DOI: 10.1002/cphc.201500281
Articles
Influence of the Metal Work Function on the Photocatalytic Properties of TiO2 Layers on Metals Janna Freitag*[a] and Detlef W. Bahnemann*[a, b] The photocatalytic properties of titanium dioxide (TiO2) layers on different metal plates are investigated. The metal–semiconductor interface can be described as a Schottky contact, and is part of a depletion layer for the majority carriers in the semiconductor. Many researchers have demonstrated an increase in the photocatalytic activity, due to the formation of a metal– semiconductor contact that are obtained by deposition of small metal islands on the semiconductor. Nevertheless, the influence of a Schottky contact remains uncertain, sparking much interest in this field. The immobilization of nanoparticu-
late TiO2 layers by dip-coating on different metal substrates results in the formation of a Schottky contact. The recombination rate of photoinduced electron–hole pairs decreases at this interface provided that the thickness of the thin TiO2 layer has a similar magnitude to the depletion layer. The degradation of dichloroacetic acid in aqueous solution and of acetaldehyde in a gas mixture is investigated to obtain information concerning the influence of the metal work function of the back contact on the efficiency of the photocatalytic process.
1. Introduction The increasing global emission of environmental pollutants drives the development of clean and efficient methods for the purification of wastewater and ambient air polluted with hazardous compounds. In recent decades, the use of powdered semiconductors for the photocatalytic oxidation of organic and inorganic pollutants has gained much attention.[1] Titanium dioxide (TiO2) was one of the first photocatalytic materials to be investigated and is also one of the most promising. TiO2 is a cheap, stable and nontoxic semiconductor material that is able to photo-oxidize a wide range of the principal pollutants of the environment.[2–4] The use of TiO2—primarily in the form of suspensions—for wastewater treatment has been investigated. The recovery of the fine-powdered photocatalyst from treated water is, however, a time consuming and costly process involving filtration and finally resuspension of the photocatalyst powder. However, immobilization of the semiconductor typically causes a decrease in activity as a result of a decrease in area of the active catalytic surface.[5] It is known from electrochemical investigations that the substrate might affect the behavior of the immobilized photocatalyst. For example, at a metal–semiconductor back contact, which can in most cases be described as a Schottky contact, a depletion layer will be formed at the interface.[21] The shape and width of this area is [a] J. Freitag, Prof. Dr. D. W. Bahnemann Institut fìr Technische Chemie Leibniz Universitt Hannover Callinstr. 3, D-30167 Hannover (Germany) E-mail: [email protected] [email protected] [b] Prof. Dr. D. W. Bahnemann Laboratory for Nanocomposite Materials, Department of Photonics Faculty of Physics, Saint Petersburg State University Ulyanovskaya 3, Saint Petersburg, 198504 (Russia)
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determined by the metal work function Fm and the degree of doping of the semiconductor. If the photocatalytic layer is thin enough, that is, in the same range as the depletion area that is formed, the Schottky contact should enhance the charge-carrier lifetime, resulting in increased photocatalytic activity. The metal–semiconductor contact for an n-type semiconductor is depicted in Figure 1. If the semiconductor is illuminated with light possessing the same or higher energy than its bandgap Ebg, an electron is excited from the valence band to the conduction band. However, if Fm is higher than the electron affinity of the semiconductor Ec, which is usually the case for Schottky junctions, a potential gradient FB is formed between the lower edge of the conduction band and the Fermi level Ef. In many studies, the positive effect of such a metal–semiconductor contact has been shown by depositing nanosized is-
Figure 1. Diagram representing the Schottky contact between an n-type semiconductor and a metal. Ef is the Fermi level and VB and CB are the valence and conduction bands of the semiconductor, respectively. Evac is the vacuum energy, fs and fm are the electron affinities of the semiconductor and metal, respectively, and fb is the potential gradient formed between the lower edge of the conduction band and the Fermi level.
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Articles lands of metals Ag, Pd, Au, and Pt on nanostructured semiconductors.[6–13] Nosaka et al. studied the effect of the work function on metallized anatase TiO2 powders (particle diameter 10 mm) for the formation of ammonia from N3¢ and water by using various metals.[14] A dependence on Fm was observed, showing that the amount of ammonia formed during the reaction increases with an increasing work function of the metal. Nevertheless, the first step of the reaction is assumed to be the formation of H2, which might be dependent on the exchange current density for hydrogen evolution on the electrodes of these metals.[15] This might also explain the dependency found for the formation of ammonia. In contrast to these studies, Huang et al.[16] concluded that an ohmic contact enhances the photonic efficiency more than a Schottky contact. These authors have studied a system of single ZnO crystals with Ag or Pt islands deposited using magnetron sputtering onto the crystal. The contact properties of these Ag/ZnO and Pt/ZnO crystals were studied using photocurrent measurements and by examining the photocatalytic oxidation of rhodamine B, to show that the ohmic contact at the Ag/ZnO crystal enhances the degradation rate, whereas the Schottky contact slightly decreases it. This effect can be explained by the uniform direction of the built-in electric field Eb at the ohmic contact and the spontaneous polarization electric field Ep, which result in an enhanced lifetime of the photogenerated electron–hole pairs. At the Schottky contact the effect of Eb can be neglected due to the reversion of Ep, which increases the probability of recombination. Nevertheless, this study was performed using single crystals, which is a different system to that used in this study of nanostructured TiO2, in which a different behavior was assumed. To evaluate the influence of Fm on the photocatalytic activity, two degradation tests were performed here. The first was a test of the degradation of dichloroacetic acid (DCA, CHCl2CO2H) in aqueous solution, and the second was of acetaldehyde in the gas phase. DCA is toxic, carcinogenic and shows little biodegradability.[17–19] Acetaldehyde is one of the principal indoor air pollutants. Furthermore, it is one of the most abundant carbonyl compounds in the atmosphere. Thus, it is of great interest to degrade both compounds in environmentally compatible reactions.[20] Within the scope of this work, TiO2 thin films were prepared on different metal substrates [M(TiO2)] to evaluate the influence of the Schottky contact on their photocatalytic properties.
Figure 2. SEM image of a TiO2 layer on a titanium sheet. A uniform distribution of particles forming a thin layer is apparent.
Figure 3. SEM images of a) pure P25 nanoparticles on a conducting graphite holder and b) a P25 layer on a titanium sheet. The particle size and shape are unaffected by the preparation process.
good electrical contact between the metal and semiconductor particles.[21] Figure 3 a depicts a high-resolution SEM image of P25 TiO2 particles. From a comparison with a high-resolution SEM image of the layer made from these particles (Figure 3 b), it is apparent that the particle size and shape are not affected by the coating process. The primary particle size of the P25 powder remained at around 20–30 nm after formation of the film structure. The thicknesses of the layers were determined by gravimetry and are shown in Table 1 for the samples used for the DCA degradation. Additionally, the thickness of the films was verified using SEM cross-section measurements, showing accordance with the results obtained with gravimetry (Table 2). Due to the high degree of roughness of the metals it was not possible to determine the film thickness of these samples from
Table 1. Thickness of the TiO2 layers formed on different metals for the photodegradation tests. The thickness of each sample was determined at least three times by gravimetric measurement.
2. Results and Discussion 2.1. Characterization of the TiO2 Layers
Sample
Film thickness [nm]
The prepared TiO2 layers were characterized using scanning electron microscopy (SEM). The SEM analysis revealed the formation of homogeneous layers from the dip-coating process (Figure 2). The thin layers are optically near-transparent, exhibiting a slight milky appearance, and, as discussed below, they are highly photoactive. The prepared layers were also used for electrochemical measurements in another study, and showed
Ti(P25) Ti(UV100) Cu(P25) Cu(UV100) Al(P25) Al(UV100) steel(P25) steel(UV100)
245 13 198 17 196 30 183 9 264 10 144 0.3 242 13 124 11
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Articles Table 2. Thickness of the formed TiO2 layers on ITO—a comparison between SEM cross-section and gravimetry. The thickness of each sample was determined at least three times for each method.
Sample
Film thickness (SEM crosssection) [nm]
Film thickness (gravimetry) [nm]
ITO(P25) ITO(UV100)
123 28 145 9
128 7 141 3
SEM images. Therefore, TiO2 layers were prepared on indium tin oxide (ITO). Figure 4 shows a SEM cross-section image of a ITO(P25) layer, from which the film thickness was measured at different points. Figure 5 shows an image of an ITO(UV100) film. Table 2 shows the good agreement between the film thicknesses of the ITO(TiO2) layers obtained using gravimetry and SEM. Therefore, the film thickness of the M(TiO2) coatings can be measured using gravimetry.
Figure 6. Transmission spectra of TiO2 layers on glass measured versus uncoated glass. P25-5 and UV100-5 were dipped five times, and P25-10 and UV100-10 were dipped 10 times into their respective suspensions to obtain different film thicknesses.
Table 3. Thickness and percentage transmission at 365 nm of the TiO2 layers formed on glass. Both values were determined at least three times for each sample.
Figure 4. Typical SEM cross-section image of an ITO(P25) layer and film thickness at different points. Scale bar: 500 nm.
Figure 5. Typical SEM cross-section image of an ITO(UV100) layer and film thickness at different points. Scale bar: 500 nm.
The transmission spectra of P25 and UV100 layers on glass (Figure 6) were measured using UV/Vis spectroscopy (Cary 1000 Bio, Varian). To obtain films of different thicknesses, the glass was dipped five times into the suspensions containing P25 or UV100 to form the layers P25-5 and UV100-5, respectively, and 10 times to generate P25-10 and UV100-10 (Table 3). Consistent with the milky appearance of the layers, a region of visible light was scattered, resulting in a reduced transmission, and in the visible part of the spectra the transmission is mainly reduced due to interference.
2.2. Degradation of DCA The M(TiO2) layers were tested for their photocatalytic activity for DCA degradation in comparison with TiO2 layers prepared on glass with the P25 and UV100 powders. The concentration of DCA¢ was calculated from the added NaOH volume during the degradation under illumination (Figure 7 a) as well as from ChemPhysChem 2015, 16, 2670 – 2679
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Sample
Film thickness [nm]
Transmission [%]
P25-5 P25-10 UV100-5 UV100-10 glass
84 1 188 9 67 7 121 1 –
3.794 0.002 0.239 0.001 5.806 0.064 1.258 0.008 99.888 0.090
high-performance ion chromatography (HPIC) measurements (Figure 7 b). The DCA degradation studies were performed at pH 8, which was chosen due to the stability of the metals in the aqueous medium at this pH. At a lower pH, the TiO2 films prepared on aluminum and copper were unstable in the suspension, leading to dissolution of the TiO2 layers. The photonic efficiency x was calculated, taking into account the stoichiometry of the degradation reaction, according to Equation (1): x¼
VNA hc dc ¡ IlA dt
ð1Þ
where V represents the volume of the test solution, NA is the Avogadro constant (6.022 Õ 1023 mol¢1), h is the Planck constant (6.626 Õ 10¢34 J s), c is the speed of light (2.99 Õ 108 m s¢1), I is the light intensity (W m¢2), l is the illumination wavelength dc (nm), A is the illuminated area (m2), dt is the degradation rate. Both species—DCA and its anion DCA¢—are photocatalytically degraded according to Equations (2) and (3), respectively: hn, TiO2 HCCl2 COOHþO2 °°°! 2 CO2 þ2 Hþ þ2 Cl¢ hn, TiO
2 HCCl2 COO¢ þO2 °°°! 2 CO2 þHþ þ2 Cl¢
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Figure 8. Comparison of the photonic efficiencies (x) for the degradation of DCA at Ti(TiO2) layers. The test solution was adjusted to pH 3 (grey) or pH 8 (black).
Figure 9 shows the results obtained for the M(TiO2) layers on the various metals at pH 8. The photonic efficiency of the M(TiO2) layers showed no obvious dependency on Fm. By comparison to a previous study that reported the activity of the P25 powder in suspension, it could be shown that the M(TiO2) layers are somewhat active.[27] A comparison of the two methods reveals that the photonic efficiencies calculated from the HPIC analysis are twice or even Figure 7. Typical plots of DCA¢ concentration as a function of the irradiation time. The concentration was calculated from the volume of NaOH added (top) and the HPIC measurements (bottom). The initial degradation rate was calculated from the regression line.
Due to the low acidity constant of DCA (pKa = 1.29),[22] a low concentration of nondissociated acid can be assumed to be present in the system. Thus, only the degradation of the DCA¢ ion was taken into account. As CO2 will be mainly present as HCO3¢ and CO32¢ at pH 6.3–10.3, an additional mathematical correction term [Eqs. (4) and (5)] has to be introduced for calculating the degradation rate of DCA¢ :[23, 24] nðpHÞ ¼ 3 þ ¢
1 1 þ 1 þ 106:3¢pH 1 þ 1010:3¢pH
d½DCA¢ ¤ d½OH¢ ¤ ¼¢ dt dt ¡ nðpHÞ
ð4Þ
ð5Þ
During degradation H + is formed, resulting in an increased consumption of NaOH, which maintains a constant pH. The degradation was performed for some samples at pH 3 as well as at pH 8 for a better comparison to the values reported in the literature. The results for the Ti(TiO2) layers are shown in Figure 8. Previous studies have shown that the pH influences the photocatalytic degradation of DCA due to its effect on both the adsorption properties of the DCA¢ ion on the TiO2 surface and the redox potential of the photocatalyst.[25] The photonic efficiencies determined here for the Ti(P25) layers at pH 3 are in good agreement with results reported in the literature obtained for P25 and UV100 layers on glass.[26] ChemPhysChem 2015, 16, 2670 – 2679
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Figure 9. Photonic efficiencies (x) of the photocatalytic degradation of DCA (1 mm DCA, pH 8) of the M(TiO2) samples on metals compared with layers prepared on glass. a) M(P25) layers and b) M(UV100) layers. The substrates were titanium (Ti), aluminum (Al), steel (V2A), copper (Cu), and glass (G). The values of x were calculated from the results of the pH-stat and HPIC methods.
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Articles four times lower than those obtained using the pH-stat method. The TiO2 layers on glass showed similar activities as the M(TiO2) layers.
2.3. Photoelectrochemical Degradation of DCA The photoelectrochemical degradation of DCA was performed to gain a better understanding of the influence of Fm on the photocatalytic properties of the M(TiO2) films. The low applied bias facilitates electron transport through the barrier layer and enhances the charge-carrier separation due to the spacecharge layer. Thus, the applied bias should result in enhanced photonic efficiencies. The results for the M(P25) and M(UV100) layers are shown in Figure 10. The photonic efficiencies calculated from the HPIC analysis are shown in Figure 11. In comparison to the photonic efficiencies obtained without bias, the activity of the M(TiO2) layers is generally enhanced using a bias of 570 mV versus NHE. The efficiency of the M(TiO2) films show a strong dependency on Fm. However, the Cu(TiO2) film does not follow this trend. This can be explained by dissolution of the copper substrate [Eq. (6)]:[28] Cu þ 2 Hþ ! Cu2þ þ H2
ðE 0 ¼ 0:34 V vs: RHEÞ
ð6Þ
Figure 11. Photonic efficiencies (x) of the photoelectrocatalytic degradation of DCA (1 mm DCA, pH 8, 570 mV vs. NHE) of the M(TiO2) layers on metals. The efficiencies were calculated from the HPIC results. The samples were prepared with a) P25 and b) UV100 on titanium (Ti), aluminum (Al), steel (V2A), and copper (Cu).
which also affects the concentration of H + ions. Hence, the degradation rate could be misinterpreted due to the partial dissolution of the copper substrate.
2.4. Degradation of Acetaldehyde The TiO2 layers prepared on metal surfaces were tested for their photocatalytic acetaldehyde degradation activities and compared with pressed pure P25 and UV100 powder samples. A typical plot of the acetaldehyde concentration as a function of the time is shown in Figure 12. The photonic efficiency x is obtained by implementing the ideal gas law [Eq. (7)]: x¼
Figure 10. Photonic efficiencies (x) of the photoelectrocatalytic degradation of DCA (1 mm DCA, pH 8, 570 mV vs. NHE) of the M(TiO2) films on metals. The efficiencies were obtained from the pH-stat method with and without a bias of 570 mV versus NHE. The samples were prepared with a) P25 and b) UV100 on titanium (Ti), aluminum (Al), steel (V2A), and copper (Cu).
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_ d ¢ cl ÞpNA hc Vðc IlART
ð7Þ
where p [Pa] is the pressure of the gas, V_ the laminar volume flow (1.675 Õ 10¢5 m3 s¢1), R is the gas constant (8.314 J K¢1 mol¢1), and T [K] is the temperature. The change of concentration is obtained from the difference between the average concentration under dark conditions and under illumination. 2674
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Figure 12. Plot of the acetaldehyde concentration as a function of the time for the measurement of pure P25 powder under UV illumination (10 W m¢2). The concentrations with illumination (cl) and without (cd) were determined by using the average of the respective equilibrium concentrations (grey).
The results of the acetaldehyde degradation on the prepared photocatalyst layers were compared to those using pure TiO2 powder (Figure 13). Although the pure photocatalyst powders exhibited the highest photocatalytic activities, P25 was found to be the most active sample. The pure anatase powder UV100 was slightly less active, whereas the coatings prepared with this powder showed higher activities than those prepared with P25. Although the films prepared from UV100 were thinner than those prepared from P25 they all exhibited higher photonic efficiencies for the photocatalytic degradation of acetaldehyde. In comparison to the G(UV100) films, with the exception of the Cu(UV00) layer, the M(UV100) films were more active. In the case of the P25 films, the layers on glass and metal substrates showed approximately the same activity.
Figure 13. Photonic efficiencies (x) of the photocatalytic degradation of acetaldehyde (5 ppm, 10 W m¢2 light intensity) of the a) M(P25) and b) M(UV100) samples on metals in comparison with TiO2 films on glass and with the pure powders P25 and UV100 used for their preparation. The samples were prepared with P25 or UV100 on the different substrates, including titanium (Ti), aluminum (Al), steel (V2A), copper (Cu), and glass (G).
Table 4. Comparison of the ratio of the photonic efficiency of the degradation of DCA calculated using the pH-stat (xpH-stat) and HPIC (xHPIC) methods. The ratio xpH-stat/xHPIC is compared for the M(P25) and M(UV100) coatings for each substrate—titanium, aluminum, steel, copper and glass— and for the Ti(TiO2) samples at pH 3 and pH 8.
2.5. Discussion From the results obtained for the DCA degradation tests, the x values calculated using pH-stat and HPIC analyses were found to be different for each sample. This difference can be explained by considering the two measurement techniques themselves. As discussed by Bahnemann and co-workers,[29] the pH-stat method determines the change in concentration of dissolved and adsorbed H + and OH¢ ions. Using HPIC as the analytical tool, however, only yields information about dissolved DCA¢ molecules. Due to the higher catalytic surface of the powders in suspension this effect is more significant than for the layers with smaller surface area. In the suspensions, a typical loading of 2 g L¢1 is applied, whereas the layers only contain milligrams of TiO2. The difference depends on the substrate (Table 4). It can be shown that the ratio xpH-stat/xHPIC depends on the pH of the testing solution. Furthermore, this ratio is equal for the M(P25) and M(UV100) coatings for each substrate. This can be explained by substrate-specific DCA¢ adsorption. Due to this substrate-dependent ratio, in the followChemPhysChem 2015, 16, 2670 – 2679
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Substrate
xpH-stat/xHPIC M(P25) coatings
xpH-stat/xHPIC M(UV100) coatings
titanium (pH 3) titanium (pH 8) aluminum steel copper glass
1:1.3 3:1 4:1 2:1 2:1 4:1
1:1.2 3:1 4:1 2:1 2:1 4:1
ing we only discuss the x values obtained using the pH-stat method. The difference in the photocatalytic efficiencies observed for the various TiO2 layers during the DCA degradation test is not significant enough to determine an influence of the metal work function. Neither the pH-stat nor the HPIC results showed any significant differences for the activities of the layers on the metal substrates. This behavior can be expected
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Articles least 10 min. However, unfortunately, the build-up time of the metal oxide layer is fast (1–3 Õ 10¢4 s), that is, faster than the time needed for the coating process.[30] Hence, it was not possible to prepare samples without a metal oxide layer, which is thin but possibly sufficient to influence the width or even the formation of a depletion area in the semiconductor film. Aluminum oxide is known to be an insulator, which might reduce the depletion area. Therefore, the separation of the photoinduced charge carriers might also be decreased, leading to lower photonic efficiencies than expected. The metal oxide layer might hinder the charge-carrier exchange between the metal and the semiconductor. However, charge carriers can still migrate through tunneling by thermic electro-emission into the metal, provided that the thickness d of the metal oxide layer or isolation layer is thinner than or equal to 10 nm.[31, 32] Therefore, if d is in the same range as the diffusion length of the charge carriers, the metal oxide layer should not influence charge-carrier transport, which is given for all the substrates used (Table 5). Accordingly, the formation of a metal oxide layer is expected to influence neither the photonic efficiency of the semiconductor nor the charge-carrier separation through the metal–semiconductor contact.
Table 5. Thickness of the metal oxide layer of the metal substrates. The thickness is given for the dominant metal oxide at the surface of the metal. It is assumed, that initially CuO is formed at the copper surface in air and then Cu2O is formed.[36] The thickness of these two copper oxides is given in brackets.
Figure 14. Photonic efficiencies (x) of the photocatalytic degradation of DCA of the M(TiO2) films on metal substrates plotted as a function of the metal work function (Fm) of the substrates. The efficiencies were obtained from the pH-stat method a) without and b) with a bias of 570 mV versus NHE. The samples were prepared with P25 (&) and UV100 (~) on titanium (Ti), aluminum (Al), steel (V2A), and copper (Cu).
if one assumes that only the first particle layer is influenced by the metal contact. The photoelectrochemical degradation of DCA, on the other hand, was subject to an influence of Fm. Figure 14 shows the dependence of the photonic efficiency from Fm for both the photocatalytic (Figure 14 a) and the photoelectrochemical (Figure 14 b) degradation of DCA. As is evident from Figure 9, no significant difference was observed for the P25 and UV100 layers. Even the layers that were prepared on glass showed nearly the same activities as those on the metal substrates. The M(P25) films showed a higher activity than the M(UV100) layers. In a previous study the P25 and UV100 powders showed photonic efficiencies of approximately 0.5 % (pH 7, pH-stat method).[26, 27] The M(TiO2) coatings exhibited activities from about 0.4 to 1 %—the same order of magnitude as for the powders. Nevertheless, the UV100 powder was more active than the P25 powder at pH 3.[25] At pH 8, no significant difference between the P25 and UV100 powders and coatings was expected, although the M(P25) films showed a slightly higher activity than the M(UV100) coatings. Nevertheless, the influence of Fm on the photocatalytic properties might be reduced by a metal oxide layer, which is always present at the metal surface under atmospheric conditions. The metal oxide layer was electrochemically reduced in 1 m acetic acid–acetate buffer at ¢1.12 V versus NHE over at ChemPhysChem 2015, 16, 2670 – 2679
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Metal
Thickness of the metal oxide layer [nm]
Ref.
Oxide
titanium aluminum steel copper
1.5–10 1.5–5 3–5 3.3
[33] [34] [35] [36]
TiO2 Al2O3 – CuO (1.3 nm) and Cu2O (2 nm)
Due to the low applied bias a significant dependence of the metal back contact on the photonic efficiency can be observed (Figure 14 b). The recombination rate of the photogenerated e¢–h + pairs is reduced due to the applied bias, which enhances the photocatalytic reaction.[37] Furthermore, the threshold could be exceeded with the bias. The Cu(UV100) layer showed a high efficiency for the photoelectrocatalytic degradation of DCA, which could be explained by the applied bias. This high activity might, however, be partially explained by dissolution of copper during the degradation, as described above [Eq. (6)]. The electron transfer from TiO2 layers to aluminum and ITO has been investigated by Dai et al., who showed that the layers on aluminum form an ohmic contact, whereas the TiO2 layers on ITO form a Schottky contact.[38] Furthermore, these authors assumed that the aluminum is an electron donor, leading to a higher recombination rate of the photoinduced charge carriers. At the ohmic contact no potential barrier is present resulting in no charge separation, as is assumed for the Schottky contact. Due to the ohmic contact and the electron-donating ability of aluminum, these layers exhibit lower
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Articles activities for the degradation of organic pollutants such as DCA and acetaldehyde. In contrast to the results of DCA degradation test, an influence of the metal back contact on the photonic efficiency was clearly observed for the degradation of acetaldehyde. The P25 layers are less active than the UV100 layers, whereas the powders showed contrasting behavior (Figure 13). As discussed by Anpo and co-workers,[39] who studied the photocatalytic evolution of hydrogen on TiO2–M–Pt (M = Al, Fe, Ti, or other metals), an increasing Fm corresponds to a decreasing photocatalytic activity. This clearly indicates that the metal–semiconductor back contact for thin layers cannot be neglected when considering the photocatalytic activity of such systems. The films prepared with UV100 are thinner than those prepared with P25. Therefore, it can be assumed that the depletion area has a more positive effect, leading to higher photonic efficiencies. Regardless, for both materials an increasing activity in the order copper < steel < aluminum < titanium is obtained (Figure 15).
Figure 15. Influence of the metal work function (Fm) on the photonic efficiency (x) of the photocatalytic acetaldehyde degradation under UV irradiation. The samples were prepared with UV100 (~) and P25 (&).
Similar behavior was observed in a previous study concerning the degradation of acetaldehyde at M(TiO2) layers prepared by cold gas spraying. This coating method ensured a clean metal–semiconductor contact, due to the in situ removal of the metal oxide layer during the spraying process. Under illumination with UV light, increasing activities for the acetaldehyde degradation with a decreasing metal work function were observed.[40] The cold-gas-sprayed layer on aluminum showed the lowest activity, which was also observed in this study for the M(P25) layers. Therefore, it can be assumed that the thin intermediate metal oxide layer only slightly, or not even at all, influences the photocatalytic properties of the M(TiO2) coatings. Although the acetaldehyde degradation tests and the photoelectrochemical DCA degradation tests showed significant differences between the different layers on the metal substrates, this was not observed in the case of DCA degradation. ChemPhysChem 2015, 16, 2670 – 2679
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However, the different degradation processes should be taken into account. Moreover, these reactions might be influenced by the recombination rate of the photogenerated charge carriers, or the photocatalytic activity is determined by other pathways such as the adsorption of the pollutant on the surface of the particles. It seems that the adsorption processes of DCA¢ or other species such as Cl¢ ions at the TiO2 surface are important for the degradation mechanism. The photogenerated charge carriers do not influence the reaction rate in the same manner. Thus, it is the adsorption of DCA¢ , rather than this step, that determines the reaction rate. The influence of adsorption on the reaction rate is also indicated by the fact that the two measurements (pH-stat and HPIC) are yield different values for the reaction rate, as described above.
3. Conclusions From the three photodegradation tests performed here, it is obvious that the powders, that is, the pure catalysts or their suspensions, show higher photonic activities for the degradation of acetaldehyde or DCA, respectively, than the films, due to their higher active surface areas. Nevertheless, the obtained thin, almost transparent layers with a thickness of around 200– 300 nm showed good activities under UV light irradiation for all the test methods used. However, the DCA test is inadequate for obtaining information concerning the metal–semiconductor interface and thus the influence of the metal work function, due to the fact that the reaction rate is governed by the adsorption of DCA¢ on the semiconductor surface. In contrast, the acetaldehyde degradation test and the photoelectrochemical degradation of DCA clearly showed the influence of the metal work function of the back contact on the photocatalytic properties of the catalyst TiO2 ; with an increasing work function the efficiency of the photocatalyst decreases. In the case of aluminum, the formed ohmic contact and the electron-donating ability of the substrate lead to higher recombination rates of the photoinduced charge carriers, resulting in lower photonic efficiencies of the Al(TiO2) coatings. The metal–semiconductor contact only has an enhancing effect on the photocatalytic activity if a Schottky contact is formed.
Experimental Section Preparation of the TiO2 Films on Metal Substrates The TiO2 films were prepared by the dip-coating of different plate metals, such as titanium, aluminum, steel (V2A), and copper using suitable coating suspensions. These suspensions contained TiO2 (0.5 m; AEROXIDE TiO2 P25, Evonik, Leverkusen, Germany or Hombikat UV100, Sachtleben Chemicals, Duisburg, Germany; the former consists of a mixture of approximately 80 % anatase and 20 % rutile, whereas the latter is a pure anatase photocatalyst). The TiO2 powder was dispersed in distilled water (50 mL). Levasil 200/30 % (15 mL; Kurt Obermeier GmbH & Co. KG) was then successively added at pH 7. A total volume of 250 mL was achieved by adding methanol/ethanol (3:1) with vigorous stirring. After sonication in an ultrasound bath for 30 min, the suspension was used immediately for the dip-coating of the metal substrates. Prior to coating, the metal (10 Õ 5 and 4 Õ 4 cm2 for the acetaldehyde and DCA deg-
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Articles radation tests, respectively) was washed with acetone and distilled water. The substrates were dipped 10 times in the suspension using a stepping motor (Nanotec GmbH, Feldkirchen, Germany) at a speed of 5 mm s¢1 and a drawing speed of 3 mm s¢1. The metal was kept in the suspension for 5 s, with a waiting time of 5 s between the dips. The obtained coatings were calcined in an oven (Nabertherm LH 30/12 with a C30 program controller) by increasing the temperature from 25 to 250 8C in 2 h (1.88 8C min¢1). The temperature was maintained at 250 8C for 21 h and then subsequently decreased to room temperature over 3 h.
Scanning Electron Microscopy SEM images were obtained using a JSM-6700F microscope (JEOL, Tokyo, Japan) with a cold-cathode electron gun and a secondary electron image (SEI) and a lower electron image (LEI) detector for acquiring high-resolution images. The thin-film semiconductor layers on the metal substrates were fixed on an SEM holder. For better conductivity the interface was coated with an Ag paste. The powders were fixed on a conducting graphite block. All micrographs were acquired at an accelerating voltage of 2 kV and a working distance of 3 or 8 mm for the LEI or SEI detector, respectively.
The initial degradation rate was calculated from a curve fit. For a better comparison with values reported in the literature, some measurements were repeated at pH 3. Figure 16 shows a diagram of the experimental setup described above.
Figure 16. The experimental setup used for the degradation of DCA. The solution was purged with oxygen throughout the measurement. A water filter was used to cut out the IR-light of the Xe arc lamp. The employed potentiometric titrator is represented by Titrino.
Photoelectrochemical Degradation of DCA
Photocatalytic Tests Photodegradation of DCA DCA was used as a model compound to evaluate the photocatalytic activities of the prepared M(TiO2) layers in aqueous solutions. To maintain the ionic strength, an aqueous solution of KNO3 (10 mm; Sigma–Aldrich, 99.9 %) containing DCA (1 mm) was used. The reaction was carried out in a glass reactor equipped with a cooling jacket. The solution was vigorously stirred. The vessel was connected to a pH electrode combined with an Ag/AgCl reference electrode (Mettler Toledo InLab Expert). A pH-stat technique was used to follow the kinetics of the degradation.[23, 27, 41] The automatic dosing of aq. NaOH (0.1 m; Sigma–Aldrich, 98 %) was carried out using a Basic Titrino 794 potentiometric titrator (Metrohm, Filderstadt, Germany). In addition, at certain time intervals (0, 15, 30, 45, 60, 90, 120 min), samples were taken to determine the concentration of formed DCA¢ ions using HPIC (Dionex ICS-1000, equipped with a conductivity detector and an electro-regenerator suppressor). The column was a Dionex Ion Pac AS9-HC (2 Õ 250 mm) and the guard column was a Dionex Ion Pac AG9-HC (2 Õ 50 mm). The eluent was an aqueous solution of Na2CO3 (8 Õ 10¢3 m) and NaHCO3 (1.5 Õ 10¢3 m). The illumination unit for the degradation test was a Xe arc lamp (450 W) with an LAX 1450 lamp housing and an SVX 1450 power supply (both from Mìller Elektronik-Optik, Moosinning, Germany). The temperature was maintained at 25 8C using a thermostatic bath (Julabo, Seelbach, Germany). The details of this experimental setup have been described elsewhere.[23] The pH-stat method allows the concentration of degraded DCA to be determined from the amount of base added to the system to keep the pH constant. Prior to the test the pH of the solution was adjusted ChemPhysChem 2015, 16, 2670 – 2679
from 2.7–3 to 8 by the addition of aq. NaOH (0.1 m). The coated metal plates were then placed in the reactor containing a total volume of 250 mL. The solution was allowed to reach its adsorption equilibrium under vigorous stirring in the dark, while the pH to was adjusted 8 by the addition of NaOH. In addition to the tests with the coated metals, degradation tests of the two commercially available TiO2 powders used here on glass were performed in the same reactor.
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The photoelectrocatalytic degradation of DCA was performed as described above, with an additionally applied bias. Hence, M(TiO2) electrodes were prepared by contacting the metal with a wire using conductive silver lacquer and conductive epoxy resin. Afterwards, the contact and the metal were isolated using non-conductive epoxy resin. Only the TiO2 coating was exposed to the electrolyte. During the measurement a bias of 100 mV versus Ag/AgCl was applied using a potentiostat (Zahner, IM6). A Pt wire was used as the counter electrode and an Ag/AgCl reference electrode was used.
Acetaldehyde Degradation The photocatalytic degradation of acetaldehyde was followed by gas chromatography according to the ISO standard ISO/DIS 221972, to measure changes in the concentration of acetaldehyde.[42] As a reference, the pure powders used for the coatings—P25 or UV100—were dried for 2 h at 110 8C and then hand-pressed in a powder holder having the same size as the coatings (10 Õ 5 cm2). All samples were pretreated with UV illumination (365 nm, 10 W m¢2, Philips CLEO 100W-R lamp) for at least 16 h to decompose residual organic compounds at their surface. The photoreactor was made of poly(methyl methacrylate) (PMMA) with a transparent silica glass plate window on the side exposed to light. The distance between the sample and the glass was 5 0.5 mm. In accordance with the ISO standard, the concentration of acetaldehyde (99.9 %, Linde Group, Pullach, Germany) was 5 ppm in air with a relative humidity of 50 %, and was monitored using a gas chromatogram (SYNTECH Spectras GC 955) in the dark, until a concentration equilibrium was reached. The gas that flowed through the reactor
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Articles was collected and, in well-defined time steps of 5 min, measured using a photo-ionization detector (PID) to obtain the acetaldehyde concentration. After equilibration the degradation of acetaldehyde under UV irradiation (Philips CLEO 15W lamp, lmax = 365 nm, irradiation intensity 10 W m¢2) was monitored until a steady state was reached. Subsequent to the photoirradiation, the system was kept in the dark until the initial concentration of acetaldehyde (5 ppm) was restored. Figure 17 shows a diagram of the experimental setup described above.
Figure 17. The experimental setup used for the degradation of acetaldehyde. The stream of compressed air is split to give a relative humidity of 50 %. After passing through the mass-flow controller, the gases are mixed and then introduced into the reactor, which is made of PMMA. A filter can be applied at the top of the reactor to cut the undesired region of the light spectrum. The substrate is placed in the reactor and the light source is fixed directly above it. The gas is accumulated in a cold trap to be analyzed every 5 min in the GC/PID analyzer.
Acknowledgements The authors thank the Deutsche Forschungsgemeinschaft for their financial support (BA1137/8-2). Keywords: acetaldehyde degradation · dichloroacetic acid degradation · metal–semiconductor interfaces · photocatalysis · titanium dioxide [1] A. Kudo, Y. Miseki, Chem. Soc. Rev. 2009, 38, 253 – 278. [2] M. Anpo, Catal. Surv. Jpn. 1997, 1, 169 – 179. [3] A. Fujishima, X. Zhang, D. A. Tryk, Int. J. Hydrogen Energy 2007, 32, 2664 – 2672. [4] T. Noguchi, A. Fujishima, Environ. Sci. Technol. 1998, 32, 3831 – 3833. [5] A. Y. Shan, T. I. M. Ghazi, S. A. Rashid, Appl. Catal. A 2010, 389, 1 – 8. [6] W. Lu, S. Gao, J. Wang, J. Phys. Chem. C 2008, 112, 16792 – 16800. [7] B. Zhu, K. Li, J. Zhou, S. Wang, S. Zhang, S. Wu, W. Huang, Catal. Commun. 2008, 9, 2323 – 2326. [8] D. Lin, H. Wu, R. Zhang, W. Pan, Chem. Mater. 2009, 21, 3479 – 3484.
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[9] V. Subramanian, E. Wolf, P. V. Kamat, J. Phys. Chem. B 2001, 105, 11439 – 11446. [10] M. Jakob, H. Levanon, P. V. Kamat, Nano Lett. 2003, 3, 353 – 358. [11] A. Wood, M. Giersig, P. Mulvaney, J. Phys. Chem. B 2001, 105, 8810 – 8815. [12] J. Sarkar, V. T. John, J. He, C. Brooks, D. Gandhi, A. Nunes, G. Ramanath, A. Bose, Chem. Mater. 2008, 20, 5301 – 5306. [13] J. S, G. D. Arteaga, R. A. Daley, J. Bernardi, J. A. Anderson, J. Phys. Chem. B 2006, 110, 17090 – 17095. [14] Y. Nosaka, K. Norimatsu, H. Miyama, Chem. Phys. Lett. 1984, 106, 128 – 131. [15] H. Kita, J. Electrochem. Soc. 1966, 113, 1095 – 1111. [16] F. Yan, Y. Wang, J. Zhang, Z. Lin, J. Zheng, F. Huang, ChemSusChem 2014, 7, 101 – 104. [17] M. R. Hoffmann, S. T. Martin, W. Choi, D. W. Bahnemann, Chem. Rev. 1995, 95, 69 – 96. [18] D. F. Ollis, Environ. Sci. Technol. 1985, 19, 480 – 484. [19] D. W. Bahnemann, Nachr. Chem. Tech. Lab. 1994, 42, 378 – 388. [20] H. Kim, W. Choi, Appl. Catal. B 2007, 69, 127 – 132. [21] Janna Freitag, PhD thesis, Leibniz Universitt Hannover (Germany), 2015. [22] Dirk Bockelmann, Dissertation, Leibniz Universitt Hannover (Germany), 1993. [23] D. W. Bahnemann, D. Bockelmann, R. Goslich, Sol. Energy Mater. 1991, 24, 564 – 583. [24] C. Kormann, D. W. Bahnemann, M. R. Hoffmann, Environ. Sci. Technol. 1991, 25, 494 – 500. [25] M. Lindner, Dissertation, Leibniz Universitt Hannover (Germany), 1997. [26] J. Tschirch, Dissertation, Leibniz Universitt Hannover (Germany), 2009. [27] M. Lindner, D. W. Bahnemann, B. Hirthe, W.-D. Griebler, J. Sol. Energy Eng. 1997, 119, 120 – 125. [28] A. F. Hollemann, E. Wiberg, Lehrbuch der Anorganischen Chemie, 101st ed., Walter de Gruyter, Berlin, 1995. [29] I. Ivanova, J. Schneider, H. Gutzmann, J.-O. Kliemann, F. Grtner, T. Klassen, D. W. Bahnemann, C. B. Mendive, Catal. Today 2013, 209, 84 – 90. [30] M. Peters, C. Leyens, Titan und Titanlegierungen, Wiley-VCH, Weinheim, 2002. [31] Z. Jian-Jun, W. Bo, J. Ruo-Lian, Li. Cheng-Xiang, J. Xiao-Li, Xi. Zi-Li, C. Dun-Jun, H. Ping, Z. Rong, Z. You-Dou, Chin. Phys. 2007, 16, 2120 – 2122. [32] X. Zhang, Y. L. Chen, R.-S. Liu, D. P. Tsai, Rep. Prog. Phys. 2013, 76, 046401. [33] Y.-T. Sul, C. B. Johansson, Y. Jeong, T. Albrektsson, Med. Eng. Phys. 2001, 23, 329 – 346. [34] M. T. A. Saif, S. Zhang, A. Haque, K. J. Hsia, Acta Mater. 2002, 50, 2779 – 2786. [35] M. V. Diamanti, M. P. Pedeferri, Corros. Sci. 2007, 49, 939 – 948. [36] P. Keil, D. Lìtzenkirchen-Hecht, R. Frahm, AIP Conf. Proc. 2007, 882, 490 – 492. [37] D. Jiang, H. Zhao, S. Zhang, R. John, J. Phys. Chem. B 2003, 107, 12774 – 12780. [38] W. Dai, X. Wang, P. Liu, Y. Xu, G. Li, X. Fu, J. Phys. Chem. B 2006, 110, 13470 – 13476. [39] M. Kitano, K. Tsujimaru, M. Anpo, Appl. Catal. A 2006, 314, 179 – 183. [40] J. Freitag, D. W. Bahnemann, Phys. Status Solidi RRL 2014, 8, 596 – 599. [41] M. Lindner, D. W. Bahnemann, B. Hirthe, W.-D. Griebler, WLB Wasser Luft Boden 1994, 11, 38 – 44. [42] International Organization for Standardization. Fine ceramics (advanced ceramics, advanced technical ceramics)—Test method for air-purification performance of semiconducting photocatalytic materials—Part 2: Removal of acetaldehyde, 2011, ISO: 22197-2. Received: April 1, 2015 Published online on June 26, 2015
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Influence of the Metal Work Function on the Photocatalytic Properties of TiO2 Layers on Metals Janna Freitag*[a] and Detlef W. Bahnemann*[a, b] The photocatalytic properties of titanium dioxide (TiO2) layers on different metal plates are investigated. The metal–semiconductor interface can be described as a Schottky contact, and is part of a depletion layer for the majority carriers in the semiconductor. Many researchers have demonstrated an increase in the photocatalytic activity, due to the formation of a metal– semiconductor contact that are obtained by deposition of small metal islands on the semiconductor. Nevertheless, the influence of a Schottky contact remains uncertain, sparking much interest in this field. The immobilization of nanoparticu-
late TiO2 layers by dip-coating on different metal substrates results in the formation of a Schottky contact. The recombination rate of photoinduced electron–hole pairs decreases at this interface provided that the thickness of the thin TiO2 layer has a similar magnitude to the depletion layer. The degradation of dichloroacetic acid in aqueous solution and of acetaldehyde in a gas mixture is investigated to obtain information concerning the influence of the metal work function of the back contact on the efficiency of the photocatalytic process.
1. Introduction The increasing global emission of environmental pollutants drives the development of clean and efficient methods for the purification of wastewater and ambient air polluted with hazardous compounds. In recent decades, the use of powdered semiconductors for the photocatalytic oxidation of organic and inorganic pollutants has gained much attention.[1] Titanium dioxide (TiO2) was one of the first photocatalytic materials to be investigated and is also one of the most promising. TiO2 is a cheap, stable and nontoxic semiconductor material that is able to photo-oxidize a wide range of the principal pollutants of the environment.[2–4] The use of TiO2—primarily in the form of suspensions—for wastewater treatment has been investigated. The recovery of the fine-powdered photocatalyst from treated water is, however, a time consuming and costly process involving filtration and finally resuspension of the photocatalyst powder. However, immobilization of the semiconductor typically causes a decrease in activity as a result of a decrease in area of the active catalytic surface.[5] It is known from electrochemical investigations that the substrate might affect the behavior of the immobilized photocatalyst. For example, at a metal–semiconductor back contact, which can in most cases be described as a Schottky contact, a depletion layer will be formed at the interface.[21] The shape and width of this area is [a] J. Freitag, Prof. Dr. D. W. Bahnemann Institut fìr Technische Chemie Leibniz Universitt Hannover Callinstr. 3, D-30167 Hannover (Germany) E-mail: [email protected] [email protected] [b] Prof. Dr. D. W. Bahnemann Laboratory for Nanocomposite Materials, Department of Photonics Faculty of Physics, Saint Petersburg State University Ulyanovskaya 3, Saint Petersburg, 198504 (Russia)
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determined by the metal work function Fm and the degree of doping of the semiconductor. If the photocatalytic layer is thin enough, that is, in the same range as the depletion area that is formed, the Schottky contact should enhance the charge-carrier lifetime, resulting in increased photocatalytic activity. The metal–semiconductor contact for an n-type semiconductor is depicted in Figure 1. If the semiconductor is illuminated with light possessing the same or higher energy than its bandgap Ebg, an electron is excited from the valence band to the conduction band. However, if Fm is higher than the electron affinity of the semiconductor Ec, which is usually the case for Schottky junctions, a potential gradient FB is formed between the lower edge of the conduction band and the Fermi level Ef. In many studies, the positive effect of such a metal–semiconductor contact has been shown by depositing nanosized is-
Figure 1. Diagram representing the Schottky contact between an n-type semiconductor and a metal. Ef is the Fermi level and VB and CB are the valence and conduction bands of the semiconductor, respectively. Evac is the vacuum energy, fs and fm are the electron affinities of the semiconductor and metal, respectively, and fb is the potential gradient formed between the lower edge of the conduction band and the Fermi level.
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Articles lands of metals Ag, Pd, Au, and Pt on nanostructured semiconductors.[6–13] Nosaka et al. studied the effect of the work function on metallized anatase TiO2 powders (particle diameter 10 mm) for the formation of ammonia from N3¢ and water by using various metals.[14] A dependence on Fm was observed, showing that the amount of ammonia formed during the reaction increases with an increasing work function of the metal. Nevertheless, the first step of the reaction is assumed to be the formation of H2, which might be dependent on the exchange current density for hydrogen evolution on the electrodes of these metals.[15] This might also explain the dependency found for the formation of ammonia. In contrast to these studies, Huang et al.[16] concluded that an ohmic contact enhances the photonic efficiency more than a Schottky contact. These authors have studied a system of single ZnO crystals with Ag or Pt islands deposited using magnetron sputtering onto the crystal. The contact properties of these Ag/ZnO and Pt/ZnO crystals were studied using photocurrent measurements and by examining the photocatalytic oxidation of rhodamine B, to show that the ohmic contact at the Ag/ZnO crystal enhances the degradation rate, whereas the Schottky contact slightly decreases it. This effect can be explained by the uniform direction of the built-in electric field Eb at the ohmic contact and the spontaneous polarization electric field Ep, which result in an enhanced lifetime of the photogenerated electron–hole pairs. At the Schottky contact the effect of Eb can be neglected due to the reversion of Ep, which increases the probability of recombination. Nevertheless, this study was performed using single crystals, which is a different system to that used in this study of nanostructured TiO2, in which a different behavior was assumed. To evaluate the influence of Fm on the photocatalytic activity, two degradation tests were performed here. The first was a test of the degradation of dichloroacetic acid (DCA, CHCl2CO2H) in aqueous solution, and the second was of acetaldehyde in the gas phase. DCA is toxic, carcinogenic and shows little biodegradability.[17–19] Acetaldehyde is one of the principal indoor air pollutants. Furthermore, it is one of the most abundant carbonyl compounds in the atmosphere. Thus, it is of great interest to degrade both compounds in environmentally compatible reactions.[20] Within the scope of this work, TiO2 thin films were prepared on different metal substrates [M(TiO2)] to evaluate the influence of the Schottky contact on their photocatalytic properties.
Figure 2. SEM image of a TiO2 layer on a titanium sheet. A uniform distribution of particles forming a thin layer is apparent.
Figure 3. SEM images of a) pure P25 nanoparticles on a conducting graphite holder and b) a P25 layer on a titanium sheet. The particle size and shape are unaffected by the preparation process.
good electrical contact between the metal and semiconductor particles.[21] Figure 3 a depicts a high-resolution SEM image of P25 TiO2 particles. From a comparison with a high-resolution SEM image of the layer made from these particles (Figure 3 b), it is apparent that the particle size and shape are not affected by the coating process. The primary particle size of the P25 powder remained at around 20–30 nm after formation of the film structure. The thicknesses of the layers were determined by gravimetry and are shown in Table 1 for the samples used for the DCA degradation. Additionally, the thickness of the films was verified using SEM cross-section measurements, showing accordance with the results obtained with gravimetry (Table 2). Due to the high degree of roughness of the metals it was not possible to determine the film thickness of these samples from
Table 1. Thickness of the TiO2 layers formed on different metals for the photodegradation tests. The thickness of each sample was determined at least three times by gravimetric measurement.
2. Results and Discussion 2.1. Characterization of the TiO2 Layers
Sample
Film thickness [nm]
The prepared TiO2 layers were characterized using scanning electron microscopy (SEM). The SEM analysis revealed the formation of homogeneous layers from the dip-coating process (Figure 2). The thin layers are optically near-transparent, exhibiting a slight milky appearance, and, as discussed below, they are highly photoactive. The prepared layers were also used for electrochemical measurements in another study, and showed
Ti(P25) Ti(UV100) Cu(P25) Cu(UV100) Al(P25) Al(UV100) steel(P25) steel(UV100)
245 13 198 17 196 30 183 9 264 10 144 0.3 242 13 124 11
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Sample
Film thickness (SEM crosssection) [nm]
Film thickness (gravimetry) [nm]
ITO(P25) ITO(UV100)
123 28 145 9
128 7 141 3
SEM images. Therefore, TiO2 layers were prepared on indium tin oxide (ITO). Figure 4 shows a SEM cross-section image of a ITO(P25) layer, from which the film thickness was measured at different points. Figure 5 shows an image of an ITO(UV100) film. Table 2 shows the good agreement between the film thicknesses of the ITO(TiO2) layers obtained using gravimetry and SEM. Therefore, the film thickness of the M(TiO2) coatings can be measured using gravimetry.
Figure 6. Transmission spectra of TiO2 layers on glass measured versus uncoated glass. P25-5 and UV100-5 were dipped five times, and P25-10 and UV100-10 were dipped 10 times into their respective suspensions to obtain different film thicknesses.
Table 3. Thickness and percentage transmission at 365 nm of the TiO2 layers formed on glass. Both values were determined at least three times for each sample.
Figure 4. Typical SEM cross-section image of an ITO(P25) layer and film thickness at different points. Scale bar: 500 nm.
Figure 5. Typical SEM cross-section image of an ITO(UV100) layer and film thickness at different points. Scale bar: 500 nm.
The transmission spectra of P25 and UV100 layers on glass (Figure 6) were measured using UV/Vis spectroscopy (Cary 1000 Bio, Varian). To obtain films of different thicknesses, the glass was dipped five times into the suspensions containing P25 or UV100 to form the layers P25-5 and UV100-5, respectively, and 10 times to generate P25-10 and UV100-10 (Table 3). Consistent with the milky appearance of the layers, a region of visible light was scattered, resulting in a reduced transmission, and in the visible part of the spectra the transmission is mainly reduced due to interference.
2.2. Degradation of DCA The M(TiO2) layers were tested for their photocatalytic activity for DCA degradation in comparison with TiO2 layers prepared on glass with the P25 and UV100 powders. The concentration of DCA¢ was calculated from the added NaOH volume during the degradation under illumination (Figure 7 a) as well as from ChemPhysChem 2015, 16, 2670 – 2679
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Sample
Film thickness [nm]
Transmission [%]
P25-5 P25-10 UV100-5 UV100-10 glass
84 1 188 9 67 7 121 1 –
3.794 0.002 0.239 0.001 5.806 0.064 1.258 0.008 99.888 0.090
high-performance ion chromatography (HPIC) measurements (Figure 7 b). The DCA degradation studies were performed at pH 8, which was chosen due to the stability of the metals in the aqueous medium at this pH. At a lower pH, the TiO2 films prepared on aluminum and copper were unstable in the suspension, leading to dissolution of the TiO2 layers. The photonic efficiency x was calculated, taking into account the stoichiometry of the degradation reaction, according to Equation (1): x¼
VNA hc dc ¡ IlA dt
ð1Þ
where V represents the volume of the test solution, NA is the Avogadro constant (6.022 Õ 1023 mol¢1), h is the Planck constant (6.626 Õ 10¢34 J s), c is the speed of light (2.99 Õ 108 m s¢1), I is the light intensity (W m¢2), l is the illumination wavelength dc (nm), A is the illuminated area (m2), dt is the degradation rate. Both species—DCA and its anion DCA¢—are photocatalytically degraded according to Equations (2) and (3), respectively: hn, TiO2 HCCl2 COOHþO2 °°°! 2 CO2 þ2 Hþ þ2 Cl¢ hn, TiO
2 HCCl2 COO¢ þO2 °°°! 2 CO2 þHþ þ2 Cl¢
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Figure 8. Comparison of the photonic efficiencies (x) for the degradation of DCA at Ti(TiO2) layers. The test solution was adjusted to pH 3 (grey) or pH 8 (black).
Figure 9 shows the results obtained for the M(TiO2) layers on the various metals at pH 8. The photonic efficiency of the M(TiO2) layers showed no obvious dependency on Fm. By comparison to a previous study that reported the activity of the P25 powder in suspension, it could be shown that the M(TiO2) layers are somewhat active.[27] A comparison of the two methods reveals that the photonic efficiencies calculated from the HPIC analysis are twice or even Figure 7. Typical plots of DCA¢ concentration as a function of the irradiation time. The concentration was calculated from the volume of NaOH added (top) and the HPIC measurements (bottom). The initial degradation rate was calculated from the regression line.
Due to the low acidity constant of DCA (pKa = 1.29),[22] a low concentration of nondissociated acid can be assumed to be present in the system. Thus, only the degradation of the DCA¢ ion was taken into account. As CO2 will be mainly present as HCO3¢ and CO32¢ at pH 6.3–10.3, an additional mathematical correction term [Eqs. (4) and (5)] has to be introduced for calculating the degradation rate of DCA¢ :[23, 24] nðpHÞ ¼ 3 þ ¢
1 1 þ 1 þ 106:3¢pH 1 þ 1010:3¢pH
d½DCA¢ ¤ d½OH¢ ¤ ¼¢ dt dt ¡ nðpHÞ
ð4Þ
ð5Þ
During degradation H + is formed, resulting in an increased consumption of NaOH, which maintains a constant pH. The degradation was performed for some samples at pH 3 as well as at pH 8 for a better comparison to the values reported in the literature. The results for the Ti(TiO2) layers are shown in Figure 8. Previous studies have shown that the pH influences the photocatalytic degradation of DCA due to its effect on both the adsorption properties of the DCA¢ ion on the TiO2 surface and the redox potential of the photocatalyst.[25] The photonic efficiencies determined here for the Ti(P25) layers at pH 3 are in good agreement with results reported in the literature obtained for P25 and UV100 layers on glass.[26] ChemPhysChem 2015, 16, 2670 – 2679
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Figure 9. Photonic efficiencies (x) of the photocatalytic degradation of DCA (1 mm DCA, pH 8) of the M(TiO2) samples on metals compared with layers prepared on glass. a) M(P25) layers and b) M(UV100) layers. The substrates were titanium (Ti), aluminum (Al), steel (V2A), copper (Cu), and glass (G). The values of x were calculated from the results of the pH-stat and HPIC methods.
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Articles four times lower than those obtained using the pH-stat method. The TiO2 layers on glass showed similar activities as the M(TiO2) layers.
2.3. Photoelectrochemical Degradation of DCA The photoelectrochemical degradation of DCA was performed to gain a better understanding of the influence of Fm on the photocatalytic properties of the M(TiO2) films. The low applied bias facilitates electron transport through the barrier layer and enhances the charge-carrier separation due to the spacecharge layer. Thus, the applied bias should result in enhanced photonic efficiencies. The results for the M(P25) and M(UV100) layers are shown in Figure 10. The photonic efficiencies calculated from the HPIC analysis are shown in Figure 11. In comparison to the photonic efficiencies obtained without bias, the activity of the M(TiO2) layers is generally enhanced using a bias of 570 mV versus NHE. The efficiency of the M(TiO2) films show a strong dependency on Fm. However, the Cu(TiO2) film does not follow this trend. This can be explained by dissolution of the copper substrate [Eq. (6)]:[28] Cu þ 2 Hþ ! Cu2þ þ H2
ðE 0 ¼ 0:34 V vs: RHEÞ
ð6Þ
Figure 11. Photonic efficiencies (x) of the photoelectrocatalytic degradation of DCA (1 mm DCA, pH 8, 570 mV vs. NHE) of the M(TiO2) layers on metals. The efficiencies were calculated from the HPIC results. The samples were prepared with a) P25 and b) UV100 on titanium (Ti), aluminum (Al), steel (V2A), and copper (Cu).
which also affects the concentration of H + ions. Hence, the degradation rate could be misinterpreted due to the partial dissolution of the copper substrate.
2.4. Degradation of Acetaldehyde The TiO2 layers prepared on metal surfaces were tested for their photocatalytic acetaldehyde degradation activities and compared with pressed pure P25 and UV100 powder samples. A typical plot of the acetaldehyde concentration as a function of the time is shown in Figure 12. The photonic efficiency x is obtained by implementing the ideal gas law [Eq. (7)]: x¼
Figure 10. Photonic efficiencies (x) of the photoelectrocatalytic degradation of DCA (1 mm DCA, pH 8, 570 mV vs. NHE) of the M(TiO2) films on metals. The efficiencies were obtained from the pH-stat method with and without a bias of 570 mV versus NHE. The samples were prepared with a) P25 and b) UV100 on titanium (Ti), aluminum (Al), steel (V2A), and copper (Cu).
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_ d ¢ cl ÞpNA hc Vðc IlART
ð7Þ
where p [Pa] is the pressure of the gas, V_ the laminar volume flow (1.675 Õ 10¢5 m3 s¢1), R is the gas constant (8.314 J K¢1 mol¢1), and T [K] is the temperature. The change of concentration is obtained from the difference between the average concentration under dark conditions and under illumination. 2674
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Figure 12. Plot of the acetaldehyde concentration as a function of the time for the measurement of pure P25 powder under UV illumination (10 W m¢2). The concentrations with illumination (cl) and without (cd) were determined by using the average of the respective equilibrium concentrations (grey).
The results of the acetaldehyde degradation on the prepared photocatalyst layers were compared to those using pure TiO2 powder (Figure 13). Although the pure photocatalyst powders exhibited the highest photocatalytic activities, P25 was found to be the most active sample. The pure anatase powder UV100 was slightly less active, whereas the coatings prepared with this powder showed higher activities than those prepared with P25. Although the films prepared from UV100 were thinner than those prepared from P25 they all exhibited higher photonic efficiencies for the photocatalytic degradation of acetaldehyde. In comparison to the G(UV100) films, with the exception of the Cu(UV00) layer, the M(UV100) films were more active. In the case of the P25 films, the layers on glass and metal substrates showed approximately the same activity.
Figure 13. Photonic efficiencies (x) of the photocatalytic degradation of acetaldehyde (5 ppm, 10 W m¢2 light intensity) of the a) M(P25) and b) M(UV100) samples on metals in comparison with TiO2 films on glass and with the pure powders P25 and UV100 used for their preparation. The samples were prepared with P25 or UV100 on the different substrates, including titanium (Ti), aluminum (Al), steel (V2A), copper (Cu), and glass (G).
Table 4. Comparison of the ratio of the photonic efficiency of the degradation of DCA calculated using the pH-stat (xpH-stat) and HPIC (xHPIC) methods. The ratio xpH-stat/xHPIC is compared for the M(P25) and M(UV100) coatings for each substrate—titanium, aluminum, steel, copper and glass— and for the Ti(TiO2) samples at pH 3 and pH 8.
2.5. Discussion From the results obtained for the DCA degradation tests, the x values calculated using pH-stat and HPIC analyses were found to be different for each sample. This difference can be explained by considering the two measurement techniques themselves. As discussed by Bahnemann and co-workers,[29] the pH-stat method determines the change in concentration of dissolved and adsorbed H + and OH¢ ions. Using HPIC as the analytical tool, however, only yields information about dissolved DCA¢ molecules. Due to the higher catalytic surface of the powders in suspension this effect is more significant than for the layers with smaller surface area. In the suspensions, a typical loading of 2 g L¢1 is applied, whereas the layers only contain milligrams of TiO2. The difference depends on the substrate (Table 4). It can be shown that the ratio xpH-stat/xHPIC depends on the pH of the testing solution. Furthermore, this ratio is equal for the M(P25) and M(UV100) coatings for each substrate. This can be explained by substrate-specific DCA¢ adsorption. Due to this substrate-dependent ratio, in the followChemPhysChem 2015, 16, 2670 – 2679
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Substrate
xpH-stat/xHPIC M(P25) coatings
xpH-stat/xHPIC M(UV100) coatings
titanium (pH 3) titanium (pH 8) aluminum steel copper glass
1:1.3 3:1 4:1 2:1 2:1 4:1
1:1.2 3:1 4:1 2:1 2:1 4:1
ing we only discuss the x values obtained using the pH-stat method. The difference in the photocatalytic efficiencies observed for the various TiO2 layers during the DCA degradation test is not significant enough to determine an influence of the metal work function. Neither the pH-stat nor the HPIC results showed any significant differences for the activities of the layers on the metal substrates. This behavior can be expected
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Articles least 10 min. However, unfortunately, the build-up time of the metal oxide layer is fast (1–3 Õ 10¢4 s), that is, faster than the time needed for the coating process.[30] Hence, it was not possible to prepare samples without a metal oxide layer, which is thin but possibly sufficient to influence the width or even the formation of a depletion area in the semiconductor film. Aluminum oxide is known to be an insulator, which might reduce the depletion area. Therefore, the separation of the photoinduced charge carriers might also be decreased, leading to lower photonic efficiencies than expected. The metal oxide layer might hinder the charge-carrier exchange between the metal and the semiconductor. However, charge carriers can still migrate through tunneling by thermic electro-emission into the metal, provided that the thickness d of the metal oxide layer or isolation layer is thinner than or equal to 10 nm.[31, 32] Therefore, if d is in the same range as the diffusion length of the charge carriers, the metal oxide layer should not influence charge-carrier transport, which is given for all the substrates used (Table 5). Accordingly, the formation of a metal oxide layer is expected to influence neither the photonic efficiency of the semiconductor nor the charge-carrier separation through the metal–semiconductor contact.
Table 5. Thickness of the metal oxide layer of the metal substrates. The thickness is given for the dominant metal oxide at the surface of the metal. It is assumed, that initially CuO is formed at the copper surface in air and then Cu2O is formed.[36] The thickness of these two copper oxides is given in brackets.
Figure 14. Photonic efficiencies (x) of the photocatalytic degradation of DCA of the M(TiO2) films on metal substrates plotted as a function of the metal work function (Fm) of the substrates. The efficiencies were obtained from the pH-stat method a) without and b) with a bias of 570 mV versus NHE. The samples were prepared with P25 (&) and UV100 (~) on titanium (Ti), aluminum (Al), steel (V2A), and copper (Cu).
if one assumes that only the first particle layer is influenced by the metal contact. The photoelectrochemical degradation of DCA, on the other hand, was subject to an influence of Fm. Figure 14 shows the dependence of the photonic efficiency from Fm for both the photocatalytic (Figure 14 a) and the photoelectrochemical (Figure 14 b) degradation of DCA. As is evident from Figure 9, no significant difference was observed for the P25 and UV100 layers. Even the layers that were prepared on glass showed nearly the same activities as those on the metal substrates. The M(P25) films showed a higher activity than the M(UV100) layers. In a previous study the P25 and UV100 powders showed photonic efficiencies of approximately 0.5 % (pH 7, pH-stat method).[26, 27] The M(TiO2) coatings exhibited activities from about 0.4 to 1 %—the same order of magnitude as for the powders. Nevertheless, the UV100 powder was more active than the P25 powder at pH 3.[25] At pH 8, no significant difference between the P25 and UV100 powders and coatings was expected, although the M(P25) films showed a slightly higher activity than the M(UV100) coatings. Nevertheless, the influence of Fm on the photocatalytic properties might be reduced by a metal oxide layer, which is always present at the metal surface under atmospheric conditions. The metal oxide layer was electrochemically reduced in 1 m acetic acid–acetate buffer at ¢1.12 V versus NHE over at ChemPhysChem 2015, 16, 2670 – 2679
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Metal
Thickness of the metal oxide layer [nm]
Ref.
Oxide
titanium aluminum steel copper
1.5–10 1.5–5 3–5 3.3
[33] [34] [35] [36]
TiO2 Al2O3 – CuO (1.3 nm) and Cu2O (2 nm)
Due to the low applied bias a significant dependence of the metal back contact on the photonic efficiency can be observed (Figure 14 b). The recombination rate of the photogenerated e¢–h + pairs is reduced due to the applied bias, which enhances the photocatalytic reaction.[37] Furthermore, the threshold could be exceeded with the bias. The Cu(UV100) layer showed a high efficiency for the photoelectrocatalytic degradation of DCA, which could be explained by the applied bias. This high activity might, however, be partially explained by dissolution of copper during the degradation, as described above [Eq. (6)]. The electron transfer from TiO2 layers to aluminum and ITO has been investigated by Dai et al., who showed that the layers on aluminum form an ohmic contact, whereas the TiO2 layers on ITO form a Schottky contact.[38] Furthermore, these authors assumed that the aluminum is an electron donor, leading to a higher recombination rate of the photoinduced charge carriers. At the ohmic contact no potential barrier is present resulting in no charge separation, as is assumed for the Schottky contact. Due to the ohmic contact and the electron-donating ability of aluminum, these layers exhibit lower
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Articles activities for the degradation of organic pollutants such as DCA and acetaldehyde. In contrast to the results of DCA degradation test, an influence of the metal back contact on the photonic efficiency was clearly observed for the degradation of acetaldehyde. The P25 layers are less active than the UV100 layers, whereas the powders showed contrasting behavior (Figure 13). As discussed by Anpo and co-workers,[39] who studied the photocatalytic evolution of hydrogen on TiO2–M–Pt (M = Al, Fe, Ti, or other metals), an increasing Fm corresponds to a decreasing photocatalytic activity. This clearly indicates that the metal–semiconductor back contact for thin layers cannot be neglected when considering the photocatalytic activity of such systems. The films prepared with UV100 are thinner than those prepared with P25. Therefore, it can be assumed that the depletion area has a more positive effect, leading to higher photonic efficiencies. Regardless, for both materials an increasing activity in the order copper < steel < aluminum < titanium is obtained (Figure 15).
Figure 15. Influence of the metal work function (Fm) on the photonic efficiency (x) of the photocatalytic acetaldehyde degradation under UV irradiation. The samples were prepared with UV100 (~) and P25 (&).
Similar behavior was observed in a previous study concerning the degradation of acetaldehyde at M(TiO2) layers prepared by cold gas spraying. This coating method ensured a clean metal–semiconductor contact, due to the in situ removal of the metal oxide layer during the spraying process. Under illumination with UV light, increasing activities for the acetaldehyde degradation with a decreasing metal work function were observed.[40] The cold-gas-sprayed layer on aluminum showed the lowest activity, which was also observed in this study for the M(P25) layers. Therefore, it can be assumed that the thin intermediate metal oxide layer only slightly, or not even at all, influences the photocatalytic properties of the M(TiO2) coatings. Although the acetaldehyde degradation tests and the photoelectrochemical DCA degradation tests showed significant differences between the different layers on the metal substrates, this was not observed in the case of DCA degradation. ChemPhysChem 2015, 16, 2670 – 2679
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However, the different degradation processes should be taken into account. Moreover, these reactions might be influenced by the recombination rate of the photogenerated charge carriers, or the photocatalytic activity is determined by other pathways such as the adsorption of the pollutant on the surface of the particles. It seems that the adsorption processes of DCA¢ or other species such as Cl¢ ions at the TiO2 surface are important for the degradation mechanism. The photogenerated charge carriers do not influence the reaction rate in the same manner. Thus, it is the adsorption of DCA¢ , rather than this step, that determines the reaction rate. The influence of adsorption on the reaction rate is also indicated by the fact that the two measurements (pH-stat and HPIC) are yield different values for the reaction rate, as described above.
3. Conclusions From the three photodegradation tests performed here, it is obvious that the powders, that is, the pure catalysts or their suspensions, show higher photonic activities for the degradation of acetaldehyde or DCA, respectively, than the films, due to their higher active surface areas. Nevertheless, the obtained thin, almost transparent layers with a thickness of around 200– 300 nm showed good activities under UV light irradiation for all the test methods used. However, the DCA test is inadequate for obtaining information concerning the metal–semiconductor interface and thus the influence of the metal work function, due to the fact that the reaction rate is governed by the adsorption of DCA¢ on the semiconductor surface. In contrast, the acetaldehyde degradation test and the photoelectrochemical degradation of DCA clearly showed the influence of the metal work function of the back contact on the photocatalytic properties of the catalyst TiO2 ; with an increasing work function the efficiency of the photocatalyst decreases. In the case of aluminum, the formed ohmic contact and the electron-donating ability of the substrate lead to higher recombination rates of the photoinduced charge carriers, resulting in lower photonic efficiencies of the Al(TiO2) coatings. The metal–semiconductor contact only has an enhancing effect on the photocatalytic activity if a Schottky contact is formed.
Experimental Section Preparation of the TiO2 Films on Metal Substrates The TiO2 films were prepared by the dip-coating of different plate metals, such as titanium, aluminum, steel (V2A), and copper using suitable coating suspensions. These suspensions contained TiO2 (0.5 m; AEROXIDE TiO2 P25, Evonik, Leverkusen, Germany or Hombikat UV100, Sachtleben Chemicals, Duisburg, Germany; the former consists of a mixture of approximately 80 % anatase and 20 % rutile, whereas the latter is a pure anatase photocatalyst). The TiO2 powder was dispersed in distilled water (50 mL). Levasil 200/30 % (15 mL; Kurt Obermeier GmbH & Co. KG) was then successively added at pH 7. A total volume of 250 mL was achieved by adding methanol/ethanol (3:1) with vigorous stirring. After sonication in an ultrasound bath for 30 min, the suspension was used immediately for the dip-coating of the metal substrates. Prior to coating, the metal (10 Õ 5 and 4 Õ 4 cm2 for the acetaldehyde and DCA deg-
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Articles radation tests, respectively) was washed with acetone and distilled water. The substrates were dipped 10 times in the suspension using a stepping motor (Nanotec GmbH, Feldkirchen, Germany) at a speed of 5 mm s¢1 and a drawing speed of 3 mm s¢1. The metal was kept in the suspension for 5 s, with a waiting time of 5 s between the dips. The obtained coatings were calcined in an oven (Nabertherm LH 30/12 with a C30 program controller) by increasing the temperature from 25 to 250 8C in 2 h (1.88 8C min¢1). The temperature was maintained at 250 8C for 21 h and then subsequently decreased to room temperature over 3 h.
Scanning Electron Microscopy SEM images were obtained using a JSM-6700F microscope (JEOL, Tokyo, Japan) with a cold-cathode electron gun and a secondary electron image (SEI) and a lower electron image (LEI) detector for acquiring high-resolution images. The thin-film semiconductor layers on the metal substrates were fixed on an SEM holder. For better conductivity the interface was coated with an Ag paste. The powders were fixed on a conducting graphite block. All micrographs were acquired at an accelerating voltage of 2 kV and a working distance of 3 or 8 mm for the LEI or SEI detector, respectively.
The initial degradation rate was calculated from a curve fit. For a better comparison with values reported in the literature, some measurements were repeated at pH 3. Figure 16 shows a diagram of the experimental setup described above.
Figure 16. The experimental setup used for the degradation of DCA. The solution was purged with oxygen throughout the measurement. A water filter was used to cut out the IR-light of the Xe arc lamp. The employed potentiometric titrator is represented by Titrino.
Photoelectrochemical Degradation of DCA
Photocatalytic Tests Photodegradation of DCA DCA was used as a model compound to evaluate the photocatalytic activities of the prepared M(TiO2) layers in aqueous solutions. To maintain the ionic strength, an aqueous solution of KNO3 (10 mm; Sigma–Aldrich, 99.9 %) containing DCA (1 mm) was used. The reaction was carried out in a glass reactor equipped with a cooling jacket. The solution was vigorously stirred. The vessel was connected to a pH electrode combined with an Ag/AgCl reference electrode (Mettler Toledo InLab Expert). A pH-stat technique was used to follow the kinetics of the degradation.[23, 27, 41] The automatic dosing of aq. NaOH (0.1 m; Sigma–Aldrich, 98 %) was carried out using a Basic Titrino 794 potentiometric titrator (Metrohm, Filderstadt, Germany). In addition, at certain time intervals (0, 15, 30, 45, 60, 90, 120 min), samples were taken to determine the concentration of formed DCA¢ ions using HPIC (Dionex ICS-1000, equipped with a conductivity detector and an electro-regenerator suppressor). The column was a Dionex Ion Pac AS9-HC (2 Õ 250 mm) and the guard column was a Dionex Ion Pac AG9-HC (2 Õ 50 mm). The eluent was an aqueous solution of Na2CO3 (8 Õ 10¢3 m) and NaHCO3 (1.5 Õ 10¢3 m). The illumination unit for the degradation test was a Xe arc lamp (450 W) with an LAX 1450 lamp housing and an SVX 1450 power supply (both from Mìller Elektronik-Optik, Moosinning, Germany). The temperature was maintained at 25 8C using a thermostatic bath (Julabo, Seelbach, Germany). The details of this experimental setup have been described elsewhere.[23] The pH-stat method allows the concentration of degraded DCA to be determined from the amount of base added to the system to keep the pH constant. Prior to the test the pH of the solution was adjusted ChemPhysChem 2015, 16, 2670 – 2679
from 2.7–3 to 8 by the addition of aq. NaOH (0.1 m). The coated metal plates were then placed in the reactor containing a total volume of 250 mL. The solution was allowed to reach its adsorption equilibrium under vigorous stirring in the dark, while the pH to was adjusted 8 by the addition of NaOH. In addition to the tests with the coated metals, degradation tests of the two commercially available TiO2 powders used here on glass were performed in the same reactor.
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The photoelectrocatalytic degradation of DCA was performed as described above, with an additionally applied bias. Hence, M(TiO2) electrodes were prepared by contacting the metal with a wire using conductive silver lacquer and conductive epoxy resin. Afterwards, the contact and the metal were isolated using non-conductive epoxy resin. Only the TiO2 coating was exposed to the electrolyte. During the measurement a bias of 100 mV versus Ag/AgCl was applied using a potentiostat (Zahner, IM6). A Pt wire was used as the counter electrode and an Ag/AgCl reference electrode was used.
Acetaldehyde Degradation The photocatalytic degradation of acetaldehyde was followed by gas chromatography according to the ISO standard ISO/DIS 221972, to measure changes in the concentration of acetaldehyde.[42] As a reference, the pure powders used for the coatings—P25 or UV100—were dried for 2 h at 110 8C and then hand-pressed in a powder holder having the same size as the coatings (10 Õ 5 cm2). All samples were pretreated with UV illumination (365 nm, 10 W m¢2, Philips CLEO 100W-R lamp) for at least 16 h to decompose residual organic compounds at their surface. The photoreactor was made of poly(methyl methacrylate) (PMMA) with a transparent silica glass plate window on the side exposed to light. The distance between the sample and the glass was 5 0.5 mm. In accordance with the ISO standard, the concentration of acetaldehyde (99.9 %, Linde Group, Pullach, Germany) was 5 ppm in air with a relative humidity of 50 %, and was monitored using a gas chromatogram (SYNTECH Spectras GC 955) in the dark, until a concentration equilibrium was reached. The gas that flowed through the reactor
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Articles was collected and, in well-defined time steps of 5 min, measured using a photo-ionization detector (PID) to obtain the acetaldehyde concentration. After equilibration the degradation of acetaldehyde under UV irradiation (Philips CLEO 15W lamp, lmax = 365 nm, irradiation intensity 10 W m¢2) was monitored until a steady state was reached. Subsequent to the photoirradiation, the system was kept in the dark until the initial concentration of acetaldehyde (5 ppm) was restored. Figure 17 shows a diagram of the experimental setup described above.
Figure 17. The experimental setup used for the degradation of acetaldehyde. The stream of compressed air is split to give a relative humidity of 50 %. After passing through the mass-flow controller, the gases are mixed and then introduced into the reactor, which is made of PMMA. A filter can be applied at the top of the reactor to cut the undesired region of the light spectrum. The substrate is placed in the reactor and the light source is fixed directly above it. The gas is accumulated in a cold trap to be analyzed every 5 min in the GC/PID analyzer.
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