Search query: Electronic gap reduction in co-doped (Cu-N) anatase TiO 2 semiconductor: A DFT study

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abstract: TiO2 doped with metal atoms shows better photoconversion efficiency compared with undoped TiO2. In this work, we have calculated the electronic structure of monodoped and co doped anatase TiO2. The calculations were carried out using Quantum ESPRESSO package which implements density functional theory and pseudopotential method. Our results show that band gap reduction in doped anatase titania and it is compared with the previous work. The chosen GGA and ultrasoft pseudopotential provides better band gap reduction. In dopant case, Cu creates acceptor levels just above the valence band and shifts conduction band to 1.9eV from 2.05eV and N substitution creates donor level just below to the conduction band. The PDOS explains the impurity states created by dopants.
author_names: Arun Suresh

N Soundararajan

A B M Ahmed
title: Electronic gap reduction in co doped (Cu N) anatase TiO 2 semiconductor: A DFT study
year: 2014

abstract: Abstract Substitutionally and interstitially Cu/N co doped anatase TiO 2 (101) surface is investigated by using density functional theory (DFT) calculations. The results suggest improved visible light photocatalytic activity over undoped anatase TiO 2 Sizable lattice relaxation around the dopants is observed, followed by a formation of N O bond. Depending on the local arrangement of atoms, localized states above the valence band maximum, deep into the band gap, and below the conduction band minimum are found. In addition, our calculation also predict band gap narrowing. The hybridization of the Cu 3d and N 2p states within the band gap and the other electronic and optical properties suggest a synergistic effect of the dopants in the enhancement of the visible light absorption on the (101) anatase surface.
author_names: Vasil Koteski

Jelena Belosevic-Cavor

Ana Umicevic

Valentin N Ivanovski

Dragan Toprek
title: Improving the photocatalytic properties of anatase TiO 2 (101) surface by co doping with Cu and N: Ab initio study
year: 2017

abstract: Abstract First principles calculations are performed to study the electronic and optical properties of Cu doped, N doped and (Cu+2N) co doped anatase TiO 2 Strong hybridization between Cu 3 d and N 2 p orbitals above the valence band leads to the formation of an isolated intermediate band (IB) deep in the band gap of pure TiO 2 The new energetic features, predicted by DFT, have been experimentally confirmed using UV vis NIR spectroscopy. In particular, the diffuse reflectance spectrum of the co doped TiO 2 sample shows the presence of two edges which confirm the existence of IB in the band gap. This IB in the band gap of TiO 2 is responsible for high visible light absorption through a two step optical transition between valence and conduction bands via the IB. In mono doped samples, only a reduction of the band gap is observed which is consistent with the first principles calculations. X ray photoelectron spectroscopy of mono and co doped TiO 2 samples establishes the chemical states of several atomic elements and especially clarifies the key presence of O vacancies leading to a new position of conduction band minima. The presence of broad IB and the absence of dopant energy levels close to the conduction band minimum in (Cu+2N) co doped TiO 2 qualify it to be an efficient material for photovoltaic conversion, photocatalytic water splitting, and photocatalytic degradation of pollutants.
author_names: Alpa Dashora

Nainesh Patel

Dushyant C Kothari

Babu Lal Ahuja

Antonio Miotello
title: Formation of an intermediate band in the energy gap of TiO2 by Cu N codoping: First principles study and experimental evidence
year: 2014

abstract: Abstract Reducing the band gap of TiO 2 has a great potential in solar energy application. Among them, doping is a very effective method. We systematically study C/Ni mono doped and (Ni, C) co doped anatase TiO 2 using density functional calculations. We first optimize the geometric structures with pseudopotential plane wave approach. Then on the optimized structure, we perform band structure calculations with HSE06 hybrid functional calculation on the frame of Gaussian type of orbital. Therefore, we can obtain accurate band gaps close to experimental values if they exist at lower computation cost. Among three doped models, the Ni single doped model has the lowest impurity formation energy under reduction condition. Compared with the single doped models, (Ni, C) co doped titanium dioxide has the smallest direct band gap, and the absorption band edge shows an apparent bigger red shift. Through calculating the optical absorption spectra of several doped models in the visible light region (400 760 nm) (Ni, C) co doped model can absorb much more visible light. Thus it has a much higher photocatalytic activity.
author_names: H M Chen

Xuechao Li

Rundong Wan
title: Theoretical studies on the electronic structure and optical absorption property of (Ni, C) co doped anatase TiO2
year: 2017

abstract: Abstract We investigate the density of states and optical properties for Fe, 2B and (Fe, 2B) doped TiO 2 with DFT calculations. The calculated results reveal mono doping introduces midgap states which are half occupied and easy to become the recombination centers of charge carriers, thus inhibiting the enhancement of photocatalystic activity. The coupling of 2p 3d states in the (Fe, 2B) compensated co doped TiO 2 makes gap states couple with the valence bands edge, thus greatly causing the band gap narrowing and higher visible light absorption. Moreover, the gap states cannot become recombination centers of the photoexcited carriers, thus promoting the separation of electron hole pairs, prolonging the lifetime of carriers. The analysis of electron density indicates more electrons from Fe transfer to adjacent B, realizing the charge compensation and forming a stronger Fe B bond. Therefore, the (Fe, 2B) compensated co doped TiO 2 exhibits the higher visible light photocatalystic activity than those of pure and solely doped TiO 2
author_names: Xuechao Li

Jianhao Shi

Hao Chen

Rundong Wan

Chongyang Leng

Ying Lei
title: Electronic and optical properties study on FeB co doped anatase TiO2
year: 2016

abstract: Abstract The present investigation focuses on the structural properties and magnetic response of Co, Mn and Co Mn doped TiO 2 nanoparticles (within 3% dilute doping) along with their optical and electrical studies. X ray diffraction (XRD) patterns for all these samples show the formation of tetragonal TiO 2 anatase phase with no extra impurity phases. Further, Raman bands at 143 cm 1 396 cm 1 516 cm 1 and 639 cm 1 also confirm the anatase phase in these samples. Diamagnetic response has been observed in pure TiO 2 nanoparticles. However, room temperature (RT) ferromagnetism has been observed with Co and/or Mn doping which gets enhanced further with codoping of Co ions. The observed ferromagnetism has been attributed to the formation of shallow levels with doping/codoping. The Co and/or Mn doped TiO 2 nanoparticles are referred as diluted magnetic semiconductors due to their combined semiconducting and ferromagnetic response. UV Vis spectra show the band gap of these samples are in the range of 3.03 eV 3.24 eV and a reduction in band gap is observed after codoping. The temperature dependent electrical conductivity measurements also show an enhancement in conductivity after codoping at RT.
author_names: Anand Kumar

Manish K Kashyap

Namita Sabharwal

Sarvesh Kumar

Ashok Kumar

Parmod Kumar

Kandasami Asokan
title: Structural, optical and weak magnetic properties of Co and Mn codoped TiO 2 nanoparticles
year: 2017

abstract: In this study, for the first time, {111} facet exposed anatase TiO2 single crystals are prepared via both F and ammonia as the capping reagents. In comparison with the most investigated {001} {010} and {101} facets for anatase TiO2, the density functional theory (DFT) calculations predict that {111} facet owns a much higher surface energy of 1.61 J/m, which is partially attributed to the large percentage of undercoordinated Ti atoms and O atoms existed on the {111} surface. These undercoordinated atoms can act as active sites in the photoreaction. Experimentally, it is revealed that this material exhibits the superior electronic band structure which can produce more reductive electrons in the photocatalytic reaction than those of the TiO2 samples exposed with majority {010} {101} and {001} facets. More importantly, we demonstrate that this material is an excellent photocatalyst with much higher photocatalytic activity (405.2 mmol h 1) about 5, 9, and 13 times that of the TiO2 sample exposed with dominant {010} {101} and {001} facets, respectively. Both the superior surface atomic structure and electronic band structure significantly contribute to the enhanced photocatalytic activity. This work exemplifies that the surface engineering of semiconductors is one of the most effective strategies to achieve advanced and excellent performance over photofunctional materials for solar energy conversion.
author_names: Hua Xu

Pakpoom Reunchan

Shuxin Ouyang

Hua Tong

Naoto Umezawa

Tetsuya Kako

Jinhua Ye
title: Anatase TiO 2 Single Crystals Exposed with High Reactive 111 Facets Toward E ffi cient H 2 Evolution
year: 2013

abstract: Abstract Cu N codoped TiO 2 photocatalyst is synthesized by sol gel method to obtain enhanced optical absorption in the visible region. Optimum concentrations of Cu and N were obtained by maximizing the photocatalytic activity for the monodoped (Cu or N) TiO 2 These optimized concentrations were used for synthesizing (Cu, N) codoped TiO 2 The photocatalysts were characterized using XRD, micro Raman, SEM, XPS, BET surface area analyzer, UV vis diffuse reflectance spectroscopy and photoluminescence. XPS study suggests the incorporation of Cu 2+ into TiO 2 lattice, which assists N to substitutionally replace oxygen in codoped TiO 2 while maintaining the anatase phase even after doping. N doping creates minor variation in the energy band gap of TiO 2 reducing it upto 3.0 eV, while Cu doping was able to narrow the band gap to 2.2 eV mainly due to the localized levels of Cu 3d states and shifting of Ti 3d states to lower energy (due to oxygen vacancies) in the band gap as deduced from XPS data and confirmed by the DFT calculation. In (Cu, N) codoped TiO 2 visible light absorption is higher than the other TiO 2 samples, a feature that is mainly attributed to the formation of an isolated intermediate band (IB) occurring due to the strong hybridization between Cu 3d and N 2p orbitals. This IB contributes to visible light absorption by two step optical transition with the first transition from valence band (VB) to IB and the second from IB to conduction band (CB) The dopant species may also act to reduce the charge carrier recombination by acting as the trapping sites for photogenerated charges. (Cu, N) codoped TiO 2 was able to degrade Methylene Blue dye and p Nitrophenol solution under light irradiation with significantly better rate in comparison to monodoped and undoped TiO 2 High photocatalytic activity is attributed to the presence of IB in the energy band gap of TiO 2 which creates the synergic effect by higher visible light absorption and lower recombination of photogenerated charges.
author_names: R G Jaiswal

J S Bharambe

Nainesh Patel

Alpa Dashora

Dushyant C Kothari

Antonio Miotello
title: Copper and Nitrogen co doped TiO2 photocatalyst with enhanced optical absorption and catalytic activity
year: 2015

abstract: Surface modification of nanocrystalline TiO(2) particles (45 A) with catecholate type ligands consisting of an extended aromatic ring system, i.e. 2,3 dihydroxynaphthalene and anthrarobin, was found to alter the optical properties of the nanoparticles in a similar way to modification with catechol. The formation of inner sphere charge transfer (CT) complexes results in a red shift of the semiconductor absorption compared to unmodified nanocrystallites and the reduction of the band gap upon the increase of the electron delocalization on the inclusion of additional rings. The binding structures were investigated by FTIR spectroscopy. The investigated ligands have the optimal geometry for binding to surface Ti atoms, resulting in ring coordination complexes of catecholate type (binuclear bidentate binding bridging) thus restoring the six coordinated octahedral geometry of surface Ti atoms. From the Benesi Hildebrand plot, stability constants in methanol/water 90/10 solutions at pH 2 of the order 10(3) M( 1) have been determined. Quantum chemical calculations on model systems using density functional theory (DFT) were performed to obtain vibrational frequencies of charge transfer complexes, and the calculated values were compared with the experimental data.
author_names: Tatjana Savic

Ivana A Jankovic

Zoran Saponjic

Mirjana I Comor

Dusan Z Veljkovic

Snezana D Zaric

Jovan M Nedeljkovic
title: Surface modification of anatase nanoparticles with fused ring catecholate type ligands: a combined DFT and experimental study of optical properties.
venue: Nanoscale
year: 2012

abstract: Introduction Nitrogen oxides (NOx) mainly produced from combustion of fossil fuels and vehicle exhaust, are responsible for atmospheric environmental problems such as haze, photochemical smog, acid rain and so on. Semiconductor photocatalysis, as a "green" technology, has been used to remove NOx at ppb levels TiO2 has long been a promising candidate for photocatalysis applications owing to its strong photocatalytic oxidation performance, photostability, natural abundance, and nontoxicity However, the relatively large band gap of TiO2 (3.0 3.2 eV) limits its application in the visible light region (400 nm l 750 nm) which accounts for 43% of the incoming solar energy. Oxygen vacancies are the most important defects in TiO2 for photocatalysis It has been shown that the photocatalytically important properties of TiO2, including the electronic structure, charge transport and surface properties, are closely related to oxygen vacancies. In this study, a series of oxygen deficient TiO2 catalysts was prepared by a facile and low temperature method. The catalysts were utilized to remove gaseous NOx under visible light irradiation. Materials and Methods Oxygen deficient TiO2 was synthesized using a sol gel method. After drying at 110 C, the powder samples were calcined in air at 150, 200, 300, 400 and 500 C for 5 h and were labeled as TiO2 150, TiO2 200, TiO2 300, TiO2 400, and TiO2 500, respectively. The catalysts were characterized by SEM, XRD, Raman, physisorption analysis, UV vis diffuse reflection spectra and photoluminescence (PL) emission spectra. The photocatalytic experiments for the removal of NOx were performed in a homemade continuous flow reactor. A 500 W commercial Xenon arc lamp was used as the simulated solar light source. Two optical filters were used to obtain light in the 420 700 nm range and the integrated light intensity was 35.8 mW/cm. The experiment conditions were: T=25 C, mcat=50 mg, NOinitial=400 ppb, RH=55% F=1.2 L min. The concentration of NO, NO2 and NOx was continuously measured by a chemiluminescence NOx analyzer. DFT calculations were performed using the CASTEP package. The generalized gradient approximation (GGA) with the PBE exchange correlation functional was adopted. The parameters were: ultrasoft pseudo potential, k point mesh of (2x2x2) energy cutoff=380 eV, convergence tolerance: energy<5.0x10 eV/atom, force<0.01 eV/A, stress<0.02 GPa and displacement<5.0x10A. A 108 atom supercell made up by 3x3x1 unit cells was used to simulate an anatase bulk crystal. The oxygen vacancy was modeled by removing one oxygen atom in TiO2 cells, thus forming TiO2 x with x=0.028. After finishing the geometry optimizations, the DFT+U method was used to calculate the band structures and the projected density of states (PDOS) of TiO2 and TiO2 x. A self consistent Hubbard U correction of 3.3 eV for the d electrons of Ti has been calculated Results and Discussion TiO2 200 is highly active for the removal of NOx under visible light. On the basis of the characterization results, the excellent catalytic activity of TiO2 200 was attributed to its larger surface area, greater light absorption in the visible region and more effective charge transport and separation than catalysts prepared at other temperatures, which was caused by the formation of oxygen vacancies in the presence of carbonaceous species.
author_names: Jinzhu Ma

Hongming Wu

Yongchun Liu

Hong He
title: Photocatalytic removal of NO x over visible light responsive oxygen deficient TiO 2
year: 2014