Inner Layer Control of Performance in a Dye-Sensitized

Feb 28, 2017 - Interfacial charge transfer and core-shell structures play important roles in dye-sensitized photoelectrosynthesis cells (DSPEC) for wa...
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Inner Layer Control of Performance in a Dye-Sensitized Photoelectrosynthesis Cell Degao Wang, Byron H. Farnum, Matthew V. Sheridan, Seth L. Marquard, Benjamin D. Sherman, and Thomas J. Meyer* Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States S Supporting Information *

ABSTRACT: Interfacial charge transfer and core-shell structures play important roles in dye-sensitized photoelectrosynthesis cells (DSPEC) for water splitting into H2 and O2. An important element in the design of the photoanode in these devices is a core/shell structure which controls local electron transfer dynamics. Here, we introduce a new element, an internal layer of Al2O3 lying between the Sb:SnO2/TiO2 layers in a core/shell electrode which can improve photocurrents by up to 300%. In these structures, the results of photocurrent, transient absorption, and linear scan voltammetry measurements point to an important role for the Al2O3 layer in controlling internal electron transfer within the core/shell structure. KEYWORDS: core−shell, atomic layer deposition, pinholes, dye-sensitized photoelectron transfer synthesis cell

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In our experiments, we utilized transparent, conductive, antimony-doped tin oxide Sb:SnO2 (ATO) as the substrate on FTO by spinning coating followed by ALD depositing TiO2 to prepare the core/shell(ATO/TiO2) structures.13 ALD is a self-limiting technique that allows for precise control and deposition by controlling mesoporous structures.14 In the ALD experiments, a subnanometer Al2O3 layer was deposited between the internal Sb:SnO2 core and external TiO2 layers in the cells. We find that variations in the thickness of the internal Al2O3 thin blocking layer from 0 to 0.99 nm can improve the photocurrent at the surface by up to ∼300%. We also cite the results of transient absorption (TA) measurements that illustrate the impact of the added Al2O3 layer on back electron transfer between layers and the results of linear scan voltammetry (LSV) measurements that reveal the influence of the inner layer on electron transfer through the core−shell structure.

he dye-sensitized photoelectrosynthesis cell (DSPEC) provides a basis for reducing protons to H2 or for the reduction of CO2 to reduced forms of carbon.1−4 In these devices, molecular light absorption and catalysis are integrated with surface-bound molecular assemblies.5−8A limiting factor dictating efficiencies in these devices is the accumulation of multiple oxidative equivalents at catalyst sites that meet the 4e−/4H+ demands for water oxidation, 2H2O → O2 + 4e− + 4H+.9 As shown by the excitation sequence in eq 1, oxidation is in competition with back electron transfer (BET) of injected electrons into the semiconductor oxide from the assembly. Following excitation and injection, back electron transfer can greatly decrease overall cell efficiencies.10,11 In controlling interfacial dynamics, the use of core/shell nano-ITO/TiO2 and SnO2/TiO2 electrodes have been used to greatly enhance photoconversion efficiencies.10−12 Here, we introduce a new concept, utilization of a thin blocking layer between the layers of the core/shell to enhance efficiencies. oxide|TiO2 |‐assembly

Special Issue: Hupp 60th Birthday Forum

→ oxide|TiO2 (e−)|‐assembly( +) → oxide|TiO2 |‐assembly © XXXX American Chemical Society

Received: January 7, 2017 Accepted: February 28, 2017 Published: February 28, 2017

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DOI: 10.1021/acsami.7b00225 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. (A) Core−Shell Electrode Configuration for the Cell, Sb:SnO2/0-0.99 nmAl2O3/4.5 nm TiO2|-Rua2+-Rub2+-OH2 on ATO Showing the Assembly Structure and Excitation and Current Flow Events Leading to Water Oxidation; (B) Excitation and Current Flow within the Core−Shell: pH 4.65, 0.1 mM Acetate Solution with 0.4 M NaClO4 (Conduction Band, CB)

The assembly, [(4,4′-(PO3H2)bpy)2Ru(4-mebpy-4′-bimpy)Ru(tpy) (OH 2)]4+ (1; -Rua2+-Rub2+-OH2, bpy = 2,2′bipyridine; 4-mebpy-4′-bimpy = 4-(methylbipyridin-4′-yl)-N(benzimid)-N′-pyridine); tpy = 2,2′:6′,2″-terpyridine), shown in Scheme 1, has been used for light-driven water oxidation at core/shell electrodes. It was prepared and characterized as described previously and utilizes an external carbene water oxidation catalyst.15,16 In a second series of experiments, the related chromophore, [Ru II (bpy) 2 (4,4′-(PO 3H 2 ) 2 -bpy)]2+ (RuP2+, bpy = 2,2′-bipyridine) (Chart 1), was used as the light absorber and its electron-transfer dynamics were explored on Sb:SnO2−TiO2 core−shell films with added Al2O3. Chart 1. [RuII(bpy)2(4,4′-(PO3H2)2-bpy)]2+ (RuP2+, bpy = 2,2′-bipyridine)

Figure 1. TEM image of a core−shell Sb:SnO2/0.99 nm Al2O3/4.5 nm TiO2 electrode structure.

The assembly structure is shown in Scheme 1. The scheme also illustrates the interface and structure of the chromophore− catalyst assembly (left-hand side) and the dynamic events that occur at the derivatized interface following light absorption by the surface-bound chromophore in the chromophore-catalyst assembly. A transmission electron micrograph (TEM) of a, Sb:SnO2/ 0.99 nm Al2O3/4.5 nm TiO2, core/shell electrode, is shown in Figure 1. It illustrates a core/shell electrode prepared by spinning coating a uniformly Sb:SnO2 nanoparticle film followed by ALD 9 cycles of Al2O3 and 75 cycles of TiO2.17 For comparison, a Sb:SnO2 nanoparticle film with 9 cycles of Al2O3 (0.99 nm) and ATO nanoparticle films with 75 cycles of

TiO2 (4.5 nm) are shown in Figure S1. Both images show that Al2O3 and TiO2 are uniformly coated on the ATO nanoparticles. In photoelectrochemical experiments, 3 μm thick mesoporous Sb:SnO2 films were subjected to sequential exposures (ALD cycles) with trimethylaluminum (Al(CH3)3) and water (H2O) to deposit Al2O3 films (eqs 2 and 3). A thin Sb:SnO2 film was chosen to ensure uniform Al2O 3 deposition throughout the film, not limited by the diffusion length of the metal organic precursor (Al(CH3 )3 ) in the ATO mesopores. The reference (0 cycle) and the Al2O3-coating ATO samples were characterized by X-ray photoelectron measurements (XPS). Both the reference and passivated films show two binding energies, one at 458.4 ± 0.1, 464.5 ± 0.1 eV, corresponding to the Ti 2p3/2 and Ti 2p1/2 transitions, respectively, consistent with preservation of the Ti4+ oxidation state after ALD processing. A peak at 74.4 ± 0.1 eV is also observed for Al 2p3/2 confirming the presence of Al3+ in Al2O3 (Figure S2). The Al 2p3/2 binding energies are constant with the number of ALD cycles. The growth rate of the Al2O3 cycle by ALD was estimated as ∼1.1 Å per cycle by spectroscopic ellipsometry measurements on Si wafers. Growth rates for the B

DOI: 10.1021/acsami.7b00225 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. (a)Current−time (I−t) traces over 20 s dark−light cycles for Sb:SnO2/0.11−1.0 nm Al2O3/4.5 nm TiO2|-Ru2+a-Rub2+-OH2 electrodes at an applied bias of 0.2 V versus Ag/AgCl, pH 4.65, 0.1 M acetate, 0.4 M NaClO4. (b) Current−time plots for Al2O3 thicknesses measured at the end of three 20 s dark−light cycles with an applied bias of 0.2 V versus Ag/AgCl.

first few cycles are dependent on surface chemistry with slight variations between Sb:SnO2 films and Si. Sb:SnO2 ‐OH + Al(CH3)3 → Sb:SnO2 ‐O‐Al(CH3)2 + CH4

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Sb:SnO2 ‐O‐Al(CH3)2 + 2H 2O → Sb:SnO2 ‐O‐Al(OH)2 + 2CH4

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The molecular dyad assembly was loaded onto the 3 μm electrodes with 4.5 nm TiO2 outer shells. Current density−time (I−t) traces for varying thicknesses of the Al2O3 interlayer, Sb:SnO2/0−0.99 nm Al2O3/4.5 nm TiO2|-Rua2+-Rub2+-OH2, were observed with 445 nm excitation. Results are shown in Figure 2 as a function of the thickness of the added overlayer. With 0.55 nm Al2O3 slides, an enhancement in photocurrent density from 27 to 76 μA/cm2 was observed for three 20 s light on/off cycles. Increasing the thickness of Al2O3 past 0.55 nm decreased the photocurrent densities. The effect of the lower TiO2 shell thicknesses of 1.5 and 3.0 nm was also investigated. At 1.5 nm, a similar trend was observed as the Al2O3 layer was varied with the highest photocurrent densities reached at 0.77 nm (Figure S4). At higher film thicknesses, the most favorable distances were ∼0.55 for both the 3.0 (Figure S5) and 4.5 nm outer layer films (Figure 2). IPCE traces reveal the same trends with added Al2O3. As shown in the IPCE traces in Figure 3, the Sb:SnO2/0.55 nm Al2O3-4.5 nm TiO2|-Rua2+-Rub2+-OH2 electrode has a value of 5.7% at 450 nm, twice the value for Sb:SnO2/0 nm Al2O3/4.5 nm TiO2|-Rua2+-Rub2+-OH2. Other configurations without the inner layer failed to show related enhancements, Figures S6 and S7. To explore the microscopic details of the Al2O3 overlayer effect, transient absorption measurements were carried out with the dye, [RuII(bpy)2(4,4′-(PO3H2)2-bpy)]2+ (RuP2+), on the same core/shell electrodes. In these experiments, core−shell slides of 1.5 and 3.0 nm were loaded from 1 mM solutions of the dye in MeOH for 24 h (Figure S3).18,19 In the TA experiments variations in the outer layer TiO2 shell were limited to 1.5 and 3.0 nm because of light scattering from thicker shell electrodes. Experiments were carried out in a three-electrode spectroelectrochemical cells (Ag/AgCl reference, Pt counter) in pH 4.6 aqueous solutions (0.1 M acetate

Figure 3. IPCE measurement for Sb:SnO2/0 nmAl2O3/4.5 nm TiO2|Rua2+-Rub2+-OH2 and Sb:SnO2/0.55 nm Al2O3/4.5 nm TiO2|-Rua2+Rub2+-OH2 electrodes at an applied bias of 0.2 V versus Ag/AgCl, pH 4.65, 0.1 M acetate, 0.4 M NaClO4. A 400 nm cutoff filter was used to mimic the conditions shown in Figure 2.

buffer, 0.4 M NaClO4) at a constant applied potential of 0.2 V versus Ag/AgCl. On the bais of the available TA data (Figure 4), -RuP2+* undergoes rapid, subnanosecond injection on the outer shell of the TiO2 electrode to give -RuP3+, as shown by the loss of the MLCT absorption bleach at 450 nm.20 Following excitation and injection, the oxidized state persists on the surface for microseconds to milliseconds, eq 5.21,22 On the basis of the results in Figures 4 and 5, the Al2O3 inner layer plays a significant role in dictating core/shell dynamics after the laser flash. Figure 4 shows time-dependent transient absorption spectra for both 1.5 and 3.0 nm core−shell electrodes at thicknesses of the inner Al2O3 layer of either 0 or 0.99 nm. On the basis of the marked decrease in amplitude for the scans with added Al2O3 layer, electron transfer though the core−shell to the core, eq 6, is greatly decreased relative to back electron transfer to -RuP3+, eq 7. A similar conclusion was reached based on the timedependent, absorption-time data in Figure 5 for a series of 1.5 nm core−shell electrodes with the Al2O3 layer varied from 0 to 0.99 nm. These data show that an integrated absorption increase occurs for a 0.11 nm Al2O3 layer with the added layer slowing back electron transfer to the surface presumably due to trapping in the ATO substrate. In Figure S8a, the prenormalized decay traces show that the magnitude of the C

DOI: 10.1021/acsami.7b00225 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Transient absorption difference spectra for (a) Sb:SnO2|0−0.99Al2O3|1.5 nm TiO2|RuP2+ and (b) Sb:SnO2|0−0.99 nm Al2O3|3.0 nm TiO2|RuP2+ measured in pH 4.6 (0.1 M acetate buffer, 0.4 M NaClO4) aqueous solutions following 488 nm excitation with an applied bias of 0.2 V versus Ag/AgCl.

ATO|0−0.99 nm Al 2O3|TiO2 |RuP2 + + hv → ATO|0−0.99 nm Al 2O3|TiO2 |RuP2 +*

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ATO|0−0.99 nm Al 2O3|TiO2 |RuP3 + → ATO|0−1.0 nm Al 2O3|(e−)TiO2 |RuP3 +

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Sb:SnO2 |0−0.99 nm Al 2O3|TiO2 (e−)|−RuP3 + → Sb:SnO2 (e−)|0−0.99 nm Al 2O3|TiO2 | − RuP3 +

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Sb:SnO2 /0−0.99 nm Al 2O3 /TiO2 (e−)|−RuP3 + → Sb:SnO2 |0−0.99 nm Al 2O3|TiO2 | − RuP2 +

Figure 5. Normalized single wavelength ΔA−time traces recorded at 450 nm as a function of Al2O3 inner-layer thickness for the electrode, Sb:SnO2/0.55 nm Al2O3/1.5 nm TiO2|-RuP2+, with additional data shown in S9. The TiO2−Al2O3 cycle numbers cite the deposition rates of 0.06 nm/cycle for TiO2 and 0.11 nm/cycle for Al2O3.

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Sb:SnO2 (e−)/0−0.99 nm Al 2O3 /TiO2 |−RuP3 + → Sb:SnO2 |0−0.99 nm Al 2O3|TiO2 | − RuP2 +

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The origin of the bilayer effect was also explored by linear scan voltammetry (LSV) measurements on the films -RuP2+ with 1,4-hydroquinone (H2Q) added as an external quencher (8.1 mM). Under these conditions, excitation and injection by -RuP2+, is followed by rapid reduction of -RuP3+ by added hydroquinone, eq 9, with E1/2 = 0.25 V vs for the Q/H2Q, couple at pH 4.5.

absorption bleach beyond 0.1 ms is significantly larger for 0.11 and 0.33 nm than for 0 nm of the Al2O3 layer. Figure S8b shows a more detailed plot of the bleach magnitude as a function of Al2O3 thickness for 0.1 and 1 ms delay times. It is clear that the addition of Al2 O3 layers increases the concentration of oxidized -RuP3+ on the surface at longer lifetimes relative to 0 nm Al2O3. The initial decrease in the absorption bleach for thicker Al2O3 layers is believed to arise from rapid back electron transfer from the TiO2 outer-layer due to slow electron transfer across the Al2O3 layer and into the Sb:SnO2 core. The same trend is observed for thicker layers but with a decrease in overall amplitude arising from competitive back electron transfer to the surface. As in related examples with similar substrates, attempts to fit the lifetime data to standard kinetic models were unsuccessful. Lifetimes were evaluated for half of the transient events that occurred with an average lifetime of τ1/2.23The data, as a function of thickness of the Al2O3 layer, are shown in Figure S10 with the kinetic parameters listed in Table S1. From the data, τ1/2 values vary with the inner-layer from τ1/2 = 120 ns at 1.5 nm to 2.8 μs at 3.0 nm. Both thicknesses are less than the maximum core/shell thickness of >4 nm required to maximize the core−shell effect and both are a measure of the time scale for back electron transport through the shell to -RuP3+.

Sb:SnO2 (e−)|0−0.99 nm Al 2O3|TiO2 | − RuP3 + + 1/2H 2Q → Sb:SnO2 (e−)|0−0.99 nm Al 2O3|TiO2 | − RuP2 + + 1/2Q + H+

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In a photoelectrochemical cell with an external Pt cathode under the same conditions, LSV measurements were used to monitor the extent of electron transfer through an external circuit to the cathode, eq 10, compared to back electron transfer to -RuP3+ followed by H2Q reduction. H+Pt || Sb:SnO2 (e−)|0−0.99 nm Al 2O3|TiO2 | − RuP3 + → 1/2H 2 + Pt || Sb:SnO2 |0−0.99 nm Al 2O3|TiO2 | − RuP3 +

(10)

The LSV results are shown in Figure 6 as the thickness of the Al2O3 layer was increased from 0 to 0.99 nm. On the basis of the data, the electrochemical response for the -RuP3+/2+ wave was steadily decreased with as the thickness of the Al2O3 layer D

DOI: 10.1021/acsami.7b00225 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was solely supported by the UNC EFRC Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0001011.

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Figure 6. Linear scan voltammograms (LSV) vs Ag/AgCl for Sb:SnO2| 0−0.99 nmAl2O3|TiO2|RuP2+ electrodes with 8.1 mM added H2Q in pH 4.5, 0.4 M acetate buffer, 0.3 M in NaClO4 at 2 cm2. On ATO films with 0 (black), 0.33 (red), 0.66 (blue), and 0.99 (green) nm layers of Al2O3; v = 10 mV/s. The voltammogram in the absence of H2Q is shown as the bottom CV.

was increased. By 0.99 nm, the current for reduction of -RuP3+ approached the background current for the ATO film at the scan rate of 10 mV/s. By way of comparison, in Figure S10, the current levels at 0.9 V, compared to the electrode with no added Al2O3, were ∼1, ∼0.11, and ∼0.02% for 0.33, 0.66, and 0.99 nm loadings of Al2O3. The dependence of the data on film thickness reinforces the importance of the inner Al2O3 layer in dictating electron transfer dynamics in the modified core/shell structures. We presume that a significant contributor to the Al2O3 distance effect arises from inhomogeneities in the interfacial surface and expected distance dependence of electron transfer over the range of distances used in the oxide layer. The appearance of pathways for electron transfer within the inner Al2O3 layer, Figure 2, provide a basis for controlling electron flow into the TiO2 outer layer, from either -RuP2+ or -Rua2+-Rub2+-OH2, by the inner Al2O3 layer. The photocurrent efficiencies also point to long-lived electrons in the core−shell that can dominate photocurrent contributions for water oxidation in molecular assembly devices for water oxidation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00225. Detailed ALD recipe, TEM pictures of the core/shell structures, photocurrent measurement, transient absorption test and electrochemical analysis (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Benjamin D. Sherman: 0000-0001-9571-5065 Thomas J. Meyer: 0000-0002-7006-2608 Author Contributions

The manuscript was written through contributions of all of the authors E

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DOI: 10.1021/acsami.7b00225 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX