Formation of Co–O bonds and reversal of thermal annealing effects induced by X-ray irradiation in (Y, Co)-codoped CeO2 nanocrystals

We report an unconventional effect of synchrotron X-ray irradiation in which Co–O bonds in thermally annealed (Y, Co)-codoped CeO2 nanocrystal samples were formed due to, instead of broken by, X-ray irradiation. Our experimental data indicate that escaping oxygen atoms from X-ray-broken Ce–O bonds may be captured by Co dopant atoms to form additional Co–O bonds. Consequently, the Co dopant atoms were pumped by X-rays from the energetically-favored thermally-stable Co-O4 square-planar structure to the metastable octahedral Co-O6 environment, practically a reversal of thermal annealing effects in (Y, Co)-codoped CeO2 nanocrystals. The band gap of doped CeO2 with Co dopant in the Co-O6 structure was previously found to be 1.61 eV higher than that with Co in the Co-O4 environment. Therefore, X-ray irradiation can work with thermal annealing in opposing directions to fine tune and optimize the band gap of the material for specific technological applications.


Scientific Reports
| (2022) 12:1578 | https://doi.org/10.1038/s41598-022-05691-0 www.nature.com/scientificreports/ of Co oxygen coordination to optimize the properties of the material for specific applications. As demonstrated in our previous works, introduction of Co dopant atoms can switch-on substantial decrease of band gap with increasing Y codopant concentration 15 and rearrangement of oxygen coordination around Co dopant atoms can lead to a dramatic band gap variation of 1.61 eV 14 in the CeO 2 host. We have thus developed a unique method to fine tune the band-gap-related electronic and optical properties of (Y, Co)-codoped CeO 2 nanocrystals using thermal annealing and X-ray irradiation for various technological applications.

Experimental
Samples of (Y, Co)-codoped CeO 2 nanocrystals were synthesized using a polyol method 15 . The as-grown samples were then annealed under pure O 2 at 300 °C for 30 min with a ramping rate of 10 °C/min such that the square-planar oxygen coordination is dominant surrounding the Co dopant atoms 14 . The Y and Co concentrations of the sample are 36.9 and 0.6 at. %, respectively, as determined from inductively coupled plasma mass spectrometry (ICPMS) measurements. As shown in Fig. 1, the synchrotron-based X-ray powder diffraction (XRD) data indicate that both the as-grown and annealed samples have good crystallinity. Despite the heavy Y doping, the XRD patterns match well with that of cubic CeO 2 at the (111), (200), (220), (311), (222) and (400) Bragg peaks. The crystallite sizes of the as-grown and annealed samples were determined from XRD using the Scherrer equation to be 3.7 and 3.9 nm, respectively. To investigate the effect of X-ray irradiation, the thermally annealed sample was irradiated with synchrotron X-ray beam at photon energy 7 keV. The spot size of the X-ray beam on the sample is 1.5 mm × 0.5 mm and the photon flux is 1.2 × 10 12 photons per second. Local structures surrounding the cobalt dopant atoms in the as-grown, annealed, and X-ray irradiated samples were probed by using Co-K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques. For comparison, the Ce-L 3 -edge and Y-K-edge XAFS (XANES and EXAFS) data for the above samples were also measured and analyzed, shown as supplementary information online. Cerium L 3 -edge and Co K-edge XANES data of samples irradiated for different time durations were measured to elucidate the relation between the increase of oxygen vacancies surrounding the Ce constituent atoms of the host due to X-ray irradiation and the change of oxygen coordination surrounding the Co dopant atoms. All X-ray measurements and treatments were performed at beamline 07A of Taiwan Light Source (TLS) at National Synchrotron Radiation Research Center (NSRRC) in Taiwan.

Results and discussion
As shown in Fig. 2a, the XANES curve of the as-grown sample is very different from that of the sample annealed under O 2 at 300 °C for 30 min. When the annealed sample was further treated with X-ray irradiation for 4 h, its XANES curve largely resumed the signatures of that of the as-grown sample. Since the Co K-edge XANES data basically represent the average local chemical environment surrounding the Co dopant atoms, we can see that the Co local structure in the as-grown sample was substantially altered by thermal annealing and then restored by the subsequent X-ray irradiation. As reported in one of our papers, the local structures of Co dopant atoms exhibit bistability in the CeO 2 host. Thermal annealing was found to switch the local coordination environment from the initial octahedral structure (Co-O6) to the thermally stable square-planner (Co-O4) structure 14 . The XANES curves in Fig. 2a for the as-grown sample and the thermally annealed sample after X-ray irradiation resemble that theoretically simulated for the octahedral structure while that for the annealed sample before irradiation resembles that simulated for the square-planner structure. Detailed theoretical simulation using the FDMNES package is described in the supplementary information online 16,17 . Therefore, our XANES results indicate that X-ray irradiation can restore the octahedral oxygen coordination surrounding the Co atoms in the sample after it was switched to the square-planner coordination by thermal annealing the as-grown sample.  Table 1 demonstrate that the pronounced peak in the FT for the as-grown sample is due to 6.0 ± 0.3 O neighboring atoms at a distance 2.07 ± 0.01 Å from the central Co atom while that for the thermally annealed sample is a composite peak due to 4.0 ± 0.4 O and 1.4 ± 0.3 Ce atoms at distances 1.87 ± 0.01 Å and 3.21 ± 0.01 Å from the Co atom, respectively. The Co K-edge EXAFS results for the as-grown and thermally annealed sample are consistent with the octahedral and square-planner models of O coordination surrounding Co, respectively. After the annealed sample is treated with X-ray irradiation, the pronounced peak in the FT is ascribed to 5.4 ± 0.1 O atoms at a distance of 2.04 ± 0.01 Å indicating the O coordination around Co was largely switched from the square-planner structure of the annealed sample back to the octahedral structure of the asgrown sample.
N is the coordination number. R is the bond length. σ 2 is the Debye-Waller-like factor serving as a measure of local disorder. ΔE 0 is the difference between the zero kinetic energy value of the sample and that of the theoretical model used in FEFF. R-factor is a residual factor representing the goodness of fit. Uncertainties were estimated by the double-minimum residue (2χ 2 ) method. The amplitude reduction factor ( S 2 0 ) representing the central atom shakeup and shakeoff effects used in the curve-fitting is 0.72 as obtained from our previous papers 14 .
The Ce L 3 -edge and Co K-edge XANES curves of samples irradiated for different time durations are plotted in Fig. 3a and b, respectively. Curve-fittings using an arctangent function to simulate the edge jump and Gaussian functions for peak features were carried out to extract Ce 3+ concentration from the Ce L 3 -edge XANES spectra. Details of the curve-fitting method have been reported in one of our previous papers 19 . The Co-O4 composition for each irradiated sample was estimated by curve-fitting its respective Co XANES spectrum as a linear combination of the XANES spectra for the as-grown and thermally annealed samples as the model curves for the Co-O6 and Co-O4 structures, respectively. As shown in Fig. 4, the Ce 3+ concentration increases with irradiation time  The Ce L 3 -edge and Y K-edge XAFS (XANES and EXAFS) data for the as-grown, annealed, and X-ray irradiated samples are shown as supplementary information online. The χ ranges used in Fourier transforms are 2 to 9 Å −1 and 3 to 11.5 Å −1 for Ce and Y EXAFS data, respectively. As shown in Supplementary Table S1 online, our Ce-L 3 -edge EXAFS curve-fitting revealed that the coordination number 5.6 ± 1.1 of the first (O) shell surrounding Ce in the X-ray irradiated sample is appreciably smaller than that in the as-grown sample (6.5 ± 1.3). This indicate that X-ray irradiation can indeed break Ce-O bonds, consistent with the Ce XANES results. The Y K-edge EXAFS results shown in Supplementary Table S1 online also show slightly decreased coordination number (from 6.9 ± 0.7 to 6.3 ± 0.8) of the first (O) shell surrounding Y as a result of X-ray irradiation, similar to the effect on the Ce-O bonds.
As shown in Fig. 5, we propose the following model for the correlation between the breakaway of O atoms from Ce and the switching of Co-O4 structure to Co-O6 structure around Co atoms due to X-ray irradiation on the thermally annealed sample. Irradiating the sample with X-rays can break the Ce-O bonds, leading to

Conclusion
We have previously shown that Co in (Y, Co)-codoped nanocrystals of CeO 2 can have two distinct types of oxygen coordination. After thermal annealing at elevated temperatures, the Co dopant atoms tend to form the energetically favored Co-O4 square-planar structure in the CeO 2 host. Consequently, the band gap of the codoped CeO 2 undergoes a dramatic reduction. In this work, we observed that X-ray irradiation can induce reversal of thermal annealing effects that additional Co-O bonds were formed and the Co dopant atoms were pumped back to the metastable octahedral Co-O6 environment as a result of X-ray irradiation. Ionizing X-ray radiation is normally known to break chemical bonds and cause radiation damage. The formation of Co-O bonds observed in this work appears to be a peculiar example of unconventional effects of X-rays on condensed matters. It also provides a special opportunity to fine tune and optimize the electronic properties of codoped CeO 2 nanocrystals using thermal annealing and X-ray irradiation in opposing directions for technological applications.

Methods
A polyol method was used to synthesize (Y, Co)-codoped CeO 2 nanocrystal samples. Synchrotron-based XRD data was used to reveal the crystal structures of the samples and to determine particle sizes using the Scherrer equation. The Y and Co concentrations in the samples were obtained from inductively coupled plasma mass spectrometry (ICPMS) measurements. Thermally annealed samples were irradiated by synchrotron X-ray beam of spot size 1.5 mm × 0.5 mm and photon flux 1.2 × 10 12 photons per second to investigate the effect of X-ray irradiation. To probe the local structural variation due to X-ray irradiation, Co K-edge and Ce L 3 -edge X-ray absorption near edge structure (XANES) and Co K-edge extended X-ray absorption fine structure (EXAFS) data were measured for the as-grown, annealed, and X-ray irradiated samples at beamline 07A of Taiwan Light Source (TLS) at National Synchrotron Radiation Research Center (NSRRC) in Taiwan.