Development of a visible-light photocatalytic membrane material
H. Mamane*, D. Avisar*, I. Horovitz*, I. Zucker*, J. Pulpytel**, W. Smith**, D. Di Camillo***,
F. Ruggieri***, M.A. Baker****, G. Heinicke*****, A.D. Enevoldsen*****, O. Grønborg******,
E. Çetinörgü -Goldenberg*, F. Arefi-Khonsari**, L. Lozzi***
* Tel-Aviv University, Tel Aviv, 69978, Israel, [email protected], [email protected],
[email protected], [email protected], [email protected]
** University Pierre & Marie Curie, ENSCP, 11 rue Pierre et Marie Curie, 75231 Paris cedex 05, France, [email protected], [email protected], [email protected]
*** University of L'Aquila, Department of Physics, Via Vetoio, 67010 Coppito, L'Aquila, Italy,
[email protected], [email protected], [email protected]
**** University of Surrey, The Surface Analysis Laboratory, Guildford, Surrey, GU2 4DL, UK, [email protected]
***** DHI, Agern Allé 5, 2970 Hørsholm, Denmark, [email protected], [email protected]
****** Skjølstrup and Grønborg ApS, Niels Jernes Vej 2-4, 9220 Aalborg Ø, Denmark, [email protected]
Abstract: The objective of the EU project NATIOMEM is to develop a membrane material with
photocatalytic properties in visible light, based on nitrogen-doped titanium dioxide (TiON). Sol-gel
coating, filtered vacuum arc deposition and sputter deposition have been tested in several variants.
The photocatalytic activity of the produced coatings has been quantified on glass slides in batch
reactors, measuring the degradation of persistent pharmaceutical Carbamazepine. Results varied
widely between methods and conditions applied. Sol-gel dip coating and oxidised sputtered
titanium nitride (TiN) samples produced promising coatings that were chosen for further
investigations of coated membranes in a flow cell.
Keywords: batch reactor, coating, nitrogen-doped titanium dioxide, photocatalytic activity, solar
simulator
Introduction
The use of semi-conductors in combination with sunlight irradiation (i.e. photocatalysis) for the
treatment of water and wastewater has attracted growing attention and intense research interest over
the last decade (Rizzo et al., 2009). The degradation of organic compounds and inactivation of
microbes by photocatalysis has been widely investigated (Mozia, 2010). Titanium dioxide (TiO2) is
photocatalytic in UV light, but may be doped with either transition metals or non-metal anions.
Nitrogen-doped TiO2 (TiON) has some photocatalytic activity in visible light (Morikawa et al.,
2005).
NATIOMEM is a collaborative research project co-funded by the European Commission. Its
objective is to develop a membrane material with photocatalytic properties in visible light, to
combine filtration and oxidation in one material. The membrane material will be integrated in a
simple unit to treat water using solar irradiation, preferably without the use of electricity or
chemicals (NATIOMEM, 2012).
In the project, seven coating methods have been investigated at three university laboratories,
including pulsed magnetron and radio frequency sputter deposition (Zhang et al., 2004; Mardare
and Rusu, 2000), dielectric barrier discharge (Nasanova et al., 2011), atmospheric pressure cold
plasma jet (Huang et al., 2010), filtered vacuum arc deposition (Çetinörgü et al., 2009), sol-gel
deposition (Lev et al., 1997) by dip coating and spray deposition, and electrospinning (Li and Xia,
2003). Membrane substrates range from ceramic and metal membranes to clay filter materials. This
publication describes the methods for the investigation of coated membrane substrates, and reports
the results of the first characterisations.
The photocatalytic effect of coated surfaces or photocatalytic nanoparticles is commonly quantified
by the degradation of specific substances such as dyes, refractive chemicals such as some
pharmaceuticals, and specific radical markers. Dyes such as methylene blue are frequently used for
tests, since they are easily quantified by spectrophotometry. The substances are however susceptible
to degradation by photolysis, i.e. without photocatalysis, which must be taken into account when
conducting the experiments. Several pharmaceuticals have been used for quantifying the
photocatalytic effect, such as Carbamazepine (5H-Dibenz[b,f]azepine-5-carboxamide), Propranolol
(2-Propanol,1-(isopropyl-amino)-3-(1-naphthyloxy)-, hydrochloride), and Ibuprofen (2-(4isobutylphenyl) propionic acid.
Carbamazepine (CBZ) is the most frequently detected pharmaceutical in various water sources.
Usually, carbamazepine is excreted with <3% remaining in its unaltered form and flushed directly
to the wastewater treatment plants (WWTPs) through the sewage system. Studies determined that
carbamazepine is persistent, and its removal efficiencies by the WWTPs are mostly below 10%. In
the classification scheme for pharmaceutical biodegradation, the removal status of carbamazepine is
classified as “no-removal”. Joss et al., 2006, Clara et al. (2004), and Kosjek et al. (2009) reported
that CBZ is persistent to biodegradation and shows almost no elimination during conventional
wastewater treatment. Carbamazepine has been detected in wastewater (Miao et al., 2005
Castiglioni et al., 2006 and Reemtsma et al., 2006), surface waters (Tixier et al., 2003 Dsikowitzky,
et al., 2004 and Wiegel, et al., 2004), and groundwater (Scheytt et al., 2006 and Fenz et al., 2005).
The main goal of this study is to determine the photocatalytic activity of N doped TiO2 coating, for
the best coating technique. To achieve this goal, initial screening tests were conducted and the most
effective deposition techniques for nanostructured photocatalytic TiON films were identified.
Material and Methods
Materials and experimental set-up
Carbamazepine (CBZ) (>99% purity), was obtained from Sigma-Aldrich. Stock solutions of CBZ
were prepared by dissolution in deionized (DI) water (Direct-Q3 UV system, Millipore, France) at a
concentration of 50 mg/L.
N-doped TiO2 coatings were deposited on 25 × 76 × 1 mm commercial microscope glass substrates
by the various techniques. The photocatalytic coated sample was immersed in a 90 mm x 50 mm
Pyrex glass plate, containing 30 mL of aqueous solution of CBZ at initial concentration of 1 mg/L
(4.24x10-6 M). Experiments were carried out at the neutral pH=7 adjusted using phosphate buffer
saline (Na2HPO4/NaH2PO4) at a concentration of 1mM. The examined solution was stirred for 60
minutes in the dark to ensure adsorption/desorption equilibrium of CBZ on the catalyst prior to its
irradiation. Afterwards, the sample was irradiated under a solar simulator for 90 minutes. Subsamples of 0.25mL were taken at predetermined times and analysed using a High Performance
Liquid Chromatography (HPLC). The HPLC system (Agilent, model 1100) was equipped with a
vacuum solvent degassing unit, a quaternary high-pressure gradient pump, an automatic sample
injector, a thermostatic column compartment and of UV-Diode Array Detection (UV-DAD) with
two lamps (deuterium and tungsten) to ensure the highest light output from 190 to 950 nm. The
selected LC column was a Phenyl Reverse Phase column (ACE-RP, 2.1 mm × 250 mm, 5 µm). The
mobile phase consisted of water and methanol, at pH 3 (0.1% formic acid). In order to obtain a high
sensitivity for the detected pharmaceuticals (by obtaining a reduced peak width), a gradient program
was used at a flow rate of 0.5 mL/min.
Light source:
Irradiation experiments were carried out by a 150 W Ozone free xenon arc lamp solar simulator
(Sciencetech Inc., SS150W, Canada) with a sharp cut off at 256 nm and an average irradiance of
approximately 800 W/m2. The light beam was filtered with a 1.5 Global air mass (AM) filter to
enable the output spectrum of the solar simulator to be equivalent to natural sunlight at 48.2°
latitudes at sea level. Incident irradiance as a function of wavelength was measured by the
calibrated spectroradiometer (International light, ILT 900R, USA) with a horizontal detector. This
device allows estimating the amount of intensity by wavelength, from 250 nm to 950 nm with a
radiometric accuracy of ±5% between 400-950 nm and ±10% between 250-400 nm. Table 1
summarises the incident irradiance values for various spectrum regions of the solar simulator.
Table 1: Incident irradiance values of solar simulator measured by international light, ILT 900R,
USA
Radiation type
total VIS
total UV
UV-B
UV-A
VIS
IR
λ(nm)
400-700
280-400
280-320
320-400
400-500
500-600
600-700
700-800
800-900
900-950
I (µW/cm2)
26360.91
2468.31
74.71
2382.53
9400.90
8973.08
7986.94
6953.04
9642.27
5754.50
I (W/m2)
263.61
24.68
0.75
23.83
94.01
89.73
79.87
69.53
96.42
57.55
Also, the solar spectrum was measured on the building roof at 12 pm and plotted against the solar
simulator spectrum in Figure 1, and experimental conditions in Table 2.
Incident irradiance [µW/cm2/nm]
1000
solar simulator
solar spectrum
800
600
400
200
0
200
300
400
500
600
700
800
900
1000
Wavelength [nm]
Figure 1: Comparison between the spectrums obtained from the solar simulator and real sun
Table 2: Experimental conditions for photocatalytic degradation of carbamazepine using a solar
simulator.
Parameter
Carbamazepine initial concentration, C0
Cbuffer Na2HPO4/ NaH2PO4 (65:35)
pH
Initial volume, V0
Vessel dimensions
Light source
Coated area
Mixing
Analysis
Sample volume (mL)
Sorption time before irradiation
Irradiance time
Value
1 ppm
1 mM
7 (tested at t0 and final)
30 ml
90 x 50mm Pyrex glass plate
150W Ozone free xenon arc lamp
25 x 75 mm (microscope slide)
Magnetic stirrer
HPLC/MS
0.2
60 min
90 min
The sequence of the coating methods and their examination included the following steps:
1.
2.
3.
4.
Coatings Deposited by Reactive Magnetron Sputtering, using Dual Gas, and annealed TiN
Coatings Deposited by Reactive Arc Evaporation
Coatings and Fibres Deposited by Sol Gel and Electrospinning
Determination of the Contamination Removal Kinetics of N-doped TiO2 coatings
Results and Discussion
Radio-frequency (RF) magnetron reactive sputtering: Sputtering is a physical vapour deposition
process wherein a target is bombarded by energetic gas ions, kinetically ejecting atoms from the
surface of the target which can subsequently deposit on a substrate. In reactive sputtering, the
deposited film is formed by chemical reaction between the target material and a gas which is
introduced into the vacuum chamber. TiON coatings were prepared in Laboratoire de Genie des
Procedes Plasmas et Traitement de Surface at Pierre et Marie Curie ENSCP (Chimie Paristech)
University, France (UPMC).
Filtered vacuum arc deposition (FVAD): In filtered vacuum arc deposition (FVAD), a high current
discharge is sustained at naturally occurring, minute hot spots on the cathode surface. These
‘cathode spots’ emit hypersonic jets of fully ionized plasma of the cathode material and a spray of
liquid droplets, known as macroparticles (MPs). The MPs are filtered out by magnetically bending
the plasma beam through a bent duct which occludes any straight line path between the cathode and
the substrate. TiON coatings were deposited in Electrical Discharge and Plasma Laboratory (EDPL)
at Tel Aviv University (TAU).
Sol-Gel deposition: “Sol-gel” describes a broad class of processes in which a solid phase is formed
through gelation of a colloidal suspension (sol). The technique uses two consecutive reactions of a
precursor (e.g. a metal-alkoxide monomer), hydrolysis and condensation, which are catalysed under
acidic or alkaline conditions and can be achieved by adding acids or bases to the solution.
Alternatively, this can be performed electrochemically by altering the pH at the electrode interface:
Cathodic reduction of oxygen consumes protons and therefore increases pH, while anodic oxidation
of water releases protons and decreases pH. Drying of the gel after condensation can yield the “dry
gel” (xerogel) state and subsequent heat treatment can be used to remove unreacted organic
residues, stabilize, densify, or crystallize the gel. TiON sol-gel coatings were prepared by
hydrolysis of Ti-alkoxide in alcoholic solutions containing various nitrogen compounds (e.g. urea)
in the Department of Physics at University of L’Aquila (UNIAQ), Italy.
Under the solar simulator at TAU’s water laboratory, the photocatalytic degradation of
carbamazepine (CBZ) was quantified (Figure 2). The photocatalytic efficiency can be expressed in
many terms. One of the most common is removal percentage:
Removal (%) =
C0 −Ct
C0
× 100 Equation (1)
where C0 is the initial pollutant concentration and Ct is pollutant concentration after t hours of
irradiation (mg/L).
1.00
0.90
blank
0.80
FVAD _TiO2
0.70
C / C0
FVAD _0%N
0.60
FVAD _33%N
FVAD _41%N
0.50
FVAD _91%N
0.40
Sol-Gel
0.30
Sputter
Deposition
0.20
-60
-45
-30
-15
0
15
30
45
60
75
90
t (min)
Figure 2: Photocatalytic degradation of CBZ by TiO2 and TiON coatings, using a solar simulator.
The non-annealed FVAD TiO2 sample in Figure 2 did not have photocatalytic activity. Several
concentrically-coated FVAD TiON samples (0-91% N) had weak photocatalytic activity. The
electospun fibres and the spray coating sample could not be investigated, as these coatings did not
have sufficient adhesion to the surface.
Despite CBZ’s relative stability under direct UV photolysis (data not shown), it is susceptible to
degradation by hydroxyl radicals and therefore a good candidate for AOP based technologies, such
as the one investigated in this study. Other researchers received similar results. For example, Vogna
et al. (2004) investigated advanced oxidation of CBZ with the UV/H2O2 system under low-pressure
mercury lamp (emitting light at 254 nm) and reported a high decay rate associated with 35%
removal of total organic carbon (TOC) when substrate degradation was complete (~4 min) with
170 ppm H2O2. Over the same reaction time, direct photolysis of CBZ was negligible, and highly
susceptible to radical induced reactions.
Sorption to the catalysts surface proved to be an important parameter for powders. Compared to
powder catalysts, the surface area and porosity is much lower for thin film catalytic substrates.
Consequently, adsorption of the catalyst to target pollutant molecules showed be examined. Doll
and Frimmel (2005) investigated photodegradation of CBZ, iomeprol and clofibric acid on TiO2
P25 (Degussa) and Hombikat UV100 and found a connection between adsorption to the catalyst and
% removal, with lowest adsorption value for CBZ for both catalyst type. Carbamazepine is a neutral
molecule and therefore may not adsorb to the catalyst surface by charge attraction forces. CBZ
showed no sorption after 24 hour period, thus results presented in the table below shows the net
photocatalytic activity removing the impact of sorption and photolysis, after 90 minutes of exposure
to the solar simulator. To conclude, CBZ susceptibility to degradation by AOP processes as
opposed to its stability to direct solar photolysis makes it a good candidate for this research purpose.
There was a large spread in the photocatalytic activity between the coated and annealed samples.
Several of the investigated coatings degraded about two thirds of the initial CBZ concentration
within 90 minutes, as detailed in the following tables 3 through 5.
Table 3 Results summary for CBZ degradation by Sol Gel (by UNIAQ):
Sample Preparation conditions
13.11.2011
Electrospun (N/Ti=1) annealed at 400°C/1h in air. The substrate was
N1
etched by plasma (RIE)
Fe (0.1%) doped TiO2 prepared by electrospinning annealed at 500°C/1h
O1
in air. The substrate was etched by plasma (RIE)
25.09.2011
Sol-gel deposited by dip-coating 150°C/12h & 600°C/1h in air
H1
Sol-gel deposited by dip-coating 150°C/12h 510°C/1h in air_550°C/3h in
I1
N2
Sol-gel deposited by dip-coating 150°C/12h_510°C/1h in air_550°C/3h in
L1
NH3
Sol-gel deposited by dip-coating
M1
% removal,
photocatalysis
only
7
49
66.6
59.8
<1
27.3
100°C/1h_500°C/1h in air
12.06.2011
TiO2+N electrospun nanofibers (1N:1Ti) annealed at 400°C/1hr in air
A2
Spray TiO2+N thin film annealed at 500°C/1hr in air
F2
Sol-gel TiO2+N thin film annealed at 150°C/12hr_510°C/1hr in air
E2
39.6
26.6
66.1±5.9
Sol-gel dip coating produced mechanically stable coatings with promising photocatalytic
capabilities around 66% degradation of CBZ.
Table 4 Results summary for CBZ degradation by TiN oxidized samples (by UPMC)
Sample
No.
26.10.2011
1
2
3
4
5
6
7
8
9
10
18.10.2011
1
2
3
4
5
10
11
9
Conditions
Pressure(mtorr) N2/O2
(cm3/min)
Annealing
Temp
Time
(°C)
(hr)
% removal, photocatalysis only
14
14
14
14
14
14
14
14
10
10
1.4
1.4
1.5
1.5
1.5
1.5
1.4
1.6
2
2
450
450
450
450
450
350
350
350
350
350
1
0.5
0.5
1.5
1
8
8
8
8
4
26.3
<1
8.3
13
18.3
1.9
6.5
12
14.8
2.3
14
14
14
14
14
14
14
14
3.6
3
3.6
2.7
2.3
2.7
2.3
2.3
350
350
350
350
350
350
350
450
8
8
8
8
8
24
24
1
8.3
13.6
3.4
11.9
21.1
25.2
34.1
45.9
Table 5 Results summary for CBZ degradation by FVAD N-doped TiO2 coatings (by TAU)
% removal, photo-catalysis
only
15.02.2012 Deposited at 8 mtorr pressure. Annealed at 400°C for 15min in air
Environment (gasses %): 90% N2 (70 s). 1.5
TiON_c34
Environment (gasses %): 50% N2 (90 s). 1.1
TiON_m27
Environment (gasses %): 100%N2 (50
2.7
TiON_c45
s).
22.05.2011: Deposited at 8 mtorr pressure Annealed at 400°C for 15min in air.
150s deposition time. 41% N.
7.1
Feb_TiON_35
150s deposition time. 33% N.
5.1
Feb_TiON_68
150s
deposition
time.
0%N.
1.8
Feb_TiON_79
240s deposition time 0%N.
10.2
TiO2-full
Sample
Preparation conditions
coverage
In addition to the coatings shown in table 5, there were six more samples of N-doped TiO2 prepared
by FVAD that showed CBZ removal below 1% (data not shown). The best FVAD-coating under
solar light was a non N-doped titanium dioxide annealed at 400°C in air.
The best coatings on glass slides examined by CBZ removal are as follows:
1.
2.
Sol-gel samples deposited by dip-coating showed 66-67% removal of CBZ in the 90-minute
batch test. These were annealed for 150°C for 12 h and then at 510°C or 600°C for 1h in air.
TiN oxidized sample prepared at 14 mtorr, and annealed at 450°C for 1 hr, giving a
photodegradation of 45.9% CBZ degradation.
The best coatings were transferred to ceramic membrane substrates and metal filter materials.
Samples sized (35 x 55 mm) are currently being characterised in a custom-made flow cell described
by Heinicke et al. (in press).
Conclusions
Photocatalytic coatings were prepared on microscope glass slides by the three coating laboratories
involved in the NATIOMEM project, by sol-gel coating, filtered vacuum arc deposition and sputter
deposition in several variants. Photocatalytic activity was quantified by a batch test that quantified
the degradation of Carbamazepine (CBZ) under a solar simulator.
Results varied widely between the coating methods and conditions applied. The batch test applied
was useful for screening the samples for photocatalytic activity. Sol-gel dip coating and oxidised
sputtered titanium nitride (TiN) samples produced promising coatings that were chosen for further
investigations of coated membranes in a flow cell.
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The research leading to these results has been coco-funded by the 7th Framework Programme of
the European Community (FP7) under grant
agreement No. 245513.