Decoration of PdAg Dual-Metallic Alloy Nanoparticles on Z-Scheme α-Fe2O3/CdS for Manipulable Products via Photocatalytic Reduction of Carbon Dioxide

Metal nanoparticles have been extensively used as co-catalysts in photocatalytic systems in order to pursue improvements in both reaction kinetics and selectivity. In this work, PdAg dual-metallic nanoparticles synthesized by the co-reduction method were decorated on a well-established α-Fe2O3/CdS Z-scheme photoactive material as a co-catalyst to study their performance for promoting the photoreduction of CO2. Herein, α-Fe2O3 and CdS were in situ synthesized on fluorine-doped tin oxide (FTO) glass by hydrothermal and SILAR (successive ionic layer adsorption and reaction) methods, respectively. The direct Z-scheme charge transfer path between Fe2O3 and CdS and the effective electron migration toward the PdAg mainly contributed to the excellent photocatalytic CO2 reduction performance. The controllable work function based on Pd (5.12) and Ag (4.26) constructed an appropriate band alignment with α-Fe2O3/CdS and displayed favorable production for CH4 rather than CO. The optimum ratio of PdAg 1:2 performed a 48% enhancement than pure Pd for photoreduction of CO2. Meanwhile, the enhanced charge separation improved the photoelectrochemical performance and photocurrent generation, and reduced the electrical resistance between components. This work provided insights into the dual-metallic co-catalyst for boosting the activity and selectivity of photocatalytic CO2 reduction.


INTRODUCTION
Solar-driven chemical reactions based on photoactive materials have attracted massive attention due to the concerns with regard to the current energy and environmental issues (Baharuddin et al., 2021). The fundamental research summarized some key features that were capable of improving the photoactive materials for practical solar-driven chemical reactions. These were broad light absorption range, rapid charge separation, good stability, abundant surface area, and reaction sites. Hematite (α-Fe 2 O 3 ) was considered as an outstanding photocatalyst due to its good ability of light response and light harvesting (absorbs ca. 30% light) (Kuang et al., 2017;Yi et al., 2021). However, its short lifetime of photogenerated charge was attributed to the limited hole diffusion distance (Shen et al., 2016). In order to improve the charge migration rate of α-Fe 2 O 3 , a previous study focused on constructing heterostructures with different bandgap materials for further improving photogenerated charge separation (Kanwal et al., 2021). For instance, Zhang et al. reported that CdS/α-Fe 2 O 3 heterojunction showed great photocatalytic activity and had a prominent charge separation rate due to the fast diffusion of photogenerated charge between Fe 2 O 3 and CdS (Zhang et al., 2013). Based on previous studies, we considered an optimization where the role of CdS was to help Fe 2 O 3 with a quick charge transfer except for light absorption. Previous research demonstrated that noble metal nanoparticles possessed good chemical stability, large surface area, and fruitful reaction sites. Among the heterogeneous metallic co-catalysts, Pd-based nanoparticles have presented an outstanding performance for promoting photocatalytic reactions under mild conditions (Naina et al., 2021;Peng et al., 2021;Cui et al., 2022). This is due to the intensive electronic interactions between metal nanoparticles and semiconductive photocatalysts (Mott-Schottky effect) (Li and Antonietti, 2013). In comparison to pure Pd, the formation of the PdAg alloy was found to enhance the catalytic performance by applying electronic and geometric effects (Antolini, 2009). Furthermore, the PdAg alloy with different lattice constants of the dual metal atoms (Ag, 408.53pm and Pd, 389.07pm) could subsequently improve the absorption of visible light and reaction rate due to the d-band shifting up (Ghiabi et al., 2018).
The photocatalytic reduction of CO 2 into valuable solar fuels was regarded as an efficient means to address the energy supply and CO 2 emission (Habisreutinger et al., 2013). In addition, using semiconductors such as hematite (α-Fe 2 O 3 ) to achieve this conversion goal had attracted extensive attention, since they were an environmentally friendly low-cost technology (Mu et al., 2021). It is well-known that the main products via photoreduction of CO 2 are CO and CH 4 . In addition, controlling the chemical reaction conditions (Qiao et al., 2014) (pH, temperature, sacrificial reagent, et al.) is the common means to manipulate the selectivity, although it might be complex.
However, a dual-metallic co-catalyst could be an alternative strategy to manipulate the selectivity of CO 2 photoreduction, as recent modulating research (Li et al., 2019) demonstrated that the dual-metal sites can dissociate CO production. This is because the C and O atoms from the adsorbed CO 2 species would coordinate with two individual metal atoms, leading to a configuration of  (Wang et al., 2020).
Of direct relevance to our work, PdAg NPs have been successfully applied to g-C 3 N 4 (Liu et al., 2019), carbon tube (Benipal et al., 2017), and reduced graphene oxide (Martins et al., 2017) for photocatalytic reactions. These studies demonstrated that PdAg was capable of promoting photocatalytic reactions and focused on the role of PdAg under different reaction conditions. However, the Pd:Ag atomic ratio and its influence on the photocatalytic selectivity and activity was still unexplored. Thus, in this work, an α-Fe 2 O 3 /CdS Z-scheme photocatalyst was employed as a photoactive platform to support a layer of PdAg NPs. The impact of the elemental ratio of PdAg NPs on photoelectrochemical features and photoreduction selectivity over CO 2 was experimentally investigated.

Synthesis of Fe 2 O 3
The Fe 2 O 3 nanorod arrays were prepared following a modified literature report (Ma et al., 2010). The cleaned FTO was entirely immersed in a 50-ml hydrothermal reactor with FeCl 3 (0.15 M) aqueous solution and heated to 100°C for 6 h. After the hydrothermal reaction, the samples were collected and rinsed with deionized water and N 2 stream drying. The sample was then annealed in a muffle furnace at 550°C for 2 h (10°C/min) and collected after cooling down to room temperature.

Fabrication of Fe 2 O 3 /CdS
The electrode of Fe 2 O 3 nanorods deposited by CdS was fabricated via the SILAR (successive ionic layer adsorption and reaction) method (Liu et al., 2013). The as-prepared Fe 2 O 3 film was first soaked in Cd(Ac) 2 (25 mM) ethanol solution for 1 min, dried by N 2 stream. Next, it was immersed in Na 2 S (25 mM) aqueous solution for 1 min and then washed with deionized water, dried by N 2 stream. The deposition of CdS was accomplished by following chemical reactions for 10 SILAR cycles. Then, the electrode was finally annealed under Ar stream at 400°C for 30 min (1°C/min).

Fabrication of Fe 2 O 3 /CdS/PdAg
The preparation of PdAg was more complicated through the coreduction of Pd and Ag using DODAC as a structure-directing agent (Zhou et al., 2020). AgNO 3 (10 mM) and H 2 PdCl 4 (20 mM, prepared from PdCl 2 and HCl aqueous solution) were added to DODAC (2 mM) which was dissolved in deionized water. In the next step, PVP (10 mg/ml) was added for better dispersion. During the co-reduction process of Pd and Ag, ascorbic acid (0.3 M) aqueous solution was added with the color change (slight yellow to black) under 60°C for 30 min.
The dispersant absorption method was employed for the deposition of PdAg on Fe 2 O 3 /CdS heterojunction. To be specific, the as-prepared Fe 2 O 3 /CdS film was dipped into the PdAg solution for 1 min and then dried by N 2 steam; this process was repeated 5 times. Then, the electrode decorated with Fe 2 O 3 / CdS/PdAg was annealed under an Ar stream at 500°C for 30 min (10°C/min). It was noted that the Pd:Ag ratio was controlled by the molar amount of each precursor.

Characterization
The phase information was characterized by X-ray diffraction with Cu Kα radiation. The morphologies and structures of the ternary composites were observed on a scanning electron microscope (Hitachi S-4800 field emission SEM, Tokyo, Japan) and a transmission electron microscope (TEM, JEOL, JEM 2100), respectively. X-ray photoelectron spectroscopy (VG ESCALAB XPS System with a monochromatized Al Kα X-ray source) was carried out to investigate the chemical state. UV-vis diffuse reflectance spectroscopy and absorption spectroscopy equipped with a DH-2000-BAL lamp (deuterium/helium) of wavelength ranging from 200 to 1,100 nm were used to characterize the optical properties and bandgap. The photoreduction of CO 2 was conducted in a home-made 100-ml Pyrex reactor with an optical window. The reactor was sealed using a rubber septum. A Xe lamp with 300 W and an AM 1.5-G filter were used as the simulated solar light. The samples were scraped from the FTO slides and sonicated for 1 h before the reaction. During the reaction, 1 ml of gas was acquired from the reactor at a specified interval (1 h), and subsequent gas concentration analysis was performed in a GC-2014C Shimazu equipped with a flame-ionization detector. The photoelectrochemical (PEC) performances were measured in an alkaline solution (0.25 M Na 2 S/0.35 M Na 2 SO 3 , pH = 13.1) using a standard three-electrode PEC cell, under the simulated sunlight (with AM 1.5-G filter). The sample, Ag/AgCl, and Pt wire were employed as the working electrode, reference electrode, and counter electrode, respectively. The potential vs.

RESULT AND DISCUSSION
The α-Fe 2 O 3 film was first grown on the FTO glass substrate, which exhibited the nanorod morphology in SEM images (see supporting information in Supplementary Figure S1). In detail, the as-prepared film presented a light orange color due to the formation of FeOOH after hydrothermal reaction and then turned red after annealing under air at 500°C in a muffle furnace; the thickness of α-Fe 2 O 3 was ca. 500 nm (Figure 2A) (Ahn et al., 2016;Yi et al., 2021). Subsequently, CdS was in situ synthesized on α-Fe 2 O 3 using the SILAR method (Sankapal et al., 2000). The CdS with a thickness of ca. 270 nm ( Figure 2B) was homogeneously coated on the α-Fe 2 O 3 nanorod, and the α-Fe 2 O 3 /CdS heterojunction film presented a crimson color. Furthermore, PdAg alloy NPs were obtained through coreduction of Pd and Ag precursors in an aqueous solution (Zhou et al., 2020). Then, a thin layer of PdAg 2:1 (the molar ratio between Pd and Ag is 2:1 in precursor) alloy NPs was coated above the CdS layer. The α-Fe 2 O 3 /CdS/PdAg on FTO was used as an electrode for photoelectrochemistry tests, and then Fe 2 O 3 / CdS/PdAg were scrapped from the substrate ( Figure 1D) for gas chromatography tests. XRD patterns (Supplementary Figure  S2A) of α-Fe 2 O 3 and CdS were confirmed to be indexed to JCPDS No. 33-0664 and 41-1049, respectively. Moreover, the two components, Fe 2 O 3 and CdS, were both in a hexagonal structure with slightly different lattice parameters (Zhang et al., 2013). PdAg NPs with an average particle size of 6.14 nm (calculated from 100 random particles) were found to be uniformly distributed on the α-Fe 2 O 3 /CdS (Supplementary Figure S2B). Furthermore, the spacing of the lattice fringe related to PdAg NPs was measured to be 2.48 nm between (111) Figure S3). X-ray photoelectron spectroscopy (XPS) tested the Pd 3 d and Ag 3 d when PdAg was coupled with α-Fe 2 O 3 /CdS, which presented a slight shift toward higher binding energy (Supplementary Figure S4). Moreover, the pristine PdAg NPs were further qualitatively confirmed by SCHEME 1 | The modulated CO 2 reduction process on single and dual metal sites.
Frontiers in Chemistry | www.frontiersin.org July 2022 | Volume 10 | Article 937543 energy-dispersive spectrum (EDS), where the atomic ratio between Pd and Ag in PdAg 2:1 was measured to be 62.1 and 37.9%, respectively (Supplementary Figure S2D). In addition, it is known that the successful connection between metal cocatalysts and photocatalysts would result in improved charge separation (Lan et al., 2021). Therefore, transient resolved photoluminescence (TRPL) was exploited to analyze the lifetime of charge after the decoration of PdAg, from which an enhanced lifetime (from 0.465 to 1.396 ns, Supplementary Figure S5) was observed, suggesting that the charge transfer was built between PdAg NPs and α-Fe 2 O 3 /CdS. The photoelectrochemical properties of α-Fe 2 O 3 , CdS, and α-Fe 2 O 3 /CdS were tested as an anodic electrode in a standard PEC cell. In accordance with a previous report (Shen et al., 2020), an improvement in the maximum photocurrent density and on-set potential was observed when coupling α-Fe 2 O 3 with CdS (Supplementary Figure S6). Next, to evaluate the influence of PdAg on the photoelectrochemical features, α-Fe 2 O 3 /CdS/PdAg was set as the cathodic electrode in the PEC cell. Among samples, the higher Pd content led to a lower on-set potential to trigger the cathodic photocurrent generation (Figure 2A), due to the higher work function of Pd (5.12) than Ag (4.26). The largest photocurrent density was observed from the sample with PdAg 1:2. This sample also presented smaller radii as revealed by EIS tests (Figures 2B,C) compared to others, suggesting lower electronic resistance in favor of charge transfer and thus enhancing the photocurrent generation. In addition, the stability test was executed by electrochemical characterization. The relative photocurrent displayed a slight decrease (ca. 0.1) after 10 min of reaction in Supplementary Figure S7, which can be attributed to the consumption of sacrificial agents and impeded the photoactivity. Combining with the XRD data shown in Supplementary Figure S8, the position of each peak presented no obvious change before and after the reaction, which certified that the catalyst possessed good stability during the photocatalytic reactions. As for the CO 2 reduction, gas chromatography (GC) confirmed the main products (CH 4 and CO) from the samples decorated with different Pd:Ag ratios. As shown in Figure 3, the gas production rates of the samples were proportional to their photocurrent density in PEC tests. The highest production rate was observed from the sample with PdAg 1:2. Meanwhile, compared to the control with pure Pd (6.62 μmol g −1 h −1 ), PdAg 1:2 (9.80 μmol g −1 h −1 ) presented a 48% enhancement in the total conversion rate. More importantly, as the modulating research predicted, by controlling the Pd:Ag ratio, the selectivity toward CH 4 was optimized from 58.7 to 83.2%.
As shown in Scheme 2, an interfacial charge region was constructed via an α-Fe 2 O 3 /CdS heterojunction, and a CO 2 reduction reaction occurred on the PdAg sites. In detail, the adsorbed CO 2 reacted with electrons accumulated on the PdAg sites to form CO and CH 4 , respectively (Ješić et al., 2021). The advantage of this ternary structure was a synergic effect between the Z-scheme α-Fe 2 O 3 /CdS and co-catalyst PdAg, which helped in prompting the selectivity of CO and CH 4 .

CONCLUSION
Overall, α-Fe 2 O 3 /CdS decorated by PdAg NPs of different Pd: Ag ratios was successfully prepared. The decoration of PdAg alloy NPs led to an enhanced photoreduction rate of CO 2 , but also ended up with an improvement in the selectivity toward CH 4 production. The improved photocatalytic activity was contributed by the accumulation of electrons on the PdAg sides, which resulted in a 48% enhancement in comparison to the pure Pd (6.62 μmol g −1 h −1 ). On the other hand, the selectivity was attributed to the dual-metallic active sites supplied by PdAg NPs, which conducted continuous protonation of C atoms in M 1 -C-O-M 2 intermediates. As a result, the sample with a ratio of PdAg 1:2 was in favor of producing CH 4 instead of CO, which was consistent with the best photocurrent and smallest radii.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding authors. SCHEME 2 | The band alignment of the α-Fe 2 O 3 /CdS/PdAg heterojunction and mechanism of photocatalytic CO 2 reduction.