Xukai
Shen
a,
Chaoran
Li
*ab,
Zhiyi
Wu
a,
Rui
Tang
a,
Jiahui
Shen
a,
Mingyu
Chu
a,
Ao-Bo
Xu
a,
Bingchang
Zhang
*c,
Le
He
*ab and
Xiaohong
Zhang
ab
aInstitute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou, 215123, Jiangsu, PR China. E-mail: lehe@suda.edu.cn; crli@suda.edu.cn
bJiangsu Key Laboratory of Advanced Negative Carbon Technologies, Soochow University, Suzhou, 215123, Jiangsu, PR China
cSchool of Optoelectronic Science and Engineering, Key Laboratory of Advanced Optical Manufacturing Technologies of Jiangsu Province, Key Laboratory of Modern Optical Technologies of Education Ministry of China, Soochow University, Suzhou, 215123, PR China. E-mail: zhangbingchang@suda.edu.cn
First published on 19th July 2022
It is of emerging interest to convert CO2 and green H2 into solar fuels with great efficiency through photothermal CO2 hydrogenation. However, designing photothermal catalysts with improved sunlight harvesting ability, intrinsic catalytic activity, and thermal management to prevent heat dissipation still remains rather challenging. Herein, we report a facile structural engineering strategy for preparing efficient nanoarray-based photothermal catalysts with strong light absorption ability, high metal dispersity, and effective thermal management. Optimizing the 120 μm-SiNCs@Co catalyst allowed it to reach a record high Co-based photothermal CO2 conversion rate of 1780 mmol gCo−1 h−1. This study provides insight into the structural engineering of photothermal catalysts for enhanced catalytic performance and lays a foundation for efficient photothermal CO2 catalysis.
Supported metal catalysts are widely used in photothermal CO2 catalysis.18,19 Structural engineering of metals and supports provides an effective way to optimise photothermal catalytic properties.20,21 For example, classical nanoarray-structured materials have been widely used as a typical kind of substrate in photothermal catalysis due to their unique anti-reflective effects.22 Furthermore, combined with the surface plasmon resonance (SPR) effect of loaded metal nanoparticles, nanoarray-based catalysts often exhibit excellent light-absorption ability.23 Initially, Ozin and his co-workers reported a Ru/SiNW photothermal catalyst that uses black silicon nanowire arrays as supports to enhance light absorbance.24 Recently, our group has developed a catalyst with plasmonic superstructure nanoarrays that can maximize the utilization of the entire sunlight spectrum.25 However, poor dispersity of active metal nanoparticles in these catalysts contributed to poor intrinsic reactivity. Typically, the metals of array-based catalysts are loaded by magnetron sputtering.26,27 Although tuning the sputtering deposition conditions can reduce the particle size, the straight up-and-down structure of nanowire and nanorod arrays will inevitably cause a wider distribution of metal particles. Agglomeration and overgrowth of metal nanoparticles can be found at the top of array structures, and these phenomena will also result in poor dispersity and low atomic utilization of decorated metals.27,28 In addition, nanoarray structures and metal nanoparticles absorb light and act as active sites while the substrate beneath the arrays is almost useless for light absorption and catalysis but accelerates heat dissipation and weakens the catalytic performance. Therefore, it is still a big challenge to optimize nanoarray-based photothermal catalysts and obtain superior photothermal catalytic performance with enhanced dispersion of active sites and good light-to-heat conversion properties.
Herein, we report the use of silicon nanocone arrays (SiNCs) as supports to increase the dispersity of Co nanoparticles. A thinning strategy to further improve light-to-heat conversion was also studied. Due to the combination of good light absorption, high metal dispersity, and low heat dissipation, the photothermal CO2 conversion rate of the optimized catalyst (120 μm-SiNCs@Co) reached as high as 1780 mmol gCo−1 h−1. Our research reveals the importance of optimizing the dispersion of metal nanoparticles via structural engineering and improving the photothermal conversion of the system via heat management to maximize the photothermal catalytic performance.
Cobalt was then loaded on the surface of both nanoarray structures by magnetron sputtering under vacuum. According to our previous research, the optimal sputtering time of 15 min was first employed.23 The as-prepared composite catalysts are denoted as SiNCs@Co-1 and SiNWs@Co-1 (Fig. S3†). As shown in Fig. 2a, the entire surface of SiNCs@Co-1 became rough, confirming that the metal particles were successfully dispersed. TEM was used to further observe the Co nanoparticles. As we expected, 3.7 ± 0.9 nm particles were free from overgrowth and evenly distributed on the Si nanocones (Fig. S4†). In contrast, the top regions of SiNWs@Co-1 were clearly rougher than the bottom regions (Fig. 2f and g). Moreover, the energy dispersive X-ray (EDX) mapping results further confirm the successful preparation of SiNCs@Co-1 and SiNWs@Co-1 (Fig. 2b–e and h–k). Obviously, SiNCs can facilitate a better distribution of Co nanoparticles than SiNWs with Co tending to accumulate on the top region. Samples with different loading amounts were obtained by controlling the sputtering time. Two samples with longer sputtering time (25 min) were also prepared, denoted as SiNCs@Co-2 and SiNWs@Co-2 (Fig. S5 and S6†). The actual loading amounts of cobalt element were measured by ICP-MS with 0.16 mg for 15 min sputtering and 0.29 mg for 25 min sputtering. The above results clearly demonstrate the effectiveness of our structural-engineering strategy in improving the dispersity of metal particles.
Light harvesting ability is an important factor to evaluate the quality of a photothermal catalyst. Therefore, UV–vis–NIR DRS was used to assess the light absorption ability of the composite catalysts before carrying out catalytic tests. We first evaluated the light absorbance of nanoarray substrates. Although the length of SiNCs is shorter than that of SiNWs, the light absorption abilities of these two arrays are nearly the same and are much stronger than those of planar Si wafers in the 250–1100 nm wavelength region (Fig. S7†). After loading Co nanoparticles, the absorbance was greatly improved, especially in the NIR region (Fig. 3a). Overall, the composite catalysts exhibited strong light-absorption ability over the entire solar spectrum due to the combination of the light-trapping effect of the nanoarray structures and the SPR of Co nanoparticles. This means that the catalysts can achieve efficient utilization of the full solar spectrum, which will be beneficial for obtaining good photothermal catalytic performance.
Fig. 3 (a) Diffuse reflectance spectra and (b) photothermal catalytic performances of SiNCs@Co-1, SiNWs@Co-1, SiNCs@Co-2 and SiNWs@Co-2. |
Testing of photothermal catalytic CO2 conversion was conducted at room temperature in a batch reactor with a gas mixture (CO2:H2 = 0.5 bar:0.5 bar). A 300 W Xe lamp was used to simulate the concentrated sunlight and the intensity of incident light was adjusted to 25 suns. In order to exclude any effects that the supports may have had, catalytic tests of SiNCs and SiNWs were first conducted. After reacting for 20 min, no products were detected within the limits of the gas chromatography instrument, suggesting that the pure Si arrays are inert for CO2 hydrogenation (Fig. S8†). Afterwards, we investigated the catalytic performance of the aforementioned composite catalysts. Before carrying out the catalytic tests, the chemical states of surface cobalt in SiNCs@Co were investigated by X-ray photoelectron spectroscopy (XPS). The Co 2p3/2 and 2p1/2 peaks are located at 778.3 and 793.8 eV, respectively, which can be assigned to metallic cobalt (Fig. S9†). Additional peaks located at higher binding energies (780.7 and 797.2 eV) for Co 2p are likely to be Co in CoOx due to the inevitable exposure of the sample to air during the XPS test. Furthermore, a hydrogen temperature programmed reduction (H2-TPR) experiment was performed. As shown in Fig. S10,† there are two peaks of 203 °C and 357 °C on the H2-TPR curve, which can be assigned to the reduction of weakly bonded Co3+ species and CoOx, respectively. As the samples for catalytic tests are freshly prepared and light irradiation under working conditions will also help in promoting the reduction of surface-oxidized Co, the working state of cobalt in SiNCs@Co is mainly Co0. Since the contributors of catalytic activity are mainly Co nanoparticles, the CO2 conversion rate (RCO2) was normalized by the mass of Co for better comparisons among the samples. SiNCs@Co-1 exhibited a RCO2 of 1310 mmol gCo−1 h−1, which was about 300 mmol gCo−1 h−1 higher than that of SiNWs@Co-1 under similar metal loading, light utilization, and surface temperature (Fig. 3 and Fig. S11†). The rate difference can be attributed to the intrinsic activity arising from the difference in metal dispersity. Notably, SiNCs@Co-2 and SiNWs@Co-2 with higher Co loading amounts showed better light absorption, but the dramatic decrease in activity further emphasizes the importance of metal dispersity.
The SEM images can be used to further evaluate the catalytic performance of the catalysts. Due to the straight up-and-down structure and the small intervals between the nanowires, the sputtered Co nanoparticles are mainly concentrated at the upper part of SiNWs (Fig. 2f and Fig. S3b†). Large particles accumulate mostly at the top of the structure while the rest of SiNWs look smooth, as if there was a complete absence of Co nanoparticles (Fig. 2g). In contrast, SiNCs can facilitate better distribution of Co nanoparticles over the entire structure and thus can expose more active sites due to the larger intervals between the nanocones (Fig. S3a†), resulting in a higher catalytic activity than SiNWs@Co with the same loading amount. Unfortunately, although the SiNC structure serves to improve the Co nanoparticle dispersion, the increase in the loading amount will still inevitably lead to agglomeration and overgrowth of Co nanoparticles both on SiNCs and SiNWs (Fig. S5 and S6†), lowering the atomic utilization and catalytic activity. Meanwhile, an increase in the loading amount also leads to a small decrease of CO selectivity which further indicates the overgrowth of Co nanoparticles. The results reveal that optimizing the dispersion of metal nanoparticles on the supports plays a crucial role in improving the photothermal catalytic activity.
It is clear that the SiNCs@Co-1 composite catalyst exhibits strong light absorption ability over the full solar spectrum and fairly strong catalytic activity. Our previous study has shown the importance of substrate effects on the activity of photothermal catalysts. The Si substrate in the composite catalyst system is a material with good thermal conductivity (149 W m−1 K−1) but may be partially responsible for the heat loss.30 On the basis of this, it is reasonable to thin the Si substrate (Fig. 4a), thus reducing this loss, improving light-to-heat conversion and further increasing the photothermal catalytic activity. Therefore, we chose SiNCs@Co-1, which exhibited the highest activity in all the above samples, replaced the SiNCs etched from 480 μm Si wafers with 210 μm and 120 μm ones, and then carried out photothermal catalytic tests under the same conditions (Fig. 4b–d). Considering that the change of substrate thickness may affect the light absorption of composite catalysts, DRS of the three catalysts was first performed. Fig. 4e shows the near-coincident curves of the three catalysts with different Si substrate thicknesses, indicating that the light absorption ability is mainly determined from the nanoarray structure and loading amount of metals. Meanwhile, the results of the catalytic tests show that a smaller thickness of the Si substrate will lead to a higher photothermal catalytic activity (Fig. 4f). Surprisingly, 120 μm-SiNCs@Co-1 exhibits the highest RCO2 of 1780 mmol gCo−1 h−1 among the reported Co-based photothermal catalysts (Table 1). Fig. 4g shows the surface temperature profiles of different catalysts under 25-sun irradiation. Likewise, it shows that a higher surface temperature of the composite catalyst can be obtained with a smaller thickness of the Si substrate. Moreover, a smaller thickness of the Si substrate will also lead to a slightly higher CO selectivity because the reverse water gas shift (RWGS: CO2 + H2 → CO + H2O) is a typical endothermic reaction and a higher temperature will facilitate the reaction. This indicates that the strategy of thinning the substrate can indeed reduce the heat loss and thus increase the photothermal catalytic activity, which is consistent with our hypothesis.
Catalyst | Light source | H2:CO2 | R max [mmol gCo−1 h−1] | Ref. |
---|---|---|---|---|
SNAs@Co | 300 W Xe light (2.5 W cm−2) | 1:1 | 433 | 23 |
Co–PS@SiO2 | 300 W Xe light (2.0 W cm−2) | 1:1 | 612.4 | 25 |
Co@dpAAO-400 | 300 W Xe light (2.5 W cm−2) | 1:1 | 1666 | 27 |
Co@SiO2-array | 300 W Xe light (2.4 W cm−2) | 1:1 | 407.3 | 28 |
Co/Al2O3 | 300 W Xe light | 4:1 | 900 | 31 |
Co@CoN&C-1 | 300 W Xe light | 1:1 | 144.9 | 32 |
Co–SiO2 | 300 W Xe light (2.7 W cm−2) | 1:1 | 1522 | 33 |
120 μm-SiNCs@Co | 300 W Xe light (2.5 W cm−2) | 1:1 | 1780 | This work |
To further verify the thinning strategy, we also reduced the substrate thickness of SiNWs@Co-1 from 485 μm to 210 μm and 120 μm and then tested the light absorbance (Fig. S12†), photothermal catalytic activity (Fig. S13†) and surface temperature (Fig. S14†) of the catalysts. The results are consistent with the patterns exhibited by the catalysts with SiNCs of different substrate thicknesses as supports. Overall, the results emphasize the importance of thermal management in reducing heat dissipation and improving both photothermal conversion and catalytic activity in photothermal catalysis.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nr02680e |
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