Enhanced Solar Light Absorption and Photoelectrochemical Conversion Using TiN Nanoparticle-Incorporated C3N4-C Dot Sheets

Plasmonic Pt-PPCN/TiN Ohmic Junction for Photo-Thermo Catalytic Hydrogen Production

The rational design of photocatalysts for improving the conversion of solar energy into hydrogen is a promising route for achieving carbon neutrality. Herein, we couple plasmonic titanium nitride (TiN) with highly crystalline potassium-doped polymeric carbon nitride (PPCN) to construct a PPCN/TiN ohmic junction. Such an ohmic junction not only broadens the absorption spectrum but also inhibits the recombination of electrons and holes. In addition, Pt nanoparticles are introduced into this ohmic junction to form plasmonic Pt-PPCN/TiN, improving the capture of hot electrons generated by TiN and thereby promoting the dissociation of the O-H bond in H2O. The energy barrier decreases from 0.7 to 0.2 eV. Enhanced separation of carrier, activation of water molecules, and capture of hot electrons are jointly promoting photo-thermo catalytic hydrogen production. Therefore, under full-spectrum irradiation, the hydrogen production rate of Pt-PPCN/TiN reaches 19 085 μmol g-1 h-1. This novel plasmonic photocatalyst is promising for full-spectrum photo-thermo catalytic hydrogen production.

Enhanced plasmonic photocatalytic performance of C doped TiN nanocrystals through ultrathin carbon layers

Carbon-doped TiN nanoparticles on an ultrathin carbon layer, were successfully used for photocatalytic dye degradation synthesised by a simple calcination process. The resulting catalyst exhibited remarkable plasmonic photocatalytic performance under visible light irradiation. In comparison with benchmark rutile TiO2 and g-C3N4/TiO2 heterostructure catalysts, the first-order reaction rate constant of the developed catalyst improved approximately 34.2 and 6.5 times, respectively. The doping concentration of carbon and the crystal size of TiN nanoparticles, predominantly influenced by the amount of urea and calcination temperature, were identified as crucial factors governing the plasmonic photocatalytic activity. Density functional theory (DFT) calculations indicated that the introduction of carbon-sp bands into the TiN band structure promoted interband excitation of electrons and facilitated the generation of hotter holes, thereby enhancing the degradation of dyes and ultimately contributing to the superior photocatalytic activity observed.

A novel TiO2-x/TiN@ACB composite for synchronous photocatalytic Cr(VI) reduction and water photothermal evaporation under visible/infrared light illumination

Relatively large band-gap, fast charge carriers recombination, and mono-functionality of photocatalytic materials are still representing stumbling hurdles against their optimal usage for water cleaning. Herein, a novel black titanium oxide/plasmonic titanium nitride@activated coconut biochar (TiO2-x/TiN@ACB) composite was designed to have both photocatalytic and photothermal functions. Intermediate states of black TiO2-x, plasmonic effect of TiN, and high electrons (e-) capacity of biochar enhanced band-gap narrowing, light absorbance extension, and charge carriers separation respectively. Black TiO2-x and plasmonic TiN sensitization via visible/infrared (Vis/IR) portion of photonic spectrum in addition to the confirmed close contact of composite constituents explained the demonstrated major role of e- in photocatalytic mechanism through efficient excitation and facile transfer. Thanks to black photocatalytic semiconductor and carbonic materials for their ultimate photons harnessing and efficient photothermal conversion where the composite exhibited a remarkable photothermal water evaporation upon Vis/IR illumination as well. TiO2-x/TiN@ACB composite revealed 92.8 and 89.7% photocatalytic reduction of hexavalent chromium (Cr(VI)) and water evaporation efficiencies up to 92.9 and 51.1% upon IR and Vis light illumination respectively. This study proposes a new approach for efficient water cleaning by coupling of oxygen deficient and plasmonic semiconductors supported on naturally derived carbonic material as a broad spectrum harvester and bi-functional photocatalytic and photothermal material.

van der Waals integration of mixed-dimensional CeO2@Bi heterostructure for high-performance self-powered photodetector with fast response speed

Heterostructures have been extensively investigated for optoelectronic devices owing to their fantastic physicochemical properties. Herein, a mixed-dimensional van der Waals heterostructure (vdWH) CeO₂@Bi, 1D ceria (CeO₂) loaded with 0D bismuth quantum dots (Bi QDs), is synthesized through a facile hydrothermal bottom-up method. It is found that the fabricated CeO₂@Bi-based photoelectrochemical (PEC)-type photodetector (PD) shows self-powered photodetection capability with a fast photoresponse speed of 0.02 s. Besides, a photocurrent of 2.00 μA cm^-2 and a photoresponsivity of 888.89 μA W^-1 under 365 nm illumination are obtained. Furthermore, good long-term cycle stability is also observed after 1 month in a harsh environment, indicating the great potential for practical applications. These results are further supported by density functional theory (DFT) calculations. We believe that the presented work is expected to provide a new pathway for the future utilization of vdWHs for high-performance optoelectronics.

Effective modification of photocatalytic and piezocatalytic performances for poly(heptazine imide) by carbon dots decoration

As the high-crystalline phase of carbon nitride, poly(heptazine imide) (PHI) has attracted much attention in recent years, considering the more effective light absorption, better charge carrier behavior, and higher surface area of PHI compared with its counterpart with a melon structure that is commonly synthesized through thermal polymerization. Nevertheless, exploring effective strategies to further improve the performance of PHI is still highly desirable. In this work, it is revealed that the photocatalytic as well as piezocatalytic performances of PHI are greatly promoted by coupling with carbon dots (CDots) through a facile ultrasonication process. Detailed structure characterizations indicate that a very low content of CDots (0.05%) decoration can double the light absorbance and achieve the efficient separation and transfer of photogenerated charge carriers. The optimal photocatalytic hydrogen evolution rate of PHI/CDots is about 2.49 and 2.81 times that of PHI, under UV-Visible and visible light irradiation, respectively. Moreover, the piezocatalytic H₂O₂ generation and KMnO₄ degradation activities of PHI/CDots are around 2 times that of PHI. The results obtained in this work provide references for the modification of PHI and may inspire new strategies for the design of highly efficient carbonaceous photocatalysts.

Flower-Shaped C-Dots/Co3O4{111} Constructed with Dual-Reaction Centers for Enhancement of Fenton-Like Reaction Activity and Peroxymonosulfate Conversion to Sulfate Radical

Novel flower-shaped C-dots/Co3O4{111} with dual-reaction centers were constructed to improve the Fenton-like reaction activity and peroxymonosulfate (PMS) conversion to sulfate radicals. Due to the exposure of a high surface area and Co3O4{111} facets, flower-shaped C-dots/Co3O4{111} could provide more Co(II) for PMS activation than traditional spherical Co3O4{110}. Meanwhile, PMS was preferred for adsorption on Co3O4{111} facets because of a high adsorption energy and thereby facilitated the electron transfer from Co(II) to PMS. More importantly, the Co–O–C linkage between C-dots and Co3O4{111} induced the formation of the dual-reaction center, which promoted the production of reactive organic radicals (R•). PMS could be directly reduced to SO4−• by R• over C-dots. On the other hand, electron transferred from R• to Co via Co–O–C linkage could accelerate the redox of Co(II)/(III), avoiding the invalid decomposition of PMS. Thus, C-dots doped on Co3O4{111} improved the PMS conversion rate to SO4−• over the single active site, resulting in high turnover numbers (TONs). In addition, TPR analysis indicated that the optimal content of C-dots doped on Co3O4{111} is 2.5%. More than 99% of antibiotics and dyes were degraded over C-dots/Co3O4{111} within 10 min. Even after six cycles, C-dots/Co3O4{111} still remained a high catalytic activity.

Enhanced Broadband Plasmonic Absorbers with Tunable Light Management on Flexible Tapered Metasurface

The growing attention in solar energy has motivated the development of highly efficient solar absorbers, and a metasurface absorber with broadband optical absorption is one of the main research interests. In this study, we developed an efficient metasurface absorber on a flexible film with a simple fabrication process. It consists of a polyimide nanocone substrate coated with gold and tungsten layers, exhibiting over 96% optical absorption in the visible range and a tunable absorption performance in the long wave range. From the analysis of experiment and simulation, the enhanced optical absorption is attributed to the synergistic effects of localized nanoparticle plasmon resonance and cavity plasmon resonance, and tunable light management comes from the strong infrared reflection of a gold layer and intrinsic absorption of variable tungsten layers. Meanwhile, the polarization-independent and omnidirectional optical absorption properties are demonstrated in the fabricated absorbers. Furthermore, this absorber shows the robustness against bending, maintaining the stable and excellent absorption performance after hundreds of bending tests. Our work offers a low-cost and straightforward tactic to design and fabricate flexible solar absorbers, and this metasurface absorber is a promising candidate for many exciting applications, such as emissivity control and flexible energy-related devices.

Direct Observation of Photoinduced Charge Separation at Transition-Metal Nitride-Semiconductor Interfaces

Optically excited hot carriers from metallic nanostructures forming metal-semiconductor heterostructures are advantageous for enhancing photoelectric conversion in the sub-band gap photon energy regime. Plasmonic gold has been widely used for hot carrier excitation, but recent works have demonstrated that plasmonic transition-metal nitrides have higher efficiencies in injecting hot electrons to adjacent n-type semiconductors and are more cost-effective. To collect direct evidence of hot carrier excitation from nanostructures, imaging of hot carriers is essential. In this work, photoexcited Kelvin probe force microscopy (KPFM) is used to image hot carriers excited in transition-metal nitride nanostructures forming heterostructures with semiconductors. Among available transition-metal nitrides, we select zirconium nitride (ZrN) for this study. Additionally, both p-type and n-type titanium dioxides (TiO₂) are selected to study the transport of hot holes and hot electrons. The KPFM results indicate that for ZrN and p-type TiO₂ heterostructures, hot holes are injected into the p-type TiO₂ across the Schottky contact. In the case of ZrN and n-type TiO₂ heterostructures, hot electrons are injected into the n-type TiO₂ across the ohmic contact. Because transition-metal nitrides are known to be more effective than gold at injecting hot carriers into adjacent semiconductors, unambiguously determining the mechanisms of hot carrier transportation of transition-metal nitrides using photoexcited KPFM will facilitate additional studies on hot carrier applications with transition-metal nitrides.

Marimo-Bead-Supported Core-Shell Nanocomposites of Titanium Nitride and Chromium-Doped Titanium Dioxide as a Highly Efficient Water-Floatable Green Photocatalyst

The release of untreated industrial wastewater creates a hazardous impact on the environment. In this regard, the development of an environmentally friendly catalyst is of paramount importance. Here, we report a highly efficient and reusable core-shell TiN/SiO₂/Cr-TiO₂ (TSCT) photocatalyst that is composed of SiO₂-cladded titanium nitride (TiN) nanoparticles (NPs) decorated with Cr-doped TiO₂ NPs for the removal of organic contaminants from water. The TiN NPs serve as the main light absorber component with excellent visible-light absorption along with Cr-TiO₂ NPs. The TSCT shows remarkable improvement in the photodecomposition of methylene blue (MB) over Cr-TiO₂ and TiO₂ NPs. An efficient structural design is proposed by adopting calcium alginate beads (P-Marimo beads) as a transparent scaffold for supporting our TSCT, which floats nature on the water surface and realizes easy handling as well as excellent reusability for multipurpose water purification. Surprisingly, our TSCT is found to keep its catalytic activity even after the illumination is turned off. Our proposed P-Marimo-encapsulated TSCT can be utilized as an excellent green photocatalyst with high photocatalytic performance, good recyclability, and easy handling.

Plasmonic ternary hybrid photocatalyst based on polymeric g-C3N4 towards visible light hydrogen generation

Surface plasmon resonance (SPR) effect of noble metal nanoparticles (NPs) for photocatalysis has a significant enhancement. In this system, a plasmonic ternary hybrid photocatalyst of Ag/AgBr/g-C₃N₄ was synthetized and used in water splitting to generation H₂ under visible light irradiation. 18%Ag/AgBr/g-C₃N₄ showed the highest photoactivity, with the efficiency of hydrogen generation as high as 27-fold to that of pristine g-C₃N₄. Compared to simple mixture of Ag/AgBr and g-C₃N₄, hetero-composite Ag/AgBr/g-C₃N₄ showed a higher photoactivity, even though they contained same content of Ag/AgBr. We find that significant factors for enhancing properties were the synergistic effect between Ag/AgBr and g-C₃N₄, and the light absorption enhancing by SPR effect of Ag NPs. Ag/AgBr NPs firmly anchored on the surface of g-C₃N₄ and their high dispersion were also responsible for the improved activity and long-term recycling ability. The structure of Ag/AgBr/g-C₃N₄ hybrid materials and their enhancement to photocatalytic activity were discussed. Meanwhile, the possible reaction mechanism of this system was proposed.

Tunable plasmonic HfN nanoparticles and arrays

We present the fabrication of tunable plasmonic hafnium nitride (HfN) nanoparticles. HfN is a metallic refractory material with the potential of supporting plasmon resonances in the visible range, similar to silver and gold, but with the additional benefits of high melting point, chemical stability, and mechanical hardness. However, the preparation of HfN nanoparticles and the experimental demonstration of their plasmonic potential are still in their infancy. Here, high quality HfN thin films were fabricated, for which ellipsometry shows their plasmonic potential. From these thin films, nanorods and nanotriangles were milled using a focused ion beam and the plasmon resonances were identified using cathodoluminescence mapping. As an alternative fabrication strategy, an optimized electron-beam lithography procedure was used to prepare arrays of HfN nanoparticles, which also exhibited clear surface plasmon resonances. These results pave the way to further explore HfN nanoparticles in plasmonically-powered applications where materials robustness is essential.

Sub-Band Gap Photodetection from the Titanium Nitride/Germanium Heterostructure

Photoexcited hot carriers through nonradiative decay offer new opportunities for harnessing longer wavelength light. Here, we have demonstrated a hot-carrier-mediated sub-band gap photodetection in germanium-based planar heterojunction devices. The planar samples that form in situ germanium/titanium nitride (Ge/TiN) interfaces are fabricated by the dc sputtering technique, and the generation of photocurrent by near-infrared (NIR) light illumination is confirmed up to 2600 nm, well exceeding the absorption limit of Ge. The photocurrent obtained with nickel contacts is 3 orders larger than that obtained without metal contacts or with gold contacts in similar structures. The specific detectivity ( D*) value for the TiN/Ge photodetector is obtained to be 6.32 × 10⁵ Jones at the sub-band gap excitation wavelength of 2000 nm without applying any bias. The superior performances of our device are attributed to the broad absorption of the TiN, the plasmonic hot carrier transfer from the TiN to Ge, and built-in potential of the TiN/Ge non-Ohmic junction, which allows efficient separation of photoexcited electron-hole pairs. Our results further support the use of TiN, which is robust and cost-effective, as an alternative to metals for NIR photodetection and photovoltaics when it forms a heterostructure with Ge.