On the structural evolution of nanoporous optically transparent CuO photocathodes upon calcination for photoelectrochemical applications.

Copper oxides are promising photocathode materials for solar hydrogen production due to their narrow optical band gap energy allowing broad visible light absorption. However, they suffer from severe photocorrosion upon illumination, mainly due to copper reduction. Nanostructuring has been proven to enhance the photoresponse of CuO photocathodes; however, there is a lack of precise structural control on the nanoscale upon sol-gel synthesis and calcination for achieving optically transparent CuO thin film photoabsorbers. In this study, nanoporous and nanocrystalline CuO networks were prepared by a soft-templating and dip-coating method utilizing poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127) as a structure-directing agent, resulting for the first-time in uniformly structured, crack-free, and optically transparent CuO thin films. The photoelectrochemical properties of the nanoporous CuO frameworks were investigated as a function of the calcination temperature and film thickness, revealing important information about the photocurrent, photostability, and photovoltage. Based on surface photovoltage spectroscopy (SPV), the films are p-type and generate up to 60 mV photovoltage at 2.0 eV (0.050 mW cm-2) irradiation for the film annealed at 750 °C. For these high annealing temperatures, the nanocrystalline domains in the thin film structure are more developed, resulting in improved electronic quality. In aqueous electrolytes with or without methyl viologen (as a fast electron acceptor), CuO films show cathodic photocurrents of up to -2.4 mA cm-2 at 0.32 V vs. RHE (air mass (AM) 1.5). However, the photocurrents were found to be entirely due to photocorrosion of the films and decay to near zero over the course of 20 min under AM 1.5 illumination. These fundamental results on the structural and morphological development upon calcination provide a direction and show the necessity for further (surface) treatment of sol-gel derived CuO photocathodes for photoelectrochemical applications. The study demonstrates how to control the size of nanopores starting from mesopore formation at 400 °C to the evolution of macroporous frameworks at 750 °C.

Thermodynamically Stable Functionalization of Microporous Aromatic Frameworks with Sulfonic Acid Groups by Inserting Methylene Spacers.

Porous aromatic frameworks (PAFs) are an auspicious class of materials that allow for the introduction of sulfonic acid groups at the aromatic core units by post-synthetic modification. This makes PAFs promising for proton-exchange materials. However, the limited thermal stability of sulfonic acid groups attached to aromatic cores prevents high-temperature applications. Here, we present a framework based on PAF-303 where the acid groups were added as methylene sulfonic acid side chains in a two-step post-synthetic route (SMPAF-303) via the intermediate chloromethylene PAF (ClMPAF-303). Elemental analysis, NMR spectroscopy, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy were used to characterize both frameworks and corroborate the successful attachment of the side chains. The resulting framework SMPAF-303 features high thermal stability and an ion-exchange capacity of about 1.7 mequiv g-1. The proton conductivity depends strongly on the adsorbed water level. It reaches from about 10-7 S cm-1 for 33% RH to about 10-1 S cm-1 for 100% RH. We attribute the strong change to a locally alternating polarity of the inner surfaces. The latter introduces bottleneck effects for the water molecule and oxonium ion diffusion at lower relative humidities, due to electrolyte clustering. When the pores are completely filled with water, these bottlenecks vanish, leading to an unhindered electrolyte diffusion through the framework, explaining the conductivity rise.

Ultrafast Proton Conduction in an Aqueous Electrolyte Confined in Adamantane-like Micropores of a Sulfonated, Aromatic Framework.

Sulfonated, cross-linked porous polymers are promising frameworks for aqueous high-performance electrolyte-host systems for electrochemical energy storage and conversion. The systems offer high proton conductivities, excellent chemical and mechanical stabilities, and straightforward water management. However, little is known about mass transport mechanisms in such nanostructured hosts. We report on the synthesis and postsynthetic sulfonation of an aromatic framework (SPAF-2) with a 3D-interconnected nanoporosity and varying sulfonation degrees. Water adsorption produces the system SPAF-2H20. It features proton exchange capacities up to 6 mequiv g-1 and exceptional proton conductivities of about 1 S cm-1. Two contributions are essential for the highly efficient transport. First, the nanometer-sized pores link the charge transport to the diffusion of adsorbed water molecules, which is almost as fast as bulk water. Second, continuous exchange between interface-bound and mobile species enhances the conductivities at elevated temperatures. SPAF-2H20 showcases how to tailor nanostructured electrolyte-host systems with liquid-like conductivities.

Multi-scale morphology characterization of hierarchically porous silver foam electrodes for electrochemical CO2 reduction.

Ag catalysts show high selectivities in the conversion of carbon dioxide to carbon monoxide during the electrochemical carbon dioxide reduction reaction (CO2RR). Indeed, highly catalytically active porous electrodes with increased surface area achieve faradaic conversion efficiencies close to 100%. To establish reliable structure-property relationships, the results of qualitative structural analysis need to be complemented by a more quantitative approach to assess the overall picture. In this paper, we present a combination of suitable methods to characterize foam electrodes, which were synthesised by the Dynamic Hydrogen Bubble Templation (DHBT) approach to be used for the CO2RR. Physicochemical and microscopic techniques in conjunction with electrochemical analyses provide insight into the structure of the carefully tailored electrodes. By elucidating the morphology, we were able to link the electrochemical deposition at higher current densities to a more homogenous and dense structure and hence, achieve a better performance in the conversion of CO2 to valuable products.

Self-Assembled Fluorescent Block Copolymer Micelles with Responsive Emission.

Responsive fluorescent materials offer a high potential for sensing and (bio-)imaging applications. To investigate new concepts for such materials and to broaden their applicability, the previously reported non-fluorescent zinc(II) complex [Zn(L)] that shows coordination-induced turn-on emission was encapsulated into a family of non-fluorescent polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer micelles leading to brightly emissive materials. Coordination-induced turn-on emission upon incorporation and ligation of the [Zn(L)] in the P4VP core outperform parent [Zn(L)] in pyridine solution with respect to lifetimes, quantum yields, and temperature resistance. The quantum yield can be easily tuned by tailoring the selectivity of the employed solvent or solvent mixture and, thus, the tendency of the PS-b-P4VP diblock copolymers to self-assemble into micelles. A medium-dependent off-on sensor upon micelle formation could be established by suppression of non-micelle-borne emission background pertinent to chloroform through controlled acidification indicating an additional pH-dependent process.

Magnetic NiFe2 O4 Nanoparticles Prepared via Non-Aqueous Microwave-Assisted Synthesis for Application in Electrocatalytic Water Oxidation.

Phase-pure spinel-type magnetic nickel ferrite (NiFe2 O4 ) nanocrystals in the size range of 4 to 11 nm were successfully synthesized by a fast and energy-saving microwave-assisted approach. Size and accessible surface areas can be tuned precisely by the reaction parameters. Our results highlight the correlation between size, degree of inversion, and magnetic characteristics of NiFe2 O4 nanoparticles, which enables fine-tuning of these parameters for a particular application without changing the elemental composition. Moreover, the application potential of the synthesized powders for the electrocatalytic oxygen evolution reaction in alkaline media was demonstrated, showing that a low degree of inversion is beneficial for the overall performance. The most active sample reaches an overpotential of 380 mV for water oxidation at 10 mA cm-2 and 38.8 mA cm-2 at 1.7 V vs. RHE, combined with a low Tafel slope of 63 mV dec-1 .

A highly porous and conductive composite gate electrode for OTFT sensors.

Ionic/protonic to electronic transducers based on organic thin film transistors have shown great promise for applications in bioelectronic interface devices and biosensors, and development of materials that exhibit mixed ionic/electronic conduction are an essential part of these devices. In this work, we investigated the proton sensing properties of an all solid-state and low voltage operating organic thin film transistor (OTFT) that uses the organic mixed conductor poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) as the gate electrode. To address the limited sensitivity due to the lack of porosity in PEDOT:PSS base sensors, we proposed a composite gate electrode material composed of PEDOT:PSS and proton conducting mesoporous SO3H-Si-MCM-41 nanoparticles for improved proton sensitivity. The composite gate electrode doubles the proton sensitivity of the OTFT, indicating a clear advantage of adding SO3H-Si-MCM-41 in the PEDOT:PSS gate. Moreover, the OTFTs with the composite gate electrode maintained OTFT characteristics similar to that of the PEDOT:PSS gated OTFT. A detailed and systematic study of the effect of variation in the composition of PEDOT:PSS:SO3H-Si-MCM-41 on OTFT characteristics and sensing properties is carried out. Our results open up the possibility of combining inorganic nanomaterials with organic conductors in the development of highly efficient bioelectronic sensing platforms.

A crystalline and 3D periodically ordered mesoporous quaternary semiconductor for photocatalytic hydrogen generation.

We have prepared the first crystalline and 3D periodically ordered mesoporous quaternary semiconductor photocatalyst in an evaporation-induced self-assembly assisted soft-templating process. Using lab synthesized triblock-terpolymer poly(isoprene-b-styrene-b-ethylene oxide) (ISO) a highly ordered 3D interconnected alternating gyroid morphology was achieved exhibiting near and long-range order, as evidenced by small angle X-ray scattering (SAXS) and electron microscopy (TEM/SEM). Moreover, we reveal the formation process on the phase-pure construction of the material's pore-walls with its high crystallinity, which proceeds along a highly stable W5+ compound, by both in situ and ex situ analyses, including X-ray powder diffraction (XRPD), Fourier transform infrared spectroscopy (FTIR) and electron paramagnetic resonance (EPR). The resulting photocatalyst CsTaWO6 with its optimum balance between surface area and ordered mesoporosity ultimately shows superior hydrogen evolution rates over its non-ordered reference in photocatalytic hydrogen production. This work will help to advance new self-assembly preparation pathways towards multi-element multifunctional compounds for different applications, including improved battery and sensor electrode materials.

Sensory suppression of brain responses to self-generated sounds is observed with and without the perception of agency.

Stimuli caused by our own movements are given special treatment in the brain. Self-generated sounds evoke a smaller brain response than externally generated ones. This attenuated response may reflect a predictive mechanism to differentiate the sensory consequences of one's own actions from other sensory input. It may also relate to the feeling of being the agent of the movement and its effects, but little is known about how sensory suppression of brain responses to self-generated sounds is related to judgments of agency. To address this question, we recorded event-related potentials in response to sounds initiated by button presses. In one condition, participants perceived agency over the production of the sounds, whereas in another condition, participants experience an illusory lack of agency caused by changes in the delay between actions and effects. We compared trials in which the timing of button press and sound was physically identical, but participants' agency judgment differed. Results show reduced amplitudes of the auditory N1 component in response to self-generated sounds irrespective of agency experience, whilst P2 effects correlate with the perception of agency. Our findings suggest that suppression of the auditory N1 component to self-generated sounds does not depend on adaptation to specific action-effect time delays, and does not determine agency judgments, however, the suppression of the P2 component might relate more directly to the experience of agency.

Motor intention determines sensory attenuation of brain responses to self-initiated sounds.

One of the functions of the brain is to predict sensory consequences of our own actions. In auditory processing, self-initiated sounds evoke a smaller brain response than passive sound exposure of the same sound sequence. Previous work suggests that this response attenuation reflects a predictive mechanism to differentiate the sensory consequences of one's own actions from other sensory input, which seems to form the basis for the sense of agency (recognizing oneself as the agent of the movement). This study addresses the question whether attenuation of brain responses to self-initiated sounds can be explained by brain activity involved in movement planning rather than movement execution. We recorded ERPs in response to sounds initiated by button presses. In one condition, participants moved a finger to press the button voluntarily, whereas in another condition, we initiated a similar, but involuntary, finger movement by stimulating the corresponding region of the primary motor cortex with TMS. For involuntary movements, no movement intention (and no feeling of agency) could be formed; thus, no motor plans were available to the forward model. A portion of the brain response evoked by the sounds, the N1-P2 complex, was reduced in amplitude following voluntary, self-initiated movements, but not following movements initiated by motor cortex stimulation. Our findings demonstrate that movement intention and the corresponding feeling of agency determine sensory attenuation of brain responses to self-initiated sounds. The present results support the assumptions of a predictive internal forward model account operating before primary motor cortex activation.

Cortical brain states and corticospinal synchronization influence TMS-evoked motor potentials.

Transcranial magnetic stimulation (TMS) influences cortical processes. Recent findings indicate, however, that, in turn, the efficacy of TMS depends on the state of ongoing cortical oscillations. Whereas power and phase of electromyographic (EMG) activity recorded from the hand muscles as well as neural synchrony between cortex and hand muscles are known to influence the effect of TMS, to date, no study has shown an influence of the phase of cortical oscillations during wakefulness. We applied single-pulse TMS over the motor cortex and recorded motor-evoked potentials along with the electroencephalogram (EEG) and EMG. We correlated phase and power of ongoing EEG and EMG signals with the motor-evoked potential (MEP) amplitude. We also investigated the functional connectivity between cortical and hand muscle activity (corticomuscular coherence) with the MEP amplitude. EEG and EMG power and phase in a frequency band around 18 Hz correlated with the MEP amplitude. High beta-band (∼34 Hz) corticomuscular coherence exhibited a positive linear relationship with the MEP amplitude, indicating that strong synchrony between cortex and hand muscles at the moment when TMS is applied entails large MEPs. Improving upon previous studies, we demonstrate a clear dependence of TMS-induced motor effects on the state of ongoing EEG phase and power fluctuations. We conclude that not only the sampling of incoming information but also the susceptibility of cortical communication flow depends cyclically on neural phase.

Sensation of agency and perception of temporal order.

After adaptation to a fixed temporal delay between actions and their sensory consequences, stimuli delivered during the delay are perceived to occur prior to actions. Temporal judgments are also influenced by the sensation of agency (experience of causing our own actions and their sensory consequences). Sensory consequences of voluntary actions are perceived to occur earlier in time than those of involuntary actions. However, it is unclear whether temporal order illusions influence the sensation of agency. Thus, we tested how the illusionary reversal of motor actions and sound events affect the sensation of agency. We observed an absence of the sensation of agency in the auditory modality in a condition in which sounds were falsely perceived as preceding motor acts relative to the perceived temporal order in the control condition. This finding suggests a strong association between the sensation of agency and the temporal order perception of actions and their consequences.

The N1-suppression effect for self-initiated sounds is independent of attention.

BACKGROUND: If we initiate a sound by our own motor behavior, the N1 component of the auditory event-related brain potential (ERP) that the sound elicits is attenuated compared to the N1 elicited by the same sound when it is initiated externally. It has been suggested that this N1 suppression results from an internal predictive mechanism that is in the service of discriminating the sensory consequences of one's own actions from other sensory input. As the N1-suppression effect is becoming a popular approach to investigate predictive processing in cognitive and social neuroscience, it is important to exclude an alternative interpretation not related to prediction. According to the attentional account, the N1 suppression is due to a difference in the allocation of attention between self- and externally-initiated sounds. To test this hypothesis, we manipulated the allocation of attention to the sounds in different blocks: Attention was directed either to the sounds, to the own motor acts or to visual stimuli. If attention causes the N1-suppression effect, then manipulating attention should affect the effect for self-initiated sounds. RESULTS: We found N1 suppression in all conditions. The N1 per se was affected by attention, but there was no interaction between attention and self-initiation effects. This implies that self-initiation N1 effects are not caused by attention. CONCLUSIONS: The present results support the assumption that the N1-suppression effect for self-initiated sounds indicates the operation of an internal predictive mechanism. Furthermore, while attention had an influence on the N1a, N1b, and N1c components, the N1-suppression effect was confined to the N1b and N1c subcomponents suggesting that the major contribution to the auditory N1-suppression effect is circumscribed to late N1 components.

An Asymmetry in the Automatic Detection of the Presence or Absence of a Frequency Modulation within a Tone: A Mismatch Negativity Study.

The infrequent occurrence of a transient feature (deviance; e.g., frequency modulation, FM) in one of the regular occurring sinusoidal tones (standards) elicits the deviance related mismatch negativity (MMN) component of the event-related brain potential. Based on a memory-based comparison, MMN reflects the mismatch between the representations of incoming and standard sounds. The present study investigated to what extent the infrequent exclusion of an FM is detected by the MMN system. For that purpose we measured MMN to deviances that either consisted of the exclusion or inclusion of an FM at an early or late position within the sound that was present or absent, respectively, in the standard. According to the information-content hypothesis, deviance detection relies on the difference in informational content of the deviant relative to that of the standard. As this difference between deviants with FM and standards without FM is the same as in the reversed case, comparable MMNs should be elicited to FM inclusions and exclusions. According to the feature-detector hypothesis, however, the deviance detection depends on the increased activation of feature detectors to additional sound features. Thus, rare exclusions of the FM should elicit no or smaller MMN than FM inclusions. In passive listening condition, MMN was obtained only for the early inclusion, but not for the exclusions nor for the late inclusion of an FM. This asymmetry in automatic deviance detection seems to partly reflect the contribution of feature detectors even though it cannot fully account for the missing MMN to late FM inclusions. Importantly, the behavioral deviance detection performance in the active listening condition did not reveal such an asymmetry, suggesting that the intentional detection of the deviants is based on the difference in informational content. On a more general level, the results partly support the "fresh-afferent" account or an extended memory-comparison based account of MMN.