Photocatalytically Active Amorphous and Crystalline TiO 2 Prepared by Atomic Layer Deposition

In this work, the photocatalytic properties of amorphous and crystalline TiO2 deposited on oxide and polymer nanoparticles by atomic layer deposition (ALD) were studied. Beside TiO2, as reference, both ALD grown amorphous Al2O3 and crystalline ZnO layers were also examined. When choosing the carrier, the priority was that it had no effect on the photocatalytic activity of TiO2; therefore the oxide layers were deposited on SiO2 and poly(methyl-methacrylate) (PMMA) nanoparticles. The amorphous SiO2 particles were synthetized by the Stöber method, while the PMMA particles were prepared by emulsion polymerization. Both the bare and core/shell composite nanoparticles were investigated by SEM-EDX, TEM, FT-IR, and XRD. Finally, the photocatalytic activity of PMMA, SiO2 and the core/shell nanoparticles was measured by the decomposition of methylene orange, monitored by UV-Vis spectroscopy. Based on the results, the SiO2 was uniformly coated with the deposited oxide films, while in the case of the PMMA a contiguous web-like polymer/TiO2 matrix was formed. During the photocatalytic reactions, the amorphous Al2O3 was not active, while the crystalline ZnO and TiO2 showed good photocatalytic activity. The amorphous TiO2 deposited by ALD on the SiO2 and PMMA nanoparticles had smaller, but a clearly detectable photocatalytic effect on the photodegradation of methylene orange.


Introduction
It has long been an important issue how the energy of the sunlight can be effectively converted into chemical energy [1], and in recent years the interest in semiconductor oxide photocatalysts such as WO 3 , ZnO, and TiO 2 has grown exponentially [2].These photocatalysts are widely used in water purification [3,4], decomposition of organic contaminants [5][6][7] or in the preparation of self-cleaning surfaces [8,9].TiO 2 is the most widespread out of the semiconductor oxides in these fields, due to its low cost, good physical and chemical stability and being environmentally friendly.TiO 2 has three crystalline forms: anatase, rutile, and brookite.Since the 1960s all forms of crystalline TiO 2 have been thoroughly investigated in photocatalytic reactions, e.g. in the splitting of water and in many other reactions [10,11].It was revealed that generally anatase TiO 2 was more active than rutile [12], while brookite was not investigated thoroughly due to the difficulties in obtaining it as a pure phase.In some cases it was proved that brookite could have even higher photocatalytic activity than anatase [13]; however, its application is hindered, thanks to the complicated and relatively expensive preparations methods [14].It has also been found that the photocatalytic activity of crystalline TiO 2 can be increased and shifted to visible light by doping it with heteroatoms (e.g.C, N, S) [15,16] and other metal atoms [17,18] or by using it in composites with different semiconductor oxides [19][20][21].
Amorphous TiO 2 in itself is considered not to have any photocatalytic property, because the defects in the crystal structure can serve as recombination centers [22].Nevertheless, it was observed that if magnetron sputtered amorphous TiO 2 was coupled with other semiconductor oxides, the amorphous material alone was inactive, but the composites showed photo-reactivity and the response could be expanded to the visible region [23].Recently it was also discovered that amorphous TiO 2 grown by atomic layer deposition (ALD) on lotus leaves could show some photocatalytic activity [24].ALD is a self-limiting, sequential thin film deposition method; owing to the alternating surface-controlled reactions in the ALD reactor the growth of thin films from the gas phase is possible within nanometer range precision [25][26][27].This technique allows the coating of flat surfaces, nanoparticles and even nanofibers [28,29] with amorphous or crystalline oxide layers depending on the deposition temperature and among other applications these core/shell nanocomposites can be used as photocatalysts.
Hence, we investigated ALD grown amorphous and crystalline TiO 2 , deposited onto SiO 2 and PMMA nanoparticles and compared the photocatalytic properties.SiO 2 and PMMA were chosen as carriers, because they did not affect the course of the photocatalytic reactions, and hence the efficiency was only dependent on the TiO 2 shell.First, the carrier particles were prepared, i.e. the SiO 2 by the Stöber method, while the PMMA nanoparticles by emulsion polymerization.After that, the TiO 2 was deposited onto them, and as a reference, amorphous Al 2 O 3 and crystalline ZnO layers were also grown on the SiO 2 nanoparticles by ALD.All samples were investigated by SEM-EDX, TEM, FT-IR, and XRD.Finally, the photocatalytic activities were measured by the decomposition of methylene orange, monitored by UV-Vis spectroscopy.

Experimental
The SiO 2 nanoparticles were prepared by the Stöber method [30,31] from tetraethyl orthosilicate (TEOS).8 cm 3 TEOS was added to a mixture of 200 cm 3 ethanol and 12 cm 3 25 % ammonium hydroxide.The solution was stirred for 24 h at room temperature, during which in various hydrolysis and polycondensation steps SiO 2 nanoparticles were formed.
The poly(methyl methacrylate) (PMMA) nanoparticles were prepared by emulsion polymerization [32] from methyl methacrylate monomer, using potassium peroxodisulfate as the initiator and sodium lauryl sulfate as the emulsifier.From the methyl methacrylate 25.81 g was mixed with 180 g distilled water, 0.18 g sodium lauryl sulfate, and 0.18 g potassium peroxodisulfate.
The reaction mixture was continuously stirred at 80 °C under nitrogen atmosphere.The completion of the reaction was indicated by the disappearance of the characteristic odor of the monomer.After cooling the mixture to room temperature the polymer nanoparticles were precipitated with the addition of 400 cm 3 1 M HCl solution.The SiO 2 and PMMA particles were separated from the liquid phases by centrifuging (3000 rpm, 10 min), then those were washed 3 times with distilled water and dried in air at room temperature.
The deposition of ALD thin films on the nanoparticles was done in a Picosun SUNALE R-100 reactor.The overall pressure in the reactor chamber was around 10 mbar during all the reactions.The TiO 2 films were prepared using titanium tetraisopropoxide (TTIP) and H 2 O as precursors.Before coating the nanoparticles, it had to be determined from what deposition temperature the produced TiO 2 layer was crystalline.For this several depositions were done onto Si wafers at different temperatures from 80 °C to 300 °C (80 °C, 160 °C, 200 °C, 250 °C, 300 °C).In this case, the precursor pulse time was 0.1 s for both TTIP and H 2 O with a 3 s purge after the TTIP and a 4 s purge after the H 2 O pulses.The TiO 2 was deposited in 1000 cycles.After this, the preparation of the TiO 2 layers was done at 80 °C and 300 °C on the SiO 2 and only at 80 °C on the PMMA particles.Beside the TiO 2 coatings, as reference, Al 2 O 3 and ZnO were also grown at 80 °C and 250 °C.The Al 2 O 3 films were synthetized from trimethylaluminium (TMA) and H 2 O as precursors, while ZnO from diethylzinc (DEZ) and H 2 O.In the preparation of the nanocomposites, the TTIP, DEZ, TMA and H 2 O pulse times were all 0.5 s.The purge times were uniformly 15 s after the precursors.The TiO 2 was deposited in 250, while the Al 2 O 3 and ZnO in 100 cycles.
The composition of the samples was studied with energy dispersive X-ray (EDX) analysis in a JEOL JSM-5500LV scanning electron microscope.The measurements were done at 20 kV voltage.Before the measurement, the nanocomposites were coated with a thin Au/Pd layer in a sputter coater.
X-ray photoelectron spectra (XPS) were taken on a PHOIBOS HSA3500 100 R6 device.Before the measurement the powders were pressed to obtain pellets, the surface of the pellets were argon sputter cleaned.
The XRD patterns were recorded with a PANalytical X'pert Pro MPD X-ray diffractometer using Cu K α irradiation.
The FTIR spectra of the composites were measured by a Biorad Excalibur Series FTS 3000 FTIR spectrophotometer between 400 and 4000 cm -1 in KBr pellets.
TEM images were taken by an FEI Morgagni 268 TEM microscope.
The photocatalytic properties of the samples were studied by decomposing methyl orange (MO) dye. 1 mg of the nanocomposites was put into 3 cm 3 aqueous 0.04 mM MO solution, kept in the dark for 1 h for adsorbing the dye, then illuminated by two parallel UV backlights (Osram blacklight UV-A, 18 W) that were placed 5 cm from the samples on either side.The measured wavelength of the lamp is between 350-390 nm (maximum intensity at 375 nm) (Fig. 1), and the estimated power at the samples is 0.5 W. The UV-Vis spectra of the MO solutions were measured every 30 minutes during the photocatalytic reactions by a Jasco V-550 UV-VIS spectrophotometer.The relative absorbance values, which were plotted versus time, were determined at the 464 nm absorption peak.As a reference the samples were compared to P25 TiO 2 (Aeroxide) measured under the same conditions.

Result and Discussion
Before the preparation of the composites, the ALD deposition temperature was determined in order to grow both amorphous and crystalline TiO 2 .The layers were synthetized onto Si wafers and the crystallinity was checked by XRD.On Fig. 2 it is visible that till 200 °C only the peaks referring to crystalline Si are visible (33° and 62°, ICDD 00-040-0932), therefore the TiO 2 layer is amorphous, but from 250 °C, the pattern of anatase TiO 2 (25° and 48°, ICDD 01-083-2243) appears on the diffractogram.Based on these findings, later the deposition of TiO 2 onto the nanoparticle substrates was done at 80 °C and 300 °C in order to be able to compare the properties of the amorphous layers with that of the crystalline ones.The reference Al 2 O 3 and ZnO layers were grown at 80 °C and 250 °C.
The preparation of SiO 2 nanoparticles by sol-gel method and the synthesis of the PMMA particles by emulsion polymerization were successful.It can be seen in the TEM images (Fig. 3a-b and Fig. 4a-b) that in both cases the particles were spherical; the SiO 2 nanoparticles had a diameter of 100-150 nm (Fig. 3 a-b), while the size of the PMMA particles was 50-100 nm (Fig. 4 a-b).
After the deposition, the composites were investigated with TEM again (Fig. 3 and Fig. 4).In the case of the SiO 2 substrate (Fig. 3 c-h), it was visible that core/shell structured nanoparticles were successfully prepared, the thickness of the deposited oxide layers was about 10-20 nm in all samples.This showed that the growth-per-cycle (GPC) [34] value for these oxides vary greatly, i.e. achieving the same The EDX results (Table 1) also proved that the ALD depositions were successful; Ti, Al, and Zn could be found in the samples.The amount of Al 2 O 3 grown at different temperatures was the same, but at 250 °C smaller amount of ZnO was deposited than at 80 °C.This can be explained by that at 250 °C the upper limit of the ZnO ALD window was reached [35].The SiO 2 /TiO 2 nanocomposite sample prepared at higher temperature contained more TiO 2 than the 80 °C composite, while when the PMMA was used as carrier, the amount of deposited TiO 2 was the lowest.This can be explained by the presence of less functional groups on the surface of PMMA, than on the SiO 2 .
From the XRD patterns (Fig. 5) it can be seen that after the deposition the TiO 2 is amorphous when deposited at 80 °C and crystalline at 300 °C, but the Al 2 O 3 is amorphous and the ZnO is crystalline (zincite, ICDD 00-036-1451) at both temperatures (80 °C and 250 °C).Based on the Scherrer-equation the crystallite size of the ZnO was about 20 nm, which was in accordance with the ZnO particle size on the TEM images.This was determined for the crystalline TiO 2 layer as well, but the calculated value of about 20 nm is rather an estimate for the thickness of the   TiO 2 film, since it was a continuous layer, and not made up by individual particles like the ZnO shell.In the case of amorphous TiO 2 during the TEM measurements the electron diffraction was investigated as well to confirm that the samples did not contain nanocrystalline domains.There were no observable electron diffraction patterns, thus these samples were completely amorphous.
In the FTIR spectra (Fig. 6) of the samples containing SiO 2 , the peaks at 3410 cm -1 and 1634 cm -1 correspond to the vibrations of the surface O-H groups and the absorbed water.The 949 cm -1 is the stretching vibration of the Si-OH bond and the peaks at 1099 cm -1 , 799 cm -1 , 468 cm -1 are the stretching, bending and out of plane rocking of Si-O and Si-O-Si bonds [36,37].In the spectra of the PMMA, the 3441 cm -1 peak refers again to the adsorbed water.The peaks at 2997 cm -1 and 2951 cm -1 can be assigned to the stretching, the 1449 cm -1 peak to the bending vibrations of C-H bonds.The 1730 cm -1 peak can indicate the presence of the acrylate carboxyl group, while the 750 cm -1 one is referring to the α-methyl groups.The 1242 cm -1 and 1150 cm -1 peaks can be attributed to the C-O-C stretching vibrations.The bands at 989 cm -1 and 841 cm -1 are characteristic PMMA absorption vibrations [38,39].The TiO 2 , ZnO and Al 2 O 3 lattice vibrations are expected below 1000 cm -1 .On the spectra, the slight shift in the position of the peaks between 1000-400 cm -1 and the changes in the ratio of intensity can refer to the presence of the ALD deposited oxide.
For the samples containing TiO 2 some further measurements were carried out.The specific surface area of the samples was determined based on nitrogen adsorption/  2).After the deposition, for the PMMA/TiO 2 composite the surface area decreased slightly, which can be attributed to the change in the shape of the PMMA substrate.In the case of the SiO 2 /TiO 2 composites, at 80 °C the surface area did not differ much from the pure SiO 2 substrate, while after the 300 °C deposition there was a considerable decrease.This was possibly due to the sintering of the SiO 2 /TiO 2 particles at the higher deposition temperature.
Also the chemical composition of the ALD TiO 2 layer was further investigate by XPS measurements (Fig. 7), for this the Ti 2p peaks were decomposed by a standard nonlinear least-squares fitting method.Based on the results, next to TiO 2 , Ti was present in the form of Ti 2 O 3 suboxide phase as well on the surface of the sample.The formation of this phase on ALD layers and other TiO 2 samples was already described in the literature, and in some cases it was found to be thermodynamically favored [40,41].
The result of the photocatalytic measurements (Fig. 8) of the composites containing Al 2 O 3 and ZnO have met the expectations.The amorphous Al 2 O 3 layer had no photocatalytic property (Fig. 8a), i. e. the SiO 2 /Al 2 O 3 prepared both at 80°C and 250 °C had the same effect as the substrate SiO 2 , while the crystalline ZnO layer had good photocatalytic properties (Fig. 6b).The SiO 2 /ZnO composite that was synthetized at 80 °C had a higher efficiency, because it contained more crystalline ZnO.The SiO 2 /crystalline TiO 2 sample (Fig. 8c) had similar activity as the SiO 2 /ZnO samples.Interestingly this sample had a very low specific surface area, but a good photocatalytic efficiency.The SiO 2 /amorphous TiO 2 core/shell nanoparticles had low activity; however, it clearly had higher decomposition rate than the SiO 2 substrate, pointing to a small, but observable photocatalytic effect of amorphous TiO 2 .In the case of PMMA/amorphous TiO 2 nanocomposite (Fig. 8d), the small, but detectable photocatalytic property of amorphous TiO 2 was even more visible.This was a surprising effect, since previously it was only observed when charge separation was possible between the substrate and the coating [24,42].The photocatalytic properties of the two samples that had the highest efficiency (SiO 2 /ZnO -80 °C, SiO 2 /TiO 2 -300 °C) were compared to P25 TiO 2 (Fig. 9a).Both samples that were synthetized by ALD had a higher photocatalytic activity than the P25 TiO 2 measured under the same conditions.The apparent reaction rate constants, which were indicated by the slope of the fitted lines on Fig. 9b, were determined assuming pseudo first order reaction kinetics.The reaction rate constants in the case of the SiO 2 /ZnO -80 °C, SiO 2 /TiO 2 -300 °C samples were more than two times higher than that of the P25 TiO 2 .

Conclusion
The photocatalytic properties of amorphous and crystalline TiO 2 thin films deposited by atomic layer deposition (ALD) were studied.With ALD the amorphous and crystalline layers could be prepared with the same morphology and their photocatalytic properties could be investigated in comparison with each other.As carrier SiO 2 and PMMA nanoparticles were chosen, because those did not interact with the TiO 2 during photocatalytic reactions.About 20 nm thick TiO 2 layers were grown at different deposition temperatures (80 °C, 300 °C); and as reference, the SiO 2 nanoparticles were covered with amorphous Al 2 O 3 and crystalline ZnO as well at 80 °C and 250 °C.The TEM images revealed that the SiO 2 nanoparticles were uniformly coated with the deposited oxide films and the PMMA formed a web-like structure with the TiO 2 .During the photocatalytic reactions, the amorphous Al 2 O 3 was not active, while the crystalline ZnO and TiO 2 showed good photocatalytic activity.The amorphous TiO 2 deposited by ALD on the SiO 2 and PMMA had smaller, but clearly detectable photocatalytic effect.This was an unexpected and previously not described phenomenon.For this behavior we assume three possible reasons: (1) the coating interacts with the substrate during the photocatalytic reactions, (2) the samples contain nanocrystalline domains or (3) there is a small amount of dopant C atoms left over from the ALD precursor (TTIP) in the TiO 2 layers.In our case, between the TiO 2 and chosen carriers (amorphous SiO 2 and PMMA) there is no possibility for charge separation, and it was also confirmed by XRD and electron diffraction measurements that the samples were completely amorphous.A detailed computational chemistry investigation could provide useful answers to these questions; however, that is beyond the scope of the present work.According to the results, the synthesis of amorphous TiO 2 layers does not require high deposition temperature, so the films might be grown by ALD on heat sensitive, highly structured substrates and used for e.g. in the preparation of self-cleaning surfaces.

Fig. 2
Fig. 2 XRD results of TiO 2 deposited at different temperatures onto Si wafers Fig. 1 Emittance of the UV blacklight source

Fig. 4
Fig. 4 TEM images of a-b) the PMMA nanoparticles and c) the PMMA/ TiO 2 nanocomposites

Fig. 8
Fig. 8 Photocatalytic efficiency of the prepared nanocomposites

Table 1
Composition of the core/shell nanoparticles measured by EDX

Table 2
Specific surface area of the pure nanoparticles and the samples XPS results of the samples containing TiO 2