Photoelectrochemical Hydrogen Evolution Driven by Visible-to

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Photoelectrochemical Hydrogen Evolution Driven by Visible-to-UV Photon Upconversion Mariam Barawi, Fernando Fresno, Raul Perez-Ruiz, and Víctor A. de la Peña O'Shea ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01916 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 24, 2018

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ACS Applied Energy Materials

Photoelectrochemical Hydrogen Evolution Driven by Visible-to-UV Photon Upconversion Mariam Barawi, Fernando Fresno,* Raúl Pérez-Ruiz* and Víctor A. de la Peña O’Shea* Photoactivated Processes Unit, IMDEA Energy Institute, Av. Ramón de la Sagra 3, 28935, Móstoles, Madrid, Spain

*[email protected];*raul.perez [email protected];*[email protected]

Keywords: photoelectrochemistry, photon upconversion, triplet-triplet annihilation, TiO2, hydrogen, solar fuels Abstract Activation of ultraviolet (UV) energy-bandgap semiconductors for solar fuel production using visible light as energy source is one of the most challenging tasks in the artificial photosynthesis field. Triplet-triplet annihilation (TTA) based on photon upconversion (UC) generates frequently high energy (i.e. UV) from lower energy (visible). Thus, an efficient and appropriate TTA-UC system can successfully use visible light to power a photoelectrochemical (PEC) cell using TiO2, leading to photovoltages, photocurrents and photoelectrocatalytic hydrogen production. Here, for the first time, visible-to-UV TTAUC is demonstrated to be a useful strategy for performing artificial photosynthesis processes by means of UV energy-bandgap semiconductors. Graphical Abstract e−

TiO2

TTA-UC System UV

Pt

Ph O PPO = acceptor (A) Emitter O Ph

H2 H2O SO3−2 SO4−2

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Solar energy is by far the largest exploitable renewable source; however, its dilute nature and its inherent discontinuity makes necessary not only to convert it but also to store it into manageable and transportable forms of energy. Among various forms of solar energy utilization, photoelectrocatalysis, which involves a direct solar-to-chemical energy conversion by light-matter interaction, exhibits relatively high theoretical efficiencies.1-5 In this context, powerful photo(electro)catalytic materials would be those with a broad range of solar spectrum absorption, high photo(electro)chemical stability, efficient use of photogenerated electrons and holes, suitable band edge positions, low overpotential for the pursued reactions, non-toxicity and low cost. Titanium dioxide (TiO2) fulfills all the aforementioned criteria6,7 with the exception of absorbing only in the ultraviolet (UV) region (4-5% of the solar spectrum). As a matter of fact, a plethora of materials have been studied, most of them based on metal oxides or chalcogenides,8 to expand the action spectrum.9,10 Some photocatalytic materials are capable per se of absorbing visible light but suffer from severe photo-instability issues by comparing with TiO2 or other d0 metal oxides with UV energy-bandgap.11 Although different strategies, such as metal and nonmetal doping, dye-sensitization, surface plasmon resonant metal nanoparticle loading or semiconductor coupling,12 have been employed to expand the action spectrum of UV energy-bandgap semiconductors, alternative methodologies for addressing this problem would be desired. One quite novel possibility could be the use of photon upconversion (UC),13,14 that converts low-energy into higher-energy radiation either by two-photon absorption (TPA) or by triplet-triplet annihilation (TTA) processes.15 In particular, the TTA-UC mechanism16 operates with lower incident light power than TPA, implying an energy transfer between a sensitizer (donor) and an annihilator (acceptor) and ultimately leading to anti-Stokes shifted fluorescence (see Scheme S1 in the Supporting Information). Last decade has witnessed the renaissance of the TTA-UC


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phenomenon as one of the most attractive wavelength conversion technologies with real applications, for example in displays,17 bioimaging,18 phototherapy,19 photoredox catalysis20-22 or integrated TTA-UC solar cells.23 Only a few investigations on TTA-UC-driven PEC cells for solar fuels production using visible light active photocatalysts such as WO3 (Eg = 2.7 eV)24-28 or Zn0.3Cd0.7S (Eg = 2.36 eV)29 have been published to date. However, the development of vis-to-UV TTA-UC to photoactivate UV energy-bandgap semiconductor appears more challenging. Here, we present a proof of concept of a TTA-UC-powered PEC device using TiO2 as photoanode for H2 production (Figure 1A and Figure 1B). Photoelectrochemical measurements have been performed into a standard 3-electrode PEC cell where TTA-UC system is integrated, using TiO2 as semiconductor photoanode (Eg = 3.2 eV) and a laser pointer (λexc = 445 ± 10 nm, 2W, 1.64 mW cm-2) as irradiation source (see more details in the Supporting Information). Based on our previous studies,21,22 we have selected 2,3-butanedione (biacetyl, BA) and 2,5-diphenyloxazole (PPO) as appropriate TTA-UC pair system. Thus, after visible light absorption of BA (λexc = 430-450 nm), observation of a band centered in the UV at 370 nm is safely attributed to the PPO delayed emission (1PPO*) with an excited singlet energy (ES) of 3.6 eV in DMF (Figure 1C). This 1PPO* is then randomly collected by a TiO2 thin film supported on an indium tin oxide (ITO) electrode (Figure 1D). From a strategical point of view, these data together with a remarkable anti-Stokes emission shifted by 0.82 eV with respect to the excitation make BA/PPO system the ideal choice for activating UV energy-bandgap semiconductors such as TiO2, which its bandgap overlaps with the PPO upconverted emission (Figure 1C).



e−

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TiO2 UV Pt

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H2 H 2O SO3−2

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Light Source Pt (cathode)

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SO4−2

Normalized Absorbance/Emission / au

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O BA = donor (D) Sensitizer

E(S1) = 3.60 eV

PPO abs

Anti-Stokes 2.78 eV 0.82 eV

PPO em

BA abs

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Vis

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SO3-2 aqueous solution

TiO2 electrode

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 / nm

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Figure 1. A: Schematic illustration of the TTA-UC powered PEC cell: selective visible light irradiation to the TTA system releases a high energetic emission (UV) that activates TiO2 generating photocurrent. B: Photograph of the TTA-UC powered PEC cell. C: Normalized absorbance and emission spectra of BA and PPO in dimethylformamide. The blue region shows the diode laser pointer emission recorded by radiometry. The violet arrow indicates the singlet energy of PPO whereas the black arrow shows the anti-Stokes shift of the couple BA/PPO as TTA-UC system. D: Atomic Force Microscopy (AFM) of TiO2 supported over ITO (thickness ~ 4 μm). Organic media play a critical role in the TTA-UC system and therefore in the subsequent light mediated processes. To evaluate this behavior we have tested the photocurrents upon voltages changes relying on the BA/PPO system in different organic media, where dimethylformamide has been clearly found to be the best medium with notably higher intensities in comparison with other solvents: acetonitrile > toluene > heptane (Figure 2).30 It is worth mentioning that these experiments depend exclusively on the stochastic collection of photons at the TiO2 photoanode, justifying the obtained photoresponses. To validate the protocol, the photocurrents at diverse potentials in the absence of any 4 ACS Paragon Plus Environment

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chromophore from the composition have been also plotted (control), showing that the TiO2 photocurrent is negligible in all cases due to the insignificant absorption of the laser emission by TiO2 (Figure 1C). Furthermore, photoresponse of the cell employing only the upconversion light harvester BA without the annihilator PPO presented none difference with respect to the blank sample (Figure S4 in the Supporting Information). 8

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Figure 2. Linear sweep voltammetry of TiO2 at 10 mV s-1 in 0.5 M Na2SO3 recorded upon irradiation of a close optical cuvette without (blue) and with a BA (0.04 M) + PPO (0.013 M) mixture (red) with a laser pointer (λexc = 445 nm ± 10, 2W, 1.64 mW cm-2) in dimethylformamide (A), acetonitrile (B), heptane (C) and toluene (D).

In order to explain these differences, laser flash photolysis (LFP) measurements of BA/PPO system in the μs regime have been achieved in all solvents. After selective BA excitation (λexc = 445 nm), the characteristic band centered at 370 nm of 1PPO* is observed (Figure 3). In principle, BA emission (1BA*) should be only detected with the incident laser pulse;21 however, results at 0.5 μs after the laser pulse reveal the 5 ACS Paragon Plus Environment


undoubtedly detection of 1BA* in both toluene and heptane and a broader shoulder at this region in acetonitrile, whereas no fluorescence of BA is detected in dimethylformamide. These observations could be attributed to that some of the upconverted singlet energy could be consumed by undesirable acceptor-to-donor singlet-singlet back energy transfer. Actually, this fact takes a great importance when the optical density of the donor is higher than that of the acceptor as in our optimal experimental conditions. Therefore, a good correlation could be established between the results from the photocurrents and LFP experiments in the different solvents. 300

PPO-to-BA 1

Delayed Emission / au

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PPO*

singlet-singlet back energy transfer

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BA*

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0 350

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 / nm

Figure 3. Delayed emission spectra (λexc = 445 nm) of a mixture of BA (0.04 M) and PPO (0.013 M) under anaerobic conditions in dimethylformamide (red), acetonitrile (black), toluene (magenta) and heptane (blue) recorded at 0.5 μs after the laser pulse.

A more exhaustive characterization of the PEC cell under optimal conditions has been performed. Hence, measurements of open circuit potential (OCP) photovoltages exhibit an enhancement of 250 mV (Figure 4A), indicating that the UV energy-bandgap output is certainly resulting from the sensitization afforded by the BA/PPO upconversion composition. Thus, a chronoamperometry of the TTA-UC-powered PEC device biased at 0.4 V vs. Ag/AgCl (the best photocurrent achieved) has been accomplished in order to investigate the stationary photocurrent and the stability of the system (Figure 4B). The 6 ACS Paragon Plus Environment

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TiO2 photoanode produces photocurrent density at ca. 4.5 μA cm-2 together with a suitable stability along the time. As far as we are aware, this system gives a considerably higher value than similar reports using other semiconductor photoanodes,24 with a 40fold increase in the current density with respect to the highest reported values. The utilization of a laser pointer as irradiation source makes this methodology outstandingly attractive since not only photocurrents can be controlled by the intensity of light source (see Figure S5 in the Supporting Information) but also all the optical power density contained in the excitation is exclusively absorbed by the BA (Figure 1B). Finally, the lack of response in the blank experiment demonstrates unambiguously the photoelectrocatalysis mechanism: under visible light irradiation, emitted UV photons by the upconversion system activates the TiO2 photoanode, inducing a charge separation in it. The holes in the valence band are captured by sulfite forming sulfate, while the electrons in the conduction band are transferred to the Pt counter-electrode, where they reduce water to produce H2, as demonstrated by the hydrogen evolution detected in the in-line analysis of the argon flow at the cell outlet in a continuous irradiation experiment (Figure 4C and 4D).



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Figure 4. Photoresponses of the TiO2 film and hydrogen evolution reaction. A: Photovoltages of TiO2 of both blank and active solution with the TTA-UC system. B: Photocurrents of TiO2 at 0.4V vs Ag/AgCl. C: Hydrogen flow obtained at 0.6 V vs Ag/AgCl bias under continuous illumination. D: Current density during the hydrogen evolution reaction. All the experiment were recorded in 0.5 M Na2SO3 electrolyte upon irradiation with a laser pointer (λexc = 445 nm ± 10, 2W, 1.64 mW cm-2) of a close optical cuvette containing oxygen-free dimethylformamide solution (blue) and oxygen-free dimethylformamide with a BA (0.04 M) + PPO (0.013 M) mixture (red).

In summary, activation of the UV energy-bandgap semiconductor TiO2 using visible-toUV photon upconversion in a PEC cell has been successfully demonstrated. The generation of UV radiation from visible light has been achieved by the combination of two simple organic chromophores following a triplet-triplet annihilation mechanism. Thus, the emitted UV photons are capable to efficiently activate a TiO2 photoanode (Eg = 3.2 eV) as elucidated by a complete characterization of the PEC device, occasioning photocurrents (ca. 4.5 μA cm-2) from the hydrogen generation in a sulfite aqueous 8 ACS Paragon Plus Environment

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solution. These promising results may pave the way for a scientific breakthrough in the research towards a sustainable alternative strategy to the current energy production system. Considering the great versatility of inorganic semiconductors absorbing UV light and other possible combinations of sensitizer/annihilator to afford visible-to-UV upconversion, there is plenty of room for further development of these type of TTA-UCpowered PEC devices for solar fuels production. Associated Content Supporting Information The Supporting Information is available free of charge on https://pubs.acs.org/ at DOI:xxxxxxxx. Material specifications; detailed explanation for the requirements for a suitable TTAUC system; description of the laser flash photolysis technique and the PEC cell set-up. Author Information Corresponding author *Email: [email protected] *Email: [email protected] *Email: [email protected] ORCID Mariam Barawi: 0000-0001-5719-9872 Fernando Fresno: 0000-0001-6622-6721 Raúl Pérez-Ruiz: 0000-0003-1136-3598 Víctor A. de la Peña O’Shea: 0000-0001-5762-4787 Notes The authors declare no competing financial interest. Acknowledgements



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Financial support from the Spanish Ministry of Science, Innovation and Universities (Project SOLPAC: ENE2017-89170-R, MCIU/AEI/FEDER, EU and Juan de la Cierva Program: FJCI-2016-30567) and from the Community of Madrid (Grant 2016-T1/AMB1275) is gratefully acknowledged. References [1] Wang, F.; Li, Q.; Xu, D. Recent Progress in Semiconductor-Based Nanocomposite Photocatalysts for Solar-to-Chemical Energy Conversion. Adv. Energy Mater. 2017, 7, 1700529/1−1700529/19. [2] Wang, Q.; Hisatomi, T.; Jia, Q.; Tokudome, H.; Zhong M.; Wang, C.; Pan, Z.; Takata, T.; Nakabayashi, M.; Shibata, N.; Li, Y.; Sharp, I. D.; Kudo, A.; Yamada, T.; Domen, K. Scalable Water Splitting on Particulate Photocatalyst Sheets with a Solar-to-Hydrogen Energy Conversion Efficiency Exceeding 1. Nat. Mater. 2016, 15, 611−615. [3] Sivula, K. Solar-to-Chemical Energy Conversion with Photoelectrochemical Tandem Cells. Chimia, 2013, 67, 155−161. [4] Dufour, J.; Serrano, D. P.; Gálvez, J. L.; González, A.; Soria, E.; Fierro, J. L. G. Life Cycle Assessment of Alternatives for Hydrogen Production from Renewable and Fossil Sources. Int. J. Hydrogen Energy, 2012, 37, 1173−1183. [5] European Commission, Directorate-General for Research & Innovation, Artificial Photosynthesis: Potential and Reality, Nov. 2016, doi:10.2777/410231. [6] Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature, 1972, 238, 37−38. [7] Inoue, T.; Fujisima, A.; Konishi, S.; Honda, K. Photocatalytic Reduction of Carbon Dioxide in Aqueous Suspensions of Semiconductor Powders. Nature, 1979, 277, 637−638.


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Photoelectrochemical Hydrogen Evolution Driven by Visible-to

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