Dual role of the reactant MOH (M=Li, Na or K) in the growth of ZnO quantum dots under a sol–gel process: promoter and inhibitor

Abstract

ZnO quantum dots (QDs) with tunable size and photoluminescence can be achieved simply by controlling their growth during the sol–gel synthesis process. However, the role of alkali metal hydroxide MOH (M = Li, Na or K) as the reactant in the growth of ZnO QDs is still controversial. In this work, we carried out a comprehensive study on the growth of ZnO QDs by using different types of MOH as the reactant under different reaction temperatures and in different solvents. The results reveal that the growth of ZnO QDs is considerably governed by MOH through the co-effect of OH⁻ and M⁺ ions, which can facilitate and hinder the growth, respectively. Further investigations suggest that the hindering effect caused by M⁺ cations is strongly dependent on the amount of OH⁻ anions around the ZnO QD surface and follows the trend of Li⁺ > Na⁺ > K⁺. Moreover, the dual role of the reactant MOH can also be confirmed when altering reaction temperature and solvent. The photoluminescence of ZnO QDs shows size-dependence and can be easily tuned over a wide range from purple to yellow via adjusting the reaction parameters. This research establishes a fundamental understanding on the growth of ZnO QDs from the role of the reactant MOH point of view, which may benefit the controllable growth of ZnO QDs.

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1 Introduction

Quantum dots (QDs), also called semiconductor nanocrystals, are one of the most researched materials that cater to the needs of present day applications in bio-imaging,¹ solar cells,² light emitting diodes,³ catalysts,⁴ sensors,⁵ etc. Among the II–IV semiconductor QDs, ZnO QDs with a wide direct band gap of 3.37 eV and a large exciton binding energy of 60 meV have gained considerable attention due to their unique electrical and optical properties, as well as non-toxicity and low cost compared with other traditional II–IV semiconductor QDs such as CdS, CdSe and CdTe.⁶

Generally, an immediate optical property of QDs is photoluminescence (PL). The typical PL of ZnO usually includes two competitive emission bands: a narrow ultraviolet (UV) emission band and a broad visible emission band. The former is definitely attributed to the near-band-edge exciton transition and the latter is related to many point defects,⁷ such as oxygen vacancies, zinc vacancies, oxygen interstitials, zinc interstitials, and antisite oxygen.⁸ Because of the high surface volume ratio, ZnO QDs possess a large quantity of defects. As a result, the UV emission is strongly quenched while the visible emission becomes overwhelming. Another attractive luminescent property for QDs is their size-dependent emission wavelength due to the quantum confinement effect. In this regard, the emission wavelength can be controlled simply by changing ZnO QDs size. Meanwhile, it is convenient for us to monitor the variation of particle size via UV-vis absorption spectrum combining with PL spectrum.

Up to now, many methods have been developed to synthesize ZnO QDs, such as sol–gel method,⁹ thermal decomposition method,¹⁰ vapor phase transport process¹¹ and nonthermal plasma method.¹² Among them, priority should be given to the sol–gel method, as it provides an easy way to adjust the reaction parameter throughout the whole synthesis process and has been regarded as the most successful approach to obtain ZnO QDs in dispersion.¹³ A popular sol–gel method for the synthesis of ZnO QDs generally involves the basic hydrolysis of zinc acetate mediated by an alkali metal hydroxide in alcoholic solvent.

So far, many researches have been carried out to explicate the growth of ZnO QDs during the sol–gel process.^14–18 However, the exact role of alkali metal hydroxide MOH (M = Li, Na or K) as the reactant is still unclear. It is widely accepted that the size of ZnO QDs can be effectively reduced through increasing LiOH amount. But the mechanism that is responsible for this experimental result is still controversial. Asok et al. ascribed this phenomenon to the hindering effect brought by counter ions,¹⁹ as it has been reported that the cations Li⁺ can form a virtual capping layer and further inhibit the growth of ZnO QDs.²⁰ Nevertheless, Tang et al. and Li et al. owed it to the influence of pH values.^21,22 Particularly, Han et al. pointed out the significance of OH⁻ ions on the growth of ZnO QDs, as they found that smaller ZnO QDs can be achieved only in the co-existence of Li⁺ cations and excessive OH⁻ anions.²³ Additionally, the difference among MOH in controlling the growth of ZnO QDs also need profound discussion. It was reported that compared with LiOH, the ZnO QDs prepared by using NaOH or KOH possessed narrower band gap and larger size, which was explained by the adsorption of Li⁺ ions on the surface of ZnO QDs, resulting in restraining the growth ZnO QDs.^23,24 This explanation seems to be unreasonable when considering the previous conclusion from Sarma et al., because they claimed that besides Li⁺ ions, both of Na⁺ and K⁺ ions can also hinder the growth of ZnO QDs.^20,25 More puzzlingly, when using KOH as the reactant, the particle size of ZnO QDs increases with increasing the amount of OH⁻ ions,²⁶ showing a quite different behavior with LiOH. Therefore, there still lacks a comprehensive investigation and especially a self-consistent explanation that can apply for all above cases.

In this work, we carried out a comprehensive investigation on the growth of ZnO QDs synthesized using different types of MOH as the reactant under different reaction temperatures and in different solvents, which reveal the dual role of the reactant MOH as promoter and inhibitor in the growth of ZnO QDs. Furthermore, our experimental results indicate that the inhibitor role arising from M⁺ cations strongly relies on the amount of OH⁻ anions and follows the trend of Li⁺ > Na⁺ > K⁺. Unusually, the growth behaviors of ZnO QDs prepared with different types of MOH are found to be different when altering reaction temperature or solvent, which also confirm the dual role of the reactant MOH. As the recent researches only focused on the sole role of M⁺ ions or OH⁻ ions of MOH, the explanation for the effect of MOH on the growth of ZnO QDs was still controversial. Based on our conclusions, the previously reported experimental results can be well explained.

2 Experimental

2.1 Materials

Zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O, 99%), lithium acetate (CH₃COOLi, 99.9%), lithium hydroxide monohydrate (LiOH·H₂O, 99%), sodium hydroxide (NaOH, 97%) and potassium hydroxide (KOH, 90%) were purchased from Aladdin Chemistry Co., Ltd. Ethanol (≥99.7%), 1-propanol (≥99.0%), 1-butanol (≥99.5%), 1-pentanol (≥98.5%) and n-hexane (≥97%) were supplied by Sinopharm Chemical Reagent Co., Ltd. All the chemicals were used as starting materials without further purification.

2.2 Synthesis of ZnO quantum dots

ZnO QDs were synthesized by a sol–gel method similar to that described in ref. 27. The procedure consisted of two major steps: (1) preparing the precursor by dissolving zinc acetate in alcoholic solvent and (2) hydrolyzing the precursor by using an alkali metal hydroxide MOH (M = Li, Na or K). In a typical synthesis, 2.5 mmol Zn(CH₃COO)₂·2H₂O was added in 25 mL alcoholic solvent (ethanol–1-pentanol) and the solution was refluxed at its boiling temperature under continuous stirring to obtain a clear solution. Meanwhile, a given amount of MOH (1.875–10 mmol) was dissolved in 25 mL of the alcoholic solvent under the assistance of ultrasonic technique. The synthetic reaction was initiated by dropping MOH solution into the Zn(CH₃COO)₂ precursor. Then, the mixture was hydrolyzed at a given temperature (0–50 °C) under vigorous stirring for 1 h.

For UV-vis absorption, photoluminescence (PL) and PL excitation (PLE) measurements, a small amount of solution was directly extracted from the reactants and diluted by corresponding alcoholic solvent after the reaction. For X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) measurements, the gel was precipitated by adding excess hexane into the solution and then purified by washing with alcohol–hexane and centrifugation three times to remove unreacted precursors.

2.3 Characterization

The XRD data were collected using a Rigaku D/MAX-RA 12 kW X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å), on the synthesized samples dried below 4 °C. The HRTEM images were obtained using a JEM 2100F high-resolution transmission electron microscope. The PL and PLE spectra were acquired on a Edinburgh FLS920 fluorescence spectrophotometer with a Xe lamp as the excitation source. The PLE and PL spectra were obtained in the following steps: firstly, several lights with different wavelengths were used to record the PL emission spectra to obtain the optimal wavelength of the PL emission spectra; then, the obtained optimal wavelengths were used to record the PLE spectra, and the optimal wavelengths of the excitation spectra were finally used to measure the PL spectra. The UV-vis absorption spectra were recorded by a UV765 UV-vis Spectrophotometer.

3 Results and discussion

Fig. 1(a) shows the XRD patterns of ZnO QDs synthesized in ethanol with different types of alkali metal hydroxide MOH (M = Li, Na and K, 10 mmol) at 0 °C. All the diffraction peaks and interplane spacings can be well indexed to the standard hexagonal wurtzite ZnO structure (JCPDF card, no. 36-1451). Fig. 1(b)–(d) demonstrate the HRTEM images of the as-synthesized ZnO QDs, from which it can be directly observed that the particle size is smaller than 8 nm and part of particles aggregate to a certain degree due to their high surface energy. Besides, the high magnification images (top right corner) show the space between the adjacent lattice planes is about 0.28 nm, which is in accordance with the distance of the (100) plane of the standard wurtzite ZnO structure. The selected area electron diffraction (SAED) patterns exhibit concentric diffused ring patterns due to the nanocrystalline nature of ZnO. The above results confirm the formation of ZnO QDs when using different types of MOH as the reactant. However, one thing should be noticed from the XRD results that there exists an obvious difference in the broadening of the diffraction peaks, indicating the different particle size of the obtained ZnO QDs. It is well-known that the diameter of QDs can be calculated by using the Debye–Scherrer formula:²⁸where D is the diameter of the particle, λ is the X-ray wavelength (λ = 1.5406 Å), k = 0.89, B is the full width at half-maximum, and θ is the Bragg diffraction angle. Based on this equation, the average particle diameters of ZnO QDs synthesized with LiOH, NaOH and KOH are calculated to be 2.7, 4.5 and 5.8 nm, respectively.

Fig. 1 (a) XRD patterns of ZnO QDs synthesized in ethanol with 10 mmol MOH (M = Li, Na and K) at 0 °C, and HRTEM images of ZnO QDs synthesized with (b) LiOH, (c) NaOH and (d) KOH. The insets show the corresponding high magnification images (top right corner) and selected area electron diffraction patterns (bottom right corner).

The variation of ZnO QDs size can be also verified by the UV-vis absorption spectra, because when the ZnO QDs size is smaller than their exciton Bohr size of about 6 nm,²⁹ their band gap will become size-dependent due to the quantum confinement effect. From the typical UV-vis absorption spectra in Fig. 2(a), we can find that as the reactant changes from LiOH to NaOH and KOH, the absorption edges show a remarkable red shift, which indicates the increased average diameter of the as-prepared ZnO QDs. It is widely accepted that for a direct band gap semiconductor of ZnO, the relationship of the absorption coefficient and the band gap energy can be described by following equation:³⁰

(αhν)² = A(hν − E_g) (2)

where A is a constant, and α, hν and E_g are denoted as the absorption coefficient, photon energy and optical band gap, respectively. The optical band gap (E_g) is achieved by plotting (αhν)² versus (hν) and extrapolating the tangent of the curve to (αhν)² = 0. As shown in Fig. 2(b), the optical band gaps for ZnO QDs synthesized with LiOH, NaOH and KOH are determined as 4.19, 3.63 and 3.50 eV, respectively. Knowing the optical band gap for ZnO QDs, the particle radius can be calculated through the effective mass model approximation:³¹where E_{(gap, dot)} represents the band gap of ZnO QDs, E_{(gap, dot)} is the band gap of the bulk material (3.37 eV), R is the particle radius, ħ is the reduced Plank's constant, e is the charge of the electron, ε is the semiconductor dielectric constant, m_e and m_h are effective masses of electron and holes, respectively, and m_o is the free electron mass. With the effective masses of electrons m_e = 0.26 m_o and holes m_h = 0.59 m_o, the particle diameters of the as-prepared ZnO QDs that are double as big as radii can be calculated by adopting eqn (3), which are 2.84, 4.61 and 6.00 nm for LiOH, NaOH and KOH, respectively. This is in good agreement with the XRD result.

Fig. 2 (a) UV-vis absorption spectra, (b) (αhν)² versus (hν) plot obtained from absorption curve, (c) normalized PLE spectra and (d) normalized PL spectra of ZnO QDs synthesized with MOH (M = Li, Na and K). The inset in (d) is the photograph of the as-synthesized ZnO QDs under a 302 nm UV lamp excitation.

The normalized PLE and PL of ZnO QDs synthesized with different types of MOH are shown in Fig. 2(c) and (d). The excitation peaks for the ZnO QDs synthesized with LiOH, NaOH and KOH are located at 304, 331 and 342 nm, respectively. When excited by the corresponding optimal excitation wavelength, the emission peaks show an obvious red shift from 439 to 514 and 538 nm. The photograph in the inset of Fig. 2(d) demonstrates different emission colours of ZnO QDs under UV lamp excitation. The peaks of excitation and visible emission of ZnO QDs are all red-shifted as the particles size increases, which are generally considered to be due to the quantum confinement effect.³²

The above results suggest that the type of MOH has a remarkable effect on the final size of ZnO QDs. In order to reveal the exact role of MOH in controlling the growth of ZnO QDs, a series of reactions using different amounts of MOH were carried out. For better understanding, the molar ratio of MOH to Zn(CH₃COO)₂ ([MOH]/[Zn], 0.75–4.0) is used to denote the MOH amount (1.875–10 mmol). Fig. 3 demonstrates the [MOH]/[Zn] ratio dependence of ZnO QDs size determined from the absorption edge using the eqn (2) and (3), and the emission peak obtained from the PL spectrum. It can be clearly seen that the ZnO QDs size undergoes an initial increase with increasing the [MOH]/[Zn] ratio up to a critical value, which are 1.0, 1.5 and 1.7 for LiOH, NaOH and KOH, respectively, and then follows by a rapid decrease. The decrease of ZnO QDs size indicates that the growth is strongly inhibited by the reactant MOH when the [MOH]/[Zn] ratio exceeds a critical value. Such a phenomenon has been already reported previously, which can be ascribed to the introduction of excess cations M⁺ when increasing the MOH amount.^20,25 These positively charged ions can form a layer around ZnO QDs surface, resulting in inhibiting the further growth. The shifts of emission peaks keep pace with the variations of particle size, which can be explained by the quantum confinement effect.

Fig. 3 [MOH]/[Zn] ratio dependences of ZnO QDs size and emission peak when using (a) LiOH, (b) NaOH and (c) KOH as the reactant, respectively. All samples were prepared in ethanol at 0 °C.

Interestingly, with further increasing the [MOH]/[Zn] ratio, the size of ZnO QDs synthesized with LiOH continuously decreases, which however shows a rapid increase when using NaOH or KOH as the reactant. Generally, more small particles are expected to be formed with increasing MOH amount, or in another word, raising the [MOH]/[Zn] ratio. At sufficiently high concentration of small particles, the inter-particle distance will be short enough to facilitate the growth, which ultimately lead to a large particle size.³³ This can be evidently reflected when using NaOH or KOH as the reactant, as shown in Fig. 3(b) and (c). Since the addition of more OH⁻ ions can lead to accelerating the growth while the introduction of excess M⁺ ions can lead to hindering the growth, the decrease of particle size with further increasing the [LiOH]/[Zn] ratio should be due to the difference in the hindering effect between Li⁺ and M⁺ (M = Na and K). To prove this, two independent reactions were designed, in which (1) the [Li]/[Zn] ratio was set as a constant of 4.0 and adjusted the [OH]/[Zn] ratio, using LiOH and CH₃COOLi as Li⁺ source (see Table 1); (2) the [NaOH]/[Zn] ratio was set as a constant of 4.0 and additional Li⁺ ions were introduced into the initial NaOH solution by adding CH₃COOLi (see Table 2). The result of independent reaction (1) indicates that the particle size rapidly decreases with increasing the [OH]/[Zn] ratio, as shown in Fig. 4(a). This suggests that the hindering effect resulting from M⁺ ions strongly relies on the amount of OH⁻ ions, which can be interpreted with the help of the schematic diagram presented in Fig. 5. As the reaction medium is basic, a large amount of OH⁻ ions will be adsorbed on the particle surface so that the initially formed ZnO QDs are negatively charged.^20,34

Table 1 LiOH and CH₃COOLi were used to keep the [Li]/[Zn] ratio constant at 4.0 while increasing [OH]/[Zn] ratio from 1.0 to 4.0

[OH]/[Zn]	1.0	1.5	1.6	1.7	2.0	2.5	3.0	3.5	4.0
[LiOH]/[Zn]	1.0	1.5	1.6	1.7	2.0	2.5	3.0	3.5	4.0
[CH3COOLi]/[Zn]	3.0	2.5	2.4	2.3	2.0	1.5	1.0	0.5	0.0

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Table 2 NaOH and CH₃COOLi were used to keep the [OH]/[Zn] ratio constant at 4.0 while increasing [Li]/[Zn] ratio from 0 to 4.0

[Li]/[Zn]	0	0.5	1.0	2.0	3.0	4.0
[NaOH]/[Zn]	4.0	4.0	4.0	4.0	4.0	4.0
[CH3COOLi]/[Zn]	0	0.5	1.0	2.0	3.0	4.0

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Fig. 4 (a) Variations of particle size and emission peak of ZnO QDs with the ratio of [OH]/[Zn], when keeping the [Li]/[Zn] ratio constant at 4.0; (b) variations of particle size and emission peak of ZnO QDs with the ratio of [Li]/[Zn], when keeping the [OH]/[Zn] ratio constant at 4.0.

Fig. 5 Schematic diagram showing the ZnO QDs surrounded by OH⁻ and M⁺ (M = Li, Na or K) ions and the dual role of MOH as the promoter and inhibitor in the growth of ZnO QDs.

During the reaction process, M⁺ ions are attracted towards the negatively charged ZnO QDs surface, which can result in hindering the approach of Zn²⁺ ions required for the further growth of ZnO QDs. The higher the [OH]/[Zn] ratio, the more the OH⁻ ions will be adsorbed so that the increased amount of M⁺ ions will be attracted towards the surface of ZnO QDs. When the hindering effect resulting from M⁺ ions is stronger than the accelerating effect resulting from OH⁻ ions, the ZnO QDs size will decrease (when using LiOH as the reactant). Otherwise, the QDs size will increase (when using NaOH or KOH as the reactant). Based on the above discussion, a conclusion can be deduced that Li⁺ has a stronger inhibiting effect on ZnO QDs growth than Na⁺ and K⁺, which is surprisingly proved by the result of independent reaction (2), as shown in Fig. 4(b). With the increase of Li⁺ ions and at the fixed [OH]/[Zn] ratio of 4.0, the ZnO QDs size rapidly decreases from 4.6 to 3.2 nm and then slowly decreases to 2.8 nm. In view of the fact that in the presence of excessive MOH, the ZnO QDs synthesized with LiOH are much smaller than those synthesized with NaOH and the QDs prepared with KOH hold the biggest size, it is reasonable to believe the hindering effect follows the trend of Li⁺ > Na⁺ > K⁺. This conclusion can be well explained from two aspects: the electronegativity (Li⁺ > Na⁺ > K⁺) and the ionic radius (Li⁺ < Na⁺ < K⁺). A larger electronegativity helps M⁺ ions to be effectively attracted towards the negatively charged ZnO QDs surface, and a smaller ionic radius enables M⁺ ions to form a more compact passivation layer.

From the above results and discussion, we can find that the reactant MOH exhibits a dual role in growth of ZnO QDs. On the one hand, the reactant MOH can provide OH⁻ ions that can react with Zn²⁺ ions, resulting in forming the product of ZnO QDs. On the other hand, the presence of M⁺ ions, especially for Li⁺ ions, can lead to hindering the growth of ZnO QDs. More importantly, this hindering effect will become more effective if excessive amount of OH⁻ ions is introduced.

It should be pointed out that the growth of ZnO QDs is determined not only by the reactant MOH but also by reaction temperature as well as solvent. It has been widely reported that higher reaction temperature will lead to the larger particles size due to the faster growth rate.^22,26,35 Besides, the growth rate can also be enhanced with increasing carbon chain length of alcoholic solvent due to the influence of solvent viscosity, surface energy and the solubility of ZnO.³⁶ Consequently, bigger ZnO QDs tend to be formed in an alcoholic solvent with longer carbon chain length.^18,36 Nevertheless, only one type of MOH was considered in the previous researches when investigating the effect of reaction temperature or solvent on the growth of ZnO QDs, and the dual role of MOH was not discussed. Thereby, we carried out reactions at different temperatures and in different solvents, in which all types of MOH were taken into consideration and the [MOH]/[Zn] ratio was fixed at 4.0. Fig. 6 shows the size and emission peak of ZnO QDs synthesized at different temperatures and with different types of MOH. Interestingly, the particle size and emission peak show different temperature-dependent behaviors when using different types of MOH as the reactant. When using LiOH as the reactant, the particle size monotonously increases with increasing temperature, which however firstly decreases with increasing reaction temperature up to 15 °C and then shows a rapid increase with further increasing reaction temperature when using NaOH or KOH as reactant. Such different solvent-dependent particle size or emission peak variations when using different types of MOH as the reactant can also be observed, as shown in Fig. 7. In the cases of NaOH and KOH, the PL spectra show a red shift as the solvent changes from ethanol to 1-pentanol, indicating an increase of the size. Especially, the ZnO QDs synthesized in 1-pentanol using KOH grow to 11.4 nm and emit very strong UV fluorescence (see ESI, Fig. S1†), which exhibits the luminescent property of bulk ZnO. Differently, in the case of LiOH, the PL spectra show a blue shift as the solvent changes from ethanol to 1-propanol, indicating the decreased particle size of ZnO QDs. These abnormal results have not been reported to the best of our knowledge. Since other reaction parameters were kept the same, the above different growth behaviors can be explained reasonably from the dual role of MOH point of view as well as the difference among the reactants MOH.

Fig. 6 Reaction temperature dependences of ZnO QDs size and emission peak when using (a) LiOH, (b) NaOH and (c) KOH as the reactant, respectively. All samples were prepared in ethanol and the [MOH]/[Zn] ratio was fixed at 4.0.

Fig. 7 (a) Solvent dependence of ZnO QDs size. (b), (c) and (d) are normalized PL spectra of ZnO QDs synthesized in different alcoholic solvent using LiOH, NaOH and KOH as the reactant, respectively. All samples were prepared at 0 °C and the [MOH]/[Zn] ratio was fixed at 4.0.

Generally, the bigger ZnO QDs should be obtained at higher reaction temperature due to a higher growth speed. However, the growth of ZnO QDs is controlled by not only reaction temperature but also the type and amount of MOH. A fact should not be omitted that less MOH amount is dissolved into the solution at higher temperature due to the decreased solubility (see Fig. S2,† taking LiOH as an example). In this case, the actual [MOH]/[Zn] ratio is lower than 4.0 at high temperature although 10 mmol MOH was added. According to the experimental results presented in Fig. 3, when the [MOH]/[Zn] ratio gradually reduces from 4.0, the ZnO QDs size will obviously increase or decrease when using LiOH or MOH (M = Na, K) as the reactant, respectively. Therefore, when using NaOH or KOH as the reactant, the reduce of ZnO QDs size with increasing temperature up to 15 °C actually results from the decreased amount of OH⁻ ions that act as promoters for the growth of ZnO QDs. Once reaction temperature is high enough to dominate the growth rate, the size monotonously increases when raising reaction temperature.

When using different solvents, the inhibitor role of MOH should also be considered to explain the different growth behaviors. With the decrease of dielectric constant from ethanol to 1-pentanol, the dissociation constant of MOH decreases, which means the easier combination for M⁺ and OH⁻ ions so that the M⁺ ions are easier to be attracted towards the OH⁻ ions surrounding the surface of ZnO QDs. According to the above conclusion, M⁺ ions can hinder the growth of ZnO QDs and the hindering effect of Li⁺ ions is stronger than that of Na⁺ and K⁺ ions. When using LiOH as the reactant, the inhibiting effect is stronger than the accelerating effect caused by changing the solvent from ethanol to 1-propanol. Hence, ZnO QDs tend to be smaller. It is worthy of mentioning that the decrease of LiOH dissociation constant can also bring about the decline of the actual LiOH amount dislocated in solution. But the result suggests that the hindering effect arising from Li⁺ ions is dominated, which may be ascribed to the low diffusion rate of Zn²⁺ ion in solvent with high viscosity.

4 Conclusions

In summary, a sol–gel method was employed to synthesize ZnO QDs with different types of MOH (M = Li, Na and K) as the reactant. The role of MOH in the growth of ZnO QDs was systematically investigated by using XRD, HRTEM, UV-vis, PL and PLE measurements. The experimental results reveal that the growth of ZnO QDs is significantly controlled by MOH through the co-effect of OH⁻ and M⁺ ions, which can promote the growth through accelerating the hydrolysis of precursors and hinder the further growth due to the formation of a passivation layer around the negatively charged ZnO QDs surface, respectively. Further investigations suggest that the hindering effect follows the trend of Li⁺ > Na⁺ > K⁺ due to their different electronegativities and ionic radii. Besides, the presence of excessive OH⁻ ions is the essential condition to restrain the growth of ZnO QDs, as which can effectively facilitate the formation of high-performance passivation layer. Moreover, the different temperature and solvent dependent ZnO QDs growth when using different types of MOH as the reactant also confirm the dual role of MOH. Due to the quantum confinement effect, the emission peak of ZnO QDs shows size-dependence and can be precisely tuned from 407 to 545 nm by adjusting the reaction parameters. This research sheds light on the understanding of the growth of ZnO QDs from the role of the reactant MOH point of view, which may benefit the controllable synthesis of ZnO QDs and pave a way for their practical applications.

Acknowledgements

This work was financially supported by the Shanghai Scientific Research Innovation Projects (No. 14ZZ037), the Basic Research Project of Shanghai Science and Technology Commission (No. 12JC1408500), the Fundamental Research Funds for the Central Universities (No. 2011KJ018).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06660g

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