Cold sprayed WO3 and TiO2 electrodes for photoelectrochemical water and methanol oxidation in renewable energy applications

Article abstract of DOI:10.1039/c7dt02063e

Films prepared by cold spray have potential applications as photoanodes in electrochemical water splitting and waste water purification. In the present study cold sprayed photoelectrodes produced with WO₃ (active under visible light illumination) and TiO₂ (active under UV illumination) on titanium metal substrates were investigated as photoanodes for the oxidation of water and methanol, respectively. Methanol was chosen as organic model pollutant in acidic electrolytes. Main advantages of the cold sprayed photoelectrodes are the improved metal-semiconductor junctions and the superior mechanical stability. Additionally, the cold spray method can be utilized as a large-scale electrode fabrication technique for photoelectrochemical applications. Incident photon to current efficiencies reveal that cold sprayed TiO₂/WO₃ photoanodes exhibit the best photoelectrochemical properties with regard to the water and methanol oxidation reactions in comparison with the benchmark photocatalyst Aeroxide TiO₂ P25 due to more efficient harvesting of the total solar light irradiation related to their smaller band gap energies.

Full text of DOI:10.1039/c7dt02063e

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Schneider, M. Fleisch, H. Gutzmann, T. Klassen and D. W. Bahnemann, Dalton Trans., 2017, DOI:
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0.1039/C7DT02063E.
Volume 45 Number
1
7
January 2016 Pages 1–398
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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
rsc.li/dalton

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DOI: 10.1039/C7DT02063E
Cold Sprayed WO and TiO Electrodes for Photoelectrochemical Water
3
2
and Methanol Oxidation in Renewable Energy Applications
Affiliations:
a
a
a
b
b,c
Christoph Haisch , Jenny Schneider , Manuel Fleisch , Henning Gutzmann , Thomas Klassen and
a,d
Detlef W. Bahnemann
a
Institut für Technische Chemie, Leibniz Universität Hannover, Callinstrasse 5, Dꢀ30167 Hannover,
Germany
b
Institute of Materials Technology, Helmut Schmidt University, Holstenhofweg 85, Dꢀ22043
Hamburg, Germany
c
Institute of Materials Technology, HelmholtzꢀZentrum Geestacht, MaxꢀPlanckꢀStr. 1, Dꢀ21465
Geestacht, Germany
d
Laboratory “Photoactive Nanocomposite Materials“ (Director), SaintꢀPetersburg State University,
Ulyanovskaya str.1, Peterhof, SaintꢀPetersburg, 198504 Russia
Key Words: cold spray, photoelectrochemical reaction, visible light, waste water purification, organic
pollutant degradation, methanol oxidation, water oxidation, cold sprayed photoanodes
Abstract:
Films prepared by cold spray have potential applications as photoanodes in electrochemical water
splitting and waste water purification. In the present study cold sprayed photoelectrodes produced with
WO (active under visible light illumination) and TiO (active under UV illumination) on titanium
3 2
metal substrates were investigated as photoanodes for the oxidation of water and methanol,
respectively. Methanol was chosen as organic model pollutant in acidic electrolytes. Main advantages
of the cold sprayed photoelectrodes are the improved metalꢀsemiconductor junctions and the superior
mechanical stability. Additionally, the cold spray method can be utilized as a largeꢀscale electrode
fabrication technique for photoelectrochemical applications. Incident photon to current efficiencies
reveal that cold sprayed Ti/WO
regard to the water and methanol oxidation reactions in comparison with the benchmark photocatalyst
Aeroxide TiO P25 due to more efficient harvesting of the total solar light irradiation related to their
smaller band gap energies.
3
photoanodes exhibit the best photoelectrochemical properties with
2

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1. Introduction
Since the first attempts concerning photoelectrochemical water splitting by Fujishima and Honda in
1
1
972 there has been a tremendous development in the field of heterogeneous photocatalysis. A wide
variety of semiconductor films have been investigated during the past 40 years and titanium dioxide
was found to be the most stable and powerful photocatalyst leading to the degradation of organic
2
, 3
compounds. The degradation of organic pollutants combined with the reduction of water yielding
molecular hydrogen is an important application for photoelectrochemical cells. A considerable amount
of research has been done regarding the decomposition of persistent organic chemicals which are
4
, 5
present as pollutants in industrial and household wastewater.
In this work tungsten oxide and
titanium dioxide photoanodes prepared by cold spray were investigated in the wellꢀknown reference
systems of water oxidation and methanol degradation (as a model pollutant), respectively. The aim of
the work is the development of a chemically stable photoanode for the industrial scale oxidation of
organic pollutants.
The technique of cold spray has previously been employed for the preparation of photoelectrodes.
6
Initially, cold sprayed TiO
Emmler et al. and HerrmannꢀGeppert et al. performed investigations of cold sprayed TiO
2
layers on metal surfaces were characterized by Kliemann et al. in 2011.
and Fe
2
7
2
O
3
, 8
photoelectrodes for photoelectrochemical water oxidation in 2013 and 2016, respectively. Ivanova
et al. employed TiO cold sprayed films on metal substrates for the photocatalytic degradation of
organic acids. However, to the best of our knowledge WO films prepared by cold spray have not
been investigated so far. Compared to other coating techniques employed to prepare WO films like
doctor blade, magnetron sputtering, atomic layer deposition, spray pyrolysis, and solvothermal
2
9
3
3
1
0
11
12
13
1
4
deposition, the cold spray technique readily enables an upscaling for the manufacturing of
mechanically stable films on metal substrates to an industrial scale with variable electrode sizes.
2
One disadvantage of employing TiO as a photocatalyst is its large band gap energy of 3.0 eV (rutile)
which corresponds to an absorption of light with wavelengths ≤ 413 nm resulting in the utilization of
1
5
only a very small part of the incoming sunlight (4 % ꢀ 5 %). Since the early works by Hodes et al.
and separately by Butler et al.
1
6, 17
tungsten trioxide has been considered as an alternative and
promising photoanode material. The smaller optical band gap of tungsten oxide (2.6 eV) allows a
more efficient harvesting of the total solar light irradiation due to some visible light absorption.
1
8
Despite the smaller band gap, the valence band position of tungsten oxide is still matching the redox
potentials for the four hole oxidation of water (H O/O , 1.23 V vs. NHE, pH 0), the one hole oxidation
of water (H O/ꢁOH, 2.73 V vs. NHE, pH 0) and the one hole oxidation of methanol (CH OH/·CH OH,
.45 V vs. NHE, pH 0). Like for TiO the location of the valence band is determined according to the
location of oxygen 2pꢀorbitals with an energy of 2.94 V vs. NHE. In 2001, the photoelectrochemical
degradation of methanol in acidic solution has been evaluated for highly transparent WO films
2
2
2
3
2
19
1
2
2
0
3
2
1
prepared the by deposition from colloidal tungstic acid solutions.
In this work tungsten oxide and titanium dioxide photoanodes were investigated in the wellꢀknown
reference systems of water oxidation and methanol degradation, respectively. The activity of the
photoelectrodes was investigated by determining the incident photon to current efficiency (IPCE) and
by chopped light voltammetry (CLV) in acidic media in the presence and in the absence of methanol.
The CLV measurements were carried out under simulated solar AM 1.5ꢀglobal irradiation.
2. Experimental Section
2.1 Materials. All chemicals were of analytical grade and used as received without further
purification. Tungsten oxide was obtained from SigmaꢀAldrich. Concentrated sulfuric acid and
methanol were obtained from Carl Roth Company. Aeroxide TiO P25 and VP Aeroperl P25/20 (both
2
22
TiO
2
: 80 % anatase and 20 % rutile ) were obtained from Evonik Industries AG. Titanium metal
substrates were obtained from ThyssenKrupp Schulte GmbH.

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2
.2 Preparation of Electrodes. Photoanodes of TiO and WO were prepared by two methods, screen
2 3
printing and cold spray. For the screen printing method, P25 was employed and corresponding pastes
23
were prepared as described in the literature. The pastes were coated on substrates of Ti metal and
FTO (Fluorineꢀdoped tin oxide SnO :F) glass and subsequently annealed at 450 °C for 2 hours. For the
cold spray technique, Aeroxide TiO P25 had to pass an agglomeration process to match the
2
2
requirements for cold spray. The resulting VP Aeroperl P25/20 powders (Evonik Industries AG)
provide the desired flowability, which usually requires particle sizes above 10 µm and spherical
particles.
7
These P25/20 agglomerates show flattening and partial breaking after the cold spray process. The
cold spray process was performed with the HSU CGT Kinetiks 8000 prototype by using nitrogen as
process gas at a temperature of 800 °C and a pressure of 4 MPa. A type 24 WCꢀCo nozzle and a
powder feeder PF4000 (CGT, Cold Gas Technology GmbH) were employed. The traverse speed of the
spray gun was 240 mm/s and the powder feed rates were 0.16 g/s and 0.47 g/s for TiO and WO ,
2
3
respectively. Within the cold spray process the solid powders were accelerated in the N gas stream to
2
very high velocities towards the Ti metal substrates at a standꢀoff distance of 60 mm.
The name of the so called “cold spray” technique derives from the comparison to other common
thermal spray techniques like plasma spray or high velocity oxygen fuel (HVOF), where relatively
high temperatures of at least 2000 °C are applied during the spray process to the particles. In contrast
to this, the process gas temperatures of the cold spray method usually do not exceed the employed
8
00 °C. After the nitrogen process gas has passed the smallest cross section of the Laval nozzle used
2
4
in cold spray, the gas temperature decreases strongly in the diverging section. Under the used spray
conditions, the nitrogen gas has a temperature of around 150 °C when it leaves the nozzle. It should
also be noted here, that the solid semiconductor particles are only exposed to the hot nitrogen process
gas on a time scale of a few milliseconds until they reach the metal substrate. Consequently, no phase
transformations could be observed and the crystallographic structure of the semiconductor powder
could be preserved. Accordingly the presence of a predominant anatase phase was confirmed by Xꢀray
diffraction (XRD) measurements (Figure 2) in the cold sprayed P25 films. In the cold spray process
the powder particles show pseudoꢀdeformation upon impact, presumably by a fracture and fusion
mechanism leaving residual cracks, while the metal substrates deform plastically and the interface area
25
is stretched out. As a result, an intimate contact is formed and the metal oxide is bonded to the metal
substrate as illustrated in Figure 1. No change in TiO modification during the cold spray process
2
6
ꢀ8
could be observed because the particle’s dwell time in the gas stream is very short. An SEM image
of a cold sprayed P25/20 photoanode film on a titanium substrate is shown in Figure 4. The cold
sprayed electrodes were also annealed at 450 °C for 2 hours in air to facilitate direct comparison with
the screenꢀprinted electrodes.
2
Figure 1. Scheme of the impact of a TiO cold sprayed particle (white) on a Ti metal substrate (grey).
The powder particle deforms and partially cracks during impact while the metal substrate deforms
2
plastically. As a result, the TiO particle is bonded to the metal without the help of any additive or
binder.

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2.3 Materials Characterisation. The optical properties of the powders and deposited films were
determined using UVꢀVis diffuse reflectance spectroscopy (Varian Cary 100 Bio). The structural and
morphological properties of the films were analysed using Xꢀray diffraction (Bruker D8 Advance) and
scanning electron microscopy (JEOL JSMꢀ6700F) with a LEI detector (Lower Secondary Electron
Image).
2.4 Electrochemical and Photoelectrochemical Measurements. The measurements were carried out
with a three compartment cell consisting of a working (photoꢀ) electrode (WE), a Pt wire counter
electrode (CE) and a Ag/AgCl reference electrode (RE). 0.1 M KCl solution was used as electrolyte
for MottꢀSchottky capacitance analysis. The MottꢀSchottky measurement was carried out at a single
frequency of 10 Hz. This frequency was derived from an electrochemical impedance measurement
over a wide frequency range where the imaginary part of the impedance was found to be purely
capacitive. In this work the flatband potentials and donor densities of the photoanodes were calculated
ꢀ2
from the resulting MottꢀSchottky plots (C vs NHE). 0.5 M acid solution of H
electrolyte for photoelectrochemical measurements. This system provides the most useful information
for comparing TiO and WO photoelectrodes, as WO itself is not stable at pH values higher than 5.
2 4
SO was used as the
2
3
3
The photocurrents were measured using a Zennium potentiostat and a PECCꢀ2 photoelectrochemical
cell (ZahnerꢀElektrik GmbH & Co. KG). The light source was a solar simulator (LOTꢀ
QuantumDesign GmbH) with a 300 W Xe lamp and an AM 1.5ꢀglobal filter providing an intensity of
680 W/m² in the regime of 280 – 1000 nm. The intensity of the lamp was measured with a SpectraRad
xpress spectral irradiance meter (BWTEK Inc.). During the chopped light voltammetry (CLV)
measurement the illumination source was chopped on and off with a frequency of 200 mHz. After the
photoelectrochemical oxidation experiments samples were taken from the electrolyte for directly
monitoring the produced formaldehyde as expected main product for methanol oxidation. Upon the
addition of the Nash reagent (0.02 M acetylacetone, 0.05 M acetic acid, 2 M ammonium acetate) to the
samples the formaldehyde will react to form 3,5ꢀdiacetylꢀ1,4ꢀdihydroꢀ2,6ꢀlutidine (DDL), which can
2
6, 27
be detected by fluorescence spectroscopy.
The DDL containing samples were excited with a
wavelength of 405 nm and showed an emission peak at 510 nm. This emission was used to calculate
the concentration of the produced formaldehyde from the photoꢀoxidation of methanol. For the
photoelectrochemical efficiency measurements a tuneable LED light source array system TLS03
(
ZahnerꢀElektrik GmbH & Co. KG) was employed. The efficiencies were received as wavelength
depended photocurrent efficiencies (A/W) and incident photon to current efficiencies (IPCE; %), while
only the latter will be shown in this work since the information provided by both values is identical.
The IPCE was calculated using the following equation 1:
mA
cm
ꢄjph
ꢅ
2ꢆꢄ ꢀꢇꢀ1239.8ꢀꢁVꢀꢇnmꢂ
mW
(
1)
IPCEꢀꢁλꢂꢀꢃꢀ
Pmonoꢀ ꢅ 2ꢆ ꢇꢀλꢀꢁnmꢂ
cm
where
j
ph denotes for the photocurrent density and
P
mono is the incident light intensity. The value of
and the speed of light . The spectra were
1239.8 (V ×
ꢀnm) was calculated from the Planck constant
h
c
measured in 1 nmꢀsteps by means of cubic spline interpolation between the samples (26 different
LEDs from 309 – 1019 nm). For better comparison of water and methanol oxidation efficiencies all
incident photon to current efficiency measurements were carried out at the same external bias of 1 V
vs. NHE. The photoelectrochemical measurements were carried out at room temperature (298 K).
3. Results
3.1 Electrode Characterization
2 3
XRD diffraction patterns of TiO und WO photoelectrodes prepared by cold spray technique are
shown in Figure 2.

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3 2
Figure 2. XRD patterns of WO (blue) and TiO (red) films deposited on titanium metal substrates by
cold spray, where W denotes for WO pattern, T for titanium metal pattern, R for rutile TiO pattern
3
2
2
and A for anatase TiO pattern.
All diffraction peaks on the XRD pattern could be addressed to the expected materials WO
TiO , anatase TiO , and Ti metal by DIFFRAC.EVA software from Bruker Corporation. The cold
spray technique has the advantage that only the carrier gas N is needed to form a coating. No other
3
, rutile
2
2
2
additives such as binders or solvents are needed for the film preparation. Hence, both types of
electrodes show distinct diffraction peaks of the titanium metal substrate and of the employed
2
semiconductor powders. The XRD pattern of the TiO electrode shows reflexes from both, the anatase
2
Θ: 25.1 ° (110), 2Θ: 47.9 ° (200), and the rutile modification 2Θ: 27.2 ° (110) as expected, since
2
2
P25/20 consists of about 80 % anatase and 20 % rutile. Two diffraction peaks (2Θ: 62.8 ° and 70.4 °)
could only be addressed to anatase or titanium metal, because both patterns have overlaying reflexes at
3
these angles. WO shows a considerable number of very close multiple diffraction peaks in the
regimes of 2Θ: 23 – 24 °, 33 – 34 °, 44 – 63 °, and 67 – 83 °. These reflexes partially overlap with
titanium metal reflexes with higher intensity as displayed in the diffractogram (see Figure 2, blue
curve). At 2Θ: 27.3 ° (110) the diffraction peak of the thermodynamically most stable TiO
modification rutile was observed in both diffractograms. This is an indication for the presence of a
TiO passivation layer on the surface of the Ti metal substrate for both photoanodes. However, during
2
2
the impact process, the particles have a very high kinetic energy, so that the agglomerates can be
8
assumed to penetrate the thin TiO passivation layer of the Ti metal substrate. After the coating
2
processes the investigated electrodes were annealed at 450 °C for 2 hours in air. The Ti metal substrate
was only partially oxidized as the deposited semiconductor material layer covers the Ti metal
substrate, while the FTO substrate was not oxidized at all.
The absorption spectra of the employed semiconductor powders and of the cold sprayed
photoelectrodes are shown in Figure 3 (left side). The absorption edge of the yellowꢀgreen WO
3
powder (blue curve) is about ~ 70 nm red shifted compared to the white TiO powder (red curve).
2
From the spectra it is obvious that the WO powder is able to harvest the visible light much more
3
efficiently while the TiO
trend was observed in the absorption spectra of the cold sprayed photoelectrodes (Figure 3, right side).
Moreover, it is obvious from the spectra that the cold sprayed TiO /Ti (black curve) and WO /Ti
green curve) photoelectrodes absorb much more of the incident visible light as compared to the
2
powder shows about 5 % higher absorption in the UV regime. A similar
2
3
(
semiconductor powders. This behaviour is attributed to the Ti metal substrates and the dark
colourations of the photoelectrodes.

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3 2
Figure 3. Absorption spectra of the employed WO (blue) and TiO P25/20 (red) powders (left side)
and of the cold sprayed Ti/WO (green) and Ti/TiO (black) photoelectrodes (right side). The purple
3
2
curve on the right side shows the absorption spectra of the employed blank Ti substrate.
The optical band gap of powders and electrodes were determined from diffuse reflectance
28
measurements employing the Tauc equation. Tungsten trioxide powder has an optical band gap E
.6 eV (λos ≈ 477 nm) while the tungsten trioxide electrode has an optical band gap of 2.76 eV
+ 6.15 %) which is slightly higher. The same effect is observed for the TiO photoelectrodes. The
g
of
2
(
2
optical band gap of the P25/20 powder is 3.0 eV (λos ≈ 413 nm) and the optical band gap of the
electrode is 3.13 eV (+ 4.33 %). It is worth to mention here that the optical properties of the powders
and electrodes were studied under the same experimental conditions. There are big changes in the
visible light absorption as presented in Figure 3 following the cold spray preparation method. Above
the band edges there is an increased visible range absorption. This can be attributed to the effect of the
Ti substrate (purple curve), which is absorbing more than 50 % of the incident light irradiation.
Moreover there was also a colour change of the WO powder from yellowꢀgreen to dark grey and a
3
2
colour change of TiO from white to light blueꢀgrey after the cold spray process. These observed
colour changes and band gap broadenings are attributed to an increased oxygen vacancy density
produced in the film by the high temperature treatment during the cold spray and the additional
calcination processes. Consequently, the absorption edge of the semiconductor films is blue shifted,
29, 30
resulting in a widening of the optical band gap energy according to the BursteinꢀMoss effect.
Figure 4. A) SEM surface morphology image of a cold sprayed TiO film. B) Crossꢀsectional SEM
2
3
image of a cold sprayed WO film on a titanium substrate at a magnification of 1400x. C) Crossꢀ

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3
sectional SEM image of the interface between the cold sprayed WO film and the titanium substrate at
a magnification of 8500x.
As second method for the preparation of the electrodes, the screen printing technique has also been
applied. This technique is well known as an easy and highly reproducible method for the fabrication of
electrodes with film thicknesses in the same order of magnitude as the cold sprayed electrodes.
However, the surfaces of the screen printed films show very different characteristics considering
2
3, 31, 32
morphology, transparency, and film thickness distribution.
2
The employed TiO screen printed
films were transparent, highly porous and had a wellꢀdefined film thickness of 2.9 ± 0.1 µm due to the
fixed mesh scale of the employed screen. They were attached to the FTO glass substrate via a binder.
Electron microscopy images reveal that the technique of cold spray is leading to a plastic deformation
of the employed smooth metal substrate (Figure 4). Additional SEM images and studies of the impact
6
morphologies of cold sprayed TiO
2
layers are given in the literature. The impact of the powder
particles with high velocities generates heat resulting in bonding as well as in a partial breakꢀup of the
7, 8
P25 agglomerates, namely P25/20. The metal oxides are bonded to the metal substrate without the
assistance of any additive or binder that might affect the photocatalytic process. Nevertheless, the
coupling of film and substrate could even be improved by additional calcination leading to higher
cycle stability for the electrodes prepared by both techniques. The cold sprayed films have a rough
6
surface with broad film thickness distribution of 1 – 10 µm on each electrode. The Figures 4B and C
display crossꢀsectional SEM images of the fabricated cold sprayed electrodes. A complete film
coverage of the metal substrates could be achieved employing the cold spray method.
2
Figure 5. A) SEM image of a cold sprayed TiO (P25/20) film on Ti metal substrate. B) SEM image
of a screen printed TiO (P25) film on FTO substrate.
2
Figure 5 shows electron microscopy images of the semiconductor particles in the film. The images
reveal that the particle sizes of the TiO particles in the screen printed film are smaller than the particle
sizes in the cold sprayed TiO film, as expected from P25/20. The morphology of screen printed TiO
films is well described in the literature.
2
2
2
2
3, 31, 32
For screen printed P25 photoanodes prepared under very
similar conditions surface areas of 52 m²/g, porosities of 52 % and an average pore diameter of
31
2
1.8 mm were reported. The SEM image of the cold sprayed film on the other hand shows that the
particles are sintered to a very dense film with no visible porosity. This will generally lead to lower
surface areas, but at the same time to improved electrical conductivity compared to the screen printed
film.
3.2 Photoelectrochemical Oxidation of Water and Methanol

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The photoelectrodes were investigated employing the wellꢀknown reference systems of water and
3
3
methanol oxidation. Hence, the observed photocurrents for all photoelectrochemical measurements
can be attributed to water or methanol oxidation, respectively. The accumulation of four long lived
oxidizing equivalents (holes) is necessary for the water oxidation reaction leading to the formation of
34
molecular oxygen with a redox potential of 1.23 V vs. NHE:
+
+
2
H
2
O + 4 h → O
2
+ 4 Haq
(2)
It should be noted here that the two hole oxidation of water leads to the formation of hydrogen
peroxide (H ) via
2
O
2
+
+
2
H
2
O + 2 h → H
2
O
2
+ 2 Haq
(3)
3
4
with a redox potential of 1.776 V vs. NHE. While H
photocatalytic) systems, the direct formation of O
state (photoelectrochemical) systems.
2
O
2
formation is often encountered in particulate
(
2
appears to be the predominant process in solid
The oxidation mechanism of methanol is even more complicated as methanol is oxidized in three steps
35
to the final oxidation product, carbon dioxide, as shown in equations 4 – 6:
+
+
CH OH + 2 h → HCHO + 2 H
(4)
(5)
(6)
3
aq
+
+
HCHO + H
2
O + 2 h → HCOOH + 2 Haq
+
+
2
HCOOH + 2 h → CO + 2 Haq
In principle, each step requires two electronꢀhole pairs, that is, the absorption of two photons. Often,
the reactions are initiated by the transfer of one hole to the reactant forming a very reactive
intermediate which will further react via certain reaction pathways to the respective product. For
example, the first reaction of CH OH with one hole results in the formation of a hydroxymethyl
3
•
•
19
radical CH
2 3 2
OH as intermediate (CH OH/ CH OH, 1.45 V vs. NHE, pH 0). This radical is able to
inject an electron into the conduction band of the semiconductor, a process called “current doubling”,
leading to formation of formaldehyde. However, in the presence of O
3
6
•
2 2
in the system CH OH is more
likely to react with O to produce formaldehyde as well.
2
3
.2.1 TiO Photoelectrodes: Cold Spray Technique vs. Conventional Methods
2
In the following the photoelectrochemical properties of TiO photoelectrodes prepared by cold spray
2
and by screen printing, respectively, will be compared. In contrast to the conventional laboratory
methods cold spray is a method which is suitable for industrial scale photoelectrode fabrication. For
example, the fabrication of one TiO photoanode in the dimension of 240 mm width and 120 mm
2
height takes just one minute. The resulting photoelectrodes exhibit a high mechanical stability due to
8
the titanium metal substrate and a mechanically stable semiconductor film. As compared with screen
printed or doctor bladed films, the calcinated cold sprayed films are much more stable towards
scratching with materials softer than metals and therefore easier to handle. TCO glass substrates,
however, normally cannot be coated with metal oxides by cold spray, because at least one plastically
deformable material is required to achieve bonding. Following frequently photoelectrochemical
characterizations of the same photoelectrodes for more than 1 month, the effectively detected weight
loss of the cold sprayed films amounted to less than 3.5 %. The change in the observed photocurrents
were as comparably low (< 5 %). On the other hand, after the same or even a smaller amount of
experiments the screen printed films showed visual damage and a breakdown of the photocurrent.
Consequently, much more screen printed photoelectrodes had to been used for this study in
comparison to the very stable cold spray photoelectrodes (Figure 11).

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After the coating processes the investigated screen printed and cold sprayed photoelectrodes were
annealed in air at 450 °C for 2 hours. For the screen printed electrodes this is necessary for removing
the terpineol and the ethyl cellulose from the semiconductor film. Otherwise, no stable photocurrent
could be observed from these photoanodes. For the cold sprayed photoelectrodes it was found that the
annealing process leads to a partial oxidation of the uncoated parts of the Ti substrate. This could also
be observed in voltammetry measurements, as cold spray photoelectrodes without subsequent heat
treatment show additional oxidizing currents. Moreover, the stable photocurrent densities of the
tempered cold sprayed photoelectrodes were increased as compared with the untreated cold spray
electrodes. This could be attributed to the reduction of defects. The donor densities for both, the screen
19
ꢀ3
19
ꢀ3
printed (N
D D 2
= 7.06 cm ) and the cold sprayed (N = 1.43 cm ) TiO photoelectrodes were found to
be similar.
Figure 5. Chopped light currentꢀpotential curves of cold sprayed TiO
printed TiO on FTO (black) and screen printed TiO on titanium metal (green) photoanodes in 0.5 M
SO with a scan rate of 20 mV/s. Current densities were measured upon irradiation with AM 1.5ꢀ
2
on titanium metal (red), screen
2
2
H
2
4
global light (300 W Xe lamp).
The currentꢀpotential curves for water and methanol oxidation are shown in Figures 6 and 7,
respectively. These measurements reveal that the preparation of photoelectrochemically active cold
sprayed TiO
with cold sprayed TiO
For a better comparison of the different film preparation methods TiO
2
films on Ti metal substrates was successful. The photocurrents that could be achieved
photoelectrodes for water oxidation are shown in Figure 6, by the red curve.
was also screen printed on Ti
2
2
metal substrates. The results for the water oxidation photocurrents show that screen printing on metal
substrates (green curve) is not practicable as the powder is apparently just homogenously distributed
on the thin TiO passivation layer of the Ti metal. During the photoelectrochemical cycling the TiO2
2
passivation layer might act as an additional barrier for the electron flow at the interface of the
semiconductor and the metal, resulting in a quite low photoelectrochemical activity. Consequently,
2
screen printing of TiO films was carried out on FTO glass substrates as well.
When comparing the photoelectrochemical activity of the TiO
2
films deposited by different methods,
the obtained results for water oxidation (Figure 6) show highest activies for the screen printed TiO
2
films on FTO glass (black curve) as compared to the other techniques employed here. The
photocurrent achieved by the nonꢀporous cold sprayed photoanodes are slightly lower (red curve). The
2
higher photoelectrochemical activities of screen printed TiO electrodes (Figures 6 – 9) are attributed
to their higher surface areas and porosities, as shown in Figure 5.

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2
Figure 6. Chopped light currentꢀpotential curves of cold sprayed TiO on titanium metal (red) and
screen printed TiO on FTO (black) photoanodes in 0.5 M H SO with addition of 10 volꢀ% of
2
2
4
methanol (~ 2 mol/L). Measured with a scan rate of 20 mV/s. Current densities were measured upon
irradiation with AM 1.5ꢀglobal light (300 W Xe lamp).
Recently, it was reported that the observed photocurrent from photoanodes in 1M H SO could be
2
4
2
ꢀ
2ꢀ 10
8
partially assigned to the anodic oxidation of sulfate (SO ) from the electrolyte to persulfate (S O ).
4
2
2 4
Consequently, the concentration of the electrolyte was reduced to 0.5 M H SO and the cold sprayed
2 2
TiO on titanium metal and the screen printed TiO on FTO photoanodes were also tested for methanol
(
model pollutant) oxidation, as displayed in Figure 7. The measured photocurrent densities increased
by more than 300 % compared to the methanol free system. Moreover, the anodic photocurrent is
almost stable over the whole potential region from 0.2 – 1.6 V vs. NHE. This indicates a very high
reaction rate for the methanol oxidation on the TiO surface. Measurements at higher light intensities
2
than the solar AM 1.5ꢀg irradiation reveal that the photocurrent is apparently limited due to the light
intensity above 0.2 V vs. NHE.
Figure 7. IPCE spectra for water oxidation measured for cold sprayed TiO on titanium metal (red)
2
and screen printed TiO
conducted in an aqueous 0.5 M H
2
on FTO (black) photoanodes. Measurements for water oxidation were
SO solution at a bias of 1 V vs. NHE.
2
4
The IPCE spectra (displayed in Figures 8 and 9) measured for the TiO photoelectrodes show a
2
photocurrent onset of around 410 nm, in good agreement with the reported band gap energy of TiO
2
1
and the observed absorption edge (Figure 3). However, the higher visible light absorption introduced
into the TiO films by the cold spray method was not able to contribute to any enhancement of the
2
photocatalytic efficiency in the visible light regime. Consequently, the change in the visible light
absorption of the electrode (Figure 3) should be assigned mostly to the metal substrate, which is not
contributing to the photoelectrochemical activity of the semiconductor film. For the screen printed

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photoanodes (black curves) the efficiencies for water and methanol oxidation at the optimal
wavelength of 325 nm are about 8 % and 13 % higher, respectively. As discussed above, this is
2
3, 31, 32
attributed to the higher surface area of screen printed films.
Figure 8. IPCE spectra for methanol oxidation measured for cold sprayed P25/20 on titanium metal
red) and screen printed P25 on FTO (black) photoanodes. Measurements for methanol oxidation were
conducted in an aqueous solution of 0.5 M H SO with addition of 10 volꢀ% of methanol (~ 2 mol/L)
(
2
4
at a bias of 1 V vs. NHE.
When methanol is added to the acidic electrolyte the incident photon to current efficiency at the
optimal wavelength of 325 nm is about 50 % and 45 % higher for screen printed and cold sprayed
photoanodes, respectively, in good agreement with the photocurrent measurements shown in Figures 6
and 7. These observations indicate that TiO is a much better photocatalyst for environmental water
2
3
cleaning applications than for photoelectrochemical water splitting. However, the IPCE
measurements reveal that UV light irradiation is needed in order to obtain notable
2
photoelectrochemical activities on TiO photoelectrodes.
3
.2.2 Visible Light Active Cold Sprayed WO Photoelectrodes
3
For photoelectrochemical applications employing solar light irradiation it is useful to utilize
photoanodes that are harvesting as much sunlight as possible. At the same time the photocatalysts
should still have suitable band gaps matching the energies required for the oxidation processes. With a
band gap of around 2.6 eV (Figure 3) WO
oxidation strength of the photogenerated holes. This work therefore focuses on cold sprayed WO
films on Ti metal substrates since FTO/WO photoanodes have already been well characterized in
3
is matching these requirements of visible light activity and
3
3
21, 37, 38
literature.
It was found that WO photoelectrodes exhibit higher activities upon illumination with regard to the
3
water and methanol oxidation reactions as compared with the benchmark photocatalyst Aeroxide TiO2
P25. Both, the photocurrent densities (Figures 10, 12) and the IPCEs (Figures 13, 14) were found to be
higher on the tungsten oxide electrodes than on all investigated TiO
explained by the red shift in the absorption edge of WO of ~ 70 nm and by an improved coupling of
the Ti metal substrate and photocatalyst WO film prepared by cold spray. Nevertheless, it is worth to
2
P25 electrodes. This is readily
3
3
mention here that TiO photoelectrodes perform better at relatively low potentials (≤ 0.6 V vs. NHE)
2
due to earlier photocurrent onsets for water and methanol oxidation. Another advantage of the cold
2
2
ꢀ3
sprayed WO
3 D
photoelectrodes might derive from its higher donor density of N = 4.04 cm
19
ꢀ3
compared to the donor density of the cold sprayed TiO
2
D
film (N = 1.43 cm ).

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3 2
Figure 9. Chopped light currentꢀpotential curves of cold sprayed WO (blue) and TiO (red) on
titanium metal photoanodes in 0.5 M H SO with a scan rate of 20 mV/s. Current densities were
2
4
measured upon irradiation with AM 1.5ꢀglobal light (300 W Xe lamp).
Based upon observations made during the measurements it is evident that the observed photocurrent
density originates from the oxidation of water or methanol, respectively, and not from the
photocorrosion of the semiconductor layers. First of all, while large photocurrent densities were
obtained, there was no observable dissolution of the semiconductor film from the photoanode. The
investigated photoanodes were cycled several times until the chopped light curves (Figures 10 and 12)
were obtained. Assuming that the hypothetical dissolution of WO occurs by a two electron transfer,
3
this should result in the dissolution of a large amount of powder and in a change in the observed
photocurrent. However, the observed changes in current (< 5 %) and weight of the film on the
3
substrate (< 3.5 %; with an average weight of the WO film: ~ 1.87 mg) were quite low. In order to
verify the photochemical stability of the Ti/WO photoanode longꢀterm polarization measurements
3
were made. Figure 11 displays the current density at anodic polarization at 1 Volt vs. NHE. During the
measuring time of 10 hours there is a variation of the photocurrent of less than 100 µA. After 6 hours
the current density reaches a value of 1.33 mA/cm² that remains stable until the end of the
2
measurement. During the measurement a gas formation (CO ) could be observed at the photoanode,
which is responsible for the repetitive waves in the current density curve.
3 2 4
Figure 11. Polarization curve of cold sprayed WO on titanium metal photoanode in 0.5 M H SO
with addition of 10 volꢀ% of methanol (~ 2 mol/L). Measured at a constant anodic potential of 1 Volt
vs. NHE over 10 hours. Current densities were measured upon irradiation with AM 1.5ꢀglobal light
(
300 W Xe lamp). A linear fit (red) was made after 6 hours to estimate the stability of the
ꢀ4 2
photocurrent. The slope of the fitting curve is – 4.137*10 mA/h*cm .

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3 2
Figure 12. Chopped light currentꢀpotential curves of cold sprayed WO (blue) and TiO (red) on
titanium metal photoanodes in 0.5 M H SO with addition of 10 volꢀ% of methanol (~ 2 mol/L).
2 4
Measured with a scan rate of 20 mV/s. Current densities were measured upon irradiation with AM 1.5ꢀ
global light (300 W Xe lamp).
The currentꢀpotential curves illustrated in Figure 10 clearly show that cold sprayed WO
exhibit the highest photocurrents for the oxidation of water in acidic electrolytes. The
photoelectrochemical performance of the TiO photoelectrode is comparably low when no organic
compound is available as h acceptor in the system. The same trend is observed for the methanol
oxidation as shown in Figure 12. Upon methanol addition to the electrolyte the WO on the Ti
3
electrodes
2
+
3
photoelectrode exhibits the highest activity and the highest absolute increase in the photocurrent
density (+ 0.65 mA/cm²) detected in this work. Nevertheless, the relative increase in photocurrent
density for methanol as compared with water oxidation is higher on the TiO electrodes as shown in
2
Table 1.
When comparing the light current curves from Figures 10 and 12 with those shown in Figures 6 and 7
it is obvious that cold sprayed WO
illumination. The light currents of the TiO
comparable low potentials, e.g. 0.2 V vs. NHE for methanol oxidation. In contrast, the WO
photoelectrodes show a photocurrent which is continuously increasing with the applied potential up to
.6 V vs. NHE.
3
2
and TiO photoelectrodes behave completely different upon
2
photoelectrodes are reaching a stable current density at
3
1
Table 1. Photocurrents taken from Figures 6, 7, 10 and 12 for the different semiconductor
photoanodes at 1.6 V vs. NHE in 0.5 M H SO with and without methanol as sacrificial reagent.
2
4
Photoanodes prepared by cold spray (CS) and screen printing (SP) are shown. The increase of
photocurrent in % upon addition of methanol to the system is also given.
Electrode
Photocurrent Photocurrent
(
mA/cm²)
(mA/cm²)
with methanol
1.90
WO
metal (CS)
TiO on Ti
metal (CS)
TiO on
FTO (SP)
3
on Ti
1.25
Increase: + 52 %
2
0.14
0.63
Increase: + 350 %
2
0.16
0.76
Increase: + 375 %
In both systems that is with and without methanol the photocurrent onset of the TiO
2
electrodes is
shifted to more negative potentials compared to WO . Methanol is oxidized by TiO with a relatively
3
2
stable photocurrent density over the entire positive potential region with a photocurrent onset of about

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–
0.1 V vs. NHE. The water oxidation starts about 0.05 V vs. NHE, while the photocurrent onset of the
WO electrode is about 0.4 V vs. NHE, both for water and for methanol oxidation. The observed
photocurrent onsets of around 0.4 V vs. NHE are very consistent with literature values reported for
3
1
0, 11, 21, 39
thick and thin film TCO/WO
3
photoanodes in acidic media.
The observed photocurrent onset
potentials are slightly more positive than the measured flatꢀband potentials of the cold sprayed
electrodes. They were calculated from MottꢀSchottky plots to be EFB = ꢀ 0.35 ± 0.05 V vs. NHE for
Ti/TiO
similar to reported values obtained by the suspension method, employing redox couples for the
estimation of the flatꢀband potentials. The flatꢀband potential of TiO P25 suspensions were measured
employing methylviologen, whereas the Cu /Cu redox couple was used to determine the flatꢀband
2 3
and EFB = 0.24 ± 0.04 V vs. NHE for Ti/WO . The values for the flatꢀband potentials are
2
4
0
2+
+
4
1
potentials of WO suspensions.
3
Due to the initial increase of the photocurrent density it is estimated that the photocurrent for WO
Figure 12 completely arises from the oxidation of methanol, and not from oxidation of water. The
initial slope of the illuminated current curve between 0.4 and 0.6 V vs. NHE for the WO photoanode
3
in
3
is much higher for the system with methanol (2.05 mA/V*cm²) compared to the system without
methanol (0.84 mA/V*cm²), in good agreement with the fact, that methanol is oxidized more easily
than water. The same photocurrent onsets of WO electrodes were reported for measurements carried
3
10, 15
out in sulfuric acid electrolytes in the absence of methanol
the presence of methanol.
and in different acidic electrolytes in
21, 42
Moreover formaldehyde, the main product of the photoꢀoxidation process of methanol, was monitored
by fluorescence spectroscopy with the help of the Nash reagent after measuring chopped light currentꢀ
26, 27
potential curves, as displayed in Figure 12.
2
The TiO cold sprayed photoanode was cycled five
times between photocurrent onset and 1.6 V vs. NHE and the WO cold sprayed photoanode was also
3
cycled five times between photocurrent onset and 1.6 V vs. NHE for comparable results. After these
measurements samples from both electrolytes were analysed. From the fluorescence analysis total
amounts of formaldehyde of 0.993 µmol and 1.736 µmol were produced from methanol photoꢀ
oxidation by Ti/TiO
chopped on and off during the measurement this leads to formaldehyde production rates of
.0023 µmol/s (Ti/TiO ) and 0.0053 µmol/s (Ti/WO ). Additionally, the theoretically produced
2 3
and Ti/WO photoanodes, respectively. Considering that the light was regularly
0
2
3
amounts of formaldehyde were calculated from the area of the anodic currents vs. the time of the
measured CLV plots. If assuming, that all measured anodic current arises from the oxidation of
methanol to formaldehyde a total amount of formaldehyde of 0.944 µmol and 1.921 µmol should have
2 3
been produced from methanol photoꢀoxidation by Ti/TiO and Ti/WO photoanodes, respectively. The
WO photoanode produced slightly less formaldehyde (1.736 µmol) than expected from the total
3
anodic current (1.921 µmol). The yield of formaldehyde from photocurrent is above 90 %. The
additional photocurrent is attributed to the oxidation of formaldehyde and the oxidation of formic acid,
as CO gas evolution was observed during the longꢀterm experiment in Figure 11. The TiO
2 2
photoanode produced slightly more formaldehyde (0.993 µmol) than expected from the total anodic
current (0.944 µmol). This small difference (± 5 %) can be addressed to the inaccuracy of both
measurements. However, from the analysis of the electrolyte after the measurement it is clear that
formaldehyde is the main product of the photoꢀoxidation of methanol on cold sprayed photoelectrodes.
The displayed results from the photocurrent measurements indicate that methanol oxidation generally
occurs more easily on the surface of TiO when illuminated with UV light, whereas WO is surely the
2 3
better photocatalyst for water oxidation. This observation is supported by the IPCE spectra shown in
Figures 13 and 14.

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3 2
Figure 13. IPCE spectra for water oxidation employing cold sprayed WO (blue) and TiO (red) on
titanium metal photoanodes. Measurements for water oxidation were conducted in an aqueous 0.5 M
SO solution at a bias of 1 V vs. NHE.
H
2
4
Besides stability and current density, another important property of a suitable photoanode is the ability
to absorb as much light from the solar spectrum as possible. The absorption spectra of the powders
(
Figure 3) show that compared with TiO
longer wavelengths. This shift improves the photoelectrochemical efficiency of WO
harvesting as displayed in Figures 13 and 14 by the IPCE spectra. As expected, the IPCE of the
photoanodes increased around the absorption edges of 480 and 410 nm for the WO and TiO
materials, respectively. Similar IPCE photoresponses have been reported for WO film
electrode are explained by the fact
passivation layer on its surface. For a
2
the absorption edge of WO
3
is shifted ~ 70 nm towards
3
for solar light
3
2
3
1
2, 39
photoelectrodes.
that the titanium metal substrate is covered with a thin TiO
The presence of two IPCE peaks for the WO
3
2
better comparison of the overall photoelectrochemical efficiencies the geometrical areas under the
photocurrent efficiency (A/W) curves (data not shown) were calculated. These absolute values indicate
the overall efficiencies of the photoanodes to employ incident photons from solar light irradiation for
the desired oxidation processes. They are found to be 8.22 A*nm/W (Ti/WO
.32 A*nm/W (Ti/TiO Electrode). In accordance with the observed photocurrents the highest overall
water oxidation efficiencies were achieved with the cold sprayed WO photoanodes.
3
Electrode) and
2
2
3
Figure 14. IPCE spectra for methanol oxidation employing cold sprayed WO
on titanium metal photoanodes. The IPCE curve of screen printed TiO on FTO photoelectrode is also
given (light grey) for comparison. Measurements for methanol oxidation were conducted in an
3 2
(blue) and TiO (red)
2
aqueous solution of 0.5 M H
vs.
2
SO
4
with addition of 10 volꢀ% of methanol (~ 2 mol/L) at a bias of 1 V
NHE.

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The situation is different when methanol is added to the system as displayed in Figure 14. The IPCEs
clearly show that the catalytic water oxidation occurs more easily at a WO surface whereas methanol
oxidation occurs more easily on the TiO surface. The IPCE data from the screen printed TiO on FTO
electrode were also added for comparison (light grey). The IPCE of the TiO transparent film electrode
for methanol oxidation has a maximum of 73 % at 325 nm which is around 30 % higher than the
maximum efficiency of the cold sprayed WO electrode. Nevertheless, the overall IPCE for methanol
oxidation is still higher on WO photoanodes due to the visible light activity of tungsten oxide. The
3
2
2
2
3
3
geometrical areas of the photocurrent efficiency curves (data not shown) for methanol oxidation were
calculated to be 12.27 A*nm/W, 11.50 A*nm/W, and 9.41 A*nm/W for the electrodes made of cold
sprayed Ti/WO
. Discussion
Time resolved investigations have shown that photogenerated charge carriers in TiO
3 2 2
, screen printed FTO/TiO , and cold sprayed Ti/TiO , respectively.
4
2
recombine
4
1
within nanoseconds if they are not separated from each other, e.g., by an electric field. In a
photoelectrochemical cell the driving force for charge carrier separation is provided by the band
4
4
bending and/or the external bias. All photoelectrodes investigated here exhibit a bend bending at the
semiconductorꢀelectrolyte interface and another band bending at the metalꢀsemiconductor interface
depending on the respective work functions of the metal substrate and the semiconductor film. The
2 3
latter is of more importance when comparing cold sprayed Ti/TiO and Ti/WO photoelectrodes with
different metalꢀsemiconductor junctions and their effects on the charge carrier recombination rate.
As this work has been performed with nꢀtype semiconductors the electron affinities are almost equal to
the work functions of the semiconductors. When comparing the cold sprayed electrodes the work
3
4
function of titanium metal (Φ = 4.33 eV) is constant, but there are differences in the electron
M
affinities of the employed semiconductors. Butler et al. calculated the electron affinities of TiO and
2
4
5
WO
junction a Schottky contact is assumed in the literature.
metalꢀsemiconductor interface (ꢂ ≈ 0 eV) of Ti/TiO , rather indicating the presence of an Ohmic
contact at this junction. For the cold sprayed Ti/WO junction a Schottky contact with a barrier height
3
to be 4.33 eV and 5.13 eV, respectively. For the cold sprayed Ti/TiO
2
metalꢀsemiconductor
4
6, 47
However, there is almost no barrier at the
Φ
2
3
1
7
of ꢂ ≈ 0.8 eV is readily calculated. The resulting energy band behaviour is schematically shown in
Φ
Figure 15.
However, in an electrochemical device a Schottky barrier generally presents a parasitic resistance to
current flow that lowers the performance of the system. Within this work higher photocurrents were
found for the Ti/WO
explanation could derive from the high doping level of the WO
3
anode with the expected Schottky junction, as displayed in Figure 12. An
2
2
ꢀ3
3
D
photoelectrode of N = 4.04 cm .
At high donor densities the depletion layer thickness becomes very small which allows a tunnelling of
charge carriers through the junction resulting in high electrical conductivity, equal to that of an Ohmic
19
ꢀ3
contact. For the TiO
2 D
photoelectrode a lower donor density of N = 1.43 cm was observed.
The effect of metalꢀsemiconductor junctions on the photocatalytic activity has also been discussed in
48, 49
the literature.
2
Dai et al. investigated the electron transfer of TiO films on transparent conductive
4
8
oxides and metal substrates. The observed differences in photocatalytic activities were addressed to
the different metalꢀsemiconductor junctions. When an Ohmic contact exists between the phases, there
is no barrier for the electrons to migrate between the semiconductor and the metal. This interface
would lead to a higher charge carrier recombination rate in the semiconductor, because both, electrons
and holes can migrate from the metal to the photocatalyst film thus reducing the photocatalytic
activity. On the other hand, a high Schottky barrier at the metalꢀsemiconductor surface is promoting
the photocatalytic efficiencies by a more effective charge carrier separation due to the band bending.
The migration of electrons from the metal to the semiconductor surface is energetically inhibited.
4
8, 49
Consequently, the lifetime of photogenerated charge carriers is expected to be improved.
Within
this work the same trends of the metalꢀsemiconductor junctions have been observed. The measured

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formaldehyde production rates of 0.0023 µmol/s and 0.0053 µmol/s were found for Ti/TiO and
2
3
Ti/WO photoanodes, respectively.
Figure 15. Schematic energy band diagram for nꢀtype semiconductor photoelectrodes under
illumination and after the equilibration of Fermi levels at the metalꢀsemiconductor and the
semiconductorꢀelectrolyte interfaces with the appearance of band bending. For the Ti/WO
3
junction a
Schottky contact is assumed, while for the Ti/TiO junction an Ohmic contact is readily expected. The
2
3
theoretical Schottky barrier height for the Ti/WO junction is ꢂΦ ≈ 0.8 eV.
Nevertheless, the penetration depth of the incident light, the depletion layer width, and the film
thicknesses of the photoelectrodes have to be taken into account as well. The optical penetration
3
depths (1/α) in WO are about 1 µm at 380 nm, 2.2 µm at 420 nm, and 4.5 µm at 440 nm matching
more or less the photoelectrode film thicknesses of 1 – 10 µm for the cold sprayed and 2.9 µm for the
3
7
screen printed photoelectrodes. The penetration depth of UV light (350 nm) in TiO is comparable
2
50
43
with 1/α ≈ 2 µm. However, like for TiO , the small charge carrier mobility and short charge carrier
2
17
diffusion length in WO
generated within the depletion layer that will contribute to the observed photocurrent. The depletion
layer thickness of about 0.75 µm for a WO film is comparably small in contrast to the film
3
will lead to a high recombination rate. It is mainly the charge carriers
3
37
thicknesses and the optical penetration depths. Therefore, the effect of charge carrier separation by
bend bending in the metalꢀsemiconductor depletion layer should be rather small because of the thick
5
1
semiconductor films.
For (highly doped) nꢀtype semiconductors the flatꢀband potentials are a direct indication of the
2
0
conduction band positions. Consequently, the conduction band position of the investigated
photoanodes are almost equal to the obtained flatꢀband potentials. By this means the conduction band
2
position of the investigated Ti/TiO photoanode is ~ 0.59 V more negative than the conduction band of
the Ti/WO electrode. The difference of the conduction band positions also has an impact on the
3
anodic photocurrent onsets for water and methanol oxidation. A photocurrent can only be measured at
+
the photoanode (WE), if the reduction of H takes place simultaneously at the platinum wire cathode
(
3 2
CE). Due to the low conduction band position of WO compared to TiO , there is a need of additional
0.4 V external bias (≈ photocurrent onset) to run the overall reaction in the photoelectrochemical cell.
When comparing the substrates FTO glass and Ti metal it has to be taken into account that the latter is
not lightꢀtransmissive, but at the same time reveals some big advantages such as lower sheet resistance
and higher mechanical stability. However, the particle to substrate bonding has to be taken into
account because this strongly depends on the employed coating technique. The most important
2
difference between the substrates is the thin passivation layer of TiO on the Ti metal surface which
can act as a barrier between the photocatalyst and the metal back contact. When the screen printing
method is employed for metal substrates the semiconductor powder is homogeneously distributed on
the oxide layer of the Ti metal. The nanostructure of the film enables the electrolyte passing to the
substrate and thus dissolve the coated film. It was observed that bonding of screen printed TiO2
particles to FTO glass is electrochemically more stable compared to screen printing on Ti metal as
detachment of the particles is hindered. The situation is different when cold spray is used to prepare
the photoelectrodes. Within this process semiconductor particles are accelerated to high velocities in
2
the N gas stream, shot through the passivation layer of the metal and directly bonded to the substrate

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7
, 8
(
Figure 1). The outcome is an improved electrical contact of the photocatalyst particles with the
metal back contact and a higher mechanical stability of the semiconductor films compared to the other
coating techniques. Consequently, an improved electrical conductivity between semiconductor film
and metal substrate will result in higher photoelectrochemical activity.
Moreover, according to the optical properties of the semiconductor films (Figure 3) the cold sprayed
2
9, 30
photoelectrodes show the BursteinꢀMoss effect.
The high amount of oxygen vacancies is
responsible for the colour change and might promote the conductivity of the electrode by nꢀtype
doping. The thick semiconductor films compared to the penetration depth of the incident light, ensure
that all of the light is absorbed by the photocatalyst film and not by the metal back contact.
However, despite of the very practicable preparation method the photoelectrochemical conversion
efficiencies of cold spray Ti/WO photoanodes still need to be improved to contest with recently
3
published efficiencies of 1 – 2 µm thin film FTO/WO
parameters. For solꢀgel prepared FTO/WO photoanodes IPCEs of max. 65 % water oxidation
efficiency at 400 nm in 1 M H SO and at 1.5 V vs. RHE were reported. The photocurrent
efficiencies of spin coated FTO/WO photoanodes reach a similar IPCE maximum at 360 nm in 0.1 M
SO and at 1.74 V vs. NHE for water oxidation. The lower photocurrent efficiencies for the cold
3
photoanodes measured with comparable
3
5
2
2
4
3
39
H
2
4
sprayed Ti/WO photoanodes are attributed to the higher film thickness, as well as to the less
3
nanoporous structure. Nevertheless, it should be mentioned that the reported IPCEs were measured at
much higher external potentials compared to this work (1 V vs. NHE). In contrast to other visible light
active photoanode materials like αꢀFe
2
O
3
the WO
photocurrent onset potential. Moreover, reported studies of charge carrier dynamics show that for
WO photoanodes, in contrast to αꢀFe photoanodes, there is no positive effect on the photohole
lifetime by applying external bias in water oxidation systems.
increasing the photohole lifetime in WO photoanodes by applying an external bias.
3
photoanodes have the advantage of a less anodic
53
3
2 3
O
3
9, 54
Consequently, there is no need of
3
5. Conclusion
In this study TiO
comparison, additional photoelectrodes of TiO
2
and WO
3
electrodes were fabricated using the cold spray technique. For
P25 were prepared with the screen printing method.
2
The asꢀprepared electrodes were calcinated at 450 °C in air for 2 h. It has been shown that cold
spraying produces mechanically and photoelectrochemical stable films without the need of any
additive or binder. Regarding the photoelectrochemically water oxidation activity of the photoanodes,
the highest photocurrents were observed for cold sprayed WO
potential range of 0.6 – 1.6 V vs. NHE. Cold sprayed TiO
3
on titanium metal substrates in the
photoelectrodes, on the other hand,
2
displayed better photocurrent onsets and thus higher photoelectrochemical activity in the low potential
range from 0.0 – 0.6 V vs. NHE. When methanol was added to the acidic electrolytes as model
pollutant for waste water, all observed current densities further increased upon simulated solar
irradiation. Consequently, the application of water purification seems more suitable for the
investigated photoanodes.
3 2
However, the biggest advantage of cold sprayed WO /Ti compared to cold sprayed TiO /Ti and screen
printed TiO /FTO was revealed during the incident photon to current efficiency measurements. The
2
absorption edge of WO is shifted ~ 70 nm to the red, allowing a more efficient harvesting of the total
3
solar light irradiation. Consequently, the photocurrent onset is shifted to longer wavelengths as well,
resulting in higher IPCEs in the regime of UVA and visible light (310 – 780 nm) of + 3.50 % and
+
2
0.47 % compared to the most active TiO photoelectrode for water and methanol oxidation,
respectively. Moreover, it could be shown that the cold spray technique is capable of producing
mechanically and chemically stable photoelectrodes that showed longer lifetimes for
photoelectrochemical measurements than the laboratory scale screen printed anodes. The results show
that cold sprayed tungsten trioxide is a promising material for use as photoanode for
photoelectrochemical cells.

Page 19 of 20
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DOI: 10.1039/C7DT02063E
Author Information
Corresponding Author: Detlef Bahnemann, Eꢀmail: bahnemann@iftc.uniꢀhannover.de
Acknowledgements
The authors wish to thank Stephanie Melchers for carrying out the SEM measurements and
H.C. Starck GmbH for the cooperation in the present study. The authors gratefully acknowledge
financial support from the Federal Ministry of Education and Research BMBF (Project “DuaSol” Nr.
03SF0482C
and
03SF0482).
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