Photochromic Color Tuning of Copper‐Doped Zinc Sulfide Nanocrystals by Control of Local Dopant Environments

Abstract

Inorganic photochromic materials offer several advantages over organic compounds, including relatively inexpensive and higher thermal stability. However, tuning their color with the same component has remained a significant challenge. In this study, we demonstrate that the photochromic color of Cu‐doped ZnS nanocrystals (NCs), which is initially pale yellow before light irradiation, can be tuned from gray to brown by adjusting the surface stoichiometry of Zn and S, which is controlled through the use of thiol and non‐thiol ligands. Several experiments and quantum chemical calculations using model clusters revealed that the color change is determined by the distribution of Cu, which significantly contributes to the coloration, specifically whether it resides on the Zn‐rich or S‐rich surface. In contrast, particle size and Cu concentration were found to have little effect on the photochromic color. This study expands the diversity of photochromic responses in inorganic NCs and marks an important step toward the development of further advanced photochromic nanomaterials.

This study achieved tunable photochromic colors (gray to brown) in Cu‐doped ZnS nanocrystals by controlling surface Zn and S stoichiometry using thiol and non‐thiol ligands. Experiments and quantum chemical calculations revealed that Cu distribution on Zn‐rich or S‐rich surfaces determines color, with minimal influence from particle size or Cu concentration.

Introduction

Thermally reversible type (T‐type) photochromic molecules, compounds that form colored forms when exposed to light and revert to their original state with moderate heat, have been utilized as ophthalmic lenses, arts, and toys, and they also have various potential applications including anti‐counterfeiting and bioimaging.[ ¹ , ² , ³ ] Currently, organic photochromic molecules have been used in industrial applications because their photochromic properties can be precisely controlled by rational designs of molecular frameworks. On the other hand, they have several disadvantages including multiple synthetic steps, cost, and low heat resistance.

Inorganic photochromism offers several advantages over organic photochromic molecules, such as being relatively inexpensive, and highly durable against heat and light irradiation. ^[2] Photochromic materials based on inorganic compounds and inorganic nanocrystals (NCs, which could also be categorized by organic–inorganic nanocomposites) reported so far include some natural minerals, ^[4] silver halide glass,[ ⁵ , ⁶ ] metal‐doped TiO₂,[ ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹ ] TiO₂,[ ¹² , ¹³ ] WO₃,[ ¹⁴ , ¹⁵ ] ZnS,[ ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ ] ZnO[ ²⁰ , ²¹ , ²² , ²³ , ²⁴ , ²⁵ ] and so on.[ ²⁶ , ²⁷ ] However, most of these compounds face challenges such as slow decoloration reactions, lack of response in air, low coloring efficiency, and difficulty in a transparent coating.

Recently, we reported that 3‐mercaptopropionic acid (MPA)‐capped colloidal zinc sulfide nanocrystals doped with monovalent copper ions (MPA‐capped Cu‐ZnS NCs) exhibit relatively fast photochromism in air. ^[28] Upon illumination by 365‐nm light, the color of the powder of MPA‐capped Cu‐ZnS NCs changes from pale yellow to dark gray even in the presence of air. The color reverts to its original color within tens of seconds to a few minutes. Compared to previous inorganic compounds and organic–inorganic nanocomposites, the photochromism of these NCs exhibits a fast color‐fading process and the decoloration reaction is less sensitive to temperature (apparently offset by the desorption of adsorbed water). ^[29] Additionally, these NCs can be synthesized in large quantities relatively easily, have low toxicity, and are composed of elements abundant on earth, making them promising for large‐scale industrial applications such as outdoor use.

As demonstrated in the aforementioned research, the control over the decoloration rate in photochromic reactions has been gradually improving. However, achieving color modulation in photochromic reactions remains challenging in materials where the same inorganic component serves as the chromophore. This difficulty arises from the fact that the coloration in many inorganic photochromic reactions is often driven by redox reactions of specific atoms within the periodic structure or by the desorption of oxygen atoms, both of which are inherently difficult to control. Photochromic colors of inorganic photochromic materials are mostly dictated by the compositions of host materials and dopants. ^[27] In the major market of photochromic materials for eyewear, gray and brown colors are particularly desired. Some organic molecules have been reported to exhibit black or brown colorations with a single chromophore.[ ³⁰ , ³¹ , ³² ] However, all of them are limited to solid‐state materials or under deoxygenated conditions. Therefore, achieving these colors in transparent solutions through simple compositional adjustments under ambient conditions would significantly advance industrial applications.

In this study, we synthesized Cu‐doped ZnS NCs using hydrophobic ligands and found that these NCs, initially pale yellow before light irradiation, exhibit brown‐color photochromic reactions (Figure 1a), whereas previous studies had only reported gray‐color photochromism (Figure 1b). We elucidated that the different coloration originates from the different local environments of the doped Cu, i.e., S‐rich or Zn‐rich surface controlled by organic ligands.

Figure 1.

(a) Plausible photochromic reaction scheme and pictures before and after 365‐nm light irradiation (500 mW cm⁻² for 10 s) of OLA‐capped Cu‐doped ZnS NCs in chloroform. (b) Conceptual schematics and pictures of MPA‐ and OLA‐capped Cu‐ZnS NCs after light irradiation.

Results and Discussion

X‐ray diffraction (XRD) measurements show that the crystal structures of all Cu‐doped ZnS NCs were zincblende (Figure S1). The diameters of the NCs synthesized in this study range from 3.2 to 4.1 nm estimated by the peak width of the XRD patterns using the Scherrer equation (Figure S2). ^[33] In the Fourier transform infrared (FTIR) spectra of oleylamine (OLA)‐capped Cu‐ZnS NCs, a peak at 3320 cm⁻¹ is ascribed to the N−H stretching mode of the amino group of OLA. A small peak at 3006 cm⁻¹ is characteristic of the sp² C−H stretching mode, indicating that OLA is coordinated to the surface of NCs. ^[34] Peaks at 2922 and 2851 cm⁻¹ can be assigned to the sp³ C−H stretching modes. Peaks at 1561 and 1409 cm⁻¹ are ascribed to the symmetric and asymmetric C=O stretching modes of a carboxylate anion originating from a stearate anion. A peak at 1466 cm⁻¹ is ascribable to the C−H bending mode because it was also observed in other alkyl organic ligands.[ ³⁴ , ³⁵ ] In the FTIR spectra of oleic acid (OA)‐capped Cu‐ZnS NCs, the C=O stretching modes were observed at 1546 and ~1412 cm⁻¹ most probably originating from oleate and stearate anions. ^[36] The characteristic peak of the sp² C−H stretching mode at 3006 cm⁻¹ indicates that the oleate anion is coordinated to the surface of NCs in addition to the stearate anion. The other peaks are essentially the same as those of OLA‐capped Cu‐ZnS NCs.

The proton nuclear magnetic resonance (¹H NMR) spectrum of OLA‐capped ZnS NCs in deuterated chloroform exhibited several peaks associated with OLA and stearate anion (0.88, 1.26, 2.00, and 5.35 ppm), ^[37] and few peaks originating from other compounds were observed (Figure S24a). The peaks associated with OLA and stearate anion were broadened, and particularly, the peaks associated with the proton adjacent to the functional groups to bind the NCs, which are observed at 2.35 and 2.68 ppm before binding to the NCs, were not observed. The absence of corresponding sharp peaks suggests that all the OLA and stearate anions of the synthesized NCs are coordinated on the surface of the NCs with few free ligands present.

Figure S5 shows the steady‐state absorption spectra of OLA‐capped Cu‐doped ZnS NCs in chloroform at room temperature. The absorption band at ~300–320 nm in all Cu‐ZnS NCs is ascribable to the first excitonic absorption band of ZnS NCs, which depends on the size and polydispersity of NCs (Figure S5). In Cu‐doped ZnS NCs, additional absorption tails were observed at 340–380 nm. On the other hand, the absorption band ascribed to the Cu_xS domain was not observed in the visible to near‐infrared light region, which is consistent with the XRD patterns (Figure S1). The absorption band at the longer wavelength has been assigned to the formation of midgap states dominated by Cu(3d) above the valence band. ^[38]

No electron spin resonance (ESR) signals associated with Cu²⁺ were observed before light irradiation (Figure S10). Moreover, the X‐ray absorption spectrum at the L edge of Cu is similar to that of Cu in Cu₂O (Figure S11) and Cu 3d‐X‐ray photoelectron spectroscopy (XPS) is also similar to that of Cu₂O (Figure S12). ^[39] These results suggest that the doped Cu is monovalent, where a covalent character is stronger than Cu₂O because of the smaller electron affinity of S atoms than O atoms. This result is consistent with the results of ESR measurements.

Photochromic reactions of OLA‐capped Cu‐ZnS NCs (D=3.4 nm, Cu: 1.1 %) in chloroform were investigated by absorption spectra after 365‐nm light irradiation (160 mW cm⁻²) for 30 s. The absorbance of the solution in a 10‐mm quartz cuvette at 365 nm was set to be 0.2. A broad absorption band was observed at 460 nm after the light irradiation, consistently with the brown color (Figure 2a). It slowly decayed over 30 min (inset of Figure 2a). This photoinduced absorption change can be repeatedly observed more than 18 times with a slight decrease in the amplitude of the coloration (Figure 2c), where 365‐nm light was periodically irradiated for 30 sec with an interval of 10 min. After the long‐term light irradiation, the photoinduced signal reverts to zero completely over long periods. Moreover, ¹H NMR spectra and XRD pattern were almost identical irrespective of the long‐term light irradiation (610 mW cm⁻² for 5 min, Figure S24). It shows that the chloroform solution of OLA‐capped Cu‐ZnS NCs exhibits photochromic reactions. A slight decrease in the coloration was observed during repeated light irradiation. However, the ¹H NMR spectra and XRD patterns before and after the light irradiation are identical (Figure S24). The slight decrease in the coloration by repeated light irradiation may be due to a minimal photochemical reaction of surface organic molecules and/or surface reorganization of NCs. The photochromic reaction can be also initiated by 280‐nm LED and solar simulators (Figure S19).

Figure 2.

(a) Time evolution of the absorption spectra of OLA‐capped Cu‐doped ZnS NCs (D=3.4 nm, Cu: 1.1 %) in chloroform excited at 365 nm. The inset shows the time profile of the photoinduced absorbance at 500 nm. (b) The effect of water on the time profiles of the photoinduced absorbance at 500 nm. (c) The repeatability of the photoinduced absorbance at 500 nm. The intensity and duration of irradiation are 160 mW cm⁻² for 30 sec for all experiments.

Similar photochromic reactions were observed in OLA‐capped Cu‐ZnS NCs in different sizes and Cu concentrations (Figure S13–17). The spectral shape of the photoinduced absorption of OLA‐capped Cu‐ZnS NCs is almost identical irrespective of the Cu concentration and NC diameters (Figure S16). Additionally, similar photochromic properties were observed even when Cu sources were added at room temperature after synthesizing undoped OLA‐ZnS NCs (Figure S18). It indicates that surface Cu on ZnS NCs is crucial for the photochromic properties of OLA‐capped Cu‐ZnS NCs. Similar behavior has also been observed in MPA‐capped Cu‐ZnS NCs.

On the other hand, the spectral shape, color, and rate are different from those of water‐soluble MPA‐capped Cu‐ZnS NCs (D=2.3 nm, Cu: 1.4 %) reported previously (gray color, Figure 1b). Since the decoloration progresses through the hole trapping of NCs, it is assumed that holes in the NCs can exist longer in a hydrophobic environment. To confirm this, a small amount of water was added to the chloroform solution, which increased the decoloration rate (Figure 2b). Therefore, the result shows that the slower decoloration of OLA‐capped Cu‐ZnS NCs in chloroform is due to the fewer hole‐trapping agents such as water.

Several experiments were conducted to reveal the mechanism of the photochromic reaction of hydrophobic Cu‐doped ZnS NCs. In the case of the powder of hydrophilic MPA‐capped Cu‐doped ZnS NCs, electron hopping between particles is crucial for the photochromic reaction. On the other hand, OLA‐capped Cu‐ZnS NCs are dispersed in chloroform, and no scattering derived from the aggregations was observed in the absorption measurements (Figure S5). If the NCs were microscopically aggregated and electron hopping between them played an important role in the coloration, the amount of coloration would be expected to follow nonlinear dependence on the NC concentration, i.e., the power function‐like behavior is expected to be observed. However, even when the NC solution was gradually diluted from a certain concentration (absorbance of the solution was 0.251 at 365 nm using a 10‐mm quartz cuvette), the power function‐like behavior was not observed (Figure 3a). That is, the signal intensity shows the linear dependence on the concentration. These results suggest that photoinduced reactions within a single particle are important for the photochromism of OLA‐capped Cu‐ZnS NCs. The MPA‐capped Cu‐ZnS NCs also show coloration in solution (Figure 1b), suggesting that the photochromic reaction proceeds through a mechanism different from that in the solid state.

Figure 3.

(a) Concentration dependence of the photoinduced absorbance at 500 nm after 365‐nm light irradiation (156 mW cm⁻²) for 30 sec. (b) The ESR spectrum of the chloroform solution of OLA‐capped Cu‐ZnS NCs (Cu: 1.3 %, D=4.1 nm) under continuous 365‐nm light irradiation (68.5 mW cm⁻²) at room temperature.

To investigate the photogenerated transient species in more detail, ESR measurements were conducted at room temperature on the chloroform solution of OLA‐capped Cu‐doped ZnS NCs (D=4.1 nm, Cu: 1.3 %,) under continuous irradiation with 365‐nm light. Before light irradiation, no ESR signals were observed (Figure S10). However, broad and relatively sharp signals were observed under light irradiation at 330 and 338 mT (g=2.032 and 2.003), respectively (Figure 3b). The broad signal is ascribable to Cu²⁺, as observed in previous studies on hydrophilic Cu‐doped ZnS NCs solids. ^[28] On the other hand, the peak at g=2.003 was not observed previously. The g value suggests that this peak originates from organic radicals. These results indicate that the organic molecules on the NC surface, probably organic ligands and/or solvent, somehow receive electrons upon light irradiation, while the holes are used for the oxidation of Cu⁺ to Cu²⁺ within the NCs (Figure S51, S53, S54, and S55).

Interestingly, when ligands of OLA‐capped Cu‐ZnS NCs (D=4.1 nm, Cu: 1.3 %) were partly replaced with 1‐dodecanethiol (DT), the DT‐capped Cu‐ZnS NCs exhibited an additional photoinduced absorption band at the longer wavelength side by 365‐nm light irradiation, resulting in a spectral shape closer to that of MPA‐capped Cu‐ZnS NCs (Figure 4). Considering that the ligand exchange with DT is partial, it is reasonable that the spectrum of DT‐capped Cu‐ZnS NCs lies intermediate between those of OLA‐ and MPA‐capped Cu‐ZnS NCs. In contrast, the spectral shape of OA‐capped Cu‐ZnS NCs (D=4.0 nm, Cu: 4.4 %) was nearly identical to that of OLA‐capped Cu‐ZnS NCs although the signal is weak. These results suggest that the presence or absence of thiol groups on the surface likely influences the coloration. Regarding the amount of coloration, OLA‐capped Cu‐ZnS NCs exhibited the highest level, followed by DT, with OA being much lower (Figure S21, S22, and S23). The much smaller coloration in OA‐capped Cu‐ZnS NCs probably originates from the higher Cu concentration than others as reported previously. ^[28] Variations in the chemical reaction characteristics on the nanointerface depending on the surface ligands may also contribute to the difference in coloration.

Figure 4.

Normalized photoinduced absorption spectra of DT‐capped (D=4.4 nm, Cu: 1.8 %), OLA‐capped (D=3.4 nm, Cu: 1.1 %), and OA‐capped Cu‐ZnS NCs (D=4.0 nm, Cu: 4.4 %) in chloroform after 365‐nm light irradiation (160 mW cm⁻²) for 30 sec. The photoinduced absorption spectrum of MPA‐capped Cu‐ZnS NCs (D=2.3 nm, Cu: 1.4 %) in water after 365‐nm light irradiation (160 mW cm⁻²) for 10 sec is overlapped on the spectra.

To further investigate the details of the coloration process, quantum chemical calculations were performed on model clusters of ZnS. The undoped ZnS model clusters with varying Zn/S ratios were prepared from the supercell structure of the zincblende ZnS crystal, as shown in Figure S27. A single Zn atom in the undoped ZnS cluster was replaced with a Cu atom to prepare Cu‐doped ZnS clusters. By varying the position of the Cu atom, the model Cu‐ZnS clusters with different configurations were prepared (Figure S28–S30). The surface metal atoms of the model clusters with the general chemical formula [Cu₁Zn_xS_y‐nNH₃]^q, q=+1 for the colorless state and q=+2 for the colored state, were passivated by amine ligands. Although these are smaller than the actual NCs, examining multiple structures as molecular models is expected to provide essential insights into the molecular science aspect. In a previous study, the coloration process was associated with the Cu²⁺ state, indicating that Cu‐ZnS NCs were oxidized. ^[28] In addition, the photoinduced ESR signals associated with Cu²⁺ were observed in this study as well. Therefore, Cu‐ZnS clusters whose spin degeneracy is a doublet (D₀) state were constructed, 140where one electron is removed from the closed‐shell electronic state. OLA was replaced with amines to reduce the calculation costs.

First, we performed structural optimization and absorption spectrum calculations for Cu‐ZnS clusters with varying Zn and S compositions (Figure 5). The structural optimization was executed by the density functional theory (PBE/PAW‐PBE) implemented in the VASP program without any geometrical restriction. Then the orbitals and energies of the optimized structures were further improved with the PBE0 hybrid functional and def2‐TZVP basis set. The spectral properties were computed by the TD‐DFT method at the PBE0/def2‐TZVP level of theory with 200 excited states included. The calculations of energies, orbitals, and spectral properties were done with the Gaussian09 package. The details are described in the Supporting Information. Interestingly, the cluster with a larger (Zn+Cu)/S ratio ((1) [Zn₁₅S₁₃Cu‐12NH₃]²⁺) showed an absorption peak at ~500 nm, while the one with a smaller (Zn+Cu)/S ratio ((4) [Zn₁₂S₁₆Cu‐12NH₃]²⁺) exhibited a broader absorption at ~850 nm. For clusters with a (Zn+Cu)/S ratio close to 1, we found two types of Cu binding. A model cluster (2) [Zn₁₅S₁₆Cu‐15NH₃]²⁺, in which Cu binds to two S atoms (Cu_2S‐bonded), showed an absorption peak around 500 nm. In contrast, a model cluster (3) [Zn₁₅S₁₆Cu‐15NH₃]²⁺, in which Cu binds to three S atoms (Cu_3S‐bonded), exhibited a broader absorption at longer wavelengths. The Cu_3S‐bonded are substituted at Zn atomic sites surrounded by S‐rich facets. In contrast, the Cu_2S‐bonded are substituted at Zn atomic sites on Zn‐rich facets (see Figure S27 for details). This suggests that Cu atoms with three S coordination are predominantly observed in S‐rich ZnS NCs, while those with two S coordination are more frequently observed in Zn‐rich ZnS NCs. In addition, Cu‐ZnS clusters with a larger (Zn+Cu)/S ratio show larger absorbance.

Figure 5.

Effect of stoichiometry on the photoinduced absorption spectra of Cu‐ZnS NCs. (a) Optimized structures and (b) simulated absorption spectra of the oxidated state of Cu‐ZnS model clusters. Gray, yellow, orange, blue, and white balls in Figure 5a indicate Zn, S, Cu, N, and H atoms, respectively. Cu_2S‐bonded and Cu_3S‐bonded indicate that the doped Cu atom binds to two and three S atoms, respectively.

Similar calculations were performed without amine ligands for all compositions (Figure S47). It was found that amine ligands did not significantly affect the oscillator strength or spectral shape. This result indicates that the functional groups of the ligands have little effect on the photochromic properties.

Experimentally, hydrophobic Cu‐ZnS NCs in this study have an absorption peak at 500 nm, while the hydrophilic Cu‐ZnS NCs reported previously exhibit a broad absorption around 800 nm. In hydrophilic Cu‐ZnS NCs, MPA is used as a ligand, which results in a smaller (Zn+Cu)/S ratio. The smaller (Zn+Cu)/S ratio was experimentally confirmed by X‐ray fluorescence (XRF) measurements. On the other hand, the hydrophobic Cu‐ZnS NCs synthesized in this study have a relatively larger (Zn+Cu)/S ratio. Moreover, the spectral change by ligand exchange from OLA to DT shows that the surface compositional change affects the photochromic color (Figure 4). In addition, the photoinduced absorption band observed at 700 nm is weaker than the absorption band at 500 nm, which is consistent with the simulated oscillator strengths of these bands. Therefore, the difference in colors of the photochromic reaction between the hydrophobic Cu‐ZnS NCs and hydrophilic Cu‐ZnS NCs indicates that the local Cu environment plays a key role, i.e., whether Cu is located at the Zn‐rich (the Cu atom binds to two S atoms: Cu_2S‐bonded) or S‐rich (the Cu atom binds to three S atoms: Cu_3S‐bonded) environments.

Furthermore, calculations were performed on NCs with the same composition but different Cu doping positions (Figure 6). It was found that when Cu is located inside the NC (labeled as Cu at the core, in which the Cu atom binds to four S atoms: Cu_4S‐bonded), the absorption shifts toward the near‐infrared region. Such near‐infrared absorption has not been experimentally observed in OLA‐capped Cu‐ZnS NCs, suggesting that Cu primarily coordinates on the surface of the NCs and plays a key role in the coloration. These results are consistent with the experimental results that similar photochromic properties were observed even when Cu sources were added at room temperature to undoped OLA‐ZnS NCs (Figure S18).

Figure 6.

Effect of the Cu location on the photoinduced absorption spectra of Cu‐ZnS NCs. (a) Optimized structures and (b) simulated absorption spectra of the oxidated states of Cu‐ZnS model clusters. Gray, yellow, orange, blue, and white balls in Figure 6a indicate Zn, S, Cu, N, and H atoms, respectively. Cu_2S‐bonded, Cu_3S‐bonded, and Cu_4S‐bonded indicate that the doped Cu atom binds to two, three, and four S atoms, respectively.

Performing the same calculations for larger Cu‐ZnS NCs yielded results consistent with the findings from smaller NCs (Figure S49). Specifically, NCs with a larger (Zn+Cu)/S ratio or with Cu that binds two S atoms exhibited absorption around 500 nm, while those with a smaller (Zn+Cu)/S ratio or with Cu that binds to three S atoms showed broad absorption on the long‐wavelength side. Thus, the results of the quantum chemical calculations indicate that these findings are not limited to small clusters but are also applicable to NCs. These computational results suggest that controlling the position of Cu within the NCs and the distribution of holes could enable precise control over the coloration of semiconductor NCs.

Conclusions

In summary, we successfully revealed that the Cu local environment dictates the photochromic color of Cu‐ZnS NCs and achieved a brown photochromic color, which had previously only exhibited gray coloration. Combinations of experiments and quantum chemical calculations using model clusters revealed that the color change is determined by the distribution of Cu, which significantly contributes to the coloration, specifically whether it resides in the Zn‐rich or S‐rich environments. In contrast, particle size and organic ligands were found to have little effect on the photochromic color. This work broadens the diversity of photochromic responses in inorganic nanocrystals and marks an important step toward the development of further advanced photochromic nanomaterials.

Supporting Information

The supporting document contains detailed experimental procedures, analyses, and results of quantum chemical calculations. The authors have cited additional references within the Supporting Information.

Conflict of Interests

The authors declare no conflict of interest.

Supporting information

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Supporting Information

Supporting Information

Kimura M., Yoshioka D., Chang I-Y., Irizawa A., Shibata D., Imada S., Kobayashi Y., Angew. Chem. Int. Ed. 2025, 64, e202423776. 10.1002/anie.202423776

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.