Studies on the adsorption and photocatalytic degradation of an EuIII(TTFA)3(MePhTerpy) complex on the TiO2 surface

Article abstract of DOI:10.1016/j.jphotochem.2018.06.016

The luminescent dye Europium^III(thenoyltrifluoroacetone)(4′-(4-methylphenyl)-2,2′:6′,2′′-terpyridine) (Eu^III(TTFA)₃(MePhTerpy)) has been proposed as a probe molecule for a fast photocatalytic test of TiO₂-materials. For an industrial application of this test method the underlying photocatalytic reaction mechanism and the degradation products of the dye need to be known. Hence, in this study the adsorption and the photocatalytic degradation of the luminescent dye on the TiO₂ surface were investigated by means of ATR-FTIR spectroscopy and GC/MS analysis. The dye is adsorbed by the thenoyltrifluoroacetone (TTFA) ligand on the TiO₂ surface as evinced by the respective ATR-FTIR spectra. It is assumed that TTFA and TiO₂ form hydrogen bonds. Upon UV illumination the ATR-FTIR spectra reveal a degradation of the TTFA ligand forming trifluoroacetic acid (TFAA) and 2-thiophenecarboxylic acid (TCA). GC/MS analysis confirms the ATR-FTIR results because TCA and TFAA are also detected here. Additionally, 2-thiophenecarboxaldehyde is observed, which is proposed to be an intermediate that is further oxidized to TCA.

Full text of DOI:10.1016/j.jphotochem.2018.06.016

Accepted Manuscript
Title: Studies on the Adsorption and Photocatalytic
Degradation of an EuIII(TTFA)3(MePhTerpy) Complex on the
TiO2 Surface
Authors: Stephanie Melchers, Yamen Alsalka, Jenny
Schneider, Detlef W. Bahnemann
PII:
S1010-6030(17)31703-3
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2018.06.016
JPC 11333
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
17-11-2017
1-6-2018
5-6-2018
Please cite this article as: Melchers S, Alsalka Y, Schneider J, Bahnemann DW, Studies
on the Adsorption and Photocatalytic Degradation of an EuIII(TTFA)3(MePhTerpy)
Complex on the TiO2 Surface, Journal of Photochemistry and Photobiology, A:
Chemistry (2018), https://doi.org/10.1016/j.jphotochem.2018.06.016
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Studies on the Adsorption and Photocatalytic Degradation of an
EuIII(TTFA)3(MePhTerpy) Complex on the TiO2 Surface
Affiliations
Stephanie Melchersa*, Yamen Alsalkab, Jenny Schneiderc*, and Detlef W. Bahnemannd
a* Institut für Technische Chemie, Leibniz Universität Hannover, Callinstr. 5, 30167
Hannover, Germany. Email: melchers@iftc.uni-hannover.de
b Institut für Technische Chemie, Leibniz Universität Hannover, Callinstr. 5, 30167
Hannover, Germany. Email: alsaka@iftc.uni-hannover.de
c* Institut für Technische Chemie, Leibniz Universität Hannover, Callinstr. 5, 30167
Hannover, Germany. Email: schneid@iftc.uni-hannover.de
d Institut für Technische Chemie, Leibniz Universität Hannover, Callinstr. 5, 30167
Hannover, Germany. Laboratory „Photoactive Nanocomposite Materials“, Saint-Petersburg
State University, Ulyanovskaya str. 1, Peterhof, Saint-Petersburg, 198504 Russia. Email:
bahnemann@iftc.uni-hannover.de
Graphical abstract

Highlights


EuIII(TTFA)3(MePhTerpy) is adsorbed by the TTFA ligand on the TiO2 surface.
Upon UV illumination the TTFA ligand of the EuIII(TTFA)3(MePhTerpy) complex is
degraded into trifluoroacetic acid, 2-thiophenecarboxaldehyde and 2-
thiophenecarboxylic acid.

EuIII(TTFA)3(MePhTerpy) is proposed to be oxidized by photogenerated OH˙ rather than
by holes.
Abstract
The luminescent dye EuropiumIII(thenoyltrifluoroacetone)(4′-(4-methylphenyl)-2,2′:6′,2′′-
terpyridine) (EuIII(TTFA)3(MePhTerpy)) has been proposed as a probe molecule for a fast
photocatalytic test of TiO2-materials. For an industrial application of this test method the
underlying photocatalytic reaction mechanism and the degradation products of the dye need to
be known. Hence, in this study the adsorption and the photocatalytic degradation of the
luminescent dye on the TiO2 surface were investigated by means of ATR-FTIR spectroscopy
and GC/MS analysis. The dye is adsorbed by the thenoyltrifluoroacetone (TTFA) ligand on
the TiO2 surface as evinced by the respective ATR-FTIR spectra. It is assumed that TTFA and
TiO2 form hydrogen bonds. Upon UV illumination the ATR-FTIR spectra reveal a
degradation of the TTFA ligand forming trifluoroacetic acid (TFAA) and 2-
thiophenecarboxylic acid (TCA). GC/MS analysis confirms the ATR-FTIR results because
TCA and TFAA are also detected here. Additionally, 2-thiophenecarboxaldehyde is observed,
which is proposed to be an intermediate that is further oxidized to TCA.
Keywords: Europium complex, luminescence, dye, degradation, TiO2
Key Words: luminescent europium dye, photocatalytic activity, degradation mechanism,
TiO2, photocatalysis, ATR-FTIR, GC/MS
Introduction
In the last decades the amount of pollutants in air and in water has increased mainly due to the
population increase and the industrial growth. Semiconductor photocatalysis might be a
sustainable and environmentally friendly solution in order to decrease the amount of these
pollutants. Herein, TiO2 appears to be an ideal photocatalyst, since by UV illumination TiO2
is able to catalyze the decomposition of organic compounds yielding CO2 and H2O and traces
of mineral acids as reaction products.[1] Besides its high reactivity, it is a low cost material, it
is nontoxic and prevalent. Hence, recently TiO2 containing building materials in different
types and forms such as tiles, paints or concrete have shown great potential.
Several standard methods already exist to test the photocatalytic activity of photocatalytic
surfaces such as the degradation of NOx (ISO 22197-1)[2] or acetaldehyde (ISO 22197-2)[3].
However, such methods require an expensive analytical equipment. Moreover, these setups do
not have a high mobility, therefore measuring the photocatalytic activity of building materials
outdoors still remains challenging.[4]

Böttger et al.[5] developed a new method to test the photocatalytic activity of photocatalytic
surfaces based on a luminescent europium complex dye. In this work the dye
EuropiumIII(thenoyltrifluoroacetone)(4′-(4-Methylphenyl)-2,2′:6′,2′′-terpyridine)
(EuIII(TTFA)3(MePhTerpy)) was investigated. Its molecular structure is shown in Figure 1.
The dye can be excited by UV light at 365 nm, therefore the same excitation can be used to
test the photocatalytic activity since both TiO2 and the dye can be excited at the same
wavelength. Upon UV light absorption the organic ligands are excited to the singlet S1 state,
immediately followed by a migration to the triplet state T1. Afterwards, the energy is
transferred to the Eu3+ cation, thus Eu3+ is excited. Upon relaxation from the occupied
electronic state to the ground state an emission at λmax= 617 nm is induced.[6,7] When a
decomposition of the complex by TiO2 occurs, the interaction between the ligand and Eu3+ is
weakened leading to a decrease in the emission intensity that can be detected.[5] Several dyes
used for photocatalytic tests are already known from literature such as methyl orange,
rhodamine B, or methylene blue (ISO 10678:2010).[8,9] However, using for example the
methylene blue degradation as a standard method to test the photocatalytic activity is
challenging because several assumptions (e.g. dye purity, pH of the solution or the stirring
time) need to be considered, since all these processes influence the bleaching of the dye.[4]
Furthermore, these dyes are only applicable to test the photocatalytic activity for water
purification, therefore they cannot be applied for air purification tests.
Figure 1: Molecular structure of EuIII(TTFA)3(MePhTerpy).
EuIII(TTFA)3(MePhTerpy)[10] can be used as a probe molecule to test the photocatalytic
activity of building materials for air purification.[11] The idea is to link the dye on a foil and
to bring the latter in contact to the surface of a building material. Upon UV illumination the
dye is degraded and a decrease in emission can be detected.
For an industrial application of this dye as a test molecule the reaction mechanism of the
decomposition by TiO2 should be known. Hence, in this work the degradation of the dye was
investigated using ATR-FTIR (Attenuated Total Reflection - Fourier Transform Infrared)
spectroscopy and GC/MS (Gas chromatography - mass spectrometry) analysis. The obtained
results of the measurements were correlated with each other. In this context the adsorption
mode of the dye on TiO2 was figured out and the degradation products were identified. To the
best of our knowledge, herein the photocatalytic degradation mechanism of a luminescent
europium complex by TiO2 is investigated for the first time.
Experimental section
Materials
The commercial TiO2 photocatalyst KRONOClean® 7050 was kindly provided by
KRONOS®. EuIII(TTFA)3(MePhTerpy) and NaTTFA were synthesized at the Institut für
Hochfrequenztechnik at the Technische Universität Braunschweig. Trifluoroacetic acid was
supplied by Carl Roth® and 2-thiophenecarboxylic acid was purchased from Merck.

TiO2 film preparation
A suspension of 5.75 g L-1 KRONOClean® 7050 in deionized water was sonicated for 15 min
in an ultrasonic bath. Subsequently, an aliquot of 400 µl was placed on a ZnSe crystal and
was distributed by gently balancing the crystal. After evaporation of the water, a homogenous
film was obtained which had a coverage of 2.3 g m-2 and a thickness of 1.7 ± 0.3 µm
according to Hug and Sulzberger.[12]
100 µl of an 1 mM EuIII(TTFA)3(MePhTerpy) solution in acetonitrile was distributed over the
TiO2 film. After evaporation of acetonitrile, the dry film was analyzed by ATR-FTIR
spectroscopy. The sample preparation of the other compounds was similar: 300 µl TTFA
(1 mM), 300 µl TCA (1 mM) or 100 µl TFAA (10 mM) in acetonitrile were deposited on the
TiO2 film.
ATR-FTIR spectroscopy
ATR-FTIR spectra were recorded on a Bruker IFS 66 equipped with a deuterated triglycine
sulfate (DTGS) detector and an internal reflection element made of ZnSe with an angle of
incidence of 45° and 9 reflections on the upper face. The interferometer and the infrared light
path in the spectrometer were constantly purged with argon in order to avoid H2O and CO2
contamination. Each spectrum was an average of 300 scans with a resolution of 4 cm-1. Before
each experiment a blank spectrum of KRONOClean® 7050 was taken in the dark.
Subsequently, spectra in the dark (2h) and upon UV illumination (6h) with
EuIII(TTFA)3(MePhTerpy) were recorded. The final spectra were obtained by subtraction the
blank from the ones with EuIII(TTFA)3(MePhTerpy). As UV light source a LED lamp was
used, supplied from OMICRON with a wavelength of 365 nm and an intensity of 1 mW cm-2.
For equal reaction conditions a closed compartment made of Plexi-Glass® (PMMA,
Polymethylmethacrylate) was attached to the upper part of the ZnSe crystal (see Figure 2),
where a gas flow of 100 ml min-1 (SIERRA®) of synthetic air was passed through.
Figure 2: Experimental set up for the ATR-FTIR measurement.
GC/MS analysis
50 mmol NaTTFA in acetonitrile (5 ml) mixed with K7050 (0.4 g/L) were illuminated by UV
light (Philips CLEO 75 W) with an intensity of 1 mW cm-² for 12 h in the presence of O2.

Afterwards the mixture was filtrated in order to remove TiO2. The liquid extract was analyzed
by GC/MS using a GCMS-QP5000 (Shimadzu) coupled with an AOC-5000 Plus auto sampler
and employing parameters based on Method 8270D (SW-846): Semivolatile Organic
Compounds by Gas Chromatography/ Mass Spectrometry (GC/MS). A capillary RXI®-5MS
column (30 m × 0.32 mm, dF= 1 µm, Restek) was used to perform the separation procedure.
In a typical analysis 2 µl of the liquid extract were injected in a split/splitless injector of the
GC/MS system. The column oven was held at 40 °C for 4 min, followed by an increase in a
rate of 10 °C/min to reach a final temperature of 270 °C. The temperature of both, the
injection and the MS interface was adjusted to 280 °C. The MS scanned from 35 m/z to 500
m/z.
For the adsorption experiments solutions of TFAA (150 mmol) and TCA (25 mmol) in
acetonitrile were prepared and analyzed by GC and GC/MS. K7050 (0.4 g/L) was added then
to the solutions in order to allow TCA and TFAA to adsorb on the TiO2 surface. After 2 h in
the dark the suspensions were filtrated and the filtrate was analyzed again by GC and GC/MS.
The GC used was a Shimadzu GC-2010 equipped with an AOC-20i autosampler und with a
Rtx-5 (30m x 025 mm ID x 0.25 um) capillary column from Restek. A split injection mode
was applied at 280 °C with a split ratio of 10 and a total flow of 19.1 ml/min with helium as
carrier gas. The temperature of the column had been kept constant at 40 °C for 4 min before it
was increased to 270 °C with a rate of 10 °C/min. TFAA was detected at a temperature of 300
°C by a flame ionization detector (FID).
Results and Discussion
In Figure 3 the ATR-FTIR spectrum (black) of the dye adsorbed on the TiO2 surface is shown
in the wavenumber range between 1100 and 1800 cm-1. This fingerprint region has been
analyzed since here the characteristic bands corresponding to the dye occur. Different bands
are observed which are referred to specific molecular vibrations of the dye as listed in Table
1. The bands can be assigned to molecular vibrations of the TTFA ligand such as ν(CF3), ν(C-
O) and ν(C4H3S). Since only the bands for TTFA appear, it is assumed that the dye is
adsorbed by the TTFA ligand on the TiO2 surface. For verification of this assumption, sodium
thenoyltrifluoroacetone (NaTTFA) was deposited on the anatase film and an ATR-FTIR
spectrum was recorded (see Figure 3, red). The spectra of NaTTFA and the dye correlate with
each other exhibiting equal band positions. Apparently, the dye is adsorbed on the TiO2
surface by the TTFA ligand. A schematic illustration of the dye interacting with the TiO2
surface is shown in Figure 4.[13] It is assumed that the dye shows rather a physical adsorption
than a chemisorption on the TiO2 surface due to steric hindrance and due to the molecular
structure of the dye. As illustrated in Figure 4 a polar H-atom of the hydroxylated TiO2
surface and a polar F-atom of the CF3 group form hydrogen-bonds. Furthermore, the H-atom
(marked in red) of the CH is acidic, because CH is located between two electron-withdrawing
carbonyl groups causing a high negative inductive effect. Thus, the polarity of the H-atom
increases favoring a hydrogen-bond formation to the O-atom of a hydroxyl group from the
TiO2 surface. Distinct polar atoms of the TTFA ligand form hydrogen-bonds with the OH
groups of the TiO2 surface, while the nonpolar MePhTerpy ligand has less contact to the polar
TiO2 surface.

Figure 3: Comparison between the ATR-FTIR spectra of the dye (black) and NaTTFA (red)
deposited on the anatase film in the dark.
Table 1: Vibrational frequencies and mode assignments of the observed FTIR bands (cm-1) of
the dye and NaTTFA deposited on the anatase film in the dark.
Position / cm-1
1145
Assignment[14,15]
ν(CF3) TTFA
1203
ν(CF3) TTFA
1307
ν(CF3) TTFA
1409
1544
1604
ν(C4H3S) TTFA
ν(C4H3S) TTFA
ν(C-O) TTFA

Figure 4: Proposed adsorption mode of the dye on the TiO2 surface based on the ATR-FTIR
spectra.
As adsorption mode of the dye on the TiO2 surface was clarified, the decomposition of the
dye and the formation of intermediates and products were investigated by ATR-FTIR
spectroscopy upon UV-light illumination. For this purpose the complex was deposited on the
TiO2 surface and was illuminated by UV light for 6 h. Figure 5 illustrates the ATR-FTIR
spectra of the dye before (red) and after 6 h of UV illumination (black). The intensity of the
bands characteristic for distinct molecular vibrations of the dye such as the ν(C-O) at 1604
cm-1, the ν(C4H3S) at 1544 cm-1 and the ν(CF3) at 1307 cm-1 strongly decrease indicating a
nearly complete decomposition of the TTFA ligand. Furthermore, the intensity of the band of
ν(CF3) at 1203 cm-1 increases and the band maximum of ν(CF3) at 1145 cm-1 is shifted to
higher wavenumbers. Additionally, a new band appears at ~1670 cm-1. These changes are
highlighted by arrows in Figure 5. The results indicate a decomposition of the TTFA ligand
accompanied by the formation of intermediates and products on the anatase surface.

Figure 5: ATR-FTIR spectra of the dye deposited on the anatase film before (red) and after 6
h of UV ilumination (black). The arrows show the formation of intermediates/products,
indicated by a shift of a band, an increase in intensity of bands and the new appearance of
bands.
Potential reaction products formed from the complex were deposited on the TiO2 surface and
ATR-FTIR spectra were recorded in order to compare them with the spectrum of the dye.
Figure 6 demonstrates the spectrum of the dye after 6 h of UV illumination, of trifluoroacetic
acid (TFAA) and of thiophenecarboxylic acid (TCA). A correlation between the spectrum of
TFAA and the dye is observed because the ν(CF3) bands at 1150 cm-1 and 1203 cm-1 appear
similar in position and in intensity. Furthermore, the band at 1670 cm-1 can be probably
referred to the ν(C-O) of TFAA.[16] Consequently, the formation of TFAA as a product of
the dye degradation could be proven which is accumulated on the anatase surface. When
comparing the spectra of the dye before and after UV illumination (see Figure 5) a small
increase in intensity at ~1380 cm-1 and at ~1420 cm-1 is observed. These bands can be referred
to TCA (see Figure 6).[17] However, a clear correlation between TCA and the dye is difficult
because of the small changes in intensity of the bands of the dye.

Figure 6: ATR-FTIR spectra of the dye after 6 h upon UV illumination, of TFAA and of TCA
deposited on the anatase film.
For the identification of the further reaction products GC/MS measurements were performed.
The ATR-FTIR analysis already evidenced that the TTFA ligand of the dye was decomposed,
therefore a reaction suspension containing NaTTFA and TiO2 in acetonitrile was illuminated
by UV light for 12 h. Subsequently, the filtrated liquid extract was analyzed by GC/MS. For
comparison a liquid sample before UV illumination was also analyzed by GC/MS. In Figure 7
the GC/MS chromatograms of the liquid extract before (inset) and after UV illumination are
shown. The peaks separated by GC were further analyzed by mass spectrometry to assign
them to the related compounds. The MS spectral patterns were compared with standard mass
spectra of the National Institute of Standards and Technology (NIST) library. The
corresponding mass spectra together with the observed compounds are shown in Figure 8.
Table 2 presents an overview of the different degradation products of NaTTFA determined by
the GC/MS analysis, their retention times and their spectral matching to the mass spectra of
the NIST library. The peak in the inset as well as the one with a retention time (RT) of 15.7
min in Figure 7 is referred to the educt TTFA because the mass spectra highly correlate with
the standard TTFA (see Table 2). After 12 h of UV illumination the intensity of the TTFA
peak strongly decreases from 40 to 1 revealing the photocatalytic degradation of TTFA. The
most intense peak in the GC/MS chromatogram with a RT of 15.3 min is referred to TCA
with a high similarity of 87 % of the mass spectra of the compound and the standard TCA.
Hence, the GC/MS analysis confirms the assumption derived from the ATR-FTIR results that
TCA is formed as a degradation product. The second intense peak that elutes at a RT of 11.1
min shows matching with 2-thiophenecarboxaldehyde. It is assumed, that 2-
thiophenecarboxaldehyde is an intermediate which can be oxidized forming TCA.
Furthermore, at a RT of 9.6 min a peak with small intensity is observed showing the highest

similarity to TFAA. Apparently both ATR-FTIR and GC/MS analyses evince the formation of
TFAA. TFAA seems to be strongly adsorbed on the TiO2 surface while TCA does not show
such a high interaction because the intensity of the peaks observed by GC/MS is much smaller
for TFAA than for TCA. For verification of these assumptions, adsorption experiments in the
dark with TCA and TFAA were performed. For this purpose solutions of TCA (25 mmol) and
TFAA (150 mmol) in acetonitrile were prepared and analyzed by GC/MS and GC, then TiO2
was added allowing TCA and TFAA to adsorb on the TiO2 surface. After 2 h in the dark the
suspensions were filtrated and the filtrate was analyzed again by GC/MS and GC. When
comparing the peaks before and after the dark adsorption (see Figure S1 and Figure S2), the
intensity of the peak of TFAA disappears after the dark adsorption therefore confirming the
ATR-FTIR results of a strong adsorption of TFAA on the TiO2 surface. The intensity of the
peak of TCA also decreases to 32 % of its original value after the dark adsorption, indicating
a partially adsorption of TCA on the TiO2 surface. As illustrated in Figure 9 TFAA and TCA
are both probably adsorbed in a bidentate coordination on the TiO2 surface.[18,19] However,
TFAA is a rather small molecule having one polar CF3 group left, while TCA has a nonpolar
thiophene rest which should not interact with the polar surface. Hence, the interaction
between TFAA and the TiO2 surface is stronger as compared to that of TCA with TiO2.
Figure 7: Gas chromatogram of NaTTFA before (inset) and after UV illumination for 12 h.

Figure 8: Mass spectra of TFAA (RT 9.6 min), 2-thiophenecarboxyaldehyde (RT 11.1 min),
TCA (RT 15.3 min) and TTFA (RT 15.7 min).
Table 2: Degradation products of NaTTFA identified by GC/MS.
Retention
Time / min
9.6
Name
Chemical
Structure
Molecular
weight / gmol-1
114
Similarity
81
Trifluoroacetic
acid
11.1
15.3
15.7
2-Thiophene-
carboxaldehyde
112
128
222
77
87
87
2-Thiophene-
carboxylic acid
Thenoyltrifluoro-
acetone
Figure 9 summarizes the ATR-FTIR and GC/MS results in form of a reaction scheme. It is
most likely that upon UV illumination the dye is oxidized by the OH˙ radicals rather than
directly by holes since in the latter case a strong interaction between the dye and the TiO2
surface is required.[20] Accordingly, the generated OH· first attacks the acidic CH group of
the TTFA ligand forming a TTFA radical. In the next reaction steps it is assumed that OH·,
O2˙- and O2 are involved in the reactions attacking the carbonyl groups rather than the
thiophene, because the detected reaction intermediates still contain thiophene. In addition CO2
should be released because TTFA contains one C-atom more than TFAA and TCA.

Figure 9: Photocatalytic degradation of the dye in the presence of TiO2 forming TCA, TFAA
and 2-thiophenecarboxaldehyde as concluded from the ATR-FTIR and GC/MS results.
Conclusions
In the present study, for the first time, the adsorption and the photocatalytic degradation of the
luminescent dye EuIII(TTFA)3(MePhTerpy) on the TiO2 surface was investigated by ATR-
FTIR spectroscopy and by GC/MS analysis. The dye is found to be adsorbed through the
TTFA ligand on the anatase surface. It is most likely that distinct polar atoms of the TTFA
ligand form hydrogen bonds with OH groups of the TiO2 surface. Upon UV illumination the
ATR-FTIR spectra reveal a decomposition of the TTFA to TFAA and TCA, with the former
being generated predominantly. The formation of these products could be confirmed by
GC/MS analysis. Furthermore, 2-thiophenecarboxaldehyde is identified which is proposed to
be an intermediate that is further oxidized forming TCA. TFAA is detected only to a small
extent because it is strongly adsorbed on the TiO2 surface. It is proposed here that upon UV
illumination the dye is not directly oxidized by holes, but that rather the photogenerated OH˙
favorably attack the acidic CH group. In the next step, an oxidative attack of the carbonyl
group of the TTFA ligand occurs. Additionally, CO2 should be generated through the
decarboxylation of TTFA which has one C-atom more than the products TFAA and TCA.
Acknowledgement
This work was funded by the German Federal Ministry of Education and Research (contract
no. 13N13350, PureBau – Untersuchung von Werkstoffsystemen für photokatalytisch
hocheffiziente Baustoffe- Teilvorhaben: Oberflächenchemie der Photokatalysatoren und der
Werkstoffe). Furthermore, Y. Alsalka gratefully acknowledges the financial support from the
DAAD (Deutscher Akademischer Austauschdienst). Additionally, we thank KRONOS® for
providing the TiO2 materials and Dr. Johannes from the Technische Universität Braunschweig
for the synthesis of the dye.
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