In situ EPR studies of electron trapping in a nanocrystalline rutile

A Unique Descriptor for Electron Polaron Migration: Larmor Radius

Polarons in metal oxides are in localized charge carriers that significantly influence material properties. The critical role of polarons in photocatalytic processes arises from their spatial distribution and dynamic properties. In this study, we propose a physically meaningful descriptor based on the potential of charged particles in a polarization field to quantify the relative stability of polaron configurations during migration. Taking a widely studied Rutile-phase TiO2 as a typical model, we focused on electron addition, where Ti4+ centers are reduced to Ti3+ centers, leading to the formation of small polarons. Using the Rutile (110) surface, we employed a constrained density functional theory (CDFT) hybrid with a projection-operator diabatization (POD) method to investigate the migration barriers between different polaron configurations induced by oxygen vacancies. Our results demonstrate the accuracy and robustness of the proposed descriptor, establishing its potential for broader applications in understanding polaron behavior in transition metal oxides.

Do Small Hole Polarons Form in Bulk Rutile TiO2?

Hole transport and localization through small polarons are essential to the performance of TiO2 in photocatalysis applications. The existence of small hole polarons in bulk rutile TiO2 has been, however, controversial with contradicting evidence from theory and experiments. Here, we use first-principles computations and more specifically a Koopmans' compliant hybrid functional and charge correction to study small hole polarons in bulk rutile. We find that a fraction of exchange exists, satisfying Koopmans' compliance for the polaron state and reproducing the band gap provided that charge correction is used. We clearly show that first-principles computations indicate that the small hole polaron is only slightly stable, with a formation energy tens of millielectronvolts below that of the delocalized state in bulk for rutile and more stable for anatase TiO2.

Identification of large polarons and exciton polarons in rutile and anatase polymorphs of titanium dioxide

Titanium dioxide (TiO2) is a wide-gap semiconductor with numerous applications in photocatalysis, photovoltaics, and neuromorphic computing. The unique functional properties of this material critically depend on its ability to transport charge in the form of polarons, namely narrow electron wavepackets accompanied by local distortions of the crystal lattice. It is currently well established that the most important polymorphs of TiO2, the rutile and anatase phases, harbor small electron polarons and small hole polarons, respectively. However, whether additional polaronic species exist in TiO2, and under which conditions, remain open questions. Here, we provide definitive answers to these questions by exploring the rich landscape of polaron quasiparticles in TiO2 via recently developed ab initio techniques. In addition to the already known small polarons, we identify three species, namely a large hole polaron in rutile, a large quasi-two-dimensional electron polaron in anatase, and a large exciton polaron in anatase. These findings complete the puzzle on the polaron physics of TiO2 and pave the way for systematically probing and manipulating polarons in a broad class of complex oxides and quantum materials.

The Role of Water Molecules on Polaron Behavior at Rutile (110) Surface: A Constrained Density Functional Theory Study

Understanding the behavior of a polaron in contact with water is of significant importance for many photocatalytic applications. We investigated the influence of water on the localization and transport properties of polarons at the rutile (110) surface by constrained density functional theory. An excess electron at a dry surface favors the formation of a small polaron at the subsurface Ti site, with a preferred transport direction along the [001] axis. As the surface is covered by water, the preferred spatial localization of the polarons is moved from the subsurface to the surface. When the water coverage exceeds half a monolayer, the preferred direction of polaron hopping is changed to the [110] direction toward the surface. This characteristic behavior is related to the Ti3d-orbital occupations and crystal field splitting induced by different distorted structures under water coverage. Our work describes the reduced sites that might eventually play a role in photocatalysis for rutile (110) surfaces in a water environment.

Redox mediators boost NOx reduction via trade-off electron charges using a cube-shaped (cMn@rGO) catalyst; mechanism and electrochemical study

Gaseous pollutants like sulfur dioxide and nitrogen oxide(s) (SO2, NOx) have been increasing exponentially for the last two decades, which have had adverse effects on human health, aquatic life, and the environment. Recently, for air pollution taming, manganese/oxide (Mn/MnO) has become a very promising heterogeneous catalyst due to its environment-friendly, low-price, and remarkable catalytic abilities for toxic gases. In this work, cube-shaped Mn nanoparticles (cMn NPs) were decorated on the surface of reduced graphene oxide (rGO) by the solvothermal method. The resulting cMn@rGO composite was employed for electrochemical NOx reduction. However, the microscopic (TEM/HRTEM) and structural analysis were utilised to investigate the morphology and characteristics of the cMn@rGO composite. This electrochemical-based treatment for NOx reduction is employed by using electron shuttle or redox mediators. Here, four distinct redox mediators are used to address electrochemical obstacles, which effectively facilitate electron transportation and promoted NOx reduction on the electrode surface. These mediators not only significantly enhanced the NOx conversion into valuable products, i.e., N2 and N2O, but also made the process smooth with high performance. Among these mediators, neutral red (N.R) exhibited extraordinary potential in enhancing NOx reduction. The obtained results indicated that the remarkable catalytic performance (∼93%) of the cMn@rGO can be attributed to several factors, including the catalyst's three-dimensional architecture structure and abundant active sites. The designed catalyst (cMn@rGO) is not only cost-effective and sustainable but also exhibits excellent potential in effectively reducing NOx, which could be beneficial for large-scale NOx abatement.

Multimodal Biofilm Inactivation Using a Photocatalytic Bismuth Perovskite-TiO2-Ru(II)polypyridyl-Based Multisite Heterojunction

Infectious bacterial biofilms are recalcitrant to most antibiotics compared to their planktonic version, and the lack of appropriate therapeutic strategies for mitigating them poses a serious threat to clinical treatment. A ternary heterojunction material derived from a Bi-based perovskite-TiO₂ hybrid and a [Ru(2,2'-bpy)₂(4,4'-dicarboxy-2,2'-bpy)]²⁺ (2,2'-bpy, 2,2'-bipyridyl) as a photosensitizer (RuPS) is developed. This hybrid material is found to be capable of generating reactive oxygen species (ROS)/reactive nitrogen species (RNS) upon solar light irradiation. The aligned band edges and effective exciton dynamics between multisite heterojunctions are established by steady-state/time-resolved optical and other spectroscopic studies. Proposed mechanistic pathways for the photocatalytic generation of ROS/RNS are rationalized based on a cascade-redox processes arising from three catalytic centers. These ROS/RNS are utilized to demonstrate a proof-of-concept in treating two elusive bacterial biofilms while maintaining a high level of biocompatibility (IC₅₀ > 1 mg/mL). The in situ generation of radical species (ROS/RNS) upon photoirradiation is established with EPR spectroscopic measurements and colorimetric assays. Experimental results showed improved efficacy toward biofilm inactivation of the ternary heterojunction material as compared to their individual/binary counterparts under solar light irradiation. The multisite heterojunction formation helped with better exciton delocalization for an efficient catalytic biofilm inactivation. This was rationalized based on the favorable exciton dissociation followed by the onset of multiple oxidation and reduction sites in the ternary heterojunction. This together with exceptional photoelectric features of lead-free halide perovskites outlines a proof-of-principle demonstration in biomedical optoelectronics addressing multimodal antibiofilm/antimicrobial modality.

Wavelength Dependence of the Transformation Mechanism of Sulfonamides Using Different LED Light Sources and TiO2 and ZnO Photocatalysts

The comparison of the efficiency of the commercially available photocatalysts, TiO2 and ZnO, irradiated with 365 nm and 398 nm light, is presented for the removal of two antibiotics, sulfamethazine (SMT) and sulfamethoxypyridazine (SMP). The •OH formation rate was compared using coumarin, and higher efficiency was proved for TiO2 than ZnO, while for 1,4-benzoquinone in O2-free suspensions, the higher contribution of the photogenerated electrons to the conversion was observed for ZnO than TiO2, especially at 398 nm irradiation. An extremely fast transformation and high quantum yield of SMP in the TiO2/LED398nm process were observed. The transformation was fast in both O2 containing and O2-free suspensions and takes place via desulfonation, while in other cases, mainly hydroxylated products form. The effect of reaction parameters (methanol, dissolved O2 content, HCO3- and Cl-) confirmed that a quite rarely observed energy transfer between the excited state P25 and SMP might be responsible for this unique behavior. In our opinion, these results highlight that "non-conventional" mechanisms could occur even in the case of the well-known TiO2 photocatalyst, and the effect of wavelength is also worth investigating.

Intrinsic charge trapping in amorphous oxide films: status and challenges

We review the current understanding of intrinsic electron and hole trapping in insulating amorphous oxide films on semiconductor and metal substrates. The experimental and theoretical evidences are provided for the existence of intrinsic deep electron and hole trap states stemming from the disorder of amorphous metal oxide networks. We start from presenting the results for amorphous (a) HfO₂, chosen due to the availability of highest purity amorphous films, which is vital for studying their intrinsic electronic properties. Exhaustive photo-depopulation spectroscopy measurements and theoretical calculations using density functional theory shed light on the atomic nature of electronic gap states responsible for deep electron trapping observed in a-HfO₂. We review theoretical methods used for creating models of amorphous structures and electronic structure calculations of amorphous oxides and outline some of the challenges in modeling defects in amorphous materials. We then discuss theoretical models of electron polarons and bi-polarons in a-HfO₂ and demonstrate that these intrinsic states originate from low-coordinated ions and elongated metal-oxygen bonds in the amorphous oxide network. Similarly, holes can be captured at under-coordinated O sites. We then discuss electron and hole trapping in other amorphous oxides, such as a-SiO₂, a-Al₂O₃, a-TiO₂. We propose that the presence of low-coordinated ions in amorphous oxides with electron states of significant p and d character near the conduction band minimum can lead to electron trapping and that deep hole trapping should be common to all amorphous oxides. Finally, we demonstrate that bi-electron trapping in a-HfO₂ and a-SiO₂ weakens Hf(Si)-O bonds and significantly reduces barriers for forming Frenkel defects, neutral O vacancies and O^2- ions in these materials. These results should be useful for better understanding of electronic properties and structural evolution of thin amorphous films under carrier injection conditions.

On the redox mechanism operating along C2H2 self-assembly at the surface of TiO2

The interaction of acetylene with the TiO2 surface at room temperature entails a complex set of self-assembly reactions with the formation of products having relatively high molecular weight. In a previous paper by some of us (Jain, S. M.; et al. J. Mater. Chem. A 2014, 2, 12247-12254), the C2H2-TiO2 reaction has been monitored, essentially by Fourier transform infrared spectroscopy, at the surface of P25 (a mixture of anatase and rutile, typical benchmark material in the field of photocatalysis) in order to elucidate the nature of the products of this surface reaction. In the present paper, the same process was followed, for the first time, using electron paramagnetic resonance (EPR) and monitoring by the thermogravimetric analysis the weight loss of the material upon heating in order to further investigate the complex mechanism of the surface reaction. This was done using pure anatase and comparing the EPR results with those concerning both rutile and P25. The self-assembly mechanism occurring at the interface is accompanied by the formation of EPR visible Ti(3+) centers due to electrons injection in the TiO2 substrate. This finding clarifies that at least one of the reaction channels of this complex process (namely, the formation of polycyclic aromatic hydrocarbons) is based on the heterolytic dissociative chemisorption of acetylene, followed by a redox interaction between the adsorbate and the solid, which allows the creation of the building blocks necessary to assemble polyaromatic molecules.

Magnetic resonance study of annealed and rinsed N-doped TiO2 nanoparticles

Nanoparticles of nitrogen-modified TiO2 (N-doped TiO2) calcined at 300°C and 350°C, have been prepared with and without water rinsing. Samples were characterized by x-ray diffractrometry (XRD) and optical spectroscopy. The electron paramagnetic resonance (EPR) spectra from centers involving oxygen vacancies were recorded for all samples. These could be attributed to paramagnetic surface centers of the hole type, for example to paramagnetic oxygen radicals O−, O2−etc. The concentration of these centers increased after water rising and it further increased for samples annealed at higher temperature. Additionally, for samples calcined at 300°C, and calcined at 350°C and rinsed, the EPR spectra evidenced the presence of magnetic clusters of Ti3+ ions. The photocatalytic activity of samples was studied towards phenol decomposition under unltraviolet-visible (UV-Vis) irradiation. It was found that, in comparison to the starting materials, the rinsed materials showed increased photocatalytic activity towards phenol oxidation. The light absorption (UV-Vis/DRS) as well as surface Fourier transform infrared/diffuse reflectance spectroscopy (FTIR/DR) studies confirmed a significantly enhanced light absorption and the presence of nitrogen groups on the photocatalysts surfaces, respectively. A significant increase of concentration of paramagnetic centers connected with oxygen vacancies after water rising has had an essential influence on increasing their photocatalytic activity.

Particle Networks from Powder Mixtures: Generation of TiO(2)-SnO(2) Heterojunctions via Surface Charge-Induced Heteroaggregation

We explored the impact of interfacial property changes on aggregation behavior and photoinduced charge separation in mixed metal oxide nanoparticle ensembles. TiO(2) and SnO(2) nanoparticles were synthesized by metal organic chemical vapor synthesis and subsequently transformed into aqueous colloidal dispersions using formic acid for adjustment of the particles' surface charge. Surface charge-induced heteroaggregation was found to yield blended nanoparticle systems of exceptionally high mixing quality and, after vacuum annealing, to extremely high concentrations of heterojunctions between TiO(2) and SnO(2) nanoparticles with dehydroxylated surfaces. For tracking charge transfer processes across heterojunctions, the photogeneration of trapped charge carriers was measured with electron paramagnetic resonance (EPR) spectroscopy. On blended nanoparticles systems with high concentrations of SnO(2)-TiO(2) heterojunctions, we observed an enhanced cross section for interparticular charge separation. This results from an effective interfacial charge transfer across the interfaces and gives rise to substantially increased concentrations of electrons and hole centers. The here presented insights are key to the rational design of particle-based heterojunctions and mesoporous nanoparticle networks and help to engineer composite nanomaterials for photocatalysis and solar energy conversion.