1. Preface
The unique cation exchange in the nanocrystal matrix is carried out by overcoming the chemical kinetic energy barrier through surface vacancy engineering, which refers to reducing the chemical kinetic energy barrier of cations on the crystal surface by introducing vacancies on the surface of nanocrystals, so as to realize the exchange of cations in the nanocrystal matrix. This method can be used to prepare nanomaterials with unique structures and properties, such as catalysts and adsorbents with high specific surface area, high activity, and high stability.
Surface vacancy engineering has played an important role in adjusting the structure and improving the performance of semiconductor nanocrystals, and the development of controllable vacancy engineering strategies to overcome the kinetic energy barrier in multi-step reactions is expected to further explore the synthesis mechanism and functional nanomaterials.
2. Unique cation exchange triggered by the SVEICE strategy
We try to achieve complete cation exchange from CIS to CdSSNC at 160 °C for 30 min at 160 °C by using Cd precursors that have 3 times more atomic weight than Cu and In. However, no slight blueshift of the SNC absorption edge obtained by the 3-fold Cd precursor was detected, demonstrating that the 3-fold Cd precursor does not promote intact cation exchange. More Cd precursors and longer reaction times are then used in this cation exchange reaction.
Unfortunately, complete cation exchange cannot be achieved due to the hard blue shift of the absorption edge in the SNC spectrum obtained by the Cd precursor.
Initially, we assumed that the complete cation exchange reaction from CIX to CdX or ZnX was a thermodynamically unfavorable reaction. However, thermodynamic simulations show that intact cation exchange reactions are thermodynamically advantageous due to the exothermic reaction from CIX to CdX or ZnX.
Further theoretical simulations, showing the high endothermic energy required to remove Cu and the low mobility of In on the surface, impede complete cation exchange. The creation of surface cation vacancies to promote guest cation adsorption and host and guest cation diffusion is thought to trigger kinetically unfavorable cation exchange from ternary to binary SNCs.
The SVEICE strategy consists of three sequential steps. Firstly, surface Cu vacancies are generated by selective coordination of oleic acid with surface Cu+. Tributylphosphine and trace Cd2+ ions are subsequently introduced into OA-treated CIXSNCs, and surface In vacancies can be specifically generated and stabilized by the synergistic effect of TBP ligands and Cd2+.
Due to the coexistence of Cu and In vacancies, guest cations are absorbed and diffused into the inner lattice, while the host cations are extracted during the cation exchange process, effectively overcoming the kinetic energy barrier of conversion from CIX to Cu, In double-doped CdXSNCs.
3. Surface copper vacancies are generated by ligands
High-angle annular darkfield scanning, transmission electron microscopy, and inductively coupled plasma emission spectroscopy (ICP-OES) are also used to confirm the absence of cation vacancies. The figure below shows the planar crystal structure of (001) and CIS superunit cells (5×1×5), with equal probability that Cu and In atoms occupy a metal atomic site.
As shown in the structure (02 ̄2 ̄0) planes, the ortho has a different number of metal atoms and the same number of S atoms, which can lead to a slight contrast of the orthos. Whereas metasites have the same metal atoms and the same number of S atoms, this does not result in a difference in contrast between the metasites.
4. Create a surface in a vacancy by cations
To further generate In vacancies in CISSC, trace Cd2+ and TBP ligands are introduced into the OA-treated CISSCNC system. ICP-OES characterization showed that the In/S ratio of TBP and Cd2+-treated CISSANCs decreased to 0.981/2 compared to OA-treated CISSNCs, indicating that some In3+ was removed.
There is a Cu atom vacancy on the original surface, Cd2+ binds into this vacancy to exothermic 2.97eV, TBP is adsorbed on the In atom, and the In-P bond length is 2.72Å. The In3+ is then removed from the surface by a TBP and the In-TBP3 complex is formed by exothermic 1.70 eV, which results in an In atom vacancy on the surface.
The continuous absorption of TBP on Cu atoms and the formation of individual Cu-S bonds is 0.17 eV of endothermic energy at VIn. In this process, V In preferentially acts as a warehouse to hold Cu+, and the second Cd2+ enters the 0.67 eV exothermic heat of the new Cu vacancy formed in the previous step.
Removing 1 TBP from the surface to form Cu-TBP2 of Cu atoms exothermics by 0.27 eV, introducing Cd2+ into the V In site to neutralize the surface charge, which may contribute to TBP removal of Cu, we emphasize that this VIn-assisted mechanism is the first reported here, and it promotes a feasible process of cation diffusion in multi-component SNCs.
5. Cu structure in double-doped CdSSNC triggered by SVEICE strategy
After synergistic generation of ligands and cations at surface cation vacancies, complete cation exchange from CIS to CdSSNC can be triggered when excess Cd2+ and ligands are added.
We investigated the UV-vis-NIR absorption spectra of four SNCs converted from four different treatments of CISNCs: oa/tbp-CD2+ co-treatment, OA treatment, TBP-CD2+ treated CISNC and untreated CISNC, and the UV-vis-NIR absorption spectra of SNCs converted from oa/tbp-CD2+ compared with CISNCs, the co-treated CIS showed a significant blue-shifted absorption edge (from 800 nm to 500 nm). This strongly indicates the successful transition from CIS to CdSSNCs.
At the same time, the TEM image confirms that the L/C-SNC maintains uniform and good monodispersity. In contrast, CIS SNCs treated with OA or TBP-Cd2+ showed significant absorption tailing after 500 nm, despite blueshifting of the absorption edge, indicating that the incomplete conversion from CIS to CdS SNC was demonstrated.
In addition, the absorption spectra of untreated CIS SNCs directly exchanged SNCs, similar to those of CIS SNCs, indicating that almost no Cu+ and In 3+ were replaced by Cd 2+ in this system, and the structure of Un-, L-, C- and L/C-SNC was further confirmed using ICP-OES characterization, and the highest amount of Cd and the lowest amount of Cu and In were obtained in L/C-SNCs, proving that L/C-SNCs are CdS SNCs. Both absorption spectroscopy and ICP-OES data confirm that the coexistence of Cu and In vacancies generated by ligand/cation synergy can trigger a complete conversion from CIS to CdS SNC.
6. Tunable PL characteristics of Cu in double-doped CdSSNC triggered by SVEICE strategy
In the final conversion step from OA/TBP-CD2+ co-treated CISSCNC to CdSSNC, OA+TBP, OAm+TBP or pure TBP were used as exchange ligands, E-4G showed that CdSSNC was obtained by PL spectra, respectively, and the converted OA+TBP-induced CdSSNC from OA/TBP-CD2+ co-processed SNC had a wide PL peak nanometer composed of two peaks.
ICP-OES showed that the doping concentrations of Cu+ and In3+ in OA-CdSSNC were 1.27% and 2.15%, respectively. The sharp diffraction peak and distinct absorption edge at 500 nm indicate that OAm+TBP and pure TBP-induced CdSSNCs, with fibrillic zinc structures and completely converted from CISNCs, indicate that OAm- and TBP-CdSSNCs have the same structure and band gap compared to OA-CdSSNCs.
PL spectra of OAm-CdS and TBP-CdSSNC showed that the two PL peaks also crossed the visible-NIR range, and the doping concentrations of Cu+ and In3+ were 1.49% and 2.04%, and 1.60% and 1.85% in OAm-CdS and TBP-CdSSNC, respectively. We used XPS and theoretical simulation characterization to further determine the oxidation state and doping structure of Cu and In in the OA-CdSSNCs matrix, respectively. In the XPS spectra of Cu LMM and In3d in OA-CdSSNC, the Auger peak of CuLMM is located at 569.9 eV, indicating that the oxidation state of Cu is +1.
Theoretical simulations revealed the coordination environment of Cu and In in OA-CdSSNC, and due to the lowest formation energy of displacement (E f = −1.013 eV), Cu and In-doped OA-CdSSNC, Cu and In dopants preferentially coordinate with S atoms and occupy the Cd site.
Benefiting from this ligand/cation synergy-induced cation exchange, controlled deep residual Cu+ and In3+ can serve as effective dopants in the matrix of obtained CdSSNCs, forming stable heterovalent double-doped CdSSNCs, where we define visible peaks as peak 1 and near-infrared peaks as peak 2.
The PL emission peak area of these two peaks can be adjusted by different ligands (OA+TBP, OAm+TBP, or pure TBP) cation exchange processes. The table below lists the locations and areas of peak 1 and peak 2 of the different CdS:Cu/InSNC produced. OA may help increase the area ratio of peak 1, and OAm and TBP, especially TBP, may help increase the area of peak 2.
7. Unique energy level structure of Cu in double-doped CdSSNC triggered by SVEICE strategy
Tauc diagram of PL spectroscopy, In-doped CdSSNC and OA-CdSSNC based on OA-CdS, schematic diagram of the energy level structure of OA-CdS, that is, CdS: Cu/InSNCs.
CdS: Cu/InSNCs have a band gap of 2.55 eV, in doped energy levels are below 0.39 eV CdS:Cu/InSNC conduction band, for Cu-doped CdS, Cu doped energy levels are divided into six degenerate d orbitals, t levels with five electrons and e levels with four fully filled d d orbitals.
Peak 1PL emission is attributed to radiation recombination from In-doped level to Cu-doped level at e level, and peak 2PL emission is attributed to radiation recombination from In-doped level to t level.
According to the PL spectrum of OA-CdS:Cu/InSNC, the t and e energy levels in CdS:Cu/InSNC are 0.69 and 0.51 eV higher than the valence band (VB), respectively, and the transient IR absorption-excitation energy scanning spectrum is used to further verify the positions of t- and e-CdS:Cu/InSNC.
Cation exchange ligands can play an important role in adjusting the intensity of emission peaks and the area of peaks 1 and 2. Ligands not only help bind and disperse SNCs in solution, but also act as electron-giving or electron-absorbing groups.
In the cation exchange process, OA acts as a typical electron-absorbing group that can attract electrons from the electron energy level doped with Cu+. Due to the lack of electrons at the e-level, the e-level can accept additional excited electrons from the In-doped level, resulting in an increase in the intensity of Peak 1.
In contrast, OAm and TBP act as electron-giving groups and fully filled orbitals that inhibit the acceptance of excited electrons from the In-doped level, which results in an increased intensity ratio of peak 2 to peak 1. Since the electronegativity of nitrogen in OAm is stronger than that of phosphorus in TBP, TBP ligands can further increase the peak area of peak 2.
8. SVEICE strategy has good versatility
The energy of this cation exchange, while full cation exchange from CIS to CdS is theoretically advantageous, the impossibility of removing Cu and In as a kinetic barrier actually hinders CIX access to CdX (ZnX). Not only is the energy change of 1TBP removing Cu reduced from +2.54 to +1.29eV after the creation of Cu and In vacancies, but In can also be removed by endothermic energy +0.71eV, which helps to complete cation exchange under easy conditions.
This strategy of overcoming the kinetic energy barrier by continuously generating cation vacancies has shown excellent versatility in other cation exchange reactions.
In addition, there is no significant tailing in the PLE spectrum after 480 nm, which also proves that CISSNC is fully converted to ZnS:Cu/InSNC. To further investigate the versatility of this strategy, a new cation exchange was performed from another ternary (CISe) SNC with another crystal structure (sphalerite).
We prepared sphalerite CISeSNCs with monodispersity and homogeneous morphology as starting SNCs, and highly crystalline sphalerite CdSe:Cu/InSNCs were co-induced by ligand/cation synergy.
Interestingly, the Cu,In double-doped binary SNCs obtained still have the same crystal phase structure and similar morphology compared to the original CIXSNCs, which exhibit the special advantages of cation exchange reactions.
During cation exchange, the main cation is partially or completely replaced by the guest cation, and the anion backbone does not have any rearrangement. Therefore, an unchanging anionic framework helps to maintain the stacking and morphology of anionic atoms.
conclusion
We develop a controlled SVEICE strategy in multi-component SNCs and demonstrate that the coexistence of Cu and In vacancies can overcome some complex cation exchange kinetic energy barrier processes. Supported by absorption spectroscopy, HAADF-STEM, theoretical simulations, ICP-OES characterization, and reference experiments, we delve into the mechanism of ligand/cation-induced surface vacancy processes and confirm the existence of surface vacancies.
Subsequently, energy analysis and complete cation exchange from CIX to Cu/In double-doped II-VI chalcogenide SNCs demonstrated the good versatility of the strategy. This strategy has strongly promoted the progress of topological chemical synthesis and surface vacancy engineering. In addition, the production of SNCs with highly efficient heteropent double-doping enables potential optical and electronic characterization applications.
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