# XPS on Co\({}_{0.95}\)R\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) Nanoparticles with R = Gd or Ho
## 2 Materials and Methods  
### Sample Preparation  
Cobalt ferrite nanoparticles doped with 5 mol% gadolinium or holmium were synthesized using a sol-gel approach incorporating bio-based agents. The substitution followed the formula Co\({}_{0.95}\)R\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\), where R represents either Gd\({}^{3+}\) or Ho\({}^{3+}\). Despite replacing divalent Co\({}^{2+}\) with trivalent rare-earth ions, the system preserves structural integrity, likely through local charge compensation mechanisms within the spinel lattice.  
The precursors used were Fe(NO\({}_{3}\))\({}_{3}\cdot\)9H\({}_{2}\)O, Co(NO\({}_{3}\))\({}_{2}\cdot\)6H\({}_{2}\)O, Gd(NO\({}_{3}\))\({}_{3}\cdot\)6H\({}_{2}\)O, and Ho(NO\({}_{3}\))\({}_{3}\cdot\)5H\({}_{2}\)O, sucrose, and pectin, all of high-purity analytical grade and sourced from Alfa Aesar. A total of 2 mmol of final product was targeted per batch. Stoichiometric quantities of the nitrate salts were dissolved in deionized Milli-Q water. Each component was dissolved separately for half an hour, adding water gradually until full dissolution. The as-obtained solutions were poured together and stirred at 60 \({}^{\circ}\)C for 1 hour to ensure full dissolution.  
To support a green synthesis process, 14.9 mmol of sucrose (approx. 5 g) was introduced as a renewable polycondensation agent. The solution was further stirred for another 30 min. After homogenization, pH was adjusted to approximately 2 by using a 65% HNO\({}_{3}\) solution. One gram of citrus-derived pectin was gradually added to the solution. Pectin was used to stabilize the polymeric growth to prevent agglomeration of spinel nanoparticles during the thermal treatment. Continuous stirring was maintained throughout to preserve homogeneity.  
The pH facilitated the polycondensation of the sucrose-metal polymeric network that was formed in the preceding synthesis step. Sucrose and pectin were selected as suitable agents for polycondensation and chelation due to several compelling advantages. Primarily, their non-toxic nature poses no threat to either the environment or human health, making them exceptionally safe choices. Furthermore, their abundance in natural sources ensures a sustainable and readily available supply. Last but not least, their use is supported by favorable economic considerations, offering a cost-effective solution for these processes.  
The solution was poured into ceramic crucibles and heated in a sand bath at 240 \({}^{\circ}\)C for 24 h to evaporate the water and induce gelation. The gradual evaporation of the water resulting from heat coming from under the crucible facilitates the homogenous growth of a spongy, porous gel. The resulting xerogel was then calcined in ambient air at 700 \({}^{\circ}\)C for 2 h to remove organics and form crystalline ferrite nanoparticles.  
This synthesis strategy provides an effective, sustainable route for incorporating rare-earth elements into cobalt ferrite structures. The use of natural complexing agents--sucrose and pectin--not only eliminates the need for hazardous chemicals like alkoxysilanes, tetraethyl orthosilicate (TEOS), poly-alcohols, or ammonia, but also aligns with principles of green chemistry by reducing ecological impact.