With the continuous depletion of non-renewable resources, the combustion of fossil fuel, such as coal, oil, and natural gas, becomes one of the most important issue for the human society development. Furthermore, the emissions resulting from burning fossil fuels severely resulting to air pollution, greenhouse gas accumulation, and climate change^[1]. Under the vision of the goal of carbon peak and carbon neutrality, the search for clean and renewable energy sources becomes paramount. Among the potential alternatives, hydrogen has emerged as a highly promising option^[2]. Notably, the combustion of hydrogen produces only water vapor as a byproduct, making it an environmentally friendly energy carrier^[3]. Moreover, the heat released during the combustion of hydrogen is more than twice that of an equivalent mass of methane, making it a high-energy fuel with vast applications across various sectors, including transportation, electricity generation, and industrial processes^[4].

However, the widespread adoption of hydrogen as a fuel is hindered by challenges such as high production costs and storage difficulties^[5]. In recent years, researchers have explored innovative approaches to produce hydrogen sustainably and efficiently. Among these, the photoelectrochemical (PEC) water splitting has gained significant attention^[6−8]. This process uses semiconductor materials to convert solar energy into chemical energy, which drives the splitting of water molecules into hydrogen and oxygen. Among various PEC structures, three-dimensional (3D) nanostructures have garnered significant attention due to their ability to increase the surface area of materials, which enhances light capture efficiency, and promotes efficient electron-porous separation^[6−8]. Among various semiconductor materials, 4H silicon carbide (4H-SiC) stands out as an exceptional candidate for PEC water splitting, duo to the advantages of high electron mobility, high melting point, chemical inertness, and radiation resistance^[9−13]. For 4H-SiC used in PEC water splitting, it is critical to fabricate 4H-SiC micro/nanostructures, which is capable of increasing the light absorption^[14], and guarantees efficient and stable hydrogen production of 4H-SiC.

Recent studies have unveiled the PEC activity of SiC photoanodes with different structures^[15−19]. It is well known that the preparation of micro-nanostructures of SiC photoanodes is crucial to the PEC performance of SiC. At early development stage, low-voltage anodization has been adopted to manipulate the porosity of 4H-SiC porous structures. In 1993, porous SiC was successfully prepared using anodization in HF^[20], and later researchers reported the preparation of triangular porous 4H-SiC using ultraviolet (UV)-light assisted electrochemical etching to tune the porosity of SiC nanostructures from 0.08 to 0.28^[21]. Metal-assisted photochemical etching has also been employed to fabricate porous 4H-SiC with different porositie^[22], metals can introduce band bending to catalyze hole generation and serve as masks to enhance the diversity of etching morphologies. Subsequently, high-voltage anodization has been used to tune the depth of 4H-SiC nanoporous array. The pulsed electrochemical etching has been proposed to prepare longitudinally uniformly distributed 4H-SiC mesopores with the depth of 19 μm^[23]. Researchers used anodization (pulsed 20 V) to prepare 4H-SiC nanoporous array with the depth of 3 μm^[24], and later a two-step anodization process (17 V pulse, followed by 17 V) was developed to transfer 4H-SiC nanoporous array films onto indium-tin oxide (ITO) for PEC water splitting^[12]. However, the interface between the photoanode and the current collector limited the PEC performance. To address this issue, a two-step electrochemical etching method (pulsed 19 V, followed by 30 V) was developed to prepare interface-free 4H-SiC nanoporous array photoanodes with the depth of 16 μm^[25], resulting in enhanced PEC performance. However, the porosity of 4H-SiC nanoporous array is uncontrollable in these high-voltage anodization researches. It might be attractive to combine the low-voltage and high-voltage anodization processes to fabrication 4H-SiC nanoporous arrays with controlled porosity and depth.

In this work, we prepare 4H-SiC nanoporous arrays with different porosity and depth via a noval two-step anodizing approach. By combining a constant low-voltage etching followed by a pulsed high-voltage etching, 4H-SiC nanoporous arrays with the depth of 15 to 20 μm have been obtained. Raman spectra indicates that the nanoporous arrays preserves the 4H polymorph of SiC. The longer the constant-voltage etching time, the higher the light absorption intensity because of the enhanced anti-reflective capability. It is found that the nanoporous arrays with medium porosity has the highest PEC current, because of the enhanced light absorption and the optimized transportation of charge carriers along the walls of the nanoporous arrays. The performance of the PEC water splitting of the nanoporous arrays is stable after the electron irradiation with the dose of 800 and 1600 kGy, which indicates that 4H-SiC nanoporous arrays has great potential in the PEC water splitting under harsh environments.

Commercial n-type 4H-SiC single-crystal wafers were bought from Hangzhou Qianjing Semiconductor Co., Ltd. Firstly, 4H-SiC wafer was cut into 0.5 cm × 1.5 cm samples. These samples were then thoroughly cleaned using acetone, ethanol, and deionized water by ultrasound. Subsequently, the 4H-SiC samples were immersed in a mixed solution of HF and H₂O₂ for 2 min to remove surface oxides. Titanium is plated on the silicon surface to ensure a robust electrical contact between the sample and the electrode clip, which reduces the contact resistance throughout the entire circuit and thereby enhancing etching efficiency. The etching process was conducted in self-made polytetrafluoroethylene container using HF, C₂H₅OH, and H₂O₂ with the volume ratio of 3 : 6 : 1. The 4H-SiC sample and graphite are served as the anode and cathode, respectively. The nanoporous array of 4H-SiC was prepared by a two-step etching process. Low-voltage etching can generate nanopores gently, and the density of nanopores can be modulated by changing the etching time. Therefore, the initial etching was performed at 3 V, with UV light illumination on the [0001¯] surface. Under the combined influence of UV light and applied bias voltage, hole accumulation occurs on the [0001¯] surface, and participates in the etching process^[21]. This removes the capping layer and generating uniformly distributed small porous structure on the C surface of 4H-SiC. Following this step, a pulsed voltage (T = 0.8 s, Toff = 0.4 s) was applied for further etching, which generated vertically uniform arrays of nanoporous arrays^[23]. The samples were labeled as S2, S5, S10, S20, and S30, which corresponded to etching times of 2, 5, 10, 20, and 30 min. Fig. 1 illustrates the preparation details.

The prepared 4H-SiC nanoporous arrays photoelectrodes were tested for their PEC performance using an electrochemical workstation. In a typical three-electrode setup, a 0.5 M Na₂SO₄ (pH = 6.8) aqueous solution was used as the electrolyte. The experiments were conducted by using a 50 W Xe lamp as the sunlight of AM 1.5 G (light intensity = 100 mW/cm²) illumination. The 4H-SiC photoelectrode, Ag/AgCl (saturated KCl solution), and graphite were used as the working electrode, reference electrode, and counter electrode, respectively. Linear sweep voltammetry (LSV) were performed at a scanning rate of 25 mV/s. Electrochemical impedance spectroscopy (EIS) was conducted in the frequency range of 1 to 10⁵ Hz. The I−t curve is utilized to assess the stability of PEC activity, with an applied potential of 1.23 V vs reversible hydrogen electrode (RHE). The experimentally obtained Ag/AgCl potential is converted to the RHE using the following formula^[25].

1ERHE=EAg/AgCl+0.059pH+E0Ag/AgCl,

where E_Ag/AgCl is the potential set on the electrochemical workstation. "pH" is the pH value of the chemical solution. E_0Ag/AgCl is the potential of the Ag/AgCl reference electrode relative to the standard hydrogen electrode, which is determined to be 0.25 V. All PEC characterizations were measured five times, and the averages were calculated to reduce errors.

The microstructures of the samples were characterized using a scanning electron microscope (SEM, Sigma300, Zeiss, UK). The chemical states and compositions of the nanoporous arrays were analyzed using a high-resolution multifunctional spectrometer excited with a 532 nm laser (Raman, LabRAM Odyssey, Horiba, Japan). The optical absorption was characterized using a UV−Vis spectrophotometer (UV−2600, UV−Vis Spectrophotometer, Bruker, USA).

Figs. 2(a)–2(f) present the SEM images of original 4H-SiC, S2, S5, S10, S20, and S30. Evidently, longer durations of the constant-voltage etching result in higher nanoporous density and larger nanoporous diameters. Hence, there exists a positive correlation between the porosity of the 4H-SiC surface and the constant-voltage etching duration. During the constant-voltage etching process, the combination of simulated solar illumination and applied bias voltage results in a randomly distributed electric field on the 4H-SiC surface, leading to the generation of randomly distributed nanoporous structure. The etched 4H-SiC sheet exhibits a larger surface area, which not only increases the contact area with the electrolyte solution but also facilitates light collection. For comparison, Fig. S1 displays the surface SEM images of 4H-SiC wafers subjected only to constant-voltage etching. It is evident that pulsed voltage etching does not alter the aperture or pore density.

Figs. 2(g) and 2(h) display cross-sectional SEM images of sample S10 at various magnification levels. It is clear that the depth of the nanoporous arrays reaches approximately 15 μm. This well-organized and oriented nanoarrays configuration plays a crucial role in facilitating efficient electrolyte transport and promoting active participation in catalytic reactions within the nanoporous arrays^[25]. At the same time, from the magnified cross-sectional SEM images, it can be observed that the nanopores exhibit a gourd shape. This is because the applied pulsed voltage triggers a periodic etching process, leading to fluctuations in pore diameter and resulting in the formation of a gourd-shaped nanoporous arrays^[24]. Remarkably, a close examination from the bottom of the porous indicates a tight and smooth interface between the nanoporous arrays and the unetched 4H-SiC region. This can be attributed to the uniform distribution of pore sizes, ensuring an even transmission of the etching solution to the bottom of the nanoporous arrays, which facilitates uniform and controlled longitudinal etching. The moderate pulsed voltage also contributes to the mild longitudinal etching of the nanoporous arrays, maintaining their structural integrity.

Fig. 3(a) presents the Raman spectra of original 4H-SiC, S2, S5, S10, S20, and S30. In the etched 4H-SiC samples, folded modes are observed in the transverse acoustic branches (FTA) at 204 and 266 cm⁻¹, in the longitudinal acoustic branch (FLA) at 610 cm⁻¹, and in the folded optical branches (FTO) at 776 and 796 cm⁻¹. The peaks of the FLA mode of the etched samples all locate at 204 cm⁻¹, indicating that the post-etched nanoporous arrays still retains the 4H-SiC polymorph. The intensities of FTA and FTO modes in all etched 4H-SiC samples are lower than those in the original 4H-SiC, indicating a certain degree of structural distortion in the etched 4H-SiC^{[26, 27]}. Due to the n-type doping, the folded mode of the longitudinal optical branch (FLO) of both the original 4H-SiC and nanoporous arrays shift to 984 cm^−1[28]. Fig. 3(b) shows the optical absorption curves of original 4H-SiC, S2, S5, S10, S20, and S30. Compared to the original 4H-SiC, the nanoporous arrays photoanodes subjected to constant voltage etching exhibits significantly enhanced light absorption^[14]. The longer the constant-voltage etching time, the higher the light absorption intensity. This is attributed to the increased porosity, which enhances its anti-reflective capability. We note that both the FTA mode locating at 204 cm⁻¹ and the maintaining of strong absorption in the UV region indicate that the 4H polymorph is preserved after the PEC etching.

Using micro-nano statistical methods, the diameter of the nanopores and the thickness of the walls of nanopores are separately analyzed for 100 adjacent targets within regular regions on different samples. Statistical distribution plots for S5, S10, S20, and S30 are shown in Fig. S2, S3, and the corresponding average values of these parameters are presented in Table 1. We note that the porosity of S2 is not calculated, because the nanopores of S2 are not evenly distributed throughout the surface of 4H-SiC. As the etching duration increases, the thickness of the walls of nanopores decreases, and the nanopores gradually enlarge before reaching a stable shape. This agree well with the pattern inferred from the SEM images of the nanoscale pore arrays mentioned above. In summary, the porosity of the 4H-SiC nanoporous arrays increase with the increase of the etching duration.

The PEC performance of 4H-SiC nanoporous arrays is then evaluated under simulated sunlight illumination. Fig. 4(a) shows the photocurrent density of the original 4H-SiC and S2, S5, S10, S20, S30 under simulated sunlight illumination. As the constant-voltage etching duration increases, the photocurrent density of nanoporous samples firstly increases and then decreases, with S10 having the highest photocurrent density. Fig. 4 (b) shows the charge transfer resistance of the original 4H-SiC and S2, S5, S10, S20, S30 under simulated sunlight illumination. It is clear that S10 has the smallest arc radius, indicating the best enhancement in the separation and transport of charge carriers of S10. It is clear that S10 has the best PEC water splitting performance among constant-voltage etched nanoporous arrays. Fig. 4(c) shows the transient photocurrent response curve of S10 under simulated sunlight illumination at a potential of 1.23 V (vs RHE). The 4H-SiC nanoporous arrays exhibits nearly zero current density in dark, and a distinct enhancement of the current density once the sunlight illumination is applied. As the duration of the transient photocurrent response test increases, the photocurrent density decreases by 17%, which indicates that the 4H-SiC nanoporous arrays exihibit rapid light response and high stability.

During the PEC catalytic reaction, a space charge zone with a thickness of d_sc is formed at the interface between the electrolyte solution and the 4H-SiC nanoporous arrays. We define the thickness of the walls of nanopores being d_w. As shown in Figs. 4(d)–4(f), when d_w > 2d_sc, the center of the porous wall maintains the n-type doping and enables effective charge carrier transport. However, when d_w≤ 2d_sc, the space charge regions on both sides of the porous wall gradually approach and even overlap, transforming the entire porous wall into a neutral region with high resistance. This disables the transportation of charge carrier^{[21, 29, 30]}.

When the constant-voltage etching duration increase from 2 to 10 min (that is, S2 to S10), the initial enhancement of the PEC catalytic performance is attributed to the increase of porosity, which leads to improved solar light utilization by 4H-SiC nanoporous arrays. Subsequently, the increase in the porosity reduces the PEC catalytic performance when the constant-voltage etching duration increases from 10 to 30 min (that is, S10 to S30). This is mainly due to the decrease of the thickness of the walls of nanopores which hinders the carrier transport in the space charge region. Therefore, the constant-voltage etching step plays a vital role in enhancing the overall PEC performance of the 4H-SiC nanoporous arrays photoanodes. By introducing nanoporous and adjusting the porosity, it facilitates better light absorption. The pulsed voltage etching step is identified as a critical factor contributing to the superior performance of the 4H-SiC nanoporous arrays. It effectively refines the nanoarrays and optimizes the surface properties, leading to enhanced charge carrier separation and transport, which is essential for efficient PEC reactions.

We then evaluate the radiation hardness of the 4H-SiC nanoporous arrays based PEC catalytic reaction. Fig. 5(a) shows the LSV curves of S10 under different electron irradiation with different doses. It is clear that the PEC catalytic characteristics of S10 is resistive to the electron irradiation of 800 kGy. As the irradiation dose increases, the photocurrent density gradually decreases. This is caused by the electron irradiation creates defects on the surface of the 4H-SiC nanoporous arrays^[31−33]. The photo-generated carriers are captured by the irradiation-induced defects, rather than participate in the photocurrent. Under the same electron irradiation, the degradation of the photocurrent density is more obvious under low voltage, because the percentage of defect captured photo-generated carriers is higher than what happens in high-voltage condition. We also compared the effect of electron irradiation on the transient photocurrent response of 4H-SiC nanoporous arrays. As shown in Fig. 5(b), although the transient photocurrent decrease upon electron irradiation, the rapid light response and high stability of the 4H-SiC nanoporous arrays is preserved after electron irradiation with the dose of 800 and 1600 kGy. Only when the dose of the electron irradiation increases to 2400 kGy, the photocurrent in dark increases, which may be caused by the high-dose generated dissociative defects participate the dark-environment current.

In conclusion, we have demonstrated the irradiation-resistant PEC water splitting based on 4H-SiC. The new two-step anodizing approach, that is, a constant low-voltage etching followed by a pulsed high-voltage etching is adopted to prepare 4H-SiC nanoporous arrays with different porosity. The constant-voltage etching and pulsed-voltage etching are found to effectively affect the diameter of the nanopores and the depth of the nanoporous arrays, respectively. The longer the constant-voltage etching duration, the higher the light absorption intensity because of the enhanced anti-reflective capability. It is found that the nanoporous arrays with medium porosity has the highest PEC current, because of the enhanced light absorption and the optimized transportation of charge carriers along the walls of the nanoporous arrays. Only when the dose of the electron irradiation increases to 2400 kGy, the photocurrent in dark increases, which may be caused by the high-dose generated dissociative defects participating the dark-environment current. The performance of the PEC water splitting of the nanoporous arrays is stable after the electron irradiation with the dose of 800 and 1600 kGy, which indicates that 4H-SiC nanoporous array has great potential in the PEC water splitting under harsh environments. As the single-crystal growth and wafer-processing solutions of 4H-SiC continue to mature, the diameter and thickness of 4H-SiC single crystals are gradually increasing. The processing efficiency and yield of 4H-SiC substrates are continuously improving. The cost of 4H-SiC wafers is rapidly decreasing, which paves the way for the application of low-cost and irradiation-resistant 4H-SiC based PEC water splitting.