating, formed with 0.5g B2 powder, before and after rinsing procedure
The assessment of the self-cleaning properties of the coated substrate was performed by the measurements of the initial contact angle, θci (
The noted differences of the θci values before and after water rinsing procedure were the result of a slight physical removal of the deposited ZnAl-LDH/TiO2 coating. This presents some insignificant negative influence of rinsing procedure on the self-cleaning efficiency, since the same trend of the θci value, in both cases of the experiment, was identified. The existing decreasing trend of the θci values after water rinsing procedure proves a good durability and a significant compatibility of the synthesized B2 coating with the substrate of clay roofing tiles.
Based on the comparative investigation of particle size distribution and XRD analysis of the designed nano-composite ZnAl-LDH/TiO2 powders, it was concluded that the samples impregnated with TiO2 by vacuum evaporation prior to the mechanical activation, possessed a three modal particle size distribution and smaller crystallinity, whereas the samples impregnated only by mechanical activation were characterized with a fine mono-modal particle sized distribution and a significantly higher level of crystallinity. These results indicate a strong influence of the way of TiO2 impregnation on the photocatalytic powder activity.
The amount of the impregnated TiO2 affected the photocatalytic activity of the obtained powders. An optimal loading of 0.5 g powder catalyst/100ml suspension of ZnAl-LDH/TiO2 was found to be the most appropriate. The value of the photochatalitic activity was found to be 17.57 % after 3.5h of radiation and 62.16 % after 24h radiation. This coating shows significant durability to water rinsing, indicating a good adhesion with the surface of the porous clay roofing tile.
Based on the obtained results, the newly synthetized coating composed of ZnAl-LDH/TiO2 presents a potential material that may find its use as a protective coating for inorganic porous substrates.
The financial support from Serbian Ministry of Education, Science and Technological Development (Contract No. III45008) is gratefully acknowledged.