The structural and optical behavior of Ag+ and Gd3+ ions in CdWO4

INTRODUCTION
Cadmium tungstate CdWO4 (CWO) is an interesting material in several respects including its optical, chemical, and structural features [1]. The optical properties of CWO have attracted extensive interest since CWO has been used as an inorganic scintillator for detecting ionizing radiations such as X- and gamma-rays [2], especially in medical imaging (computed tomography (CT) and positron emission tomography (PET)) [3], fluorescent substances, memory devices [4], and as a photocatalyst for water cleansing [5]. CWO has diverse morphologies and properties, and its optical properties strongly depend on its morphology and defects (shallow levels and deep levels produced by dopant ions), concentration, and valence of dopants [6, 7]. Scintillators may be either solid or liquid, intrinsic or extrinsic, and microcrystalline or nanocrystalline [3]. The CWO advantages over other scintillator materials include its almost high X-ray absorption coefficient and scintillation output; however, its response time is relatively long [1]. An intrinsic scintillator is a transparent single crystal free of impurities and electronic energy levels in its bandgap [3]. Most scintillators are extrinsic types doped with activator ions. Activators introduce energy levels in the forbidden bandgap. In recent years most investigated scintillators were usually in the form of single crystals. The processes of crystal growth, such as Bridgman [8] and Czochralski [7, 9], have their problems including doping dopants being at the bottom of the crystal, cracking the crystal during the cooling process, and introducing twins and macro defects such as light scattering centers and blocks in the crystal structure [10]. Chemical methods for synthesizing CWO nanopowders are more time-saving, affordable, applicable, and simple [11]. Up to now, tungstate nanoparticles have been synthesized by using various chemical methods such as sol-gel [12, 13], co-precipitation [11, 14, 15], and hydrothermal [10]. In addition, doping by rare earth elements (RE) is a practical method for enhancing the electronic and optical properties of CWO for the new generation of optoelectronic and scintillation applications. Gadolinium (Gd) is a rare earth element (Lanthanide) having an electron configuration of [Xe] 4f75d16s2 and numerous energy levels used as an element in television screens, medical imaging, and diagnosing cancerous tumors. Ag is a transition metal with an electron configuration of [Kr] 4d105s1 having optical and plasmonic advantages widely used in producing sensors and optical detectors [11, 14, 16]. Ag nanoparticles have broad-spectrum antibacterial activities and good inhibitory effects on Gram-positive and Gram-negative bacteria, fungi, viruses, and cancer cells, making them one of the most important antimicrobial materials [17]. Ag3PO4 and CuWO4-Ag3PO4 nanoparticles considerably remove rhodamine B dye from effluents and water sources [18]. The excitation level of Ag has overlapped with the 3T1u level of the WO66- complex [11], which may lead to the fluorescence resonance energy transfer (FRET) in CWO: Ag nanopowders. Co-doping is another feasible way of enhancing the optical properties of the materials by introducing more surface defects incorporating additional bands near the valence/conduction band edges. Few papers have reported the cost-effective and easy method for synthesizing the CWO, single-doped, and co-doped CWO nanopowders [11, 16]. In this research, CWO, CWO: Ag/Gd, Ag and Gd-doped CWO (co-doped CWO) fluorescent nanopowders were synthesized by using the low-cost co-precipitation technique. Afterward, we investigated their structural and optical properties by applying various methods. Further, the synthesized nanopowders were examined for their novel optical properties against UV-Vis and ion beams. They have been suitable candidates for future photonic applications to produce flexible scintillator nanocomposites and light-emitting diodes (LEDs) on a large scale.

EXPERIMENTAL DETAILS 
Samples preparation
Cadmium acetate dihydrate (Cd (CH3COO)2·2H2O), sodium tungstate (Na2O4W), silver nitrate (AgNO3), and gadolinium (III) nitrate (Gd(NO3)3) were purchased from Sigma-Aldrich. Based on our previous works, CWO, CWO: 0.5 at%Ag / 0.5 at%Gd, and 0.5 at%Ag, 0.5 at% Gd-doped CWO nanoparticles were synthesized by using an affordable co-precipitation method [11, 16]. To synthesize the CWO, first, the equal molar ratio of cadmium acetate dihydrate Cd(CH3COO)2·2H2O and sodium tungstate (Na2WO4) were discretely dissolved in distilled water. Then, sodium tungstate solution was added to cadmium acetate dihydrate solution, and the white precipitate was centrifuged for 5 times and dried at 80ºC for 24 h. The powder was calcined at 600 ºC for 3hr. In addition, to synthesize the CWO: 0.5 at% Ag / 0.5 at% Gd nanopowders, first, 0.5at% silver nitrate/0.5%at gadolinium (III) nitrate were individually dissolved in distilled water, then silver nitrate/ gadolinium (III) nitrate and sodium tungstate solutions were mixed and added dropwisely to the cadmium acetate dihydrate solution. Furthermore, the washing, drying, and calcination steps were conducted as mentioned above. To prepare co-doped CWO nanopowders, a mixture of silver nitrate, gadolinium (III) nitrate, and sodium tungstate solutions was added dropwisely to cadmium acetate dihydrate solution. The rest of the steps were performed as mentioned above.

Instrumentation
The crystalline structure of synthetic nanopowders was characterized by X-ray diffraction (Bruker D8 diffractometer with Cu Kα (λ= 1.54 Å.) The elemental states and surface composition of nanopowders were analyzed by X-Ray Photoelectron Spectroscopy (XPS)-BESTEC (EA10). The morphology and size of the nanoparticles were investigated by Transmission Electron Microscope (TEM-Ziess EM900) and Filed Emission-Scanning Electron Microscope (FE-SEM-TESCAN MIRA). The optical properties of nanopowders were investigated using UV–Vis absorbance and Photoluminescence (PL) spectra (Avaspec-2048-TEC and Perkin-Elmer LS-5 Fluorescence Spectrometer). Ion-Beam Induced Luminescence (IBIL) measurement was performed at room temperature with focused micro-beam irradiation conditions with an energy of 2.2 MeV and electrical current of around 4nA.

RESULTS AND DISCUSSION
Characterizations
XRD
Figure 1(a, b) presents the diffractograms for the CWO and CWO: Ag/ Gd and co-doped CWO and 01-087-1114 data of CWO nanopowders, respectively.
According to the XRD diffractograms (Figures 1(a, b)), the structure of nanopowders was monoclinic wolframite belonging to the space group P2/c, matched well with the standard JCPDS files (No. 01-087-1114) [11]. Based on Figure 1(c), the intensity of primary peaks (-111) and (111) in single- and co-doped CWO has increased due to the increased crystallinity. A slight shift in the co-doped CWO diffraction peaks was related to the substitution of Cd2+ with Ag+ and Gd3+ ions, which introduced some crystallographic defects into the CWO lattice [16]. XRD diffractograms of single-doped CWO shifted toward lower angles due to the increase in the d space between crystallographic planes. 

XPS
The elemental and chemical analyses of nanopowders were performed using XPS, shown in Figure 2.
According to Figure 2a, all nanopowders exhibited the characteristic peaks of the constituent elements. Five strong peaks at 534, 409, 418, 43, and 620eV correspond to the O1s, Cd3d5/2, Cd3d3/2,W4f, and W4s, respectively. The peaks at 19, 262, 433, and 658eV correspond to the O2s, W4d, W4p, and Cd3p, respectively. The auger peaks of O KLL and W N4N67N7 were located at 937 and 1110eV, respectively. According to Figure 2(b, c, and d), the Cd3d5/2, Cd3d3/2, W4p, W4f7/2, W4f5/2, W5p3/2, and O1s in doped nanopowders shifted toward lower binding energies. The peaks at 584 and 1120 correspond to Ag3p and Gd3d shown in single- and co-doped CWO (Figure 2(a, e)). All results are consistent with the findings in the articles [19-22]. 

SEM and TEM
FE-SEM and TEM images of 4 nanopowders are presented in Figure 3.
According to the FE-SEM images, nanoparticles in all samples were agglomerated. As shown in Figure 3 (e, f, g. h), the mean particle sizes of CWO, CWO: Gd, CWO: Ag, and co-doped CWO were 63, 42,58, and 24.2nm, respectively, which were smaller than the sizes of the particles synthesized using the sol-gel method (average size of 100 nm) by Alamdari et al. [13]. The dopant ions affected the size and shape of nanoparticles. The shape of the CWO nanoparticles was irregular while CWO: Ag and co-doped nanoparticles have nanorod shapes. The CWO: Gd nanoparticles were approximately circular and smaller than the CWO nanoparticles. The particle size distribution of CWO: Ag and co-doped CWO was less than that of the others. Ag+ ions introduced some nanorods to the particles while Gd3+ ions introduced some spheres to the system. The size of particles in the co-doped CWO was approximately ½ that of CWO.

Luminescence properties 
UV-Visible absorbance spectra
The bandgap energy of nanomaterials can be inferred from various measurement techniques including photoacoustic spectroscopy (PAS) technique [23], UV–Vis spectroscopy (UV–vis) [24], reflectance spectroscopic ellipsometry [25, 26], photoluminescence spectroscopy (PLS) [26], and valence electron energy-loss spectroscopy (VEELS) by monochromated electrons [27]. Among these techniques, the UV–Vis is the easiest, most cost-effective, and most preferred method for bandgap energy determination. For this purpose, the discrete solutions of CWO, CWO: Ag/Gd, and co-doped CWO nanopowders were exposed to UV-Vis light. The UV-Vis absorbance and Tauc plots of 4 samples are shown in Figure 4.
As shown in Figure 4(a), the absorption edge of all samples was around 239-244nm. The optical bandgap energy of nanopowders was evaluated according to the Tauc method by simply fitting a straight line (Tauc line) to the linear (or steepest) region of the optical spectrum to intersect the photon energy (h) axis [11, 24]. The bandgap values for CWO, CWO: Gd, CWO: Ag, and co-doped CWO nanopowders were about 5.3, 5.5, 5.55, and 5.48eV, respectively, as shown in Figures 4(b, c, d, e). The difference among bandgap energy of 4 samples was about 0.1-0.2eV relating to the additional states produced by Ag+ and Gd3+ ions [16]. Ag+ ions introduced excited-state levels near the 3T1u levels and participated in FRET [11], leading to the increase of the bandgap energy (Eg) in CWO: Ag by 0.25. Gd3+ ions introduced traps (deep levels) near the 1T1u level and captured the excited electrons of the WO66- complex [11], resulting in increasing the Eg in CWO: Gd by 0.2. The estimated bandgaps are more than that of CWO single crystal (2.9-4.15eV) [28, 29]. Besides other physical and chemical properties of nanomaterials, the change in the bandgap of a semiconductor from its bulk value were due to different reasons such as doping, the synthesis parameters, strain in the lattice, crystallite size, calcination temperature, microstructure, and composition [24].

IBIL spectrum
Ion beam-induced luminescence characterization is an advantageous method for investigating defects, impurities, dopants, and chemical compounds in material. The ionoluminescence response of nanopowders was investigated at room temperature. CWO, CWO: Gd, CWO: Ag, and co-doped CWO nanopowders as a target exposed to the focused micro-beam irradiation applying 2.2 MeV protons and 4nA current coupled to PerkinElmer LS-5 Fluorescence Spectrometer. The photons that emitted from the target materials were recorded, and the scintillation response of nanopowders was investigated and reported in Figure 5(a).
All samples exhibited a peak at around 497nm, as shown in Figure 5(a). Total counts in CWO: Ag drastically increased while their value decreased in CWO: Gd and co-doped CWO. Based on the XRD results, Ag+ ions increased the crystallinity more than Gd3+ ions did. Localized surface plasmon resonance band of Ag+ overlapped with the energy level of 3T1u within the WO66-complex; as a result, excited electrons in Ag+ transferred to the WO66- complex and participated in fluorescence resonance energy transfer. In CWO: Gd, the 6IJ level of Gd3+ almost overlapped the energy level of 1T1u in the WO66- complex; therefore, the excited electrons were trapped in deep levels of Gd3+ and tolerated non-radiative transition. In co-doped CWO, the effect of Gd3+ was more than that of Ag+; thus, the intensity of IBIL spectra decreased. The IBIL spectra of all samples showed a shoulder at 462nm related to the PL emission peak. The results are consistent with the results of other reseach [30, 31].
PL spectrum
Figure 5(b) shows the PL spectra of CWO, CWO: Gd, CWO: Ag, and co-doped CWO nanopowders. All samples exhibited a peak in the blue-green range at 469nm. X-ray-induced-luminescence measurements by Alamdari et al. also showed a strong emission peak at 400-550 nm [13]. The total PL intensity in CWO: Ag nanoparticles with rod shapes increased while its value in both CWO: Gd and co-doped CWO with circular and rod shapes decreased.  The blue-green emission was attributed to the 3T1u1A1u transition inside the WO66- complex [11]. The characteristic peaks of dopant ions have not been seen in IBIL and PL spectra since the excited electrons in the SPR band of Ag+ preferred to transfer to the 3T1u level of the WO66- complex. When the concentration of the Ag+ ions in CWO nanopowder is higher than the threshold, some electrons of the 3T1u may go to the SPR band of Ag+ ions. Therefore, the characteristic peak of Ag+ion may appear and the intensity of blue-green emission may decrease. In CWO: Gd, the captured electrons in the 6IJ level of Gd3+ tolerated non-radiative transitions; therefore, the characteristic peaks of Gd3+ have not been seen in IBIL and PL spectra. The effect of Gd3+ in co-doped CWO was more than the effect of Ag+; thus, the total PL intensity reduced to CWO and CWO: Ag while it increased to CWO: Gd. 

CONCLUSION
In this paper, the microstructure and optical response of CWO, CWO: Gd, CWO: Ag, and co-doped CWO nanopowders synthesized by using a simple and affordable co-precipitation method were investigated. Ag+ions increased the crystallinity of CWO, and introduced nanorods to the nanoparticles as well. They introduced defect levels and enhanced the optical properties of CWO by surface plasmon resonance (SPR). Gd3+ ions also increased the crystallinity of CWO and produced some spheres in nanoparticles, introduced some non-radiative levels to the CWO, and reduced the optical properties of CWO. Co-doping CWO with Ag+and Gd3+ ions also increased the crystallinity, introduced nanorods to the particles and defects to the electronic structure; the optical properties of co-doped CWO were more than the CWO: Gd’s. The synthesized nanopowders with high density and stopping power, high purity, high total light output, high PL and IBIL intensities, and non-hygroscope are good choices for medical imaging and CT scanners for high and low energies rather than NaI(Tl). The nanopowders can be dispersed in the polymer or glass matrices and used on large scales for detecting the gamma, X, and alpha rays.