Spin effects in regulating the adsorption characteristics of metal ions

Abstract

Understanding the adsorption behavior of intermediates at interfaces is crucial for various heterogeneous systems, but less attention has been paid to metal species. This study investigates the manipulation of Co³⁺ spin states in ZnCo₂O₄ spinel oxides and establishes their impact on metal ion adsorption. Using electrochemical sensing as a metric, we reveal a quasi-linear relationship between the adsorption affinity of metal ions and the high-spin state fraction of Co³⁺ sites. Increasing the high-spin state of Co³⁺ shifts its d-band center downward relative to the Fermi level, thereby weakening metal ion adsorption and enhancing sensing performance. These findings demonstrate a spin-state-dependent mechanism for optimizing interactions with various metal species, including Cu²⁺, Cd²⁺, and Pb²⁺. This work provides new insights into the physicochemical determinants of metal ion adsorption, paving the way for advanced sensing technologies and beyond.

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Introduction

Adsorption and desorption of intermediates on solid surfaces are fundamental components in various heterogeneous systems, particularly in catalysis,^1,2 environmental science,^3,4 and materials engineering.^5,6 These processes are critical in understanding reaction pathways that occur at solid-involved interfaces and optimizing device efficiencies.^7,8 For instance, typical catalysis on solid surfaces consists of the adsorption of reactants, the chemical transformations of intermediates, and the desorption of products.⁹ Based on these processes, the Sabatier principle demonstrates the importance of optimal catalyst surfaces that bind reactants neither too strongly nor too weakly.¹⁰ This balance ensures sufficient interaction for the reaction to occur while allowing products to desorb efficiently, which indicates a primary strategy in catalyst design.^11–13 The subsequent development of d-band theory further enriches the catalysis toolbox by explaining how the electronic properties of catalysts affect their adsorption characteristics with adsorbates.^14–16 In addition, the impact of geometric properties for these processes has also been well studied, providing further guidance for the optimization of catalyst surfaces.^17,18 While understanding these behaviors is essential for the design of solid surfaces to improve heterogeneous systems, current research primarily focuses on non-metallic adsorbates (such as C,¹⁹ H,²⁰ and O²¹), often overlooking their interaction with metal species.

Unveiling the physicochemical parameters that govern the adsorption and desorption processes of metal adsorbates at interfaces holds great promise in metallurgy,²² metal-ion batteries,^23,24 and sensor technology.²⁵ Electrochemical sensing of metal ions involves adsorption, migration, electron transfer, and desorption processes of metal species at the electrochemical interface, enabling effective investigation of their interaction with solid interfaces.^26,27 However, research on the adsorption processes of metallic species, similar to that of the non-metallic adsorbates, is still limited in tuning the electronic structures through doping, morphology, crystal planes, and defects.^28,29 Recently, spin, an intrinsic property of electrons, has emerged as another promising degree of electronic freedom to regulate the electronic structure of various catalyst surfaces.^30–32 However, the spin state of active centers is influenced by splitting energy and electron pairing energy, making it challenging to directly modulate the spin state.³³ Consequently, the correlation between spin properties and adsorption characteristics of metal species remains largely unexplored.

Herein, we address this challenge by correlating the spin state of Co³⁺ sites with their adsorption characteristics for various metal ions, including Cu²⁺, Cd²⁺, and Pb²⁺ ions. Transition metal oxides are employed due to their strong interdependence between spin, charge, orbital, and lattice degrees of freedom, creating a highly interactive system for manipulating spin states through various pathways.³⁴ Spinel oxide ZnCo₂O₄ is an ideal choice because Co³⁺ cations are confined to octahedral sites, allowing exclusive investigation of CoO₆ octahedra. The manipulation of the Co³⁺ spin state is achieved by calcination-induced lattice distortion, which is carefully verified through density functional theory (DFT) calculations, Fourier-transform infrared spectroscopy (FTIR), X-ray absorption spectra (XAS) and Superconducting Quantum Interference Device (SQUID) analysis. Increasing the calcination temperature leads to an increase in the spin state of Co³⁺ cations, causing the d-band center to shift downward relative to the Fermi level. The lower energy level of the d-band center results in the antibonding orbitals being occupied by more electrons, weakening the adsorption affinity between the Co³⁺ sites and metal ions. As the desorption process plays a major role in determining the sensing signal, weakened metal ion adsorption leads to enhanced sensing performances. This positive correlation between the spin state and sensing activity applies to various metal ions, including Cu²⁺, Cd²⁺, and Pb²⁺ ions. This work sheds light on the understanding of spin properties in optimizing the adsorption characteristics of metal species and would spur new strategies in optimizing electrochemical sensors by spin engineering.

Results and discussion

In the spinel oxide ZnCo₂O₄, Zn²⁺ and Co³⁺ cations occupy four-coordinated tetrahedral and six-coordinated octahedral sites, respectively (Fig. 1a). Zn²⁺ cations are considered inert with limited electrochemical activity due to their filled 3d orbital and lower electron affinity compared to other catalytically active 3d-transition metals. Therefore, the Co³⁺ sites in octahedral units play a dominant role during the electrochemical adsorption and desorption processes of metal ions.²⁷ The spin state of metals is determined by the crystal field splitting energy and electron pairing energy.³⁵ Due to the high crystal field splitting energy, Co³⁺ typically favors the low-spin state (t⁶_2ge⁰_g) in an octahedral geometry, where the t_2g orbitals are fully occupied and the e_g orbitals are completely empty (Fig. 1b). The empty e_g orbitals provide enormous opportunities for spin state engineering. As shown in Fig. 1b, the e_g orbitals can accommodate up to two unpaired electrons, resulting in the intermediate-spin state (t⁵_2ge¹_g) and high-spin state (t⁴_2ge²_g), respectively. Thus, the electronic structure of Co³⁺ sites can be effectively regulated through spin engineering, resulting in significant changes in electrochemical activity.

Fig. 1 Structural characterization. (a) Crystal structure of spinel ZnCo₂O₄ with Zn²⁺ and Co³⁺ cations occupying tetrahedral and octahedral sites, respectively. (b) d-Electron configurations of the Co³⁺ cation in different spin states in the spinel ZnCo₂O₄. (c) AC-HAADF-STEM image of ZCO-600, viewed along the [011̄] zone axis. Theoretical models with octahedral and tetrahedral arrangements are overlaid on the image, with blue and pink atoms representing Zn²⁺ and Co³⁺ cations, respectively. (d) FFT pattern of the TEM image in (c). (e) XRD patterns of the as-prepared spinel ZnCo₂O₄ samples, corresponding to the standard pattern PDF# 23-1390. (f)Co L_2,3-edge XAS spectra of the as-prepared ZnCo₂O₄ oxides and Co₂O₃ reference. The red area indicates the Co³⁺ high-spin state peak,⁴⁴ and the blue area indicates the Co³⁺ low-spin state peak.⁴⁵ (g) Octahedral distortion (ε) associated with an increase in the spin state. The specific computational details are summarized in the ESI.† The inset shows the model changes caused by lattice distortion. (h) FTIR spectra of the as-prepared spinel ZnCo₂O₄ oxides.

A series of ZnCo₂O₄ spinel oxides with varied Co³⁺ spin states was obtained by tuning the calcination temperatures to induce lattice distortion during the sol–gel synthesis (i.e. 300, 400, 500, and 600 °C, denoted as ZCO-300, ZCO-400, ZCO-500, and ZCO-600, respectively). Current methods for controlling the spin state include lattice distortion,^36,37 defect engineering,^38,39 heteroatom doping,^40,41 and metal–carrier interaction.⁴² Among these, the lattice distortion-induced method offers a more straightforward and scalable approach to modulate the volume proportion of high-spin Co³⁺.³³ This facilitates the detailed exploration of the structure–activity relationship, which is critical for advancing spin-state-dependent applications. The as-obtained ZnCo₂O₄ oxides show irregular morphology according to transmission electron microscopy (TEM) characterization (Fig. S1†). Increasing calcination temperature leads to slight sintering and increased particle sizes. Fig. 1c and d show the aberration-corrected high angular annular dark-field scanning TEM (AC-HAADF-STEM) image and the corresponding fast Fourier transform (FFT) pattern of the ZCO-600 oxide. The atomic arrangement of ZCO-600 matches well the spinel lattice with the space group Fd3m, where Co³⁺ cations occupy the octahedral sites and Zn²⁺ cations occupy the tetrahedral sites.⁴³ As shown in Fig. 1e, the X-ray diffraction (XRD) analysis further confirms the successful synthesis of the spinel phase (JCPDS no. 23-1390) at different calcination temperatures. In particular, the main peaks located at about 31.21, 36.81, 59.28 and 65.15° are indexed as 220, 311, 511 and 440 Bragg reflections of the spinel crystal structure, respectively. This aligns with the space group Fd3m, and agrees well with the AC-HAADF-STEM analysis. No obvious diffraction peaks of impurities were observed within the calcination temperature range of 300 to 600 °C, indicating excellent structural robustness of the ZnCo₂O₄ spinel oxide.

Further analysis of the chemical and spin states of ZnCo₂O₄ spinel oxide was performed using X-ray photoelectron spectroscopy (XPS) and XAS (Fig. 1f and S2–S4†). The XPS analysis of the Co 2p signal (Fig. S3†) confirms that Co within all ZnCo₂O₄ spinel oxides exhibits a trivalent oxidation state. The Co L₂ and L₃ XAS spectra of ZnCo₂O₄ are presented in Fig. 1f, together with Co₂O₃ as a Co³⁺ reference. The center of the L₃ spectrum of ZnCo₂O₄ aligns with that of Co₂O₃, demonstrating that the Co valence in ZnCo₂O₄ is indeed trivalent. Previous studies have shown that the presence of a low-energy shoulder (red area) at the Co³⁺ L₃ edge is characteristic of the high-spin state,⁴⁴ while the high-energy shoulder (blue area) is indicative of the low-spin state.⁴⁵ This analysis proves that Co³⁺ cations in ZnCo₂O₄ exist predominantly in the high-spin state. Hence, the consistent trivalent oxidation state of Co ensures an exclusive examination of the spin-dependent electrochemical activity in ZnCo₂O₄ spinel oxide.

The emergence of the Co³⁺ high-spin state is ascribed to lattice distortion induced by high-temperature calcination. We explore the spin-structure correlation using both DFT calculations and FTIR characterization. Fig. 1g shows the relationship between the number of unpaired electrons and the octahedral distortion (ε), which is consistent with previous reports.³³ The lattice distortion becomes more pronounced with an increase in the number of unpaired spins in ZnCo₂O₄. This is because the increased number of unpaired electrons leads to the occupation of e_g orbitals, resulting in a degenerate electronic ground state. This electronic state can induce the Jahn–Teller effect, which lowers the energy and symmetry of the system. Furthermore, FTIR analysis was utilized to investigate the local chemical bond changes in the as-prepared ZnCo₂O₄ (Fig. 1h). The absorption bands at 575 cm⁻¹ and 670 cm⁻¹ correspond to vibrations of the Co–O and Zn–O bond stretching, respectively. As the calcination temperature increases, the peak shapes of the metal–oxygen bonds broaden and shift to higher wavenumbers (Fig. S5†), confirming the emergence of lattice distortion.

To gain insights into the spin-related electronic configuration in spinel ZnCo₂O₄, the magnetic properties and related spin information were investigated using a SQUID, which consists of a superconducting loop with two parallel Josephson junctions capable of detecting incredibly small changes in magnetic flux.⁴⁶ As shown in Fig. 2a, the magnetic properties of the ZnCo₂O₄ oxides were investigated by measuring magnetization as a function of the magnetic field (M–H). All the as-prepared ZnCo₂O₄ oxides exhibit similar magnetization curves with no hysteresis feature, indicating paramagnetic behavior under ambient conditions. However, the four ZnCo₂O₄ oxides exhibit distinct magnetic susceptibilities (χ = M/H), confirming variations in their spin structures.

Fig. 2 Spin-related magnetic characterization. (a) Hysteresis loops of the as-prepared ZnCo₂O₄ oxides recorded at room temperature (300 K). (b) Temperature-dependent magnetic characterization of the as-prepared ZnCo₂O₄ oxides at H = 1000 Oe, performed using field-cooling procedures between 5 and 300 K. The inset shows an enlarged view. (c) Fitted susceptibility versus temperature based on the Curie–Weiss law for the as-prepared ZnCo₂O₄ oxides. (d) Calculated volume fractions of high-spin state and low-spin state Co³⁺ in the four ZnCo₂O₄ oxides.

Temperature-dependent magnetization characterization experiments were carried out to further explore the spin-related properties (Fig. 2b). When the temperature is above 150 K, the magnetic susceptibility of magnetic field induction follows the Curie–Weiss law: χ = C/(T − θ), where C is the Curie constant (in emu K mol⁻¹ in centimeter-gram-second units) and θ (in K) is the Curie–Weiss temperature.⁴⁶ As shown in Fig. 2c, the slope of the fitting curve (susceptibility vs. T) represents C⁻¹. The Curie constant C is directly related to the number of unpaired electrons and, once determined, can be used to calculate the effective magnetic moment (, with units of Bohr magnetons, μ_B). Based on recent experimental and theoretical studies, we only consider changes between the low-spin state (t⁶_2ge⁰_g) and high-spin state (t⁴_2ge²_g) without introducing the intermediate-spin state (t⁵_2ge¹_g).^33,47–51 Therefore, the volume fractions of Co³⁺ cations in high-spin and low-spin states can be obtained using the following equation:

where S_LS (S_low-spin = 0) and S_HS (S_high-spin = 2) are the number of unpaired spins in the e_g orbitals for the low-spin state and high-spin state Co³⁺ cation, respectively; g (=2) is the Landé g-factor. Fig. 2d shows the volume fractions of Co³⁺ in low-spin and high-spin states (more details are displayed in Table S1†). As the calcination temperature increases, the volume fraction of high-spin Co³⁺ increases gradually, indicating that more low-spin Co³⁺ is converted to the high-spin state, thus verifying the success of spin engineering.

After confirming the increase in the high-spin state of the Co³⁺ cation, electrochemical sensing of metal ions was used to explore the intrinsic determinants of metal ion adsorption and desorption affinity on the oxide surface (Fig. 3a). First, the electrochemical impedance spectra (EIS) of ZnCo₂O₄ confirmed the enhanced ion transport at the electrode–electrolyte interface in spin-engineered ZnCo₂O₄ spinel oxides (Fig. S6†). Then the electrochemical performance of the ZnCo₂O₄ oxides was studied using square wave anodic stripping voltammetry (SWASV). Specifically, a 0.1 M HAc–NaAc buffer solution (pH = 6) was employed as the electrolyte, and the deposition process was performed at −1.0 V for 150 s. The electrochemical sensing of metal ions is essentially a surface reaction, where the adsorption and desorption of metal ions occur only on the catalyst surface. Considering that changes in the calcination temperature could affect the specific surface area of the oxides, specific activity is employed to evaluate the electrochemical performance by normalizing the current to the catalyst surface area. In this study, the specific surface area of ZnCo₂O₄ oxides was analyzed by N₂ physisorption according to the Brunauer–Emmett–Teller model (Fig. S7†). The electrocatalytic performance of the ZnCo₂O₄ spinel oxides toward different metal ions (Cu²⁺, Cd²⁺, and Pb²⁺) is shown in Fig. 3b and S8–S10.† The anodic stripping peaks for Pb²⁺, Cd²⁺, and Cu²⁺ ions appear at around −0.55 V, −0.79 V, and −0.05 V respectively, and the peak current increases linearly with the concentration of metal ions. According to the linear equations, the Pb²⁺ electrochemical detection sensitivities of the ZnCo₂O₄ oxides are 0.34, 0.575, 0.982 and 1.78 mA cm_oxide⁻² μM⁻¹ for ZCO-300, ZCO-400, ZCO-500 and ZCO-600, respectively. And the detection sensitivity of Cd²⁺ and Cu²⁺ ions also gradually improved with increased calcination temperature (more details are displayed in Table S2†). Notably, we found a quasi-linear relationship between the electrochemical performance and the volume fraction of the high-spin state Co³⁺. As shown in Fig. 3c, sensitivity increases with the volume fraction of the high-spin state Co³⁺, indicating the significance of the Co³⁺ spin state in electrochemical activity. When we normalized the current to the electrochemically active surface area, we obtained the same trend that the sensitivity increased with the volume fraction of high-spin Co³⁺ (Fig. S11–S13†).

Fig. 3 Electrochemical performance and mechanism. (a) Schematic illustration of the adsorption and desorption processes in electrochemical sensing. (b) SWASV peaks for Pb²⁺ desorption (left) and linear relationship (right) of ZCO-600. (c) Relationship between the volume fraction of high-spin state Co³⁺ and the electrochemical detection sensitivity for Pb²⁺, Cd²⁺ and Cu²⁺ cations. (d) XPS spectra of Co 2p before and after the adsorption of *Pb species for ZCO-600 (top) and XPS spectra of Pb 4f after the adsorption of *Pb species for ZCO-600 and Pb(NO₃)₂ (bottom). (e) SWASV desorption response for 1 μM Pb²⁺ sensing and the 2^nd and 3^rd anodic stripping curves without re-deposition.

To explore how spin properties affect electrochemical performance, XPS analyses were conducted to reveal the surface chemical interactions between ZnCo₂O₄ oxide and *Pb species. As shown in Fig. 3d (top), the characteristic peak of Co 2p shifts to higher energies after adsorption of *Pb species, implying a possible charge transfer from Co to O due to the binding with *Pb species. Additionally, the high-resolution XPS spectra of Pb 4f in Pb(NO₃)₂ and ZCO-600/Pb are shown in Fig. 3d (bottom), where the Pb 4f peak shifts toward lower binding energy after adsorption, indicating that the adsorbed *Pb species accept electrons from the oxide. As the high-spin state increases, the Pb 4f peak shifts less toward lower binding energy, indicating that fewer electrons are obtained and the binding force decreases (Fig. S14†). Since electrochemical sensing involves the adsorption and desorption of metal ions at the electrode surface, previous studies predominantly consider a single stripping as the only sensing signal, often overlooking the potential interactions between the metal ion and the electrode.²⁷ Here, multiple stripping analyses were performed on the adsorbed *Pb species (1 μM). Interestingly, we found that the adsorbed *Pb species cannot be completely stripped in a single cycle (Fig. 3e). As the number of stripping cycles increases, the response current gradually decreases, indicating that the oxide surface still contains unstripped *Pb species after the square wave voltammetry (SWV) process. This confirms that the desorption of metal ions plays a major role in electrochemical sensing performances. These results also suggest that the weaker the binding force between *Pb species and the electrode, the higher the detection performance.

Spin-polarized DFT calculations were further conducted to elucidate the correlation between the Co³⁺ spin state and electrochemical activity (Fig. 4 and S15–S17†). According to the calculated binding energy diagrams of ZnCo₂O₄ with varied Co³⁺ spin states (Fig. 4b), the adsorption affinity of metals on ZnCo₂O₄ surfaces decreases with the increase of the spin state regardless of the adsorbed metal species (i.e., Pb, Cu and Cd). This is consistent with the experimental electrochemical sensing performances. We further evaluated the projected density of states (PDOS) of the Co 3d orbitals for both low-spin and high-spin states of the ZnCo₂O₄ surfaces. As shown in Fig. 4c, the high-spin state exhibits a downshift of the d-band center (−2.01 eV) as compared with that of the low-spin state (−0.92 eV). According to the d-band center theory, it is suggested that lowering the d-band center relative to the Fermi level will weaken the surface chemisorption of adsorbed species, thereby enhancing the electrochemical desorption of metal ions and sensing performances (the d-band center discussed here is the average of the spin-up and spin-down d-band centers). The resulting trend indicates that the spin state alters the electronic properties of the Co sites, leading to a moderate affinity toward the intermediate metal species.

Fig. 4 DFT calculations. (a) Side and top views of the ZnCo₂O₄ structure models after the adsorption of *Pb species. Color scheme: Zn, blue; Co, pink; O, red; Pb, brown. (b) Calculated binding energy profiles of different spin state ZnCo₂O₄ oxides for *Pb, *Cu and *Cd species. (c) Partial density of states and d-band center of the oxide with both low-spin and high-spin states. (d) COHP for the Co–Pb bond of the low-spin and high-spin state models. (e) Differential charge density and Bader charge analysis of the Pb-adsorbed ZnCo₂O₄ surface. The blue areas represent electron depletion, while the yellow areas represent electron accumulation. The isosurface level is set to 0.1 electrons per ų.

Furthermore, Crystal Orbital Hamilton Population (COHP) analysis was performed to evaluate the binding strength of the Co–Pb interaction after adsorption. The integral area of the COHP (ICOHP) is proportional to the bond strength, with higher electron orbital overlap (lower ICOHP) indicating higher bond strength. Compared to the low-spin state, the high-spin state exhibits decreased occupancy of the bonding state, resulting in a decrease in the -ICOHP (as a measure for the bond strength) of the Co–Pb bond from 0.82 to 0.71 (Fig. 4d), which is consistent with the DFT calculated Co–Pb binding trend. In addition, the charge density difference diagram of both the low-spin state and high-spin state of ZnCo₂O₄ structures shows that the charges accumulate on the *Pb species side (yellow region) and deplete on the ZnCo₂O₄ side (green region), representing spontaneous electron migration from the Co site to the *Pb species. As illustrated in Fig. 4e, the ZnCo₂O₄ structure with the low-spin state transfers more electrons and has stronger adsorption energy than that of the structure with the high-spin state. These results demonstrate the electronic effects under the influence of spin states in ZnCo₂O₄ in optimizing the binding strengths of adsorbed metal species.

Conclusions

In summary, we investigated the correlation between the spin state of Co³⁺ sites and their adsorption characteristics with various heavy metal adsorbates. The manipulation of the spin state is achieved using ZnCo₂O₄ spinel oxide as a model based on calcination-induced lattice distortion. By using the electrochemical sensing technique as a measure, the adsorption affinity of metal ions on the ZnCo₂O₄ surfaces shows a quasi-linear correlation with the volume fraction of high-spin state sites. The high-spin state dominated ZCO-600 shows a Pb²⁺ electrochemical sensing performance of 1.78 mA cm_oxide⁻² μM⁻¹, which is approximately 5-fold higher than that of the low-spin state dominated ZCO-300. Similar spin-based correlations were further demonstrated for various metal species, including Cu²⁺ and Cd²⁺. DFT calculations show that the d-band center of Co sites gradually shifts downward relative to the Fermi level with the increase of the Co³⁺ spin state in ZnCo₂O₄, which weakens the adsorption affinity for metal species. This accounts for the enhanced electrochemical sensing performance toward heavy metal ions. This work unveils the correlation between spin properties and adsorption characteristics of metal species on spinel oxides and points out a potential avenue for the development of next-generation metal-involved technologies, such as sensors, metal batteries and beyond.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

C. G., S. Z. and B. C. designed and carried out the experiments and wrote the manuscript. L. W. collected the TEM data. Y. W. and Y. C. carried out the electrochemical tests. B. C. and L. Z. supervised the whole study and revised the manuscript. C. G. and S. Z. contributed equally to this work. All the authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (52201262 and 22373055), the Natural Science Foundation of Shandong Province (ZR2023YQ039 and ZR2022QE001), the Taishan Scholars Program of Shandong Province (tsqn202211042), and the Science and Technology Innovation Committee of Shenzhen Municipality (GJHZ20220913142604008). The authors acknowledge the support from the Tsinghua University Initiative Scientific Research Program, the State Key Laboratory of Intelligent Green Vehicle and Mobility under Project No. ZZ2023-063, and the computation resources from the Tsinghua Center of High-Performance Computing. We acknowledge X-ray magnetic circular dichroism (XMCD) beamline BL12B at the National Synchrotron Radiation Laboratory.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4sc06477a

‡ These authors contributed equally.

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