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Article

BiVO4-Based Systems Magnetron Sputtered with Silver Nanoparticles for the Artificial Photosynthesis Reaction

by
Eva Naughton
1,
Emerson C. Kohlrausch
2,
Jesum Alves Fernandes
2 and
James A. Sullivan
1,*
1
School of Chemistry, University College Dublin, Belfield, D18 V1W8 Dublin, Ireland
2
School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK
*
Author to whom correspondence should be addressed.
Sustain. Chem. 2025, 6(1), 4; https://doi.org/10.3390/suschem6010004
Submission received: 3 December 2024 / Revised: 8 January 2025 / Accepted: 11 January 2025 / Published: 17 January 2025
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Abstract

The incorporation of Ag nanoparticles onto BiVO4 (a known H2O oxidising photocatalyst) through magnetron sputtering to form a composite was studied. ICP-OES results showed that the loading of Ag on BiVO4 was below 1% in all cases. UV-Vis DRS and CO2-TPD analyses demonstrated that upon incorporation of Ag onto BiVO4, an increase in the extent of visible light absorption and CO2 adsorption was seen. TEM imaging showed the presence of Ag particles on the surface of larger BiVO4 particles, while XRD analysis provided evidence for some doping of Ag into BiVO4 lattices. The effect of the composite formation on the activity of the materials in the artificial photosynthesis reaction was significant. BiVO4 alone produces negligible amounts of gaseous products. However, the Ag-sputtered composites produce both CO and CH4, with a higher loading of Ag leading to higher levels of product formation. This reactivity is ascribed to the generation of a heterojunction in the composite material. It is suggested that the generation of holes in BiVO4 following photon absorption is used to provide protons (from H2O oxidation), and the decay of an SPR response on the Ag NPs provides hot electrons, which together with the protons reduce CO2 to produce CH4, CO, and adsorbed hydrocarbonaceous species.

Graphical Abstract

1. Introduction

Currently, about 80% of global energy is generated through the combustion of fossil fuels such as coal, oil, and natural gas [1]. With projected population growth, energy demand is expected to rise further [2]. However, this continued reliance on fossil fuels will not only lead to their eventual depletion but also cause significant environmental harm, including localised air pollution, and the intensification of the greenhouse effect, contributing to global warming [3].
While CO2 (a greenhouse gas (GHG)) is naturally generated from fossilised materials at a very slow rate, human activities (e.g., fossil fuel combustion and land-use changes) have disrupted the carbon cycle, resulting in an excess accumulation of CO2 in the atmosphere [4].
One promising long-term strategy to reduce atmospheric CO2 emissions is carbon capture and utilisation (CCU), which involves capturing CO2 after its formation and using it to produce valuable chemicals, materials, or fuels [5]. However, because CO2 has strong carbon–oxygen (C=O) bonds, converting it into most other compounds is an endothermic process, requiring the input of energy [6].
Photocatalysis, which uses light to drive chemical reactions in the presence of a catalyst, has gained significant attention in recent years, particularly in environmental and sustainable chemistry applications like artificial photosynthesis (AP). The reaction of CO2 with water to produce carbon monoxide (CO) or hydrocarbons using a sustainable energy source is particularly appealing, as it could reduce atmospheric CO2 levels while generating renewable fuels or chemical precursors. Success in this area would also support the broader goal of establishing a circular carbon economy.
In this context, the use of semiconductor materials has attracted significant research interest, particularly in the field of photocatalysis and its application to AP [7,8,9,10,11,12].
To perform AP using a semiconductor photocatalyst, the catalyst must meet two key energetic criteria. First, the conduction band (CB) minimum must be at a higher energy level (more negative potential) than the energy required for CO2 reduction. Second, the valence band (VB) maximum must be at a lower energy level than the potential needed for oxygen (O2) evolution. As illustrated in Figure 1, while some semiconductors, such as TiO2 and TaNO, possess suitable band energies for catalysing AP, others do not. For example, BiVO4 lacks a CB minimum with sufficient energy to reduce CO2, and Si’s VB maximum is not low enough to effectively oxidise water (H2O). To overcome these limitations and enable AP, such materials may be paired with a ‘co-catalyst’ [13,14,15].
BiVO4 is a promising semiconductor for photochemical water splitting as it is stable, low cost, non-toxic, and has a moderate band gap (Eg) allowing it to absorb visible light [17]. The Eg of BiVO4 is approximately 2.46 eV (which equates to a visible light wavelength of 504 nm), with a valence band maximum of 2.86 V, and a conduction band minimum of 0.4 V vs. NHE at pH = 0 [18]. This means it has the potential to split water and act as an oxidation photocatalyst as the VB maximum lies below the oxidation potential of water. However, it cannot reduce CO2 as its CB energy is not sufficiently high.
In an attempt to overcome this limitation, in this work, BiVO4 was magnetron sputtered with Ag nanoparticles. The addition of noble metals to semiconductors is a well-studied method of increasing photocatalytic efficiency. Among the noble metals, silver is a popular choice due to its high photo-absorption and electrical conductivity, as well as the fact it is non-toxic [19]. Ag/BiVO4 composites have proven active in many different types of photocatalytic reactions. For example, Ag-doped BiVO4 thin films prepared by magnetron sputtering were 2.3 times more active in the photodegradation of Rhodamine B than pristine BiVO4 [20]. Hydrothermal synthesis of Ag-doped BiVO4 microspheres yielded a material that could photodegrade 76% of methylene blue in a solution in 6 h, compared to a photodegradation efficiency of approximately 40% over pure BiVO4 [21]. A BiVO4 material coated with 7 wt% graphene and 0.003 wt% particulate Ag exhibited photocatalytic activity more than five times that of a BiVO4 analogue in an AP reaction under flow conditions [22]. These increases in activity are attributed to the lower rates of electron/hole recombination in the Ag-containing systems due to the heterojunctions formed at the Ag-BiVO4 interfaces which lead to more effective charge carrier separation, the generation of plasmonic electrons that can be catalytically useful following absorption of visible light, and variations in the band structure of BiVO4 which may allow the material to both oxidise water and reduce CO2 [20,21,22,23].
Plasmonic nanostructures can function as co-catalysts alongside different semiconductors. According to Zhang et al., plasmonic nanoparticles enhance the transfer of excited electrons to acceptor molecules and reduce the recombination of photogenerated charges, leading to improved photocatalytic efficiency [24]. For example, the photocatalytic reduction of Cr(VI) by BiVO4 was greatly enhanced by the addition of Ag due to both the enhanced visible light absorption from the surface plasmonic resonance of Ag and the increased lifetime of the charge carriers. Photocatalytic electrons can easily transfer from BiVO4 to Ag at the metal–semiconductor interfaces, suppressing the recombination of electrons and holes [25]. Similar results have been seen when Ag was added to BiVO4 for the photocatalytic degradation of 2,4-dichlorophenoxyacetic acid [26], and when Ag was added to g-C3N4 for photocatalytic H2 evolution [27]. Previously in our lab, the addition of plasmonic RuO2 was shown to increase the reactivity of various semiconductors when acting as a co-catalyst in the AP reaction [28,29].
Sputter deposition is a widely used technique employed to deposit thin films on substrates. Sputtering is a technique where ion bombardment of a source material (the target, Ag in this case) results in the ejection of surface atoms which are then deposited onto a second material (the substrate, BiVO4 in this case). Magnetron sputtering uses plasma to complete ion bombardment. The term “magnetron” arises due to the use of magnets to confine the plasma’s electrons near the surface of the target. This leads to a high density of the plasma which increases the deposition rate but also prevents damage to the substrate from ion bombardment. The advantages of using magnetron sputtering to deposit metals onto substrates over other techniques is that we can easily get uniformity, it is fast, cheap, needs only low temperatures for operation, and a high purity can be obtained.
While Ag/BiVO4 and similar composites are widely reported in the literature for various reactions, including AP, to the best of our knowledge, no studies have explored the preparation of Ag/BiVO4 composites via magnetron sputtering for the AP reaction. Existing research on Ag/BiVO4 materials prepared by magnetron sputtering primarily focuses on their use in the photodegradation of organic dyes [20,23]. In contrast, for AP or related photocatalytic reactions, other synthesis methods such as hydrothermal [30,31], photodeposition [22], and electrodeposition [32] are typically employed.
This research looks at Ag/BiVO4 materials prepared by magnetron sputtering of BiVO4 with Ag to hold two different loadings of Ag particles. The loading of Ag NPs was changed by varying the sputtering current and these materials were characterised and then tested in the AP reaction to see if the above-mentioned reaction-enhancing mechanisms occur, leading to higher CO2 conversions and photocatalytic efficiencies.

2. Materials and Methods

2.1. Materials

Bismuth nitrate pentahydratea (Bi(NO3)3·5H2O) (99% pure), ammonium metavanadatea (NH4VO3) (99% pure), ethylenediaminetetraacetic acidb ([CH2N(CH2CO2H)2]2) (99% pure), sodium hydroxidec (NaOH) (99% pure), and nitric acidc (HNO3) (69%) were purchased from Flukaa (Fluka Chemicals Ltd, Gillingham, Dorset SP84JL, UK), Sigma-Aldrichb (Vale Road, Arklow, Co. Wicklow, Y14 EK18, Ireland), and Riedel de Haënc (Wunstorfer Str. 40, 30926 Seelze, Germany) and used without further purification. Deionised water was used throughout. (The superscript letters (a, b, c) indicate the supplier for each chemical: (a) Fluka, (b) Sigma-Aldrich, and (c) Riedel-de Haën).

2.2. Methods

2.2.1. Synthesis of Photocatalysts

BiVO4 was synthesised according to the preparation methods reported by Bai et al. [33] and Min et al. [17]. BiVO4 was firstly synthesised by a facile hydrothermal method. Bi(NO3)3·5H2O (2.42 g, 4.99 × 10−3 moles) was added to 2M nitric acid (2 mL, 4 × 10−3 moles) under constant stirring (denoted as solution A). Then, NH4VO3 (0.58 g, 4.96 × 10−3 moles) was dissolved in a solution of 4M NaOH (10 mL, 0.04 moles) and EDTA (1 g, 3.42 × 10−3 moles), which was denoted as solution B. EDTA can complex with metal cations in solution to form stable water-soluble complexes. Then, solution B was added dropwise to solution A under vigorous stirring, and the pH of the solution was adjusted to pH 7 using a HNO3 solution (2M). Finally, the obtained mixed solution was sealed in a Teflon-lined stainless steel autoclave and heated for 10 h at 180 °C. The resulting solid was collected by filtration and washed with water.
Silver was then deposited onto the surface of the BiVO4 catalyst using magnetron sputtering. The instrument used in this work was an ATC-Orion 5 UHV with load-lock by AJA International, Inc. Approximately 2.0 g of catalyst was placed in a glove box connected to the load-lock prior to deposition. The catalyst was then placed in the sample holder and loaded into the load-lock (which has a pressure of approximately 6 × 10−6 torr). The sample was held there for 20 min before being transferred to the main chamber for sputter deposition. This process was to ensure that any adsorbed species on the substrate surface were desorbed. In the chamber, the sample holder was rotated in order to get a uniform coating of the target on the powder substrate. DC sputtering was used. The working distance between the target and the substrate was 120 mm. The power used was 33 W, the potential was 333 V, the flow of Ar was 20 mL/min, and the deposition time was 30 min for each sample. To get a higher loading of Ag on the samples, the current was altered. The different currents used were 100 and 250 mA. The formed composite samples were labelled Ag/BiVO4 100 and Ag/BiVO4 250.
The catalyst characterisation methods and parameters can be found in the Supplementary Materials.

2.2.2. Photocatalytic Reactions

The catalytic performances of the materials were analysed under batch-reaction conditions using simulated solar light as the energy input (using an Atlas Suntest CPS+ solar simulator unit equipped with a Xe arc lamp with UV filters). The experiments were carried out with the reactant species (CO2 and H2O) in the gas phase. All the gas flows to the reactor were controlled and adjusted using previously calibrated mass flow controllers.
Approximately 20 mg of catalyst were loaded into a Pyrex reactor, and the reactor was placed in a furnace in the dark. The temperature was increased to 110 °C for 30 min under a flow of argon (100 mL/min) to ensure surface cleanliness. The temperature was then lowered to 50 °C, and the rate of Ar flow was lowered to 45 mL/min.
A gas mixture of water vapour and argon was produced by flowing pure Ar (at a rate of 50 mL/min) through a gas bubbler vessel containing water at 60 °C. This mixture of Ar and H2O(g) was then combined with a CO2 flow (5 mL/min), and the total mixture passed over the material for 1 h. The composition of the gas mixture was as follows: Ar (91.4% v/v), CO2 (5% v/v), and H2O (3.6% v/v). The system was then allowed to cool to 25 °C, the taps closed to isolate the reactant gases, and the reactor placed in the solar simulator. Gas aliquots were extracted periodically using a 500 μL Swagelok™ gastight syringe. A Varian 450-GC equipped with TCD and FID detectors was used to identify and quantify the gaseous products evolved during the photocatalytic tests.

3. Results and Discussion

3.1. Characterisation

Inductively coupled plasma combined with optical emission spectroscopy (ICP-OES) was used to determine the loading of Ag NPs deposited onto the semiconductor materials following the sputtering process. The results of this analysis are displayed in Table 1.
From Table 1, we can see that both Ag-containing samples have a wt. % of Ag below 1%. This was desired as, according to the literature, high loadings of Ag NPs on these semiconductors can decrease reactivity of the generated composites in photocatalytic reactions by blocking active sites on the surface of the semiconductor and acting as recombination centres which reduces charge carrier separation efficiency [22,34,35,36].
The results of this analysis show that, as expected, when the current used during the sputtering process increased from 100 mA to 250 mA, so too did the mass of Ag NPs deposited on the materials. We should then, in turn, be able to measure how these different loadings affect the photocatalytic activity of these materials in the AP reaction.
Ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) was used to study the light absorption characteristics of these materials and estimate the band gap of BiVO4 before and after sputtering with silver. The spectra are shown in Figure 2.
Each material absorbs strongly in the UV and visible regions of the electromagnetic spectrum. This was expected as the colour of the samples goes from a yellow to a dark green (see Table 1). This agrees with the literature [37]. Pure BiVO4 absorbs the least intensely out of the materials at longer wavelengths. However, upon the incorporation of Ag NPs onto the surface of the materials, the extent of visible light absorption increases greatly in the 800–500 nm (visible) region, with the Ag/BiVO4 250 sample absorbing more intensely than the 100 mA analogue. The spectra of the sputtered materials show an additional peak at ~600 nm (highlighted in the plot). This is due to the plasmonic absorption by Ag NPs [38]. This SPR band is attributed to longitudinal oscillation of electrons on the NP surface [39], i.e., a surface plasmon.
Tauc plots were used to determine the band gaps of these materials. The results of this analysis are displayed in Table 2. The band gap of BiVO4 was determined to be 2.49 eV. This agrees with related calculations previously shown in the literature [40,41].
Furthermore, these measurements showed that there is a relatively small apparent decrease in the band gaps of the materials following sputtering of silver onto the surfaces of the semiconductors. The more silver that is deposited on the BiVO4 surface, the greater the apparent reduction in the band gap. Shifting of the VB and CB positions of BiVO4 following doping of Ag into the BiVO4 lattice has been reported where Ag-doping is reported to have moved the positions of both the CB and VB to higher energies (measured using VB-XPS and UV-Vis DRS techniques) [21]. Therefore, this is a possibility that may allow for BiVO4 to (i) absorb more of the visible solar light, and (ii) increase the reduction potentials of the excited electrons in its CB following photon absorption.
Powder X-ray diffraction (XRD) was carried out to confirm the crystalline phases of the materials by comparing their diffraction profiles to reference profiles. The diffractograms of BiVO4, the Ag-sputtered materials, and reference profiles of Ag0 and BiVO4 are displayed in Figure 3.
From Figure 3, we can see that the profile of BiVO4 matches that of the BiVO4 reference profile (JCPDS 83-1698) very well. The sharp peaks indicate high crystallinity of the materials and suggest a large particle size. The BiVO4 materials show several characteristic peaks, for example at 18.9°, 29.1°, and 30.5°, which correspond to reflections from the (011), (121), and (040) planes, respectively [42].
No peaks relating to diffractions from Ag0 were observed in the profiles of the composite. This was to be expected as the loading of Ag on the BiVO4 surfaces is lower than the limit of detection of XRD (~5%). Interestingly, there was a negative shift of 0.28° to lower diffraction angles between the peak maxima in profiles of BiVO4 and the same peaks in the Ag-modified analogues. This suggests that some Ag was substitutionally doped within the BiVO4 lattice. When the dopant atoms are larger in size than those they replace in the lattice (Ag0 has an atomic radius of 144 pm, while Bi3+ has an ionic radius of 103 pm [43]), this leads to expansion of the lattice which causes peak shifts to lower angles. This small shift to lower angles of 2θ upon doping of BiVO4 with Ag has been reported in the literature [44]. As Ag is doped within the BiVO4 lattice, this may lead to altering of the band positions which may affect the activity of these materials in the AP reaction.
Transmission electron microscopy (TEM) images of the pure and modified BiVO4 materials were obtained. These were used to gain information on the morphology and sizes of the particles and provide evidence for the existence of Ag NPs on the BiVO4 surfaces. The average particle sizes of BiVO4 and Ag were estimated using ImageJ analysis.
From Figure 4a, we can see that the BiVO4 particles are large and non-uniform in size and shape. The addition of smaller silver particles following magnetron sputtering generates semiconductor:Ag interfaces which may aid reactivity.
In the TEM images of the Ag-sputtered samples (Figure 4b,c), smaller particles were seen on the surface of the larger BiVO4 particles. These were not observed in the TEM images of the unmodified BiVO4 sample and, therefore, are likely Ag particles. The interfaces between the two materials may allow for the transfer of electrons between the two materials, generating heterojunctions and increasing the lifetime of the charge carriers. This should promote reactivity.
Overall, this analysis showed that BiVO4 particles are large (~683 nm) and, following sputtering, smaller Ag particles (23 nm) are seen decorating the surface of BiVO4, leading to the generation of interfaces between the two materials which may promote photocatalysis.
Carbon dioxide temperature programmed desorption (CO2-TPD) profiles are displayed in Figure 5, and the amounts of CO2 desorbed from each material are tabulated in Table 3. Each material showed several desorption events with varying CO2(ads) stabilities and CO2(ads) concentrations. Following deconvolution of the TPD profiles and analysis of the peak areas, it was determined that only 10 μmol of CO2 desorbed per gram of BiVO4. This is likely due to the large particle size and the accompanying low surface area of the material (see BET, Table 4). This result agrees with the literature where CO2-TPD from BiVO4 nanosheets showed that BiVO4 adsorbed negligible amounts of CO2 [45].
The shape of the TPD profiles from the composite materials differed greatly from the CO2 desorption profile from pure BiVO4, suggesting that the contribution of Ag to the adsorption/desorption process was significant. The addition of Ag led to the development of strong CO2 binding sites. This is evident as the two composite profiles exhibited large desorption peaks centred at approximately 585 °C. This peak was not present in the CO2 desorption profile from BiVO4. The area under this peak increases slightly as the loading of Ag in the composites increases, showing that the adsorption site is either on the Ag particles or at Ag/BiVO4 interfaces. Studies have shown that Ag NPs can adsorb CO2, especially when the Ag has been exposed to oxygen. The O atoms on the surface of Ag can interact with CO2(g) to form carbonate species [46,47,48]. As these materials were not stored in a protective or inert environment, it is probable that atmospheric oxygen had adsorbed on the surface of the materials, increasing their basicity and, in turn, their ability to adsorb CO2. However, as the proportion of silver on the surface of BiVO4 is low (<1%), the increase in the extents of CO2 adsorption between the samples is not large. Nevertheless, the trend is there.
Furthermore, as the concentration of Ag increases, the BiVO4-derived CO2 adsorption sites (seen at 350–500 °C) became less important and we see desorption from weaker CO2 adsorption sites (peaking at 300–400 °C). This is likely due to the blocking of surface sites on BiVO4 by Ag, but also the generation of new sites at the interfaces. In addition to this, the concentration of CO2 that can be adsorbed increases. Therefore, the addition of Ag leads to the generation of a higher number of weaker CO2 adsorption sites compared to those on pure BiVO4.
From Table 3, we can see that the composites desorb over three times more CO2 than BiVO4. This, along with the increased light absorption of these materials (see Figure 2), should result in higher photocatalytic activity in the AP reaction.
Figure S2 shows the Brunauer–Emmett–Teller (BET) surface area plots collected from the analysis of BiVO4 and the two Ag/BiVO4-sputtered samples. These plots were used to determine the BET surface area of the samples (presented in Table 4).
The surface area of pure BiVO4 was found to be 1.46 m2/g. This low surface area was to be expected when considering the TEM and CO2-TPD results. The surface area of the BiVO4 samples before and after silver sputtering remained unchanged despite the addition of smaller particles to the composite (seen in TEM, Figure 4, and the CO2-TPD analysis, Figure 5).

3.2. Photocatalytic Activity Measurements

The photocatalytic activity of these materials in the AP reaction was measured. The results of these experiments are displayed in Figure 6 and the production of CO(g) and CH4(g) over each material is displayed in Table 5. It should be noted that no products resulted from reactions that were carried out (i) in the dark or (ii) in the absence of catalyst. Additionally, a longer experiment was conducted with Ag/BiVO4 250, where the material was exposed to irradiation for 72 h. However, this extended exposure did not produce significantly higher levels of reduced CO2 products, indicating that a plateau was reached after 24 h. As a result, subsequent experiments were limited to a maximum duration of 24 h.
Since gaseous CO2 is initially present in the reactor at time = 0, the concentration at this point is considered to be 0 μmol/g. Any subsequent increase in the levels of CO2 in the reactor results from the desorption of CO2(ads) from the catalysts’ surfaces, or from the oxidation of reduced hydrocarbonaceous products. Figure 6c) shows the plot of CO2 levels in the reactor over time. The CO2 “evolution” from BiVO4 was very low, reflecting the results from the TPD analysis where BiVO4 only adsorbed/desorbed approximately 10 μmol/g. The other two samples show higher levels of CO2 desorption with time, again in line with the TPD results. As we can see in Figure 6d), the O2 levels over time are not consistent with the CO and CH4 evolution levels, i.e., the levels of O2 are not 50% the levels of CO as would be expected from CO2 decomposition reactions (CO2 → CO + 0.5O2). This suggests that perhaps some adsorbed carbon-containing CO2 reduction products also remained adsorbed on the surfaces of the materials and were not detected using GC analysis.
The levels of O2 evolved over the Ag-containing-sputtered composites were much higher than those generated over pure BiVO4. This shows that the modified BiVO4 materials are more active than the unmodified analogue in the AP reaction (O2 being a byproduct of AP). However, the evolution of O2(g) may also reflect decomposition of Ag2O. It has been reported that Ag2O can decompose into Ag and O2 following absorption of photons [49,50].
The carbon-containing gaseous product that was produced to the greatest extent over the Ag/BiVO4 250 material was CO. As the level of Ag in the composite increased, so too did the levels of produced CO. This shows that the more “hot” electrons being produced (from excitation of the greater loading of plasmonic Ag), the more CO2 molecules were reduced. CO generation was also observed (albeit to a much lesser extent) over pure BiVO4. Under artificial photosynthesis conditions, BiVO4 has been shown to produce minor amounts of both CO and CH3OH from CO2 and H2O [51]. No methanol was observed here; however, Figure S3 shows that BiVO4 post-reaction lost more mass during a TGA experiment compared to the mass lost from a pre-reaction sample, suggesting the presence of combustible surface-adsorbed moieties following reaction, as this extra mass loss occurred between 200 and 300 °C (i.e., the temperature range at which adsorbed hydrocarbonaceous species oxidise/combust).
Possible reasons for the production of CO over pure BiVO4 (despite the fact that the CBm is not high enough in energy) could be that upon adsorption of CO2, interactions between the semiconductor and the CO2 π-orbitals lead to a decrease in the reduction activation energy, or perhaps this is merely due to a reaction between CO2 and surface-adsorbed oxygens and hydroxyl groups on BiVO4. Whatever the reason, the level of evolved CO is low, and the conversions carried out over the sputtered materials are higher.
CH4 production was also observed over the composites. This was not noted over unmodified BiVO4. Therefore, we can conclude that the “hot” electrons from Ag play an important role in this reaction, and the mechanism depicted in Figure 7 below may be at play. These hot electrons, combined with the protons produced from water splitting over BiVO4, produce CH4. However, over the material with the higher loading of Ag, the production of CH4 fell. This may be because as less BiVO4 is exposed to reactants (as proportionately more is covered with Ag nanoparticles), there are fewer protons produced and available for this reaction. This agrees with previous reports on the reactivity of Ag/BiVO4 photocatalysts for CO2 reduction using LED light sources [31].
CO was produced in greater quantities than CH4. Thermodynamically, the production of CO is more energy demanding than that of CH4 (see Figure 1). The reduction of one molecule of CO2 may involve as many as eight protons and electrons, the cleavage of C=O bonds, and/or the formation of C-H bonds [52]. Proton-assisted multiple electron transfers are considered more favourable for CO2 reduction reactions [53]. However, CO formation requires only two proton and electron transfers, whereas CH4 formation requires eight (see equations below), which is more challenging from a kinetic point of view [54].
CO2(g) + 8H+ + 8e− → CH4(g) + 2H2O
CO2(g) + 2H+ + 2e− → CO(g) + H2O
These reactions are very slow, with the materials requiring approximately 4–6 h of irradiation before any product is observed. Therefore, it is possible that these reactions produce CO as it is kinetically less challenging to produce than CH4. The greater production of CO(g) compared to CH4(g) over Ag/BiVO4 materials is also seen in a study which utilised hydrothermally prepared triangular silver nanoplate-BiVO4 composites for photocatalytic CO2 reduction under LED light sources [31].
Overall, these results show that combining Ag nanoparticles with BiVO4 is beneficial to the reactivity as Ag/BiVO4 100 and Ag/BiVO4 250 showed an increase in reactivity for the production of CO(g) and CH4(g) by a factor of 5.2 and 6.3, respectively, compared to the reactivity of pure BiVO4.

3.3. Post-Reaction Characterisation

Infrared (IR) analysis was carried out post-reaction on the sputtered samples to determine whether some adsorbed carbon-containing products had been formed over the Ag-containing materials during the reaction. These are displayed below in Figure 8.
The pre-reaction spectra match those seen in the literature [55,56]. Several characteristic peaks can be observed such as those at 475, 600, and 835 cm−1, which correspond to the symmetrical stretches of VO43−, Bi-O-Bi, and V-O, respectively.
The presence of adsorbed hydrocarbonaceous species is confirmed by the post-reaction IR spectra which exhibit peaks relating to C-H vibrations that were not present in the spectrum of the material before the reaction. This includes a peak between 3000 and 2840 cm−1, which is due to C-H stretching, and another at 1400 cm−1, which corresponds to C-H bending. Additionally, a further peak at 1070 cm−1 also appears. The C-O stretch of a primary alcohol typically appears between 1050 and 1080 cm−1, suggesting methanol or higher alcohols may have been formed and adsorbed onto the surface of the catalyst during the reaction.
This result suggests that not only did these materials produce gaseous CO and CH4, as detected by GC analysis, but also unidentified adsorbed hydrocarbonaceous materials.

4. Conclusions

BiVO4 was prepared using hydrothermal synthesis methods. Ag particles were magnetron sputtered onto the BiVO4 surface at two different Ag loadings. ICP-OES was used to determine the % loading of Ag on the two samples. Powder XRD gave confirmation of the synthesis of BiVO4 and showed that although the loading of Ag was too low to be detected using XRD, there was some evidence of Ag doped into the BiVO4 lattice. UV-Vis spectroscopy was used to calculate the band gaps of the materials and show how the absorption of light increases following incorporation of Ag onto (and into) BiVO4. Furthermore, the presence of the SPR band in the UV-Vis DRS spectra suggests that the Ag is present predominantly in its metallic form (although XPS would be needed to confirm this). CO2-TPD experiments showed that incorporating Ag NPs onto the surface of BiVO4 led to higher levels of desorbed (and, hence, adsorbed) CO2.
Finally, the AP results demonstrated that sputtering BiVO4 with Ag led to an increase in the photocatalytic activity of the materials compared to the reactivity of BiVO4 alone. Both CO and CH4 were produced by the composites, showing a synergistic relationship between the two materials where BiVO4 can split water to provide protons and Ag particles can produce plasmonic “hot” electrons which can be used to reduce CO2. Future experiments to further elucidate the mechanism of reaction would include irradiating with a monochromatic light source with insufficient energy to excite the Ag plasmon in order to confirm the role of “hot” electrons in the AP reaction.

Supplementary Materials

The following supporting information can be downloaded at: www.mdpi.com/article/10.3390/suschem6010004/s1, Catalyst Characterisation Methods and Parameters, Figure S1: Particle size distribution histograms across all three samples of BiVO4 (left) and Ag (right), Figure S2: BET plots for BiVO4 and the Ag-sputtered materials, Figure S3: TGA profile of BiVO4 pre- and post-AP reaction. References [32,56] is cited in the Supplementary Materials.

Author Contributions

Conceptualisation, E.N. and J.A.S.; methodology, E.N., E.C.K. and J.A.F.; formal analysis, E.N.; investigation, E.N.; data curation, E.N.; writing—original draft preparation, E.N.; writing—review and editing, E.N. and J.A.S.; supervision, J.A.S.; funding acquisition, J.A.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by funding from Science Foundation Ireland under grant number 16/RC/3889 and was co-funded under the European Regional Development Fund and by BiOrbic industry partners.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors thank Julia Bruno for collection of XRD profiles, Ben Young for his help with ICP-OES, and Wolfgang Schmitt and Sebastien Vaesen for obtaining the BET plots. The authors also thanks the Engineering and Physical Sciences Research Council (EPSRC) Programme Grant ‘Metal Atoms on Surfaces and Interfaces (MASI) for Sustainable Future’ (EP/V000055/1) for the financial support.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Suschem 06 00004 g001
Figure 1. Valence (VB) and conduction (CB) band-edge positions of some semiconductors relative to the energy levels of the redox couples involved in the reduction of CO2 and oxidation of H2O vs. NHE at pH = 7. Values for CBm and VBm obtained from Tamirat et al. [16].
Figure 1. Valence (VB) and conduction (CB) band-edge positions of some semiconductors relative to the energy levels of the redox couples involved in the reduction of CO2 and oxidation of H2O vs. NHE at pH = 7. Values for CBm and VBm obtained from Tamirat et al. [16].
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Figure 2. UV-Vis DRS spectra of the silver-sputtered BiVO4 materials (top left) and their Tauc plots.
Figure 2. UV-Vis DRS spectra of the silver-sputtered BiVO4 materials (top left) and their Tauc plots.
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Figure 3. XRD profiles of the sputtered BiVO4 materials, and their reference profiles (left), and peak shifts between the profiles of unmodified BiVO4 and the Ag-sputtered samples (right).
Figure 3. XRD profiles of the sputtered BiVO4 materials, and their reference profiles (left), and peak shifts between the profiles of unmodified BiVO4 and the Ag-sputtered samples (right).
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Figure 4. TEM micrographs of (a) BiVO4, (b) Ag/BiVO4 100, and (c) Ag/BiVO4 250, where the red circles highlight Ag NPs present on BiVO4 surfaces. Particle size distribution histograms can be found in the Supplementary Material (Figure S1).
Figure 4. TEM micrographs of (a) BiVO4, (b) Ag/BiVO4 100, and (c) Ag/BiVO4 250, where the red circles highlight Ag NPs present on BiVO4 surfaces. Particle size distribution histograms can be found in the Supplementary Material (Figure S1).
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Figure 5. CO2-TPD profiles from all three materials.
Figure 5. CO2-TPD profiles from all three materials.
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Figure 6. Gaseous reactant and product evolution vs. time over the BiVO4 and Ag/BiVO4 catalysts, (a) CH4, (b) CO, (c) CO2, and (d) O2.
Figure 6. Gaseous reactant and product evolution vs. time over the BiVO4 and Ag/BiVO4 catalysts, (a) CH4, (b) CO, (c) CO2, and (d) O2.
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Figure 7. Proposed mechanism of a photocatalytic redox reaction over Ag/BiVO4, where “A” is an electron acceptor, “D” is an electron donor, and “VB” and “CB” are valence and conduction bands, respectively.
Figure 7. Proposed mechanism of a photocatalytic redox reaction over Ag/BiVO4, where “A” is an electron acceptor, “D” is an electron donor, and “VB” and “CB” are valence and conduction bands, respectively.
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Figure 8. Pre- and post-reaction IR spectra of the two Ag-sputtered BiVO4 samples.
Figure 8. Pre- and post-reaction IR spectra of the two Ag-sputtered BiVO4 samples.
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Table 1. Wt % loadings of Ag determined from ICP analysis.
Table 1. Wt % loadings of Ag determined from ICP analysis.
SampleWt. % AgColour
BiVO40Yellow
Ag/BiVO4 1000.26Khaki
Ag/BiVO4 2500.73Dark Green
Table 2. Band gaps of BiVO4 and the Ag-sputtered materials.
Table 2. Band gaps of BiVO4 and the Ag-sputtered materials.
SampleBand Gap (eV)
BiVO42.49
Ag/BiVO4 1002.47
Ag/BiVO4 2502.46
Table 3. Concentrations of desorbed CO2 from each material.
Table 3. Concentrations of desorbed CO2 from each material.
Sample[CO2] (μmol/g)[Ag] (moles/20 mg)
BiVO4100
Ag/BiVO4 100324.8 × 10−7
Ag/BiVO4 250341.4 × 10−6
Table 4. BET surface areas of each material.
Table 4. BET surface areas of each material.
SampleSurface Area (m2/g)
BiVO41.46
Ag/BiVO4 1001.45
Ag/BiVO4 2501.45
Table 5. Accumulated production of CO and CH4 over each prepared material.
Table 5. Accumulated production of CO and CH4 over each prepared material.
SampleCO/μmol g−1CH4/μmol g−1
BiVO40.950
Ag/BiVO4 1002.492.49
Ag/BiVO4 2505.190.65
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Naughton, E.; Kohlrausch, E.C.; Alves Fernandes, J.; Sullivan, J.A. BiVO4-Based Systems Magnetron Sputtered with Silver Nanoparticles for the Artificial Photosynthesis Reaction. Sustain. Chem. 2025, 6, 4. https://doi.org/10.3390/suschem6010004

AMA Style

Naughton E, Kohlrausch EC, Alves Fernandes J, Sullivan JA. BiVO4-Based Systems Magnetron Sputtered with Silver Nanoparticles for the Artificial Photosynthesis Reaction. Sustainable Chemistry. 2025; 6(1):4. https://doi.org/10.3390/suschem6010004

Chicago/Turabian Style

Naughton, Eva, Emerson C. Kohlrausch, Jesum Alves Fernandes, and James A. Sullivan. 2025. "BiVO4-Based Systems Magnetron Sputtered with Silver Nanoparticles for the Artificial Photosynthesis Reaction" Sustainable Chemistry 6, no. 1: 4. https://doi.org/10.3390/suschem6010004

APA Style

Naughton, E., Kohlrausch, E. C., Alves Fernandes, J., & Sullivan, J. A. (2025). BiVO4-Based Systems Magnetron Sputtered with Silver Nanoparticles for the Artificial Photosynthesis Reaction. Sustainable Chemistry, 6(1), 4. https://doi.org/10.3390/suschem6010004

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