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Rinnakkaistallenteet Luonnontieteiden ja metsätieteiden tiedekunta
2019
Protective stainless steel micropillars for enhanced photocatalytic activity of TiO2 nanoparticles during wear
Temerov, Filipp
Elsevier BV
Tieteelliset aikakauslehtiartikkelit
© 2019 Published by Elsevier B.V.
CC BY-NC-ND https://creativecommons.org/licenses/by-nc-nd/4.0/
http://dx.doi.org/10.1016/j.surfcoat.2019.125201
https://erepo.uef.fi/handle/123456789/23993
Downloaded from University of Eastern Finland's eRepository
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Protective stainless steel micropillars for enhanced photocatalytic activity of TiO
2nanoparticles during wear
Filipp Temerov1*, Lena Ammosova1*‡, Janne Haapanen2, Jyrki M. Mäkelä2, Mika Suvanto1, and Jarkko J. Saarinen1‡
1Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland
2Aerosol Physics Laboratory, Department of Physics, Tampere University, P.O. Box 692, FI-33101 Tampere, Finland.
* Equal contribution
‡ Corresponding authors: lena.ammosova@uef.fi, jarkko.j.saarinen@uef.fi
Graphical abstract
Highlights
• Stainless steel microtextured arrays were prepared by metal injection molding (MIM)
• Liquid flame spray (LFS) was used to deposit TiO2 nanoparticles (NPs)
• MIM micropillar structures increase photocatalytic activity of TiO2 NPs
• MIM micropillars function as a weight carrying support protecting TiO2 NPs
• Micropillar TiO2 NP structures remain photocatalytically active after a wear
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Abstract
Enhanced photocatalytic activity of liquid flame spray (LFS) deposited TiO2 nanoparticles under tribological wear were investigated on metal injection molded (MIM) stainless steel substrates having micropillars as the load carrying support. A combination of LFS with MIM microtextures on the surface is a simple and cost-efficient way for manufacturing efficient photocatalytic substrates. Computer controlled microworking technique in combination with MIM was used to produce stainless steel micropillar surfaces that were functionalized by the LFS deposition of photocatalytically active TiO2 nanoparticles. The photocatalytic activity was measured in gas-phase with an in-house built photoreactor. Our results show that micropillars with controlled spacing we can control the surface area and increased photocatalytic activity of the micropillar substrate was observed compared to flat reference. The wear test confirmed that micropillars not only increase the surface area but they also provide protective support against wear whereas flat reference substrate lost the photocatalytic activity completely during wear.
Keywords
: photocatalysis, liquid flame spray (LFS), metal injection molding (MIM), TiO2nanoparticles, stainless steel, micropillar textures.
1 Introduction
Titanium dioxide (TiO2 or titania) is a well-known and widely used photocatalytic semiconductor material. TiO2 is cost-effective material with excellent properties such as photostability [1], strong oxidation power [2], non-toxicity [3], antibacterial activity [4], and low cost [5]. Three distinct polymorph phases of TiO2 exist: anatase, rutile, and brookite from which the brookite phase is known as a non-photo-active titania. Anatase has the best photocatalytic efficiency, and it has been found that anatase mixed together with a rutile phase is more photocatalytic effective compare to a single anatase phase [6]. For anatase phase the energy between the valence band (VB) and the conduction band (CB) is 3.2 eV (387 nm), which corresponds to ultraviolet A (UVA) part of the solar spectrum. Photons with higher energy than the bandgap can excite electrons from the valence band into the conduction band.
The light absorption is followed be generation of electrons and holes, which can further facilitate various reduction and oxidation reactions. Fujishima and Honda in their seminal work in 1972 [7] water splitting into hydrogen and oxygen by irradiating TiO2 electrode with UVA light in a photochemical cell. Since that discovery, the photocatalytic activity of TiO2 has been widely investigated from controlled wettability of surfaces to selective conversion of CO2 into
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solar fuels (CH4, CH3OH and HCHO) [8]. Moreover, TiO2 nanostructures have been used for production of hydrogen from water [9] and decomposition of organic molecules (pollutants) [10].
The photocatalytic activity of TiO2 nanoparticles make them suitable candidates for surface functionalization for potential applications, for example, in self-cleaning, antifouling, and biomedical applications. There exist several routes for deposition of TiO2 nanoparticles and thin films such as atomic layer deposition (ALD), [11] metal-organic chemical vapor deposition (MOCVD) [12], sol-gel processing [13], pulsed laser deposition (PLD) [14], magnetron sputtering [15], and liquid flame spray (LFS) [16] on various substrates ranging from glass [17], silicon [18], indium tin oxide [19], sapphire [20], and LaAlO3 [21] to paper [22], fluorine tin oxide (FTO) [23], and stainless steel [24].
In this paper we used liquid flame spray (LFS) for TiO2 nanoparticle deposition that is an excellent tool for depositing various metal and metal oxide nanoparticles on large-area substrates. The organometallic precursor is supplied into a high velocity and high temperature hydrogen-oxygen flame. The precursor undergoes several stages in the flame: evaporation, nucleation, and finally formation of solid nanoparticles. The deposited TiO2 nanoparticles in this paper are in anatase crystalline form and photocatalytically active: we have shown previously [25] that the temperature and nucleation mechanism in the LFS process favor the formation of anatase crystalline form that was also supported by the X-ray diffraction data [26].
By comparing with TiO2 thin film techniques such atomic layer deposition (ALD) and chemical vapor deposition (CVD), LFS method provides TiO2 nanoparticles with rather uniform and precise coverage, high deposition rate, relatively easy to handle, operation at atmospheric conditions, and cost-effective deposition suitable for roll-to-roll process flow [27,28].
Recently, LFS has been widely used in several photocatalytic applications [29-31] including doping with ions such as copper [32] and silver [33]. The LFS process parameters can easily be varied for a deposition of nanoparticles with controlled size, composition, and amount on different substrates.
Stainless steel has high strength, hardness, and corrosion resistance, which make it widely used material in a worldwide such as skyscrapers, constructions, building facades, and roofs. In big cities there are several problems related to pollution such as exhaust gases but no adequate solution has yet been presented. Stainless steel is an appropriate substrate for TiO2
functionalization [15], [21], [34]. However, deposited TiO2 nanostructures on flat surface are
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susceptible for environmental wear as they may possess low adhesion and in contact with environmental factors such as wind, rain or friction they may be completely removed. A possible and promising solution is formation of periodical texture (micropillar array) since they can both increase surface area but also provide protective support against wear.
In this paper, we show that MIM manufactured micropillar arrays on stainless increase the surface area that is also seen in a higher photocatalytic activity after the LFS deposition of TiO2
nanoparticles. We investigated the protective micropillars in combination with TiO2
nanoparticles under wear, and observed that the micropillars can prevent removal of TiO2
nanoparticles in between the pillars under wear. The photocatalytic activity was lost for the flat surfaces under wear due to complete removal of nanoparticles whereas the controlled micropillar array remained photocatalytically active as only the top of the pillars experienced the wear. The observed photocatalytic activity after the tribological wear followed the relative surface areas of the micropillar arrays.
2 Experimental
2.1 Preparation of stainless steel micropillar arrays
Ni foils (99.99 %) with thickness of 0.25 mm (Good Fellow, England) were used for preparation of the insert mold in injection molding process. Computer controlled RP-1AH microworking robot technique (Mitsubishi Electric) with the CR1 control and a feedback unit from Delta Enterprise Ltd was employed for preparation of the microtextured insert mold with a needle having a top diameter of 200 µm (Fodesco, Ltd., Finland). Tungsten carbide needle was assembled in the robot arm and used for texturing with the working speed of 1 000 mm/s.
Round shaped micropits were textured on Ni foil and overall size of the textured area was 9×41.5 mm2 that was used for further metal injection molding (MIM) process.
HAAKE® MiniLab II micro compounder (Thermo Fisher Scientific) was used for the MIM process for precipitation of hardened granulated 17-4 PH stainless steel feedstock (PolyMIM GmbH, Germany). The feedstock was injected into the mold cavity under the injection pressure of 450 bar with injection time of 8 s. The obtained green compact with micropillar arrays was followed with the solvent debinding process in distilled water bath at 60°C for 10 hours. The samples were dried for 2 hours in air conditions after the solvent debinding. After debinding, so called brown part was sintered in hydrogen atmosphere in the sintering furnace (Carbolite
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GERO, model is HTK 8 MO/16, United Kingdom). The sintering cycle was from room temperature to 600°C in 2 hours holding time, then from 600°C to 1 380°C in 8 hours hold, and finally cooling from 1 350°C to 80°C in 2 hours.
The active surface area was calculated before and after tribological wearing. The surface area before tribological wear was calculated using
𝑆 = 𝐴 ∗ 𝐵 − (𝑁1(2)−𝐷12∗ 𝜋
4 ) + (𝑁1(2)−𝐷22∗ 𝜋 4 ) + (𝑁1(2)∗ 𝜋 ∗ 𝐻 ∗ (𝐷1
2 +𝐷2 2)),
(1)
where A and B are the width and length of TiO2 deposited area (9.0 mm and 45.0 mm), N1(2)
is the number of micropillars (for high density 5 565, for low density 2 592), D1 and D2 are the bottom and top diameters of micropillars (for high density 235.8 µm and 192.0 µm, for low density 228.0 µm and 190.4 µm), H is the height of micropillars (for high density is 134.7 µm, for low density 133.9 µm). The surface area after tribological wear was calculated by
𝑆 = 𝐴 ∗ 𝐵 − (𝑁1(2)−𝐷12∗𝜋
4 ) + (𝑁1(2)−(𝐷1−𝐷0)2∗𝜋
4 ) + (𝑁1(2)∗ 𝜋 ∗ 𝐻0∗ (𝐷1
2 +𝐷0
2)), (2)
where all elements are the same as before but D0 is the top diameter of micropillars after wear (for high density is 186.9 µm, for low density 185.9 µm), and H0 is the height of micropillars after wear (for high density is 131.2 µm, for low density 130.5 µm).
For flat sample, the surface area was calculated using
𝑆 = 𝐴 ∗ 𝐵 (3)
The photocatalytic activity of TiO2 nanoparticles was assumed to be the same for the flat areas with the side walls of the pillars as also reflected in Eqs. (1-2).
2.2 Liquid flame spray (LFS) deposition of TiO
2nanoparticles
TiO2 nanoparticles were deposited using an LFS process that allows a cost-efficient deposition of various metal and metal oxide nanoparticles in atmospheric conditions on large areas [35].
LFS contains a high temperature and a high velocity flame in which an organometallic
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precursor evaporates, nucleates, and forms solid nanoparticles of the final material. In this work, TiO2 nanoparticles were deposited on stainless steel microstructures array using a rotating carousel at the Tampere University. The LFS nozzle was placed 20 cm away from the sample surface. Hydrogen and oxygen were used for the combustion gases with gas flow rates of 50 l/min and 15 l/min, respectively. The precursor, Ti (IV) isopropoxide (50 mg/ml, TTIP, Alfa Aesar, > 97%) in 2-propanol (VWR, HPLC grade), was injected with a feed rate of 2 ml/min into the flame. The micropillar arrays were exposed to the depositing flame for 15 times which corresponds to thickness of the layer in the range of 600−700 nm [36]. The yield of the process, i.e., the relative number of deposited nanoparticles, is typically in the range of 10% to 50% depending on the substrate and the process parameters [37].
2.3 Tribological wear
A tribological wear test was performed using a tribometer (TRN S/N 18-347) with a square tip (area of 10 × 10 mm2) covered with a rubber layer. Five cycles with a load of 5 N were applied for 10 s each. The pin removed first nanoparticles from the top of micropillars, but it did not reach the TiO2 nanoparticles in between the micropillars. The friction data was also collected with a ModelIX software.
2.4 Photocatalytic activity characterization
Traditionally optical dyes such as methylene blue (MB) or methyl orange (MO) in water have been used as photocatalytic activity characterization markers based on a color transformation [38]. However, such indirect methods may result in an erroneous result due to dye bleaching by the incident light [39] but they also induce mechanical stress on nanostructures at surfaces.
Therefore, photocatalytic activity is measured here by gas-phase detection i.e. mineralization of acetylene (C2H2) into carbon dioxide (CO2) using an in-house gas-phase reactor. The reactor was equipped with CO2 concentration detector (Vaisala GMP343), temperature and humidity sensor (Thorlabs TSP01), and pressure meter (Wika PGT10, USB mode). All detectors were USB connected to the PC for real-time, online measurements.
The method is based on acetylene (C2H2) oxidation into CO2 that is monitored by CO2 detector.
A mixture of acetylene and technical air was purged through reactor system for constant CO2
concentration. The ultraviolet A (UVA) excitation was performed through the UVA transparent quartz glass reactor window using a high-intensity (100W) UVA lamp (UVP Black-Ray® B- 100AP High Intensity, peak emission at 365 nm) located 17 cm above the reactor window. The UVA light excited the photocatalytic activity of TiO2 followed by decomposition of C2H2. All
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data collected from the CO2 detector was used directly without any refinement since the reaction temperature, humidity and pressure were kept constant during the measurement time.
Moreover, the in-house built gas-phase reaction mimics the potential outdoor application.
2.5 Scanning and scanning transmission electron microscopy (SEM / STEM)
The morphology and average structural sizes of TiO2 nanoparticles and stainless steel micropillars were observed with Hitachi S-4800 FE-SEM (field emission scanning electron microscope). Scanning electron microscope (SEM) images of TiO2 nanoparticles were acquired before and after tribology wearing.
Scanning transmission electron microscope (STEM) images of the nanoparticles were acquired after drying a significantly diluted sample on a copper grid coated with a Lacey carbon film.
3 Results and Discussion
To compare photocatalytic activity and to study the effect of protective micropillars three different samples were prepared: high-density micropillars, low-density micropillars, and a flat reference sample without micropillars. The photocatalytic activity of these samples was measured before and after the tribological wear. A schematic picture of the sample manufacturing and the tribological wear is shown in Fig. 1.
3.1 Characterization of composite structures before tribological wear
Figure 2 (A, D, G) shows the SEM images of high-density micropillars, low-density micropillars, and flat reference sample, respectively. The high-density micropillar array has 9 micropillars per 1 mm2, whereas low-density micropillar array has 4 micropillars per 1 mm2 and flat sample does not have micropillars. Roughness of the surface induced by TiO2
nanoparticles is clearly visible both at low and high magnifications. It is important to note that TiO2 layers form not only on perpendicular to flame surface but also around the micropillars.
The low magnification images (Fig. 1 B, E, H) show that TiO2 nanoparticles form a rather uniform and porous layer. From high magnification images (Fig. 1 C, F, I) individual nanoparticles can be observed, and the size distribution is rather high within 20-100 nm.
Moreover, some nanoparticles tend to agglomerate during nucleation in flame during LFS coating [35]. It is worth emphasizing that there is not distinct variation in nanoparticles content in all three samples and they look rather identical. A more detailed imaging was carried out
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using the STEM mode of the used microscope that is displayed in Fig. 3. STEM images confirm that there are no significant changes in the nanoparticle morphology with different substrates.
Photocatalytic activity was measured using the in-house built gas-phase photoreactor. Two identical samples were placed in the reactor chamber to amplify the signal level by increasing the active surface area. UVA light excites the sample surface, and photocatalytical decomposition of acetylene (C2H2) starts immediately once the illumination is turned on.
Results of the photocatalytic activity are displayed in Fig. 4.
Figure 4 shows a higher photocatalytic activity with high-density micropillars (15.6 ppm/h) whereas sample with low density micropillars has a bit lower activity (8.6 ppm/h). The lowest photocatalytic activity is observed for flat sample without micropillars (4.2 ppm/h). The observed increase in the photocatalytic activity is well in agreement with samples having a higher active surface area.
3.2. Characterization of composite structures after tribological wear
The nanoparticle functionalized micropillar arrays were treated by tribological wear to investigate the durability of LFS deposited TiO2 nanoparticles against wear. Tribological wear was carried out with a special square rubber tip (10×10 mm2). Figure 5 A, B show the SEM images of individual micropillars in high and low-density samples, respectively. One can clearly see that wear was performed in such a way that it removed TiO2 nanoparticles on top of micropillars but not at the bottom flat surface. This confirms the load carrying protective properties of micropillars for at least against a wear of 5N. It is noteworthy that although TiO2
nanoparticles were completely removed from top of the micropillars, they still remain untouched on the side walls of micropillars forming a ring from the top view (Figure 1 and 5).
Figure 6 displays SEM images of three samples after tribological wear. Figure 6 A, C shows micropillar array. It is observed that wearing occurred only on top of micropillars and surface below micropillars is untouched. Figure 6 E displays a flat reference sample without micropillars on which TiO2 nanoparticles are almost completely removed. The black dots and patches are worn parts of the rubber tip as there was a load on the top of tip. Under a higher magnification the surface morphology on the top of micropillars (Figure 6 B, D) and flat surface (Figure 6 F) show that the TiO2 nanoparticles were completely removed. However, due to a rough surface of the stainless steel, some nanoparticles have remained.
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Figure 7 presents three samples under a high magnification to study the nanoparticle morphology that were not removed tribological wear. The top images (A, C, E) confirm that TiO2 layers were basically completely removed, and the background stainless steel surface was observed. On the other hand, Fig. 7 (B, D) show TiO2 nanoparticles that have been flattened between the rubber tip and top surface of micropillars. It can be concluded that the nanoparticles are not any more porous and separated from each other. During tribological wear the TiO2 nanoparticles were first forced to close contact with each other and then were spread into the grooves in an imperfect stainless-steel surface.
An important parameter for photocatalytic activity studies is the active surface area. Based on the obtained diameters of the tops of micropillars and their heights from the SEM images, the results are summarized in Table 1. The active surface area of the remaining TiO2 layers after tribological wear was calculated and also presented in Table 1. There is a clear decrease in the active surface area in samples with micropillar arrays whereas for the flat reference sample the active surface area was completely removed, although there some nanoparticles remain on the surface. It is worth noting that the actual shape of the used pillars is not important whereas the increase of the surface area is reflected in the increased photocatalytic activity.
Table 1. The results of calculated active surface area before and after tribological wear.
Before (mm2) After (mm2) Decrease (%)
High-density micropillars 7.94 6.24 21.5%
Low-density micropillars 5.70 4.89 14.2%
Without micropillars 3.74 0.00 100 %
Photocatalytic activity was measured after tribological wear to investigate the influence of reduced active surface area. Figure 8 shows photocatalytic activity results of the three samples after tribological wearing. From the flat sample the active surface area was completely removed that was followed by negligible photocatalytic activity below detection limit (0.0 ppm/h). This is in good agreement with the calculated surface area. The small amount of remaining nanoparticles do not have significant effect on photocatalytic activity because of their small area with smeared structure [40]. However, we do not claim that the crystal phase of nanoparticles has been changed [41]. The photocatalytic activity results of the micropillar arrays are in good agreement with calculated active surface areas. The high-density micropillar arrays have the highest activity (10.6 ppm/h) and the low-density micropillar arrays have (7.2
10
ppm/h). For the flat reference sample without micropillars active surface area reduced from 3.74 mm2 to almost 0 mm2 (100%) and the corresponding photocatalytic activity from 4.22 ppm to 0 ppm (100%). For the high-density micropillar arrays the active surface area reduced from 7.95 mm2 to 6.24 mm2 (21.5%) and photocatalytic activity from 15.6 ppm to 10.6 ppm (32.0%) whereas for the low-density micropillar arrays the active surface area reduced from 5.70 mm2 to 4.89 mm2 (14.2%) and the corresponding photocatalytic activity from 8.6 ppm to 7.2 ppm (16.2%). These results are also summarized in Table 1.
Photons with energy greater than the band gap of TiO2 (3.2 eV) can induce generation of electron–hole pairs in the bulk that can diffuse onto surface of the TiO2 nanoparticle. Water molecules can react with oxygen in Ti-O-Ti bond forming a hydroxyl ion and a proton (eq. 3).
Electrons formed upon absorption of light can react with absorbed oxygen generating highly reactive oxygen species (ROS, eq. 4), whereas holes can interact with hydroxyl ion producing a hydroxyl ion radical (eq. 5). Finally, hydroxyl ion radical can react with absorbed acetylene C2H2 followed by degradation into CO2 and H2O (eq.6). The overall reactions (1) – (6) are presented below:
TiO2 + hν → TiO2 (e- + h+) (1)
TiO2 (e- + h+) → TiO2* + heat (2)
TiO2* + H2Oads → TiO2 + OH- + H+ (3)
e- + O2ads → O2- (4)
h+ + OH- → OH• (5)
2OH• + C2H2 +2O2→ 2CO2 + 2H2O (6)
4 Conclusion
As a summary we have successfully prepared stainless steel micropillar arrays that were functionalized with photocatalytically active TiO2 nanoparticles by LFS. The samples were exposed to tribological wear, and the photocatalytic activity was characterized before and after wear. Our results showed that the photocatalytic activity decreases after tribological wear as expected but more importantly, the micropillar arrays function as load carrying structures protecting the LFS deposited TiO2 nanoparticles between the pillars. Additionally, the micropillars also play another important role by increasing the active surface area. The high-
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density micropillar arrays had the highest photocatalytic activity with the highest active surface area whereas the flat reference sample without micropillars had the lowest activity.
The high- and low-density micropillar arrays showed a decreased photocatalytic activity after tribological wear that was consistent with the reduction of the active surface area. On the other hand, the flat reference sample without micropillars lost the photocatalytic activity completely.
This confirms that the MIM stainless steel micropillar arrays can both enhance the photocatalytic activity after being functionalized with the LFS deposited TiO2 nanoparticles but they also provide mechanical support against environmental wear. It is believed that the results of the current work can find applications in photocatalytic applications in outdoor and harsh conditions.
Acknowledgments
FT wishes to thank the Finnish Cultural Foundation for a research grant. LA and MS acknowledge the European regional development fund. JJS acknowledges the Faculty of Science and Forestry at the University of Eastern Finland for the financial support (grant no.
579/2017).
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Figure Captions
Figure 1. A schematic picture of sample preparation and tribological wear.
Figure 2. SEM images of high-density micropillars (A-C), low-density micropillars (D-F) and sample without micropillars (G-I) under different magnifications before tribological wear.
Figure 3: STEM images of (A) high-density micropillars, (B) low-density micropillars and (C) without micropillars.
Figure 4. Measured photocatalytic activity of the three prepared samples during one hour.
Figure 5. SEM images of individual high-density (A) and low-density (B) micropillars.
Figure 6. SEM images of high-density (A, B), and low-density (C, D) micropillar arrays and flat reference (E, F) without micropillars under different magnifications after tribological wear.
Figure 7. SEM images of high-density (A) and low-density (C) micropillar arrays and flat reference without micropillars (E), and samples with remained TiO2 nanoparticles high-density (B) and low-density micropillar arrays (D) and flat reference without micropillars (F) after tribological wear.
Figure 8. Measured photocatalytic activity of the three prepared samples during one hour after tribological wear.
