1 Introduction

Laser ablation of a solid-state target in liquid attracts much attention of many researchers as a novel method for synthesizing functional nanoparticles [1–6]. A technical advantage of liquid-phase laser ablation is a low equipment cost since it requires no vacuum systems. Another technical advantage is easy collection of synthesized nanoparticles since they are stored in the liquid as a form of colloidal solution. On the other hand, liquid-phase laser ablation has physical uniqueness in addition to the technical advantages. The physical uniqueness is owing to the high temperature and the high pressure of reaction fields for the synthesis of nanoparticles. A high-temperature, high-pressure reaction field is the laser ablation plasma. Since the expansion of particles ejected from the target is restricted significantly in liquid-phase laser ablation because of the tight confinement effect of ambient liquid, the density of the laser ablation plasma is much higher than that in gas-phase laser ablation. Another high-temperature, high-pressure reaction field is a cavitation bubble, which is induced by the production of the dense laser-ablation plasma. It is known that the cavitation bubble expands and shrinks with time, and the second high-pressure reaction field is obtained at the collapse of the cavitation bubble [7]. It is suggested by several reports that the high-temperature, high-pressure reaction fields play helpful roles in the synthesis of nanoparticles with crystalline structures [8–11].

A problem of liquid-phase laser ablation is the difficulty in the physical control of the reaction fields. Although it is known that the size of synthesized nanoparticles is controlled by the laser fluence used for ablation [12–14], we need another knob for controlling the synthesis characteristics of nanoparticles. In previous works [15, 16], we proposed applying external high pressure to ambient liquid in liquid-phase laser ablation as a method for controlling the dynamics of the laser ablation plasma and the cavitation bubble. In addition, we demonstrated the control of the size of nanoparticles by the external high pressure using an in situ laser-light scattering apparatus [17]. In this work, we examined the effect of the external high pressure on the synthesis characteristics of ZnO nanoparticles. Since our principal interest was the positive effect of the high-temperature, high-pressure reaction fields on the synthesis of crystalline nanoparticles, we mainly focused on the structures of ZnO nanoparticles synthesized at various liquid pressures.

2 Experiment

The details of the chamber developed for laser ablation in pressurized water have been described in previous papers [15, 16]. The chamber was compatible with a pressure up to 35 MPa. It was made of hastelloy, and the inner surfaces of the chamber as well as other components were covered with gold films. The chamber was filled with distilled water, and a mechanical pump was used for pressurizing water inside the chamber. A Zn target installed in the chamber was irradiated by Nd:YAG laser pulses at a wavelength of 1064 nm from the normal direction. The repetition frequency and the duration of the YAG laser pulse were 10 Hz and 10 ns, respectively. The YAG laser beam was focused using a lens. The target was fixed on a rotating holder to avoid a deep ablation crater due to the irradiations of many laser pulses. The colloidal solution, which included nanoparticles, was collected after the laser irradiation for one hour.

The optical characteristics of the colloidal solution were examined by optical absorption spectroscopy and photoluminescence spectroscopy. The excitation wavelength for photoluminescence spectroscopy was 320 nm. The crystalline structure of nanoparticles was examined by X-ray diffraction (XRD). The sample for the XRD analysis was prepared by dropping a droplet of the colloidal solution on a silicon wafer. The same sample was used for taking pictures of nanoparticles using a scanning electron microscope.

3 Results

Figure 1 shows XRD spectra of nanoparticles prepared at water pressures of 0.1 and 3 MPa. The samples for the XRD analyses were prepared by dripping the same numbers of droplets onto silicon substrates. The energy of the YAG laser pulse used for ablation was 44 mJ. The peaks corresponding to the (100), (002), (101), and (102) planes indicate ZnO nanoparticles with the wurtzite crystalline structure. As shown in the figure, higher amplitudes of the XRD peaks were observed when we pressurized ambient water at 3 MPa. The effect of the high pressure of ambient water was observed more remarkably when we used weaker laser pulses for ablation. Figure 2 shows XRD spectra of nanoparticles prepared at a laser energy of 10 mJ. The samples were prepared via the same procedures of enrichment and dripping. In this case, the structure of nanoparticles synthesized at a water pressure of 0.1 MPa was amorphous as shown in Fig. 2(a). Nanoparticles prepared at a water pressure of 30 MPa had XRD peaks as shown in Fig. 2(b), indicating a crystalline structure of nanoparticles. The spectrum shown in Fig. 2(b) has four peaks, three of which can be assigned as the (100), (002), (101) planes of ZnO with the wurtzite crystalline structure. The fourth peak at 2θ=33∘ may be assigned to ZnOOH [18].

Fig. 1
figure 1

XRD spectra of ZnO nanoparticles prepared at water pressures of (a) 0.1 MPa and (b) 3 MPa. The laser energy was 44 mJ

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Fig. 2
figure 2

XRD spectra of ZnO nanoparticles prepared at water pressures of (a) 0.1 MPa and (b) 30 MPa. The laser energy was 10 mJ

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Figure 3 shows SEM images of nanoparticles prepared at 0.1 and 30 MPa. The laser energy was 7 mJ. As shown in the figure, the morphologies of nanoparticles was significantly different between the two samples. As shown in Fig. 3(a), we observed no spherical nanoparticles when the water pressure was 0.1 MPa. According to the XRD analysis and the electron diffraction pattern observed by transmission electron microscopy, nanoparticles having the morphology shown in Fig. 3(a) have an amorphous structure. In contrast, we observed aggregated spherical nanoparticles, as shown in Fig. 3(b), when we pressurized ambient water at 30 MPa. The spherical nanoparticles had a crystalline structure [19].

Fig. 3
figure 3

SEM images of ZnO nanoparticles prepared at water pressures of (a) 0.1 MPa and (b) 30 MPa. The laser energy was 7 mJ

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The absorbance spectra of colloidal solutions prepared at water pressures of 0.1, 3, and 30 MPa are shown in Fig. 4. The laser energy was 44 mJ. Spectra which are normalized at the wavelength corresponding to the shoulder of the absorbance curve are shown in Fig. 4 to show the difference in the absorbance curves clearly. The absolute absorbance of the colloidal solution prepared at 30 MPa was weaker than those prepared at 0.1 and 3 MPa. As indicated in the figure, it was observed that the wavelength corresponding to the shoulder of the absorbance curve became shorter with the water pressure increase.

Fig. 4
figure 4

Optical absorbance spectra of colloidal solutions prepared at water pressures of 0.1, 3, and 30 MPa. The laser energy was 44 mJ

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Figure 5 shows the photoluminescence spectra of the same colloidal solutions as those shown in Fig. 4. The photoluminescence spectra were composed of two components caused by water and ZnO nanoparticles. We decomposed the two components using a curve fitting procedure. The photoluminescence spectra of water, which had a peak at a wavelength of 360 nm, are illustrated by broken curves in Fig. 5. As indicated in the figure, it was observed that the wavelength corresponding to the peak of the photoluminescence spectrum of ZnO shifted toward the shorter wavelength side with the water pressure.

Fig. 5
figure 5

Photoluminescence spectra of colloidal solutions prepared at (a) 0.1 MPa, (b) 3 MPa, and (c) 30 MPa. The laser energy was 44 mJ

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4 Discussion

A remarkable effect of the high pressure applied to ambient water was the enhancement of the crystallinity of ZnO nanoparticles, as shown in Figs. 1 and 2. The change in the structure of ZnO nanoparticles from a crystal to amorphous one with the decrease in the laser energy has been reported by Zhang et al. [19]. A physical explanation for the structure change with the laser energy is that higher pressures of the laser ablation plasma and the collapsed cavitation bubble are expected when a laser pulse with a higher energy is used for ablation. The importance of the pressure of the collapsed cavitation bubble is supported by an experimental finding on the growth of nanoparticles inside the cavitation bubble [20]. It has been observed that nanoparticles are stored inside the cavitation bubble until the collapse, and hence, nanoparticles are located in the high-pressure reaction field which is realized at the collapse of the cavitation bubble.

A similar mechanism may be applicable for the enhancement of the crystallinity by the external high pressure. We have reported a slight compression of the laser ablation plasma by applying a pressure of 30 MPa to ambient water, suggesting the increase in the plasma pressure by the external high pressure [15]. In addition, we have reported the change in the dynamics of a cavitation bubble by applying the external high pressure [16]. We have observed that the maximum size and the period from the formation to the collapse of the cavitation bubble are inversely proportional to the water pressure. A theoretical work, which reproduced the dynamics of the cavitation bubble observed experimentally, suggested the increase in the pressure of the collapsed cavitation bubble with the water pressure [21]. Therefore, the enhancement of the crystallinity of ZnO nanoparticles by applying the external high pressure to ambient water may be caused by the enhancement of the pressures of the laser ablation plasma and the collapsed cavitation bubble. The lower nanoparticle density in the colloidal solution prepared at a water pressure of 30 MPa is attributed to the extinction of the growth of the nanoparticle density due to the early collapse of the cavitation bubble [17].

Another effect of the external high pressure applied to ambient water is the decrease in the size of nanoparticles, which is seen from the shifts of the wavelengths corresponding to the shoulder of the optical absorbance spectrum and the peak of the photoluminescence spectrum (Figs. 4 and 5). The shifts of the wavelengths toward the shorter wavelength side indicate the decrease in the size of nanoparticles with the water pressure [22, 23]. The change in the size of nanoparticles is consistent with the result of in situ laser-light scattering [17]. However, we observed no clear variation in the size of nanoparticles (several tens of nanometers) by the direct observation using a transmission electron microscope, which may be due to the influence of agglomeration. It is known that changing the laser energy is a method for controlling the size of nanoparticles synthesized by liquid-phase laser ablation [12–14]. It has been shown by the present work that the water pressure is another knob for controlling the size of nanoparticles.

5 Conclusions

We have examined effects of water pressure on the characteristics of ZnO nanoparticles synthesized by liquid-phase laser ablation. The effects of the pressurized water were (i) the enhancement of the crystallinity of nanoparticles and (ii) the decrease in the size of nanoparticles. The water pressure is a valuable knob for controlling the synthesis of nanoparticles by liquid-phase laser ablation.