Laser direct overall water splitting for H₂ and H₂O₂ production

Significance

Hydrogen (H₂) and hydrogen peroxide (H₂O₂) are important raw materials for industry. However, traditional catalytic methods require a variety of complex and expensive catalysts and the efficiency is relatively low. Here, laser direct overall water splitting is put forward to generate H₂ and H₂O₂ simultaneously. Laser-induced cavitation bubbles have two characteristics: transient high-temperature (10⁴ K) and ultrafast cooling (10⁸ K/s). They provide a very suitable microenvironment for water decomposing. A remarkable energy conversion efficiency of light-to-chemical products (2.1%) with H₂ production rates of 2.2 mmol/h and H₂O₂ production rates of 65 µM/h can be achieved under ambient conditions without any catalyst. It can be served as a demonstration of potentially solar-pumped catalyst-free hydrogen extraction and other chemical synthesis.

Abstract

Hydrogen (H₂) and hydrogen peroxide (H₂O₂) play crucial roles as energy carriers and raw materials for industrial production. However, the current techniques for H₂ and H₂O₂ production rely on complex catalysts and involve multiple intermediate steps. In this study, we present a straightforward, environmentally friendly, and highly efficient laser-induced conversion method for overall water splitting to simultaneously generate H₂ and H₂O₂ at ambient conditions without any catalysts. The laser direct overall water splitting approach achieves an impressive light-to-hydrogen energy conversion efficiency of 2.1%, with H₂ production rates of 2.2 mmol/h and H₂O₂ production rates of 65 µM/h in a limited reaction area (1 mm²) within a short real reaction time (0.36 ms/h). Furthermore, we elucidate the underlying physics and chemistry behind the laser-induced water splitting to produce H₂ and H₂O₂. The laser-induced cavitation bubbles create an optimal microenvironment for water-splitting reactions because of the transient high temperatures (10⁴ K) surpassing the chemical barrier required. Additionally, their rapid cooling rate (10¹⁰ K/s) hinders reverse reactions and facilitates H₂O₂ retention. Finally, upon bubble collapse, H₂ is released while H₂O₂ remains dissolved in the water. Moreover, a preliminary amplification experiment demonstrates the potential industrial applications of this laser chemistry. These findings highlight that laser-based production of H₂ and H₂O₂ from water holds promise as a straightforward, environmentally friendly, and efficient approach on an industrial scale beyond conventional chemical catalysis.

Hydrogen (H₂) and hydrogen peroxide (H₂O₂) have become increasingly important in industrial production and everyday life as green energy and oxidants (1, 2). H₂, as a clean and renewable energy with high mass–energy density and high calorific value, is essential in solving the current energy and environment crisis (3). H₂ can be obtained from both nonrenewable and renewable energy sources, with the former typically being derived from fossil fuels and the latter from water or renewable materials. Unfortunately, H₂ production from fossil fuels generates carbon emissions, making it unsuitable as a solution to current energy and environmental challenges (4). Currently, electrocatalysis and photocatalysis are common methods used to produce H₂ from water, but usually require complex catalysts and involve tedious catalytic processes (5). On the other hand, H₂O₂ is a vital raw material and disinfectant in the medicine and healthcare industries (6). The traditional synthesis of H₂O₂ through the anthraquinone process is an energy-intensive process that generates pollution (7). Given the need for mild, simple, and efficient production of both H₂ and H₂O₂, researchers have turned to new approaches to explore green production (8).

Here, we report a laser-induced conversion for directly splitting water to simultaneously produce H₂ and H₂O₂ without any catalysts. Remarkably, this laser-induced water splitting yields the results without the use of catalysts, producing 2.2 mmol h⁻¹ of H₂ and 65 µM h⁻¹ of H₂O₂ in a limited reaction space and a very short action time of 0.36 ms/h while operating at room temperature and atmospheric pressure. It is fascinating that the conversion efficiency of light-to-hydrogen (LTH) efficiency in water splitting can be as high as 2.1%. In the present work, we focused the laser below the surface of pure water using a pulsed laser with a small focused spot size of approximately 1 mm². Upon irradiation of pulsed laser, liquid water molecules can be violently decomposed into energetic particles, leading to nano- and microbubbles on the focus (9–11). It is notable that the temperatures inside the laser-induced microbubbles can reach up to 10⁴ K (12–14). This extreme state inside the bubbles creates a favorable microenvironment for water splitting. Thermodynamically, water molecules can decompose rapidly at such high temperatures, regardless of chemical barriers (15–17). Kinetically, the fast quenching of bubbles at a high rate of approximately 10¹⁰ K/s (18–20) can rapidly freeze the reactions and prevent reverse reactions. Quick cooling to room temperature also provides the stability required for decomposable H₂O₂ to exist. The synergistic effect of thermodynamics and kinetics produces a high water-splitting rate and results in a high yield of products (21, 22). Therefore, in the present study, we successfully developed a simple, clean, and efficient approach for generating H₂ and H₂O₂ from direct water splitting beyond catalytic chemistry under conventional conditions, which can be served as a demonstration of potentially solar-pumped catalyst-free hydrogen extraction and other chemical synthesis (SI Appendix, Fig. S1).

Results and Discussion

Illustration of the Laser-Induced Water Splitting.

The water-splitting process occurs in a custom-built closed cylindrical quartz reactor containing 250 mL of pure water. A digital picture in Fig. 1A displays laser-induced water decomposition to produce H₂ and H₂O₂. The beam of a Q-switched Nd:YAG laser with a wavelength of 532 nm is focused below the water surface. The generated gas is carried out from the system by Ar flow, as shown in Movie S1. Many small bubbles around focus can be captured by a digital camera equipped with a filter in Fig. 1B. Based on the observation, a schematic diagram of laser acting on water is displayed in Fig. 1C. These bubbles created by laser contain production gas and are carried out of the reaction system by Ar flow. Instead of collapsing in water, these macroscopic bubbles float upward to escape the water, as demonstrated in Movie S2. In the process of laser-induced water splitting, transient high temperature and rapid cooling are vital for the production of H₂ and H₂O₂.

Fig. 1.

Schematic diagram of water splitting by laser. (A) Digital image of the device for laser-induced water splitting to produce hydrogen and hydrogen peroxide. (B) A digital image of the laser focus when it is applied to water. Protective filters were installed in front of the camera during the laser process. (C) Diagram of microbubbles created by a pulsed laser near the focus when the laser acts on water. (D) Laser pulse. (E) Time flow of the preparation of hydrogen and hydrogen peroxide by laser water splitting corresponding to the laser pulse in (D). The production process of hydrogen and hydrogen peroxide by pulsed lasers is different from traditional continuous preparation techniques, such as thermal catalysis, photocatalysis, and electrocatalysis. Water can be split into hydrogen and hydrogen peroxide directly by a laser via one pulse by one pulse.

To more vividly understand the process of laser action on water, several laser pulse periods are depicted in Fig. 1D. A high-energy pulsed laser is capable of heating water molecules at the focus point to extremely high temperatures. For this work, a laser pulse duration of 10 ns and a frequency of 10 Hz were used, meaning that the laser acted on the water in real time for 0.36 ms within 1 h. Fig. 1E displays a diagram corresponding to the laser pulse that depicts water splitting. A laser applied to water can induce cavitation bubbles at the focal point, which contain active particles that expand and cool rapidly. With the expanding and cooling process, H₂ and H₂O₂ are formed and released into the surrounding water. The temperature inside the microbubble can reach 10⁴ K, creating a suitable microenvironment for water splitting. Because of the confining and cooling effects from the surrounding liquid water, the microbubble can be cooled with a fast-quenching process of 10¹⁰ K/s, which is vital for producing H₂ and H₂O₂. Fast-cooling is crucial to maintaining a high yield of H₂ and H₂O₂, as the reverse reaction can reform them back into water. Unlike traditional continuous reactions, this method is a discontinuous process for water splitting that is similar to the “batch-by-batch” method. Compared with the traditional catalytic H₂ and H₂O₂ production reaction, this laser chemistry is simple and clean and can be operated at room temperature and conventional pressure without any catalysts.

The Production Rates of H₂ and H₂O₂.

Laser-induced water splitting performance is characterized, as shown in Fig. 2. Fig. 2A displays H₂ and H₂O₂ production rates over the laser energy. H₂ concentration was quantified by gas chromatography (GC). The concentration of hydrogen peroxide was measured by spectrophotometer and obtained by a standard curve through chromogenic method (SI Appendix, Fig. S2). The production of H₂ and H₂O₂ increases as the pulsed laser energy increases (Fig. 2A and SI Appendix, Fig. S3). When the laser energy reached 710 mJ/pulse, the H₂ and H₂O₂ production rates reached 2.1 mmol/h and 65 µM/h, respectively. Higher laser energies may yield higher yields, but due to the power of the lasers used, higher energies than 710 mJ/pulse were not tested. It should be noted that in 1 h, the production rates of H₂ and H₂O₂ should be the apparent value, because the real-time laser acting on water is only 0.36 ms, which can be obtained by multiplying the laser pulse width by the laser pulse repetition frequency and then by 3,600 s/h. The extremely short duration of effective action implies that by increasing the laser pulse repetition frequency, the effectiveness of laser action on water can be significantly enhanced within an hour, thus increasing the product yield. Fig. 2B shows the variation in H₂ production with the laser pulse repetition frequency at 710 mJ/pulse. When the laser pulse repetition frequency is 1 Hz, the H₂ production rate is only 0.2 mmol/h. When the laser pulse repetition frequency increases to 10 Hz, the H₂ production increases to 2.2 mmol/h. There is a linear relationship between H₂ production and laser pulse repetition frequency. As the pulse width of the laser is 10 ns, it makes the real time for the laser acting on water far less than the operation time. With further development of laser science and techniques, the laser pulse repetition frequency can be greatly increased, thereby increasing the yield of the product. For the extrapolating to high laser pulse repetition frequency region, the highest limit frequency is 10¹⁰ Hz or 100 MHz because of 10 ns of laser pulse width. Thus, we also give the theoretical limit yield of hydrogen (2.2 × 10⁷ mmol/h) under the experimental conditions (710 mJ/pulse). Although such a high frequency is difficult to achieve at present in the case of high laser pulse energy, the predicted hydrogen production should be theoretically achievable if the laser can achieve such high repetition frequency.

Fig. 2.

(A) H₂ and H₂O₂ production rates with different laser energies. (B) H₂ evolution rate with different laser frequencies. (C) The conversion efficiency of laser LTH. (D) The yield rate of H₂ and H₂O₂ at different times under 600 mJ/pulsed laser energy. (E) Comparison of H₂ and H₂O₂ evolution rates between direct laser-induced water splitting and other catalysis.

Light energy utilization efficiency is one of the important evaluation indexes for water splitting. The “bench by bench” approach utilizing pulse laser further confirms the exceptional LTH efficiency. Fig. 2C illustrates the energy conversion efficiency of laser light into H₂ and H₂O₂ at varying pulse energies, utilizing the calculation method for photocatalytic hydrogen production. Notably, the highest observed LTH energy conversion rate reaches an impressive 2.1%. Additionally, the stability of the laser-induced water decomposition method has important influence on the practical application. Stability testing was carried, as depicted in Fig. 2D and SI Appendix, Fig. S4, demonstrating that the H₂ and H₂O₂ yields remained consistent at a laser energy of 600 mJ/pulse. Furthermore, water splitting was performed using a femtosecond laser with an 800 nm wavelength, as shown in SI Appendix, Fig. S5 and Movie S3. To further demonstrate the excellent efficiency of the laser-induced water splitting method, we conducted a comparative analysis of H₂ and H₂O₂ precipitation rates between laser method and photocatalytic/electrocatalytic approaches in one reaction cell, as presented in Fig. 2E. In particular, only pure water was used in our work, while the comparative reports all need catalysts and sacrificial agents. Notably, within the confined space of laser action on water, the laser method exhibits a remarkable yield of H₂ (23–31). The production rate of H₂O₂ is also comparable with the catalytic method (32–40).

Mechanisms for Laser-Induced Water Splitting.

To investigate the physical and chemical mechanisms of laser-induced water splitting to produce H₂ and H₂O₂, we utilized real-time time-dependent density functional theory (rt-TDDFT) methods to simulate the photoinduced dynamics of liquid water (41). During the laser-induced water-splitting process, the high-energy laser ionizes water molecules, to a significant number of active particles that ultimately react to yield the final products. As shown in Fig. 3A, the intense laser with wavelengths of 532 and 800 nm and field intensity of E₀ = 2.4 V/Å excites a large number of valence electrons to the conduction band and heat liquid water (Fig. 3B), followed by electron–electron and electron–ion recombination interactions, which is manifested as a decrease in the fraction of excitations and a continual rise in ionic temperature. The temperature evolution under different laser intensities with a laser wavelength of 800 nm is shown in SI Appendix, Fig. S6. A higher laser intensity will result in a higher valence electron excitation ratio (SI Appendix, Fig. S7). At 10 fs, the water molecules have not yet been fully excited and still show a transient density similar to that of the intrinsic water molecules (SI Appendix, Fig. S8). At around 100 fs, the ratio of excited valence electrons is 12% for 800 nm and 8% for 532 nm. The transient density of states and its electron occupancy reveal zero-band phenomenon because of the production of free radicals, as shown in Fig. 3C.

Fig. 3.

(A) Temperature evolution of particles excited by laser at different wavelengths (λ = 800 and 532 nm). (B) The change in the excitation ratio of valence electrons at different wavelengths (λ = 800 and 532 nm). (C) The occupation number and density of states of the system at 100 fs with 800 nm laser excitation. (D) The number changes of different particles during the annealing process. The annealing rate was 1 K/fs. (E) The final retention number of various particles in the final system with different annealing rates. A faster annealing rate is more conducive to product retention. (F) One of the possible final images of the molecular system at different annealing rates. (G) Gibbs free energy change for various reactions at different temperatures. Thermodynamically, it is much easier to generate water in the cooling process. Therefore, only a fast enough cooling rate can retain more hydrogen and peroxide products. (H) The change in free energy for bubble nucleation under different pressures induced by laser shock waves. (I) Normalized concentration of critical bubbles as a function of shock wave pressure. (J) Bubble nucleation rate as a function of shock wave pressure.

After 500 fs photoexcitation, the ultimate structure is selected for annealing simulation. For the fast-annealing rate of 1 K/fs, the temporal evolutions of the system temperature and energy are shown in SI Appendix, Fig. S9. The annealing process can be divided into three stages (Fig. 3D). In the first stage, hydrogen ions are rapidly converted into water molecules. Then, the conversion rate of hydrogen ions to water obviously slows down in the second stage, and the number of hydrogens increases continuously. Finally, the system reaches the ultimate stable state, which contains H₂, O₂, and H₂O₂, and the ratio of H₂ is up to 12%. According to the water decomposition reaction energy, the light energy conversion efficiency to chemical energy was up to 8%. The formation of H₂ and H₂O₂ can be observed in the annealing process (SI Appendix, Fig. S10).

Transient high temperature and fast cooling are the core of this technology. The pulsed laser shows a superfast nonequilibrium process, and the cooling rate reflects the degree of nonequilibrium of the system. Therefore, we simulate the nonequilibrium degree of this process by changing the annealing rate. The effect of the annealing rate on the final product distribution is obtained by counting the retention number of various particles under different annealing rates (SI Appendix, Table S1). As shown in Fig. 3E, when the annealing rate is 2 K/fs, the average H₂ retention is 12%. When the annealing rate is reduced to 0.2 K/fs, the average H₂ retention drops significantly to 4%. The results show that the pulsed laser’s enhanced nonequilibrium degree of water splitting could significantly enhance the final H₂ and H₂O₂ production efficiency. The possible molecular structure distribution of one of the systems is shown in Fig. 3F when finally stabilized at different annealing rates.

To further elucidate the process of water splitting induced by the pulsed laser, we analyzed the whole process in thermodynamics. Fig. 3G shows the Gibbs free energy of different reactions as a function of temperature. H*, OH*, OOH*, O*, and other active species can only exist stably at high temperatures. During the cooling process, the Gibbs free energy of the decomposition reaction of H₂ and H₂O₂ reaches zero later than that of water, implying the active species preferentially produce water over H₂ and H₂O₂ in the cooling process. Therefore, only if the system temperature falls fast enough can H₂ and H₂O₂ be preserved in the reaction system.

The high-energy laser first ionizes water molecules during the laser-induced water splitting process, which generates a significant number of active particles in the local region of the focal point by the interaction between strong fields and water molecules. The active particles with high concentrations then reach the supersaturation state in a transient manner and undergo explosive nucleation through the LaMer mechanism, which finally results in the formation of massive nano- and microbubbles in the region of the focal point. The microbubbles containing active particles inside are expanded and cooled to generate the final products.

The nucleation of the active particles to form bubbles is modeled (the detailed model, labeled The Nucleation of Laser-Induced Bubbles by the LaMer Mechanism, is provided in SI Appendix). First, we explored the change in free energy during microbubble nucleation at ambient temperature, as depicted in Fig. 3H. When a bubble with radius R nucleates in the liquid, the change in free energy is

$$\mathrm{\Delta }G=4\pi R^2{\sigma }_{\mathit{LG}}-\frac{4}{3V_m}\pi R^3\mathrm{\Delta }\mu ,$$ [1]

where $R$ and ${\sigma }_{\mathit{LG}}$ are the radius of the bubble and air–liquid surface tension, respectively, and $V_m$ and $\mathrm{\Delta }\mu$ denote the volume of 1 mol gas and chemical potential, respectively. It is obvious from Fig. 3H that the change in free energy first gradually increases and then decreases as the bubble radius increases at any shock wave pressure induced by a pulsed laser; this is mainly due to the competition of free energy of the bulk phase (4πR³p_bubble/3) and interfacial part (4πR²σ_LG). Additionally, the maximum value is 5.42 × 10⁻¹⁰ J, which appears at 23 nm for p_laser = 2 MPa, indicating that the critical size of the bubble and the nucleation work are 23 nm and 5.42 × 10⁻¹⁰ J, respectively. Bubbles with radii larger than or equal to the critical size (R ≥ R_c) will continue to nucleate and grow, while bubbles with R < R_c will liquefy and disappear. Furthermore, the critical bubble size decreases as the pressure increases. Therefore, the bubble size will be smaller at high pressure than at low pressure. Fig. 3I shows the normalized concentration of the critical bubble as a function of shock wave pressure induced by a pulsed laser. When the critical bubble formation condition is satisfied, the critical nuclear concentration is an important parameter that can be expressed as

$$n_c=n_0\mathrm{e}\mathrm{x}\mathrm{p}\left[\frac{-\mathrm{\Delta }G\left(R_c\right)}{k_{B}T}\right],$$ [2]

where n₀ represents the density of nucleating states in the liquid and $k_B$ is the Boltzmann constant. It is obvious that the normalized concentration ratio (n_c/n₀) presents a monotonic increase with increasing pressure, mainly due to the reduction in nucleation work (ΔG(R_c)), which greatly increases the number of bubbles. Unsurprisingly, under the combined effect of pressure and nucleation work, the nucleation rate (J ) of bubbles shows a similar trend as the critical bubble concentration (Fig. 3J). The nucleation rate of bubbles can be obtained as

$$J={\rho }_L\sqrt{\frac{2{\sigma }_{\mathit{LG}}}{\pi m}}\mathrm{e}\mathrm{x}\mathrm{p}\left[\frac{-\mathrm{\Delta }G\left(R_c\right)}{k_{B}T}\right].$$ [3]

These results indicate that the higher the pressure, the greater the number of stable bubbles obtained. Therefore, to obtain more bubbles, the pressure of the shock wave should be as strong as possible, indicating that high pulsed energy is necessary for the laser.

Based on the above experimental and theoretical analysis, the physical and chemical processes of laser-induced water splitting are systematically described in Fig. 4A. The pulsed laser is focused under the water’s surface in stage I. When the pulsed laser is applied to water, the water is instantly excited, ionized, and heated to extremely high temperatures, producing a high density of energetic particles at the laser’s focus. In stage II, these energetic ions nucleate through the LaMer mechanism to form many nano- and microbubbles. The temperature inside these microbubbles can reach up to 10⁴ K. At such high temperatures, water can be easily decomposed without any catalyst, producing large quantities of hydrogen and oxygen atoms, ions, free radicals, and other active substances. These active particles can exist stably under such high temperatures. In stage III, these microbubbles, rich in high-energy particles, continue to expand and cool. During expansion and cooling, these active particles react to form the final product. The time of laser action on water is only 10 ns. Thus, when one laser pulse ends, the temperature inside the microbubble can be reduced to room temperature at a superfast cooling rate of 10¹⁰ K/s due to the confinement and cooling effect of the surrounding liquid water. In stage IV, H₂ and H₂O₂ are released into the surrounding environment when these microbubbles collapse.

Fig. 4.

(A) Physical and chemical processes of laser-induced water splitting. (B) The proposed concept for the scale-up of this laser chemistry technique through a laser beam splitter. (C) Device image of H₂ and H₂O₂ production by laser-induced conversion under laser beam splitting conditions. The total energy is 655 mJ/pulse. The split beam energies are 193, 196, and 261 mJ/pulse. (D) The yield of H₂ and H₂O₂ under laser beam splitting conditions.

Transient high temperature and fast cooling are two essential characteristics of water splitting by a pulsed laser to produce H₂ and H₂O₂. The exceptionally high temperature inside the laser-induced microbubble provides a suitable microenvironment for water splitting. Thermodynamically, such a high temperature enables water to decompose rapidly without any catalyst, and the chemical barrier becomes insignificant. Kinetically, such a high temperature allows chemical reactions to occur at extremely high rates. In addition, due to the super-rapid quenching and cooling, the water-splitting reaction can be frozen in time without considering the occurrence of a reverse reaction during the slow cooling process by the traditional continuous process. Therefore, a large amount of H₂ and H₂O₂ can be produced from water splitting by a laser in a limited space and very short real action time. The synergy of thermodynamics and kinetics leads to a high yield and high efficiency of H₂ and H₂O₂ evolution from water splitting driven by the pulsed laser.

Envisioning Scale-Up.

This technique is simple, clean, and does not require complex catalysts. Another important advantage of the technique is that it can be operated under normal conditions, such as room temperature and atmospheric pressure. The performance of the pulsed laser is crucial to this technique. The increased laser energy and pulse repetition frequency can effectively improve the efficiency of water splitting to produce H₂ and H₂O₂. With the progress of laser equipment, the energy of a single laser beam is high enough to exceed the threshold needed for water splitting. Thus, a single laser beam can be divided into multiple beams to obtain a higher product yield, as shown in Fig. 4B and Movie S4. Fig. 4C is a demonstration of the experimental equipment. A laser beam is divided into three beams to achieve a stable H₂ and H₂O₂ output. Fig. 4D shows the H₂ and H₂O₂ production rates as a function of time. The cumulative production of H₂ and H₂O₂ increases steadily over time (SI Appendix, Fig. S11).

The hydrogen yield and energy conversion efficiency of laser-induced water splitting is dependent on the laser used, thus relying on advancements in laser science and technology. Higher laser pulse energy and laser pulse repetition frequency can effectively enhance the efficiency of laser-induced water splitting. It is believed that as laser science and technology development, the performance and cost of lasers can be rapidly reduced. In fact, the price of lasers has significantly decreased over the past few decades.

Indeed, the laser utilized in this study is electrically powered. However, electricity is not the sole means of driving a laser. Numerous previous studies have demonstrated that solar light can be efficiently converted into laser, with an evaluated optical–optical conversion efficiency reaching 63% (42–44). This presents a unique and viable approach for utilizing solar-powered lasers to drive water splitting. It is reasonable to anticipate the realization of water splitting by solar-light-powered lasers as a practical solution. Therefore, this research establishes an innovative method employing solar light for water splitting, which holds significant implications for efficient solar light conversion and storage through laser-induced water splitting method.

Conclusion

In summary, we have presented the laser production of H₂ and H₂O₂ via direct water splitting without any catalysts. This laser chemistry is straightforward, environmentally friendly, and high-yielding and can be performed under normal temperature and pressure. A high H₂ yield of 2.2 mmol/h and a H₂O₂ yield of 65 µM/h were achieved. Additionally, a remarkable conversion efficiency of 2.1% of light to chemical energy was obtained in a limited reactive space and a very short real action time (0.36 ms/h) upon laser-acting water. In thermodynamics, the high temperature in the laser-induced bubble enables water splitting to proceed efficiently without any catalysts. In kinetics, the ultrashort quenching and cooling time of the laser-induced bubble inhibits the reverse reaction of H₂ and H₂O₂ produced in the initial stage. This synergy of thermodynamics and kinetics leads to the high evolution efficiency of H₂ and H₂O₂ from direct water splitting driven by the pulsed laser. Therefore, in this work, we propose a simple, clean, high-yielding, and easily operable laser chemistry to produce H₂ and H₂O₂ from water under mild conditions beyond traditional chemical catalysis.

Method

Synthesis and Characterization.

A 532 nm Nd:YAG laser producing a maximum energy of 710 mJ with 10 ns pulses was focused to a point approximately 10 mm in diameter in pure water by a lens with a focal length of 500 mm. The pulse laser was adjusted to different energies (200, 300, 400, 500, 600, and 710 mJ/pulse) to create bubbles, which are regarded as microreactors. The pulse energy was monitored by a light intensity meter. Pure water (250 mL) was filled into a tight round container for the pulsed laser process. The focus of the pulsed laser beam was fixed underwater at a distance of 10 mm from the water’s surface. During pulse laser operation, high-purity argon was fed into water continuously with a flow of 2 L/min to carry out hydrogen. The out-gas was collected and analyzed by GC (GC-2014C, Shimadzu) with a TCD (5 Å molecular sieve column) for H₂. The concentration of hydrogen peroxide was measured by spectrophotometer through the hydrogen peroxide test box (Beyotime, light absorbance method determined by spectrophotometer based on ferrous ions and xylenol orange).

Calculation and Simulation.

Laser excitation and annealing simulation.

The nonadiabatic ab initio molecular dynamics simulations were performed via the rt-TDDFT algorithm (45) as implemented in the plane-wave code Quantum Espresso (46). We used the projector augmented-waves method (47) and the Perdew–Burke–Ernzerhof exchange–correlation functional (48) in both Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TDDFT) calculations. Pseudopotentials were generated using pslibrary, and the van der Waals density functional using the optB88-vdW method (49) was chosen to account for the hydrogen-bond networks of liquid water.

According to the change of the Gibbs free energy of the substance, the energy storage efficiency of the laser is calculated as follows:

$${\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+{1/2\mathrm{O}}_2;\mathrm{\Delta }G=237\text{kJ}/\text{mol},$$

$${2\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{H}}_2;\mathrm{\Delta }G=472.2\text{kJ}/\text{mol}.$$

The formula for calculating the energy conversion of laser LTH and hydrogen oxide is as follows:

$$\begin{array}{c}\eta \left(\%\right)=\frac{\mathrm{the} \mathrm{change} \mathrm{of} \mathrm{the} \mathrm{Gibbs} \mathrm{free} \mathrm{energy} \mathrm{of} \mathrm{chemicals}}{\mathrm{laser} \mathrm{energy} (\mathrm{mJ}/\mathrm{pulse})\times 10 \mathrm{Hz}\times 3,600 \mathrm{s}/\mathrm{h}}\times 100\%.\hfill \end{array}$$

Nucleation of laser-induced bubbles by the LaMer mechanism.

For bubble nucleation, the free energy difference between the gas–liquid phases before and after nucleation can be understood as volume work, that is, the work done by the bubble to resist the external resistance and displace the liquid per unit volume. For the detailed calculation steps, see SI Appendix.

Beam splitting.

With the development of lasers, an increasing number of high-energy lasers have been developed. When a single laser beam has more pulse energy than needed, it can be split to increase the product yield. When n (product yield under a single beam laser) < n (product yield under multiple beam lasers) × the number of beam splits, beam splitting can be performed. A simple criterion formula for beam splitting is as follows:

$$N\times n_{E_N}<\left(N+1\right)\times n_{E_{N+1}};E_{\mathit{total}}=N\times E_N,$$

where N is the number of beam splits, $n_{E_N}$ is the product yield rate under $E_N$ beam energy, and $E_N$ is the single beam energy with N beam splits.

In addition, $E_{\mathit{total}}$ is the total beam energy.

Supplementary Material

Appendix 01 (PDF)

Movie S1.

The water splitting process occurs in a custom-built closed cylindrical quartz reactor containing 250 mL of pure water.

Movie S2.

The observation of bubbles containing gaseous products generated around the focus point during laser-induced water splitting process.

Movie S3.

The process of fs-laser-induced water splitting. It can be observed from the video that when the femtosecond laser was applied on water, a large number of bubbles containing products are generated and escape the system. Femtosecond laser parameters. Pulse width: 35 femtoseconds; Frequency: 1000 Hz; pulse energy: about 3.7 mJ.

Movie S4.

A preliminary amplification experiment to demonstrate the potential industrial applications of this laser chemistry.

Data, Materials, and Software Availability

All study data are included in the article and/or supporting information.