Flexible polypyrrole activated micro-porous paper-based photoanode for photoelectrochemical water splitting

We herein demonstrate polypyrrole decorated micro-porous laboratory filter paper (PFP) as photoanode (PA) for efficient and stable water splitting. The straddling band position with water redox and the measured band gap of ~1.98 eV, make these PFP-PAs effective for water splitting reactions. The results manifest excellent photo-anodic PEC activity of these PFP-PAs, yielding a photocurrent density of ~9.5 mA/cm² (at 1.23 V vs. RHE) in a three-electrode configuration. The incident photon-to-current efficiency (IPCE) and applied bias photon-to-current efficiency (ABPE) was measured to be 43.19% and ~1%, respectively. Moreover, the robustness of these flexible PFP-PAs was visualized by the provided stability for more than ~160 min in alkaline conditions. The current study provides a proof-of-concept for the realization of a cost-effective, flexible, and efficient paper-based artificial catalyst (like a natural leaf) for solar-driven water splitting.     

Simultaneously efficient light absorption and charge transport of CdS/TiO2 nanotube array toward improved photoelectrochemical performance

CdS has been widely used to modify TiO₂-based photoanodes for photoelectrochemical (PEC) water splitting. Due to the poor interface contact between chalcogenides and oxides, however, such CdS modified TiO₂ materials usually exhibit inefficient separation and transport of charges, leading to an unsatisfactory efficiency during the PEC water splitting process. Addressing this issue, we herein report a CdS/TiO₂ nanotube array (CdS/TNA) photoanode that was fabricated through a successive ion layer absorption and reaction (SILAR) method with an additional subsequent annealing. This post-annealing process is essential to enhance the interface contact between the CdS and the TNAs, resulting in an accelerated transfer of photogenerated electrons from the CdS to the TNAs. In addition, the post-annealing also improves the light absorption capability of the CdS/TNA photoanode. The simultaneous enhancement of charge transport and light absorption provided by the post-annealing is essential for improving the PEC performance of the CdS/TNA photoanode. The CdS/TNA photoanode obtained by this strategy exhibits a much enhanced PEC performance in water splitting, and its photocurrent density and solar-to-hydrogen conversion efficiency could reach 4.56 mA cm⁻² at 1.23 V vs. reversible hydrogen electrode and 5.61%, respectively. This simple but effective route can provide a general strategy for obtaining high-performance oxide-based photoelectrodes.     

Nitrogen-doped carbon dot anchored 1-D WO3 for enhanced solar water splitting: A nano surface imaging evidence of charge separation and accumulation

Combining WO₃ with suitable materials to form heterojunction is essential to overcome the limitations of WO₃ to enhance its photoelectrochemical (PEC) water splitting activity. Moreover, a clear understanding of photo-response and charge behavior of materials could lead to the rational design of efficient photoelectrodes. Given this, an efficient strategy is applied to fabricate WO₃ heterojunction with nitrogen-doped carbon dots (NCDs) and in-depth characterization to investigate the surface charge dynamics using nano imaging in a relation to the enhanced PEC water splitting activity. The optimized NCDs loading to the WO₃ NRs exhibited the enhanced photocurrent density of 1.54 mA cm⁻² at 1.23V vs RHE under AM 1.5 G illumination, highest IPCE of ~82 % (at 308.32 nm). The Kelvin probe force microscopy and electrostatic force microscopy reveal that after loading NCDs to the WO₃, a relatively smooth charge transport has been observed, which improves the PEC. Furthermore, this work demonstrates the effect of photogenerated charges caused by the NCDs that assist in enhancing the increased photocurrent, hydrogen production efficiency, and stability of the PEC water splitting system. Significantly, the nano imaging characterization utilized in this work could be extended to various photoanodes to study the surface charge dynamics.     

Photoactivity of MWCNTs modified α-Fe2O3 photoelectrode towards efficient solar water splitting

MWCNTs (Multiwalled Carbon Nanotubes) modified α-Fe₂O₃ (hematite) photoelectrodes have been investigated for their possible application in hydrogen generation via photoelectrochemical (PEC) splitting of water. Enhanced photoresponse seen in comparison to the pristine α-Fe₂O₃ films is credited to the effective charge facilitation and charge separation provided by MWCNT conducting support. 0.2 wt% MWCNTs modified α-Fe₂O₃ thin film exhibited the maximum photocurrent density of 2.8 mA/cm² at 0.75 V/SCE. Measured values of flat band potential, donor density, resistance, Applied bias photon-to-current efficiency (ABPE) and Incident-photon-to current-conversion efficiency (IPCE) support the observed enhancement in photocurrent.     

An efficient tandem photoelectrochemical cell composed of FeOOH/TiO2/BiVO4 and Cu2O for self-driven solar water splitting

An integrated solar water splitting tandem cell without external bias was designed using a FeOOH modified TiO₂/BiVO₄ photoanode as a photoanode and p-Cu₂O as a photocathode in this study. An apparent photocurrent (0.37 mA/cm² at operating voltage of +0.36 V_RHE) for the tandem cell without applied bias was measured, which is corresponding to a photoconversion efficiency of 0.46%. Besides, the photocurrent of FeOOH modified TiO₂/BiVO₄–Cu₂O is much higher than the operating point given by pure BiVO₄ and Cu₂O photocathode (∼0.07 mA/cm² at +0.42 V_RHE). Then we established a FeOOH modified TiO₂/BiVO₄–Cu₂O two-electrode system and measured the current density-voltage curves under AM 1.5G illumination. The unassisted photocurrent density is 0.12 mA/cm⁻² and the corresponding amounts of hydrogen and oxygen evolved by the tandem PEC cell without bias are 2.36 μmol/cm² and 1.09 μmol/cm² after testing for 2.5 h. The photoelectrochemical (PEC) properties of the FeOOH modified TiO₂/BiVO₄ photoanode were further studied to demonstrate the electrons transport process of solar water splitting. This aspect provides a fundamental challenge to establish an unbiased and stabilized photoelectrochemical (PEC) solar water splitting tandem cell with higher solar-to-hydrogen efficiency.     

Photoelectrochemical water splitting at nanostructured α-Fe2O3 electrodes

In this study, nanostructured α-Fe₂O₃ thin films were deposited by simple electrodeposition for photoelectrochemical water splitting. Post-annealing temperature was found to have drastic effect on photoactivity of these films. SEM analysis illustrated that size of nanoparticles increases with annealing temperature. The current–potential characteristics showed that the water-splitting photocurrent strongly depends on post-annealing temperature. A maximum photocurrent density of 0.67 mA/cm² was observed at 1.23 V versus reversible hydrogen electrode (RHE) under standard illumination conditions (AM 1.5 G 100 mW/cm²), and the water-splitting current was over 1.0 mA/cm² before the dark current flow starts (at 1.55 V versus RHE). The electrode shows an onset potential as low as 0.8 V (versus RHE) for water photooxidation, which is one of the best results reported for hematite photoanodes. This high photoactivity of electrodes is attributed to the preferential growth of hematite nanostructures along the most conductive plane (001) and incorporation of Sn in film from the substrate at high annealing temperature. The best-performing electrode shows an incident photon conversion efficiency (IPCE) of 12% at 400 nm (in 1 M NaOH at 1.23 V versus RHE), which indicate the improved light-harvesting properties of these nanostructures.     

Photoelectrochemical performance of bismuth vanadate photoanode for water splitting under concentrated light irradiation

Photoelectrochemical (PEC) water splitting is a promising way to convert solar energy into hydrogen energy. It is typically carried out at room temperature (RT) and 1 sun illumination. The PEC water splitting under concentrated light is expected to be an effective route to improve PEC performance, but there are few studies on it. Herein, CoPi/Mo:BiVO₄ photoanode was selected to investigate the effect of concentrated light and the reaction temperature on its PEC performance. It was revealed that CoPi/Mo:BiVO₄ showed enhanced PEC performance under concentrated light. The photocurrent density was enhanced with increased light intensity and increased reaction temperature. At a high temperature (60 °C), the normalized photocurrent density (3.31 mA cm⁻² at 1.23 V vs. RHE) was found to be optimal at 4 suns, which was attributed to the synergistic effect of concentrating light and heating. It is proved that concentrated light can effectively improve the PEC performance, which has important guiding significance to realize the low-cost and efficient PEC water splitting.     

Three-dimensional ZnO@TiO2 core-shell nanostructures decorated with plasmonic Au nanoparticles for promoting photoelectrochemical water splitting

A novel three-dimensional (3D) core-shell nanostructure decorated with plasmonic Au nanoparticles (NPs) was prepared for photoelectrochemical water splitting. In the new nanostructure, ZnO nanorods (NRs) are perpendicular to ZnO nanosheets (NSs), and the ZnO NSs-NRs are coated with a thin TiO₂ shell formed by liquid phase deposition. The plasmonic Au NPs were formed in situ on the surface of ZnO NSs-NRs@TiO₂ by thermal reduction. A thin TiO₂ shell and uniformly distributed Au NPs were successfully obtained. The photoconversion efficiency and photocurrent density of the 3D ZnO NSs-NRs@TiO₂-Au nanostructure respectively reached 0.48% and 1.73 mA cm⁻² at 1.23 V vs. reversible hydrogen electrode, 4.80 and 4.33 times higher than those of ZnO NSs, respectively. The thin TiO₂ shell effectively promoted charge separation, while the surface plasmon resonance effects of the Au NPs improved the photocurrent density. The findings suggest that the 3D ZnO NSs-NRs@TiO₂-Au nanostructure is a promising photoanode for photoelectrochemical water splitting.     

Copper oxide nanoparticle made by flame spray pyrolysis for photoelectrochemical water splitting - Part II. Photoelectrochemical study

A scalable method for hydrogen generation by splitting water via a photoelectrochemical cell was studied. Flame spray pyrolysis and spin coating processing methods were used for preparing copper oxide nanoparticles and copper oxide photocathodes. Copper oxide p-type semiconductor nanoparticles made by flame spray pyrolysis were spin coated on conducting ITO substrates and served as photocathodes for photoelectrochemical splitting of water. The film thickness was controlled by the concentration of the CuO suspension solution and numbers of layer deposited on the substrate. As sintering temperature increased to 600 °C, crystalline diameter increased from 28 nm (before sintering) to 110 nm and the bandgaps decreased from 1.68 eV to 1.44 eV. A 387 nm thickness CuO film with bandgap 1.44 eV was demonstrated to have 1.48% total conversion efficiency and 0.91% photon-to-hydrogen generation efficiency. The net photocurrent density (photocurrent - dark current) was measured to be 1.20 mA/cm² at applied voltage of −0.55 V vs. Ag/AgCl in 1 M KOH electrolyte with 1 sun (AM1.5G) illumination. Based on the Mott-Schottky plot, the carrier density was estimated to be 1.5 × 10²¹ cm⁻³ and the flatband potential to be 0.23 V vs. Ag/AgCl. Furthermore, the valence band edge and conduction band levels were found to lie at −5.00 eV and −3.56 eV respect to the vacuum respectively.     

Self-organized TiO2 nanotube arrays with uniform platinum nanoparticles for highly efficient water splitting

Highly efficient water splitting electrode based on uniform platinum (Pt) nanoparticles on self-organized TiO₂ nanotube arrays (TNTAs) was prepared by a combination of multi-step electrochemical anodization with facile photoreduction process. Uniform platinum (Pt) nanoparticles with an average diameter of 8 nm are distributed homogeneously on nanoporous top layer and underneath TiO₂ nanotube wall. In comparison to pristine TNTAs, Pt@TNTAs show substantially enhanced photocurrent density and the incident photon-to-current conversion efficiency (IPCE) in the entire wavelength window. The maximum photocurrent density and IPCE from the optimized Pt@TNTAs photoelectrode (Pt, ～1.57 wt%) were about 24.2 mA cm⁻² and 87.9% at 350 nm, which is much higher than that of the pure nanotubes sample (16.3 mA cm⁻² and 67.3%). The resultant Pt@TNTAs architecture exhibited significantly enhanced photoelectrochemical activities for solar water splitting with hydrogen evolution rate up to 495 μmol h⁻¹ cm⁻² in 2 M Na₂CO₃ + 0.5 M ethylene glycol under the optimal external bias of −0.3 V_SCE.     

Photocorrosion-less stable heterojunction photoanode for efficient visible-light driven solar hydrogen generation

In this work, a heterostructure CdS/TiO₂ nanotubes (TNT) photoelectrode is decorated with Ni nanoparticles (NPs) to enhance hydrogen generation via the photoelectrochemical method. Herein, we report a systematic study of the effect of Ni NPs heterostructure photoelectrode to improve light absorption and photoelectrochemical (PEC) performance. The fabricated photoelectrodes were evaluated for photoelectrochemical hydrogen generation under simulated sunlight. The optimized Ni/CdS/TNT photoelectrode exhibited an improved photocurrent density of 6.5 mA cm^?2 in poly-sulfide aqueous media at a low potential of 0 V. Owing to the enhanced photocurrent density, Ni NPs also played a significant role in improving the stability of the photoelectrode. The synergistic effect with semiconductor ternary junction incites the surface plasmon resonance (SPR) for light-harvesting to enhance photoelectrochemical hydrogen generation.     

An in-situ cation exchange approach to stabilize Zn-MOF: Understanding the role of nickel ions for photoelectrochemical performance

The exposed active sites of ZIF-8 is considered as the key to boost the kinetics of photoelectrochemical (PEC) water splitting. However, the terrible stability under irradiation and aqueous conditions significantly limit the practical application for fabricating the efficient photoeletrode. Herein, a novel strategy is employed for solving the stability of Zn-MOF (ZIF-8) by partially replacing Zn²⁺ ions by Ni²⁺ ions. The TiO₂ photoelectrode is selected as a testing model to study the superiority of TiO₂–ZnNi MOF for PEC water splitting. TiO₂–ZnNi MOF exhibits a strong photocurrent density of 1.00 mA/cm² (1.23 V vs. RHE), which is 2.7 and 1.7 times that of TiO₂ and TiO₂–Zn MOF, respectively. More importantly, the photocurrent density of TiO₂–ZnNi MOF maintains almost 100% for 2 h, while TiO₂–Zn MOF significantly decays to 81.6% for just 8.3 min, indicating the successful achievement for promoting the stability of Zn-MOF. Therefore, this work proposes a reasonable route to simultaneously enhance the PEC efficiency and stability of TiO₂–Zn MOF for the first time, which provides a new guidance for the design of highly stable photoelectrode.     

Tailoring morphological characteristics of zinc oxide using a one-step hydrothermal method for photoelectrochemical water splitting application

In the present paper describe the zinc oxide (ZnO) with various morphologies have been synthesized using the one-step hydrothermal method, in which the growth of ZnO nanostructures are significantly tailored by adjusting the pH level between 9 and 12 using 0.1 M Sodium hydroxide (NaOH). Significant results reveal the morphological properties of ZnO nanostructures varied with different pH values with the formation of ZnO nanostructures have different morphological such as a baton, star, flower, and rod-like structures. The present results show the rod-like structure of ZnO nanostructures exhibits the highest photocurrent density of 746.61 μAcm⁻² (at 1.23 V vs RHE) under simulated solar AM 1.5G illumination in Potassium hydroxide (KOH) medium, also the other morphologies. The dependent of the photoelectrochemical (PEC) water splitting properties on the different morphological of ZnO nanostructures are studied. Achieving the morphological evolution mechanism has become one of the method to obtain the production of the hydrogen growth regime used for solar energy conversion and their applied storage potentials. The application of the ZnO nanostructures for PEC water splitting was proposed with the adoption of screen-printed carbon electrodes (SPCEs). These are to quantify the best degree of the highest photocurrent density with one-step tailoring with an ideal modeling system to enhance PEC water splitting performances. Thus, the screen-printed carbon electrodes (SPEs) has been used as an alternative method for fabrication and photoelectrodes testings.     

Probing plasmonic Ag nanoparticles on TiO2 nanotube arrays electrode for efficient solar water splitting

Decoration of semiconductors with plasmonic nanoparticles provides a new direction for efficient solar water splitting for hydrogen production. Herein, Ag nanoparticles as the plasmonic metal were electrodeposited on a TiO₂ nanotube arrays (TNTA) photoelectrode by controlling deposition-charge density. The resulting Ag/TNTA electrode with 10–50 nm diameter Ag nanoparticles exhibited a higher photocurrent density and hydrogen production rate than a bare TNTA electrode under AM 1.5 irradiation. The origins of this enhancement were explored by analyzing the photoelectrochemical behaviors with the relevant optical properties. The absorption of these Ag/TNTA electrodes in the visible light region increased owing to the surface plasmon resonance (SPR) effect of the Ag nanoparticles, and only low photocurrent densities under visible light irradiation were observed. The overall enhancement is owing to the greater incident photon-to-electron conversion efficiency in the 300–400 nm range that is more dependent on the interfacial charge transfer from TNTA to Ag nanoparticles. The localized electronic field of the SPR also reduces the electron transport time in the Ag/TNTA electrodes.     

CIS/CdS/ZnO/ZnO:Al modified photocathode for enhanced photoelectrochemical behavior under visible irradiation: Effects of pH and concentration of electrolyte solution

In this paper, the CuInS₂ films were firstly modified with CdS and CdS/ZnO/ZnO:Al/Au layers in order to improve the photoelectrochemical (PEC) water splitting efficiency. The CuInS₂ photoelectrode was synthesized by electrodeposition method as a facial and green method, on the FTO substrate. The effects of pH and concentration of Na₂S electrolyte solution on the photocurrent density of photoelectrode samples were studied. As a p-n junction photocathode, the CIS/CdS/ZnO/ZnO:Al/Au photoelectrode indicates the enhanced PEC activity. The photocurrent density of CIS/CdS/ZnO/ZnO:Al/Au photoelectrode reaches to 1.91 mA/cm², while is about 2.5 times higher than that for CuInS₂ film at pH = 8 (−0.6 V vs Ag/AgCl). The formation of a p-n junction at the CuInS₂ photoelectrode surface not only reduces the recombination of electron-hole pairs but also increases the PEC response and water splitting performance of the as-prepared CIS/CdS/ZnO/ZnO:Al/Au photoelectrode.     

A heterostructured catalyst composed of poly-2,6-diaminopyridine and TiO2 microspheres used as a photoanode for efficient water splitting

Photoelectrocatalytic (PEC) water splitting provides an alternative to direct solar-to-fuel production. In this study, a novel heterostructure formed between a conjugated polymer [poly-2,6-diaminopyridine (PDAP)] and three-dimensional TiO₂ microspheres was grown in situ on a Ti substrate (PDAP-3DTiO₂MSs/Ti) and used as photoanode for water oxidation in alkaline media under AM 1.5G illumination. The PDAP-3DTiO₂MSs/Ti can produce applied bias photon-to-current efficiency of 0.85% at 0.44 V vs. Pt and a photocurrent density of 1.56 mA cm⁻² at 1.23 V vs. RHE. Moreover, PDAP-3DTiO₂MSs/Ti displays impressive photoelectrochemical stability with 93% of its initial photocurrent being retained after 4 h of reaction. Based on physical-chemical characterization and photo-/electro-chemical measurements, the superior PEC water splitting performance of PDAP-3DTiO₂MSs/Ti should benefit from the coexistence of Ti³⁺ and Ti⁴⁺ in 3DTiO₂MSs, the light harvest capability of PDAP and the type II heterojunction formed between 3DTiO₂MSs and PDAP, which result in the enhanced generation and separation of photocarriers.     

Enhanced solar-driven water splitting by ZnO/CdTe heterostructure thin films-based photocatalysts

Solar-driven hydrogen production by water splitting using a photocatalyst is considered the most effective approach to produce hydrogen. Hydrogen is the most suitable renewable energy source. The efficiency of hydrogen production is still low. The efficiency of hydrogen production through photocatalysis can be enhanced by preparing a suitable and efficient photocatalyst. In this work, ZnO thin films were deposited on CdTe thin films at 600 °C, 650 °C, and 700 °C temperatures to form ZnO/CdTe heterostructure thin films by chemical vapor deposition (CVD) as photoelectrodes for water splitting. The photoelectrochemical properties showed that ZnO/CdTe heterostructure thin films have better photocurrent response compared to pristine ZnO and CdTe thin films. EIS results showed that the charge transfer at the electrode-electrolyte interface for ZnO/CdTe heterostructure thin films is much better than that of the pristine ZnO film. The ZnO/CdTe-700 °C heterostructure thin film has a 112-fold higher rate of photocatalytic hydrogen generation than pure ZnO.     

Molybdenum doped bilayer photoanode nanotubes for enhanced photoelectrochemical water splitting

Conversion of solar energy into hydrogen energy via photoelectrochemical (PEC) water splitting is one of the most promising approaches for generation of clean and sustainable hydrogen energy in order to address the alarming global energy crisis and environmental problems. To achieve superior PEC performance and solar to hydrogen efficiency (STH), identification, synthesis, and development of efficient photoelectrocatalysts with suitable band gap and optoelectronic properties along with high PEC activity and durability is highly imperative. With the aim of improving the performance of our previously reported bilayer photoanode of WO₃ and Nb and N co-doped SnO₂ nanotubes i.e. WO₃-(Sn_0.95Nb_0.05)O₂:N NTs, herein, we report a simple and efficient strategy of molybdenum (Mo) doping into the WO₃ lattice to tailor the optoelectronic properties such as band gap, charge transfer resistance, and carrier density, etc. The Mo doped bilayer i.e. (W_0.98Mo_0.02)O_3-(Sn_0.95Nb_0.05)O₂:N revealed a higher light absorption ability with reduced band gap (1.88 eV) in comparison to that of the undoped bilayer (1.94 eV). In addition, Mo incorporation offered improvements in charge carrier density, photocurrent density, with reduction in charge transfer resistance, contributing to a STH (～3.12%), an applied bias photon-to-current efficiency (ABPE ～ 8% at 0.4 V), including a carrier density (N_d ～ 7.26 × 10²² cm^?3) superior to that of the undoped bilayer photoanode (STH ～2%, ABPE ～ 5.76%, and N_d ～5.11 × 10²² cm^?3, respectively). The substitution of Mo⁶⁺ for W⁶⁺ in the monoclinic lattice, forming the W–O–Mo bonds altered the band structure, realizing further enchantments in the PEC reaction and charge transfer kinetics. Additionally, doped bilayer photoanode revealed excellent long term PEC stability under illumination, suggesting its robustness for PEC water splitting. The present work herein provides a simple and effective Mo doping approach for generation of high performance photoanodes for PEC water splitting.     

(In,Cu) Co-doped ZnS nanoparticles for photoelectrochemical hydrogen production

In and Cu co-doped ZnS nanoparticles were successfully synthesized in DI water and ethanol solvent by a sonochemical approach using citric acid as surfactants in aqueous medium. FESEM micrographs show that In and Cu co-doped ZnS crystallites have a rough surface nanostructure and the as-synthesized photocatalysts were tested for the photocatalytic hydrogen evolution from water splitting via the irradiation of simulated sunlight. Among In and Cu co-doped ZnS products, 4In4CuZnS photocatalyst can achieve the maximum hydrogen production rate (752.7 μmol h⁻¹ g⁻¹) in 360 min under simulated sunlight illumination. Meanwhile, we separated the hydrogen and oxygen cells using an ion exchange membrane. Both electrodes (working electrode and Pt electrode) are dipped into each cell containing an aqueous solution containing 0.1 M Na₂S at pH 3 to convert water into hydrogen and oxygen under solar irradiation. As expected, the photoelectrochemical water splitting cells could significantly improve the photocatalytic activity, where the 4In4CuZnS nanoparticles shows the photoelectrochemical performance with photocurrent density of 12.2 mA cm⁻² at 1.1 V and hydrogen evolution rate of 1189.4 μmol h⁻¹ g⁻¹.     

Photocurrent and hydrogen production by overall water splitting based on polymeric composite Calix[n]arene/Cyanin Dye/IrO2 nanoparticle

In this study, a new photoelectrochemical cell based on overall splitting of water into oxygen and hydrogen is constructed to obtain an improved photocurrent under a visible range of light. The photoanode was obtained by a gold electrode (GE) modified with carboxylic acid functionalized SH-Calix-4-arene-COOH and IrO₂ nanoparticles attached light absorbing cyanine dye via polymeric oligoaniline linkages. The conductive polymer, 4- (4H-Dithieno [3,2-b: 2 ′, 3′-d] pyrrol-4-yl)aniline, was coated on GE using electropolymerization and used as a photocathode after platinum nanoparticles (Pt) were attached on the surface. The system was illuminated under the visible light, and the water was oxidized via IrO₂ catalyst to produce hydrogen on the photocathode side while oxygen on the photoanode. A photocurrent density of 182.03 μA cm⁻² was obtained by direct transfer of electrons without using a mediator. The bilirubin oxidase (BOx) enzyme was successfully used to remove excess oxygen from the reaction chamber and a further increase in photocurrent was reached up to 272.44 μA cm⁻². Hydrogen production in the reaction chamber was measured by gas chromatography at different time intervals and a maximum of 1.25 × 10⁻⁸ mol cm⁻² was obtained.