Facile Zn and Ni Co-Doped Hematite Nanorods for Efficient Photocatalytic Water Oxidation.

In this work, we report the effect of zinc (Zn) and nickel (Ni) co-doping of hydrothermally synthesized hematite nanorods prepared on fluorine-doped tin oxide (FTO) substrates for enhanced photoelectrochemical (PEC) water splitting. Seeded hematite nanorods (NRs) were facilely doped with a fixed concentration of 3 mM Zn and varied concentrations of 0, 3, 5, 7, and 9 mM Ni. The samples were observed to have a largely uniform morphology of vertically aligned NRs with slight inclinations. The samples showed high photon absorption within the visible spectrum due to their bandgaps, which ranged between 1.9-2.2 eV. The highest photocurrent density of 0.072 mA/cm2 at 1.5 V vs. a reversible hydrogen electrode (RHE) was realized for the 3 mM Zn/7 mM Ni NRs sample. This photocurrent was 279% higher compared to the value observed for pristine hematite NRs. The Mott-Schottky results reveal an increase in donor density values with increasing Ni dopant concentration. The 3 mM Zn/7 mM Ni NRs sample produced the highest donor concentration of 2.89 × 1019 (cm-3), which was 2.1 times higher than that of pristine hematite. This work demonstrates the role of Zn and Ni co-dopants in enhancing the photocatalytic water oxidation of hematite nanorods for the generation of hydrogen.

Mono-Doped and Co-Doped Nanostructured Hematite for Improved Photoelectrochemical Water Splitting.

In this study, zinc-doped (α-Fe2O3:Zn), silver-doped (α-Fe2O3:Ag) and zinc/silver co-doped hematite (α-Fe2O3:Zn/Ag) nanostructures were synthesized by spray pyrolysis. The synthesized nanostructures were used as photoanodes in the photoelectrochemical (PEC) cell for water-splitting. A significant improvement in photocurrent density of 0.470 mAcm-2 at 1.23 V vs. reversible hydrogen electrode (RHE) was recorded for α-Fe2O3:Zn/Ag. The α-Fe2O3:Ag, α-Fe2O3:Zn and pristine hematite samples produced photocurrent densities of 0.270, 0.160, and 0.033 mAcm-2, respectively. Mott-Schottky analysis showed that α-Fe2O3:Zn/Ag had the highest free carrier density of 8.75 × 1020 cm-3, while pristine α-Fe2O3, α-Fe2O3:Zn, α-Fe2O3:Ag had carrier densities of 1.57 × 1019, 5.63 × 1020, and 6.91 × 1020 cm-3, respectively. Electrochemical impedance spectra revealed a low impedance for α-Fe2O3:Zn/Ag. X-ray diffraction confirmed the rhombohedral corundum structure of hematite. Scanning electron microscopy micrographs, on the other hand, showed uniformly distributed grains with an average size of <30 nm. The films were absorbing in the visible region with an absorption onset ranging from 652 to 590 nm, corresponding to a bandgap range of 1.9 to 2.1 eV. Global analysis of ultrafast transient absorption spectroscopy data revealed four decay lifetimes, with a reduction in the electron-hole recombination rate of the doped samples on a timescale of tens of picoseconds.

A Promising Three-Step Heat Treatment Process for Preparing CuO Films for Photocatalytic Hydrogen Evolution from Water.

Copper (II) oxide (CuO) nanostructures were prepared on fluorine-doped tin oxide (FTO) using a three-step heat treatment process in a sol-gel dip-coating method. The precursor used for the dip-coating process was prepared using copper acetate, propan-2-ol, diethanolamine, and polyethylene glycol 400. Dip-coated films in layers of 2, 4, 6, 8, and 10 were prepared by drying each layer at 110 and 250 °C for 10 and 5 min, respectively, followed by calcination at 550 °C for 1 h. The films were applied toward photocatalytic hydrogen evolution from water. The X-ray diffraction (XRD) pattern of the films confirmed the tenorite phase of pure CuO. Raman spectroscopy revealed the 1Ag and 2Bg phonon modes of CuO, confirming the high purity of the films produced. The CuO films absorb significant photons in the visible spectrum due to their low optical band gap of 1.25-1.33 eV. The highest photocurrent of -2.0 mA/cm2 at 0.45 V vs reversible hydrogen electrode (RHE) was recorded for CuO films consisting of six layers under 1 sun illumination. A more porous surface, low charge transfer resistance, and high double-layer capacitance at the CuO/electrolyte interface observed for the films consisting of six layers contributed to the high photocurrent density attained by the films. CuO films consisting of six layers prepared using the conventional two-step heat treatment process for comparative purposes yielded 65.0% less photocurrent at 0.45 V vs RHE compared to similar films fabricated via the three-step heating method. The photocurrent response of the CuO nanostructures prepared using the three-step heat treatment process is promising and can be employed for making CuO for photovoltaic and optoelectronic applications.

Electrically Enhanced Transition Metal Dichalcogenides as Charge Transport Layers in Metallophthalocyanine-Based Solar Cells.

Transitional metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2) have found application in photovoltaic cells as a charge transporting layer due to their high carrier mobility, chemical stability, and flexibility. In this research, a photovoltaic device was fabricated consisting of copper phthalocyanine (CuPc) as the active layer, exfoliated and Au-doped MoS2, which are n-type and p-type as electron and hole transport layers, respectively. XRD studies showed prominent peaks at (002) and other weak reflections at (100), (103), (006), and (105) planes corresponding to those of bulky MoS2. The only maintained reflection at (002) was weakened for the exfoliated MoS2 compared to the bulk, which confirmed that the material was highly exfoliated. Additional peaks at (111) and (200) planes were observed for the Au doped MoS2. The interlayer spacing (d002) was calculated to be 0.62 nm for the trigonal prismatic MoS2 with the space group P6m2. Raman spectroscopy showed that the E 2 1 g (393 cm-1) and A1g (409 cm-1) peaks for exfoliated MoS2 are closer to each other compared to their bulk counterparts (378 and 408 cm-1, respectively) hence confirming exfoliation. Raman spectroscopy also confirmed doping of MoS2 by Au as the Au-S peak was observed at 320 cm-1. Exfoliation was further confirmed by SEM as when moving from bulky to exfoliated MoS2, a single nanosheet was observed. Doping was further proven by EDS, which detected Au in the sample suggesting the yield of a p-type Au-MoS2. The fabricated device had the architecture: Glass/FTO/Au-MoS2/CuPc/MoS2/Au. A quadratic relationship between I-V was observed suggesting little rectification from the device. Illuminated I-V characterization verified that the device was sensitive and absorbed visible light. Upon illumination, the device was able to absorb photons to create electron-hole pairs and it was evident that semipermeable junctions were formed between Au-MoS2/CuPc and CuPc/MoS2 as holes and electrons were extracted and separated at respective junctions generating current from light. This study indicates that the exfoliated and Au-MoS2 could be employed as an electron transporting layer (ETL) and hole transporting layer (HTL), respectively in fabricating photovoltaic devices.

Photoelectrochemical performance and ultrafast dynamics of photogenerated electrons and holes in highly titanium-doped hematite.

Elemental doping of hematite has been widely performed to improve its mobility, electrical conductivity as well as to suppress electron-hole recombination in photoelectrochemical applications. When hematite is doped with high titanium concentrations, above 5%, pseudobrookite layers may be formed as overlayers leading to improved photocurrent while further doping beyond 15% could lead to the formation of a titania overlayer which has an effect of suppressing photocurrent. In this study, we observed that doping hematite with titanium improves photocurrent, reaching a maximum of 1.83 mA cm-2 at a titanium concentration of 15%, the highest achieved photocurrent with spin coating method. Further titanium incorporation to 20% resulted in a decrease of the photocurrent. XRD measurements shows that a Fe2TiO5 layer formed at 15% Ti concentration which resulted in the observed increase in photocurrent while a reduction in photocurrent at 20% Ti concentration could have resulted from the formation of a TiO2 layer. Analysis of the transient absorption spectroscopy data was achieved using a four-component sequential analysis scheme in the Glotaran software. We observed major doping concentration dependent lifetimes in the τ3 and τ4 values where the 15% doped samples had the slowest recombination rates. We also observed a blueshift in the spectra with increasing doping concentration, suggesting the occurance of the Burstein-Moss effect. This work shows that doping hematite with titanium leads to structural changes of the photoanodes at Ti concentrations of over 10%, in addition to the well documented conductivity enhancement.

Strong plasmonic fluorescence enhancement of individual plant light-harvesting complexes.

Plasmonic coupling of metallic nanoparticles and adjacent pigments can dramatically increase the brightness of the pigments due to the enhanced local electric field. Here, we demonstrate that the fluorescence brightness of a single plant light-harvesting complex (LHCII) can be significantly enhanced when coupled to a gold nanorod (AuNR). The AuNRs utilized in this study were prepared via chemical reactions, and the hybrid system was constructed using a simple and economical spin-assisted layer-by-layer technique. Enhancement of fluorescence brightness of up to 240-fold was observed, accompanied by a 109-fold decrease in the average (amplitude-weighted) fluorescence lifetime from approximately 3.5 ns down to 32 ps, corresponding to an excitation enhancement of 63-fold and emission enhancement of up to 3.8-fold. This large enhancement is due to the strong spectral overlap of the longitudinal localized surface plasmon resonance of the utilized AuNRs and the absorption or emission bands of LHCII. This study provides an inexpensive strategy to explore the fluorescence dynamics of weakly emitting photosynthetic light-harvesting complexes at the single molecule level.