A mesoporous atomic layer deposition (ALD) double-shell electrode, Al2O3 (insulating core)//ALD ZnO|ALD TiO2, on a fluorine-doped tin oxide (FTO) conducting substrate was explored for a photoanode assembly, FTO//Al2O3 (insulating core)//ALD ZnO|ALD TiO2|–chromophore–catalyst, for light-driven water oxidation. Photocurrent densities at photoanodes based on mesoporous ALD double-shell (ALD ZnO|ALD TiO2|) and ALD single-shell (ALD ZnO|, ALD TiO2|) electrodes were investigated for O2 evaluation by a generator–collector dual working electrode configuration. The high photocurrent densities obtained based on the mesoporous ALD ZnO|ALD TiO2 photoanode for O2 evolution arise from a significant barrier to back electron transfer (BET) by the optimized tunneling barrier in the structure with the built-in electric field at the ALD ZnO|ALD TiO2 interface. The charge recombination is thus largely decreased. In the films, BET following injection has been investigated through kinetic nanosecond transient absorption spectra, and the results of energy band analysis are used to derive insight into the internal electronic structure of the electrodes.

Dye-sensitized photoelectrochemical cells (DSPECs) provide a strategy for solar energy conversion by integrating light absorption, photoelectron conversion and electron transfer to drive electrochemical reactions including water splitting.1–12 Charge separation is a critical factor in these DSPEC devices for electron transfer to carry out electrochemical reactions. Multiple strategies have been developed to slow the rates of back electron transfer (BET) in these dye-sensitized systems in order to enhance charge separation and photoelectrochemical efficiencies.13–15 Construction of core/shell oxide, layer-by-layer structures, by atomic layer deposition (ALD) has led to decreased charge recombination (CR) (back electron transfer) and substantially improved photocurrent densities due to proper band alignment and interfacial barriers.16–24 

A typical photoanode in a DSPEC device consists of a mesoporous nano-TiO2 or SnO2 semiconductor serving as a high-surface-area film for efficiently loading molecular chromophores with catalysts to create an extermal charge-transfer layer.25–32 As a way to enhance water-splitting efficiencies in DSPEC devices, atomic layer deposition (ALD) has been used to form nano ITO(tin-doped indium oxide)/ALD TiO2 or SnO2/ALD TiO2 core/shell-based photoanodes on fluorine-doped tin oxide (FTO) substrate.33 The latter configuration results in greatly improved photoelectron conversion efficiencies by decreasing BET rates.34–36 However, the overall efficiencies in these devices are still low due to rapid recombination by back electron transfer. Overcoming the problem of large BET rates is a major challenge in maximizing water-splitting performance.

In this study, atomic layer deposition (ALD) has been used to prepare derivatized electrodes on the surface of mesoporous Al2O3 films on FTO to give the electrodes FTO//Al2O3//ALD ZnO|ALD TiO2 with the ZnO|TiO2 double shell added by atomic layer deposition. The final electrodes were surface-modified by the addition of the molecular ruthenium(II) polypyridyl chromophore, RuP2+ [Ru(4,4′-PO3H2-bpy)(bpy)2]2+: bpy = 2,2′-bipyridine, Fig. S1(a), and the Ru-based water oxidation catalyst C1 according to the Sun group [Ru(bda)L2: bda = 2,2′-bipyridine-6,6′-dicarboxylate, L = 4-O(CH2)3PO3H2-pyr, pyr = pyridine, Fig. S1(b)].37–44 The resulting photoanode structure, FTO//Al2O3//ALD ZnO|ALD TiO2|–(RuP2+)2–C1, was further explored to include an optimized tunneling barrier with a built-in electric field at the ALD ZnO|ALD TiO2 interface to decrease BET and enhance photoinduced charge transfer. The mesoporous Al2O3 film is insulated with the external ZnO|TiO2 film providing a mesoporous double-shell electrode surface to the conducting FTO substrate. As shown in Fig. 1, the combined layered structure, FTO//Al2O3//ALD ZnO|ALD TiO2, provides the basis for improving the performance of a dye-sensitized photoanode toward light-driven water oxidation. This work used a rare assembly of a two-layer ALD layer system on an insulating core as a charge collection electrode, which resulted in greatly long recombination lifetime.

FIG. 1.

(a) Chromophore−catalyst assemblies on a mesoporous metal oxide film for water oxidation following photoexcitation: FTO//Al2O3//ALD ZnO|ALD TiO2|–(RuP2+)2–C1. The black arrows illustrate photoinduced electron transfer. (b) SEM (scanning electron microscopy) images of a mesoporous Al2O3//ALD ZnO|ALD TiO2 core–double shell electrode on a FTO substrate. Left: surface; right: cross section.

FIG. 1.

(a) Chromophore−catalyst assemblies on a mesoporous metal oxide film for water oxidation following photoexcitation: FTO//Al2O3//ALD ZnO|ALD TiO2|–(RuP2+)2–C1. The black arrows illustrate photoinduced electron transfer. (b) SEM (scanning electron microscopy) images of a mesoporous Al2O3//ALD ZnO|ALD TiO2 core–double shell electrode on a FTO substrate. Left: surface; right: cross section.

Close modal

Nanosecond transient absorption (TA) measurements were used to investigate charge recombination dynamics to explore the role of the structure within the photoanode. The available results show that optimizing the double-shell structure by ALD results in remarkable enhancements in charge recombination lifetime in the mesoporous double-shell based photoanodes relative to the mesoporous single-shell based photoanodes. This would demonstrate the decreased BET arising from interfacial built-in electric fields and the tunneling barrier. Based on the photophysical results, photocurrent densities for light-driven water oxidation, 2H2O → O2 + 4H+ + 4e, were also substantially enhanced in the mesoporous double-shell (FTO//Al2O3//ALD ZnO|ALD TiO2) based dye-sensitized photoanode, compared with that in the mesoporous single-shell (FTO//Al2O3//ALD ZnO, FTO//Al2O3//ALD TiO2) based photoanodes, with O2 detected with a generator–collector dual working electrode.

Figure 1(b) shows scanning electron microscopy (SEM) images of a mesoporous Al2O3//ALD ZnO|ALD TiO2 film on FTO substrates with a film thickness of ∼4.5 µm. The electrode was prepared from an ∼4.5 µm mesoporous Al2O3 film on a FTO substrate with a typical doctor-blading method followed by ALD deposition of the ZnO and TiO2 shells. Figures S2(a) and S2(b) display TEM images of a Al2O3//ALD ZnO|ALD TiO2 core–double shell structure with a total shell thickness of 6.2 nm and a Al2O3//ALD ZnO core–shell structure with a shell thickness of 3.3 nm.

The effect of shell thickness on electrode performance for light-driven water oxidation in DSPEC devices was also investigated. The comparison showed that, FTO//Al2O3//ZnO(∼3 nm)|TiO2(∼3 nm), the mesoporous double-shell electrode gave the highest photocurrent densities compared with single-layered structure and double-layered one with other thicknesses for light-driven water oxidation, and it was this structure that dominated subsequent studies. The working electrode is an equivalent of the FTO/ALD ZnO|ALD TiO2 double-shell electrode, considering about the insulating Al2O3 film that serves as a surface for dye and catalyst loading in subsequent studies. The effect of shell thickness on photocurrent densities under DSPEC experimental conditions for light-driven water oxidation is shown in Fig. 2. For creating the surface assemblies, the molecular dye RuP2+ was co-loaded with the molecular water oxidation catalyst C1 at a molar ratio of 2:1 as previously described.37,38,45 The samples were excited by 1 sun illumination with light on-light off cycles in pH 4.6, 0.1M acetate buffer with 0.4M NaClO4 supporting electrolyte at an applied bias of 0.25 V vs. Ag/AgCl as the reference electrode. Effect of film thickness of the ZnO shell (0, 3, and 6 nm) and the TiO2 shell (0, 1.5, 3, 4.5, 6, and 7.5 nm) was explored with the maximum photocurrent density obtained in the configuration FTO//Al2O3(core)//ZnO(∼3 nm)|TiO2(∼3 nm)|–(RuP2+)2–C1.

FIG. 2.

[(a)–(c)] Photocurrent density (j)–time traces for FTO//Al2O3//ALD ZnO|ALD TiO2|–(RuP2+)2–C1 photoanodes with different double-shell thicknesses in pH 4.6, 0.1M acetate buffer with 0.4M NaClO4 supporting electrolyte at an applied bias of 0.25 V vs. Ag/AgCl reference electrode. RuP2+ was co-loaded with C1 at a molar ratio of 2:1.37,38,45 The samples were excited by 1 sun illumination with light-on-light off cycles with a 400 nm cut-off filter to avoid direct bandgap excitation of the metal oxide layer.27 

FIG. 2.

[(a)–(c)] Photocurrent density (j)–time traces for FTO//Al2O3//ALD ZnO|ALD TiO2|–(RuP2+)2–C1 photoanodes with different double-shell thicknesses in pH 4.6, 0.1M acetate buffer with 0.4M NaClO4 supporting electrolyte at an applied bias of 0.25 V vs. Ag/AgCl reference electrode. RuP2+ was co-loaded with C1 at a molar ratio of 2:1.37,38,45 The samples were excited by 1 sun illumination with light-on-light off cycles with a 400 nm cut-off filter to avoid direct bandgap excitation of the metal oxide layer.27 

Close modal

The photocurrent densities obtained for the developed double−shell structures prepared by ALD are comparable to those obtained previously based on nanoparticle oxide core−ALD shell structures.34,38 The ALD double−shell electrodes result in a greatly enhanced photocurrent response compared to single−shell electrodes prepared by ALD. In ALD double−shell electrodes, there are optimized tunneling barrier shells with the built−in electric field at the ALD ZnO|ALD TiO2 interface that inhibit back electron transfer, enhancing charge recombination lifetime. With increasing shell thicknesses, the photocurrent densities are layer thickness dependent, indicating a tunneling mechanism in the process. Photocurrent densities first increase because of a decrease in the BET rate by increasing the tunneling barrier,17,38,46 and then decrease due to diffusion-limited BET from electrons localized in trap states in the shells46 and from reduced loading of the molecular dye due to decreased surface areas with smaller pore size.47 

Figure S3 shows a UV–visible absorption spectrum of RuP2+ on different mesoporous shells to estimate the surface coverage of RuP2+. The surface coverage (Γ) of RuP2+ for FTO//Al2O3//ZnO(∼3 nm)|TiO2(∼3 nm)|–(RuP2+)2–C1 was determined by subtracting the background spectra [Fig. S4(b)],47,48 which gave Γ(RuP2+) 6 × 10−8 mol/cm2.

The Faradic efficiency for O2 formation in FTO//Al2O3//ZnO(3 nm)|TiO2(3 nm)|–RuP2+–C1, was determined by a previously reported generator–collector dual working electrode method,38,49 as shown in Fig. 3(a). In these experiments, the samples were illuminated for 10 min with the input of a one sun lamp. Photoelectrochemical O2 evolution at the photoanode generator was monitored using a parallel fluorine-doped tin oxide (FTO) collector cathode with the two separated by 1 mm, with a pre-established collection efficiency of 70%.38,49 The Faradic efficiency (η) for O2 generation was evaluated from current density (j)−time plots by using Eq. (1). In Eq. (1), Qcollector refers to the total charge that passed at the collector electrode; Qgenerator is the total charge passed at the generator electrode. The Faradic efficiency (η) for FTO//Al2O3//ZnO(3 nm)|TiO2(3 nm)|–(RuP2+)2–C1 was 75.5%

ηt=(Qcollector/Qgenerator)/0.7=0tjcollector0.7×0tjgenerator.
(1)
FIG. 3.

(a) Current density (j)−time traces in a collector−generator (C–G) cell for FTO//Al2O3//ZnO(3 nm)|TiO2(3 nm)|–(RuP2+)2–C1 photoanode in pH 4.6, 0.1M acetate buffer with 0.4M NaClO4 supporting electrolyte at an applied bias of 0.25 V vs. Ag/AgCl reference electrode. The photoanode was excited by 1 sun illumination from 100 to 700 s. In the cell, the generator current density for water oxidation (black) under illumination and the collector current density for oxygen reduction (gray) are shown. (b) Normalized transient absorption (TA) difference spectra (ΔOD)–time traces at 480 nm for photoanode assemblies: FTO//Al2O3//ZnO(3 nm)|TiO2(3 nm)|–RuP2+, FTO//Al2O3//ALD ZnO|–RuP2+, and FTO//Al2O3//ALD TiO2|–RuP2+ electrodes following excitation at 425 nm in pH 4.6, 0.1M acetate buffer with 0.4M NaClO4 as the supporting electrolyte at an applied bias of 0.25 V vs Ag/AgCl as the reference electrode. Scatter: TA data; dark lines: fitted curves based on Eq. (3).

FIG. 3.

(a) Current density (j)−time traces in a collector−generator (C–G) cell for FTO//Al2O3//ZnO(3 nm)|TiO2(3 nm)|–(RuP2+)2–C1 photoanode in pH 4.6, 0.1M acetate buffer with 0.4M NaClO4 supporting electrolyte at an applied bias of 0.25 V vs. Ag/AgCl reference electrode. The photoanode was excited by 1 sun illumination from 100 to 700 s. In the cell, the generator current density for water oxidation (black) under illumination and the collector current density for oxygen reduction (gray) are shown. (b) Normalized transient absorption (TA) difference spectra (ΔOD)–time traces at 480 nm for photoanode assemblies: FTO//Al2O3//ZnO(3 nm)|TiO2(3 nm)|–RuP2+, FTO//Al2O3//ALD ZnO|–RuP2+, and FTO//Al2O3//ALD TiO2|–RuP2+ electrodes following excitation at 425 nm in pH 4.6, 0.1M acetate buffer with 0.4M NaClO4 as the supporting electrolyte at an applied bias of 0.25 V vs Ag/AgCl as the reference electrode. Scatter: TA data; dark lines: fitted curves based on Eq. (3).

Close modal

Ultraviolet Photoelectron Spectra (UPS) and Kubelka-Munk function transformed spectra from absorption measurements on FTO//Al2O3//ZnO(3 nm) and FTO//Al2O3//TiO2(3 nm) films are shown in Fig. S4 with the energy band data shown in Table S1. From the comparison, it can be seen that the Fermi level for the ALD TiO2 layer (−0.09 V vs. NHE (Normal Hydrogen Electrode)) is lower than that for the ALD ZnO layer (−0.34 V vs. NHE), due to which the built-in electric field can form at the ALD ZnO|ALD TiO2 interface (Fig. S5). Based on these results, there is an internal driving force for electron transfer from the TiO2 layer to the ZnO layer, which creates an internal barrier to back electron transfer through the shells to increase recombination lifetimes and enhance photoanode performance for light-driven water oxidation. Given the difference in conduction band potentials between the TiO2 shell (−1.08 V) and the MLCT (Metal-to-Ligand Charge-Transfer) excited state RuP3+/2+* (−0.81 V), photoinduced electrons from exited states RuP3+/2+* are favored energetically tunneling through the 3 nm TiO2 layer, creating a strong barrier to back electron transfer, consistent with earlier results on core-shell electrodes.46 Following electron excitation and transfer through the TiO2 shell, electrons will subsequently be transferred through the 3 nm ZnO layer to the conducting FTO substrate [Fig. 1(a)] favored energetically by the interfacial electric field.

The data obtained by analysis of the electronic spectra for the ZnO(6 nm) and TiO2(6 nm) films (Fig. S6 and Table S2) show that electronic energy levels of metal oxide thin layers vary with thicknesses, which further confirms that photoinduced electrons from RuP3+/2+* are injected through the ALD thin layer by electron tunneling with barriers that vary with thickness.

Nanosecond transient absorption (TA) experiments were carried out to analyze the details of back electron transfer from the electrodes to RuP3+ following electron excitation and injection by RuP2+ with a 425 nm laser in pH 4.6, 0.1M acetate buffers with 0.4M NaClO4 supporting electrolyte. As shown in Fig. 3(b), normalized transient absorption (TA) difference spectra (ΔOD)–time decay traces were obtained due to BET and analyzed by application of the Kolrausch–Williams–Watts (KWW) stretched exponential function [Eq. (2)]. The bi-exponential KWW fitting model for back electron transfer describes absorbance-time changes as the sum of two exponential decay functions

At=A1e(tτ1)B1+A2e(tτ2)B2
(2)

with a characteristic distribution lifetime, τ, and distribution width, B. A1 and A2 are the initial absorbances at time 0 for the two amplitudes in the exponential sum in Eq. (2).2,5,50–53 The average charge recombination (CR) lifetime, tCR, was calculated from Eq. (3) with Γ, the gamma function Γx=0tx1etdt, and τ and B derived from KWW fit to the kinetic traces.54,55 The obtained fitting parameters for the decay traces are shown in Table I,

tCRi=(τiBi)×Γ(1Bi).
(3)
TABLE I.

KWW biexponential kinetic fitting parameters for absorbance decay traces.a

FittingALD ZnO|ALDALDALD
parametersTiO2|-RuP2+TiO2|-RuP2+ZnO|-RuP2+
A1 (%) 94 81 83 
τ1 (s) 1.7 × 10−4 7.9 × 10−6 1.0 × 10−8 
B1 0.29 0.28 0.20 
tCR1 (s) 2.0 × 10−3 1.1 × 10−4 1.2 × 10−6 
A2 (%) 6.0 19 17 
τ2 (s) 0.048 0.011 0.0020 
B2 0.87 0.48 0.32 
tCR2 (s) 0.051 0.024 0.013 
FittingALD ZnO|ALDALDALD
parametersTiO2|-RuP2+TiO2|-RuP2+ZnO|-RuP2+
A1 (%) 94 81 83 
τ1 (s) 1.7 × 10−4 7.9 × 10−6 1.0 × 10−8 
B1 0.29 0.28 0.20 
tCR1 (s) 2.0 × 10−3 1.1 × 10−4 1.2 × 10−6 
A2 (%) 6.0 19 17 
τ2 (s) 0.048 0.011 0.0020 
B2 0.87 0.48 0.32 
tCR2 (s) 0.051 0.024 0.013 
a

In pH 4.6, 0.1 M acetate buffer with 0.4 M NaClO4 supporting electrolyte at an applied bias of 0.25 V vs. Ag/AgCl.

Kinetic absorbance decay traces for back electron transfer to RuP3+ in the assemblies are shown in Eqs. (4)–(7): Eqs. (4) and (5) for assembly 1, FTO//Al2O3//ALD ZnO|ALD TiO2|–RuP2+; Eq. (6) for assembly 2, FTO//Al2O3//ALD TiO2|–RuP2+; and Eq. (7) for assembly 3, FTO//Al2O3//ALD ZnO|–RuP2+.

ZnO|TiO2e|RuP3+ZnO|TiO2|RuP2+,
(4)
ZnOe|TiO2|RuP3+ZnO|TiO2|RuP2+,
(5)
TiO2e|RuP3+TiO2|RuP2+,
(6)
ZnOe|RuP3+ZnO|RuP2+.
(7)

The kinetic fitting results show that the kinetics of back electron transfer for all three electrodes in Eqs. (4)–(7) is complex, following KWW distribution kinetics. The complexity in the kinetics is a characteristic feature of the heterogeneous, thin-film structure with short term components arising from recombination between surface-trapped electrons and holes at RuP3+, tCR1, and long term components from recombination between electrons hopping through a distribution of trap states within the reduced metal oxide and holes at RuP3+, tCR2.56–59 For the assembly FTO//Al2O3//ALD ZnO|ALD TiO2|–RuP2+, the contribution to the long-term component for charge recombination arises from slow back electron transfer from reduced ZnO [ZnO (e)] through the shells to RuP3+, and the relatively short-time component results from rapid BET from reduced TiO2 [TiO2 (e)] to RuP3+. For the assembly FTO//Al2O3//ALD TiO2|–RuP2+ and FTO//Al2O3//ALD ZnO|–RuP2+, the long term component is due to slow BET from electrons trapped in a distribution of localized states within the oxide layer, and the short term component is from rapid recombination between surface-trapped electrons and holes at RuP3+. Further work still needs to be done to illustrate the recombination phases in kinetic absorbance decay traces of back electron transfer to RuP3+ in the assemblies.

Based on the recombination lifetime tCR in Table I with 2 ms for tCR1 and 51 ms for tCR2 for the optimized assembly in FTO//Al2O3//ZnO(3 nm)|ALD TiO2(3 nm)|–RuP2+, higher photocurrent densities are obtained for light-driven water oxidation than the other two. The origin of the effect arises from an increase in recombination lifetime due to the significant optimized barrier with a built-in electric field and the extended separation distance for back electron transfer. In addition, the difference between ALD TiO2 and ALD ZnO/TiO2 at time scales longer than 2 ms where the recombination seems to be faster for the bilayer system compared to the single layered one could be due to the surfaces’ charge accumulations which may screen the built-in electric field effect.

The electrode, FTO//Al2O3//ALD ZnO|ALD TiO2, with a mesoporous double-shell structure on an FTO conducting substrate, was developed for chromophore-catalyst assembly for light-driven water oxidation. Photocurrent density for photoanode assembly, FTO//Al2O3//ZnO|TiO2|–(RuP2+)2–C1, were largely enhanced comparable with that for mesoporous single-shell photoanodes, revealing a Faradaic efficiency of 75.5% for O2 evolution. The resulting assembly is important in decreasing charge recombination time. Nanosecond time-dependent transient absorption (TA) measurements show that the mesoporous ALD double-shell structure results in greatly decreased back electron transfer based on an optimized electron tunneling barrier with the interfacial built-in electric field.

Supplementary material includes the detailed experimental section, TEM pictures, and XPS information.

The research on mesoporous double-shell based photoanode for light-driven water oxidation was supported by the University of North Carolina Energy Frontier Research Center (UNC EFRC): Center for Solar Fuels, an Energy Frontier Research Center supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001011. The transmission electron microscopy (TEM), scanning electron microscopy (SEM), and ultraviolet photoelectron spectroscopy (UPS) experiments were performed at the Chapel Hill Analytical and Nanofabrication Laboratory, CHANL, a member of the North Carolina Research Triangle Nanotechnology Network, RTNN, supported by the National Science Foundation, Grant No. ECCS-1542015, as part of the National Nanotechnology Coordinated Infrastructure, NNCI.

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