The alkalinity of the precursor solution is an important parameter for controlling the morphology of the formed products.At higher concentration of the alkali, it was assumed that a largenumber of [Zn(OH)4]2units which act as the growing nuclei forthe rods are formed. At low and medium concentration of thepotassium hydroxide, it was considered that both Zn(OH)2 and[Zn (OH)4]2species are present in the solution. Although the A3and A2 rods form with the same mechanism, the A1 rods have thelowest aspect ratio with smaller diameter and shorter length.As the OHconcentration increases, the nucleation rate isincreased. But the growth rate is inhibited. Hence the aspect ratioof the rods formed is as follows: A1oA3oA2. Also, when theOHconcentration exceeds a saturation limit, the growth unitsare shielded by the excess OHions before they are incorporatedinto the crystal unit and hence the growth rate of the rods ishighly inhibited. For this reason, the crystallite size of A1 issmaller and has the lowest aspect ratio compared to the othertwo products. The same effect was well explained by QiXiao et al.[24]. Thus rods obtained at higher pH are shorter with lowestaspect ratio and aggregated. At lower alkalinity of the solution saypH of 11 and 9, well separated hexagonal nano rods along withfew tetrapod and flower-like morphology were exhibited by assynthesized products A2 and A3, respectively[25]. The formationof tetrapod and flower-like morphology could be attributed to theformation of some of the activated ZnO nuclei that differ intheir size.
3.5. Study of optical properties3.5.1. Band gap determinationTo investigate the optical properties and to determine theband gap of the as-synthesized nano rods, UV–visible spectrumwas recorded in the DRS mode using the UV–visible spectrometer(Model: UV–vis 2450). The percentage of reflection was transformed to absorbance using the Kubelka–Munk formula F(Ra)
whereRa¼Rsample/RstandardFRa ðÞhn2¼C2ðhnEgÞð3ÞwhereRis the reflection intensity, his Planck’s constant, nis thewave number,C2is the constant and Egis the band gap energy[26].Fig. 5shows plot made between [F(Ra)hn]2andhn. The opticalband gap was determined by extrapolating the steep segment ofthe sliding curve. The band gap energy calculated decreases in theorder A14A24A3 as the size of the products increases and theirvalues are listed inTable 2.To verify the effect of crystallinity of the products on UVemission wavelength, the Room Temperature Photo Luminescence spectra were recorded at the excitation wavelength of325 nm for the nano ZnO rods A2 and A3 with Xe light as theexcitation source. InFig. 6, the PL spectrum for synthesized nanorods A2 and A3 consists of high intense peaks centered at378.35 nm and 380.34 nm and peaks at 408 nm (NBE).The PL signal at400 nm is a typical ZnO UV emission wavelength [27]. It was observed that the UV emission peaks at380.34 nm and 408 nm of the A3 rods have higher intensity(337.3/au and 256/au) compared to those of A2 rods (302.2/auand 207/au). This also confirms the data obtained from XRD thatA3 rods have high crystallinity. The PL spectrum also has twoweak emission peaks at474 nm and 518 nm. The UV emissionpeak which is also known as near band emission (NBE) could beattributed to the recombination of free excitons through anexciton–exciton collision process [28]. The blue transition at474 nm is caused due to transition from the level of ionizedoxygen vacancy to the valence band. The green emission, calleddeep-level emission occurring by the recombination of the photogenerated holes with singly ionized oxygen vacancies in ZnO, iscentered at518 nm[29]. As the ratio of the intensity of the NBEpeaks and green emission peaks are high, our ZnO rods synthesized by a microwave assisted simple chemical precipitationtechnique have good optical properties [30] and so have lessconcentration of oxygen vacancies. Besides, the increased intensity of the deep-level emission in A3 compared to A2 reveals thatthe increased concentration of OHions enhances the surfacedefects A3 rod greatly[31].
The alkalinity of the precursor solution is an important parameter for controlling the morphology of the formed products.At higher concentration of the alkali, it was assumed that a largenumber of [Zn(OH)4]2units which act as the growing nuclei forthe rods are formed. At low and medium concentration of thepotassium hydroxide, it was considered that both Zn(OH)2 and[Zn (OH)4]2species are present in the solution. Although the A3and A2 rods form with the same mechanism, the A1 rods have thelowest aspect ratio with smaller diameter and shorter length.As the OHconcentration increases, the nucleation rate isincreased. But the growth rate is inhibited. Hence the aspect ratioof the rods formed is as follows: A1oA3oA2. Also, when theOHconcentration exceeds a saturation limit, the growth unitsare shielded by the excess OHions before they are incorporatedinto the crystal unit and hence the growth rate of the rods ishighly inhibited. For this reason, the crystallite size of A1 issmaller and has the lowest aspect ratio compared to the othertwo products. The same effect was well explained by QiXiao et al.[24]. Thus rods obtained at higher pH are shorter with lowestaspect ratio and aggregated. At lower alkalinity of the solution saypH of 11 and 9, well separated hexagonal nano rods along withfew tetrapod and flower-like morphology were exhibited by assynthesized products A2 and A3, respectively[25]. The formationof tetrapod and flower-like morphology could be attributed to theformation of some of the activated ZnO nuclei that differ intheir size.
3.5. Study of optical properties3.5.1. Band gap determinationTo investigate the optical properties and to determine theband gap of the as-synthesized nano rods, UV–visible spectrumwas recorded in the DRS mode using the UV–visible spectrometer(Model: UV–vis 2450). The percentage of reflection was transformed to absorbance using the Kubelka–Munk formula F(Ra)
whereRa¼Rsample/RstandardFRa ðÞhn2¼C2ðhnEgÞð3ÞwhereRis the reflection intensity, his Planck’s constant, nis thewave number,C2is the constant and Egis the band gap energy[26].Fig. 5shows plot made between [F(Ra)hn]2andhn. The opticalband gap was determined by extrapolating the steep segment ofthe sliding curve. The band gap energy calculated decreases in theorder A14A24A3 as the size of the products increases and theirvalues are listed inTable 2.To verify the effect of crystallinity of the products on UVemission wavelength, the Room Temperature Photo Luminescence spectra were recorded at the excitation wavelength of325 nm for the nano ZnO rods A2 and A3 with Xe light as theexcitation source. InFig. 6, the PL spectrum for synthesized nanorods A2 and A3 consists of high intense peaks centered at378.35 nm and 380.34 nm and peaks at 408 nm (NBE).The PL signal at400 nm is a typical ZnO UV emission wavelength [27]. It was observed that the UV emission peaks at380.34 nm and 408 nm of the A3 rods have higher intensity(337.3/au and 256/au) compared to those of A2 rods (302.2/auand 207/au). This also confirms the data obtained from XRD thatA3 rods have high crystallinity. The PL spectrum also has twoweak emission peaks at474 nm and 518 nm. The UV emissionpeak which is also known as near band emission (NBE) could beattributed to the recombination of free excitons through anexciton–exciton collision process [28]. The blue transition at474 nm is caused due to transition from the level of ionizedoxygen vacancy to the valence band. The green emission, calleddeep-level emission occurring by the recombination of the photogenerated holes with singly ionized oxygen vacancies in ZnO, iscentered at518 nm[29]. As the ratio of the intensity of the NBEpeaks and green emission peaks are high, our ZnO rods synthesized by a microwave assisted simple chemical precipitationtechnique have good optical properties [30] and so have lessconcentration of oxygen vacancies. Besides, the increased intensity of the deep-level emission in A3 compared to A2 reveals thatthe increased concentration of OHions enhances the surfacedefects A3 rod greatly[31].
การแปล กรุณารอสักครู่..
The alkalinity of the precursor solution is an important parameter for controlling the morphology of the formed products.At higher concentration of the alkali, it was assumed that a largenumber of [Zn(OH)4]2units which act as the growing nuclei forthe rods are formed. At low and medium concentration of thepotassium hydroxide, it was considered that both Zn(OH)2 and[Zn (OH)4]2species are present in the solution. Although the A3and A2 rods form with the same mechanism, the A1 rods have thelowest aspect ratio with smaller diameter and shorter length.As the OHconcentration increases, the nucleation rate isincreased. But the growth rate is inhibited. Hence the aspect ratioof the rods formed is as follows: A1oA3oA2. Also, when theOHconcentration exceeds a saturation limit, the growth unitsare shielded by the excess OHions before they are incorporatedinto the crystal unit and hence the growth rate of the rods ishighly inhibited. For this reason, the crystallite size of A1 issmaller and has the lowest aspect ratio compared to the othertwo products. The same effect was well explained by QiXiao et al.[24]. Thus rods obtained at higher pH are shorter with lowestaspect ratio and aggregated. At lower alkalinity of the solution saypH of 11 and 9, well separated hexagonal nano rods along withfew tetrapod and flower-like morphology were exhibited by assynthesized products A2 and A3, respectively[25]. The formationof tetrapod and flower-like morphology could be attributed to theformation of some of the activated ZnO nuclei that differ intheir size.
3.5. Study of optical properties3.5.1. Band gap determinationTo investigate the optical properties and to determine theband gap of the as-synthesized nano rods, UV–visible spectrumwas recorded in the DRS mode using the UV–visible spectrometer(Model: UV–vis 2450). The percentage of reflection was transformed to absorbance using the Kubelka–Munk formula F(Ra)
whereRa¼Rsample/RstandardFRa ðÞhn2¼C2ðhnEgÞð3ÞwhereRis the reflection intensity, his Planck’s constant, nis thewave number,C2is the constant and Egis the band gap energy[26].Fig. 5shows plot made between [F(Ra)hn]2andhn. The opticalband gap was determined by extrapolating the steep segment ofthe sliding curve. The band gap energy calculated decreases in theorder A14A24A3 as the size of the products increases and theirvalues are listed inTable 2.To verify the effect of crystallinity of the products on UVemission wavelength, the Room Temperature Photo Luminescence spectra were recorded at the excitation wavelength of325 nm for the nano ZnO rods A2 and A3 with Xe light as theexcitation source. InFig. 6, the PL spectrum for synthesized nanorods A2 and A3 consists of high intense peaks centered at378.35 nm and 380.34 nm and peaks at 408 nm (NBE).The PL signal at400 nm is a typical ZnO UV emission wavelength [27]. It was observed that the UV emission peaks at380.34 nm and 408 nm of the A3 rods have higher intensity(337.3/au and 256/au) compared to those of A2 rods (302.2/auand 207/au). This also confirms the data obtained from XRD thatA3 rods have high crystallinity. The PL spectrum also has twoweak emission peaks at474 nm and 518 nm. The UV emissionpeak which is also known as near band emission (NBE) could beattributed to the recombination of free excitons through anexciton–exciton collision process [28]. The blue transition at474 nm is caused due to transition from the level of ionizedoxygen vacancy to the valence band. The green emission, calleddeep-level emission occurring by the recombination of the photogenerated holes with singly ionized oxygen vacancies in ZnO, iscentered at518 nm[29]. As the ratio of the intensity of the NBEpeaks and green emission peaks are high, our ZnO rods synthesized by a microwave assisted simple chemical precipitationtechnique have good optical properties [30] and so have lessconcentration of oxygen vacancies. Besides, the increased intensity of the deep-level emission in A3 compared to A2 reveals thatthe increased concentration of OHions enhances the surfacedefects A3 rod greatly[31].
การแปล กรุณารอสักครู่..