ARTICLES
PUBLISHED ONLINE: 21 DECEMBER 2015 | DOI: 10.1038/NMAT4511
Water reduction by a p-GaInP2 photoelectrode
stabilized by an amorphous TiO2 coating and a
molecular cobalt catalyst
Jing Gu1†, Yong Yan1†, James L. Young1,2, K. Xerxes Steirer1, Nathan R. Neale1 and John A. Turner1*
Producing hydrogen through solar water splitting requires the coverage of large land areas. Abundant metal-based molecular
catalysts offer scalability, but only if they match noble metal activities. We report on a highly active p-GaInP2 photocathode
protected through a 35-nm TiO2 layer functionalized by a cobaloxime molecular catalyst (GaInP2 –TiO2 –cobaloxime). This
photoelectrode mediates H2 production with a current density of ∼9 mA cm−2 at a potential of 0 V versus RHE under 1-sun
illumination at pH 13. The calculated turnover number for the catalyst during a 20-h period is 139,000, with an average turnover
frequency of 1.9 s−1 . Bare GaInP2 shows a rapid current decay, whereas the GaInP2 –TiO2 –cobaloxime electrode shows ≤5% loss
over 20 min, comparable to a GaInP2 –TiO2 –Pt catalyst particle-modified interface. The activity and corrosion resistance of the
GaInP2 –TiO2 –cobaloxime photocathode in basic solution is made possible by an atomic layer-deposited TiO2 and an attached
cobaloxime catalyst.
T
he future of a clean energy infrastructure depends on efficient,
low-cost, long-lasting systems for the conversion and storage
of solar energy. Hydrogen production directly from water
and sunlight is a leading approach to provide an energy carrier for
transportation, large-scale energy storage for intermittent resources,
and a feedstock for ammonia synthesis1–3 . The photoelectrochemical
(PEC) process is an innovative approach for hydrogen production in
which the energy collection (solar absorber) and water electrolysis
(catalysis) functions are mated into a single device. However, the
bulk of the research over the past 40+ years has focused on metal
oxides, where the efficiency of PEC solar hydrogen generation is low
owing to the collective challenges of poor solar absorption and poor
electronic properties4,5 . More suitable semiconductors for highefficiency photoconversion—such as silicon and the III–Vs—suffer
from material instability and slow interfacial kinetics towards the
hydrogen evolution reaction (HER) and oxygen evolution reaction
(OER). Incorporation of proper electrocatalysts onto the solar
energy-collecting semiconductor surfaces is an innovative approach
to stabilize the photoelectrode surface and reduce the interfacial
OER and HER kinetic barriers, thus enhancing the overall device
performance6 . However, few studies have focused on building a
functional photocatalytic system consisting of a low-cost, scalable,
and transparent molecular catalyst attached onto a semiconductor
surface rather than noble metals, such as platinum6,7 . Here, we
explore a covalently attached molecular HER electrocatalyst on
a semiconductor surface that will allow a better understanding
of the catalyst/semiconductor interface and provide opportunities
to systematically optimize the PEC water-reduction performance
towards higher efficiency, longer life renewable fuel generation.
The p-GaInP2 photocathode with a direct bandgap of 1.83 eV,
when paired with a GaAs solar cell in a tandem PEC/photovoltaic
arrangement, can split water with an ∼12% solar-to-hydrogen
efficiency8,9 . Unfortunately, the current is not maintained owing
to the material’s instability, causing it to degrade/corrode in the
aqueous solutions9 . The integration of an active HER catalyst
and a protection scheme for the high-efficiency semiconductor
photoelectrode would be a significant advance. One approach is to
use noble metals in addition to applying a nitrogen ion implantation
surface-passivation treatment to stabilize the p-GaInP2 surface10 .
Another approach would be to protect the surface and graft on
an efficient, low-cost catalyst11,12 . Reaching a sustainable current
density of >10 mA cm−2 to meet the aforementioned requirements
is necessary for a viable commercial device. This raises important
questions regarding the discovery and the integration of the best
possible materials that are able to harvest light with materials to
catalyse H2 evolution for a long-lasting system.
Innovative developments based on cobalt molecular catalysts
have emerged for both the water reduction and oxidation reactions13,14 . These catalysts are attractive because they not only exhibit
excellent catalytic properties but are also cost-effective and sustainable15,16 . In this work, we study the application of a molecular catalyst
as a surface-attached catalyst at the p-GaInP2 /electrolyte interface.
Surface modification of GaInP2
Our approach to studying this molecular attachment scheme is
shown in Fig. 1. An Earth-abundant first-row transition metal
complex, namely (HOOCpy)Co(dmgH)2 (Cl), where (dmgH)2
is bis-glyoxime and HOOCpy is isonictonic acid (Fig. 1), was
chosen as the molecular catalyst for binding to the surface.
However, attaching the catalyst directly to the surface of the III–V
semiconductor is very challenging owing to the absence of a stable
oxide layer and surface hydride17 . It is well known that the carboxylic
acid group will form a strong attachment to a TiO2 surface18,19 ;
therefore, an atomic layer-deposited (ALD) TiO2 coating was
applied to the GaInP2 photoelectrode to facilitate the binding of the
catalyst. ALD TiO2 was selected in this study primarily because it
can directly modify the surface with a conformal layer of the oxide
with controlled thickness20 . An additional benefit is that the TiO2
exhibits remarkable properties in terms of stability under basic
conditions21,22 , and operating under basic conditions is a preferred
1 National Renewable Energy Laboratory, Chemistry and Nanoscience Center, Golden, Colorado 80401, USA. 2 Material Science and Engineering Program,
University of Colorado, Boulder, Colorado 80309, USA. †These authors contributed equally to this work. *e-mail: [email protected]
456
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ARTICLES
NATURE MATERIALS DOI: 10.1038/NMAT4511
E5
E2
O
N
TiO2
Pt
H
Cl
O
N
Co
N
N
O O
H
N
Co catalyst =
E4
E2′
a.u.
E1
COOH
(py)Co(dmgH)2(Cl) reference
(HOOCpy)Co(dmgH)2(Cl) reference
E2: GaInP2−TiO2
GaInP2−TiO2 treated with (py)Co(dmgH)2(Cl)
E2′: GaInP2−TiO2 treated with (HOOCpy)Co(dmgH)2(Cl)
E3: GaInP2−TiO2−(OOCpy)Co(dmgH)2(Cl)−TiO2
1,000
1,200
1,400
1,600
1,800
Wavenumber (cm−1)
E3
Figure 2 | ATR-IR spectra of electrode surfaces.
Figure 1 | Surface modification strategies for the p-GaInP2
photoelectrodes.
environment for the OER counter reaction23 . For the work presented
here, the TiO2 layer was used as-grown, so it remained amorphous.
We investigated the bare p-GaInP2 electrode (electrode 1) towards
water splitting and as it was protected by a nominally 35-nm-thick
layer of ALD TiO2 , denoted as GaInP2 –TiO2 (electrode 2). The
catalyst loading procedure, adapted from published literature24,25 ,
was simply immersing the electrode in an ethanol solution of the
catalyst overnight. The linkage is presumably through a covalent
coordination of the carboxylate group to the TiO2 surface26,27 ,
denoted as GaInP2 –TiO2 –(OOCpy)Co(dmgH)2 (Cl) (electrode 2’).
To protect the linkage stability, an additional 10-cycle (∼0.4 nm)
TiO2 ALD layer was deposited after catalyst attachment (electrode 3:
GaInP2 –TiO2 –(OOCpy)Co(dmgH)2 (Cl)–TiO2 ). Several other
electrodes were also manufactured for comparison: electrode 4
GaInP2 –TiO2 –Pt and electrode 5 GaInP2 –Pt.
Characterization of the attached surface catalyst
Successful loading of the cobalt catalyst onto the TiO2 -modified
GaInP2 (electrode 2) was confirmed and characterized by attenuated total-reflection infrared spectroscopy (ATR-IR; Fig. 2). Compound (py)Co(dmgH)2 (Cl) (py = pyridine) without the carboxylate
linkage was also synthesized for comparison. The infrared spectra of (HOOCpy)Co(dmgH)2 (Cl) and (py)Co(dmgH)2 (Cl) were
distinguished by a characteristic absorption at about 1,750 cm−1 ,
denoted as the carboxylate stretch. Other frequencies, that is, 1,580,
1,430, 1,240 and 1,100 cm−1 , are identical to the literature report
of (py)Co(dmgH)2 (Cl) (ref. 6). Electrode 2 was immersed in the
ethanol solution of these two compounds for 24 h, followed by consecutive washing with pure ethanol. The ATR-IR data in Fig. 2 show
that (HOOCpy)Co(dmgH)2 (Cl) chemisorbs to the GaInP2 –TiO2
surface (as evidenced by similar vibrations to its parent compound), whereas no peaks from surface-bound (py)Co(dmgH)2 (Cl)
are observed. This demonstrates that the carboxylate covalent linkage is necessary to robustly attach the catalyst onto the ALD
TiO2 . The ATR-IR spectrum of a (HOOCpy)Co(dmgH)2 (Cl)treated GaInP2 –TiO2 sample following 10 cycles of TiO2 (electrode 3) also exhibits signatures from the molecular cobalt catalyst (Fig. 2), suggesting that this treatment does not significantly
affect its structure. The X-ray photoelectron spectroscopy (XPS)
analysis of the electrode 3 surface (Supplementary Fig. 2) shows
two sharp peaks in the Co 2p3/2 and 2p1/2 region with binding
energies of 780.6 eV and 795.6 eV respectively. The peak binding
energies and doublet separation of 15 eV provide clear evidence
of the presence of the Co(III) ion (Supplementary Fig. 2(B)28 ).
In support of the ATR-IR result, electrode 2 (Supplementary
Fig. 2(I)) and the electrode processed with (py)Co(dmgH)2 (Cl)
do not show any distinctive peaks in the Co 2p region (Supplementary Fig. 2(J)). The amount of catalyst loading was estimated
by inductively coupled plasma mass spectroscopy (ICP-MS) and
indirect ultraviolet–visible spectroscopy absorption spectroscopy
(Supplementary Fig. 3). Electrode 3 was digested in concentrated
ultrapure sulphuric acid, and the elements in the diluted solution
were analysed by ICP-MS to estimate a surface catalyst concentration as high as 12.7 ± 1.2 nmol cm−2 . This value corroborated well
with the 12.8 nmol cm−2 obtained from indirect ultraviolet–visible
absorption spectroscopy (Supplementary Fig. 3).
The carboxylate linkage to the surface of nanoparticle TiO2
was reported to be unstable owing to hydrolysis under basic
conditions29,30 . However, here we found that improved surface
stability between the carboxylic linkage group and ALD TiO2 can
be achieved by an extra 10-cycle overlayer of ALD TiO2 . This is
consistent with a previous report in which an ALD Al2 O3 treatment
(3 cycles only) on mesoporous TiO2 substantially reduced a similar
dye desorption by an order of magnitude versus one without
the overlayer31 . Pt-modified surfaces were also characterized by
scanning electron microscopy (SEM) and atomic force microscopy
(AFM) as shown in Supplementary Fig. 4. The loading amount of Pt
for electrode 4 and electrode 5 was estimated by ICP-MS to be about
13 nmol cm−2 and 102 nmol cm−2 Pt, respectively.
Photoelectrochemical profile
The Co-modified photocathode shows significant PEC performance enhancement over an untreated electrode in an argonpurged 0.1 M NaOH solution (pH = 13, this electrolyte was
used for all PEC experiments). The PEC current density (J )
versus potential (V ) behaviours of the photocathodes under 1-sun
illumination (100 mW cm−2 ) were measured to evaluate their
catalytic performance as shown in Fig. 3. Negligible currents were
observed under dark conditions for all electrodes. Note that, under
a strong negative bias of −2.0 V versus Ag/AgCl, the cobaloximemodified electrode demonstrated catalytic activity under dark
conditions whereas the bare and GaInP2 –TiO2 are non-catalytic
(Supplementary Fig. 5). Under illumination, the cobaloximemodified electrode 3 exhibited an onset potential of about −0.50 V
versus Ag/AgCl (Fig. 3). The onset potential is observed to be
∼150 mV more positive than that of the bare electrode, and the
value increased to ∼650 mV compared with TiO2 -coated electrode
2. Not surprisingly, the limiting current for the latter is reached
at the negative potential of −1.65 V versus Ag/AgCl, whereas the
limiting current is reached at about −1.05 V for the catalyst-loaded
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ARTICLES
NATURE MATERIALS DOI: 10.1038/NMAT4511
Potential V versus RHE
−0.5
0.0
0.5
E1: bare GaInP2
E2: GaInP2−TiO2
1.0
E3: GaInP2−TiO2−(OOCpy)Co(dmgH)2(Cl)−TiO2
0.8
E4: GaInP2−TiO2−Pt(ALD)
−2
E5: GaInP2−Pt(ALD)
−4
0.6
−6
IPCE
Photocurrent density (mA cm−2)
−1.0
0
−8
0.4
−10
−12
−14
−16
E1: bare GaInP2
GaInP2−TiO2−(py)Co(dmgH)2(Cl)
E3: GaInP2−TiO2−(OOCpy)Co(dmgH)2(Cl)−TiO2
−1.6
E2: GaInP2−TiO2
E4: GaInP2−TiO2−Pt(ALD)
E5: GaInP2−Pt(ALD)
−1.2
−0.8
Potential V versus Ag/AgCl
−0.4
0.2
0.0
400
440
480
520
560
600
Wavelength (nm)
640
680
Figure 3 | Linear-sweep voltammetry of electrodes 1–5 at a 20-mV s−1
scan rate, with Pt foil as the counter electrode, and Ag/AgCl as the
reference electrode.
Figure 4 | IPCE performance of electrodes 1–5 at −1 V versus Ag/AgCl in
pH 13 aqueous solution.
electrode 3. At −1.0 V versus Ag/AgCl (or 0 V versus RHE), the
current density of electrode 3 reaches ∼11 mA cm−2 , which is close
to its light-limited current of 12.5 mA cm−2 ; however, no current or
only 6 mA cm−2 was observed for electrodes 2 and 1, respectively.
These observations demonstrate at least three important points:
photo-excited electrons can pass through the ALD TiO2 layer;
TiO2 itself is a poor catalyst towards the HER (ref. 32); and the
grafted molecular cobalt HER catalyst on TiO2 is very effective
towards lowering the kinetic barrier and providing high activity
for water reduction. When the GaInP2 –TiO2 electrode was treated
with a non-covalently linked catalyst (py)Co(dmgH)2 (Cl) in the
same manner as electrode 3, it performed almost identically to
the untreated GaInP2 –TiO2 (electrode 2), indicating again that
a covalent linkage between TiO2 and the molecular catalyst is
critical. It is worth mentioning that the cobaloxime-modified
electrode also demonstrated enhanced catalytic activity and stability
in pH 7 solution as compared with the bare and TiO2 -modified
GaInP2 (Supplementary Fig. 6), which suggests that the carboxylate
chemisorption attachment is stable under both neutral and alkaline
conditions. When isonictonic acid (pyCOOH) was attached onto
the GaInP2 –TiO2 surface, the electrode performance was shown to
be similar to the unmodified electrode 2 (Supplementary Fig. 5),
indicating no catalytic activity with the pyridine moiety alone under
these conditions. The highly studied Pt HER catalyst was also
investigated and compared. As indicated in Fig. 2, Pt-modified
GaInP2 and GaInP2 –TiO2 electrodes demonstrated slightly more
positive onset than that of electrode 3. At 0 V versus RHE, their
photocurrent density was 11.5 and 8 mA cm−2 , respectively. The
current–voltage studies illustrate that in basic solution a costeffective Co molecular catalyst-modified GaInP2 –TiO2 electrode
could achieve performance approaching that of the Pt catalystmodified GaInP2 electrodes.
The efficiency of photocurrent generation under light irradiation
is usually described and evaluated as incident photon-to-current
efficiency (IPCE). As demonstrated in Fig. 4, from 470 to 680 nm,
the GaInP2 –TiO2 (electrode 2) has a lower IPCE (∼20%) compared
with the bare electrode (electrode 1) (∼45%). An enhanced IPCE for
electrode 4, electrode 5, and the Co molecule-modified electrode 3
was observed. Electrode 3 exhibits the highest IPCE of up to
∼80% across the visible range, about 50% higher than the bare
electrode, and about 20% higher than Pt-modified electrode 4. Our
interpretation for this high IPCE is twofold: that the catalyst is
transparent throughout the visible spectra; and the TiO2 layer can
act as an antireflective coating33 . The lower IPCE of electrodes 4
and 5 can be explained by the blockage of light by the Pt particles.
Material stability and hydrogen production
458
Figure 5 shows the photocurrent stability measurement on these
electrodes at −1.0 V versus Ag/AgCl for 20 min. Bare GaInP2
(electrode 1), although exhibiting a large photocurrent in the
first few seconds as reported before9 , unsurprisingly exhibited
more than one order-of-magnitude decrease in current in about
20 min. On the other hand, the high current density (∼9 mA cm−2 )
produced by electrode 3 was quite stable. Only a 5% or less
photocurrent decrease was measured during the 20-min electrolysis
experiment. Electrode 2, GaInP2 –TiO2 , was studied under the same
conditions, but only a small current was observed (0.3 mA cm−2 ),
which is consistent with the current–voltage curves in Fig. 3.
ALD TiO2 has been previously reported to protect Si, GaAs,
GaP, InP and Cu2 O photoelectrodes21,34–36 ; corroboratively, all ALD
TiO2 -coated electrodes showed a very stable current, indicating
complete coverage of the GaInP2 with few pinholes (Fig. 5). For
20 min, GaInP2 –TiO2 –Pt showed a stable photocurrent density at
∼8 mA cm−2 , but the GaInP2 –Pt (electrode 5) exhibited an unstable
current, decreasing to about half of the original photocurrent ending
up at only 5 mA cm−2 . The instability of the direct Pt-modified
GaInP2 electrode was explained by inadequate surface coverage
of the catalyst. As indicated by the SEM image of electrode 5
(Supplementary Fig. 4), platinum ALD produces small islands
and the coverage may be insufficient to stabilize the surface.
Moreover, platinum’s excellent HER catalytic activity was reported
to be diminished in basic conditions owing to the formation of a
passivated Pt hydrated species at low applied potentials37 .
The detailed electrolysis data are summarized in Table 1.
To estimate the lower bound of the turnover number (TON)
and turnover frequency (TOF) for the HER, in this case, all
loaded catalysts were assumed to contribute for the entire 20-min
electrolysis. On the basis of catalyst loadings measured by ICP-MS,
a TON of 4,070 and a TOF of 3.4 s−1 are estimated for electrode 3.
For comparison, direct Pt-modified GaInP2 (electrode 5) showed
an order-of-magnitude lower TON and TOF, of 342 and 0.28 s−1 ,
respectively. If 35 nm of TiO2 is present as an intermediate layer
between the GaInP2 and Pt (electrode 4), a TON of 3,129 and
TOF of 2.6 s−1 are obtained. In summary, our molecular catalystmodified GaInP2 –TiO2 electrode affords robust catalytic activity
and performs better than the Pt-modified electrodes under these
operational conditions.
The durability of electrode 3 has also been investigated for 20 h,
as shown in Supplementary Fig. 8. Following a slow decay from
10 to 5 mA cm−2 in the first 4 h, stable behaviour at ∼5 mA cm−2
was maintained for about 16 h. This initial current decrease might
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ARTICLES
NATURE MATERIALS DOI: 10.1038/NMAT4511
Table 1 | Summary of PEC performances for investigated GaInP2 photoelectrodes.
Entry
Electrodes
1
2
3
GaInP2 only
GaInP2 –TiO2
GaInP2 –TiO2 –(OOCpy)
Co(dmgH)2 (Cl)TiO2
GaInP2 –TiO2 –Pt
GaInP2 –Pt
4
5
Catalyst amount
(nmol)
–
–
0.504
J, 20 min at 0 V versus RHE
(mA cm−2 )
−0.6
−0.3
−8.2
Total charge
(C)
0.12
0.009
0.396
–
–
4,070
TOF
(s−1 )
–
–
3.4
0.384
6.060
−6.2
−4.7
0.232
0.400
3,129
342
2.6
0.28
Photocurrent density (mA cm−2)
0
−2
−4
−6
−8
E1: bare GaInP2
E2: GaInP2−TiO2
E3: GaInP2−TiO2−(OOCpy)Co(dmgH)2(Cl)−TiO2
E4: GaInP2−TiO2−Pt(ALD)
E5: GaInP2−Pt(ALD)
−10
−12
0
200
400
600
Time (s)
800
1,000
1,200
Figure 5 | Current density–time profile of electrodes 1–5 at −1 V versus
Ag/AgCl (or 0 V versus RHE) under 1-sun illumination for 20 min.
be attributed to several factors: molecular catalyst slowly desorbing
from the interface in the alkaline solution with some loss of the TiO2
layer38 and the deactivation of this grafted Co molecular catalyst. As
shown by XPS analysis in Supplementary Fig. 2, before electrolysis,
only peaks for Co(III) and TiO2 are observed in the spectrum
(Supplementary Figs 2(A) and 2(B)). The 20-h post-electrolysis
surface yields higher binding-energy shoulder peaks next to the
Co(III) peak, indicating alternations of grafted Co catalyst over
prolonged electrolysis39 . Moreover, Ga 3d, In 3d, and P 2p peaks
were present (Supplementary Fig. 2(C)), indicating some loss of
TiO2 in the electrolysis process and exposure of the GaInP2 surface.
However, XPS also showed that even after 20 h of electrolysis,
Co(III) and TiO2 are still present on the surface of GaInP2 .
Supplementary Fig. 9 shows the SEM images for the electrode 3
surface before and after 20 h of electrolysis. Before electrolysis,
the SEM images (Supplementary Fig. 9(A)) depict 50–150-nm
TiO2 nucleation on the electrode surface. Following electrolysis
for 20 min, no discernible change can be noticed from SEM
(Supplementary Fig. 9(B)). After electrolysis for 20 h, SEM analysis
shows that the original TiO2 bump disappeared, and this change in
surface morphology further confirms a certain degree of loss of the
cobaloxime–TiO2 protection layer (Supplementary Fig. 9(C)).
The production of hydrogen was established by the formation
of bubbles evolving from the photocathode continuously during
illumination and by gas chromatographic analysis. After passing
13.6 C through photocathode 3 at −1 V versus Ag/AgCl, 1.5 ml
of gas was measured by volume displacement and hydrogen
confirmed by gas chromatography, corresponding to a Faradaic
efficiency close to 100%. By carefully quantifying the catalyst
loading (Supplementary Table 1), we were able to calculate the
catalytic TON and TOF. A TON greater than 139,000 and a
1.9 s−1 TOF were achieved for 20 h for the Co catalyst at zero
overpotential. The remarkable turnover ability of the Co-modified
electrode was reported to be significantly increased after attaching
to a surface28 , and such an ameliorating effect of grafting molecules
TON
was derived from preventing possible solution-based dimerization
and inactivation according to ref. 28. For comparison, an analogous
cobalt complex together with ruthenium dye attaching onto the
TiO2 nanoparticle under visible-light illumination produced a
TOF of 4.17 × 10−3 s−1 . Cobalt diamine dioxime catalyst and
a biomimetic nickel complex coupled with carbon nanotube
achieved a TOF of 2.2 and 2.8 s−1 at an overpotential of 600 mV
and 300 mV, respectively28,40 . A key advantage of our system is
the coordinated coupling of the catalytic active cobalt catalyst
with a light-absorbing semiconductor unit—and thus, up to
3.4 s−1 TOF can be achieved with the semiconductor driving
the reaction. The TOF of electrode 3 under 1-sun at H2 O/H2
equilibrium potential for 20 h, to our best knowledge, is one of
the highest among molecular catalyst-modified photoelectrodes for
hydrogen production.
Conclusion
We have demonstrated the immobilization of a cobaloxime
hydrogen evolution catalyst on an ALD TiO2 -modified p-GaInP2
surface and have characterized its catalytic activity under illumination in pH 13 aqueous solution. The GaInP2 –TiO2 –cobaloxime
photocathode exhibited remarkable stability compared with a bare
GaInP2 electrode during 20 h of continuous operation at 0 V versus
RHE with the catalyst showing a TON of 139,000 and a TOF
of 1.9 s−1 during that time. A high IPCE, up to ∼80% through
the visible range (<690 nm), was observed from this structure,
clarifying the advantage of a visible-light transparent hydrogen
evolution catalyst.
This architecture allows the decoupling of the electrocatalytic
reactions from the underlying photovoltaic materials, which would
permit further improvements by independent optimization of
device parameters, that is, thickness of the ALD TiO2 layer or
other linkage groups. This approach seems quite general and should
have applications in protecting semiconductor substrates other
than GaInP2 and in integrating other molecular electrocatalysts
besides cobaloxime. Further, ALD is currently used in industrial
semiconductor device fabrication, which indicates the potential of
this device architecture to be employed on a large scale for solar
fuel production.
Methods
Methods and any associated references are available in the online
version of the paper.
Received 22 June 2015; accepted 2 November 2015;
published online 21 December 2015
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Acknowledgements
We gratefully acknowledge H. Doscher for supplying the GaInP2 wafers, C. Xiao for
AFM measurements, L. Gedvilas for ATR-IR measurements, C. Macomber for ICP-MS
measurements, S. George for advice on ALD and H. Wang and T. Deutsch for useful
discussions. This material is based on work supported by the US Department of Energy,
Office of Science, Office of Basic Energy Sciences, Solar Photochemistry Program under
Contract Number DE-AC36-08GO28308. J.L.Y. acknowledges NSF Graduate Research
Fellowship Grant No. DGE 1144083.
Author contributions
J.G., Y.Y., N.R.N. and J.A.T. designed the research; J.G. and Y.Y. performed the research;
J.L.Y. performed the ALD deposition; and K.X.S. performed the XPS study. J.G., Y.Y.,
N.R.N. and J.A.T. co-wrote the paper.
Additional information
Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at www.nature.com/reprints.
Correspondence and requests for materials should be addressed to J.A.T.
Competing financial interests
The authors declare no competing financial interests.
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ARTICLES
NATURE MATERIALS DOI: 10.1038/NMAT4511
Methods
Chemicals. Solvents and starting materials were purchased from
Sigma-Aldrich and all aqueous solutions were prepared from Milli-Q water
(18.2 M cm). The cobaloxime catalyst precursor Co(dmgH)2 Cl2 was prepared
following the literature procedure41 . First, 110 mg (0.306 mmol) of
Co(dmgH)2 Cl2 precursor was dissolved in 0.1 M NaOH (1 ml) and methanol
(5 ml). Then, 4-pyridinecarboxylic acid (37 mg, 0.306 mmol) was added and the
solution was refluxed for 1 h. Subsequently, the solution was cooled to room
temperature and stirred overnight. Next, 10 ml of water was added to the flask
after the solvent was evaporated by a rotary evaporator. The pH of the solution
was adjusted to 4.0 and a brown precipitate was obtained. The brown
product was filtered off from the solution and dried under vacuum. The
structure of the cobaloxime catalyst was characterized by nuclear magnetic
resonance (NMR) and the result is shown in Supplementary Fig. 10. Yield: 60 mg,
44%; 1 H NMR (400 MHz, CDCl3 ): δ = 8.21(dd, 2H, py), 7.64 (dd, 2H, py),
2.35 (12 H, s, CH3 ).
Preparation of GaInP2 material and photocathode. The GaInP2 (2–3-µm)
epilayers were grown on GaAs substrates following a literature procedure8 . For
photoelectrochemical measurement, the wafer was cleaved into 0.04–0.1 cm2
samples. Ohmic contact was made by etching the surface with concentrated H2 SO4 ,
followed by electron-beam deposition (Temescal FC-2000) of 10 nm of Ti, followed
by a 30 nm gold layer. For use in solution, the back side of the GaInP2 wafers was
mounted onto coiled copper wire by silver epoxy (PELCO colloid silver paste) and
heated to 65 ◦ C for 1 h. The electrical contact was isolated from the electrolyte
solution by covering with a glass tube and then sealing with two insulating and
acid-resistant epoxy coatings (Loctite 9462 Hysol and Loctite E-120 HP, cured
overnight at 65 ◦ C). Before any surface modification, the surface was cleaned by
etching with reagent-grade concentrated sulphuric acid (J.T. Baker) for 20 s,
followed by rinsing with deionized water, and dried under nitrogen or argon.
(HOOCpy)CoCl(dmgH)2 (Cl) complex was loaded onto the GaInP2 –TiO2
electrode by the following method. The GaInP2 –TiO2 electrode was immersed into
a 5 × 10−3 M (HOOCpy)Co(dmgH)2 (Cl) ethanol solution for 24 h. Subsequently,
the electrode was sonicated in 10 ml of ethanol solution for 5 min, washed with
ethanol (3 × 5 ml), and dried under a nitrogen or argon atmosphere. From
scanning electron microscopy (SEM) images, molecular Co catalyst-loaded
electrodes show no morphology difference as compared with the GaInP2 –TiO2
electrode (Supplementary Figs 1 and 9), although the Co loading was confirmed by
inductively coupled plasma mass spectroscopy (ICP-MS), indirect
ultraviolet–visible desorption, attenuated total-reflection infrared spectroscopy
(ATR-IR) and X-ray photoelectron spectroscopy (XPS).
Atomic layer deposition (ALD) of TiO2 (ref. 42). The ALD reactor is a
custom-built, viscous-flow, hot-wall-type ALD reactor. The deposition was done at
150 ◦ C at ∼1 torr pressure from a continuous flow of 100 standard cubic
centimetres per minute ultrahigh-purity N2 . The ALD coating applied in this study
is expected to be amorphous because of the fact that almost no crystalline TiO2 can
grow below 150 ◦ C (ref. 20). Amorphous, highly defective ALD TiO2 has been
reported to have a low electron-tunnelling resistance and high electronic
conductivity22,43 ; however, in this study, we are unable to distinguish whether the
conductivity originates from electronic tunnelling or conductivity. The precursors
are TiCl4 and H2 O, and the film was grown with alternating ∼100 mtorr doses of
TiCl4 and H2 O for 1 s. The doses were separated by 40 s periods of purging. The
measured growth rate on a Si witness sample is 0.44 Å per cycle and 790 cycles
resulted in ∼35 nm. The thickness of the TiO2 ALD layer on top of the GaInP2
(electrode 2) was further confirmed by atomic force microscopy (AFM)
measurements to be 35 nm (Supplementary Fig. 1), and the root-mean-square
(r.m.s.) roughness was determined to be ∼4 nm. The ∼4 nm roughness was mainly
due to 50–150 nm-diameter ‘bumps’ that were shown to be on the surface of the
TiO2 layer by both SEM and AFM. There are a limited number of hydroxyl ligands
present on the surface of the GaInP2 , which is known to induce a non-uniform
growth of TiO2 (ref. 44).
Atomic layer deposition (ALD) method for platinum nanoparticle. Electrode 4
GaInP2 –TiO2 –Pt and electrode 5 GaInP2 –Pt were made by ALD of nanoparticle Pt
by alternating exposures of trimethyl(methylcyclopentadienyl)platinum(IV) and a
hydrogen plasma. The ALD Pt deposition was performed in a custom-built,
plasma-enhanced ALD reactor. The deposition used 2 s doses of trimethyl
(methylcyclopentadienyl)platinum(IV) (MeCpPtMe3 ), alternating with 15 s of
100-W hydrogen plasma exposure. The sample stage temperature was 120 ◦ C, and
150 cycles of alternating Pt and H2 plasma were performed to target a nominal
continuous-film thickness equivalent to 2–3 nm. The loading amounts of Pt for
GaInP2 –TiO2 and bare GaInP2 were estimated by ICP-MS to be about
13 nmol cm−2 and 102 nmol cm−2 Pt, respectively. The variation of Pt deposition
amounts onto GaInP2 and GaInP2 –TiO2 using the same ALD method was
attributed to the very slow nucleation rate of ALD Pt on TiO2 as evidenced in
the literature45 .
Photoelectrochemical measurement. Linear-sweep voltammetry data were
collected using an EG&G Princeton Applied Research VersaStat II potentiostat
with a 300-W Xe-arc lamp (Newport). The intensity of the light was calibrated by a
GaInP2 photodiode to ensure an incident photon density identical to 1-sun
(100 mW cm−2 ). Photocathodic durability electrolysis experiments were performed
in a H-cell, isolated by a glass frit between the electrolysis compartments. For
electrolysis for 20 h, a steady d.c.-powered, 250-W tungsten-halogen lamp (Oriel
model 66183) with a water filter blocking infrared irradiation was used as the light
source. The GaInP2 photoelectrode was placed in the same compartment with the
Ag/AgCl reference and the Pt counter electrode was placed in the auxiliary
compartment. The headspace of the cell was continuously purged with Ar. A
gas-tight electrochemical set-up was applied to collect and measure the increased
volume caused by H2 production with a volumetric pipette. The collected gas (H2 )
was further confirmed and measured by gas chromatography (Shimadzu GC-2010
Plus) with a CarbxenTM 1010 PLOT column and a thermal conductivity detector.
The result of gas chromatography is shown in Supplementary Fig. 12. Potentials
were converted into RHE using the equation: V (RHE) = V(Ag/AgCl) + 0.23 V
(refs 46,47). The onset potential of the electrodes on the linear-sweep voltammetry
plot was calculated from the potential that generated −0.5 mA cm−2 current
density. We note that the electrode modified with catalyst but without an additional
10-cycle TiO2 layer (electrode 2’) demonstrated an unstable current, in which half
of its original current was lost during a 20-min electrolysis and this current decay
was manifested to more than 90% in a 20 h electrolysis (Supplementary Fig. 7).
SEM and XPS results of the electrode without the additional ALD cycle before and
after 20 h electrolysis further indicate substantial loss of the cobaloxime–TiO2
protection layer as compared with the electrode with an additional 10-cycle TiO2
(Supplementary Figs 2 and 9). Such observations concur with previous work on
similar protection of Ru complexes where the thin (∼0.4 nm) 10-cycle TiO2 layer
serves only to protect the carboxylic group linkage and does not cover the entire
molecule31 . This clearly indicates that the catalyst linkage-group stability has an
important role in stabilizing the current.
Indirect chemical desorption ultraviolet–visible experiment. Direct
measurement of the absorption spectroscopy on electrode 3 was not possible
because this cobalt complex absorbs only in the ultraviolet range, which
largely overlaps with the TiO2 absorption. An indirect chemical desorption
ultraviolet–visible experiment was applied here to measure the loading
amount of the Cobalt catalyst. Ultraviolet–visible absorption spectra were
collected using a Hewlett Packard 8453 spectrometer. The ultraviolet–visible
desorption measurement (Supplementary Fig. 3) was performed by immersing
a GaInP2 –TiO2 –Co(dmgH)2 (OOCpy)(Cl) electrode (0.2 cm2 ) in 1 ml of 1 M
NaOH solution and sonicating for 24 h with a Branson 5800 sonicator. The
extinction coefficient of the cobaloxime compound at 227 nm absorption
maximum was determined to be 20,764 M−1 cm−1 . After the desorption process,
the solution was analysed by an ultraviolet–visible spectrometer and the amount
determined to be 12.8 nmol cm−2 , which is close to the amount estimated by
ICP-MS as the amount of catalyst determined to be absorbed on the
electrode surface.
Inductively coupled plasma mass spectrometry (ICP-MS) measurement. The
ICP-MS experiment was conducted on a Thermo Scientific ICAP Q instrument
with a CETAC ASX-520 auto sampler. The Cobalt standard was prepared
from Inorganic Ventures MS Co-10 ppm. Owing to the many polyatomic
interferences for Co, the samples were run in standard (STD) mode and
kinetic-energy discrimination (KED) mode with Collision Cell Technology
(CCT). The results are equivalent, so either technique is valid. Here, a standard
mode value is used. The ICP-MS samples were prepared by immersing a
GaInP2 –TiO2 –(OOCpy)Co(dmgH)2 (Cl) electrode into 5 ml of concentrated
Omni trace H2 SO4 solution and heating the solution at 60 ◦ C for 20 min, followed
by sonicating the solution for 1 h to ensure the dissolution of the ALD TiO2 layer
with cobaloxime catalyst into solution. The reaction mixture was diluted to 30 ml
and sonicated for 1 h. Further, the solution was diluted to 0.1 M H2 SO4 by taking
1.67 ml of the 30 ml solution and diluting to 50 ml. The experiments were
repeated three times, and the data are reported in Supplementary Table 1. The
platinum standard was prepared from Inorganic Ventures MS Pt-10 ppm. Pt
electrodes were digested in aqua regia solution (0.58 ml) overnight and diluted to
30 ml for analysis.
Incident photon-to-current efficiency (IPCE) measurement. The IPCE
measurement was performed in a three-electrode set-up, 0.1 M NaOH aqueous
solution (pH 13) electrolyte at −1 V versus Ag/AgCl. The set-up of the instrument
was reported previously48 . The sequence at each wavelength was 6 s of dark,
following 4 s of illumination. The current was collected at 10 points per second,
with the final 10 points of each light and dark cycle averaged. For each data point at
one wavelength, the photocurrent was obtained by subtracting the dark current
from the light current. Sample photocurrent data were normalized to the output of
a calibrated silicon photodiode.
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ARTICLES
NATURE MATERIALS DOI: 10.1038/NMAT4511
X-ray photoelectron spectroscopy (XPS) experiment. Photoelectron
spectroscopy was performed on a Kratos AXIS Nova or Physical Electronics
(PHI) 5600 system depending on instrument availability. In either case,
photoelectrons were generated using monochromatic Al Kα X-rays at 1,486.6 eV.
Base pressures were better than 1 × 10−9 torr. Binding-energy calibrations were
performed by comparing measured Au 4f7/2 , Ag 3d5/2 , and Cu 2p3/2 core-level
spectra from clean metal foils and the accepted centroid positions reported in
ref. 49. For cobaloxime samples that exhibited charging, the C 1s peak was
referenced to 285.2 eV (ref. 50).
Attenuated total reflectance infrared (ATR-IR) measurement. The ATR-IR
measurement was carried out using a Nic Plan IR microscope, equipped with a
micro ATR attachment containing a Ge crystal allowing small-area sample
measurement and a MCT (mercury cadmium telluride) detector attached to a
Nicolet Magna 510 series FTIR spectrometer.
Scanning electron microscopy (SEM) measurement. The surface morphology of
the TiO2 -modified GaInP2 electrode or the Pt catalyst-modified GaInP2 was
analysed using a field-emission scanning electron microscope (FE-SEM), JEOL
JSM 7000F, operated at 5 kV.
Atomic force microscopy (AFM) experiment. The AFM images were acquired
with a Bruker AFM-D3100NSV at room temperature. Tapping mode was applied
with silicon cantilevers (Asylum Research, AC160 TS) at a resonant frequency of
200–400 kHz (Spring Constant 42 N m−1 ). The depth profile AFM sample was
prepared by employing a blade and vertically scratching the edge of the
GaInP2 –TiO2 wafer.
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