Self-healing, stretchable, and highly adhesive hydrogels for epidermal patch electrodes.

Flexible, self-healing and adhesive conductive materials with Young's modulus matching biological tissues are highly desired for applications in bioelectronics. Here, we report self-healing, stretchable, highly adhesive and conductive hydrogels obtained by mixing polyvinyl alcohol, sodium tetraborate and a screen printing paste containing the conducting polymer Poly (3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS) and diol additives. The as prepared hydrogels exhibited modelling ability, high adhesion on pig skin (1.96 N/cm2), high plastic stretchability (>10000%), a moderate conductivity, a low compressive modulus (0.3-3.7 KPa), a good strain sensitivity (gauge factor = 3.88 at 500% strain), and remarkable self-healing properties. Epidermal patch electrodes prepared using one of our hydrogels demonstrated high-quality recording of electrocardiography (ECG) and electromyography (EMG) signal. Because of their straightforward fabrication, outstanding mechanical properties and possibility to combine the electrode components in a single material, hydrogels based on PVA, borax and PEDOT:PSS are highly promising for applications in bioelectronics and wearable electronics. STATEMENT OF SIGNIFICANCE: Soft materials with electrical conductivity are investigated for healthcare applications, such as electrodes to measure vital signs that can easily adapt to the shape and the movements of human skin. Conductive hydrogels (i.e. gels containing water) are ideal materials for this purpose due softness and flexibility. In this this work, we report hydrogels obtained mixing an electrically conductive polymer, a water-soluble biocompatible polymer and a salt. These materials show high adhesion on skin, electrical conductivity and ability to self-repair after a mechanical damage. These hydrogels were successfully used to fabricate electrode to measure cardiac and muscular electrical signals.

Insights into the enhanced photoelectrochemical performance of hydrothermally controlled hematite nanostructures for proficient solar water oxidation.

In this paper, we focus on the controlled growth mechanism of α-Fe2O3 nanostructures via the hydrothermal method. The field emission scanning electron microscopy (FESEM) results reveal that at a lower hydrothermal time, the initial nucleation involves the formation of short and thin ß-FeOOH nanorods. The subsequent increase in the hydrothermal time leads ß-FeOOH to form thicker and longer nanorods. However, high-temperature quenching (HTQ) at 800 °C for 10 min causes the conversion of akaganeite to the hematite phase and activation of hematite by Sn4+ diffusion from a FTO substrate. Sn4+ diffusion from the FTO substrate to the hematite nanostructure was elaborated by X-ray photoelectron spectroscopy (XPS). An α-Fe2O3 nanorod photoanode prepared by a hydrothermal reaction for 3 h and HTQ exhibits the highest photocurrent density of 1.04 mA cm-2. The excellent photoelectrochemical performance could be ascribed to the synergistic effect of the optimum growth of α-Fe2O3 nanorod arrays and Sn4+ diffusion. Intensity modulated photovoltage spectroscopy (IMVS) studies revealed that the α-Fe2O3 photoanodes prepared at 3 h and HTQ exhibited a long electron lifetime (132.69 ms), and contribute to the enhanced PEC performance. The results confirmed that the controlled growth of the ß-FeOOH nanorods, as well as Sn4+ diffusion, played a key role in charge transfer during the photoelectrochemical application. The charge transfer mechanisms in α-Fe2O3 nanostructure photoanodes prepared at different hydrothermal times and high-temperature quenching are also investigated.

CdS/Zr:Fe2 O3 Nanorod Arrays with Al2 O3 Passivation Layer for Photoelectrochemical Solar Hydrogen Generation.

CdS-sensitized 1 D Zr:Fe2 O3 nanorod arrays were synthesized on fluorine-doped tin oxide substrates by a two-step hydrothermal method. The photoelectrochemical results demonstrate that the current density (4.2 mA cm-2 at 0 V vs. Ag/AgCl) recorded under illumination for the CdS/1 D Zr:Fe2 O3 photoanodes is 2.8 time higher than the bare 1 D Zr:Fe2 O3 . The extended absorbance spectrum, the reduced recombination, and the effective transport of photogenerated holes in CdS to the electrolyte facilitate enhancement in the photoelectrochemical performance. From X-ray photoelectron spectroscopy and TEM observations of the bare and aluminum oxide-treated CdS/1 D Zr:Fe2 O3 photoanodes, we could confirm that the 1 D Zr:Fe2 O3 nanorods were covered by the CdS layer and Al2 O3 layer present on surface of CdS. Furthermore, the photocurrent and stability of the CdS/1 D Zr:Fe2 O3 nanorods was significantly enhanced by Al2 O3 compared to bare CdS/1 D Zr:Fe2 O3 heterojunction owing to its ability to act as an effective holetransport- as well as photocorrosion-protecting layer. These remarkable enhancements in light-energy harvesting, improvement in charge transport, and stability directly suggest the usefulness of photoanodes for solar hydrogen generation.

A hydrothermally grown CdS nanograin-sensitized 1D Zr:α-Fe2O3/FTO photoanode for efficient solar-light-driven photoelectrochemical performance.

Well-defined CdS nanograin-sensitized one-dimensional (1D) Zr:α-Fe2O3 nanostructured arrays with enhanced photoelectrochemical performance are synthesized directly on F-doped SnO2 (FTO) using the hydrothermal method. Owing predominantly to the appropriate photogenerated electron-hole separation and charge collection in 1D Zr:α-Fe2O3 nanorods, hydrothermally deposited CdS/1D Zr:α-Fe2O3 samples exhibit improved photocurrent density over CdS/Fe2O3 nanosheets prepared by other methods. In our work, compared with 1D Zr:α-Fe2O3, the CdS-sensitized 1D Zr:α-Fe2O3 nanorod arrays show 1.9 times improved photoelectrochemical performance. Unfortunately, CdS nanograin-sensitized 1D Zr:α-Fe2O3 nanorod arrays suffer from instability problem. Nickel hydroxide loading, however, can boost the photoelectrochemical performance of the heterojunction and also act as a protective layer that improves the stability of the Ni(OH)2/CdS/1D Zr:α-Fe2O3 electrode compared to CdS/1D Zr:α-Fe2O3. This enhanced PEC activity may be ascribed to the strong heterojunctions between CdS nanograins and 1D Zr:α-Fe2O3 nanorod arrays as well as effective charge separation. This work will provide a new insight into the fabrication and protection of many new photosensitive electrode materials to engineer photoelectrochemical and photocatalytic devices in the near future.

Trade-off between Zr Passivation and Sn Doping on Hematite Nanorod Photoanodes for Efficient Solar Water Oxidation: Effects of a ZrO2 Underlayer and FTO Deformation.

Herein we report the influence of a ZrO2 underlayer on the PEC (photoelectrochemical) behavior of hematite nanorod photoanodes for efficient solar water splitting. Particular attention was given to the cathodic shift in onset potential and photocurrent enhancement. Akaganite (ß-FeOOH) nanorods were grown on ZrO2-coated FTO (fluorine-doped tin oxide) substrates. Sintering at 800 °C transformed akaganite to the hematite (α-Fe2O3) phase and induced Sn diffusion into the crystal structure of hematite nanorods from the FTO substrates and surface migration, shallow doping of Zr atoms from the ZrO2 underlayer. The ZrO2 underlayer-treated photoanode showed better water oxidation performance compared to the pristine (α-Fe2O3) photoanode. A cathodic shift in the onset potential and photocurrent enhancement was achieved by surface passivation and shallow doping of Zr from the ZrO2 underlayer, along with Sn doping from the FTO substrate to the crystal lattice of hematite nanorods. The Zr based hematite nanorod photoanode achieved 1 mA/cm(2) at 1.23 VRHE with a low turn-on voltage of 0.80 VRHE. Sn doping and Zr passivation, as well as shallow doping, were confirmed by XPS, Iph, and M-S plot analyses. Electrochemical impedance spectroscopy revealed that the presence of a ZrO2 underlayer decreased the deformation of FTO substrate, improved electron transfer at the hematite/FTO interface and increased charge-transfer resistance at the electrolyte/hematite interface. This is the first systematic investigation of the effects of Zr passivation, shallow doping, and Sn doping on hematite nanorod photoanodes through application of a ZrO2 underlayer on the FTO substrate.

Sn/Be Sequentially co-doped Hematite Photoanodes for Enhanced Photoelectrochemical Water Oxidation: Effect of Be(2+) as co-dopant.

For ex-situ co-doping methods, sintering at high temperatures enables rapid diffusion of Sn(4+) and Be(2+) dopants into hematite (α-Fe2O3) lattices, without altering the nanorod morphology or damaging their crystallinity. Sn/Be co-doping results in a remarkable enhancement in photocurrent (1.7 mA/cm(2)) compared to pristine α-Fe2O3 (0.7 mA/cm(2)), and Sn(4+) mono-doped α-Fe2O3 photoanodes (1.0 mA/cm(2)). From first-principles calculations, we found that Sn(4+) doping induced a shallow donor level below the conduction band minimum, which does not contribute to increase electrical conductivity and photocurrent because of its localized nature. Additionally, Sn(4+)-doping induce local micro-strain and a decreased Fe-O bond ordering. When Be(2+) was co-doped with Sn(4+)-doped α-Fe2O3 photoanodes, the conduction band recovered its original state, without localized impurities peaks, also a reduction in micro-strain and increased Fe-O bond ordering is observed. Also the sequence in which the ex-situ co-doping is carried out is very crucial, as Be/Sn co-doping sequence induces many under-coordinated O atoms resulting in a higher micro-strain and lower charge separation efficiency resulting undesired electron recombination. Here, we perform a detailed systematic characterization using XRD, FESEM, XPS and comprehensive electrochemical and photoelectrochemical studies, along with sophisticated synchrotron diffraction studies and extended X-ray absorption fine structure.