**4.4. Nanoparticle generation**

irradiation at 144 mJ/cm2 made irreducible change, and the generated gas volume decreased [20]. This hydrogen generation reaction did not proceed by the irradiation of 30 fs laser pulses

larger surface area. However, when the gas volume was compared to binchotan powders of different sizes of 1, 5, and 10 μm in mean diameter, whose BET surface areas were 120, 22,

The gas generation was observed from VIS to near-infrared (NIR) irradiation for both cases with binchotan (A) and pure carbon (B). **Figure 8** shows the generated gas volume versus the

volume generated with binchotan (A, red solid circles) was more than twice that of the pure carbon (B, black open circles) under the same irradiation conditions in the VIS-NIR range. A tendency for a reduction in gas yield at longer wavelengths was anticorrelated to the optical

**Figure 8.** Irradiated wavelength dependence of the generated gas volume for binchotan powder (A, solid circles) and

An alcohol additive in the binchotan water suspension enhanced the hydrogen generation efficiency for the laser fluences above the threshold of ca. 50 mJ/cm2 [21]. Among methanol, ethanol, and isopropanol, ethanol was the most efficient additive and raised twice the generated volume. The generated volume increased according to the increase of ethanol additive

pure carbon powder (B, red circles) obtained by pulse irradiation of 112 mJ/cm2

was adapted with permission from Ref. [20].

**4.3. Alcohol additive effect**

in **Figure 7b**. This fact implies that there are other factors affecting the reaction efficiency.

irradiated wavelength, obtained for 30 min irradiation at a laser fluence of 112 mJ/cm2

/g, the generated volume did not depend on the surface area ratio linearly as shown

[20], implying the present reaction is classified into

/g. Therefore, the higher gas generation is mainly attributed to its

/g, which was almost twice

for 30 min. The result for sample A

. The gas

/g [20] or the graphite powder of

which affords a laser fluence of 80 mJ/cm2

The BET surface area of the binchotan of 5 μm (A) was 22 ± 3 m2

276 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

that of the high-grade carbon powder of 5 μm (B) 13 ± 3 m2

the thermal ablation process [1].

less than 45 μm (C) 9 ± 3 m2

and 8 m2

reflectivity [20].

A post-irradiated solution was investigated using UV-VIS absorption and DLS methods. As shown in **Figure 9**, light extinction by UV absorption and light scattering appeared in the centrifuged solution following the irradiation. This change indicates the production of new materials by laser ablation as by-products of the hydrogen generation. DLS measurements showed that this solution contained nanoparticles of 125 nm in mean diameter (see the inset). Such nanoparticles could be measured only for the irradiation above the threshold fluence that is the same as that for gas generation. The mean diameter of the nanoparticles was independent

**Figure 9.** UV-Vis absorption spectra of centrifuged solutions after laser irradiation at 125 (black line), 150 (blue line), and 175 (red line) mJ/cm2 for binchotan in water. Inset: size distribution of nanoparticles (bars) and logarithmic normal distribution function (broken line) in the centrifuged solution created by laser irradiation at 175 mJ/cm2 , measured using the DLS method. Adapted with permission from Ref. [36].

of the laser fluence, whereas light extinction was enhanced at higher irradiation fluences corresponding to an increase in the nanoparticle number. Furthermore, the generated nanoparticles are slightly dressed by a negative ζ-potential (−15 MeV) in water. Therefore, the nanoparticles suspended in water are expected to be stable for a long time as in the case of the organic QQ nanoparticles mentioned in Section 3.

A TEM image of the nanoparticles is shown in **Figure 10a**. Nanoparticles with sizes of around 100 nm were typically observed, as indicated by a yellow circle for a typical one. The sizes of the nanoparticles are consistent with the mean diameter observed by the DLS. A selected area electron diffraction (SAED) pattern of the nanoparticles (**Figure 10b**) shows clear diffraction spots in addition to diffused halo rings, whereas the Debye-Scherrer rings from the carbon structure were observed in a SAED pattern from the nonirradiated particle ensemble (**Figure 10c**). Some of spots in **Figure 10b** was located on the rings derived from the lattice spacings of diamond, and other parts of spots were on those of the C8 and n-diamond that were produced by laser ablation of a graphite target covered by water [29]. There were still other diffraction spots that could not be assigned to diffraction patterns of known structures. These results indicate that various crystalline/amorphous carbon structures including nanocrystalline carbon/diamond were created by laser ablation of binchotan charcoal in the liquid phase.

**Figure 10.** (a) A TEM image of binchotan nanoparticles produced by laser ablation. (b) SAED pattern obtained from the nanoparticles. (c) SAED pattern obtained from binchotan powder before irradiation.

Furthermore, the dried nanoparticles on a silicon substrate showed new IR peaks at 797, 873, 1019, 1261, 1425, 2906, and 2963 cm−1 as shown in **Figure 11**. Generally, vibrations of aromatic molecules are observed in the fingerprint range of 500–1500 cm−1, and C─H stretch modes are in 2800–3000 cm−1 by an FT-IR measurement. The Raman peaks of O─H bonding were also observed during the reaction as described in Section 4.5. Therefore, the appearance of the peaks in these ranges indicated the creation of small carbon networks including the bonding of C─H and O─H groups.

**Figure 11.** FT-IR spectra of dried post-irradiated nanoparticles (a red line) and nonirradiated binchotan powder (a black line) on a silicon substrate at room temperature. For comparison, the spectrum of a silicon substrate (a blue line) is also shown.

It is known that charcoal constitutes a form of amorphous carbons consisting of sp2 and sp3 bonding [47]. For the creation of new networks, bond breaking and reconstruction occur during the laser ablation by nanosecond pulses. Light energy at the threshold is necessary for such reactions. Surprisingly, in graphite powder, no nanoparticle was measured, and no additional IR peaks were observed.

### **4.5. Mechanism of the hydrogen generation**

of the laser fluence, whereas light extinction was enhanced at higher irradiation fluences corresponding to an increase in the nanoparticle number. Furthermore, the generated nanoparticles are slightly dressed by a negative ζ-potential (−15 MeV) in water. Therefore, the nanoparticles suspended in water are expected to be stable for a long time as in the case of the

278 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

A TEM image of the nanoparticles is shown in **Figure 10a**. Nanoparticles with sizes of around 100 nm were typically observed, as indicated by a yellow circle for a typical one. The sizes of the nanoparticles are consistent with the mean diameter observed by the DLS. A selected area electron diffraction (SAED) pattern of the nanoparticles (**Figure 10b**) shows clear diffraction spots in addition to diffused halo rings, whereas the Debye-Scherrer rings from the carbon structure were observed in a SAED pattern from the nonirradiated particle ensemble (**Figure 10c**). Some of spots in **Figure 10b** was located on the rings derived from the lattice spacings of diamond, and other parts of spots were on those of the C8 and n-diamond that were produced by laser ablation of a graphite target covered by water [29]. There were still other diffraction spots that could not be assigned to diffraction patterns of known structures. These results indicate that various crystalline/amorphous carbon structures including nanocrystalline carbon/diamond were created by laser ablation of binchotan charcoal in the liquid

**Figure 10.** (a) A TEM image of binchotan nanoparticles produced by laser ablation. (b) SAED pattern obtained from the

Furthermore, the dried nanoparticles on a silicon substrate showed new IR peaks at 797, 873, 1019, 1261, 1425, 2906, and 2963 cm−1 as shown in **Figure 11**. Generally, vibrations of aromatic molecules are observed in the fingerprint range of 500–1500 cm−1, and C─H stretch modes are in 2800–3000 cm−1 by an FT-IR measurement. The Raman peaks of O─H bonding were also observed during the reaction as described in Section 4.5. Therefore, the appearance of the peaks in these ranges indicated the creation of small carbon networks including the bonding of C─H

nanoparticles. (c) SAED pattern obtained from binchotan powder before irradiation.

organic QQ nanoparticles mentioned in Section 3.

phase.

and O─H groups.

A clue to understand the mechanism behind the hydrogen generation via intense light irradiation is to clarify the nonequilibrium conditions at the irradiated site within a nanosecond time period. Investigation by time-integrated/-resolved spectroscopy during the hydrogen generation provided us crucial information regarding on-site nonequilibrium conditions including temperature increases [36].

**Figure 12.** (a) Optical emission spectra from binchotan block in water (red solid line) and in 50% ethanol aqueous solution (broken black line) excited by laser pulses with 170 mJ/cm2 energy density and 532 nm wavelength. (b) Incident laser fluence dependence of emission intensity at 470 nm for binchotan block in water (red solid circles) and in 50% ethanol aqueous solution (blue open circles). Adapted with permission from Ref. [36].

White-light emission was observed during the reaction from a binchotan block in water. As shown in **Figure 12a**, a broad spectrum over the visible range is apparent on both sides of the 532 nm excitation wavelength, across the penetration gap of the super notch filter, in water (solid red line), or in 50% ethanol aqueous solution (broken black line). No emission was observed from the water itself. The relatively narrow peaks at 650 and 630 nm are attributed to the Raman scattering lines at 3400 and 2930 cm−1, because the peak positions changed following excitation wavelengths. There was no indication of the plasma emission from neutral/ionized atoms typically observed in LIBS. The Raman scattering lines are assigned to vibration of the O─H stretch mode under a hydrogen bond and Raman-active C─H vibrational modes of ethanol [48].

The white-light emission appeared only above a threshold excitation energy density. As shown in **Figure 12b**, the emission intensity at 470 nm increased nonlinearly in accordance with variations in the incident laser fluence. The threshold at 50 mJ/cm2 was identical for both specimens in the water (red solid circles) and 50% ethanol aqueous solution (blue open circles). Note that the threshold for the appearance of the white light is coincident with the threshold for hydrogen generation (**Figure 7**). Therefore, it is reasonable to consider that the white-light emission is a simultaneous product of the hydrogen generation reaction. With a carbon electrode (99.9%), one fifth of emission intensity was observed above similar threshold excitation energy, and the generated gas volume was also small.

Spectral shape at shorter than 650 nm is well reproduced by Planck's law at a temperature 3860 K. Furthermore, time-resolved spectrum revealed a repetitive spectral change due to the temperature variation in the duration of laser pulse [36]. From these experimental facts, it was confirmed that the laser pulse supplies heat energy through optical absorption, and the whitelight emission can reasonably be attributed to blackbody radiation from the irradiated site. It implies that hydrogen generation induced by laser irradiation proceeds similarly to classical coal gasification, which features reactions at HPHT. Finally, it was concluded that the hydrogen generation induced by the laser pulse irradiation occurs under high-pressure and hightemperature conditions.
