**4.1. Hydrogen generation from carbon in water**

During a trial of nanoparticle generation by laser ablation in the liquid phase, we found that bubbles rose from the irradiated site inside a bottle which contained powder of binchotan charcoal and water. After preliminary discovery of explosive combustion of the generated gas by ignition, the collected gas was analyzed to find that hydrogen gas was contained. **Table 1** shows the ratio of the generated gas components, where argon portion from the argon-purged water was excluded. In the collected gas, roughly 50% of hydrogen and 20% of carbon monoxide were contained, whereas the amount of oxygen was very low [20, 21]. No gas was generated from pure water itself under the same irradiation conditions. From these facts, it was concluded that the reaction is due to photochemical reaction of carbon with water, instead of ideal water-splitting. Alcohol additive enhanced the generated gas volume with 56% of hydrogen concentration [21] as shown in the third row in **Table 1**. Details are described in Section 4.3.


**Table 1.** Ratio of gas component obtained from binchotan suspension by laser irradiation under argon atmosphere with argon-purged water (100% H2O) and 50 wt% ethanol/argon-purged water (50% EtOH/H2O).

From the ratio of the generated gas components, the reaction resembles to that of coal gasification, which is a classical technique of syngas production by steaming of coal under high pressure (a few MPa) and high temperature (>800°C) (HPHT) [42] via the following:

$$\rm{C}(s) + \rm{H}\_{2}\rm{O}(g) \rightarrow \rm{H}\_{2}(g) + \rm{CO}(g) \tag{1}$$

Application of Liquid Laser Ablation: Organic Nanoparticle Formation and Hydrogen Gas Generation http://dx.doi.org/10.5772/64939 275

$$\rm{C(s)} + 2H\_2O(g) \rightarrow 2H\_2(g) + CO\_2(g) \tag{2}$$

$$\text{2.3 C(s)} + \text{O}\_2(\text{g}) + \text{H}\_2\text{O(g)} \rightarrow \text{H}\_2(\text{g}) + \text{2CO(g)}\tag{3}$$

$$2\text{ C(s)} + \text{O}\_2(\text{g}) + 2\text{H}\_2\text{O(g)} \rightarrow 2\text{H}\_2(\text{g}) + 2\text{CO}\_2(\text{g})\tag{4}$$

$$2\text{ C(s)} + 2\text{H}\_2\text{O(g)} \rightarrow \text{CH}\_4\text{(g)} + \text{CO}\_2\text{(g)}\tag{5}$$

In the present laser-induced reaction, the water temperature rose from 22 to 29°C during the 30-min irradiation at 182 mJ/cm2 for 9.5 mL volume of water. Further evidence of temperature elevation in the laser pulse duration was witnessed by optical emission spectroscopy as discussed later in Section 4.5.

### **4.2. Laser fluence dependence**

This hydrogen generation reaction occurred under a lower irradiation energy than that required for plasma-state generation. It has been known that the plasma state is induced when the laser pulse energy is focused on materials with an energy density over a few joule per square centimeter [29, 32]. Such an exploded plasma gas is the result of material dissociation and has been investigated, for example, in laser-induced breakdown spectroscopy (LIBS) by measuring the luminescence from the plasma state [29, 32]. In contrast, in the present reaction, no evidence of a plasma state was observed, but temperature elevation at the irradiated site

In this reaction, a high-grade Japanese charcoal, known as binchotan in Japan, is adopted as the carbon source because of its high carbonization over 93%. Among various carbon materials, charcoal is a sustainable carbon source, because it is made of wood and intermediates Earth's

During a trial of nanoparticle generation by laser ablation in the liquid phase, we found that bubbles rose from the irradiated site inside a bottle which contained powder of binchotan charcoal and water. After preliminary discovery of explosive combustion of the generated gas by ignition, the collected gas was analyzed to find that hydrogen gas was contained. **Table 1** shows the ratio of the generated gas components, where argon portion from the argon-purged water was excluded. In the collected gas, roughly 50% of hydrogen and 20% of carbon monoxide were contained, whereas the amount of oxygen was very low [20, 21]. No gas was generated from pure water itself under the same irradiation conditions. From these facts, it was concluded that the reaction is due to photochemical reaction of carbon with water, instead of ideal water-splitting. Alcohol additive enhanced the generated gas volume with 56% of hydrogen concentration [21] as shown in the third row in **Table 1**. Details are described in

100% H2O 48.7 1.3 20.5 0.5 5.1 23.1 50% EtOH/H2O 56.2 2.7 25.2 0.0 5.5 8.5

**Table 1.** Ratio of gas component obtained from binchotan suspension by laser irradiation under argon atmosphere

pressure (a few MPa) and high temperature (>800°C) (HPHT) [42] via the following:

From the ratio of the generated gas components, the reaction resembles to that of coal gasification, which is a classical technique of syngas production by steaming of coal under high

In both rows, values show a ratio excluding the argon. Adapted with permission from Refs. [20, 21].

with argon-purged water (100% H2O) and 50 wt% ethanol/argon-purged water (50% EtOH/H2O).

**H2 (%) O2 (%) CO (%) CO2 (%) N2 (%) CH (%)**

C s H O g H g CO g ( ) + ®+ 2 2 ( ) ( ) ( ) (1)

was confirmed by spectroscopy [36] as described in Section 4.5.

**4.1. Hydrogen generation from carbon in water**

Section 4.3.

carbon cycle. Laser ablation effects are compared to other carbon materials.

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

The hydrogen-included gas was generated above a threshold fluence of nanosecond laser pulse irradiation. **Figure 7a** shows the laser fluence dependence of a generated gas volume after 30 min of irradiation at a laser wavelength of 532 nm for three kinds of carbon powders. Macroscopic gas volumes of more than 0.05 mL were detectable only above a laser fluence of ca. 50 mJ/cm2 . The gas volume generated with binchotan charcoal powder of 5 μm in diameter (A, red solid circles) was almost twice that of the high-grade carbon powder of 5 μm (B, black open circles) and graphite powder of less than 45 μm (C, blue solid triangles) under the same irradiation conditions, whereas the threshold laser fluences were nearly coincident. The generated gas volume increased by irradiation time within one hour, but the further prolonged

**Figure 7.** (a) Laser fluence dependence of generated gas volume with irradiation at 532 nm for 30 min, 26 mg of binchotan charcoal powder (A, red solid circles), high-grade carbon powder (B, black open circles), and graphite powder (C, blue solid triangles). The results for sample A and B were adapted with permission from Ref. [20]. (b) Laser fluence dependence of generated gas volume for powders of different sizes in diameter.

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 which affords a laser fluence of 80 mJ/cm2 [20], implying the present reaction is classified into the thermal ablation process [1].

The BET surface area of the binchotan of 5 μm (A) was 22 ± 3 m2 /g, which was almost twice that of the high-grade carbon powder of 5 μm (B) 13 ± 3 m2 /g [20] or the graphite powder of less than 45 μm (C) 9 ± 3 m2 /g. Therefore, the higher gas generation is mainly attributed to its 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, and 8 m2 /g, the generated volume did not depend on the surface area ratio linearly as shown in **Figure 7b**. This fact implies that there are other factors affecting the reaction efficiency.

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 irradiated wavelength, obtained for 30 min irradiation at a laser fluence of 112 mJ/cm2 . The gas 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 reflectivity [20].

**Figure 8.** Irradiated wavelength dependence of the generated gas volume for binchotan powder (A, solid circles) and pure carbon powder (B, red circles) obtained by pulse irradiation of 112 mJ/cm2 for 30 min. The result for sample A was adapted with permission from Ref. [20].

### **4.3. Alcohol additive effect**

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 and saturated at 40–50% of ethanol in water. The generated gas contained 56% of hydrogen as shown in **Table 1**.

Alcohol is known to act as an oxygen scavenger preventing the reverse reaction into water [43, 44] and/or as a current doubler [45] in photocatalytic water splitting. As the present photochemical reaction is different from the photocatalytic water-splitting reaction, the enhancement of the reaction is partially due to a photochemical reaction of ethanol itself. Endothermic reactions of a steam reformation of ethanol, C2H5OH + 3H2O → 2CO2 + 6H2 and/or C2H5OH + H2O → 2CO + 4H2, which usually progress under HPHT [40], might occur by the laser irradiation, in addition to the oxidation reactions of solid carbon.

The generated gas volume of 7.3 mL, which was obtained after 1 h of irradiation with a 209 mJ/pulse at 532 nm by the 50% ethanol additive [21], was quite small. According to the hydrogen ratio of 56%, the hydrogen amount included in the volume was calculated as 0.17 mmol. That is, one hydrogen molecule per 126 photons was generated, assuming that 64% of the irradiated laser power was used in the reaction [20]. Although this gas volume was comparable to the production by a photocatalytic water reduction with hydrogen-terminated nano-diamonds [30], it was much less than the carbon-assisted electrochemical hydrogen generation by electric power [46].
