Introducing ytterbium acetate to luminescent CsPbCl₃ nanocrystals for enhanced sensitivity of Cu²⁺ detection

Abstract

In this study, we employed a luminescenct agent CsPbCl₃ perovskite nanocrystal for Cu²⁺ detection, and achieved a prominent enhancement in sensitivity by the introduction of ytterbium acetate (Yb(OAc)₃). The introduced Yb(OAc)₃ is capable of causing a morphology transformation of CsPbCl₃ nanocrystals from initial nanocubes into one-dimensional (1D) nanowires (NWs). The formed 1D CsPbCl₃ NWs paved a direct charge transport path and exhibited low defect density as well as superior conductivity, which were beneficial to accelerating the rapid electron transport from the luminescent agent to Cu²⁺, and to give rise to an efficient inhibition of undesired charge trap, respectively. Besides, AcO⁻ rendered a reduced steric hindrance during the formation of a copper-based counter-ion pair, thus improving the adsorption capacity of Cu²⁺ on the surface of the luminescent agent, CsPbCl₃ nanocrystals. As results, the 1D Yb(OAc)₃ passivated CsPbCl₃ NWs exhibited efficient luminescence quenching for enhanced detection sensitivity. This Yb(OAc)₃-involved system exhibited a detection limit as low as 0.06 nM in the Cu²⁺ detection window range from 0 to 1000 nM, and such value is optimal in the documented works for Cu²⁺ detection based on luminescent perovskite materials.

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Introduction

Heavy metal ion pollution poses a serious threat to the environment and human health due to its high toxicity, carcinogenic effects, and accumulation in the ecosystem and human body.^1–4 This situation dictates the lasting pursuit of simultaneous precise and efficient detection of heavy metal ions considering their quantitative and qualitative recognition. In particular, demand for Cu²⁺ detection with high sensitivity is going up because of their crucial role in biological and industrial systems.^5–9 At present, numerous strategies for Cu²⁺ detection have been well-developed, such as traditional inductively coupled plasma mass spectrometry (ICP-MS), atomic absorption spectrometry (AAS), and electrochemistry. However, they suffer from a complicated sample pre-treatment, limited universality and unsatisfied sensitivity of detection.^10–12 In contrast, luminescent nanomaterials as detection agents towards highly sensitive detection of heavy metal ions have attracted intensive attention due to their low detection limit, desired universality, expected responsiveness, etc. Considering the luminescence responsiveness in the detection of metal ions, several mechanisms have been developed including fluorescence resonance energy transfer (FRET), cation exchange induced by exotic metal ions (static quenching), and electron transfer.^13–15 Thus, attaining the desired detection sensitivity is highly dependent on the selection of luminescent species, in which features, including excellent optical properties, large specific surface area, strong ion adsorption capacity, and rapid electron transfer rate, are most important.^8,16–19

Lead-halide perovskites (APbX₃, A is a cation and X is a halide anion) with brilliant photoelectric properties, adjustable bandgaps, and abundant surface landscapes are promising luminescent agents for ion detection.^20–23 Recently, there have been some reports on Cu²⁺ detection by luminescent perovskite materials. For example, Sheng et al. employed CsPbX₃ quantum dots (QDs) to detect Cu²⁺ in cyclohexane,²⁴ in which CsPbBr₃ QDs exhibited the optimal performance with a detection limit of 2 nM. In contrast, Liu et al. accomplished a more lower detection limit of 0.1 nM for the detection of Cu²⁺ in hexane using CsPbBr₃ QDs.²⁵ Although these CsPbX₃ QD-based probes could realize the sensitive detection of Cu²⁺, a higher ion detection sensitivity with satisfactory selectivity is still highly expected.

In this study, we performed CsPbCl₃ perovskite nanocrystals as luminescence agents to evaluate their detection ability of Cu²⁺. To gain a significant enhancement in detection sensitivity, ytterbium acetate (Yb(OAc)₃) was introduced. The introduction of Yb(OAc)₃ drove the anisotropic transformation from initial CsPbCl₃ nanocubes (NCs) to one-dimensional (1D) nanowires (NWs). The as-formed 1D CsPbCl₃ NWs exhibited lower defect density (0.67 × 10¹⁶ cm⁻³) compared with that of the initial CsPbCl₃ NCs (2.1 × 10¹⁶ cm⁻³), which will promote the photoluminescence quantum efficiency (PLQY) and restrain the possibility of charge trap. 1D CsPbCl₃ NWs displayed prominent conductivity (15.8 × 10^–3 S cm⁻¹), about 10-times higher than that of the initial CsPbCl₃ NCs (1.6 × 10^–3 S cm⁻¹), due to the direct electron transport path and reduced defects. This enhancement is beneficial to the efficient electron transfer between host CsPbCl₃ and Cu²⁺. Besides, AcO⁻ possess shorter alkyl chains in comparison to oleate ions (OA⁻), which coordinated with Cu²⁺ originally in the organic phase, leading to the reduced steric hindrance during the formation of a copper-based counter-ion pair, thus improving the adsorption capacity of Cu²⁺ on the surface of the luminescent agent, CsPbCl₃ nanocrystals. Through the strategy of synergistically optimizing electron transfer capacity and ion adsorption, such Yb(OAc)₃-involved system exhibits a detection limit as low as 0.06 nM in the Cu²⁺ detection window ranging from 0 to 1000 nM, of which the value was the best in the documented works for Cu²⁺ detection based on luminescent perovskite materials.

Results and discussion

In a typical synthesis, the hot-injection method was used to prepare the long-chain alkylammonium and alkylacid co-capped CsPbCl₃ nanocrystals. As displayed in Fig. 1a, b and Fig. S1a–d,† the morphology was observed by transmission electron microscopy (TEM), namely the well-defined NCs and NWs before and after the introduction of AcO⁻, respectively. The size evolution indicated that the as-formed NWs kept a constant width of ∼16 nm but an increased length of up to ∼500 nm. Further high resolution TEM (HR-TEM) image of such formed CsPbCl₃ NWs in Fig. 1c shows the clear lattice distances of 0.281 nm and 0.279 nm, which are assigned to the (002) and (200) lattice planes of tetragonal CsPbCl₃ perovskites. This indicated that the NWs grow along a preferred [200] direction, which was further verified by the powder X-ray diffractions analysis (XRD) (Fig. 1d). The XRD patterns of initial NCs and final NWs of CsPbCl₃ presented their tetragonal perovskite structure, refering to the standard card PDF # 18-0365. Although the diffraction peaks were unshifted, the relative intensity of the diffraction peaks of (200) and (002) planes increased after the formation of NWs, implying the favored growth orientation along the [200] direction during the morphology transformation.

Fig. 1 TEM images of CsPbCl₃ nanocrystals before (a) and after (b) the introduction of AcO⁻. (c) The corresponding high-resolution image of the as-formed CsPbCl₃ NWs. (d) XRD patterns of initial NCs and final NWs of the CsPbCl₃ nanocrystals, with the selected zoom-in area centered at 31.8 and 32.1 degree. (e) Elemental mapping of the as-formed CsPbCl₃ NWs.

In this morphology transformation system, AcO⁻ plays a key role, and the complete transformation can only occur when the feeding amount of Yb(OAc)₃ was up to 0.05 mmol (Fig. 1, S1a, b and Table S1†). Notably, we performed comparative trials by adding AcO⁻ (Pb(OAc)₂) and Yb³⁺ (YbCl₃) separately to evaluate the effect of AcO⁻ and Yb³⁺ on the formation of CsPbCl₃ NWs. As a result, only the co-existence of AcO⁻ and Yb³⁺ enabled by the synergistic effect could promote the formation of CsPbCl₃ NWs effectively (Fig. S1c–e†). This directed growth may involve in the AcO⁻ and Yb³⁺ corelative passivation and activation effect on the crystal plane of the CsPbCl₃ perovskites.

Furthermore, mapping images and inductively coupled plasma mass spectrometry (ICP-MS) were conducted to validate the chemical compositions and elemental distribution of CsPbCl₃ NWs. As shown in Fig. 1e, Cs, Pb, Cl and Yb elements were homogeneously distributed in CsPbCl₃ NWs. The composition of the initial CsPbCl₃ NCs and final CsPbCl₃ NWs was further explored via the investigation of X-ray photoelectron spectra (XPS) (Fig. 2a–c and Fig. S3†). The survey spectra displayed that the initial CsPbCl₃ NCs and final CsPbCl₃ NWs both included Cs, Pb, Cl and Yb elements, and the additional signal peaks of Yb elements emerged (Fig. S3a†). The high-resolution XPS spectra of the Cs, Pb, Cl and Yb elements for initial CsPbCl₃ NCs and final CsPbCl₃ NWs are revealed in Fig. 2a–c and Fig. S3b.† The binding energies related with Cs (Cs 3d_3/2, Cs 3d_5/2) exhibited the same position in the initial CsPbCl₃ NCs and final CsPbCl₃ NWs (Fig. S3b†). However, the high-resolution XPS spectra demonstrate that the binding energies of the Cl 2p and Pb 4f peaks shifted to higher binding energies in the final CsPbCl₃ NWs compared with initial CsPbCl₃ NCs. This shift verified that the Yb³⁺ has partially replaced Pb²⁺ ions in the CsPbCl₃ perovskite (Fig. 2a and b). In addition, Fig. 2c shows the depth-resolved XPS measurement results of Yb 4d. Most interestingly, after the etching treatment, the binding energy signal corresponding to Yb 4d in the final CsPbCl₃ NWs disappeared, indicating that the Yb³⁺ ions passivated only on the surface of the final CsPbCl₃ NWs.²⁶ In addition, the binding energy of Yb³⁺ corresponded to YbCl₃ that passivated on the surface of NCs, which consisted well with the previous reports.²⁷

Fig. 2 High-resolution XPS analysis of initial CsPbCl₃ NCs and final CsPbCl₃ NWs. The corresponding (a) Cl 2p (b) Pb 4f, (c) Yb 4d signals for the final CsPbCl₃ NWs with and without Ar ion etching; (d) FTIR spectra for initial CsPbCl₃ NCs and final CsPbCl₃ NWs.

Next, the capping ligands on the surface of the CsPbCl₃ perovskite were identified by Fourier transform infrared (FTIR) spectroscopy. As shown in Fig. 2d, the asymmetric stretching of C–H of the alkyl chain could be detected at the wavenumber range of 2800–3010 cm⁻¹ in the initial CsPbCl₃ NCs and final CsPbCl₃ NWs.²⁸ In addition, the peak at 1640 cm⁻¹ can be assigned to the characteristic N–H bending vibrations of alkylammonium cations (RNH₃⁺), and the peak at 1400–1560 cm⁻¹ corresponded to the C–O bending vibrations of carboxylate (RCOO⁻).^29,30 After treatment with AcO⁻, the C–O bending vibration of AcO⁻ at 1537 cm⁻¹ appeared, which indicated that the introduced AcO⁻ was located on the surface of the final CsPbCl₃ NWs.³¹

With respect to the formation mechanism of the final CsPbCl₃ NWs, we presented a schematic in Fig. 3a. After AcO⁻ was introduced, AcO⁻ was mainly passivated on the (002) facet, which decreased the surface activation energy and inhibited the growth of the CsPbCl₃ NWs along the [002] direction. In addition, AcO⁻ also played as a surface passivator, providing a sufficient precursor to promote the NWs grow along the [200] direction. However, Yb³⁺ mainly activated the (200) crystal planes. The Yb³⁺ ions with three charges will compete with alkylammonium (RNH₃⁺) ions on the perovskite surface, thus reducing the steric hindrance and promoting the anisotropic growth along the [200] direction.

Fig. 3 (a) Illustration of the formation CsPbCl₃ NWs with the aid of cooperative passivation of AcO⁻ and Yb³⁺ ions. (b) Absorption and (c) PL emission spectra for the initial CsPbCl₃ NCs and final CsPbCl₃ NWs.

To investigate the impact of AcO⁻ on the optical properties of CsPbCl₃ NCs, we have performed absorption and photoluminescence (PL) spectral measurements (Fig. 3b and c and Fig. S4†). Compared with initial CsPbCl₃ NCs, the absorption peak of final CsPbCl₃ NWs red-shifted by 11 nm. In addition, the PL peak red-shifted from 403 nm (initial CsPbCl₃ NCs) to 412 nm (final CsPbCl₃ NWs) (Fig. 3c and d), as verified by the TEM pattern in Fig. 1b. This can be attributed to the enhanced electronic coupling of excitons and energy transfer through nonradiative pathways between adjacent NCs in the perovskite system, which were enhanced after the morphology transformation from CsPbCl₃ NCs to NWs.³² The final CsPbCl₃ NWs exhibited a larger PLQY value (17.3%) than that of the initial CsPbCl₃ NCs (2.1%). We speculated that this may have resulted from the reduced defect of the final CsPbCl₃ NWs compared with initial CsPbCl₃ NCs caused by the AcO⁻ passivation. However, the Yb³⁺-related emission was not detectable, which was mainly contributed to the minimum amount of Yb³⁺ passivated on the surface of CsPbCl₃ perovskites.²¹

For this consideration, the defect density for the initial CsPbCl₃ NCs and the final CsPbCl₃ NWs by I–V measurements was further investigated (Fig. 4a). The space-charge-limited current under different bias from capacitor-like devices was measured, as shown in Fig. 4b, and the defect density can be calculated via the subsequent equation:

where ε is the relative dielectric constant of the CsPbCl₃ NCs (ε = 4.432 for the CsPbCl₃ bulk material) and ε₀ is the vacuum permittivity.³³ Besides, V_TFL is the trap-filled limit voltage, e is the elementary electronic charge, and L is the thickness of the CsPbCl₃ NC film obtained from the AFM characterization (Fig. 4c–f). The defect densities were calculated to be 2.1 × 10¹⁶ and 0.67 × 10¹⁶ cm⁻³ for the initial CsPbCl₃ NCs and the final CsPbCl₃ NWs, respectively. The final CsPbCl₃ NWs exhibited a significantly lower density of defect states compared with the initial CsPbCl₃ NCs, which was most probably attributed to the passivation of the perovskite NC surface by the AcO⁻ and Yb³⁺ ions.²⁰

Fig. 4 (a) Trap density, (b) conductivity extraction by current–voltage measurement; AFM characterization on thickness of perovskite NC film for initial CsPbCl₃ NCs (c and e) and the final CsPbCl₃ NWs (d and f), respectively.

The conductivity of the CsPbCl₃ perovskite was also characterized by the I–V measurement under the capacitor-like devices. As shown in Fig. 4b, the resistance (R) values were attained for these devices, which are listed in Table S2, ESI.† The electrical conductivity (σ) of the perovskite NC films has been calculated by the following equation:

where A is the device area (4 mm²), and the values of R and the measured thickness of the perovskite films d are provided in Table S2, ESI.† For the films made of initial CsPbCl₃ NCs and the final CsPbCl₃ NWs, the conductivities were 1.6 × 10^–3 and 15.8 × 10^–3 S cm⁻¹, respectively. CsPbCl₃ NWs demonstrated an obviously higher conductivity than the initial CsPbCl₃ NCs. This is due to the fact that the direct charge transport path in the 1D material accelerates the rapid transport of electrons.³⁴ The increased conductivity including electron transport in our study was principally the successful preparation of 1D CsPbCl₃ NWs. In addition, the lower defect intensity also promoted the electron mobility contributed by the decreased possibility of charge trap.³⁵

Besides the excellent photoelectric properties, the CsPbCl₃ perovskite is equipped with numerous active sites that can adsorb metal ions efficiently, since the formation energy of vacancy in the CsPbCl₃ perovskite is lower than that in CsPbBr₃ and CsPbI₃.³⁶ CsPbCl₃ displays brilliant stability than CsPbBr₃ and CsPbI₃ due to its desirable tolerance factor of 0.87, which is more close to 1 compared to that of CsPbBr₃ (0.81) and CsPbI₃ (0.80).³⁷ All of these enable them to be an excellent luminescence agent for metal ion detection. We therefore evaluated the practical value by studying their relative PL intensity before and after adding different metal ions, including Ag⁺, Al³⁺, Er³⁺, Yb³⁺, Cs⁺, Na⁺, Zn²⁺, Fe³⁺, Ni²⁺, Pb²⁺, Mn²⁺, Mg²⁺, Ca²⁺, K⁺, and Cu²⁺, at the same concentration level. As shown in Fig. 5a and b, only Cu²⁺ can effectively quench the luminescence of CsPbCl₃ perovskites compared with other cations. Clearly, the final CsPbCl₃ NWs exhibited a higher selectivity towards Cu²⁺ detection than that of initial CsPbCl₃ NCs, which can be attributed to the efficient electron transfer from the CsPbCl₃ NWs to Cu²⁺ adsorbed on the surface of the perovskite. The absorption and PL intensity of initial CsPbCl₃ NCs and final CsPbCl₃ NWs decreased with an increase in the concentration of Cu²⁺, as shown in Fig. S5 and S6.† In addition, the PL peaks of initial CsPbCl₃ NCs and final CsPbCl₃ NWs remained unshifted after introducing Cu²⁺, implying that the quenching mode was independent of the size and the cation exchange process caused by the introduced cations.²⁴Fig. 5c displays the function relation of the luminescent intensity of initial CsPbCl₃ NCs and final CsPbCl₃ NWs with the Cu²⁺ concentration. The detection curves present that the luminescent intensity of the CsPbCl₃ perovskite was linear with the Cu²⁺ concentration, demonstrating the reliability of the ion detection measurement.

Fig. 5 (a and b) The PL intensity of initial CsPbCl₃ NCs and final CsPbCl₃ NWs after adding different metal ions with the concentration of 1000 nM; (c) fitting curve for the PL intensity of initial CsPbCl₃ NCs and final CsPbCl₃ NWs as a function of Cu²⁺ concentration; (d and e) the PL decay curves of initial CsPbCl₃ NCs and final CsPbCl₃ NWs; (f) illustration for the copper-based counter-ion pair formed on the surface of the CsPbCl₃ NWs after introducing copper oleate.

The luminescence quenching is illustrated by the equation:³⁸

I₀/I = A + K[Q] (3)

where I₀ and I are the PL intensity values of the CsPbCl₃ perovskite in the absence and presence of Cu²⁺, respectively. A and K represent the intercept and slope, respectively, and [Q] is the final Cu²⁺ concentration in the probe solution. As shown in Fig. 5c and Fig. S6,† the PL intensity of the initial CsPbCl₃ NCs and final CsPbCl₃ NWs with the addition of Cu²⁺ decreased linearly, and the K values were 1.6 μM⁻¹ and 5.9 μM⁻¹, respectively. In addition, the theoretical detection limit was determined by utilizing 3σ/K, where σ is the standard deviation of the blank signal. σ can be calculated as:⁶where F_n is the PL intensity for the n^th time, and F is the average value of the multiple PL intensity measurements. The theoretical detection limits of initial CsPbCl₃ NCs and final CsPbCl₃ NWs were found to be 10 nM and 0.06 nM, respectively. It demonstrates that the detection limit of the final CsPbCl₃ NWs was improved by two orders of magnitude compared with that of the initial CsPbCl₃ NCs. This is the lowest detection limit of Cu²⁺ in those documented works until now (Table S3†).

In terms of the detection mechanism, the emission peaks of the CsPbCl₃ perovskite remained unchanged with an increase in the concentration of Cu²⁺, implying that Cu²⁺ preferred to adsorb on the surface of the perovskite. In addition, the PL decay curves of the CsPbCl₃ perovskite are shown in Fig. S7† and Fig. 5d and e, and the corresponding data are listed in Tables S4 and S5.† Both the initial CsPbCl₃ NCs and final CsPbCl₃ NWs exhibited a short average decay time for the exciton luminescence vs. the Cu²⁺ concentration. The reduced average decay time after the combination with the Cu²⁺ indicated an electron transfer process from the CsPbCl₃ perovskite to Cu²⁺ by the dynamic quenching mode.²⁵ In detail, the average PL lifetimes of initial CsPbCl₃ NCs and final CsPbCl₃ NWs in the absence of Cu²⁺ were 9.4 ns and 12.5 ns, respectively, while it would drop to 8.4 ns and 7.5 ns once fed with a certain amount of Cu²⁺ (Table S6†). In particular, compared with the initial CsPbCl₃ NCs, the short lifetime of NWs corresponding to the nonradiative transition path became shorter after feeding Cu²⁺. It suggests that the electron transfer process from the CsPbCl₃ NWs to Cu²⁺ was faster than that from the initial CsPbCl₃ NCs. From the previous investigations, the low defect density in the as-prepared 1D CsPbCl₃ NWs inhibited the possibility of charge trap through other nonradiative transition paths. The direct charge transport path in the 1D material could promote the rapid transport of electrons. Therefore, the successful preparation of 1D Yb(OAc)₃ passivated CsPbCl₃ NWs with reduced defect and prominent conductivity mainly promoted the electron transport. Most interestingly, when Cu²⁺ was adsorbed on the surface of the CsPbCl₃ perovskite, the copper-based counter-ion pair (Cl–Cu–OOC–R₂) species formed easily.²⁶ The introduced AcO⁻ adsorbed on the surface of the CsPbCl₃ perovskite, as confirmed by Fig. 2d, with a shorter alkyl chain can reduce the steric hindrance during the formation of a copper-based counter-ion pair (AcO⁻ and RCOOCu⁺) (Fig. 5f).^39–41 Namely, introduced Yb(OAc)₃ can not only accelerate the rapid transfer of electrons from the perovskite to Cu²⁺, but also promote the Cu²⁺ adsorption capacity on the perovskite surface. In results, the Cu²⁺ was more sensitive to the NWs with a remarkable decrease in the detection limit.

Conclusions

In this study, we report that the introduction of Yb(OAc)₃ into a luminescence agent, CsPbCl₃ perovskite nanocrystals, was beneficial to achieving a significant advancement in sensitivity for Cu²⁺ detection. On the one hand, the introduced Yb(OAc)₃ promoted the morphology transformation of CsPbCl₃ from NCs to 1D NWs. 1D CsPbCl₃ NWs exhibited low defect density and enhanced conductivity. The decreased defect density in the as-prepared 1D CsPbCl₃ NWs inhibited the possibility of charge trap. The direct charge transport path in 1D CsPbCl₃ NWs accelerated the electron transfer process. On the other hand, the adsorbed AcO⁻ on the surface of the CsPbCl₃ perovskite enhanced the adsorption capacity of Cu²⁺ by the formation of a stable Copper counter-ion pair. As a result, the PL intensity of the CsPbCl₃ perovskite was more sensitive towards Cu²⁺ in a wide range from 0 to 1000 nM with a detection limit of as low as 0.06 nM by synergistically advancing the electron transfer and ion adsorption capacity. This work is of interest not only because it exemplifies the usefulness of luminescent perovskite nanocrystals for Cu²⁺ detection, but also represents a useful step towards the rational design of luminescence agents towards high detection sensitivity of metal ions.

Author contributions

Xiufeng Wu: Methodology, software, formal analysis, investigation resources writing – original draft. Songtao Hu: Software, formal analysis, resources. He Shao: Formal analysis, investigation, data curation. Lifang Li: Resources, formal analysis. Wenda Chen: investigation, data curation. Biao Dong: Investigation, supervision, project administration. Lin Xu: Supervision, formal analysis. Wen Xu: Funding acquisition, project administration. Donglei Zhou: Investigation, resources. Zhennan Wu: Data curation, visualization, writing – review & editing. Hongwei Song: Data curation, visualization, project administration, funding acquisition. Xue Bai: Conceptualization, validation, writing – review & editing, visualization, supervision, project administration, funding acquisition.

Conflicts of interest

There is no conflict of interest for the authors to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 61822506, 11974142, 11874181, 12174151) and the Fundamental Research Funds for the Central Universities.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qi01235e

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