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

   https://doi.org/10.1073/pnas.2414203121  

   Dai, Zhenbang; Giustino, Feliciano (November 2024, Proceedings of the National Academy of Sciences)

   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 TiO₂via 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 TiO₂and pave the way for systematically probing and manipulating polarons in a broad class of complex oxides and quantum materials. 

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2. Phase transition of individual anatase TiO ₂ microcrystals with large percentage of (001) facets: a Raman mapping and SEM study

   https://doi.org/10.1039/d2cp04882e  

   Lu, Weigang; Zhu, Hao; Birmingham, Blake; Craft, Nolan; Hu, Jonathan; Park, Kenneth; Zhang, Zhenrong (January 2023, Physical Chemistry Chemical Physics)

   TiO 2 has been extensively studied in many fields including photocatalysis, electrochemistry, optics, etc. Understanding the mechanism of the anatase–rutile phase transition (ART) process is critical for the design of TiO 2 -based high-activity photocatalysts and tuning its properties for other applications. In this work, the ART process using individual anatase micro-particles with a large percentage of (001) facets was monitored and studied. Phase concentration evolution obtained via Raman microscopy was correlated with the morphological evolution observed in scanning electron microscope (SEM) images. The ART of anatase microcrystals is dominated by surface nucleation and growth, but the ART processes of individual anatase particles are distinctive and depend on the various rutile nucleation sites. Two types of transformation pathways are observed. In one type of ART pathway, the rutile phase nucleated at a corner of an anatase microcrystal and grew in one direction along the edge of the crystal firstly followed by propagation over the rest of the microcrystal in the orthogonal direction on the surface and to the bulk of the crystal. The kinetics of the ART follows the first-order model with two distinct rate constants. The fast reaction rate is from the surface nucleation and growth, and the slow rate is from the bulk nucleation and growth. In the other type of ART pathway, multiple rutile nucleation sites formed simultaneously on different edges and corners of the microcrystal. The rutile phase spread over the whole crystal from these nucleation sites with a small contribution of bulk nucleation. Our study on the ART of individual micro-sized crystals bridges the material gap between bulk crystals and nano-sized TiO 2 particles. The anatase/rutile co-existing particle will provide a perfect platform to study the synergistic effect between the anatase phase and the rutile phase in their catalytic performances. 

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3. Iodine binding with thiophene and furan based dyes for DSCs

   https://doi.org/10.1039/c8cp03065k  

   Baumann, Alexandra; Cheema, Hammad; Sabuj, Md Abdus; McNamara, Louis E.; Zhang, Yanbing; Peddapuram, Adithya; Nguyen, Suong T.; Watkins, Davita L.; Hammer, Nathan I.; Rai, Neeraj; et al (January 2018, Physical Chemistry Chemical Physics)

   Iodine binding to thiophene rings in dyes for dye-sensitized solar cells (DSCs) has been hypothesized to be performance degrading in a number of literature cases. Binding of iodine to dyes near the semiconductor surface can promote undesirable electron transfers and lower the overall efficiency of devices. Six thiophene or furan containing dye analogs were synthesized to analyze iodine binding to the dyes via Raman spectroscopy, UV-Vis studies, device performance metrics and density functional theory (DFT) based computations. Evidence suggests I 2 binds thiophene-based dyes stronger than furan-based dyes. This leads to higher DSC device currents and voltages from furan analogues, and longer electron lifetimes in DSC devices using furan based dyes. Raman spectrum of the TiO 2 surface-bound dyes reveals additional and more instense peaks for thiophene dyes in the presence of I 2 relative to no I 2 . Additionally, broader and shifted UV-Vis peaks are observed for thiophene dyes in the presence of I 2 on TiO 2 films suggesting significant interaction between the dye molecules and I 2 . These observations are also supported by DFT and TD-DFT calculations which indicate the absence of a key geometric energy minimum in the dye–I 2 ground state for furan dyes which are readily observed for the thiophene based analogues. 

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4. Integrating density functional theory with machine learning for enhanced band gap prediction in metal oxides

   https://doi.org/10.1039/D4CP03397C  

   Ezeakunne, Chidozie; Lamichhane, Bipin; Kattel, Shyam (March 2025, Physical Chemistry Chemical Physics)

   In this study, we used a combination of density functional theory with Hubbard U correction (DFT+U) and machine learning (ML) to accurately predict the band gaps and lattice parameters of metal oxides: TiO2 (rutile and anatase), cubic ZnO, cubic ZnO2, cubic CeO2, and cubic ZrO2. Our results show that including Up values for oxygen 2p orbitals alongside Ud/f for metal 3d or 4f orbitals significantly enhances the accuracy of these predictions. Through extensive DFT+U calculations, we identify optimal (Up, Ud/f) integer pairs that closely reproduce experimentally measured band gaps and lattice parameters for each oxide: (8 eV, 8 eV) for rutile TiO2; (3 eV, 6 eV) for anatase TiO2; (6 eV, 12 eV) for c-ZnO; (10 eV, 10 eV) for c-ZnO2; (9 eV, 5 eV) for c-ZrO2; and (7 eV, 12 eV) for c-CeO2. Our ML analysis showed that simple supervised ML models can closely reproduce these DFT+U results at a fraction of the computational cost and generalize well to related polymorphs. Our approach builds on existing high-throughput DFT+U frameworks by providing fast pre-DFT estimates of structural properties and band gaps. Since this work does not aim to improve the underlying DFT+U method, the ML model shares its limitations. We also note that the reported values of Up strongly depend on the choice of correlated orbitals, and caution is recommended with a different choice of correlated orbitals. 

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5. Hydrothermal synthesis of carbon nanotube–titania composites for enhanced photocatalytic performance

   https://doi.org/10.1557/jmr.2020.97  

   Everhart, Brian M.; Baker-Fales, Montgomery; McAuley, Bailey; Banning, Eric; Almkhelfe, Haider; Back, Tyson C.; Amama, Placidus B. (June 2020, Journal of Materials Research)

   null (Ed.)

   Nanosized, well-dispersed titania particles were synthesized via a hydrothermal method using multiwalled carbon nanotubes (MWCNTs) as structural modifiers during the nucleation process to decrease aggregation. Synthesized TiO 2 /MWCNT composites containing different amounts of MWCNTs were characterized using N 2 physisorption, XRD, spectroscopic techniques (Raman, UV-visible, and X-ray photoelectron), and electron microscopy to illuminate the morphology, crystal structure, and surface chemistry of the composites. Photocatalytic performance was evaluated by measuring the degradation of acetaldehyde in a batch reactor under UV illumination. Average rate constants decrease in the following order: TiO 2 /MWCNT-1% > TiO 2 > TiO 2 /MWCNT-5%. Addition of MWCNTs beyond the optimum loading ratio of 1:100 (MWCNT:TiO 2 ) diminishes the effectiveness of the photocatalyst and the synergistic effect between MWCNTs and TiO 2 . The primary mechanism for photocatalytic activity enhancement in TiO 2 /MWCNT-1% is thought to be due to increased porosity, hydroxyl enrichment on the surface, and high dispersion of TiO 2 particles. 

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