key: cord-0066027-gqipv6kk
authors: Kumar, Jitender; Kumar, Rajesh; Singh, Mukhtiyar; Kumar, Shalendra; Kumar, Ravi; Won, Sung Ok; Brajpuriya, Ranjeet; Dwivedi, Sourabh; Sharma, Ram K.; Vij, Ankush
title: Structural, diffuse reflectance and luminescence study of t-Mg(2)B(2)O(5) nanostructures
date: 2021-07-23
journal: Appl Phys A Mater Sci Process
DOI: 10.1007/s00339-021-04761-w
sha: a161fedd7ced7d222f2344c7bf2aabf8bfdf18f7
doc_id: 66027
cord_uid: gqipv6kk

We report here the structural, reflectance, photoluminescence and thermoluminescence study of t-Mg(2)B(2)O(5) nanostructures synthesized using optimized combustion method relatively at much lower temperature. The rietveld refinement of X-ray diffraction data confirms single-phase triclinic crystal structure of Mg(2)B(2)O(5) nanoparticles. The direct band gap determined using diffuse reflectance spectra (DRS) was 5.23 eV, which is contrary to earlier reports quoting Mg(2)B(2)O(5) as indirect band gap material. To elucidate the nature of band gap in Mg(2)B(2)O(5), we performed first principle calculations based on full potential linearized augmented plane-wave (FPLAPW) method, predicting the direct band gap of 5.10 eV in t-Mg(2)B(2)O(5) which is in good agreement with our experimental results. The t-Mg(2)B(2)O(5) nanoparticles were found to exhibit yellow-reddish photoluminescence peaking at 588 nm, attributed to various defects states. The combustion synthesized Mg(2)B(2)O(5) nanocrystals exhibited ultraviolet (254 nm) responsive thermoluminescence (TL). TL glow curve of Mg(2)B(2)O(5) comprises of one dominant peak around 417–428 K and less intense shoulder around 573–589 K which arouse possibility of various trapping sites or defects present in the sample. The TL analysis using general order Kitti’s equations was performed to estimate the activation energies of trapping states. Owing to already well-known mechanical and thermal properties, the direct wide band gap nature and UV responsive thermoluminescence of combustion synthesized t-Mg(2)B(2)O(5) nanostructures can pave way for its use in luminescence-based applications and UV dosimetry. As an additional application of Mg(2)B(2)O(5), anti-biofilms activity of Mg(2)B(2)O(5) nanoparticles using pseudomonas aeruginosa bacterial cells was also performed which revealed 91 ± 2.7% inhibition of biofilms formed by P. aeruginosa, respectively, at 100 μg/ml after 24 h of treatment.

The optical and luminescent materials are extensively used in variety of applications [1] [2] [3] . In particular, there is always a quest for developing luminescent materials possessing good mechanical and thermal properties. Among borate family, bulk and nanowires of Mg 2 B 2 O 5 are mechanically and thermally quite stable, and therefore find immense applications in anti-wear, anti-corrosion, scintillator, discharge lamp, reducing friction additives, turbines, aerospace components, deep submergence structure and armor plate in modern engineering field [4] [5] [6] [7] [8] [9] [10] [11] . On the other hand, MgB 4 O 7 in its pure and doped form has been extensively studied for optical and luminescencebased applications such as thermoluminescence dosimetry [12, 13] . Owing to robust mechanical and thermal properties, it becomes paramount to investigate the viability of Mg 2 B 2 O 5 nanoparticles for optical and luminescence applications, which is severely lacking in literature so far. The single-phase Mg 2 B 2 O 5 pyroborates are generally crystallized on annealing at higher temperatures. Zhu et al. reported monoclinic Mg 2 B 2 O 5 whiskers synthesized using coprecipitation technique at 832ºC but on increasing synthesis temperature monoclinic phase is found to be transforming into triclinic phase [5] . Qasarawi et al. reported synthesis of triclinic Mg 2 B 2 O 5 at 1250ºC using partial precipitation technique and found that Mg 2 B 2 O 5 has indirect band gap of 4.72 eV [8] . However, earlier Cheng et al. have theoretically shown that Mg 2 B 2 O 5 has direct band gap of 5.44 eV [14] . The nature of band gap is an important factor for any material to be suitable for optical/ luminescence applications. This led us to investigate the nature of band gap in Mg 2 B 2 O 5 nanoparticles using combined diffuse reflectance spectroscopy and first-principle calculations using density functional theory. The luminescence studies in Mg 2 B 2 O 5 pyroborates did not get much attention as compared to MgB 4 O 7 borates. There are only few reports in the literature on the luminescence properties of doped Mg 2 B 2 O 5 synthesized at optimum conditions. Oztas et al. reported the synthesis of pure magnesium pyroborate using solution combustion method (SCS) and conventional ceramic process at various stoichiometries. They concluded that SCS is a time saving and much practical method to synthesis single-phase pyroborates without compromising other properties [15] . Kawano [17] . We have very recently investigated the luminescence properties of Eu doped triclinic Mg 2 B 2 O 5 nanophosphors exhibiting intra 4f transitions in Eu 3+ when excited with 325 nm wavelength [18] . However, luminescence in undoped Mg 2 B 2 O 5 nanocrystals is still unexplored. Being wide band gap material, Mg 2 B 2 O 5 is expected to show defects assisted emission in visible region. Therefore in order to get deep insights on defects or trapping states within band gap of Mg 2 B 2 O 5 , thermoluminescence of ultraviolet-irradiated samples was also performed for the first time.

Oxide-based nanostructures also exhibit good antibacterial properties. Several findings show the antimicrobial effects of nanostructured MgO against a range of bacteria. Zhu et al. have stated the antimicrobial action of MgO nanoparticles against E. coli bacteria [19] . He et al. also reported the mechanism of antibacterial activity of magnesium oxide nanoparticles against foodborne pathogens [20] . Nanoparticles as an antimicrobial agents help in combating multi-drug resistance bacteria and also providing long-lasting effects with low toxicity as compared to conventional antibiotics. More often bacteria form biofilms and adhere to the surface of the organism, colonize and become resistant to the conventional antibiotics. To combat such bacteria, nanoparticles have proved to be very effective. Owing to robust antimicrobial properties of MgO nanoparticles, we also investigated effectiveness of combustion synthesized Mg 2 B 2 O 5 (2MgO.

In the present study, single-phase triclinic Mg 2 B 2 O 5 nanocrystals were synthesized using combustion method in the presence of urea as fuel. The combustion method has the advantage of completing chemical reaction at a relatively much lower temperature than solid-state method. We employed rietveld refinement of x-ray diffraction (XRD) data to elucidate the crystal structure of Mg 2 B 2 O 5 nanoparticles (NPs). Thermogravimetric and differential scanning calorimetry was performed in order to get insights about the thermal stability of material. The band gap and luminescence studies were investigated through combined diffuse reflectance spectroscopy (DRS), density functional theory (DFT)-based calculations using WEIN2K, photoluminescence spectroscopy and thermoluminescence (TL). The thermoluminescence was performed on ultraviolet (254 nm) irradiated Mg 2 B 2 O 5 nanostructures. Anti-biofilms activity of Mg 2 B 2 O 5 nanoparticles using pseudomonas aeruginosa bacterial cells was performed.

The Mg 2 B 2 O 5 nanostructures were prepared using solid combustion synthesis technique. The combustion synthesis is a method based on the principle that once a reaction is initiated under heating, an exothermic reaction occurs that becomes self-sustaining within a certain time interval, resulting in a powder as final product. Combustion technique is preferred over other synthesis techniques because it does not require high temperature and long time for reaction completion unlike some other techniques like solid state reaction and other methods which require high temperature annealing. Generally, metal nitrates as oxidants and fuels like urea, citric acid, glycine, etc., as reducing agents are used in combustion technique. In the present case, magnesium nitrate was mixed in stoichiometric proportion with urea, boric acid and ammonium nitrate. All chemicals used in the synthesis were of analytical grade. The probable chemical reaction is given as All the reactants were then grounded using agate mortar and pestle and put in a china dish. By calculating total valency of oxidizers and reducing agents, stoichiometric composition of redox reaction was calculated so that oxidizer to fuel ratio is unity (O/F = 1). This china dish was then put into muffle furnace, which is kept preheated at 200 °C and after some time temperature was increased to 600˚C. At this temperature, the combustion starts with decomposition of nitrates resulting in release of flames and fumes. The resulting product in china dish becomes dried. After that china dish was put on metal plate for quenching and the powder was grounded using agate mortar and pestle again. This grounded fine powder was then annealed at 400 °C for 30 min [21] .

Initially, effects of interaction pattern at Mg 2 B 2 O 5 NPsbacteria interface are studied by following growth kinetics of P. aeruginosa in the absence and the presence of NPs concentrations in the range between 10 and 100 μg/ml. Test organisms were prepared in LB broth taking loop full of bacteria from the specified slant culture and cultured overnight at 37 °C and 200 rpm agitation. The stock solutions were prepared by dispersing NPs in sterilized LB and sonicated for 10 min followed by UV radiation sterilization before use. The reaction mixtures without NPs were taken as controls. Briefly, 20 μL of bacterial cultures were added to the different reaction mixtures prepared in 96-well plate, and the reaction volumes were adjusted by LB broth to a final volume of 300 μ L with NPs. Upon addition of the NP, data collection for growth kinetic studies was immediately performed by measuring optical density (O.D.) at 620 nm using plate reader (Thermo scientific Multiskan EX, REF 51,118,170, China) at regular time interval.

The Mg 2 B 2 O 5 nanophosphors produced from combustion method were characterized by using model D-8 advance Bruker (Germany) diffractometer using Cu-Kα radiations having energy 8.04 keV and wavelength 1.54 Å. The morphology and selected area electron diffraction pattern were investigated using JEOL/JEM-F200 transmission electron microscope. Thermal analysis of sample was performed on Simultaneous Thermal Analyzer (STA-8000) of Perkin Elmer. DRS spectroscopy of powder sample was performed on Indian make RIMS (RIII India) set up in the range of 200-700 nm. Photoluminescence was performed on Renishaw In-Via Raman microscope using 325 nm line of He-Cd laser. The UVGL-58 lamp which emits UV radiations of wavelength 254 nm was used to expose the samples at room temperature. One minute of UV exposure at a distance of 3 inch from the sample corresponds to an irradiation of 55.8 mJ/cm 2 . In the present study, the synthesized nanostructures were irradiated with UV radiations of wavelength 254 nm for different interval of times such as 2 min, 5 min, 30 min and 60 min. After the covet exposure, TL curves were recorded on a Harshaw TLD reader (Model 3500). While performing TL experiment, 5 mg of sample is taken for each measurement which is placed in TLD reader under fixed heating rate of 5 K/s.

X-ray diffraction is employed to elucidate crystal structure of Mg 2 B 2 O 5 nanostructures synthesized from combustion method and rietveld refinement was performed using Fullprof software with pseudo-voigt profile shape function which is linear combination of Lorentzian and Gaussian functions with same FWHM (full wavelength at half maximum). Single phase of triclinic Mg 2 B 2 O 5 nanostructures is confirmed by refinement as shown in Fig. 1 . The inset in Fig. 1 also shows the crystal structure of triclinic polymorph of Mg 2 B 2 O 5 . Reliability factors (R) judging the quality of fit are profile R-factor (Rp), weighted profile R-factor (Rwp) and expected R-factor (Rexp) are 3.77, 4.76 and 3.55, respectively, with convergence indicators like chisquare value 1.79 and goodness of fit 1.3, which indicates the validity of refinement [22, 23] . Each Mg is surrounded by six oxygen atoms octahedrally as can also be seen from inset of Fig. 1 . Mg 2 B 2 O 5 nanophosphors is generally known as magnesium pyroborate in which both boron atoms are connected with three oxygen atoms triangularly and there are two such triangular boron atoms with one oxygen in common. Atomic positions of various elements of compound shown in Table 1 are obtained from rietveld refinement. Unit cell parameters which belongs to space group-P-1 were also determined from rietveld refinement; a = 6.1648, b = 9.2355, c = 3.1314, α = 90.51, β = 92.17, γ = 104.48. Whereas the parameters obtained by Fan et al. [24] for the similar system synthesized with different technique at 1100ºC were; a = 6.1563, b = 9.2228, c = 3.1217, α = 90.46, β = 92.16, γ = 104.39. The minor difference between these parameters may be due to variation of synthesis conditions which affects crystal structure of material. Rietveld refinement results show the presence of oxygen and magnesium vacancies in the compound while studying occupancy of atoms in Mg 2 B 2 O 5 .

We performed transmission electron microscopy (TEM) to investigate the morphology and particle size of combustion synthesized t-Mg 2 B 2 O 5 nanocrystals. As shown in Fig. 2 , TEM images of sample reveal the agglomerated non-uniform sized nanocrystals in the range of 80-110 nm with slightly elongated morphology. High resolution TEM image of sample as shown in Fig. 2 exhibits lattice planes corresponding to (0,-2,1) plane. The selected area electron diffraction (SAED) pattern has also been shown and d-spacing calculated from SAED pattern belongs to (150) and (1-21) planes of triclinic Mg 2 B 2 O 5 . The x-ray diffraction and SAED both confirm the formation of triclinic phase of Mg 2 B 2 O 5 nanocrystals.

TGA curve between weight % and temperature for Mg 2 B 2 O 5 is shown in Fig. 3 . It is observed that weight % of Mg 2 B 2 O 5 nanophosphors decreases rapidly from 50ºC to 226ºC, i.e., around 8.54% of weight (2.18 mol of crystal water) is lost. On increasing the temperature, change in weight loss % is around 3.34% in the temperature range from 226ºC to 600ºC. Further, only 1% weight is lost on raising temperature from 600 ºC to 800 ºC. These results show that combustion synthesized t-Mg 2 B 2 O 5 nanostructures are more immune to thermal decomposition i.e., thermally stable. In earlier published papers, researcher got more weight % loss during TGA analysis as compared with current scenario [10] , i.e., combustion technique is performed under optimum conditions for synthesis of Mg 2 B 2 O 5 nanophosphors in present study. DSC curve between heat flow and temperature is shown in Fig. 3 which is used for examination of transformation steps and their respective transformation enthalpies at various temperatures. These DSC-TGA experiments were Fig. 1 Rietveld refined XRD pattern of Mg 2 B 2 O 5 nanostructures and inset shows crystal structure obtained after rietveld refinement performed at 2 bar pressure of nitrogen gas at heating rate of 20ºC. Here, it is observed that till 600ºC mainly three events take place, the first one being an exothermic event with peak temperature 45.86 ºC and enthalpy change of -26.8901 j/g (-ve sign for exothermic). The second event is an endothermic with peak temperature 102.12 ºC and enthalpy change 14.3939 J/g, while third event is also endothermic with peak temperature at 225.05 ºC and has maximum enthalpy change 62.954 J/g. It can be noticed that the endothermic events are observed at temperatures where material is losing its weight which means it is the process where moisture is getting out. From 226ºC to 600ºC, there is not as much change in weight % of the sample as well as no phase change is observed in this range, so it is in agreement with structural properties that single-phase triclinic Mg 2 B 2 O 5 nanophosphor is synthesized under optimum conditions. Figure 4a shows the diffuse reflectance spectrum of t-Mg 2 B 2 O 5 nanocrystals, which shows band edge absorption around 250 nm and a broad absorption in the range 300-340 nm. The band edge absorption corresponds to band gap, while broad absorption around 300-340 may be ascribed to absorption in defect states such as oxygen and magnesium vacancies within the band gap as predicted by rietveld refinement analysis. We used Kubelka-munk function to convert reflectance into absorbance to be used in determination of band gap [3, 8] . Kubelka-munk function which is used to express intensity of diffused reflectance spectrum is as follows . 2 a TEM 

k s = 1 − R ∞ 2R 1 = f R 1 Fig

The electronic structure was calculated using density functional theory (DFT)-based Full Potential Linearized Augmented Plane-Wave (FPLAPW) method as implemented in WIEN2k program package [25] . In full potential calculations, the core states were treated fully relativistically, whereas for the valance states, a scalar relativistic approximation was used. Generalized gradient approximation (GGA) within the parameterization of Perdew-Burke-Ernzerhof was used to treat the exchange and correlation potential [26] . The radii of muffin-tin (RMT) spheres for various atoms were used in the present calculation to ensure the nearly touching spheres. Plane wave cut-off parameters were decided by RMTKmax = 7(where Kmax is the largest wave vector of the basis set). A 9*6*17 k-points mesh was used as the base for the integration in triclinic system which results in 460 k-points in the irreducible Brillouin Zone (IBZ). The energy convergence criterion was set to 0.0001Ry/cell and charge convergence was also monitored accordingly. DFT-based calculations were employed by using the experimentally determined lattice parameters to elucidate band structure of t-Mg 2 B 2 O 5 nanoparticles. The total density of states (TDOS) and the band structure of the system under investigation are shown in Fig. 5a . The system is a non-magnetic semiconductor with a direct band-gap along Γ-Γ direction. The calculated value of energy band-gap is 5.10 eV which is in good agreement with our experimental result. The slightly lower value of band gap is expected using GGA method. The TDOS of this system is mainly governed by O-atoms. Further, to have deep insight of the qualitative features of the DOS, partial (atom-resolved) DOS of Mg 2 B 2 O 5 is also explored (Fig. 5b) . It is clear that, except a deep-lying band (from ~ -8.4 eV to ~ -8.15 eV) having major contribution from B-p states, the valence band is dominated by O-p states with a small contribution from B-p states and even smaller from Mg-p states. The conduction band is composed predominantly of p states of O and B atoms.

The directed band gap nature prompted us to investigate photoluminescence (PL) of t-Mg 2 B 2 O 5 nanoparticles. Figure 6a shows the PL spectrum collected at an excitation wavelength of 325 nm using He-Cd laser. PL spectrum comprises of asymmetric broad peak with maxima at 588 nm, which may be ascribed to defects emission related to absorption in magnesium and oxygen vacancies within band gap of Mg 2 B 2 O 5 as predicted by rietveld analysis. Further spectroscopic investigations may be required to comment precise form of defects leading to emission in this system. Figure 6b also shows CIE diagram, depicting overall emission of t-Mg 2 B 2 O 5 in yellow-reddish region of visible region. CRI of sample is found to be 86 with x and y coordinates 0.44 and.043, respectively. Already well known for its mechanical and thermal properties, we have demonstrated here that t-Mg 2 B 2 O 5 nanoparticles being direct band-gap may further be tuned with suitable dopants and can be exploited in various luminescence-based applications. 

TL glow curve of UV (254 nm) irradiated Mg 2 B 2 O 5 nanostructures is shown in Fig. 7a . TL glow curve of Mg 2 B 2 O 5 nanostructures comprises of one prominent peak around 417-428 K and a shoulder around 573-589 K. TL signal, as a result of detrapping of trapped carriers in defect states due to increase in temperature which were filled upon UV irradiation, suggests that there may be two kinds of trapping or defect sites present in Mg 2 B 2 O 5 . TL glow curve for 2 min UV exposure shows that the first TL peak dominates over the second one. On increasing UV exposure time, intensity of the first TL peak decreases but there is not much of change in TL intensity of second peak as can be seen for UV exposure time of 5 and 30 min in Fig. 7b . On further increasing UV irradiation for 60 min exposure time, TL intensity of both peaks decreases. i.e., distribution of trapping sites is immensely affected by UV exposure time. Maximum TL intensity for first peak was found for 2 min UV exposure and maximum TL intensity for second peak was found for 30 min UV exposure. Figure 7b shows that intensity of low-temperature TL peak decreases with UV exposure time consistently, while second TL peak shows abrupt behavior. Further to get deep insight on the kinetic parameters such as activation energy, order of kinetics and frequency factor, we deconvoluted TL glow curves by TLanal software using Kiti's general order equation [27, 28] given as Figure 8 shows the deconvoluted TL curves of UV irradiated sample for different exposure time. The evaluated trapping parameters of Mg 2 B 2 O 5 nanophosphors exposed to UV radiation for different times are tabulated in Table 2 . It is clear that TL kinetics study of the sample shows the presence of closely lying four kinds of trapping states within the band gap of sample and their distribution changes with change in UV exposure time. The present study establishes that Mg 2 B 2 O 5 has trapping states giving rise highly UV responsive TL behavior which can be optimized for TL-based UV dosimetry.

Due to increase in drug resistance of pathogens to routinely used antibiotics, it is obligatory to find new and biocompatible antibacterial compounds. For this purpose, combustion synthesized t-Mg 2 B 2 O 5 nanostructures were examined for their antibiofilm activity against P. aeruginosa. The results of well diffusion assay on the effect of nanostructures at a dose of 10-100 μl each on the P. Aeruginosa. These results were further validated by measuring inhibition of bacterial growth kinetics based on OD620 measurement in a concentration range of 10-100 μg/ml of nanostructures. The data exhibit the dose dependent effect of nanostructures on the test strains in concentration range of 10-100 μg/ml and suggests maximum cytotoxic effect with 100 μg/ ml of nanostructures. The results in (Fig. 9) show the nanostructures concentration-dependent inhibition of biofilm formation. The data revealed 91 ± 2.7% inhibition of biofilms formed by P. aeruginosa, respectively, at 100 μg/ml after 24 h of treatment. They are regarded as causative agents of many infections in humans. The previous results consistent of this study also reported the antibacterial activity of magnesium oxide nanoparticles alone or in combination with other antimicrobials agents [29] . The mechanism of action of nanoparticles depends on their binding with bacterial surface as well as metabolism in the organism [30, 31] . The present results establish the antibacterial activity as an additional application of t-Mg 2 B 2 O 5 nanostructures which can further be optimized.

We have successfully synthesized single phase t-Mg 2 B 2 O 5 nanostructures using combustion method at relatively low temperature and optimum synthesis conditions are also supported by TGA-DSC analysis. HR-TEM and SAED images provided confirmation of triclinic phase from d-spacing corresponding to planes of triclinic structure. Experimentally calculated direct band gap (5.2 eV) from diffuse reflectance spectroscopy was found to be in good agreement with DFT calculations. The t-Mg 2 B 2 O 5 nanostructures exhibited . 9 Concentration dependent inhibition of bacterial biofilm. These data represent the mean ± SD of two independent experiments done in triplicate defects assisted PL emission in yellow-reddish region. TL kinetics analysis of UV irradiated samples shows the presence of four closely lying trapping sites and their distribution changes with UV exposure time. The present study confirms the direct band gap nature of Mg 2 B 2 O 5 nanostructures and these samples can be further optimized by adding suitable dopants for various luminescence-based applications and TL-based UV dosimetry. It has also been found that t-Mg 2 B 2 O 5 nanostructures show good antibacterial properties.

Catalyst-Free Synthesis

Zeitschrift für Anorganische und Allgemeine Chemie

WIEN2k, An Augmented Plane Wave Plus Local Orbitals Program For Calculating Crystal Properties

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.