Photocatalytic H2 Production from Water by Metal-free Dye-sensitized TiO2 Semiconductors: The Role and Development Process of Organic Sensitizers

Fabrication of Transparent Pt-TiO2 Sol and Its Photocatalytic Activity for Hydrogen Evolution

Titanium dioxide doped with Pt(IV) ions was synthesized via a sol-gel method, incorporating a sol purification process through dialysis. The doped Pt(IV) ions were reduced by UV light irradiation to obtain a transparent Pt-TiO2 sol, with Pt(0) acting as a cocatalyst for hydrogen evolution. The hydrogen evolution activity of Pt-TiO2 sol was evaluated under UV light irradiation using methanol as a sacrificial reagent. 0.05 atom % Pt-TiO2 sol remained in the sol state without precipitation after 10 h of UV light irradiation. The amount of hydrogen produced after 10 h of light irradiation was 1195 μmol, with a turnover number (TON) of 664.1, a turnover frequency (TOF) of 66.4 h-1, and an apparent quantum yield (AQY) of 0.86%. The sol-gel synthesis from the molecular level is thought to result in Pt ions being doped inside the TiO2 particles, maintaining high dispersibility by suppressing particle growth due to the aggregation of Pt(0) during the UV reduction of Pt(IV) ions. These results show that the Pt-TiO2 photocatalyst has a high renewable energy factor because it can be synthesized at room temperature, and the aggregation of Pt-TiO2 particles is suppressed by making full use of a sol-gel method.

Recent Advances of CeO2-Based Composite Materials for Photocatalytic Applications

Photocatalysis has the advantages of practical, sustainable and environmental protection, so it plays a significant role in energy transformation and environmental utilization. CeO2 has attracted widespread attention for its unique 4 f electrons, rich defect structures, high oxygen storage capacity and great chemical stability. In this paper, we review the structure of CeO2 and the common methods for the preparation of CeO2-based composites in the first part. In particular, we highlight the co-precipitation method, template method, and sol-gel method methods. Then, in the second part, we introduce the application of CeO2-based composites in photocatalysis, including photocatalytic CO2 reduction, hydrogen production, degradation, selective organic reaction, and photocatalytic nitrogen fixation. In addition, we discuss several modification techniques to improve the photocatalytic performance of CeO2-based composites, such as elemental doping, defect engineering, constructing heterojunction and morphology regulation. Finally, the challenges faced by CeO2-based composites are analyzed and their development prospects are prospected. This review provides a systematic summary of the recent advance of CeO2-based composites in the field of photocatalysis, which can provide useful references for the rational design of efficient CeO2-based composite photocatalysts for sustainable development.

Synthesis, Characterization, and Properties of a Titanium(IV)-Tetrathiafulvalene-Based Complex

To deeply investigate the interaction between a tetrathiafulvalene (TTF) unit and a Ti(IV) center, a monomeric heteroleptic octahedral Ti(IV) complex containing a diimine ligand composed of a 1,10-phenanthroline core fused with a TTF fragment (ligand 2a) was prepared. The stable complex formulated as Ti(1)2(2a), where 1 is a 2,2'-biphenolato derivative, was efficiently synthesized by following a one-step approach. This complex and its model species [Ti(1)2(2b)] were fully characterized in solution, and their solid-state structures were established by single-crystal X-ray diffraction analysis. Density functional theory calculations allowed the assignment of the frontier orbitals involved in the electronic transitions characterized by ultraviolet-visible absorption spectroscopy. Electrochemical and spectroelectrochemical studies revealed that the TTF unit within Ti(1)2(2a) can undergo two reversible one-electron oxidation processes; a reversible one-electron reduction of the Ti(IV) atom was highlighted. The photophysical measurements performed for this donor-acceptor molecular system indicated that an electron transfer process upon light excitation occurred within Ti(1)2(2a).

Antenna Effect in Noble Metal-Free Dye-Sensitized Photocatalytic Systems Enhances CO2 -to-CO Conversion

Dye-sensitized photocatalytic systems (DSPs) have been extensively investigated for solar-driven hydrogen (H2 ) evolution. However, their application in carbon dioxide (CO2 ) reduction remains limited. Furthermore, current solar-driven CO2 -to-CO DSPs typically employ rhenium complexes as catalysts. In this study, we have developed DSPs that incorporate noble metal-free components, specifically a zinc-porphyrin as photosensitizer (PS) and a cobalt-quaterpyridine as catalyst (CAT). Taking a significant stride forward, we have achieved an antenna effect for the first time in CO2 -to-CO DSPs by introducing a Bodipy as an additional chromophore to enhance light harvesting efficiency. The energy transfer from Bodipy to zinc porphyrin resulted in remarkable stability (turn over number (TON)=759 vs. CAT), and high CO evolution activity (42 mmol g-1  h-1 vs. CAT).

Dye-Sensitized Photocatalysis: Hydrogen Evolution and Alcohol-to-Aldehyde Oxidation without Sacrifical Electron Donor

There is a growing interest in developing dye-sensitized photocatalytic systems (DSPs) to produce molecular hydrogen (H2 ) as alternative energy source. To improve the sustainability of this technology, we replaced the sacrificial electron donor (SED), typically an expensive and polluting chemical, with an alcohol oxidation catalyst. This study demonstrates the first dye-sensitized system using a diketopyrrolopyrrole dye covalently linked to 2,2,6,6-tetramethyl-1-piperidine-N-oxyl (TEMPO) based catalyst for simultaneous H2 evolution and alcohol-to-aldehyde transformation operating in water with visible irradiation.

Degradation of reactive red 198 dye from aqueous solutions by combined technology advanced sonofenton with zero valent iron: Characteristics/ effect of parameters/kinetic studies

Dyes are one of the most common contaminants in industrial effluents, whose continuous release into the environment has become an increasing global concern. In this work, nanoparticles of zero-valent iron (NZVI) were synthesized using the chemical regeneration method ،and were utilized for the first time as a catalyst in the advanced Sono-Nano-Fenton hybrid method for the decomposition of Reactive Red 198 (RR198). The properties of zero-valent iron nanoparticles were analyzed using SEM and XRD. The effect of pH, initial dye concentration, nanoparticle dosage, zero-valent iron and H2O2 concentration on the decomposition efficiency of Red Reactive 198 was investigated. Comparing the efficiency of Reactivate 198 dye degradation in Sonolysis, Sono-NZVI, Sono-H2O2 and Sono-Nano Fenton processes showed that 97 % efficiency was achieved by the Sono-Nano Fenton process in 60 min. The kinetics of the removal process showed that this process follows pseudo-first-order kinetics and the Langmuir-Hinshelwood model. The results indicate that the effectiveness of the ultrasonic process in removing resistant organic pollutants such as dyes increases tremendously with the synergy of the Fenton process.

Carbon nitride quantum dots-modified cobalt phosphate for enhanced photocatalytic H2 evolution

In the present work, carbon nitride quantum dots (CNQDs)-modified cobalt phosphate (CoPi) composites CNQDs/CoPi-x (x = 1, 2, 3) were prepared at room temperature and characterized by FTIR, XRD, UV-Vis DRS, EIS, SEM, TEM/HR-TEM, XPS, and N2 gas adsorption. The morphologies and surface areas of CNQDs/CoPi-x have no remarkable change after modification of CNQDs, compared with pure CoPi. The obtained CNQDs/CoPi-x shows enhanced activity and stability of photocatalytic H2 evolution compared to pure CoPi using Eosin Y (EY) as a sensitizer and triethanolamine as an electron donor. The CNQDs/CoPi-2 possesses the highest hydrogen evolution rate, 234.5 μmol h-1  g-1 , upon visible light, which outshines that of CoPi by 2.4 times. It was believed that the enhanced photocatalytic performances of the CNQDs/CoPi-2 could result from the boosted electron transfer from radical EY·- to CNQDs/CoPi-2 by the employment of CNQDs; in addition, the visible-light activity of CNQDs contributes to hydrogen evolution. The mechanism of photocatalytic hydrogen production was discussed. This study may contribute toward the development of production of "green hydrogen" using solar.

Ligand-Centered Photocatalytic Hydrogen Production in an Axially Capped Rh2(II,II) Paddlewheel Complex with Red Light

A new series of Rh2(II,II) complexes with the formula cis-[Rh2(DTolF)2(bpnp)(L)]2+, where bpnp = 2,7-bis(2-pyridyl)-1,8-naphthyridine, DTolF = N,N'-di(p-tolyl) formamidinate, and L = pdz (pyridazine; 2), cinn (cinnoline; 3), and bncn (benzo[c]cinnoline; 4), were synthesized from the precursor cis-[Rh2(DTolF)2(bpnp)(CH3CN)2]2+ (1). The first reduction couple in 2-4 is localized on the bpnp ligand at approximately -0.52 V vs Ag/AgCl in CH3CN (0.1 M TBAPF6), followed by reduction of the corresponding diazine ligand. Complex 1 exhibits a Rh2(δ*)/DTolF → bpnp(π*) metal/ligand-to-ligand charge-transfer (1ML-LCT) absorption with a maximum at 767 nm (ε = 1800 M-1 cm-1). This transition is also present in the spectra of 2-4, overlaid with the Rh2(δ*)/DTolF → L(π*) 1ML-LCT bands at 516 nm in 2 (L = pdz), 640 nm in 3 (L = cinn), and 721 nm in 4 (L = bncn). Complexes 2 and 3 exhibit Rh2(δ*)/DTolF → bpnp 3ML-LCT excited states with lifetimes, τ, of 3 and 5 ns, respectively, in CH3CN, whereas the lowest energy 3ML-LCT state in 4 is Rh2(δ*)/DTolF → bncn in nature with τ = 1 ns. Irradiation of 4 with 670 nm light in DMF in the presence of 0.1 M TsOH (p-toluene sulfonic acid) and 30 mM BNAH (1-benzyl-1,4-dihydronicotinamide) results in the production of H2 with a turnover number (TON) of 16 over 24 h. The axial capping of the Rh2(II,II) bimetallic core with the bpnp ligand prevents the formation of an Rh-H hydride intermediate. These results show that the observed photocatalytic reactivity is localized on the bncn ligand, representing the first example of ligand-centered H2 production.

Highly efficient light-driven hydrogen evolution utilizing porphyrin-based nanoparticles

We developed dye-sensitized photocatalytic systems (DSPs) by utilizing porphyrins as a photosensitizer (PS) or as a photosensitizer-catalyst (PS/CAT) upon their chemisorption onto platinum-doped titanium dioxide nanoparticles (Pt-TiO2 NPs). The DSPs coated with Pt-Tc3CP (PS/CAT entity) exhibited a record-high stability (25 500 TONs) and H2 evolution activity (707 mmol g-1 h-1) compared to similar DSPs in the literature.

Ultrafine Vacancy-Rich Nb2O5 Semiconductors Confined in Carbon Nanosheets Boost Dielectric Polarization for High-Attenuation Microwave Absorption

The integration of nano-semiconductors into electromagnetic wave absorption materials is a highly desirable strategy for intensifying dielectric polarization loss; achieving high-attenuation microwave absorption and realizing in-depth comprehension of dielectric loss mechanisms remain challenges. Herein, ultrafine oxygen vacancy-rich Nb2O5 semiconductors are confined in carbon nanosheets (ov-Nb2O5/CNS) to boost dielectric polarization and achieve high attenuation. The polarization relaxation, electromagnetic response, and impedance matching of the ov-Nb2O5/CNS are significantly facilitated by the Nb2O5 semiconductors with rich oxygen vacancies, which consequently realizes an extremely high attenuation performance of - 80.8 dB (> 99.999999% wave absorption) at 2.76 mm. As a dielectric polarization center, abundant Nb2O5-carbon heterointerfaces can intensify interfacial polarization loss to strengthen dielectric polarization, and the presence of oxygen vacancies endows Nb2O5 semiconductors with abundant charge separation sites to reinforce electric dipole polarization. Moreover, the three-dimensional reconstruction of the absorber using microcomputer tomography technology provides insight into the intensification of the unique lamellar morphology regarding multiple reflection and scattering dissipation characteristics. Additionally, ov-Nb2O5/CNS demonstrates excellent application potential by curing into a microwave-absorbing, machinable, and heat-dissipating plate. This work provides insight into the dielectric polarization loss mechanisms of nano-semiconductor/carbon composites and inspires the design of high-performance microwave absorption materials.

Impact of the bromination of carbazole-based D-π-A organic dyes on their optical and electrochemical properties and visible-light-driven hydrogen evolution

Brominated dyes, 2C-n (n = 1-5), 3C-4 and 4C-4, were prepared through bromination of three carbazole-based D-π-A dyes, 2C, 3C and 4C with N-bromosuccinimide (NBS). The detailed structures of the brominated dyes were confirmed by ¹H NMR spectroscopy and mass spectrometry (MS). The introduction of the Br atom on the 1,8-positon of carbazole moieties led to blueshifted UV-vis and photoluminescence (PL) spectra, increased initial oxidation potentials and enlarged dihedral angles, indicating bromination enhanced non-planarity of the dye molecules. In the hydrogen production experiments, with the increase of the Br content in brominated dyes, the photocatalytic activity increased continuously (except 2C-1). The dye-sensitized Pt/TiO_2, 2C-4@T, 3C-4@T and 4C-4@T, exhibited high hydrogen production efficiencies of 655.4, 877.9 and 905.6 μmol h^-1 g^-1, respectively, which were 4-6-fold higher than those of 2C@T, 3C@T and 4C@T. The enhanced performance of photocatalytic hydrogen evolution was attributed to decreased dye aggregation resulting from the highly non-planar molecular structures of the brominated dyes.

Preparation and modification methods of defective titanium dioxide-based nanoparticles for photocatalytic wastewater treatment-a comprehensive review

The rapid population growth and industrial expansion worldwide have created serious water contamination concerns. To curb the pollution issue, it has become imperative to use a versatile material for the treatment. Titanium dioxide (TiO₂) has been recognized as the most-studied nanoparticle in various fields of science and engineering due to its availability, low cost, efficiency, and other fascinating properties with a wide range of applications in modern technology. Recent studies revealed the photocatalytic activity of the material for the treatment of industrial effluents to promote environmental sustainability. With the wide band gap energy of 3.2 eV, TiO₂ can be activated under UV light; thus, many strategies have been proposed to extend its photoabsorption to the visible light region. In what follows, this has generated increasing attention to study its characteristics and structural modifications in different forms for photocatalytic applications. The present review provides an insight into the understanding of the synthesis methods of TiO₂, the current progress in the treatment techniques for the degradation of wide environmental pollutants employing modified TiO₂ nanoparticles, and the factors affecting its photocatalytic activities. Further, recent developments in using titania for practical applications, the approach for designing novel nanomaterials, and the prospects and opportunities in this exciting area have been discussed.

Porphyrins and phthalocyanines as biomimetic tools for photocatalytic H2 production and CO2 reduction

The increasing energy demand and environmental issues caused by the over-exploitation of fossil fuels render the need for renewable, clean, and environmentally benign energy sources unquestionably urgent. The zero-emission energy carrier, H₂ is an ideal alternative to carbon-based fuels especially when it is generated photocatalytically from water. Additionally, the photocatalytic conversion of CO₂ into chemical fuels can reduce the CO₂ emissions and have a positive environmental and economic impact. Inspired by natural photosynthesis, plenty of artificial photocatalytic schemes based on porphyrinoids have been investigated. This review covers the recent advances in photocatalytic H₂ production and CO₂ reduction systems containing porphyrin or phthalocyanine derivatives. The unique properties of porphyrinoids enable their utilization both as chromophores and as catalysts. The homogeneous photocatalytic systems are initially described, presenting the various approaches for the improvement of photosensitizing activity and the enhancement of catalytic performance at the molecular level. On the other hand, for the development of the heterogeneous systems, numerous methods were employed such as self-assembled supramolecular porphyrinoid nanostructures, construction of organic frameworks, combination with 2D materials and adsorption onto semiconductors. The dye sensitization on semiconductors opened the way for molecular-based dye-sensitized photoelectrochemical cells (DSPECs) devices based on porphyrins and phthalocyanines. The research in photocatalytic systems as discussed herein remains challenging since there are still many limitations making them unfeasible to be used at a large scale application before finding a large-scale application.

Homogeneous Carbon/Potassium-Incorporation Strategy for Synthesizing Red Polymeric Carbon Nitride Capable of Near-Infrared Photocatalytic H2 Production

The efficient utilization of near-infrared (NIR) light for photocatalytic hydrogen generation is vitally important to both solar hydrogen energy and hydrogen medicine, but remains a challenge at present, owing to the strict requirement of the semiconductor for high NIR responsiveness, narrow bandgap, and suitable redox potentials. Here, an NIR-active carbon/potassium-doped red polymeric carbon nitride (RPCN) is achieved for by using a similar-structure dopant as the melamine (C₃ H₆ N₆ ) precursor with the solid KCl. The homogeneous and high incorporation of carbon and potassium remarkably narrows the bandgap of carbon nitride (1.7 eV) and endows RPCN with a high NIR-photocatalytic activity for H₂ evolution from water at the rate of 140 µmol h^-1 g^-1 under NIR irradiation (700 nm ≤ λ ≤ 780 nm), and the apparent quantum efficiency is high as 0.84% at 700 ± 10 nm (and 13% at 500 ± 10 nm). A proof-of-concept experiment on a tumor-bearing mouse model verifies RPCN as being capable of intratumoral NIR-photocatalytic hydrogen generation and simultaneous glutathione deprivation for safe and high-efficacy drug-free cancer therapy. The results shed light on designing efficient photocatalysts to capture the full spectrum of solar energy, and also pioneer a new pathway to develop NIR photocatalysts for hydrogen therapy of major diseases.

Imidazole substituted Zinc(II) phthalocyanines for co-catalyst-free photoelectrochemical and photocatalytic hydrogen evolution: influence of the anchoring group

Novel zinc phthalocyanine derivatives, ZnPc-1 and ZnPc-2, consisting of one and four imidazole units, respectively, have been synthesized and utilized as panchromatic photosensitizers for photocatalytic and photoelectrochemical H₂ evolution. The effect of the imidazole-anchoring group on the photocatalytic H₂ production has been compared with ZnPc-3, which possesses a carboxylic acid unit as the anchoring group. ZnPc-1/TiO₂ shows the best photoactivity with the highest H₂ evolution rate of 0.4006 mmol g^-1 h^-1, which is much higher than that of ZnPc-2/TiO₂ and ZnPc-3/TiO₂ (0.3319 mmol g^-1 h^-1 and 0.3555 mmol g^-1 h^-1, respectively). After 20 h of irradiation, ZnPc-1 produces an H₂ production rate of 3.4187 mmol g^-1 with a turnover number (TON) of 14863 and a solar-to-hydrogen energy (STH) conversion efficiency of 1.03%, without using a co-catalyst.

Assembly of a Titanium-Oxo Cluster and a Bismuth Iodide Cluster, a Single-Source Precursor of a p-n-Type Photocatalyst

Titanium oxides and bismuth halides or oxyhalides have been known to be excellent semiconductors with both excellent photocatalytic and photoelectric properties. The design of supersalts assembled by titanium-oxo clusters (TOCs) and bismuth iodide clusters is a hopeful strategy for exploring the chemistry and application of new titanium-oxo clusters. We report herein a series of unusual ionic TOCs with Ti₁₂ oxo cluster cations and bismuth iodide anions, [Ti₁₂O₁₅(OⁱPr)₁₇]₃[Bi₃I₁₂] (Bi₃), [Ti₁₂O₁₄(OⁱPr)₁₈][Bi₄I₁₄(THF)₂] (Bi₄), and [Ti₁₂O₁₄(OⁱPr)₁₈][Ti₁₁BiO₁₄(OⁱPr)₁₇][Bi₆I₂₂] (Bi₆). Single-crystal X-ray analysis revealed that the type and charge of the Ti₁₂ clusters varied with the charges of different bismuth iodide clusters. Taking advantage of the easy hydrolysis of the TOCs and BiI clusters in water, we used these supersalt crystals as single-source precursors to prepare a p-n-type BiOI-TiO photocatalyst. The heterojunction materials were carefully characterized by powder X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, etc. The synergistic effect of the two components of BiOI and TiO on the photocatalytic degradation of RhB in water is demonstrated. This is a very convenient method for obtaining a p-n-type BiOI-TiO heterojuction photocatalyst by just placing the ground TOC crystals into water.

Charge Carrier Processes and Optical Properties in TiO2 and TiO2-Based Heterojunction Photocatalysts: A Review

Photocatalysis based technologies have a key role in addressing important challenges of the ecological transition, such as environment remediation and conversion of renewable energies. Photocatalysts can in fact be used in hydrogen (H₂) production (e.g., via water splitting or photo-reforming of organic substrates), CO₂ reduction, pollution mitigation and water or air remediation via oxidation (photodegradation) of pollutants. Titanium dioxide (TiO₂) is a “benchmark” photocatalyst, thanks to many favorable characteristics. We here review the basic knowledge on the charge carrier processes that define the optical and photophysical properties of intrinsic TiO₂. We describe the main characteristics and advantages of TiO₂ as photocatalyst, followed by a summary of historical facts about its application. Next, the dynamics of photogenerated electrons and holes is reviewed, including energy levels and trapping states, charge separation and charge recombination. A section on optical absorption and optical properties follows, including a discussion on TiO₂ photoluminescence and on the effect of molecular oxygen (O₂) on radiative recombination. We next summarize the elementary photocatalytic processes in aqueous solution, including the photogeneration of reactive oxygen species (ROS) and the hydrogen evolution reaction. We pinpoint the TiO₂ limitations and possible ways to overcome them by discussing some of the “hottest” research trends toward solar hydrogen production, which are classified in two categories: (1) approaches based on the use of engineered TiO₂ without any cocatalysts. Discussed topics are highly-reduced “black TiO₂”, grey and colored TiO₂, surface-engineered anatase nanocrystals; (2) strategies based on heterojunction photocatalysts, where TiO₂ is electronically coupled with a different material acting as cocatalyst or as sensitizer. Examples discussed include TiO₂ composites or heterostructures with metals (e.g., Pt-TiO₂, Au-TiO₂), with other metal oxides (e.g., Cu₂O, NiO, etc.), direct Z-scheme heterojunctions with g-C₃N₄ (graphitic carbon nitride) and dye-sensitized TiO₂.

Dye-catalyst dyads for photoelectrochemical water oxidation based on metal-free sensitizers

Dye-Sensitized Photoelectrochemical Cells (DS-PECs) have been emerging as promising devices for efficient solar-induced water splitting. In DS-PECs, dyes and catalysts for water oxidation and/or reduction are typically two separate components, thus limiting charge transfer efficiency. A small number of organometallic dyes have been integrated with a catalyst to form an integrated dye–catalyst dyad for photoanodes, but until now no dyads based on metal-free organic dyes have been reported for photoanodes. We herein report the first example of dyad-sensitized photoanodes in DS-PEC water splitting based on metal-free organic dyes and a Ru catalyst. The di-branched donor–π–acceptor dyes carry a donor carbazole moiety which has been functionalized with two different terminal pyridyl ligands in order to coordinate a benchmark Ru complex as a water oxidation catalyst, affording water oxidation dyads. The two dyads have been fully characterized in their optical and electrochemical properties, and XPS has been used to confirm the presence of the catalyst bonded to the dye anchored to the semiconductor anode. The two dyads have been investigated in DS-PEC, showing an excellent faradaic efficiency (88% average across all cells, with a best cell efficiency of 95%), thus triggering new perspectives for the design of efficient molecular dyads based on metal-free dyes for DS-PEC water splitting.

Dye–catalyst dyads based on metal-free dyes were prepared for dye-sensitized photoanodes in photoelectrochemical water splitting, showing a top ranked faradaic efficiency for O₂ generation up to 95%.

Visible-Light Photocatalysts and Their Perspectives for Building Photocatalytic Membrane Reactors for Various Liquid Phase Chemical Conversions

Photocatalytic organic synthesis/conversions and water treatment under visible light are a challenging task to use renewable energy in chemical transformations. In this review a brief overview on the mainly employed visible light photocatalysts and a discussion on the problems and advantages of Vis-light versus UV-light irradiation is reported. Visible light photocatalysts in the photocatalytic conversion of CO2, conversion of acetophenone to phenylethanol, hydrogenation of nitro compounds, oxidation of cyclohexane, synthesis of vanillin and phenol, as well as hydrogen production and water treatment are discussed. Some applications of these photocatalysts in photocatalytic membrane reactors (PMRs) for carrying out organic synthesis, conversion and/or degradation of organic pollutants are reported. The described cases show that PMRs represent a promising green technology that could shift on applications of industrial interest using visible light (from Sun) active photocatalysts.