Increase In Cr Doping Research Articles

Tuning the spin-reorientation towards room temperature and the evidence for spin-phonon coupling in modified SmFeO3

The spin-reorientation phenomenon exhibited by rare-earth based ferrite systems generate lot of interest due to its application potential in related fields like spintronic devices. In this work, it was demonstrated that the spin-reorientation temperature (TSR) of SmFeO3 was tuned towards near room temperature by Cr doping. The SmFe1-xCrxO3 (x = 0.0, 0.1, 0.3 and 0.4) samples were prepared by solid state reaction method and the structural refinements carried out on the x-ray diffraction patterns revealed that the samples were crystallized in orthorhombic structure with Pbnm space group. The magnetic measurements displayed that the samples were all ordered antiferromagnetically and the ordering temperature (TN) shows decreasing trend with increase in Cr doping. Furthermore, TSR was tuned from 472 K to 317 K when the composition was changed from x = 0.0 to x = 0.4. The temperature-dependent Raman spectroscopic studies on x = 0.4 sample revealed the existence of the spin-phonon coupling. In addition, this sample also displayed an interesting negative magnetization and compensation temperature. The details are discussed in this work.

Enhancement of multifunctional optoelectronic and spintronic applications of nanostructured Cr-doped SnO2 thin films by conducting microstructural, optical, and magnetic measurements

A chemical synthesis method was used to produce Cr-doped SnO2 nanoparticles. By using the TEM technique, the nanocrystalline nature of the Sn1-xCrxO2 (0.0 ≤ x ≤ 0.1) nanopowder was confirmed. With different Cr concentrations, the electron beam deposition technique was used to deposit a series of Sn1-xCrxO2 nanocrystalline thin films on a silica substrate. X-ray diffraction (XRD), atomic force microscopy (AFM), spectroscopic ellipsometry (SE), and vibrating sample magnetometry (VSM) techniques were used to examine the physical properties of the deposited films. The nanocrystalline nature of the Sn1-xCrxO2 (0.0 ≤ x ≤ 0.1) thin film was confirmed by XRD and AFM morphology measurements. The XRD spectrum of the Sn1-xCrxO2 nanocrystalline film demonstrated a tetragonal crystal structure with no detectable extra phases. The optical measurements showed that as the Cr content increases, the direct optical energy gap Eg decreases without any sign of solubility limit up to x ≤ 0.1. The decrease in Eg is attributed to the sp-d exchange interaction. Also, the spectral behavior of the refractive index dispersion of the Cr-doped SnO2 indicates that as the Cr dopant increases, the refractive index of the deposited film also increases, which is attributed to the increase in the polarizability. Moreover, as the Cr content raises the atomic number, the density of the deposited film increases from 1.43 × 1022 cm−3 for SnO2 to 1.57 × 1022 cm−3 for Sn0·9Cr0·1O2. Further, the behavior of the refractive index dispersion of the deposited film was revealed using a single oscillator model proposed by Wemple-DiDomenico (WDD). Our calculations revealed that as the Cr concentration increases, the value of oscillator energy Eo decreases owing to the decrease in Eg, whereas the value of dispersion energy Ed increases because of the chemical structural changes such as the lattice parameters. Finally, the magnetic studies revealed that incorporating a small fraction of Cr into SnO2 produces room temperature ferromagnetism in the film, which is gradually suppressed by a further increase in Cr doping. These findings indicate that the Cr-doped SnO2 film can be recommended for optoelectronic and spintronic device applications.

Effect of Cr doping in CeO2 nanostructures on photocatalysis and H2O2 assisted methylene blue dye degradation

Optimization of Advanced Oxidation Processes (AOPs) is key for the demand in green and sustainable method for degradation of organic contaminants in aqueous solution. In this study, the effect of Cr doping on CeO2 nanostructure was investigated to evaluate the catalytic performances of the Cr doped CeO2 nanocomposites with Methylene Blue (MB) used as model contaminant. Two types of AOPs were also explored; photocatalysis and H2O2 assisted. The Cr-doped CeO2 samples were prepared by deposition precipitation method and characterized with XRD, FE-SEM-EDX, UV–vis Spectroscopy and TPR. Generally, increase in Cr doping improved the catalytic activity in MB degradation efficiency for photocatalysis and H2O2-assisted methods, where with 3% weight of Cr-doping achieved 41 % and 59 % of MB degradation across 100 min respectively. The improvement of catalytic activity was attributed to the effects on Cr doping, which lowers the CeO2 band gap and also the Cr species serving as electron trapping centers to slow down the recombination of the charge carriers. The MB degradation results also showed that for Cr-doped CeO2 system, H2O2-assisted was the more effective catalytic method compared to photocatalysis. Isopropanol has been found explicitly feasible to demonstrate a plausible scavenging properties toward hydroxyl radical by suppressing the MB degradation in the reaction media, thus, suggesting that, the MB degradation mechanism proceeds via hydroxyl radical species.

Influence of Cr on structural, spectroscopic and magnetic properties of CoFe2O4 grown by the wet chemical method

The Cr doped CoFe2O4 nano crystalline ceramics are synthesized using wet chemical method. The structural, morphological, functional, spectroscopic and magnetic properties of pure CoFe2O4 and Cr doped CoFe2O4 are characterized by XRD, FESEM, FTIR, Raman, Mössbauer spectroscopy and M-H hysteresis loop. The X-ray diffraction (XRD) patterns of undoped (x = 0) and doped (x≠ 0) samples reveal the formation of single phase cubic spinel structure. The lattice parameters and cation distribution between tetrahedral (A) and octahedral (B) sites of the samples (AB2O4) were obtained by Rietveld refinement of XRD patterns. The FESEM micrographs exhibit dense micro structure with small voids. The energy dispersive x-ray studies confirm the presence of Cr in CoFe2O4. The FTIR spectrum confirms that the peaks corresponding to the metal-oxygen bonds have shiftedtowards higher wavelength region. Raman spectrum exhibits broad peaks which confirm the existence of local disorder due to the inter-site cation migration between tetrahedral and octahedral sites. The Mössbauer spectroscopy reveals that the magnitude of magnetic hyperfine field of tetrahedral site (HA) and octahedral site (HB) decreases with increase in Cr doping in CoFe2O4 and that the decrease is more rapid in tetrahedral site (HA) compared to that in octahedral site (HB). And also confirm that the magnetic hyperfine fields (Bhf) are smaller relative to pure cobalt ferrite representing that super exchange interaction which is decreases as the Cr concentration increases. The magnetization studies reveal a decrease in the saturation magnetization (Ms) of CoFe2O4with increase in Cr substitution. A similar and non-monotonic variation of coercivity (Hc) and magneto crystalline anisotropy (K) suggests that Hc is mainly determined by K.

Cr modified Raman, optical band gap and magnetic properties of SnO2 nanoparticles

Pure and Cr (1, 3, 5, 7 and 10 at.%) doped SnO2 nanoparticles were synthesized in aqueous solution by a low cost chemical co-precipitation method without using any stabilizing agent. The effects of Cr doping on Raman, optical band gap and magnetic properties of SnO2 nanoparticles were investigated. Particle size is found to decrease with Cr doping into the SnO2 matrix which was confirmed by TEM. Besides of the fundamental mode of vibration, two additional peaks are also observed in Raman spectra which are correlated to Cr. The absorption spectra showed two peaks at 340 and 454 nm. The absorbance peak at 340 nm is assigned to the transition from valence band (VB) to conduction band (CB) and the peak at 454 nm was due to the transition from VB to mid gap energy level introduced by Cr. The optical band gap of undoped SnO2 nanoparticles is calculated to be 3 eV. With the doping of Cr in SnO2, band gap increases due to the decrease in particle size. The emission intensity is found to decrease with the increase in Cr doping due to the emission from CB to mid gap energy levels introduced by Cr between CB and VB. Undoped SnO2 nanoparticles show room temperature ferromagnetism due to the presence of defects and oxygen vacancies. The heavily doped SnO2 nanoparticles show paramagnetic nature due to the antiferromagnetic coupling between Cr and its nearest neighbour.

Bandgap modulation and hydrogen storage with Cr-doped BN sheets

The theoretical calculations indicate that the metal-doped boron nitride (BN) sheets are potential materials to store the hydrogen and tune the bandgap. It is all known that the BN sheet is a nonmagnetic wide-bandgap semiconductor. Using density function theory (DFT), the lattice parameters of Cr-doped BN sheets are optimized, which are still kept on two-dimensional (2D) planar geometry, and the bandgap and [Formula: see text] storage are studied. The simulation results show that the [Formula: see text] molecule can be easily absorbed by Cr-doped N in BN sheet. As the adsorption energy was greatly decreasing with the increasing number of Cr-doped N, B had an affinity for adsorption of [Formula: see text]. With the increase of Cr doping, the bandgap of Cr-doped BN sheet is decreasing. The bandgap decreases from 4.705 eV to 0.08 eV. So Cr-doped BN sheet is a promising material in storing [Formula: see text] and tuning the bandgap.

Cr doping assisted tuning of functional properties of ZnO nanorods prepared by fast solution route

In this work, single-crystalline Zn1−x Cr x O (0.0 ≤ x ≤ 0.05) nanorods were prepared by a facile microwave-assisted solution method, and their significant correlation between structural, optical, morphological and magnetic properties was investigated. X-ray diffraction and transmission electron microscopy (TEM) results showed that Cr-doped ZnO was comprised of single-phase nature with hexagonal wurtzite structure. Field emission scanning electron microscopy and TEM micrographs suggested that the pure nanorods with an average length of ~2 µm and a diameter in the range of 150–200 nm, respectively, were observed. Interestingly, the size of nanorods decreased with the increase in Cr concentration in ZnO. There is striking similarities between changes in the lattice volume, bandgap energy, morphology and the saturation magnetization caused by Cr doping. Unit cell volume, value of saturation magnetization and bandgap was decreased with the increase in Cr doping. Zn vacancies, together with Cr doping, played an important role in the ferromagnetic origin of Zn1−x Cr x O nanorods. Single-crystalline Zn1−x Cr x O (0.0 ≤ x ≤ 0.05) nanorods were prepared by a facile microwave-assisted solution method showing the relationship between lattice volume, bandgap energy, morphology and the saturation magnetization.

Preparation and Some Physical Properties of Zn1−xCrxO

A series of Zn1−xCrxO with different Cr concentrations were prepared by solid state reaction. The powders were characterized by X-ray diffractometer and the results exhibited that Cr-doped ZnO consisted of two phases; the major phase was zensite wurtzite structure and the second phase was zinc chromate ZnCr2O4 spinel structure. The results of the Rietveld refinement indicated that the ZnO lattice is further deformed upon Cr substitution. The crystallite size was found to decrease with increasing the Cr substitution, and this may be due to the difference in the ionic radius between Zn2+, and Cr3+, in addition to the segregation of the second phase inhabit the growth mechanism. The calculated energy band gap decreased as Cr concentrations increased. The dielectric properties of Cr doped ZnO were studied in the frequency range 0.1–5 MHz and at different temperatures (30–150 °C). The dielectric constant (e′) and loss factor (tan δ) increased with increasing Cr doping due to the increase of the number of charge carriers and interfacial polarization effects. The conductivity of the samples increases with increasing both of composition parameter x and temperature, and showing semiconducting behavior. The magnetization and the magnetic susceptibility were found to increase with the increase in Cr doping.

Doping dependent properties of Cr-doped ZnO nanostructures prepared by microwave irradiation.

In this work, undoped and Cr-doped single-crystalline ZnO nanorods were prepared by a facile microwave assisted solution method. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results showed that Cr-doped ZnO was comprised of single phase nature with hexagonal wurtzite structure up to 5% Cr doping, however, secondary phase ZnCr2O4 appeared upon further increasing the Cr dopant concentration. Field emission scanning electron microscopy (FESEM) and TEM micrographs suggested that the undoped nanorods with an average length of -~2 μm and a diameter in the range of 150-200 nm, respectively were observed. Interestingly, the size of nanorods decreased with the increase of Cr concentration in ZnO. Optical studies depicted that the energy bandgap was decreased with the increase of Cr concentration. Raman scattering spectra of Cr-doped ZnO revealed the lower frequency shift of E2(high) phonon mode with the increase in concentration of Cr dopant, suggested the successful doping of Cr into Zn site in ZnO. Magnetic studies showed that Cr-doped ZnO exhibited room temperature ferromagnetism (RTFM) and the value of magnetization was continuously decreased with the increase in Cr doping.

Structural studies and magnetic and transport properties of Cr-substituted La0.67Ba0.33Mn1−xCrxO3 (0 ≤ x ≤ 0.15) perovskites

We have investigated the structural, magnetic and electrical transport properties of a series of ABO3-type perovskite compounds, La0.67Ba0.33Mn1−xCrxO3 (0≤x≤0.15), which strongly depend on the doping level x. The slight difference between the ionic radii of Cr3+ and Mn3+ causes no change in the structure when x≤0.1, remaining rhombohedral (space group R-3C), while for x=0.15 the structure becomes cubic (space group Pm-3m). Energy dispersive X-ray analysis (EDAX) confirms the expected stoichiometry of all samples. Upon Cr doping on the Mn site, the lattice parameters, the unit cell volume and the BOB bond angle are reduced. All samples present a single magnetic transition from ferromagnetic to paramagnetic phase, showing a decrease of the Curie temperature Tc and the magnetization M when x increases (x≤0.15). However, Cr doping makes the saturation magnetization at 5K to decrease, which indicates that the Cr3+ moments tend to be antiparallel to the Mn3+ moments at low temperature.The Cr-doped manganites exhibit a large variation in resistivity values. The increase of Cr doping (x≤0.15) leads to an increase of the electrical resistivity. Below 10at.% of Cr3+, the electrical resistivity shows a metallic behavior, which is well fitted by the relation ρ=ρ0+ρ2T2+ρ4.5T4.5, indicating the importance of the grain/domain boundary, the electron–electron scattering effects and, to a lesser extent, the electron–(magnon, phonon) scattering effects in the mechanism of conduction. On the other hand, the 15at.% of Cr3+ doping makes the material to exhibit a semiconductor behavior, for which the electronic transport can be explained by a variable range hopping (VRH) and small polaron hopping (SPH) models. Results are consistent with a reduction of the number of available hopping sites for the Mn eg (↑) electron due to the substitution of Mn3+ by Cr3+, which suppresses the double exchange (DE) interactions.

Electrochemical Behavior of Cr- Doped Composite Li2MnO3-LiMn0.5Ni0.5O2 Cathode Materials

The effect of Cr-doping on the structural, physical and electrochemical properties of Li2MnO3 and LiNi0.5Mn0.5O2 is reported. The formation of the solid solution and local distortion of the structure with the increase in Cr doping (0.7Li2MnO3 − 0.3LiNi0.5Mn0.5O2) was confirmed respectively, by X-Ray diffraction and Raman spectroscopy. Morphological investigations and elemental mapping showed the formation of primary particles in the range 0.5–1.0 μm and homogeneity, respectively. Electrochemical measurements on the materials were carried out in different voltage ranges and temperatures. Structural transformation of the material from layered to spinel symmetry has been observed with cycling in case of the samples with low Li2MnO3 content (0.3Li2MnO3-0.7LiNi0.5Mn0.5O2). Cr acted as a catalyst to enhance the activation of the material with high Li2MnO3 content.

The effect of chromium impurity on the thermoelectric properties of PbTe in the temperature range 100–600 K

In this work we have investigated the effect of the chromium (Cr) impurity on the thermoelectric properties of lead telluride (PbTe) bulk crystal with different Cr-content. The structural characterization of the crystals was done by x-ray diffraction and high-resolution transmission electron microscopic analyses. The incorporation of Cr in PbTe over the soluble limit has been found to replace Pb from the lattice site, forcing it to be precipitated into nanodots, with their dimensions in the range of 2–13 nm and their numbers increasing with the increase in Cr doping. The transport properties of the samples with different Cr-content have been evaluated through temperature dependent electrical resistivity, the Hall coefficient, and thermopower measurements in the temperature range of 100–600 K. The highest mobility is obtained as 1404 cm2/V s for a sample with a carrier concentration of 3.84 × 1018 cm−3 at 300 K. The high electron mobility and moderately higher value of the thermopower are expected to produce a high power factor of the samples with the highest value of 38.49 × 10−4 Wm−1K−2 in a specimen with a Cr-content of 1.45 at. % at room temperature. The power factor of the specimen with 1.3 at. % Cr-content is found to be above 29 × 10−4 Wm−1K−2 in the wide temperature range of 300–600 K.

Variation in structural, optical and magnetic properties of Zn 1− xCr xO ( x = 0.0, 0.10, 0.15, and 0.20) nanoparticles: Role of dopant concentration on non-saturation of magnetization

Cr-doped ZnO, i.e. Zn 1− x Cr x O ( x = 0.00, 0.05, 0.10, 0.15 and 0.20) nanoparticles were synthesized by sol–gel route. The structural and morphological properties of these nanoparticles were investigated by high resolution transmission electron microscope (HRTEM). The average particle size of Zn 1− x Cr x O nanoparticles decreases from 75 to 40 nm with the increase in x from 0.00 to 0.20. The rings observed in selected area diffraction pattern revealed that up to x = 0.10 these nanoparticles have single phase ZnO. However, a secondary spinel phase of ZnCr 2O 4 was observed for higher Cr doping ( x ≥ 0.15). The optical band gap calculated using UV–visible absorption was decreased from 3.27 to 2.27 eV with the increase in Cr-doping from 0.00 to 0.20 in ZnO nanoparticles. The undoped ZnO (Zn 1− x Cr x O; x = 0.00) nanoparticles did not show any hysteresis loop at room temperature, however, clear loops were obtained for x = 0.05–0.20. Additionally, magnetization ( M) vs. applied magnetic field ( H) loops for lower Cr-concentration ( x = 0.05) saturate at 5 kOe, and while those with higher Cr concentration ( x > 0.05) do not show saturation even at 10 kOe. This may be attributed to increase in the defects at higher Cr-doping into ZnO. The value of saturation magnetization was found to decrease from 4.24 emu g −1 to 1.96 emu g −1 with the increase in Cr doping from x = 0.05 to 0.20 in ZnO and may be due to the secondary ZnCr 2O 4 phase.

Effects of the Cr doping on structure and optical properties of ZnO thin films

Cr-doped ZnO thin films are prepared on glass substrates by the magnetron sputtering technique. An X-ray diffraction (XRD) is used to analyze the structural properties of the thin films. It indicates that all the thin films have a preferential c-axis orientation. The peak position of the (002) plane shifts to the higher 2θ value, and the peak intensity decreases with the increase of Cr doping. The results of the scanning electron microscopy (SEM) show that the surface morphology becomes loose with the increase of Cr doping. Besides, it is found from the photoluminescence (PL) measurement at room temperature that the ultraviolet emission peak and green emission band are located at 375 nm and 520 nm, respectively, and both intensities of them decrease with the increase of the Cr doping concentration, while the band gap of the ultraviolet emission shifts to the lower wavelength. The experimental results confirm that the optimal Cr doping concentration is 2 at. %.

Study of photocatalytic activity in sputter‐deposited Cr‐TiO2 thin film

AbstractIn this investigation, Cr‐doped TiO2 thin films were prepared on glass substrate with various doping concentrations to study the photocatalytic activity in UV–visible light irradiation. The Cr‐doped TiO2 thin films were prepared by reactive magnetron sputtering using various sputtering pressures to control the Cr doping concentration. The optical property shows large red‐shift of transmittance edge and decrease of average transmittance with the increase of Cr doping. The photocatalytic activity, evaluated by the measurement of decomposition of methanol under visible and UV–visible light irradiation, increases for the low Cr doping concentration, but decreases for higher Cr doping concentrations. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The defect structure, sintering behavior, and dielectric responses of Cr2O3-doped Sr0.5Ba0.5Nb2O6

Sr 0.5 Ba 0.5 Nb 2 O 6 (SBN50) with and without Cr2O3 dopant had been prepared by the solid-state reaction. The structural variations of undoped and Cr2O3-doped SBN were examined by the x-ray diffractometer. As the Cr2O3 content increases, the lattice parameters remain almost unchanged initially but become greatly changed revealing the increase in a axis and the decrease in c axis when the Cr2O3 content is >1mol%, which was mainly influenced by the created oxygen vacancies. Isothermal sintering behavior of undoped and Cr2O3-doped SBN50 has been investigated. It was found that the creation of the charge-compensated oxygen vacancies and the accompanied structural change had a significant influence on the sintering behavior. The direct jumping path through vacant O sites (path 2 in Fig. 9) was further considered as the main reason for the enhancement of the densification rate and slight increase of the activation energy when the Cr2O3 content is ≦1mol% and the decrease of the densification rate but great increase of the activation energy when the Cr2O3 content is >1mol%. Tmax [temperature at the maximum dielectric constant (εmax)] decreased with the increase of Cr doping and is essentially determined by the structural change of the NbO6 units in SBN. εmax is suggested to be determined by the competition between the reduction of the polarization due to the substitution of Cr for Nb and the enhancement of the polar moment arising from the weakening of Nb–O bonds due to the creation of the oxygen vacancies.

ELECTRICAL TRANSITIONS IN PURE AND DOPED V2O3

Extensive electrical and thermal studies on transitions in (CrxV1-x)2O3, 0 ⩽ x ⩽ 0.03, and in (TixV1-x)2O3, 0 ⩽ x ⩽ 0.06, have been undertaken. An increase in Cr-doping (0 ⩽ x ⩽ 0.018) raises the temperature of the low-temperature transition and elevates the resistivity of the metallic phase ; it first sharpens and later diminishes the high-temperature transition. The latter (transition) in 1 at. % of Cr-doped V2O3 is characterized by stepwise discontinuities in resistivity and by sharp spikes in DSC scans. Experimental work on Ti-doped V2O3 shows that the size and temperature of the low-temperature transition (Tt) diminish drastically with increasing Ti content. At a Ti concentration of ca. 5.6 at. %, Tt drops abruptly from 35 to (presumably) 0 K. The activation energy of the semiconducting phase diminishes nearly exponentially to zero in this composition range. The above findings, in conjunction with other evidence obtained from resistivity and heat capacity measurements, suggest that the high-temperature transition in (CrxV1-x)2O3 system is not readily compatible with the Mott transition model but is consistent with the existence of Overhauser charge density waves. Further, the (TixV1-x)2O3 system, whose structure is essentially unaltered by Ti doping, provides a testing ground for theories on the abrupt disappearance of metal-insulator transitions.