Charge Carrier Dynamics in Solar Water Oxidation

The search for sustainable energy sources is one of the greatest problems facing mankind in the 21st century. Most renewable sources do not have adequate potential to cover the growing need for energy in order to sustain economic and population growth. Solar power is a plausible way to fully cover mankind’s continuously growing need for energy. However, sunlight is diurnal, and the amount of sunlight received at different latitudes of the Earth varies drastically. Harnessing solar energy into chemical bonds is an attractive approach to enable the storage of energy for transportation and later use. Direct photoelectrochemical water splitting produces only oxygen and hydrogen, of which hydrogen can be used to sustain a possible hydrogen based economy. The materials used in this Thesis are metal oxide semiconductors that act as photoanodes, performing the water oxidation reaction on their surface and supplying electrons for the water reduction reaction.Hematite is an n-type metal oxide semiconductor that has a band gap suitable for the absorption of a noticeable fraction of solar radiation. The absorption of light leads to the generation of electron-hole pairs that are separated due to a built-in electric field. However, the conduction band level of hematite is not suitable for unassisted water splitting and it suffers from poor intrinsic charge transport properties. For this reason the photoanodes studied in this Thesis have been modified with doping and by forming heterojunctions with other metal oxide semiconductors, namely titanium dioxide.This Thesis studies the evolution of the primary charge carriers responsible for water splitting in modified hematite photoanodes. The method selected to probe the charge carrier dynamics is transient absorption spectroscopy that enables the monitoring of charge carriers from the subpicosecond timescale up to seconds. The measurements were performed in a three electrode photoelectrochemical cell to see the effects of additional bias voltage on the charge carrier dynamics and how the recombination and oxygen evolution reaction are changed when a photocurrent is generated.The results of this Thesis indicate that the modification of hematite has a profound effect on the charge carrier behaviour. The observed effects range from changes in recombination on the picosecond timescale, to nanosecond timescale trapping of electrons into intraband or surface states, and all the way to changes in the reaction rates of long-lived holes in the hundreds of milliseconds timescale

UV-valon aikaansaama polypropeenin ja polystyreenin rappeutuminen – spektroskopinen ja DSC tutkimus

Ultraviolet (UV) radiation is an important factor affecting the life-time of polymer products. When polymers are exposed to UV radiation in the presence of oxygen they degrade due to photo-oxidative reactions. The changes in the mechanical properties and appearances of polymers are the result of chemical changes in the polymer structure. The observation of the early chemical changes in the photo-oxidative degradation of polypropylene (PP) and polystyrene (PS) was examined in this thesis. In this thesis Fourier transform infrared (FTIR) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy and differential scanning calorimetry (DSC) were used to study the changes that occurred in polypropylene and polystyrene when they were irradiated with a xenon arc lamp in air. The spectroscopic methods were used to observe the chemical changes in the polymer structure and the addition of new chemical groups and DSC was used to observe changes in the thermal properties and oxidative stability of the polymers. Differential photocalorimetry (DPC) was used as a novel method to observe changes in the oxidative stability of polymers with simultaneous irradiation. The DPC results were compared with the DSC results to examine how irradiation affects the oxidative stability of PP and PS. The largest change observed in the polymer samples during irradiation was the formation of a carbonyl group absorption band in the IR spectra located at ν ̃≈1720 cm^(-1). A substantial increase in the absorbance between λ=270–350 nm was observed in the UV-Vis spectra of PS, due mostly to the formation of conjugated double bonds. A small increase in the absorbance between λ=250–300 nm was observed in the UV-Vis spectra of PP, assigned to the formation of carbonyl groups. The melting temperature of PP decreased after irradiation, which was assigned to chain scission. The glass transition temperature of PS rose steadily for the first irradiation sequences, but decreased noticeably during the last one. The increase was assigned to crosslinking whereas the decrease was assigned to either chain scission or the photochain oxidation mechanism. The oxidation induction time (OIT) of PP decreased over 90 % during the first irradiation sequence. The oxidation occurred practically immediately for the irradiated samples, which indicates a large and fast decrease in thermal oxidative stability. The OIT test was not applicable for PS. The oxidation induction temperature (T_ox) decreased for PP and increased for PS due to photodegradation. This may be due to changes in the average molecular mass, with a decrease in mass indicating a decrease in the oxidation induction temperature and vice versa.Ultraviolettisäteily on tärkeä polymeerituotteiden elinikään vaikuttava tekijä. Kun polymeerejä altistetaan UV-säteilylle hapen läsnä ollessa, ne rappeutuvat valon aikaansaamien hapetusreaktioiden ansiosta. Muutokset polymeerien mekaanisissa ja ulkoisissa ominaisuuksissa aiheutuvat kemiallisista muutoksista polymeerirakenteessa. Tässä diplomityössä tutkittiin varhaisten kemiallisten muutosten havaitsemista polypropeenin (PP) ja polystyreenin (PS) rappeutumisessa valon vaikutuksesta. Tässä työssä käytettiin Fourier’n muunnos infrapunaspektroskopiaa (FTIR), ultravioletti-näkyvä valo (UV-Vis) -spektroskopiaa sekä differentiaalista pyyhkäisykalorimetriaa (DSC) polypropeenissa ja polystyreenissä tapahtuvien muutosten havaitsemiseen, kun niitä säteilytettiin ksenon-kaarilampulla ilman läsnäollessa. Spektroskopisia menetelmiä käytettiin polymeerirakenteessa tapahtuvien muutosten ja uusien funktionaalisten ryhmien muodostumisen havaitsemiseen, ja DSC:tä käytettiin havaitsemaan muutoksia polymeerien termisissä ominaisuuksissa ja hapettumisen kestossa. Differentiaalista fotokalorimetriaa (DPC) käytettiin uutena menetelmänä muutosten havaitsemiseen hapettumisen kestossa samanaikaisen säteilytyksen kanssa. DPC:llä mitattua hapettumisen kestoa verrattiin DSC:llä mitattuun säteilytyksen vaikutuksen määrittämiseksi PP:n ja PS:n hapettumisen kestossa. Suurin muutos säteilytetyissä polymeerinäytteissä oli IR-spektreihin aaltoluvulle ν ̃≈1720 cm^(-1) muodostunut karbonyyliryhmän absorptiovyö. Polystyreenin UV-Vis-spektreissä havaittiin huomattava absorbanssin kasvu välillä λ=270–350 nm, mikä johtui konjugoituneiden kaksoisidosten muodostumisesta. Polypropeenin UV-Vis spektreissä havaittiin pieni absorbanssin kasvu välillä λ=250–300 nm, mikä johtui karbonyyliryhmien muodostumisesta. Polypropeenin sulamislämpötila laski säteilytyksen jälkeen, minkä aiheutti polymeeriketjujen katkeaminen. Polystyreenin lasittumislämpötila kasvoi tasaisesti ensimmäiset säteilytysjaksot, mutta laski huomattavasti viimeisen aikana. Kasvu johtui polymeeriketjujen silloittumisesta, kun taas lasku oli seurausta joko ketjujen katkeamisesta tai valoketjuhapettumismekanismista. Polypropeenin hapettumisen alkamisaika (OIT) laski yli 90 % ensimmäisen säteilytysjakson aikana. Säteilytettyjen näytteiden hapettuminen tapahtui käytännössä välittömästi, mikä viittaa huomattavaan ja nopeaan laskuun termisen hapettumisen kestossa. OIT-mittaus ei soveltunut PS:lle. Hapettumisen alkamislämpötila (T_ox) laski PP:lle ja kasvoi PS:lle valon aikaansaaman hajoamisen seurauksena. Tämä voi olla seurausta keskimääräisen molekyylimassan muutoksista, jolloin massan lasku vastaa laskua hapettumisen alkamislämpötilassa ja päinvastoin

Hydrogen-Bonded Liquid Crystal Elastomers Combining Shape Memory Programming and Reversible Actuation

Materials that undergo shape morphing in response to external stimuli have numerous applications, e.g., in soft robotics and biomedical devices. Shape memory polymers utilize kinetically trapped states to, typically irreversibly, transfer between a programmed morphed shape and an equilibrium shape. Liquid crystal elastomers (LCEs), in turn, can undergo reversible actuation in response to several stimuli. This study combines the irreversible and reversible shape morphing processes to obtain LCEs that undergo shape-programming via the shape memory effect and subsequent reversible actuation of the programmed shape. This is enabled by an LCE crosslinked via dynamic hydrogen bonds that break at high temperatures and reform upon cooling, endowing the shape memory effect, while mild thermal or photothermal stimulation yields the reversible actuation. Through this combination, proof-of-concept robotic application scenarios such as grippers that can adjust their shape for grabbing different-sized objects and crawling robots that can morph their shape to adapt to constrained spaces, are demonstrated. It is anticipated that this work adds new diversity to shape-programmable soft microrobotics.Peer reviewe

Charge carrier dynamics in tantalum oxide overlayered and tantalum doped hematite photoanodes

We employ atomic layer deposition to prepare 50 nm thick hematite photoanodes followed by passivating them with a 0.5 nm thick Ta2O5-overlayer and compare them with samples uniformly doped with the same amount of tantalum. We observe a three-fold improvement in photocurrent with the same onset voltage using Ta-overlayer hematite photoanodes, while electrochemical impedance spectroscopy under visible light irradiation shows a decreased amount of surface states under water splitting conditions. The Tadoped samples have an even higher increase in photocurrent along with a 0.15 V cathodic shift in the onset voltage and decreased resistivity. However, the surface state capacitance for the Ta-doped sample is twice that of the reference photoanode, which implies a larger amount of surface hole accumulation. We further utilize transient absorption spectroscopy in the sub-millisecond to second timescale under operating conditions to show that electron trapping in both Ta2O5-passivated and Ta-doped samples is markedly reduced. Ultrafast transient absorption spectroscopy in the sub-picosecond to nanosecond timescale shows faster charge carrier dynamics and reduced recombination in the Ta-doped hematite photoanode resulting in the increased photoelectrochemical performance when compared with the Ta2O5-overlayer sample. Our results show that passivation does not affect the poor charge carrier dynamics intrinsic to hematite based photoanodes. The Ta-doping strategy results in more efficient electron extraction, solving the electron trapping issue and leading to increased performance over the surface passivation strategy.Peer reviewe

New tricks and emerging applications from contemporary azobenzene research

Azobenzenes have many faces. They are well-known as dyes, but most of all, azobenzenes are versatile photoswitchable molecules with powerful photochemical properties. Azobenzene photochemistry has been extensively studied for decades, but only relatively recently research has taken a steer towards applications, ranging from photonics and robotics to photobiology. In this perspective, after an overview of the recent trends in the molecular design of azobenzenes, we highlight three research areas where the azobenzene photoswitches may bring about promising technological innovations: chemical sensing, organic transistors, and cell signaling. Ingenious molecular designs have enabled versatile control of azobenzene photochemical properties, which has in turn facilitated the development of chemical sensors and photoswitchable organic transistors. Finally, the power of azobenzenes in biology is exemplified by vision restoration and photactivation of neural signaling. Although the selected examples reveal only some of the faces of azobenzenes, we expect the fields presented to develop rapidly in the near future, and that azobenzenes will play a central role in this development.publishedVersionPeer reviewe

Low-power/high-gain flexible complementary circuits based on printed organic electrochemical transistors

The ability to accurately extract low-amplitude voltage signals is crucial in several fields, ranging from single-use diagnostics and medical technology to robotics and the Internet of Things. The organic electrochemical transistor, which features large transconductance values at low operation voltages, is ideal for monitoring small signals. Its large transconductance translates small gate voltage variations into significant changes in the drain current. However, a current-to-voltage conversion is further needed to allow proper data acquisition and signal processing. Low power consumption, high amplification, and manufacturability on flexible and low-cost carriers are also crucial and highly anticipated for targeted applications. Here, we report low-power and high-gain flexible circuits based on printed complementary organic electrochemical transistors (OECTs). We leverage the low threshold voltage of both p-type and n-type enhancement-mode OECTs to develop complementary voltage amplifiers that can sense voltages as low as 100 $\mu$V, with gains of 30.4 dB and at a power consumption < 2.7 $\mu$W (single-stage amplifier). At the optimal operating conditions, the voltage gain normalized to power consumption reaches 169 dB/$\mu$W, which is > 50 times larger than state-of-the-art OECT-based amplifiers. In a two-stage configuration, the complementary voltage amplifiers reach a DC voltage gain of 193 V/V, which is the highest among emerging CMOS-like technologies operating at supply voltages below 1 volt. Our findings demonstrate that flexible complementary circuits based on printed OECTs define a power-efficient platform for sensing and amplifying low-amplitude voltage signals in several emerging beyond-silicon applications

Low-Power/High-Gain Flexible Complementary Circuits Based on Printed Organic Electrochemical Transistors

The ability to accurately extract low-amplitude voltage signals is crucial in several fields, ranging from single-use diagnostics and medical technology to robotics and the Internet of Things (IoT). The organic electrochemical transistor (OECT), which features large transconductance values at low operating voltages, is ideal for monitoring small signals. Here, low-power and high-gain flexible circuits based on printed complementary OECTs are reported. This work leverages the low threshold voltage of both p-type and n-type enhancement-mode OECTs to develop complementary voltage amplifiers that can sense voltages as low as 100 \ub5V, with gains of 30.4\ua0dB and at a power consumption of 0.1–2.7 \ub5W (single-stage amplifier). At the optimal operating conditions, the voltage gain normalized to power consumption reaches 169\ua0dB \ub5W−1, which is &gt;50\ua0times larger than state-of-the-art OECT-based amplifiers. In a monolithically integrated two-stage configuration, these complementary voltage amplifiers reach voltage gains of 193\ua0V/V, which are among the highest for emerging complementary metal-oxide-semiconductor-like technologies operating at supply voltages below 1 V. These flexible complementary circuits based on printed OECTs define a new power-efficient platform for sensing and amplifying low-amplitude voltage signals in several emerging beyond-silicon applications

Synergistic Effect of Multi-Walled Carbon Nanotubes and Ladder-Type Conjugated Polymers on the Performance of N-Type Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) have the potential to revolutionize the field of organic bioelectronics. To date, most of the reported OECTs include p-type (semi-)conducting polymers as the channel material, while n-type OECTs are yet at an early stage of development, with the best performing electron-transporting materials still suffering from low transconductance, low electron mobility, and slow response time. Here, the high electrical conductivity of multi-walled carbon nanotubes (MWCNTs) and the large volumetric capacitance of the ladder-type {\pi}-conjugated redox polymer poly(benzimidazobenzophenanthroline) (BBL) are leveraged to develop n-type OECTs with record-high performance. It is demonstrated that the use of MWCNTs enhances the electron mobility by more than one order of magnitude, yielding fast transistor transient response (down to 15 ms) and high uC* (electron mobility x volumetric capacitance) of about 1 F/cmVs. This enables the development of complementary inverters with a voltage gain of > 16 and a large worst-case noise margin at a supply voltage of < 0.6 V, while consuming less than 1 uW of power

Synergistic Effect of Multi-Walled Carbon Nanotubes and Ladder-Type Conjugated Polymers on the Performance of N-Type Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) have the potential to revolutionize the field of organic bioelectronics. To date, most of the reported OECTs include p-type (semi-)conducting polymers as the channel material, while n-type OECTs are yet at an early stage of development, with the best performing electron-transporting materials still suffering from low transconductance, low electron mobility, and slow response time. Here, the high electrical conductivity of multi-walled carbon nanotubes (MWCNTs) and the large volumetric capacitance of the ladder-type π-conjugated redox polymer poly(benzimidazobenzophenanthroline) (BBL) are leveraged to develop n-type OECTs with record-high performance. It is demonstrated that the use of MWCNTs enhances the electron mobility by more than one order of magnitude, yielding fast transistor transient response (down to 15\ua0ms) and high μC* (electron mobility 7 volumetric capacitance) of about 1 F cm−1\ua0V−1 s−1. This enables the development of complementary inverters with a voltage gain of &gt;16 and a large worst-case noise margin at a supply voltage of &lt;0.6\ua0V, while consuming less than 1 \ub5W of power

Design aspects of all atomic layer deposited TiO2–Fe2O3 scaffold-absorber photoanodes for water splitting

Iron and titanium oxides have attracted substantial attention in photoelectrochemical water splitting applications. However, both materials suffer from intrinsic limitations that constrain the final device performance. In order to overcome the limitations of the two materials alone, their combination has been proposed as a solution to the problems. Here we report on the fabrication of an atomic layer deposited (ALD) Fe2O3 coating on porous ALD-TiO2. Our results show that successful implementation requires complete mixing of the TiO2 and Fe2O3 layers via annealing resulting in the formation of a photoactive iron titanium oxide on the surface. Moreover, we found that incomplete mixing leads to crystallization of Fe2O3 to hematite that is detrimental to the photoelectrochemical performance. IPCE and transient photocurrent measurements performed using UV and visible light excitation confirmed that the iron titanium oxide extends the photocurrent generation to the visible range. These measurements were complemented by transient absorption spectroscopy (TAS), which revealed a new band absent in pristine hematite or anatase TiO2 that we assign to charge transfer within the structure. Taken together, these results provide design guidelines to be considered when aiming to combine TiO2 and Fe2O3 for photoelectrochemical applications.Peer reviewe