Ag-decorated GaN for high-efficiency photoreduction of carbon dioxide into tunable syngas under visible light

The photoconversion of gaseous CO₂ (that is, photocatalytic CO₂ reduction reaction (pCO₂RR)) offers an underlying option to mitigate the excessive emission of anthropogenic CO₂ [1–5]. Thus far, great advancements have been achieved towards converting CO₂ to C₁, C₂, or C₂₊ products through pCO₂RR by using solar energy [6–12]. Among them, syngas (H₂/CO mixture), is a key chemical feedstock for the industrially adopted Fischer–Tropsch (FT) route [13–15], which has been traditionally obtained by coal gasification and natural gas reforming [16]. Also, it should be noted that different CO/H₂ ratio is required for the different downstream product [9, 15]. Therefore, given the significance of syngas, the synthesis of tunable syngas via pCO₂RR is highly desirable.

Although various semiconductor pCO₂RR catalysts have been developed [1, 6, 7, 15], most studies are only focus on metal oxides (including TiO₂, Fe₂O₃ and ZnO). It has been documented that CO₂ molecules are typically adsorbed on oxygen vacancies of metal oxide surfaces [9, 17, 18]; however, at the same time, the adsorption of O₂ in solution is also strengthened, and then fill the vacancy sites, leading to an inert surface [17]. Recent studies demonstrated that GaN is a highly promising candidate for solar fuel applications due to its wide direct transition bandgap (3.42 eV) along with high carrier mobility, thermal conductivity and nontoxicity [17, 19–26]. In comparison with most reported metal oxides, the conduction band minimum of GaN is more negative, and hence it is kinetically more probable for the reduction of stable CO₂ [17, 27]. Meantime, GaN has also shown relatively high stability during harsh photocatalysis reactions with more considerable corrosion resistance [27]. In addition, the built-in electric field induces an enhanced charge separation efficiency [28]. Nevertheless, the wide bandgap of GaN limits its use to effectively harvest a wide range of solar spectrum. Currently, the construction of heterojunction by introducing plasmonic metals (e.g. Au and Ag) is an effective strategy to tune electronic and photocatalytic properties of the GaN-based photocatalysts [22, 27–30]. The resulting Mott–Schottky effect will significantly promote the transfer of photogenerated electrons from the semiconductors to the metal nanoparticles [31]. Meanwhile, the loaded Au and Ag metals can capture the irradiation energy, resulting in high energy electrons on their surface via the localized surface plasmon resonance effects [22, 32]. Of the plasmonic metals studied, Ag has been widely investigated for photocatalysis due to its relatively low cost and high electrical conductivity [33]. To date, however, the exploration of Ag/GaN hybrid photocatalyst that can precisely control the H₂/CO ratio is still lacking, which largely hinders their further industrial applications.

Inspired by the pioneering progress and our previous work [8, 10–12], we design an Ag crystals-decorated GaN nanobelts photocatalyst that is capable of efficiently producing syngas with a wide range of H₂/CO ratio (0.5–3) under visible light irradiation (λ > 400 nm). Notably, the generated syngas can further detect under light irradiation up to 600 nm. The obtained maximal turnover frequency (TOF) value and the corresponding yield rate is 3.85 h⁻¹ and 2.12 mmol h⁻¹ g⁻¹ for CO production, respectively. This performance is comparable to the recently reported catalysts (see details in table S1 (available online at stacks.iop.org/NANO/32/505722/mmedia)), such as porous metallic and magnetic Co–C composite [34], SnS₂/SnO₂ [35], Cu/Pt/TiO₂ [36], Pt/C–In₂O₃ [37], and In₂S₃–CdIn₂S₄ [38]. Interestingly, the catalyst shows pCO₂RR activity toward syngas production in low­-concentration CO₂ atmosphere. Moreover, it also shows impressive pCO₂RR stability during five cycling tests. Detailed experimental characterizations confirm that the introduced Ag not only extends the light absorption region but also promotes the charge transfer efficiency, both of which enable more electrons to participate in the conversion of CO₂ to CO, resulting in improved pCO₂RR activity and selectivity.

2.1. Chemicals

2.2. Synthesis of Ag/GaN

2.3. Characterizations

2.4. Electrochemical measurements

The details for the synthesis of hybrid photocatalysts are described in the experimental section. Here, four samples with different Ag mass loadings on GaN were synthesized and denoted as Ag/GaN-x (x = 0.8, 1.7, 2.2 or 3.5). As depicted in SEM and TEM images (figures 1(a) and (b)), GaN displayed a nanobelt-like morphology with a width of 100–500 nm and length of 1–10 μm. After in situ depositing Ag crystals on surface GaN nanobelts, a hybrid photocatalyst (namely Ag/GaN-3.5) were obtained, as confirmed by the TEM image (figure 1(c)). The particle size of Ag crystals was measured to be about 10–30 nm.

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As revealed by the high-resolution TEM (HRTEM) image (figures 1(d) and (e)), the lattice fringes with a distance of 0.272 and 0.233 nm were observed, which correspond to the (100) and (111) crystallographic planes of GaN and Ag, respectively. EDX mapping analysis also confirmed the deposition of Ag nanoparticles on the GaN surface (figure 1(f)).

XRD pattern of Ag/GaN-3.5 was shown in figure 2(a). Interestingly, all the diffraction peaks are well-matched with the hexagonal GaN (JCPDS No. 50-0792), confirming that the Ag deposition on GaN nanobelts is well-dispersed with a low loading amount. The Ag/GaN-3.5 sample was examined by Raman spectroscopy (figure S1), which yields two peaks at 561.7 and 712.2 cm^–1, corresponding to the E₂ and A₁ modes of GaN [40], respectively. The elemental compositions and valence states of Ag, Ga, and N of Ag/GaN-3.5 were then examined by XPS. The survey spectrum (figure S2) verified the presence of Ag, Ga and N in Ag/GaN, which agrees with the EDX results (figure 1(f)). The high-resolution Ag 3d XPS spectrum of Ag/GaN demonstrated two peaks centered at binding energies of 367.1 and 373.2 eV that index to the metallic state of Ag, ruling out the existence of Ag₂O or other silver oxides (figure 2(b)) [41]. In figure 2(c), the spin–orbit constituent of Ga 2p could be deconvoluted into two peaks with binding energies of 1117.2 and 1144.1 eV assigned to the 2p_3/2 and 2p_1/2 valence state of Ga³⁺, respectively. This is consistent with previous work [40]. It should also be noted that there is a negative shift of 0.5 eV for Ga 2p in Ag/GaN-3.5 in comparison to pristine GaN, suggesting the strong electronic interaction between the Ag and GaN [41]. Figure S3 presents the N 1s peak located at 397.3 eV, which could be ascribed to the N–Ga bond. In addition, the peak at 395.5 eV was originated from the Ga Auger LMM transitions. As shown in UV–vis spectra (figure 2(d)), increased absorption in the visible range was observed owing to the surface plasmon resonance effect of Ag, which is beneficial to the utilization of low photonic energy long-wavelength light [42].

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We next investigated the pCO₂RR activity of the as-obtained Ag/GaN-x in an airtight gas circulation system (figure S4) under visible-light irradiation (λ > 400 nm) with [Ru(bpy)₃]Cl₂·6H₂O as the photosensitizer and TEOA as the electron donor [42, 43]. According to GC, NMR, and HPLC measurements, CO and H₂ were confirmed to be the only gaseous products from CO₂ photoreduction (figures S5–S7). When replacing CO₂ with Ar, a much fewer amount of H₂ was detected, implying that CO was generated from pCO₂RR. In addition, when no [Ru(bpy)₃]Cl₂ was added, Ag/GaN-3.5 achieved low CO and H₂ yield rates of 0.01 and 0.005 μmol g^–1 h^–1, respectively (figure S8(a)). Furthermore, in the case of only [Ru(bpy)₃]Cl₂ being added, no CO or H₂ can be detected (figure S8(b)). Noticeably, as shown in figures 3(a) and (b), the yield rates, ratios and selectivities for CO and H₂ products depend strongly on the amount of Ag. As the Ag loading increased as 0, 0.8, 1.7, 2.2 to 3.5 in Ag/GaN-x, the CO yield rate reached 0.13, 1.16, 1.65, 1.89 to 2.12 mmol g^–1 h^–1, respectively. At the same time, the corresponding H₂ yield rate was determined to be 0.38, 2.27, 2.48, 1.85, and 1.04 mmol g^–1 h^–1, respectively. In this case, the H₂/CO ratios for the above five photocatalysts were measured to be 2.92, 1.96, 1.50, 0.98, and 0.49, respectively. This concomitant production of tunable H₂/CO mixture (from 0.5 to 3) would provide the possibility to synthesize a wide range of value-added chemicals through the FT process [15]. Of note, the achieved maximal yield rates for H₂ and CO are comparable to the best results reported in the literatures (table S1). The above results also demonstrated that the deposited Ag is the main active site for CO₂ photoreduction, which matches well with previous studies [12, 44]. To further improve the pCO₂RR performance of GaN-based materials, strategies including the introduction of defects, the construction of hierarchical morphology, and the combination of suitable p-type semiconductors have been developed. In this regard, the TOF values of Ag/GaN-x were calculated to evaluate the catalytic efficiency of Ag depositions (figure 3(c)). The results show that GaN, Ag/GaN-0.8, Ag/GaN-1.7, Ag/GaN-2.2, and Ag/GaN-3.5 gives a TOF value of 0.69, 2.61, 3.05, 3.32 and 3.85 h^–1, respectively.

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To verify the carbon source of produced CO, an isotopic experiment with ¹³CO₂ was then performed, as depicted in figure 3(d). As seen, a signal at m/z = 29 was observed, which could be ascribed to the generated ¹³CO. This strongly proved that CO was originated from photocatalysis of CO₂ via pCO₂RR. More interestingly, Ag/GaN-3.5 still displayed high pCO₂RR activity for syngas production with a CO yield rate of 0.09 mmol g^–1 h^–1 and a corresponding H₂/CO ratio of ∼1:2 when the long-wavelength light (λ > 600 nm) was applied (figure 3(e)). It should be emphasized that the usage of long-wavelength light plays an important role in promoting the utilization efficiency of solar energy [13, 42, 44].

The stability of Ag/GaN-3.5 was checked by five sequential CO₂ photoreduction experiments under visible-light irradiation (λ > 400 nm). As indicated in figure 3(f), the yield rates for CO and H₂ remained stable during the test, suggesting the good reusability of Ag/GaN-3.5. Furthermore, the post-reaction catalyst was characterized by XRD result (figure S9), both of which confirmed the compositional and structural stability of Ag/GaN-3.5.

Previous studies indicated that the conversion of low­-concentration CO₂ is of great importance for the practical application of pCO₂RR technology [43, 45]. However, to date, very few photocatalysts can effectively adsorb and activate low­-concentration CO₂ molecules [43]. Interestingly, our designed Ag/GaN-3.5 exhibited a catalytic performance toward syngas production in a 10% CO₂ atmosphere (the typical CO₂ concentration in flue gas [46]) with corresponding yield rates of 4.05 and 1.99 μmol g^–1 h^–1 for CO and H₂ production, respectively (figures S10 and S11).

To uncover the underlying reasons for the improved pCO₂RR activity of Ag/GaN-3.5, the electrochemical impedance spectra were first explored, as shown in figure 4(a). Ag/GaN-3.5 showed a smaller semicircle radius compared to GaN under visible-light irradiation, suggesting the faster charge transfer kinetics and thereby enabling more photoexcited electrons to participate in photocatalysis CO₂ [42]. This result was also confirmed by the increased photocurrent density observed on Ag/GaN-3.5 (figure 4(b)). Meanwhile, the photoluminescence (PL) spectra in figure 4(c) manifested that a suppressed recombination of the electron–hole pairs occurred on Ag/GaN-3.5, indicating an improved efficiency for charge transfer and separation resulted from the Ag deposition. Further, the Mott–Schottky plots (figure S12) of GaN and Ag/GaN-3.5 showed two positive slopes, implying Ag/GaN-3.5 still maintain an n-type characteristic [33]. Moreover, it is noted that the formation of heterojunctions in Ag/GaN-3.5 increased the conduction band position, which promoted the separation and migration of photogenerated carriers, and then induced a higher pCO₂RR performance [31].

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It has been well recognized that Ag possessed a stronger adsorption affinity toward CO₂ in comparison with H₂O, enabling it exhibits higher activity for CO₂ photoreduction [12, 47, 48]. Meantime, previous studies have shown that GaN is extremely reactive for H₂ production directly from H₂O under UV and visible light illumination [20, 27]. Taking into these considerations and the above observations and analyses, a possible mechanism for photoreduction of CO₂ and H₂O to tunable syngas on Ag/GaN has been proposed [13, 22–26, 49], as depicted in figure 4(d). Under visible-light irradiation, the photosensitizer [Ru(bpy)₃]²⁺ was photoexcited and oxidized to [Ru(bpy)₃]³⁺, producing a larger amount of electron. Then, these photoelectrons are transferred to Ag/GaN surface, where CO₂ and H₂O molecules are adsorbed. Some fraction of the generated photoelectrons were used to reduce CO₂ to CO on Ag sites, while another fraction of ones on GaN catalyzed the coupling of H⁺ to form H₂. After that, [Ru(bpy)₃]³⁺ was reduced to the initial state [Ru(bpy)₃]²⁺ by TEOA. By flexibly adjusting the Ag loading amount in Ag/GaN photocatalyst, it is easy to control the H₂/CO ratios in syngas, which is highly desirable for potential applications.

In conclusion, a hybrid photocatalyst of well-defined Ag crystals anchoring on GaN nanobelts has been prepared and was evaluated for CO₂ photoreduction. As a result, the obtained composites can produce syngas with a widely tunable H₂/CO ratio (from 0.5 to 3) under visible-light irradiation (λ > 400 nm). The maximal TOF and yield rate for CO formation was measured to be 3.85 h^–1 and 2.12 mmol g^–1 h^–1, respectively, achieved by Ag/GaN-3.5. Interestingly, the catalyst also showed activity toward syngas synthesis (with an H₂/CO ratio of 1:2) under light irradiation up to 600 nm. Moreover, Ag/GaN-3.5 demonstrated feasibility for utilization of low-concentration CO₂ (i.e. flue gas). Besides, it could remain a high activity for five sequential CO₂ reductions. Detailed experimental measurements clarified that the Ag deposition not only increases the absorption at longer wavelengths but also promotes the charge transfer and separation efficiency. Meantime, the construction of dual active sites toward CO₂ and H₂O adsorption synergistically induced remarkable improvement in pCO₂RR activity. In short, this work provides a valuable guideline for the development of efficient and robust GaN-based photocatalysts for syngas synthesis by using clean solar energy. Future investigation includes the development of GaN nanobelts for the conversion of CO₂ to valuable fuels under visible light irradiation.

This work was financially supported by National Natural Science Foundation of China (6202780115) and the Science and Technology Development Fund of Tianjin Education Commission for Higher Education (2018KJ126).

All data that support the findings of this study are included within the article (and any supplementary files).

Wei Huang and Dejin Zhou: Investigation, Data curation, Visualization, Validation, Writing—original draft. John Lee, Jiaqiang Sun, Shusheng Zhang and Hong Xu: Methodology, Resources, Funding acquisition. Jun Luo and Xijun Liu: Conceptualization, Methodology, Supervision.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.