Even though D-A dyes are largely applied in dye-sensitized solar cells and photoelectrochemical cells, there have not been enough studies to focus on the potential impact of photoinduced twisting the performance. Thus, researchers tried improving donor-accepted dye to drive H2 evolution.
Aim of the Study – To highlight the potential of TICT-controlled dyes to develop photocathodes with a simple design which can eliminate the requirement for extra catalyst in H2 evolution.
Limiting Molecular Twisting: Improving Donor-Accepted Dye to Drive H2 Evolution
Often, dye 4-(bis-4-(5-(2,2-dicyano-vinyl)-thiophene-2-yl)-phenyl-amino)-benzoic acid (P1) is used to enhance NiO photocathodes for proton reduction when paired with a catalyst. The twisting of P1 dye on NiO is influenced by the co-absorption of octadecanoic acid (C14) or myristic acid (MA). Studies have shown that twisting lowers the energy of the excited dye.
The non-polar environment from MA reduces twisting. It also slows down charge recombination after charge separation between P1 and NiO. This further boosts photocurrent by increasing electrochemical potential.
Furthermore, MA co-absorption causes H2 evolution when lightly excited even without adding a catalyst. Researchers think that H2 formation results from Ni2+ dissolution and subsequent reduction along with deposition of Ni nanoparticles as these are active catalysis sites.
Then to stabilize the excited molecular complex, after photoexcitation, twisting between the acceptor and donor occurs. This research demonstrates the potential of designing TICT-controlled dyes and realize the efficiency of photocathodes.
Results and Discussion About Improving Donor-Accepted Dye to Drive H2 Evolution
These findings further emphasize the importance of photoinduced intramolecular twisting. It also suggests that there is a possibility to improve solar fuel devices by designing twisting-limited D-A dyes. There has been more attention towards the dye-sensitized photoelectrochemical cells (DSPECs) to convert solar energy into hydrogen.
Nanostructure Characterization and Steady-State Absorption Spectra
As per the following figure, we can notice the following:
- A leaf-like porous film structure which is ≈1.8 μm thick.
- With X-ray diffraction (XRD), researchers ensure that the film consists mostly of pure NiO.
- UV-visible absorbance spectra show weak visible absorption of NiO attributed to trap states. An increase below 380 nm is shown, which indicates a bandgap of 3.4 to 4.3 eV.
P1 is the Main Influencer
- It is clear that dye P1 is mainly responsible for visible light absorption. It shows bands around 300-420 nm and 400-700 nm related to π-π* transitions.
- The absorption broadens and there is a shift due to electronic interactions when P1 absorbs NiO.
- As MA is added to the P1 solution, the visible absorption reduces likely from competitive absorption. It decreases P1 surface coverage by around 48% and light absorption efficiency by 25%.
- It is evident from unchanged spectra, that there is no noticeable effect on the electronic properties of P1 or the Ni3+ concentration in NiO.
- Raman spectra also show no significant differences without or with MA. It suggests similar Nio-dye interactions in both cases.
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Reduced Twisting on the Photodynamics: Effects
In the following figure, we can notice these points.
- Time-Dependent Density Functional Theory (TDDFT) analyzes structural changes of the free P1 dye after excitation.
- The initial excitation of S0 to S1 was around 69.6% HOMO-LUMO transition character. After relaxation, the contribution increased to 85.7% and HOMO-1 to LUMO+1 contribution decreased to 1.8%.
- The oscillator strength for the twisted dye was less than non-twisted dye, by around 2.57 vs 1.89.
- With further optimization it becomes clear that twisting angle of the reduced dye remains largely intact. It increases by 33.6% from the ground state from 47.9° to 81.5°.
- In Gibson et al., the symmetry breaking in the excited and reduced state is mentioned in this study.
Using NTOs
- Natural transition orbitals (NTOs) are more localized on the P1 dye trails in comparison to HOMO and LUMO orbitals.
- On excitation, the P1 molecule breaks its pseudo-C2 symmetry around the C-C bond which connects the phenyl and carboxyl groups.
- During optimization, the carboxylic and malononitrile acid angle increased by 46.0° (from 47.9° to 93.9°). Thus, NTOs localize entirely on the non-twisted tails and stabilize the excited state by 0.36 eV. It further decreases the S0 to S1 transition energy by 0.61 eV.
- Furthermore, NTOs indicate the charge density transfer from the triphenylamine core.
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In both states: NiO/P1 and NiO/P1-MA structures, PL intensity is low because of rapid hole injection which further complicates comparisons. For better understanding MA’s effect, researchers used ZrO2 instead of NiO. A 0.09 eV blue-shift is shown in ZrO2/P1-MA in comparison to ZrO2/P1. This indicates that MA limits twisting.
- No effect is seen on the PL spectrum as P1 loading reduces and this rules out dye aggregation.
- Even with lower P1 loading, the PL intensity of ZrO2/P1-MA is 6 times higher than ZrO2/P1.
With additional experiments, scientists found that various other acids do not reduce twisting as effectively as MA. This is why the impact of MA on NiO/P1 interface is mostly due to the apolar environment. Solvents with higher viscosity show higher blueshift and intensity in PL spectra.
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TRPL
Using time-resolved photoluminescence (TRPL), it becomes easy to study the twisting process of ZrO2/P1-MA and ZrO2/P1. There is a red shift in the spectra of ZrO2/P1 compared to ZrO2/P1-MA. However, both depict a time-dependent red shift after excitation. PL decay increases with CA-absorbency MA and this suggests minimal photoinduced charge transfer from P1 to MA.
The TRPL data reveals 4 spectral bands:
- ZrO2/P1-MA – 606 nm
- Both – 630 nm
- Only ZrO2/P1 – 645 nm and 670 nm
As the PL shift depends on the amount of MA present, the presence of MA suppresses the P1* twisting. It is also evident through femtosecond transient absorption studies that twisting occurs within an MA of approximately:
- ZrO2/P1 – 17 ps
- ZrO2/P1-MA – 35 ps
This weakens the fluorescence from the LE/ICT state.
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TA Experiments
Researchers conducted TA experiments to find the impact of co-absorbed MA on the hole injection and recombination dynamics between P1 and NiO. The following data is shown:
- The TA spectra for NiO/P1-MA and NiO/P1 in a 0.1 m pH 3.8 citrate phosphate show buffering after 500 nm excitation. It shows that TA signals decay over time due to charge recombination.
- Reduced photoinduced hole injection is indicated at 250 fs.
- With larger gap at 560 nm for NiO/P1-MA, slower hole injection is shown which can be due to the hydrophobic long alkyl chain of MA.
- Initially, NiO/P1-MA decays faster than NiO/P1, probably due to P1* decay. Later, it shows a slow decay indicating slower charge recombination. It further suggests that electron transfer from P1•− to MA is unlikely.
Effect of Reduced Twisting on the Photoelectrochemical Performance
Recording linear sweep voltammograms in a pH 3.8 citrate phosphate helps researchers study how photoinduced twisting impacts photoelectrochemical performance under chopped illumination.
- To eliminate dissolved O2 and CO2, researchers degassed the electrolyte for more than 20 minutes.
- The mass signal m/z = 2 for H2 increases with illumination and decreases when lights are off.
- The m/z = 32 signal for O2 also shows a decrease and this suggests that O2 generated at the anode reduces at the cathode to form water.
- No Co or CO2 detection indicates a minimal contribution from MA or dye decomposition to photocurrents and H2 production.
Conclusion
To further explore the factors affecting H2 generation on NiO/P1-MA, researchers conducted more experiments. They concluded that the unique behaviour of NiO/P1-MA is possibly due to the inhibited twisting of the P1 D-A dye. This further enhances its ability to reduce Ni2+ to Ni nanoparticles for effective catalyzation and H2 generation. Additionally, Ni is considered an efficient catalyst for hydrogen evolution and it can also accept electrons from the excited dye. This mechanism is further supported by the increased Ni:O ratios post-experiment.
Source: Limiting Molecular Twisting: Upgrading a Donor–Acceptor Dye to Drive H2 Evolution
