Highly active oxide photocathode for …...1 Supplementary Information: Highly active oxide...

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Supplementary Information:

Highly active oxide photocathode for

photoelectrochemical water reduction

Adriana Paracchino1, Vincent Laporte2, Kevin Sivula1, Michael Grätzel1 and Elijah Thimsen1

1 Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne,

Laboratory of Photonics and Interfaces, Station 6, CH-1015 Lausanne, Switzerland.

2 Interdisciplinary Centre for Electron Microscopy, Ecole Polytechnique Fédérale de Lausanne, Station

12, CH-1015 Lausanne, Switzerland.

Cu(0) formation on bare Cu2O Electrodes

Evidence of Cu formation in the illuminated area of the measured bare Cu2O electrodes is

provided by SEM (Figure S1), X-ray photelectron spectroscopy (XPS) spectroscopy (Figure S2), and X-

ray diffraction (Figure S4). First, bright nanoparticles were observed on the surface of the Cu2O grains

by SEM after PEC measurement, which appeared by visual inspection as a black circle in the

illuminated area (Figure S1). Depth profile XPS analysis was performed on bare Cu2O samples before

and after PEC measurement. Before PEC measurement, a native CuO surface oxide was indicated in the

Cu 2p region of the XPS spectrum by a shoulder at 933.8 eV and a satellite structure at the higher

binding energy side (not shown). The CuO signal disappeared after 30 seconds of sputtering, indicating

that the native oxide was less than 2 nm in thickness. The quantification analysis after the first

sputtering cycle for the as-deposited sample from the Cu 2p and the O 2p peaks showed a Cu/O ratio

that was constant with sputtering time and close to the expected value of 2.77 due to a preferential

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oxygen sputtering1. After PEC stability measurement, the quantification analysis showed Cu/O values

between 3.5 and 7.5, according to the duration of sputtering, indicating enrichment of copper relative to

the as-deposited Cu2O. We therefore conclude that the sample measured after PEC is a mixture of Cu

(formed via reaction (1) of the main text) and Cu2O. An XRD peak for Cu metal was also observed

after PEC measurement (Figure S4), which was absent in XRD spectra of the as-deposited sample.

Figure S1. SEM images of a bare Cu2O electrode before (a) and after (b) PEC stability measurement.

Cu nanoparticles are visible on the surface of the Cu2O grains after PEC measurement. The insets are

a

b

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digital images of the electrodes: the black circle visible on the measured electrode is the illuminated

area, where photocathodic decomposition occurred to form Cu(0).

Figure S2. Atomic concentrations from XPS analysis for Cu and O in a Cu2O electrode as-deposited

and after PEC measurement. The horizontal lines indicate the expected Cu (dark green) and O (light

green) concentrations according to the preferential oxygen sputtering in Cu2O under Ar+ bombardment1.

Role of the ZnO buffer layer and Al2O3 doping (this discussion refers to Table 1 of the main text)

The ZnO buffer layer between the Cu2O and TiO2 was critical to improve the stability of the

photocurrent. This can be seen by comparing the bare electrode (sample 1), Cu2O/11 nm TiO2 electrode

(sample 2) and the Cu2O/20 nm ZnO/11 nm TiO2 electrode (sample 3). For the bare electrode, the

photocurrent was very unstable and decayed to 0% of its initial value after 5 minutes (Figure 1 a).

Depositing 11 nm of TiO2 on the surface did little to enhance the stability, as the photocurrent again

decayed to 0% of its initial value after 20 minutes. It should be noted that increasing the TiO2 thickness

did not increase the stability, and electrodes with 30 nm TiO2 protective layers still decayed to 0% of





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their initial value after 20 minutes. However, if 20 nm of ZnO was used as a buffer layer between the

Cu2O and TiO2, the initial photocurrent was greatly improved to –7.8 mA cm-2 and retained 14% of its

initial value after 20 minutes (sample 3), a vast improvement over both the bare and TiO2-only samples.

The hypothesis is that the ZnO buffer is acting as a nucleation layer to control the mechanism of TiO2

growth. The TiO2 growth is presumably layer-by-layer on the fresh ZnO and island growth on the

electrodeposited Cu2O. The stability of the ZnO layer was further improved by inserting periodic sub-

nanometer layers of Al2O3 into the ZnO buffer layer.

Monolayers of Al2O3 were inserted into the layered structure with different configurations by

alternating TMA/H2O cycles with the DEZ/H2O cycles during ZnO deposition, which is a well known

route to synthesizing Al-doped ZnO by ALD2. First, one Al2O3 layer approximately 0.17 nm in

thickness was placed periodically every 4 nm in the ZnO, with the final Al2O3 layer at the ZnO/TiO2

interface (sample 4). Second, one Al2O3 layer approximately 0.17 nm in thickness was placed every 2

nm in the ZnO with the final layer at the ZnO/TiO2 interface (sample 5). The overall thickness of the

ZnO:Al layer was constant at approximately 21 nm. For the sample with Al2O3 layers spaced every 4

nm (sample 4), the photocurrent remained the same as the ZnO-only sample, but the stability improved

to 33% of the initial photocurrent after 20 minutes. When the Al2O3 layers were placed every 2 nm, the

stability further improved to retain 53% of its initial value after 20 minutes of illumination at 0 V vs.

RHE, but was accompanied by a drop in the initial photocurrent to –4.7 mA cm–2, presumably due to the

high aluminum content that increased the resistance of the protective layer either through tunnel barriers

or through reduced electron mobility.

To further elucidate the effect of Al, Al2O3 layers 0.9 nm in thickness were placed at the

ZnO/TiO2 interface (sample 6) and Cu2O/ZnO interface (sample 7). In both samples, the initial

photocurrent at 0 V vs. RHE dropped significantly, to –2.7 mA cm–2 for sample 6 and –2.4 mA cm–2 for

sample 7, presumably due to the rather thick Al2O3 layer that presented a large barrier to electron

tunneling. The interesting trend is in the stability. The sample with 0.9 nm of Al2O3 at the ZnO/TiO2

interface (sample 6) had approximately the same photocurrent stability after 20 minutes as the sample





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with Al2O3 layers distributed periodically every 2 nm (sample 5). However, the electrode with the 0.9

nm Al2O3 at the Cu2O/ZnO interface had approximately the same stability as the electrode with no

Al2O3 (sample 3). We therefore conclude that the Al2O3 is not stabilizing the Cu2O, but instead

stabilizing the ZnO.

Hall effect measurements were carried out at room temperature using an Ecopia HMS-3000 to

determine any electronic role Al3+ might be playing in the Al-doped ZnO layers through its effect on the

majority carrier density. Films were deposited on square quartz substrates at identical conditions to the

protective layers, and the Al content was controlled by varying the TMA/H2O to DEZ/H2O cycle ratios.

The thickness of the Al-doped ZnO layers was 21 nm in every case, as measured by elipsometry on

"witness" samples deposited on optically polished silicon. The Al atom fraction was varied from 0.0 to

4.3 %, as calculated by the “rule of mixtures”2. The electron mobility, resistivity and electron density

are plotted in Figure S3. For the as-deposited films, the carrier densities for the Al3+ atom fractions of

1.4%, 2.2% and 4.3% were –5.0 x 1019 cm–3, –7.9x1019 cm–3 and –1.9x1020 cm–3 respectively; which

was much lower than the expected Al3+ atom concentration of 5.7x1020 cm cm–3, 9.3x1020 cm–3 and

1.8x1021 cm–3 respectively. After heat treating the films for 60 min at 200 oC in air, the carrier

concentration in the ZnO dropped dramatically to –3.6x1016 cm–3, –3.8x1017 cm–3 and –1.1x1018 cm–3

for respective Al3+ atom fractions of 1.4%, 2.2% and 4.3%. In the as-deposited case, the carrier

concentration was approximately 10% of the expected Al3+ atom concentration, indicating that 90% of

the aluminum was not serving as an electron donor, but was instead serving a different role. The effect

was even more dramatic after heat treatment, where less than 0.1% of the Al3+ was serving as an

electron donor. We believe this indicates that the Al3+ was serving a structural role, perhaps as an

isolated Al2O3 phase in the periodic layers where it was deposited. This conclusion is supported by the

observation that for Al3+-containing samples, the electron mobility decreases monotonically with

increasing Al3+ atom concentration (Figure S3 a).





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Figure S3. Resistivity (a), electron mobility (b) and carrier density (c) from Hall effect measurements

on 21 nm Al:ZnO films (constant thickness) with different Al2O3:ZnO cycle ratios (0, 1:10, 1:20, 1:33)

deposited on quartz substrates by ALD. The Al fraction was calculated using the rule of mixtures and

assuming the bulk density of ZnO2.

a b

c





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Figure S4. XRD before (a) and after (b) PEC characterization for the bare Cu2O electrode and after

PEC characterization on the surface-protected Cu2O (c).

a

b

c





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Figure S5. H2 bubbles evolving from the illuminated protected photocathode biased at 0 V vs. RHE.

Figure S6. Photocurrent spectrum (IPCE) at 0 V vs. RHE (a) and current-potential characteristics (b)

for a heat treated Cu2O / 5 x (4 nm ZnO / 0.17 nm Al2O3) / 11 nm TiO2 /Pt electrode immersed in 1 M

Na2SO4 solution under AM1.5 irradiation and N2 purging. The photocurrent value calculated by

integration of the IPCE curve over the AM1.5 solar spectrum was 25% lower than the measured value

from the JV plot, but consistent with the observed photocurrent decay during chronoamperometry at 0 V

vs. RHE.

a b





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Figure S7. XPS spectra of Cu 2p as a function of sputtering time in the protective layer of a heat treated

Cu2O / 5 x (4 nm ZnO / 0.17 nm Al2O3) / 11 nm TiO2 /Pt electrode. No Cu was detected in the

protective layer. Each spectra is vertically offset for clarity.

Chemical stability of Cu2O

The chemical stabilization of Cu2O was confirmed by the position of the Auger Cu LMM signal in XPS.

The Cu LMM peak for an electrode measured for 80 minutes under irradiation at 0 V vs. RHE (Figure

S8a) showed the same position as for a bare Cu2O that wasn’t exposed to a PEC measurement (Figure

S8b). The reduction of bare Cu2O under photocathodic conditions (Figure S8c) is apparent from a peak

shift to a higher kinetic energy as well as from the narrowing of the peak. These features were not

observed for a Cu2O / 5 x (4 nm ZnO / 0.17 nm Al2O3) / 11 nm TiO2 /Pt electrode after PEC

measurement, which is consistent with the chemical stabilization of the photoactive semiconductor.





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Figure S8. Auger Cu LMM spectra obtained after XPS depth profiling for a protected Cu2O electrode

measured for stability under light at 0 V vs. RHE for 80 minutes (a), a bare Cu2O not measured (b) and

a PEC-measured bare Cu2O electrode (c).





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Formation of Ti3+ in the amorphous TiO2 layer

The theory of Ti3+ formation in the TiO2 layer was tested by a mild in-situ air oxidation

cycle at room temperature in the photoelectrochemical cell by the following procedure. Prior to any

PEC measurements, the electrolyte was purged with N2 for at least 15 minutes. A 20 minute stability

measurement was carried out under chopped illumination and N2 purging. After the stability

measurement, the electrolyte was purged in the dark with air for 15 minutes, followed by a 15 minute N2

purge, to complete an air/N2 purge cycle. Another 20 minute stability measurement was then recorded,

followed by another air/N2 purge cycle. Thus after each 20 minute stability measurement under N2

purging, an air/N2 purge cycle was completed before starting the next 20 minute stability measurement.

The results for a heat-treated Cu2O/5 x (4 nm ZnO / 0.17 nm Al2O3) / 11 nm TiO2 / Pt electrode

and a Cu2O/5 x (4 nm ZnO / 0.17 nm Al2O3) / 20 nm TiO2 / Pt electrode are presented in Figure S8. The

time axis for Figures S9b and S9c is the time under illumination (i.e. the elapsed time during the air/N2

cycle is not included). After 40 minutes under chopped illumination, the photocurrent of the electrode

with 11 nm of TiO2 decayed and was not reversible after an air/N2 purge cycle. The photocurrent of the

electrode with 20 nm of TiO2 decayed after 80 minutes of stability measurement and was not reversible

by an air/N2 purge cycle. We believe this result indicates that a critical concentration of Ti3+ traps was

reached that the air/N2 purge cycle was insufficient to reverse and thus the initial photocurrent could not

be restored.

The mechanism proposed for the photocurrent decay is consistent with the XPS Ti 2p signals

plotted in Figure S11. The peak at 459 eV is due to Ti4+ and the shoulder around 457 eV is due to Ti3+.

In an untested sample (Figure S11a), Ti3+ formation is due to the chemical modification induced by the

Ar+ sputtering required for the XPS depth profiling. The sample PEC-tested for 20 minutes (Figure

S11b) presents a large Ti3+ shoulder at already 30 s of sputtering, which conclusively indicates that it is

enriched in Ti3+ compared to the not measured sample. Owing to the constant total amount of Ti, the

intensity of the Ti4+ peak is decreasing as the Ti3+ shoulder becomes larger. The untested sample of





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Figure S11a was analyzed by XPS depth profiling some months after the sample of Figure S11b,

inducing possible slightly different sputtering conditions, but the trend in the Ti3+ and the Ti4+ signals

with increasing sputtering depth was very reproducible with other similar samples PEC-measured for 20

min or more.

c

b

a

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min or more.

c

b

a

12




min or more.

c

b

a





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Figure S9. Stability of the photocurrent at 0 V vs. RHE for a Cu2O/5 x (4 nm ZnO / 0.17 nm Al2O3) /

11 nm TiO2/Pt electrode and a Cu2O/5 x (4 nm ZnO / 0.17 nm Al2O3) / 20 nm TiO2/Pt electrode. In

chronological order, the JV plot (a) shows the initial linear sweep for the 11 nm TiO2 sample (LS-1),

linear sweep after 20 minutes of stability measurement (LS-after stability), after an air/N2 purge cycle

(LS-2) and after 40 minutes of stability measurement (LS-after stability 2). The stability measurement

in panel (b) shows that the photocurrent recovers after the first air/N2 purge cycle at 20 minutes, but

after the second air/N2 purge cycle at 40 minutes the photocurrent decay is irreversible. Panel (c) shows

the same stability experiment for the electrode with 20 nm of TiO2, which results in the photocurrent

being restored after air/N2 purge cycles at 20, 40 and 60 minutes. However, the photocurrent decay

eventually becomes irreversible after 80 minutes of light chopping at 0 V vs. RHE.

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Figure S10: Pourbaix diagram for TiO2 in 1M Na2SO4 electrolyte at T=25°C, for [Ti2+]=10 M.

Generated using Medusa (http://www.kemi.kth.se/medusa/).

Figure S11. Ti2p spectra obtained after XPS depth profiling for an untested electrode (a) and for an

electrode tested for stability for 20 minutes. The Ti 2p signals are presented for different Ar+ sputtering

times. The peak at 459 eV is due to Ti4+ and the shoulder around 457 eV is due to Ti3+. The data in b)

are the same that have been plotted as contour plot in Figure 4 of the main text.





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Figure S10: Pourbaix diagram for TiO2 in 1M Na2SO4 electrolyte at T=25°C, for [Ti2+]=10 M.

Generated using Medusa (http://www.kemi.kth.se/medusa/).

Figure S11. Ti2p spectra obtained after XPS depth profiling for an untested electrode (a) and for an

electrode tested for stability for 20 minutes. The Ti 2p signals are presented for different Ar+ sputtering

times. The peak at 459 eV is due to Ti4+ and the shoulder around 457 eV is due to Ti3+. The data in b)

are the same that have been plotted as contour plot in Figure 4 of the main text.





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References

1 Malherbe, J.B., Hofman, S., & Sanz, J.M., Preferential sputtering of oxides: A comparison of

model predictions with experimental data. Applied Surface Science 27, 355-365 (1986). 2 Elam, J.W., Routkevitch, D., & George, S.M., Properties of ZnO/Al2O3 alloy films grown using

atomic layer deposition techniques. Journal of the Electrochemical Society 150 (6), G339-G347 (2003).





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