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Strong Transient Absorption of Trapped Holes in Anatase and Rutile TiO2 at High Laser Intensities Jenny Schneider, and Detlef W. Bahnemann J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01109 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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The Journal of Physical Chemistry

Strong Transient Absorption of Trapped Holes in Anatase and Rutile TiO2 at High Laser Intensities Jenny Schneider1 and Detlef Bahnemann1,2 1

Leibniz University Hannover, Institute for Technical Chemistry, 30167 Hannover, Germany

2

Saint-Petersburg State University, Laboratory “Photoactive Nanocomposite Materials”, Saint-Petersburg,

198504 Russia

Abstract During transient absorption spectroscopic investigations we found that the intensity of the transient absorption signal of the trapped holes monitored in the µs time domain drastically increases at high excitation laser intensity. This increase has been related to the presence of the long-lived Ti3+ centers formed upon high laser exposure via a surface reorganization. The Coulomb interaction of the trapped holes with long-lived Ti3+centers leads to an increased absorption coefficient of the former resulting in much higher transient absorption signals below 450 nm rather than in the wavelength region above where the trapped electrons absorb. The surface reorganization induced via the excitation source can be avoided in case of anatase if the measurements are conducted at low laser intensities, while in case of rutile already at low excitation conditions the transient absorption enhancement of the trapped holes occurs.

1. Introduction Studies concerning the photo-induced charge carrier dynamics are essential for the understanding of the reaction mechanism and thus for a better design of photocatalytic systems. Transient Absorption Spectroscopy is one widely used method to study fundamental processes such as formation, trapping, recombination, and transfer of the charge carriers photogenerated in different semiconductors. Several reviews have been published summarizing the most important scientific publications based on this topic.1-3 Most of the previous fundamental studies dealing with the reaction dynamics of the photogenerated charge carriers have been performed on colloidal TiO2 systems3-7, nanocristalline TiO2 films812

, and bulk commercial TiO2 powders13-16. Accordingly, the entire time regime of the photo-

induced processes in TO2 has been investigated.

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The trapped charge carriers could be spectroscopically characterized with the holes absorbing below 500 nm and the electrons in the visible and IR spectral region. The transient absorption of the trapped holes has been assigned to the transition from the valence band to the trap state, while the transient absorption in the visible range is attributed to the d-d transition of localized Ti3+ species and in IR range to the intraband transition of free electrons, respectvely.9, 17 The mentioned transitions of the trapped species are symmetrically forbidden thus leading to relative small absorption coefficients. However, Katoh et al. 18 have shown that the absorption coefficient of the trapped electrons and thus the transient absorption can be affected by the surrounding positive charges, which are present at high excitation density. Furthermore, in our previous report19 we have demonstrated the effect of the high excitation intensity during the diffuse reflectance transient absorption measurements on the morphological and optical properties of TiO2 powders. Herein, the formation of nonreactive Ti3+ species accompanied by the release of oxygen atoms from the TiO2 matrix has been detected. Based on this work, the present study focused on the effect of these long lived species on the transient absorption spectra and decay kinetics of the photogenerated charge carriers. For this purpose commercially available anatase TiO2 and rutile TiO2 have been investigated. Moreover, this study provides an explanation concerning the different transient absorption spectra for rutile and anatase reported in the literature.16, 20, 21

2. Experimental Nanosecond diffuse reflectance transient absorption spectroscopy measurements were performed using an experimental set-up as reported previously.22 Briefly, the excitation of the sample proceeds with an excimer laser (LPX 200) with a wavelength of 351 nm, and with a pulse duration of 15 ns. The laser intensity per pulse varied between 7 mJ cm-2 pulse-1 (here denoted as weak excitation condition) and 23 mJ cm-2 pulse-1 (here denoted as high excitation condition). Powder in a flat quartz cuvette has been used for all experiments. Herewith, the illumination area of the laser beam and of the analyzing light are 0.5 cm2 and 0.196 cm2, respectively. The obtained transient signal is detected upon measuring the change in the intensity of the reflected light before and after laser excitation according to:

∆J =

J 0 /I0 − J x /I0 J 0 -J x = J 0 /I0 J0

(1)


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where I0 is the incident intensity of the analyzing light; J0 is the diffusely reflected light without laser pulse (ground light level); Jx is the diffusely reflected light with the laser pulse. ∆J value has been used to describe the optical changes obtained in TiO2 upon laser excitation and is given in percentage. ∆J has been attributed to the transient absorption according to the Ref.14, 23 The estimated error for the obtained ∆J values in all measurements presented here is about 5 %. In this study the widely used commercial powder Hombikat UV100 (100 % anatase TiO2) from Sachtleben and R15 (100 % rutile) from Cristal have been investigated. In the following sections UV100 is denoted as anatase TiO2 and R15 as rutile TiO2.

3. Results Figure shows the transient absorption properties of the intermediates formed in anatase TiO2 upon excitation at low (7 mJ cm-2 pulse-1) and at high (23 mJ cm-2 pulse-1) laser intensities under inert conditions. For better comparison the long lived component has been subtracted from the original signal and the transients were normalized to the respective maximum value according to the equation 2. f λ (t) =

∆J λ (t) − ∆J λ (t = 18µs) ∆J max − ∆J λ (t = 18µs) λ

(2)

The original transient absorption decays are shown in the Figure S1 (a) and (b). All obtained transient signals presented in the Figure S1 (a) and (b) do not decay to zero and rather a longlived component of the initial absorption remains. It is important to notice, that the longlasting transient absorption is present at all wavelengths studied. This correlates very well with the reported UV-vis spectrum of the laser treated anatase TiO2 exhibiting visible light absorption, which indicates the previously reported irreversible changes of the sample upon laser illumination.19 At lower laser excitation intensity (Figure 1 (a)) the decays were slow and similar to each other. A half time t1/2 of 150 ± 20 ns has been estimated. It can be concluded that electrons and holes decay simultaneously through a recombination process. As the laser intensity was increased the decay kinetics became wavelength dependent. The decay of the transients obtained

at

wavelengths

above

420 nm

was

found

to

be

slightly

accelerated

(t1/2 = 126 ± 20 ns) (see Figure 1(c)). This is in accordance with recombination kinetic law; higher laser intensity leads to a higher charge carrier number resulting in a higher



recombination rate. However, the lifetime of the transient signals detected at 390 nm was found to be increased (t1/2 = 300 ± 20 ns). 4

b

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Figure 1: (left) Transient absorption signals observed at 390 nm (green), 450 nm (blue), and 550 nm (red) and (right) transient absorption spectra observed 0.109 µs (-■-) and 17 µs (-▲-) after the laser at intensities of 7 mJ cm-2 pulse-1 (a,b) and of 23 mJ cm-2 pulse-1 (c,d). Experimental conditions: N2-saturated anatase TiO2 powder, laser excitation with λexc = 351 nm.

The transient absorption spectra measured at low laser intensity in the wavelength range between 390 nm and 750 nm are very broad and featureless (see Figure (b)). This broadness of the transient absorption spectra can be related to that fact that in absence of any electron donor or acceptor both, the trapped electrons and the trapped holes are present and their transient absorption overlaps. Similar broad transient absorption spectra for anatase have already been reported elsewhere.9, 24 By means of the experiments performed in the presence of electron donors and acceptors, respectively, it was possible to identify the specific wavelength regions where trapped holes and trapped electrons absorb. The identification of these spectral regions was in agreement with the already published results, thus the corresponding data are shown in the SI (see Figure S2). Accordingly, the wavelength region above 450 nm could be attributed to the transient absorption caused mainly by the trapped electrons. The electrons can be trapped at Ti4+ ACS Paragon Plus Environment

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cations yielding Ti3+ species. The transient absorption detected below 450 nm corresponds to the trapped holes. The formation of hydroxyl radicals •OH or of oxygen radical anions O2•- in the TiO2 lattice are usually considered to be a result of the hole trapping process.

25-27

Moreover, in the literature the presence of two different types of holes has been reported.10 The trapped holes located in the bulk are found to absorb in the visible wavelength range, while the holes trapped at the surface forming a complex with the OH- ions exhibit a transient absorption in the ultraviolet region. The transient absorption spectrum of the latter disappears in acidic medium. At conditions used in the present study both types of the trapped holes are likely to be present. However, Figure 1 (d) shows that the high excitation intensity obviously causes a strong increase of the transient absorption of the trapped holes, while only a slight increase of the transient absorption of the trapped electrons is detected simultaneously. The Jhigh/Jlow was 9 and 2 for the trapped holes and electrons, respectively. Such pronounced transient absorption feature of the trapped holes obtained at high laser intensities has been reported here for the first time. Very often different transient absorption features obtained for rutile TiO2 in comparison to anatase TiO2 have been reported. In the present study, the effect of the high excitation intensity on the trapped species formed in rutile TiO2 has been evaluated as well. For better comparison the commercial rutile powder R15 has been chosen as it contains similar particle sizes (d = 20 nm) as the anatase TiO2 used here. Figure 2 (a) presents the transient absorption signals modified according to equation 2 (here

J at t = 45 µs has been subtracted). In comparison to anatase TiO2 the difference between the decays obtained at different wavelengths was much more pronounced already at low laser intensity. The decay obtained at 450 nm was much faster with t1/2 = 720± 60 ns compared to the transient signal recorded at 390 nm with t1/2 = 4,000 ± 100 ns. Generally, the charge carriers formed in rutile exhibit a longer lifetime as compared with the charge carriers photogenerated in anatase. The transient absorption spectra measured for rutile show already at low excitation intensity conditions high transient absorption intensities with decreasing observation wavelengths (see Figure 2 (b)). The transient absorption in the visible region, however, is very low. The shape of the transient absorption spectra does not change with the decay time.



30

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Figure 2: (a) Transient absorption signals observed at 390 nm (green), 450 nm (blue) and (b) transient absorption spectra observed 0.109 µs (-■-) and 45 µs (-▲-) after the laser pulse at 7 mJ cm-2 pulse-1 excitation intensity. Experimental conditions: N2-saturated rutile TiO2 powder, laser excitation with λexc = 351 nm.

The transient absorption spectrum observed in this study for rutile corresponds very well to the reported one.15 The different transient absorption spectra for rutile and anatase recorded upon identical laser excitation conditions are usually explained by a different trapping behavior of the photogenerated charge carriers in the different phases. However, the discussion presented below provides another explanation for the observed difference between the anatase and the rutile phases.

4. Discussion For the discussion of the origin of the spectral changes obtained at high excitation intensities the density of the charge carriers photogenerated in TiO2 is the major parameter. Hence, the amount of photons or rather the amount of the electron-hole pairs generated per TiO2 particle Np during the laser pulse can be determined according to:

Np =

I ⋅ λ exc ⋅ 1/6 ⋅ π ⋅ d 3 ⋅ α c⋅h

(3)

I: Laser intensity [J m-2 pulse-1] λex: Excitation wavelength [m] d: Crystallite diameter [m] (danatase = 12 nm, drutile=15 nm) α: Absorption coefficient [m-1] (αanatase/351nm = 1.2·106 m-1, αrutile/351nm = 6·106 m-1)28 c: Speed of light [m s-1] h: Planck constant [J·s]

This equation is developed under the assumption that the particles are spherical. The light penetration depth and the illuminated area determine the illuminated volume; hereby the ACS Paragon Plus Environment

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former one is determined by the reciprocal value of the absorption coefficients representing the absorption depth. According to equation 3 the number of electron-hole pairs generated per particle is directly proportional to the absorption coefficient, which depends on the crystallographic phase of TiO2. Since rutile exhibits a much higher absorption coefficient at 351 nm as compared with anatase, the former absorbs considerably more photons per particle per laser pulse provided that all other parameters are kept constant. For example, 3000 electron-hole pairs are produced per rutile TiO2 particle at a laser intensity of 7 mJ cm-2 pulse-1, while in one anatase TiO2 particle the number of generated charge carriers is by a factor of 20 less upon identical excitation conditions. These simple calculations demonstrate that for a "fair" comparison of the transient absorption spectra observed in rutile and in anatase the laser intensity should be adjusted accordingly. This explains the already reported difference between the transient absorption spectra of rutile and anatase. The transient absorption spectra have been recorded at the same laser intensity, however, in rutile a much higher number of electron/hole pairs is formed per particle while a smaller total number of particles is excited than in anatase, different absorption spectra are monitored. In this work the transient absorption spectra observed in rutile at a laser intensity of 7 mJ cm-2 pulse-1 are comparable with the spectra observed for anatase samples at a laser intensitiy of 23 mJ cm-2 pulse-1. To explain the obtained spectral changes in the UV wavelength region we first consider the thermal effect. For example, Wilkinson and Willsher pointed out that materials exhibiting a high optical absorption coefficient such as TiO2 or Fe3O4 (i.e., with α > 104 cm-1) are likely to experience a considerable temperature rise following pulsed excitation.29 The authors assigned the transient absorption in the UV region obtained for TiO2 powders to photoinduced thermal transients formed by rapid heating and cooling of the sample. However, this effect can be ruled out in the present work. According to the experiments performed by Sieland30 the strong transient absorption in the UV region remains when performing the experiments at low excitation conditions after the sample had been exposed to high laser intensities before. Moreover, an increase of the absorption coefficient of TiO2 in the discussed wavelength range has been reported after heating TiO2 at 1000 °C.31 This change in the absorption coefficient has been explained by the formation of Ti3+ centers in TiO2 upon heating. However, in this case contributions to the transient absorption spectra should be found in the entire spectral range rather than only at wavelengths below 450 nm. Next we will consider the effect of the high laser intensity on the absorption coefficient of the trapped species. Generally, there are two parameters which can affect the amplitude of the



transient absorption, namely the concentration and the absorption coefficient of the charge carriers. In agreement with the charge neutrality principle the concentration of electrons and holes should be the same at any time of the measurement. Hence, the increase of the laser intensity should lead to an enhancement of the transient absorption in the entire wavelength region. This has not been observed in the present study. In contrast, only the transient absorption of the trapped holes was found to increase drastically, while only a slight increase of the transient absorption of the trapped electrons is detected simultaneously. Especially, it should be noted here, that the spectroelectrochemically estimated absorption coefficients for the holes and for the electrons are similar being 2,930 M-1 cm-1 and 2,440 M-1 cm-1, respectively32, 33 This result indicates that the absorption coefficient of the trapped holes has been affected by the exposure of TiO2 to the intense laser pulse. The absorption coefficient of trapped species is determined by the respective electronic transition. In contrast to the trapped electrons, the corresponding transition of the excited states of the trapped holes has so far not been identified unambiguously. For example, Zawadzki34 calculated the transient absorption spectra of trapped holes in bulk anatase TiO2 and on its 001 and 101 surfaces. According to his calculations the broad band from 300 to 800 nm can be attributed to an interpolaron transition from a hole state localized on an O− site to an adjacent localized state, whereas the excitation energy increases with increasing distance between neighboring O sites. On the contrary, Cheng et al.35 and Selloni et al.17 have assigned the transient absorption of the trapped holes to the vertical transition from the VB maximum to the trapped state. However, the transition from the excited states and thus its absorption coefficient can be influenced by different factors. For example, Katoh et al.18 obtained for nc-TiO2 films an increased transient absorption of the trapped electrons that was explained by an increased absorption coefficient. The authors considered two parameters which can affect the absorption coefficient of the trapped electron, namely, lattice deformations induced by trapped holes and Coulomb interactions. The authors ruled the former out due to the long distance between the electrons and holes at the applied laser intensities, while the Coulomb interactions of around 0.16 eV were found to be much larger than the thermal energy. Hence, the surrounding structure will be affected by the Coulomb interactions with the trapped hole under high density excitation and subsequently the absorption coefficient of the trapped electrons will be increased. The authors supported their conclusion by the report of Boschloo et al.36 who observed an increase of the absorption coefficient of the trapped electrons induced by intercalating cations such as Li+.


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In accordance to this study the transition or rather the absorption coefficient of the trapped holes can be influenced by their interaction with the counter ions. In our previous publication19 we have shown that irreversible changes of TiO2 occur during pulse illumination with high laser excitation densities. A surface reorganization accompanied with the release of oxygen atoms from the lattice has been observed followed by the formation of non-reactive Ti3+ centers in both phases that are anatase and rutile TiO2. It should be mentioned here that these non-reactive Ti3+ centers are different from those Ti3+ which were detected here by the transition absorption in the visible range. The former are created by the surface reconstruction and remain in the lattice exhibiting absorption in the entire wavelength range as it has been shown in the previous publication.19 Moreover, the formation of these long lived Ti3+ species in TiO2 is most likely complete before the measurement of the transient absorption starts, i.e., 100 ns after the laser pulse. For example, the laser-induced darkening of TiO2 due to the formation of Ti3+ centers has been monitored employing femtosecond excitation with laser intensities of less than 200 nJ.37 In contrast, the Ti3+ centers or rather trapped electrons obtained here in the µs region at probe wavelengths in the visible range disappear due to their recombination with the trapped holes. However, the absorption coefficient of the trapped holes can be affected by the Coulomb interaction with the long-lived Ti3+ species formed at high laser intensities. The Coulomb potential depends not only on the distance between the two charges but, in particular, on the charge density. In the present case the charge density is determined by the concentration of the long-lived Ti3+ centers. Upon increasing the laser intensity the number of the long-lived Ti3+ centers increases, hence their interaction with the trapped holes will become stronger resulting in an enhancement of the absorption coefficients. Moreover, in the present study it has been observed that the initial transient absorption intensity measured at 390 nm increases linearly with the laser intensity as the amount of the long-lived Ti3+ centers also grows steadily. (see Figure S3). However, as can be seen from the Figure S3 (a) the intercept of the linear fit does not cross the origin but rather exhibits negative value. This is an indication that at laser intensities below 7 mJ cm-2 pulse-1 the absorption coefficient does not depend on the laser intensity or shows another dependency. However, in contrast to the anatase TiO2, in case of rutile the obtained data could be fitted to a linear function with an intercept at zero (see Figure S3 (b)). This clearly evinces that in case of rutile already at very low laser intensities the absorption coefficient of the trapped holes is affected by the irreversibly changes. As argued before this observation corresponds to the fact that rutile can be reduced much more easily than anatase.17



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The transient absorption spectra of the trapped holes and electrons photo-generated in anatase and rutile TiO2 powders have been already reported. Yamakata et al.

15

16, 20, 21

Similar to the present results,

obtained a broad transient absorption spectrum for the anatase phase, while

for rutile TiO2 a strong increase of transient signal with decreasing wavelength has been detected. The authors have related this difference between anatase and rutile to the different trapping behavior of the charge carriers in both TiO2 phases. In contrast to the present study, in the reported experiments a lower laser intensity of 0.5 mJ pulse-1 has been applied. According to equation 3 in their study each rutile particle absorbs around 214 photons, which is still relatively high. Moreover, as the data in Figure S3 (b) shows, already low laser intensities can induce irreversible changes in rutile thus affecting the transient absorption spectra of the photogenerated charge carriers. Subsequently, it can be concluded that not only the different trapping behavior of the charge carriers can be the reason for the different transient absorption spectra of anatase and rutile but also the formation of the long lived Ti3+ species induced upon laser excitation, which in case of rutile are more likely than in anatase. However, the effect reported here should be treated with care. Similar strong transient absorption signals at wavelengths below 450 nm have been already reported by others.3, 38 For example, Lawless et al.27 have related this transient absorption to the presence of •OH radicals. Here, the •OH radicals were generated by pulse radiolysis and loaded on the TiO2 surface. Hence, the transient absorption was recorded at high •OH radical concentrations. These results correlate very well with the observation published by Katoh et al.10, who reported the ultraviolet transient absorption of the holes trapped at the surface forming •OH radicals. Taking these two scientific reports into account we can conclude that in the presence study the transient absorption of the holes trapped at the surface rather than in the bulk has been affected by the interaction with long-lived Ti3+ species formed at high laser excitation densities. Finally, the obtained dependency of the transient decay kinetics on the observation wavelength monitored at high excitation density for anatase (see Figure 1(c)) and for rutile already at low excitation conditions (see Figure 2 (a)) will be discussed. Provided that additional processes besides the charge carrier recombination would be involved a faster decay should occur. Curiously, a deceleration of the 390 nm decay has been obtained for both anatase and rutile in comparison to the transient signals measured at lower laser intensity and to signals obtained at higher probe wavelength. This behavior contradicts to any kinetic rules. l However, the transient at 390 nm monitored employing low excitation conditions ∆J 390 can


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be reproduced via the division of the 390 nm transient absorption detected at high intense h laser pulse ∆J 390 by a factor x reflecting the increase in the transient absorption intensity: l h ∆J 390 = ∆J 390 /x

(4)

Accordingly, Figure 3 (a) clearly shows that the transient decays obtained at different excitation conditions for anatase show a similar decay behavior provided that a factor of 9 is applied. (For the transients shown in Figure 3 the long-lived component has been subtracted). If we consider the second order relation, this factor represents the absorption coefficient (details concerning this equation are given in SI): ∆J = ε T ⋅

c0 + f(t) x c0k r t + 1

(5)

where c0 presents the initial concentration of the formed transient species, and f(t)x is the observed long-lasting transient absorption which follows more complicated non-second-order reaction kinetics.39 εT has been defined as the product of the absorption coefficient of the trapped holes being 2,930 M-1 cm-1 and the penetration depth of the light at 351 nm being 1.2·10-4 cm for anatase. Both decay kinetics could be fitted by the second order relation given in equation 5 and a rate constant kr of 6·107(±3·106) s-1 M-1 has been obtained for both transient signals (see Figure 3 (b)). Comparable values have been reported for the recombination rate constant by other research groups.40, 41The absorption coefficient ratio of the transients obtained at low and high excitation conditions was 1:9, respectively, thus correlating very well with the factor used in equation 4. 3

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Figure 3: (a) Transient absorption signals observed at 390 nm at low (green) and at high excitation conditions (green), while the later has been modified according to the equation 4. (b) The red line represents a fit to second order kinetics according to equation 5.

In accordance with the aforementioned discussion the deceleration of the transient signal observed at 390 nm can be explained by the increased absorption coefficient of the trapped ACS Paragon Plus Environment


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holes due to the Coulomb interaction with the long-lived Ti3+ centers formed at high excitation conditions.

5. Conclusions We have investigated the transient absorption features of the trapped charge carriers formed at high laser excitation conditions. A considerable deceleration of the decay kinetics and a strong increase of the transient absorption signal recorded in the wavelength region where the trapped holes absorb have been found. Based on the observation made by Katoh10, the obtained spectral and kinetic changes have been attributed to the enhancement of the absorption coefficient of the trapped holes induced by an internal electric field. The transient absorption in the ultraviolet wavelength region has been affected by the Coulomb interaction of the holes trapped at the TiO2 surface with the long lived Ti3+ centers formed via the photoinduced surface reorganization. In case of anatase such spectral changes disappear at weak excitation conditions, while in case of rutile they cannot be avoided. These findings are essential for the interpretation and comparison of the transient absorption spectra monitored for different TiO2 phases.

Supporting Information The original transient absorption signals obtained for anatase and rutile at different excitation intensities, the transient absorption spectra obtained for anatase in the presence of electron donor (methanol), and electron acceptor (Platinum), the plot of J390 versus laser intensity obtained for anatase and rutile, description of the second order equation (5) applied here for fitting.

Acknowledgements This work was funded in part by the German Federal Ministry of Education and Research (contract

no.

photokatalytisch

13N13350,

PureBau

hocheffiziente

–

Untersuchung

von

Baustoffe-Teilvorhaben:

Photokatalysatoren und der Werkstoffe“).

References


Werkstoffsystemen Oberflächenchemie

für der

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