Highly Crystalline Nanoparticle Suspensions for Low-Temperature

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Highly Crystalline Nanoparticle Suspensions for Low Temperature Processing of TiO2 thin films Jonathan Watté, Petra Leona Lommens, Glenn Pollefeyt, Mieke Meire, Klaartje De Buysser, and Isabel Van Driessche ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01684 • Publication Date (Web): 27 Apr 2016 Downloaded from http://pubs.acs.org on April 30, 2016

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ACS Applied Materials & Interfaces

Highly Crystalline Nanoparticle Suspensions for Low Temperature Processing of TiO2 Thin Films Jonathan Watté, Petra Lommens, Glenn Pollefeyt, Mieke Meire, Klaartje De Buysser, Isabel Van Driessche* SCRiPTS, Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281S3, 9000 Gent, Belgium KEYWORDS TiO2, Nanoparticles, Microwave-assisted hydrothermal synthesis, Photocatalytic activity, Thin Films, Low temperature deposition ABSTRACT In this work, we present preparation and stabilization methods for highly crystalline TiO2 nanoparticle suspensions for the successful deposition of transparent, photocatalytically active TiO2 thin films towards the degradation of organic pollutants by a low temperature deposition method. A proof-of-concept is provided wherein stable, aqueous TiO2 suspensions are deposited on glass substrates. Even if the processing temperature is lowered to 150 - 200°C, the subsequent heat treatment provides transparent and photocatalytically active titania thin layers. Since all precursor solutions are water-based, this method provides an energy-efficient, sustainable and environmental-friendly synthesis route. The high load in crystalline titania particles obtained after

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microwave heating opens up the possibility to produce thin coatings by low temperature processing, as a conventional crystallization procedure is in this case superfluous. The impact of the precursor chemistry in Ti4+-peroxo solutions, containing imino-diacetic acid as a complexing ligand and different bases to promote complexation was studied as a function of pH, reaction time and temperature. The nanocrystal formation was followed in terms of colloidal stability, crystallinity and particle size. Combined data from Raman and infrared spectroscopy, confirmed that stable titanium precursors could be obtained at pH levels ranging from 2 to 11. A maximum amount of 50.7% crystallinity was achieved, which is one of the highest reported amounts of anatase nanoparticles that are suspendable in stable aqueous titania suspensions. Decoloring of methylene blue solutions by precipitated nano-sized powders from the TiO2 suspensions proves their photocatalytic properties towards degradation of organic materials, a key requisite for further processing. This synthesis method proves that the deposition of highly crystalline anatase suspensions is a valid route for the production of photocatalytically active, transparent films on heat-sensitive substrates such as polymers.

INTRODUCTION

Titania, being a wide band-gap semiconductor, is a cost-effective, inert and environmentally friendly material, which is already widely used in bulk applications such as pigments in cosmetics, toothpaste and paints1. One of its most interesting properties is its remarkable photocatalytic activity under UV-irradiation2-4. Combined with its high chemical and optical stability, non-toxicity and corrosion resistance, TiO2 has many applications, including the oxidative photo-degradation of various organic pollutants5. Titania coatings are often applied to create self-cleaning, anti-bacterial surfaces for outdoor use, air and water purification6-9. One of


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the most known applications is self-cleaning glass in which the photocatalytic action of the titania together with its UV-light induced superhydrophilicity leads to the removal of dirt and organic species from window glass10-11. The ability to deposit self-cleaning titania coatings on heat sensitive substrates such as polymers opens up large markets such as displays, (touch-)screens, visors, light domes or even noise barriers on highways. Although there are already some experimental, low temperature deposition methods for titania thin films on polymers (radio frequency magnetron sputtering12 and Plasma Induced Chemical Vapour Deposition13), these techniques are relatively more expensive than chemical solution deposition methods (CSD), as no high vacuum processing is necessary in the latter14-15. Additionally, the facile and straightforward processing and scalability of CSD techniques is guaranteed and thus very interesting for industrial production16. Other state-of-theart deposition methods such as conventional CVD or sol-gel deposited coatings normally require high temperatures, mostly over 400°C, to obtain crystalline thin layers17-20. Obviously, when using polymers substrates, these techniques cannot be applied. The use of preformed titania nanoparticles could offer a solution to this problem. If crystalline nanoparticles are already present in a stable suspension, it is possible to convert deposited wet layers into crystalline thin films by using a low temperature heat treatment. Most of the sol-gel chemistry literature on TiO2 focuses on controlled hydrolysis in organic media21-23. However, industrial demands encourage the development of water-based precursor solutions. Therefore, the development of clear and stable precursor solutions in which titanium ions are stabilized, consisting of environmentally benign chemicals and using water as the primary solvent, are of upmost importance24. In this work, we succeeded in preparing stable titanium ion precursor solutions using titanium tetra-isopropoxide, a chelating agent and water as


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the main solvent. These precursors exhibit no sign of precipitation between pH levels of 2 and 11. These precursors can be further processed into nanoparticle containing suspensions after being subjected to a microwave treatment. The impact of the precursor solution chemistry on nanoparticle formation was studied and the resultant suspensions were characterized on the basis of nanoparticle size and crystallinity. The Ti4+-precursor solution synthesis was adapted from Pollefeyt et al25, which was in turn based on a typical citrato-26 or glycolato-peroxo27 precursor solution synthesis. The colloidal titania suspensions were deposited onto glass substrates by means of dip-coating and transparent titania thin films were prepared by a subsequent low temperature heat treatment. The films were characterized on their transparency and photocatalytic activity in comparison with commercially available self-cleaning coatings on glass. The work presented here focuses on the microwave-assisted hydrothermal synthesis and characterization of titania nanocrystal suspensions from stable, aqueous Ti4+ precursor solutions, suited for deposition onto heat sensitive substrates. Subsequent low temperature deposition of these suspensions on glass is a clear proof-of-concept for their ability to be deposited on polymers. EXPERIMENTAL SECTION Ti4+-precursor solution synthesis. In a typical synthesis procedure, 8,4 mL titaniumisopropoxide (TTiP, Sigma-Aldrich, ≥ 97,0%) was precipitated in 100 mL of water. The precipitate was filtered off and washed with an excess of water in order to remove any isopropanol residue from the Ti-source. After washing, the wet Ti(OH)4 precipitate was added to a 1:2 molar equivalent of iminodiacetic acid powder (IDA, Alfa Aesar, > 98%). Subsequently, H2O2 (Sigma-Aldrich, 35 wt% in H2O) was added in a 1:2 molar ratio (Ti:H2O2). A base (either


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ethanolamine (EA), Carl Roth, 99,5% or tetraethyl ammoniumhydroxide (TEAOH), SigmaAldrich, 40 wt% in H2O) was added in different ratios, resulting in an exothermic reaction. Afterwards, the precursor was stirred for 30 minutes at 60°C. A transparent, burgundy red precursor was obtained with a titanium concentration of 0.7 mol/L. The pH level of each precursor is dependent on the used base and the molar ratio in which it is added to the titanium precursor. Stepwise increase of the pH was achieved by adding the same base that was used during precursor synthesis, in order to be able to study the influence of the pH level on the stability and the reactivity of the synthesized precursors. Up to a pH of 11 precursors remained free of precipitation, yet a color change was observed from burgundy red to bright yellow with increasing pH as shown in figure 1 (left) for the Ti-EA precursor. It is clearly observable from figure 1 that the amount of the added ethanolamine, and thus the pH, determines the color of the resulting precursor solution and thus the complexation behavior. Alteration of the pH level in the Ti-EA precursor results in either the protonation (low pH) or deprotonation (high pH), because of the added base ethanolamine, of the complexing ligands, thereby influencing the coordination sphere. The absorption maximum shifts because of the changing nature of the ligand properties, thus resulting in a color change with increasing amounts of ethanolamine.

Figure 1. Color change of the Ti-EA (top) precursor by increasing the pH from 2 to 11. A similar trend is observable for the Ti-TEAOH precursor. On the right: gel of the Ti-EA precursor (pH = 4) gelled at room temperature for 24h.


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From previous work by our research group17, it was found that the use of triethanolamine during synthesis caused a yellow coloring. In order to produce clear and transparent thin titania films, any coloring effect is not desirable. Therefore, another titanium ion precursor was synthesized with tetraethyl ammoniumhydroxide as a base (Ti-TEAOH). For Raman and infrared analysis, precursor blanks were prepared according to the syntheses described above, only the addition of TTiP has been omitted during the synthesis process of the precursor blanks. All precursor solutions have a shelf life of several months in sealed containers at room temperature. In order to determine the stability of our titanium precursors during further processing, the precursor solutions were poured into a petri dish at room temperature and left standing for 24 h. Throughout this gelation step, most of the solvent evaporates, and a viscous substance is obtained as can be seen in figure 1 (right). The stability of this gel is a prerequisite for obtaining compositional homogeneity throughout the thin films. Microwave-assisted hydrothermal synthesis. In order to convert the Ti4+-precursors into crystalline titania nanoparticle suspensions, a microwave furnace (CEM Discover) was used at a frequency of 2,450 MHz. Three mL of an aqueous Ti-precursor was pipetted into a 10 mL reaction vessel, which is subsequently placed inside the microwave reactor. Different reaction conditions were explored with a reaction temperature ranging from 100 – 140°C and dwell times from 5 minutes to 3 hours. In each reaction, the power output of the microwave reactor was set at 300 W and the pressure reached within the reactor vials varied from 0.3 – 4 bar. After synthesis, the obtained suspensions were filtered with a 0.20 µm PET filter in order to remove the presence of aggregates. Purification was done for selected suspensions by adding either methanol (for TiEA suspensions) or ethanol (for Ti-TEAOH suspensions) in a threefold excess. After


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precipitation, the powders were centrifuged at 4000 rpm for 10 minutes, then filtered off and dried at 60°C overnight. Thin layer deposition and thermal processing. The TiO2 nanoparticle suspensions were dipcoated on standard microscopy glasses (Carl Roth) with dimensions of 20x20 mm, at room temperature and at a coating speed of 100 mm/s, by means of a computer controlled dip-coating unit (KSV Instruments) in a clean room facility (class 100,000/1000). Prior to coating, the substrates were thoroughly rinsed with isopropanol and the suspensions were filtered with a standard 20 µm filter. The as-coated films were first dried in a drying furnace at 60 °C for 30 min. The subsequent thermal processing of these dried films was performed in a Nabertherm muffle furnace with a P330 temperature controller. The following temperatures were selected: 200, 300, 400, 500°C for 2 hours with a heating rate of 5°C/min under air. Spectroscopic characterization. Characterization of the titanium precursors was performed by dispersive Raman spectroscopy (RamanRxn, Kaiser Optical Systems Inc., 532 nm) and ATRFTIR (PerkinElmer Spectrum 100). For Raman spectroscopy, the precursor solutions were measured as well as their gels. The transparency of the deposited films on glass substrates was determined using a UV–Vis Spectrophotometer (PerkinElmer Lambda 950). Structural characterization. XRD analysis on precipitated powders from Ti-EA and Ti-TEAOH nanoparticle suspension was collected on a Thermo X’tra diffractometer (Cu Kα = 1,5405 Å). Samples were measured in a θ–2θ geometry over an angular range of 5–70° using a 0,02° step size and a 1s step counting time. The internal standard approach was selected for the determination of the amorphous content by XRD analysis and Rietveld refinement. A ZnO (10 wt%) internal standard was added to precipitated TiO2 powders. These powders were side-loaded


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onto sample holders to reduce preferred orientation effects. Topas Academic V4.1 software was used for Rietveld refinement28. By adding a known weight fraction of a crystalline internal standard to the mixture, weight fractions for amorphous and other phases in the specimen can be calculated using the composition, instrumental settings and the cell parameters of the phases. Nanoparticle size distributions were determined by using Dynamic Light Scattering (DLS, Malvern Nano Series Zetasizer) in backscattering mode (173°). High-resolution transmission electron microscopy (HRTEM, JEM-2200FS with Cs corrector) was used to investigate particle sizes, morphology and crystallinity. A cross-section of the produced coatings on glass as a proof of concept was assessed with a FEI Nova 600 Nanolab Dual Beam FIB-SEM and a JEOL JSM7600F. Thermal analysis of TiO2 powders precipitated from nanoparticle suspensions was performed by a Netzsch STA 449 F3 Jupiter with a heating rate of 10 °C/min and an air flow rate of 120 mL/min. Photocatalytic analysis. The photocatalytic activity for the degradation of organic components by the TiO2 thin layers and powders, precipitated from the above-described titania nanoparticle suspensions, was determined by measuring the color removal of a 35 mL methylene blue aqueous solution in contact with the TiO2 powder as a function of UV irradiance. This was done according to the regulations stipulated in ISO 10678:2010(E) via UV/VIS spectrophotometry29. It is generally accepted that the bleaching of MB aqueous solutions exhibits a pseudo first-order kinetic mechanism as described in the following equation. 𝐶 ln ( ) = −𝑘𝑡 𝐶0 In which C is the concentration of methylene blue after a specific UV irradiation time t, C 0 is the initial MB concentration (10-5 mol/L), and k is the rate constant of the reaction. According to


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another ISO standard (ISO 10677:2011), the ultraviolet light source for testing the performance of semiconducting photocatalytic materials, was assessed for radiation intensity. The MB decolorization measurement set-up was equipped with a Vilber Lourmat VL-315BLB blacklight blue fluorescent light tube. The photon source has a maximum emission at 365 nm and emits 10 W/m². The titania coated glass microscopy slides were accurately cut at 4.0 cm² and inserted into the holder cell. Concerning the MB decolorization by TiO2 nano-sized powders, 25 mg powder was used per experiment. Furthermore, nitrogen sorption experiments were carried out on a TriStar 3000 (Micromeritics) at -196 °C. The specific surface area were calculated using the Brunauer-Emmett-Teller (BET) method. RESULTS AND DISCUSSION Ti4+-precursor solution chemistry. During precursor synthesis, some observations were made, which are crucial for the understanding of the reaction mechanism. Concerning the hydrolysis reaction of titanium(IV)isopropoxide (TTiP), the formed precipitate should never be completely dry since this will inhibit the formation of a clear Ti-precursor solution due to incomplete dissolution of the titanium hydroxide species. Additionally, it is of the upmost importance that complete hydrolysis of the TTiP is ensured. Any remaining isopropanol or isopropoxide results in unclear precursor solutions. Infrared and Raman measurements were performed on precursor solutions and gels in order to determine the complexation behavior of the chelating ligands to the titanium ions.


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Figure 2. Infrared (top, left axis) and Raman (bottom, right axis) spectra of the Ti-EA precursor solution and gel, as well as the EA precursor blank (without any titanium). For the T-EA precursor (Fig. 2), both IR spectra of solution and gel show corresponding absorption peaks, albeit more intense in the spectrum of the precursor gel. As known from literature30, deprotonated carboxylic acids yield absorption bands corresponding to an νas asymmetric stretch between 1540 and 1650 cm-1 and a symmetric COO- mode (νs) between 1300 and 1420 cm-1. Both bands are present in the Ti-EA IR spectra of both precursor and gel and occur at 1586 cm-1 and 1372 cm-1 for, respectively, the asymmetric and symmetric carboxylate stretch30-32. The shoulder present at 1518 cm-1 can be attributed to N+HR325. The distinct peak at 1314 cm-1 is an overlap of both δ (O-H) and a C-COO- stretching vibration. In the IR spectrum of the Ti-EA gel, a very sharp peak can be observed at 1116 cm-1, attributed to the antisymmetric CN(-C)-Mn+ stretching vibration, which can be attributed to a complexation effect of the amine function of IDA to titanium, however no further evidence of this effect could be gathered. The shoulder observable at 1142 cm-1 is connected to the C-N-C stretch of IDA25. All of this


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information implicates the formation of a Ti-IDA complex in which the deprotonated carboxylate functions, together with the amine function of either IDA or ethanolamine, act as chelating agents. The Δ value (νas(COO-) - νas(COO-)) of the Ti-EA precursor is close to the ionic Δ value of the EA precursor blank, since the νas(COO-) and νas(COO-) absorption peaks of both Ti-EA precursor and EA precursor blank are overlapping (Fig. 2). This observation is a good indication of bridging Ti-complexes27,

30, 33

, in which the delocalized electron pair of the deprotonated

carboxylic functions acts as a stabilizing ligand. Sharp peaks at 1066 and 1012 cm-1 are attributed to, respectively, the C-O and C-N stretch of ethanolamine34-35. Further analysis of the IR spectra of both the Ti-EA precursor solution and gel suggests the presence of Ti-peroxide30, 36 as (Ti-O2) asymmetric stretches located at 620 cm-1 and 724 cm-1 are observed. According to literature37, IR absorption peaks for peroxo complexes occur at 900 - 800 cm-1 for ν(O22-) and ν(Ti-O2) in the 700 - 500 cm-1 range. In accordance with IR measurements, Raman vibrations are also observed at 870, 610 and 415 cm-1. These are attributed to, respectively, ν(O-O) of H2O2, νs(Ti-O) and either the O22- → Ti(IV) charge-transfer transition38-40 or the νs(Ti-N) stretch25. The νs(Ti-O) band also has an overtone at around 1225 cm-1 (2 νs(Ti-O)). The weak band at 300 cm-1 represents the δs(O-Ti-O). The small, yet distinct peak at 945 cm-1 is assigned to a coordinated peroxo stretching vibration ν(O-O). All the observed signals of ν(O22-), ν(O-O), νs(Ti-O) and ν(Ti-O2) are in line with the signal related for non-bridged η2-peroxo coordinated ligands (η2: O22- is bonded to titanium in a triangular bidentate manner; for η1, this is single bonded to titanium in a linear monodentate fashion)41. Clearly, next to a bridging complexation behavior with IDA, also peroxide acts as a stabilizing ligand in the formation of our T-EA precursor solution.


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Figure 3. Infrared (top, left axis) and Raman (bottom, right axis) spectra of the Ti-TEAOH precursor solution (−) and gel (•••), and the TEAOH precursor blank (- -). Infrared and Raman absorption peaks of the Ti-TEAOH precursor, gel and TEAOH precursor blank are represented in Fig. 3. The IR spectrum of Ti-TEAOH is very similar to that of Ti-EA (Fig. 2). Deprotonated carboxylic acids correspond to the asymmetric stretch at 1600 cm-1 and a symmetric stretch between 1400 and 1260 cm-1. Again, the Δ value (νas(COO-) - νas(COO-)) of the Ti-TEAOH precursor is close to the ionic value of the TEAOH precursor blank. This indicates a bridging complexation of the deprotonated carboxylate functions of IDA to the titanium ion. Next to this, also the non-bridged η2-peroxo coordination effect occurs to the titanium ion, in which O22- is again triangularly bonded to a titanium ion as is the case for the TiEA precursor. IR absorption peaks of this peroxo complexation effect in the Ti-TEAOH precursor are in line with those of the Ti-EA precursor. For Raman vibrations, there is a clear difference in intensity for the 620 cm-1 peak related to a peroxo Ti-O symmetric stretch. The same is observable for O22- → Ti(IV) charge-transfer transition at 415 cm-1. This can indicate a


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larger complexation effect of peroxo ligands to titanium. Although this hypothesis should be further examined, it is not the scope of the work presented in this paper. TiO2 nanoparticle synthesis. Different microwave reaction parameters were optimized in terms of stability and crystallinity of the resulting colloidal titania suspensions. Both the Ti-EA and TiTEAOH precursors could be subjected to a maximum reaction temperature of 140°C and a maximum dwell time of 60 minutes in order for the resulting nanoparticle suspensions to remain stable. All of these nanoparticle suspensions remain stable for at least several months at room temperature. An increase of microwave reaction parameters (above 140°C reaction temperature and over 60 minutes dwell time) results in the formation of indispersable titania nano-sized powders, which is clearly not preferable for further processing. As can be seen from the digital photographs in figure 4, a color change is seen from yellow to brown-orange suspensions for increasing dwell times of a Ti-EA precursor (pH = 3.5) subjected to microwave treatment. Furthermore, the nanoparticle suspensions, microwaved for longer residence times, clearly show a Tyndall effect, which indicates the presence of agglomerates.

Figure 4. Left: color change of the Ti-EA suspensions microwaved at 140°C with increasing dwell times (pH = 3.50): a) 5 min, b) 10 min, c) 15 min, d) 20 min, e) 30 min, f) 45 min and g) 60 min. Right: Gel of a Ti-EA (h) suspension, both microwaved at 140°C for 15 minutes. The suspension was gelled at room temperature for 24h.


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In Figure 5a, the influence of microwave residence time on the size of the titania nanoparticles, obtained after microwave synthesis at 140°C, is depicted for the Ti-EA suspensions with a pH level of 3.5. Increasing the irradiation time from 5 to maximum 60 minutes results in a clear trend concerning particle (hydrodynamic) diameters. As the irradiation time increases up to 30 minutes, the particle size distribution broadens while the volume percent of smaller particles decreases. A further increase in MW residence times results in further growth or agglomeration of the titania nanoparticles. For a synthesis performed at 140 °C and 30 minutes dwell time, zeta potential measurements of a pH series, ranging from 2 to 9, gave average values in the range of -9 mV to 13 mV. This indicates that the particle surface is negatively charged and that the Ti-EA suspensions are not charge stabilized, since the absolute value of the zeta potential does not exceed 25 mV17.

Figure 5. Size distribution curves obtained from DLS measurements of (a) Ti-EA suspensions synthesized at 140°C (pH = 3.5) for 5 min, 20 min, 30 min and 60 min and (b) Ti-TEAOH suspensions at 140°C (pH = 3.5) for 5 min, 15 min and 30 min (b).


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As for the microwave synthesis of the Ti-TEAOH precursor solution, precipitate-free suspensions were obtained up to a maximum of 140°C microwave reaction temperature and 30 minutes dwell time. Figure 5b represents size distributions for T-TEAOH suspensions (pH = 3.5) microwaved at 140°C for residence times ranging from 5 to 30 minutes. Again, a clear trend is observable: at short irradiation times smaller particles of around 6 nm are being formed as well as bigger particles of 20 nm, although the smaller particles are more abundant. The more the reaction time increases, the more the hydrodynamic diameter shifts to greater values. Particle growth can be again attributed to Ostwald ripening, reaching sizes of 100 nm for 30 minutes dwell time. Zeta potentials were found to be in the range of – 6.8 mV to -9.2 mV for pH levels ranging from 2.8 to 11, resulting again in a negatively charged particle surface. It has to be noted that for Ti-TEAOH suspensions, which underwent longer MW irradiation time, large agglomerates of particles are present as indicated by DLS (Fig. 5b). To determine whether the nanoparticles obtained after microwave synthesis are crystalline, XRD was carried out on precipitated powders obtained after purification. For all the powders destabilized from either Ti-EA or Ti-TEAOH nanoparticle suspensions, wide anatase reflections can be observed at 2θ = 25.4°, 38°, 48°, 54°and 56° corresponding to the (101), (004), (200), (105) and (211) planes. Figure 6 (left) reveals that for the Ti-EA nanoparticle suspensions, reflection intensities increase with reaction temperature and time, indicating both the influence of the nanoparticle crystallinity (Figure 6, right). Analogous results were obtained for the Ti-TEAOH precursor solutions.


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Figure 6. XRD diffractograms collected for TiO2 powders obtained from precipitated Ti-EA suspensions at pH 3.5, (left) after microwaving at 140°C for (a) 5 min (b) 30 min and (c) 60 min and (right) after microwaving for 15 min at (a) 140°C (b) 160°C and (c) 200°C. By adding ZnO as an internal standard to the precipitated powders, it is possible to calculate the crystalline fraction of TiO2 present in all samples (Table 1) by applying Rietveld refinement. The results were corrected with additional TGA analysis of the same powders (see Supporting Information). The weight loss determined by TGA corresponds to the amount of organic fraction adsorbed to the precipitated powders from both Ti-EA and Ti-TEAOH nanoparticle suspensions at a pH of 3.5 for different MW synthesis parameters. In this way, a correct estimation of the titania weight fraction of all powders could be obtained and thus the actual amount of TiO2 in these powders was used in further Rietveld refinement. Rietveld analysis revealed that the amount of amorphous fraction inside precipitated powders decreases as the microwave reaction times increase, confirming the increased intensity of the reflections observed from powder XRD. From the Rietveld refinement, the crystallite sizes are extracted using the Scherrer equation, as


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well as the anatase fraction in each suspension of Ti-EA and Ti-TEAOH. These data are represented in Table 1. Anatase wt%

Crystallite size (nm)

Goodness of fit (Rietveld Refinement)

pH = 3.5

Ti-EA

Ti-TEAOH

Ti-EA

Ti-TEAOH

Ti-EA

Ti-TEAOH

140°C - 10 min

17.0

15.3

4.9 ± 0.5

4.2 ± 2.8

1.137

1.195

140°C - 30 min

29.8

44.9

3.8 ± 0.6

3.3 ± 1.0

1.252

1.252

140°C - 60 min

49.8

50.7

5.3 ± 1.1

4.6 ± 0.5

1.045

1.243

Table 1. Quantitative Rietveld analysis of the weight percentage of crystalline anatase TiO2 and crystallite sizes of the TiO2 nanoparticles obtained from precipitated Ti-EA and Ti-TEAOH nanoparticle suspensions. TEM measurements were performed on a Ti-EA nanoparticle suspensions as obtained after microwave treatment (at 140°C for 30 minutes) and purification (Fig. 7). These TEM images clearly represent the presence of aggregates (Fig. 7, a) of very small (4 - 5 nm), nearly spherical TiO2 nanoparticles. In general, these aggregates are about 100 nm in diameter. Measuring the lattice spacing resulted in an interplanar spacing of 0.352 nm (see inset Fig. 7, b), typical for the (101) plane in the anatase polymorph of titania. These results are analogous for Ti-TEAOH suspension microwaved at the same conditions. The observed agglomeration did not pose any issues related to stability of the suspensions, as stated before.


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Figure 7. TEM images collected for a Ti-EA suspensions at pH 3.5 microwaved at 140°C for 30 min: a) clustering of small titania nanoparticles with an inset of an SAED pattern corresponding to the anatase polymorph and b) HR-TEM image of anatase TiO2 nanoparticles with an inset of a 5 nm particle in which (101) lattice fringes of the anatase polymorph are clearly visible.


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Figure 8. Logarithmic plot of the reduction of MB as a function of UV irradiance time for precipitated powders from Ti-TEAOH (left) and Ti-EA (right) nanoparticle suspensions (pH = 3.5) microwaved at 140°C for 10, 30 and 60 minutes. A blank measurement is also included. The photocatalytic activity of the powders was calculated from the logarithmic plot of the pseudo first order degradation rate of a Methylene Blue (MB) solution in contact with TiO2 nano-sized powder (Fig. 8). The specific surface area of these nano-sized powders was approximately 1.0 m²/g for all samples. For each TiO2 powder, precipitated from either a Ti-TEAOH (left) or a TiEA (right) nanoparticle suspensions, the specific degradation rate constant k was calculated by performing a least-squares linear fit to all the collected data points. From these rate constants in table 2, it can be concluded that the Ti-TEAOH powders, microwaved for 30 and 60 minutes exhibit a significant faster degradation rate of the MB solution than their Ti-EA counterparts. Precipitated powders from suspensions microwaved at 140°C for 10 min or less, showed no real difference in photocatalytic activity, no matter what precursor was used. This is in accordance with the crystalline fractions calculated by Rietveld refinement (Table 1). These rate constants in


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Table 2 have been compared to a measurement with commercially available TiO2 powder (Degussa P25). It was found that the commercially available P25 powder had a rate constant of 0.02, which is a better value than the rate constants provided in Table 2. However, the purpose of this research is not to synthesize crystalline titania nano-sized powders, but to produce stable nanocrystal suspensions that can be applied in a wet chemical deposition method for the fabrication of transparent titania thin films for the photocatalytic degradation of adsorbed organic species. The MB tests on powders is only meant to illustrate the potential towards degradation of organic matter by the nanocrystals applied as a thin layer. Ti-TEAOH

Ti-EA

k

R²

k

R²

140°C – 10 min

-0.0009

0.99

-0.0009

0.98

140°C – 30 min

-0.0020

0.99

-0.0014

0.99

140°C – 60 min

-0.0026

0.99

-0.0017

0.99

Table 2. Degradation rate constants of aqueous MB solutions in contact with TiO2 powders, precipitated from Ti-TEAOH and Ti-EA nanoparticle suspensions microwaved at 140°C for increasing microwave irradiation time. Low Temperature Deposition – Proof of Concept. The synthesis route, described in this work, for stable TiO2 nanoparticle suspensions, should provide the possibility to produce optically clear and photocatalytically active thin TiO2 films on polymeric substrates. As a proof of concept, we produced transparent titania layers from TiO2 suspensions onto glass substrates using thermal processing temperatures down to 200°C. As can be seen from figure 9, the type of base used during precursor synthesis has a dramatic effect on the color of the final layer. Where TEAOH based suspensions yield colorless, transparent titania layers, the presence of ethanolamine in the Ti-EA suspensions causes a yellow-brownish color in the films processed at lower temperatures.


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This effect is attributed to the degradation of ethanolamine within the wet deposited layers in the temperature region of 200-300°C. All of the processed films were assessed for their durability by applying the Scotch® tape test as performed in literature42 (9.5 N per 25 mm of tape). All of the films remained attached to the glass substrates and no residue was found on the tape after testing.

Figure 9. Titania thin films produced on glass substrates. Left: titania layers from a Ti-EA suspension (top) were thermally processed at a) 200°C, b) 300°C, c) 400°C and d) 500°C, the layers from a Ti-TEAOH suspension (bottom) were processed at the same temperatures. Both TiEA and Ti-TEAOH suspensions were microwaved at 140°C for 15 minutes. Right: FIB-SEM cross-section of a Ti-EA layer treated at 500°C. A cross-sectional FIB-SEM analysis was performed on a Ti-EA (140°C – 15 min MW parameters) layer processed at 500°C as can be seen in figure 9, right. The layer thickness is in average 160 nm thick, uniformly coated. Furthermore, this analysis confirms the presence of porosity inside the layer, probably due to a remaining amount of peroxide and organics inside the suspension. This porosity can be detrimental for the long term stability and durability of the layers, so further research is required in order to produce more dense titania layers.


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UV–Vis spectroscopy was applied to determine the transparency of both Ti-EA and Ti-TEAOH films (Fig. 10, left and right respectively). For all samples, the steep decrease in transparency below 380 nm is due to absorption of UV light as a result of electron excitation from the valence band to the conduction band of TiO25, corresponding to the bandgap of the anatase polymorph. For the films dip-coated with the Ti-EA suspension, the average transmittance at wavelengths above 500 nm is around 80 %, except for the layer processed at 300°C. As can be seen from figure 9 (b) the 300°C processed titania coating from Ti-EA is much darker than the one processed at 200°C, thus resulting in a higher fraction of the absorbed light between 350 and 500 nm. For the coatings heated at 200°C the yellow-brown color, related to the decomposition of ethanolamine, is clearly reflected by the increased absorption between 350 and 450 nm. These results are in line with the TGA data of precipitated powders collected from Ti-EA and TiTEAOH nanoparticle suspensions (Supporting Information). When the decomposition temperature range for ethanolamine has been exceeded, at 400-500°C, the transmittance in this region starts to increase again. For the Ti-TEAOH films, the transmittance of processed films is comparable for all processing temperatures, reaching a value of 80% (Fig. 10, right). The wavy fluctuation of the transmittance curve over the entire 300–800 nm spectral region is due to interference effects which are mainly determined by the thickness of the thin film and the refractive index of the material and substrate17. The average thickness of all the films processed at different temperatures (ranging from 200 – 500°C) were assessed with FIB-SEM and are about 160 nm thick on average.


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Figure 10. UV-VIS transmittance spectra for TiO2 thin layers processed at different temperatures, synthesized from a (left) Ti-EA and (right) Ti-TEAOH nanoparticle suspensions microwaved for 15 minutes at 140°C.

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Ti-TEAOH - 300°C

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50 100 150 UV Irradiance Time (min)

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Figure 11. Logarithmic plot of the reduction of MB as a function of UV irradiance time for TiEA (left) and Ti-TEAOH (right) coatings processed at 500 and 300°C. The photodegradation rate of commercially available self-cleaning glass (Pilkington Active®) is included. Coatings heated at a temperature of 300°C or higher show a photocatalytic activity higher than the commercially available Pilkington Activ® self-cleaning glass. For both types of films, the


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photocatalytic activity of the coatings is more or less comparable for either 300 or 500°C processing temperatures (Fig. 11). This is beneficial for coatings at lower temperatures, since the photocatalytic activity remains guaranteed, thus adding to the proof of concept for low temperature deposition. Further studies are in progress to downscale processing temperatures and optimize coating and adhesion on several types of polymers, thus providing further insights into the possibility of using these coatings on heat-sensitive substrates. CONCLUSIONS Stable, aqueous suspensions containing TiO2 nanoparticles were synthesized by bottom-up synthesis from aqueous Ti4+-precursor solutions. When stored in sealed beakers at room temperature, these precursors were stable for several months. Gelation of the precursors led to the formation of stable and transparent gels. The TiO2 nanoparticles are generally small in size, ranging in the region of 4-5 nm, and exhibit an anatase crystalline fraction of 50.7 % for a TiTEAOH suspension microwaved at 140°C for 1 hour. This value is, to our knowledge, the highest reported crystalline fraction of suspendable TiO2 nanoparticles in an aqueous environment. The nanocrystal suspensions proved to be highly photocatalyticaly active towards the degradation of organic species and could be used without further processing for the deposition of photocatalytically active TiO2 thin films.

ASSOCIATED CONTENT Supporting Information Experimental details, insights towards complexation behavior and TGA analysis of TI-EA and Ti-TEAOH precipitated nano-sized powders from nanoparticle suspensions (pH = 3.5).


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +32(0)92644433. Fax: +32(0)92644983. Notes The authors declare no competing financial interest. Funding Sources One of the authors (J.W.) would like to thank the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT) for funding. ACKNOWLEDGMENTS The authors would like to thank Olivier Janssens for XRD analysis, Research group COMOC (prof. Dr. P. Van der Voort) for use of the Raman set-up and prof. dr. Van der Eycken for use of the infrared equipment. REFERENCES 1. Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Soc. Rev. 2007, 107 (7), 2891-2959. 2. Carp, O.; Huisman, C. L.; Reller, A. Photoinduced Reactivity of Titanium Dioxide. Prog. Solid State Chem. 2004, 32, 33-177. 3. Fujishima, A.; Rao, T. N.; Tryk, D. Y. Titanium Dioxide Photocatalysis. J. Photochem. Photobiol., C 2000, 1, 1-21. 4. Hashimoto, K.; Irie, H.; Fujishima, A. TiO2 Photocatalysis: A Historical Overview and Future Prospects. Jpn. J. Appl. Phys. 2005, 44 (12), 8269-8285. 5. Fujishima, A.; Zhang, X.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515-582. 6. Van de Velde, N.; Arin, M.; Lommens, P.; Poelman, D.; Van Driessche, I. Characterization of the Aqueous Peroxomethod for the Synthesis of Transparent TiO2 Thin Films. Thin Solid Films 2011, 519, 3475-3479.


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24. Duan, H.; Wang, D.; Li, Y. Green Chemistry for Nanoparticle Synthesis. Chem. Soc. Rev. 2015, 44, 5778-5792. 25. Pollefeyt, G.; Clerick, S.; Vermeir, P.; Lommens, P.; De Buysser, K.; Van Driessche, I. Influence of Aqueous Precursor Chemistry on the Growth Process of Epitaxial SrTiO3 Buffer Layers. Inorg. Chem. 2014, 53, 4913-4921. 26. Truijen, I.; Van Bael, M. K.; Van den Rul, H.; D'Haen, J.; Mullens, J. Synthesis of Thin Dense Titania Films Via an Aqueous Solution-Gel Method. J. Sol-Gel Sci. Technol. 2007, 41, 4348. 27. De Dobbelaere, C.; Mullens, J.; Hardy, A.; Van Bael, M. K. Thermal Decomposition and Spectroscopic Investigation of a New Aqueous Glycolato(-Peroxo) Ti(Iv) Solution–Gel Precursor. Thermochimica Acta 2011, Thermochim. Acta, 121-133. 28. Coelho, A. A. Topas-Academic, 4.1; Coelho Software: Brisbane, 2007. 29. Mills, A.; Hill, C.; Robertson, P. K. J. Overview of the Current Iso Tests for Photocatalytic Materials. Journal of Photochemistry and Photobiology A: Chemistry 2012, 237, 7-23. 30. Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds. 6th ed.; John Wiley & Sons: 2009. 31. Tsaramyrsi, M.; Kavousanaki, D.; Raptopoulou, C.; Terzis, A.; Salifoglou, A. Systematic Synthesis, Structural Characterization, and Reactivity Studies of Vanadium(V)-Citrate Anions [VO2(C6H6O7)]22-, Isolated from Aqueous Solutions in the Presence of Different Cations. Inorg. Chim. Acta 2001, 320, 47-59. 32. Cabaniss, S. E.; Leenheer, J. A.; McVey, I. F. Aqueous Infrared Carboxylate Absorbances: Aliphatic Di-Acids. Spectrochimica Acta Part A 1998, 54, 449-458. 33. Deacon, G.; Philips, R. Relationships between the Carbon-Oxygen Stretching Frequencies of Carboxylato Complexes and the Type of Carboxylate Coordination. Coord. Chem. Rev. 1980, 33, 227-250. 34. Tseng, C.-L.; Chen, Y.-K.; Wang, S.-H.; Peng, Z.-W.; Lin, J.-L. 2-Ethanolamine on TiO2 Investigated by in Situ Infrared Spectroscopy, Adsorption, Photochemistry, and Its Interaction with CO2. J. Phys. Chem. C 2010, 114, 11835-11843. 35. Sun, W. Theoretical Studies on Ethanolamine in Gas Phase and Solution: Conformations, Frequencies and Basicities. Int. J. Comput. Mater. Sci. Eng. 2011, 1, 55-62. 36. Kholdeeva, O.; Trubitsina, T.; Maksimovskaya, R.; Golovin, A.; Neiwert, W.; Kolesov, B.; Lopez, X.; Poblet, J. First Isolated Active Titanium Peroxo Complex: Characterization and Theoretical Study. Inorg. Chem. 2004, 43, 2284-2292. 37. Kakihana, M.; Tomita, K.; Petrykin, V.; Tada, M.; Sasaki, S.; Nakamura, Y. Chelating of Titanium by Lactic Acid in the Water-Soluble Diammonium Tris(2Hydroxypropionato)Titanate(IV). Inorg. Chem. 2004, 43, 4546-4548. 38. Vacque, V.; Sombret, B.; Huvenne, J. P.; Legrand, P.; Suc, S. Characterisation of the O-O Peroxide Bond by Vibrational Spectroscopy. Spectrochim. Acta, Part A 1997, 53, 55-66. 39. Dakanali, M.; Kefalas, E.; Raptopoulou, C.; Terzis, A.; Voyiatzis, G.; Kyrikou, I.; Mavromoustakos, T.; Salifoglou, A. A New Dinuclear Ti(IV)-Peroxo-Citrate Complex from Aqueous Solutions. Synthetic, Structural, and Spectroscopic Studies in Relevance to Aqueous Titanium(IV)-Peroxo-Citrate Speciation. Inorg. Chem. 2003, 42, 4632-4639. 40. Nour, E.; Morsy, S. Resonance Raman Studies of the Peroxotitanate(Iv) Complexes K2(Ti(O2)(SO4)2).5h2o and K2(Ti(O2)(C2O4)2).3H2O. Inorg. Chim. Acta 1986, 117, 45-48. 41. Kakihana, M.; Tada, M.; Shiro, M.; Petrykin, V.; Osada, M.; Nakamura, Y. Structure and Stability of Water Soluble (NH4)8[Ti4[C6H4O7)4(O2)4].8H2O. Inorg. Chem. 2001, 40, 891-894.


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42. Hashizume, M.; Hirashima, M. Sol–Gel Titania Coating on Unmodified and SurfaceModified Polyimide Films. J. Sol-Gel Sci. Technol. 2012, 62 (2), 234-239.


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TOC Graphic - Highly Crystalline Nanoparticle Suspensions for Low Temperature Processing of photocatalytically active TiO2 Thin Films 90x27mm (300 x 300 DPI)


Highly Crystalline Nanoparticle Suspensions for Low-Temperature

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