P4. Composite photocatalytic coatings: metallic oxide – nanocarbon materials with applications in environment technologies: self-cleaning properties and advanced treatment of organic pollutants - FOTOCAT-CARBONCOMP
Main goals of the project:
Project P4 proposes the development of thin composite layers with photocatalytic properties based on semiconductor metallic oxides, and carbon nanostructures for two applications:
- Advanced treatment for organic pollutants in used waters
- Self-cleaning surfaces
This kind of composites have an advanced degree of novelty on international level, the results being reported to laboratory stage, which is why the development/expansion of scientific knowledge is aimed by:
- Selecting the pairs of metal oxide (TiO2, ZnO) – carbon nanostructure (Graphene, G, graphene oxide, GO and reduced graphene oxide, r-GO) that are optimal for the formation of stable and extendable interfaces and of the obtaining processes and thermal treatment for the two applications.
- Technological development by testing / optimizing the sol-gel technique and by developing and testing for the first time worldwide, of the SPD technique (Spray, Pyrolysis Deposition) for the deposition of this kind of thin layers, efficient and resistant in the conditions of the selected applications.
Thus, it is possible to extend the applicability of these composites trough the possibility of developing industrial processes for obtaining thin composite layers.
Objectives:
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OCDI 1. The development and optimization of the photocatalytic composites follows:
- the development of nanostructured carbon compounds (n-C) with dimensional control and of surface loading for each type (G, GO, r-GO)
- obtaining TiO2 and ZnO sol-gel films with crystallinity control, morphology, and surface loading
- obtaining SPD films of TiO2 and ZnO with crystallinity control, morphology, and surface loading
- testing the photocatalytic and superhydrophilic properties of ZnO and TiO2 films and selecting the matrixes of the composite films
- OCDI 2. Nanocarbon based composite thin layers, using TiO2 and ZnO as matrixes
- OCDI 3. Development, characterization, and testing of photocatalytic solar-active composite structures for advanced water purification and for self-cleaning surfaces.
Results 2021
Testing the performance and durability of solar-active demonstrator thin layers as self-cleaning surfaces and their optimization
Composite layers with TiO2 matrix and rGO filling
In the last part of the P4 project, the activities started from the results obtained in 2020 on composites with TiO2 matrix and GO filling. Characterizations and tests were performed on the demonstrator structures and the materials were optimized in order to reach the proposed optimum, namely double-coated FTO / TiO2 SPD / TiO2 - GO 5%w composite that had a concentration of 5% w GO in the composite layer.
The results showed that these composites are Vis-active, but thermally stable only up to 150 oC (as in the case of the composites with GO filling). Photocatalysis tests have shown that a percentage higher than 5%w in the composite layer decreases the photodegradation efficiency under irradiation from the UV + VIS spectral ranges (G ~ 55 W / m2, of which ~ 3 W / m2 UV) of the blue methylene, MB, 10ppm, natural pH MB = 6.90.
To streamline the photocatalysis process, activities in 2021 began with the determination of PZC values of the films with rGO filling, the values being between 7.37 ... 8.34 as a result the optimal pH of the MB solution (positive ion) was selected at pH = 8.50.
In the realization of the demonstrators with rGO filling, the aim was to identify the optimal concentration at which the composite is stable and efficient in the process of photodegradation of MB at optimal pH. In this sense, photocatalysis (PC) tests were performed on layers with A = 3 cm2, the selected concentrations being 3%w and 5%w rGO in the composite layer, under irradiation with UV + VIS (G ~ 55 W / m2, of which ~ 3 W / m2 UV), UV (G ~ 3 W / m2 UV) and adsorption tests. The films were characterized before and after photocatalysis (UV + VIS) to determine the stability of these types of composites.
Vis-activation is lower for the composite with 3%w rGO and occurs after a certain irradiation time (after 4 ... 5 hours of irradiation). After photocatalysis under UV + VIS irradiation, the rGO platelets are more visible, demonstrating that part of the TiO2 aggregates pass into the pollutant solution confirmed by the decrease of RMS values.
It is recommended to use composites with a higher %w of rGO filling, but previous results have shown that a higher percentage of 5%w affects the efficiency in photocatalysis by adsorption efficiency. The optimum selected for the transition to the demonstrator level is the composite with a concentration of 5%w rGO.
To evaluate the hydrophilicity of rGO-filled films, in order to use these composites as self-cleaning surfaces, contact angle (WCA) measurements were performed on samples with 3%w and 5%w rGO after photocatalysis / adsorption tests. Then, the samples were irradiated with UV spectral range radiation (G ~ 23 W / m2) for 1h to induce superhydrophilia. Decreases in WCA below the superhydrophilicity limit (WCA <10o) are observed and irradiation of these films is recommended before reuse in successive photocatalysis cycles.
The optimal double layer material FTO / TiO2 SPD / TiO2 - rGO SG 5%w was deposited on the demonstrator substrates (Atotal = 20 x 30 cm2) and used in photocatalysis tests in continuous regime at a flow rate of 1 L/min. Prior to photocatalysis, the demonstrator films were irradiated with UV to induce superhydrophilicity (GUV ~ 23 W / m2). The photocatalysis tests were carried out under irradiation with UV + VIS simulated solar radiation (G ~ 810 W / m2, of which ~ 23 W / m2 UV) and 5L solution of MB 10 ppm was used as pollutant at pHoptim MB = 8.50. Three successive cycles of photocatalysis were performed, and after each cycle the films were regenerated by washing with distilled water continuously, followed by emptying the demonstrator and irradiating the films for 2 hours next to the simulator with UV + VIS radiation (G ~ 810 W / m2, of which ~ 23 W / m2 UV) for self-cleaning of the layers by complete elimination of unreacted MB or of any reaction products remaining on the surface of the photocatalyst. Simultaneously with the regeneration of the films, hydrophilicity was induced in order to follow the behavior of the composites as self-cleaning surfaces.
It is recommended to condition the films by irradiation for 1h with UV radiation at high irradiance before PC and for regeneration / self-cleaning after each PC cycle it is recommended to irradiate for 2h with UV + VIS radiation at high irradiance when induced and (super) hydrophilicity.
Compared to GO composites, rGO-type fillers embedded in TiO2 matrix behave inferior in photocatalysis applications such as advanced wastewater treatment and self-cleaning surfaces.
Starting from the optimized synthesis of TiO2 matrix composites and (r)GO fillers, the investigations were extended to develop TiO2 matrix composites and graphite carbon nitride filler (g-C3N4) following the increase of their thermal stability. In the synthesis, 16 mL of ethanolic dispersion of g-C3N4 (c = 10 mg / mL) were used to reach the concentration of 3%w g-C3N4 in the composite layer; the dispersed phase was synthesized at IMT by trimeration of urea at T = 550 oC, respectively T = 600 oC.
The characterization of the thin layers was performed by X-ray diffraction (XRD, crystalline structure and crystallinity degree), scanning electron microscopy, SEM and atomic force microscopy, AFM (morphology and roughness), EDX analysis (elemental composition of the layers), efficiency photodegradation of methylene blue, stability of layers in aqueous environment and under irradiation.
Significantly lower values of contact angles (θ <10o) on heat-treated TiO2-CN550 and TiO2-CN600 composites at T = 450 oC confirm their superhydrophilia. For these composites the photodegradation efficiencies of methylene blue are higher and also, the Vis-activation is accentuated. The higher photodegradation efficiencies obtained on the heat-treated composites at T = 450 oC are also due to the higher crystallinity values compared to the values of the crystallinity degree of the heat-treated layers at T = 150 oC.
In the last part of the P4 project, ZnO matrix composites and GO filling starting from the results obtained in 2020 were used. Because after the tests performed on the demonstrator it was found the instability of the composite layer in continuous flow regeneration / washing) at a flow rate of 1L / min, continued with continuous photodegradation tests on the demonstrator at a flow rate of 0.5 L / min on substrates (Total = 20 x 30 cm2) using in composite with two concentrations of GO: 1.5%w and 3%w (% GO in the composite layer). Demonstrator thin layers based on ZnO-GO were obtained using optimized reception in 2020, namely:
(1) the first layer (ZnO) was deposited using the pyrolysis spray deposition (SPD) technique on glass substrate
(2) the ZnO-GO soil matured for 48 hours and diluted in soil dilution ratio: IPA = 1: 4 was deposited over layer 1.
(3) the obtained composites were heat treated at T = 150 oC, for 2h
Photocatalysis tests have shown that a lower percentage of GO in composite decreases photocatalytic efficiency, but not significantly. Also in the case of the composite with 1.5%w GO the activation takes place later, after approximately 2 hours of process.
Thin composite layers with ZnO matrix and rGO filling
Using optimized reception on composites with GO filler, similar tests were performed on composites with reduced graphene oxide filler (rGO). The synthesis method consisted in mixing the carbon nanostructure (rGO) in the form of ethanolic dispersion of rGO (c = 26.35 mg / mL, prepared by the IMT Bucharest partner) followed by the gradual addition of isopropyl alcohol and monoethanolamine. The dispersion was added in quantities established so as to correspond to a theoretical mass ratio in the layer of 1.5%w, 3% w and 5%w rGO relative to ZnO. To obtain the ZnO-rGO soil the mixture of precursors was subjected to magnetic stirring for 1h at 60-70 oC, followed by ultrasound for 1h to obtain the ZnO-rGO soil. It was left to mature for 48 h, at room temperature then diluted with isopropyl alcohol soil: IPA = 1: 4, ultrasound 30 min and then spray deposited over layer 1 ZnO_SPD.
Although the composites with 5%w rGO show a more accentuated dream-activation, they also presented a lower stability in the aqueous environment, therefore for the testing on the demonstrator the composites with 1.5%w and 3%w rGO in the composite were selected.
The photodegradation efficiency was lower in the case of the composite with 1.5%w rGO. These results can be correlated with lower Vis-activation in the case of the composite with 1.5%w rGO in the composite layer.
In order to verify the behavior of these surfaces as self-cleaning demonstration materials, the contact angles were determined. The results showed a decrease in the contact angle indicating that it is necessary to irradiate the composites before photocatalysis. But, even after irradiation, the super-hydrophilicity of these layers was not reached.
- Composites with ZnO matrix and GO filler have higher efficiencies, but also better stability in the aqueous environment compared to composites with rGO filler.
- Compared to similar TiO2 matrix composites, ZnO matrix composites are less efficient and more fragile. Therefore, composites with (r) GO filling and TiO2 matrix are recommended as photocatalytic materials in advanced water purification and for use as self-cleaning surfaces.
Results 2020
Project 4 (P4) had included in the 2020 Plan activities for the development and characterization of demonstration structures usable for advanced wastewater treatment and for self-cleaning surfaces. For this, it was decided from the results obtained in the previous stages (2018 and 2019) and their transition to industrial research and experimental development through innovation activities. The activities carried out in 2020 are summarized in the report below, highlighting the results obtained, results with an advanced degree of novelty and competitiveness.
Composite layers with TiO2 matrix
Stability tests and pH optimization tests of the pollutant solution were performed depending on the PZC value of the layers obtained using the optimized deposition alternative in 2019, namely double layer deposited on FTO substrate with the following structure: Glass / FTO / TiO2 (obtained by SPD with 30 spray sequences, 60s pause between sequences, precursor solution TTIP: AcAc: EtOH = 1: 1: 15v, heat treated after deposition at 450 oC for 3h) / TiO2 - GO (obtained by sol-gel technique using the ratio volumetric in the precursor system TTIP: EtOH: AcAc: HAc: GO = 1: 0.8: 0.04: 0.009: 0.12v with 15 deposition sequences, break 60s, the soil being diluted with ethanol in a ratio of 1: 5 soil: ethanol).
The tests consisted in evaluating the TOC values in the degradation of methylene blue (MB) from a solution with a concentration of 10ppm, analysis of SEM micrographs, analysis of surface composition (EDX) and monitoring of variations in roughness (RMS) before and after photocatalysis under UV + VIS small irradiation (G = 55 W / m2, of which ~ 3 W / m2 UV) and in the dark using solutions of the standard methylene blue pollutant (MB), but also by similar tests using water under identical irradiation and in the dark. The results indicate an increase in TOC after one hour of immersion and a slow decrease in this value over the next eight hours of testing. The low TOC values (TOC <10 mg / L in MB solution and TOC <5 mg / L in water) demonstrate the high stability of these types of double-coated films.
Photocatalytic efficiency tests indicated an obvious improvement. It is therefore recommended to optimize the pH of the pollutant solution (in low concentrations) depending on the PZC of the photocatalytic material and the type of pollutant to support adsorption on the photocatalytic surface of the pollutant and thus to improve process efficiency.
Composite structures were also made using rGO at laboratory level (A = 3 cm2) initially varying the heat treatment temperature for the final material and then the concentration of rGO in the composite. The rest of the deposition conditions were those previously optimized for the composite layer with GO. Preliminary tests targeted a low percentage of rGO (0.15% w) and indicated that the heat treatment at 200 oC leads to layers with a zero surface percentage of C while the heat-treated layers at 150 oC indicate 1.01% C due to the organization "layer by layer" of the layers with the organization of rGO monolayers to the surface.
Photocatalysis tests with MB 10ppm solution were performed on these films and VIS-activation caused by the presence of rGO in heat-treated composites at 150 oC was proved: the photodegradation efficiency at UV-Vis irradiation was 27.74% and at UV irradiation of 20.53% while the adsorption efficiency (in the dark) was 15.99%.
The photocatalysis results indicate that a higher percentage of 5%w rGO in the composite affects the photodegradation efficiency of MB, the adsorption being very reduced. It is recommended to use 5%w rGO composites in demonstrator level tests. In this sense, the value of the PZC of the film was determined to be between the values of 4.25 and 8.34, so the optimal pH of the MB pollutant was chosen 8.50 for the tests at demonstrator level.
Preliminary tests were performed to establish the optimal conditions for the deposition of demonstrator-type structures. The tests targeted layer 2, the ZnO-GO composite layer and layer 1 was deposited according to the optimized conditions in stage 2019. The aim was to identify:
• Optimal soil concentration for layer 2 deposition.
• Influence of the deposition substrate: glass or glass / FTO. On the FTO substrate the crystallinity of the composite structure is higher but it presents non-uniform morphologies with wide cracks / pores, with more obvious discontinuities in the thicker layers, sprayed with 20 sequences of composite soil.
• Influence of ZnO-GO composite soil deposition temperature: T = 65 oC, 80 oC, 100 oC. The ZnO-GO layers deposited at 65 0C show higher photocatalytic efficiencies compared to the layers deposited at 80 0C, respectively at 100 0C on all the radiation fields investigated.
• Number of submission sequences: 15, 20 sequences. A thicker layer can have a better resistance to peeling when operating in continuous flow due to its dimensional homogeneity.
• Break between two spray sequences: 60s, 90s: Layers deposited by spraying ZnO soil. GO, with a break of 90 s between sequences shows lower photodegradation efficiencies of methylene blue by approx. 10-12% under irradiation with UV + VIS radiation compared to ZnO-GO layers deposited with a break between sequences of 60 seconds.
Realization and characterization of composite structures with selected self-cleaning properties, as demonstrator units (layers 10 x 10 cm2)
The previous activity aimed at conditioning tests in order to analyze the variation of the hydrophilic character on the optimized deposition variant in the previous stage (2019); the variation of the contact angle in the dark and after irradiation with UV radiation, the variation of the roughness (RMS), of the composition and morphology were studied.
Test conditions: a) Samples kept in the dark (under normal atmospheric conditions) - Duration: up to 7 days; b) Irradiated samples with UV spectral radiation (G = 8.3 W / m2) - Duration: up to 72 hours.
The results were correlated with the efficiencies in photocatalytic decomposition on radiation irradiation from the UV + VIS spectral ranges (G ~ 55 W / m2, of which ~ 3 W / m2 UV) of the standard methylene blue pollutant, MB (10ppm, VMB = 20 mL). Photocatalysis conditions: 1h dark + 8h UV + VIS irradiation.
Analyzing the photocatalysis results, it is recommended to condition the films before using them in photocatalysis processes, especially if they have been stored for longer periods of time.
The way to regenerate films with self-cleaning properties was also investigated. The samples were used in laboratory photocatalysis under UV + VIS irradiation with MB pollutant at natural pH = 6.85 and optimal pH = 8.50. Duration of the regeneration / self-cleaning process: 1h… 4h. Radiation used for regeneration / self-cleaning: UV + VIS at the solar simulator (G ~ 810 W / m2, of which 23 W / m2 UV). For use in the demonstrator in several photocatalysis cycles, it is recommended to regenerate the films for at least 2 hours for tests, at optimal pH = 8.50. The realization of the composites was identical to that of the demonstrators with application in advanced water purification.
ZnO matrix layers
In order to obtain composite structures based on ZnO and graphene derivative (GO, rGO) with photocatalytic self-cleaning properties, thin layers were deposited according to the presented recipes and tested for the evaluation of control properties for the application concerned:
• Hydrophilic / superhydrophilic character of the surface
• Photodegradation efficiency under irradiation with UV + VIS radiation after 8 hours of irradiation evaluated according to the procedure previously described
Recommended composite structure for future tests and optimizations for the development of self-cleaning surfaces based on ZnO and GO (layers 10 x 10cm2) corresponds to the sample st / ZnO_SPD / ZnO-GO_1: 2_650C_60s_20 secv, obtained by spraying the composite soil temperature ZnO 65 oC, in 20 sequences, with a break of 60 s on the glass covered with ZnO_SPD and heat treated 2h at a temperature of 150oC.
Testing and optimization of solar-active demonstrator thin layers in advanced water purification processes (stationary processes and continuous flow processes)
TiO2 matrix thin films
The recorded results indicate VIS-activation of GO structures for all composites containing GO and also a decrease in efficiency after the first photocatalysis cycle (due to pollutant molecules or oxidation products remaining adsorbed on the surface) as well as stabilizing efficiency around of 30 ... 35%. It is also found that the maximum efficiency values are obtained for the composite containing 5% GO and the composites with 10% GO lead, after cycle 1, practically to efficiencies similar to those with 5% GO.
ZnO matrix thin films
For continuous flow operation, demonstrators with solar-active composite structures based on ZnO and GO with dimensions of 10x10 cm2, respectively 8x10 cm2 and 2x2 cm2 integrated in the photoreactor were made.
To evaluate the photodegradation efficiency of methylene blue, a solution sample (3 mL) was taken every 15 minutes to determine the absorbance by UV-VIS spectroscopy. To keep the solution volume constant and to compensate for the evaporation of the solution from the tank, 20 mL of distilled water was added hourly. At the end of the process, the thin layers were regenerated by applying a wash cycle with distilled water for 30 min.
The adsorption efficiency of MB on the surface of the photocatalytic plates is ~5% from the first 15 min and remains almost constant until 1h of adsorption, indicating that 1h is sufficient to achieve the adsorption-desorption balance of the pollutant on the photocatalytic substrate.
The results obtained in the photodegradation of methylene blue in continuous flow processes (simulated solar radiation, G = 810 W / m2) are promising and confirm the acceleration of the process by increasing the irradiance of the radiation used; this generates a larger number of charge carriers, respectively hydroxyl radicals that degrade the pollutant. The stability of the composite layers was monitored by measurements on the control samples (2x2 cm2 samples positioned in the center of the reactor and the edge of the reactor).
The comparative analysis of the results indicates the relatively limited stability of the thin layers in the continuous flow photocatalysis processes, a consequence of the combined processes of:
• Adsorption on the surface of the thin layer of colored products resulting in photodegradation of methylene blue,
• Partial passage of compounds from the surface of the layer in the pollutant solution (more intense in continuous flow processes compared to static ones) when compounds with C / GO from the layer migrate predominantly, as demonstrated by the significant decrease of % C at the surface during photocatalysis process.
• The tested thin layers retain their dendritic morphology after 24 h of UV irradiation and show good stability under irradiation with UV radiation, as indicated by the variation of roughness over time and under irradiation, respectively the variation of the transmitter before and after irradiation.
• The stability of the layer during the photocatalysis process can be considered acceptable.
• Activation of composite structures based on ZnO-GO (type st / ZnO_SPD / ZnO-GO_1: 2_650C_60s_20 secv) at 24 h UV irradiation (G = 8.3 W / m2) takes place and simultaneously leads to increased surface hydrophilicity and to improve the photodegradation efficiency of methylene blue
• After conditioning by irradiation 24 h with UV radiation (G = 8.3 W / m2), the thin layers st / ZnO_SPD / ZnO GO_1: 2_65 0C_60s_20 secv show hydrophilic character (water contact angle of ~ 21 degrees) and the photodegradation efficiency of MB of ~36% after 8 hours of irradiation with 2UV + 5VIS radiation (G = 55 W / m2);
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