Synthesis of Ag/Cu decorated 3D self-assembled nanowire TiO2 Photocatalyst for Hydrogen Production: A Promising Pathway towards Sustainable Energy Generation

doi:10.21203/rs.3.rs-4503595/v1

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Synthesis of Ag/Cu decorated 3D self-assembled nanowire TiO2 Photocatalyst for Hydrogen Production: A Promising Pathway towards Sustainable Energy Generation

https://doi.org/10.21203/rs.3.rs-4503595/v1

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published 11 Oct, 2024

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Here synthesis and characterization of TiO₂ with different morphologies along with the cost-effective bimetallic decoration on optimized 3D self-assembled nanowire TiO₂ (NWT) photocatalyst (Ag/Cu-NWT) with overwhelming hydrogen production rate is reported. All the photocatalysts were well characterised by different characterization techniques. Initially, the effect of morphology change obtained by changing the NaOH concentration has been studied for TiO₂. Morphology obtained at 10 M NaOH solution i.e., NWT (678 μmol/g) showed better hydrogen production than morphology obtained at 5M (410 μmol/g), 15M (210 μmol/g) and 20M (160 μmol/g) NaOH solutions. Further with the aim to achieve comparable or better activity low cost photocatalyst as compared to Pt-TiO₂ system, NWT was decorated with various Cu percentages and then with minimal percentage of Ag on optimized Cu-NWT photocatalyst. The observed trend for photocatalytic hydrogen production has been found to be P25 TiO₂ < NWT < 1.0Cu-NWT < 0.5Pt-NWT ≤ 0.1Ag/1.0Cu-NWT. The marked increase by a factor of 103 in hydrogen production for the optimized bimetallic 0.1Ag/1.0Cu-NWT (10,184 μmol/g) photocatalyst compared to P25 TiO₂ (99 μmol/g), nearly threefold increment in hydrogen production than optimized 1.0 Cu-NWT (3,907 μmol/g) photocatalyst and comparable hydrogen production as compared to 0.5Pt-NWT (10,050 μmol/g) may be attributed to the successful synthesis of a highly porous NWT morphology, which offers large surface area, increased light absorption combined with the synergistic effects of surface plasmon resonance (SPR) and the Schottky barrier for H⁺ reduction to H₂ gas. The optimization of TiO₂ morphology and inexpensive bimetallic decoration strategy opens up promising opportunities for the development of cost-effective photocatalysts in the realm of energy and environment.

Photocatalytic hydrogen production

titania

photocatalysis

morphological study

SPR effect

nanowire

Synthesis of Ag/Cu-NWT photocatalysts for photocatalytic H₂ production.
3D hierarchical TiO₂ showed 2-fold surface area and 7-fold HER activity than P25.
Cost effective co-metallic decoration was better choice over costly Pt system.
Schottky junction, SPR and synergetic effect of Ag and Cu improved H₂ production.

Entire world is looking for a sustainable and alternative clean energy source in anticipation to the future high energy demand and to mitigate the issues of global warming and climate change (Ni et al. 2007; Navarro Yerga et al. 2009). The environmentally friendly nature of hydrogen as a green fuel, along with its comparable potential to fossil fuels when generated through the photocatalytic water splitting techniques has created significant interest in recent years in research community thereby presenting a feasible approach for its facile production using renewable and virtually inexhaustible solar energy. (Navarro et al. 2009; Melo and Silva 2011; Jafari et al. 2016; Baykara 2018). Although TiO₂ is a benchmark photocatalyst among various semiconductors due to its magnificent properties viz. chemical inertness, environmental compatibility, low cost, photoactivity, and excellent photostability, it has critical limitations of fast electron/hole recombination and limited photoconversion efficiency (Riegel and Bolton 1995; Ni et al. 2007; Amano et al. 2016). Therefore, it is indispensable to map out strategic approach for the betterment of TiO₂ in terms of its impediments to make it a powerful tool for technology transfer. The very first simplest approach could be a suitable morphology frame up resulting in high surface area and improved light absorption as compared to commercially available P25 TiO₂ (Amano et al. 2016). Thus to design the desired photocatalyst with favourable crystal structure and morphology for improved photoconversion efficiency has become crucial important (Bhatt and Lee 2017; Clarizia et al. 2017). The solvent and the reaction time are the key factors in determining the morphologies and dimensions of the anatase TiO₂ nanostructures (Padmanabhan et al. 2020). Das et al. demonstrated the tunability of the structures, sizes, and morphologies of anatase TiO₂ products by manipulating the reaction temperature and time, as well as utilizing mixed solvents with varying compositions (Das et al. 2008). Alshehri et al. investigated the photocatalytic reformation of methanol to hydrogen using PtOx (2.0 wt.%) deposited TiO₂ anatase with nanoparticles, nanotubes, and nanofibers morphology. Many researchers have reported that TiO₂ anatase nanotubes exhibit exceptional photocatalytic activity (Alshehri and Narasimharao 2020). A 3D Pt/TiO₂ architecture composed of numerous 1D TiO₂ nanowires was synthesized by Li et al. using a one-pot solvothermal method, with a photocatalytic hydrogen production rate of 13.33 mmol h^− 1 g^− 1. This excellent performance can be attributed to the cumulative effects of the unique TiO₂ architecture, allowing enhanced light absorption depths along with the presence of noble metal decoration (Li et al. 2015a).

Recently literature also reports the decoration of bimetallic nanoparticles to improve the photocatalytic activity of TiO₂ (Zielińska-Jurek 2014). Liu et al. have provided a comprehensive review of various bimetallic cocatalyst systems, including bi-noble and non-noble metal cocatalysts, plasmonic and non-plasmonic bimetallic cocatalysts, among others, for photocatalytic hydrogen production from water. The review aimed to underscore the synergetic effects within bimetallic systems and elucidate the impact of factors such as elemental composition, overall metal loading, structure, particle size, and more (Liu et al. 2021). Decorating semiconductor photocatalysts with cocatalysts has been emerged as a potent strategy to mitigate the recombination of photo-excited electron-hole pairs, minimize overpotential for redox reactions, enhance visible light absorption, suppress reverse reactions, and ultimately improve the overall efficiency of photocatalysis (Zhang et al. 2014; Gao et al. 2017; She et al. 2020).

Although noble metallic or noble bi-metallic cocatalyst systems such as Pt/Au (Shuang et al. 2016; Grabowska et al. 2016; Gołąbiewska et al. 2017), Pd/Au (Xin et al. 2015; Zhang et al. 2015b; Han et al. 2015; She et al. 2020), etc show high performance but their towering costs and inadequate reserves restrict their applications for large scale commercialization in future. Thus, one of the efficacious strategies would be to use a combination of noble metal with a low-cost and abundantly available non-noble transition metals, such as Ni (Tian et al. 2015), Co (Iwasaki et al. 2000; Sadanandam et al. 2013), Cu (Zhang et al. 2015a; Mani et al. 2024; Zou et al. 2024) and Sn (Gao et al. 2019) as cocatalysts without compromising with the performance for hydrogen production. The current emphasis on cost-effective and highly efficient noble and non-noble metal bimetallic cocatalyst systems, as opposed to monometallic cocatalysts, underscores the remarkable synergetic effects that can significantly enhance catalytic activity and stability (Mani et al. 2024; Li et al. 2024; Yadav et al. 2024; Ramírez et al. 2024; 2024). Wang et al. and Reddy et al. have reported comparable bimetallic systems, namely hydrogenated Ag-Cu-modified P25 TiO₂ and Cu/Ag quantum dots on TiO₂ nanotubes, demonstrating hydrogen production rates of 1.16 mmol h^− 1 g^− 1 and 56,167 µmol h^− 1 g^− 1 (Reddy et al. 2017; Wang et al. 2022) respectively

Through a comprehensive literature survey, it has been noted that no detailed study has been conducted to substantiate the morphological optimization along with plasmonic noble-non noble bimetallic cocatalyst decoration of TiO₂ for cost-effective and enhanced photocatalytic hydrogen production. Thus, at this point, it would be highly intriguing to explore the advantages offered by both aspects i.e. TiO₂ morphology optimization for improved light harvesting and high surface area and further decoration of bimetallic nanoparticles on the TiO₂ surface. Decoration of noble metal Pt has been studied extensively due to its fascinating co-catalyst attributes (Li et al. 2015a; Denisov et al. 2019)(Mani et al. 2024). But Pt decoration ends up with higher cost issues. Therefore, decoration of inexpensive non-noble metal Cu along with low-cost noble metal Ag with their individual properties of visible light harvesting ability along with an excellent electron sink capacity and surface plasmon resonance (SPR) effect respectively for improved solar photocatalytic hydrogen production would be an interesting piece of research.

In view of this, firstly, we have performed a detailed investigation on the synthesis and characterization of different morphologies of TiO₂ by solvothermal method and checked for their photocatalytic hydrogen production capacity. NWT like morphology of TiO₂ was found to be the best as compared to other obtained morphologies with a remarkable photocatalytic hydrogen production activity (678 µmol/g) which was nearly 7-fold better than benchmark P25 TiO₂ (99 µmol/g). NWT was further decorated with popular co-catalyst Pt for improved hydrogen production. In the later part, non-noble metal Cu decoration and its percentage optimization on NWT (Cu-NWT) was carried out with further co-decoration of minimal quantity of plasmonic noble metal Ag (Ag/Cu-NWT) for improved hydrogen production activity comparable with Pt-NWT with an added advantage of cost effectivity. The proposed mechanism outlined at the end emphasizes the highly porous morphology of NWT, which enhances light absorption through repeated light reflection and refraction. This, combined with increased charge transfer and separation, the surface plasmon resonance (SPR) effect from Ag nanoparticles, and the electron sink capability of Cu, collectively contributes to the improvement in photocatalytic hydrogen production.

2.1. Materials

Titanium tetraisopropoxide (Ti {OCH(CH₃)₂}₄) and Hexachloroplatinic acid (H₂PtCl₆) were purchased from Sigma Aldrich; Hydrochloric acid (HCl) and Methanol (CH₃OH) were purchased from SDFCL; Sodium hydroxide (NaOH), Ethylene glycol (CH₂-OH)₂, Copper sulphate (CuSO₄.H₂O) were purchased from Merck; Silver nitrate (AgNO₃) was purchased from Thermo Fisher Scientific India Pvt. Ltd. All chemicals used are of analytical reagents grade (AR). The chemicals were utilized as it is, without undergoing additional purification procedures.

2.2. Synthesis of catalyst materials

In the typical process, 0.2M TTIP (Titanium tetraisopropoxide) in 50 mL ethylene glycol was taken in a round bottom flask and was stirred for 2 h. 50 mL of 10M NaOH was then added slowly to this TTIP solution and stirred for 3 h to get a clear solution. The reaction mixture was then put in an autoclave for 16 h at 200 ºC. Then the precipitate was washed with 0.2M HCl and distilled water several times and then was kept in a vacuum oven overnight at 80 ºC. The product was then calcined for 2 h at 500 ºC which gave white powder of Nanowire TiO₂ (NWT) as demonstrated in Fig. 1 (Li et al. 2015b).

Different morphologies of TiO₂ were obtained as a function of NaOH concentration (ranging from 5–20M) and labelled as TiO₂ at 5M NaOH (TiO₂@5M), TiO₂ at 10M NaOH (NWT), TiO₂ at 15M NaOH (TiO₂@15M) and TiO₂ at 20M NaOH (TiO₂@20M). All these samples were further investigated for their improved photocatalytic hydrogen production capacity over P25 TiO₂. The obtained data was then taken into consideration to correlate the relationship between the morphology and photocatalytic hydrogen production capacity.

For the preparation of 0.1Ag/1.0Cu-NWT, as prepared and optimized 1.0 Cu-NWT photocatalyst (supporting information) was further decorated with a minimal quantity of 0.1% Ag, for which silver nitrate (AgNO₃) was used as a precursor. The typical synthesis of Ag decorated 1.0Cu-NWT (labelled as 0.1Ag/1.0Cu-NWT) was carried out by mixing 0.095 mL of 0.02 M AgNO₃ solution with 200 mg of 1.0Cu-NWT in 10% methanol solution. Subsequently, the mixture was subjected to UV light irradiation for 3 h, followed by washing with distilled water and drying at 60°C to obtain the desired product. Pt decorated NWT (Pt-NWT) and its percentage variation also prepared by using the photo-deposition method as mentioned in supplementary information (Reddy et al. 2017; Shinde et al. 2018a).

2.3. Photocatalytic hydrogen production setup

All the photocatalytic hydrogen production reactions were checked in simulated solar light. In the standard experimental procedure, a 10% methanol solution (100 mL) was placed in a 200 mL quartz reactor with a top loading port. Here, methanol has been used as hole scavenger as it is relatively stable compound under various experimental conditions. Further it is inexpensive and widely available which makes it a cost-effective choice for large-scale experiments, unlike some other sacrificial agents which can be more expensive or difficult to source in large quantities. (Guzman et al. 2013; Schneider and Bahnemann 2013). Due to all these reasons 10% methanol solution was the best choice. To this 100mL solution, 40 mg of catalyst was added. The quartz reactor was then packed with a rubber cork and nitrogen gas was purged into it for 10 minutes to remove the other gases present in the free space of the quartz reactor as well as dissolved in the reaction solution. The quartz reactor was then mounted in front of 1000 W Xenon lamp irradiation which is a case similar to solar irradiation. At specific time intervals, samples were extracted and the amount of generated hydrogen gas was determined using offline Shimadzu GC-2014 gas chromatography (GC) equipped with a thermal conductivity detector (TCD) and a molecular sieve/5A column. The measurement was performed at 70°C utilizing N₂ as the carrier gas (Shinde et al. 2018b). Recyclability test of optimized photocatalyst was carried out upto 5 cycles to check the stability and reusability of the photocatalyst. After completion of the first cycle, the quartz reactor was dismounted, rubber cork was removed and kept as it is overnight under ambient conditions by covering it with aluminium foil. In the second cycle, the same solution was employed and purged with N₂ gas to preserve a neutral environment within the reactor. The solution was then exposed to solar simulated light for 4 h. This procedure was repeated for the subsequent three cycles, and the hydrogen production was quantified using GC.

2.4. Characterization of the photocatalysts

The prepared photocatalysts underwent various characterization techniques. X-ray diffraction (XRD) analysis was performed using a Rigaku Ultima IV instrument with Cu K alpha radiation, scanning at a rate of 10^o/min and a sampling width of 0.02^o to determine the crystallographic information. Fourier Transform Infrared Spectra (FTIR) in the mid-IR region (400 to 4000 cm-1) were recorded using a Bruker Tensor 37 instrument. UV-visible absorption spectra (DRS) were obtained using a Shimadzu 1800 UV-visible spectrophotometer. X-ray photoelectron spectroscopy (XPS) analysis was employed using PHI 5000 Versa Probe III equipment and Al Kα radiation as the excitation source to ascertain the oxidation state of the photocatalyst. Scanning Electron Microscopy (SEM) images were acquired using a Quanta FEG 450 instrument, along with Energy Dispersive X-ray (EDX) analysis. Raman Spectroscopy was conducted on a micro-Raman spectrometer Lab RAM ARAMIS, Horiba Jobin-Yvon instrument, specifically for carbon photocatalysts characterization. Transmission Electron Microscopy (TEM) images were captured using a JEOL JEM F200, Tokyo instrument. The BET method, employing a Quantachrome Auto sorbiQ instrument, was utilized to measure porosity and surface area. The quantity of evolved hydrogen gas was measured using a Gas Chromatography (GC) Shimadzu GC 2014 equipped with a TCD detector, and a PORAPAK Q column (length 2.0 m, ID 3.13 mm) was employed (Wadhai et al. 2021).

3.1. Catalyst characterization

X-ray diffraction (XRD) analysis was conducted to investigate the crystalline structure of the photocatalysts, including NWT, Pt-NWT, Cu-NWT, and Ag/Cu-NWT (Fig. 2a and S1). The XRD pattern of NWT revealed a highly crystalline anatase structure with dominant peaks at 2θ = 25.3^◦, 37.9^◦, 48^◦, and 54.9^◦ corresponding to the (101), (004), (200), and (211) planes, respectively, as indicated by the JCPDS Card No. 21-1272 (Zhu et al. 2021). Figure 2a shows the XRD patterns of NWT, 0.5Pt-NWT, 1.0Cu-NWT and 0.1Ag/1.0Cu-NWT photocatalysts. For all these four photocatalysts the major peaks observed were of the anatase phase of TiO₂. There was no peak of Pt, Cu or Ag found in the XRD pattern because of the very small quantity of these metals in the metal-decorated NWT photocatalysts. All the optimized photocatalysts also showed the characteristic peak at 2θ = 25.3^◦, 37.9^◦, 48^◦ and 54.9^◦ corresponding to (101), (004), (200) and (211) planes respectively which not only confirms the formation of high crystallinity but also pure compound. The intensity of the reflections related to the TiO₂ anatase phase decreased following the deposition of metals, suggesting that the presence of an additional crystalline phase had modified the crystalline structure of TiO₂ (Alshehri and Narasimharao 2020). High crystallinity and co-presence of Ag and Cu on NWT photocatalysts are responsible for enhanced light harvesting capacity and ultimately the photocatalytic hydrogen production. Fig. S1a shows the XRD pattern of NWT and Pt-NWT (0.2Pt-NWT, 0.4Pt-NWT, 0.5Pt-NWT, 0.6Pt-NWT, 0.8Pt-NWT and 1.0Pt-NWT) photocatalysts and Fig. S1b shows the XRD of Cu-NWT (0.5Cu-NWT, 0.75Cu-NWT, 1.0Cu-NWT, 1.5Cu-NWT and 2.0Cu-NWT) photocatalysts. The XRD pattern of all Pt-NWT and Cu-NWT photocatalysts retained the peak of bare NWT and no other peak or impurity was observed.

Raman measurements for all four samples i.e., NWT, 0.5Pt-NWT, 1.0Cu-NWT and 0.1Ag/1.0Cu-NWT were carried out. Figure 2b exhibits the Raman spectroscopy analysis, elucidating the characterization of the Ti-O-Ti network structure of NWT materials within the wavenumber range of 100–900 cm^− 1. Notably, the well-established anatase peaks at 142, 393, 511, and 635 cm^− 1 are distinctly observed. In the case of NWT, these four pronounced peaks at 142, 393, 511, and 635 cm^− 1 correspond to the vibration modes of Eg, B1g, A1g, and Eg, respectively, aligning with the TiO₂ anatase phase. The Eg peak arises predominantly from the symmetric stretching vibration of the O–Ti–O linkages in TiO₂, whereas the B1g peak corresponds to the symmetric bending vibration of O–Ti–O and the A1g peak is attributed to the antisymmetric bending vibration of the O–Ti–O linkage (Balachandran and Eror 1982; Melvin et al. 2015). In the case of Pt-NWT, Cu-NWT, and Ag/Cu-NWT, no distinct separate peak was observed. However, a slight shift towards lower wavenumbers and broadening of the peak were noted compared to bare NWT. This phenomenon can be attributed to the strong electronic interaction between the metals and the surface of NWT (Fan et al. 2014). Thus, Ag and Cu decoration improves the crystal structure of anatase TiO₂. The phase of NWT was retained even after the metal decoration. These findings are consistent with the XRD data mentioned earlier. Fig. S2 shows the FTIR spectra of NWT, 0.5Pt-NWT, 1.0Cu-NWT and 0.1Ag/1.0Cu-NWT photocatalysts. The FTIR spectra manifest prominent and comprehensive peaks, signifying the existence of diverse functional groups within the materials. Notably, a slight dip around 3360 cm^− 1 and 1643 cm^− 1 can be attributed to the stretching and bending vibrations of adsorbed molecules and surface hydroxyls on TiO₂ particles, respectively (Li et al. 2016; Chougala et al. 2017). The intense peak at 485 cm^− 1 is assigned to the Ti-O stretching band which is the characteristic peak of TiO₂ (Zhang et al. 2014). There is no significant peak-shift or extra peak observed in optimized metal decorated NWT photocatalysts because of a very small percentage of metal present in it and/or can be mainly attributed to the metal loading effect.

The optical and electronic properties of materials have been characterized by using Diffuse Reflectance Spectroscopy (DRS). To gain insights into the optical absorbance of the optimized photocatalysts, namely NWT, 0.5Pt-NWT, 1.0Cu-NWT, and 0.1Ag/1.0Cu-NWT, Fig. 2c presents the UV-visible diffuse reflectance spectroscopy (DRS) data. The strong absorption bands around 380 nm, attributed to the intrinsic absorption of TiO₂, are thought to result from the excitation of O2p electrons to the Ti3d level (Xu et al. 2010; Zhang et al. 2015b). Absorption edges of NWT, 0.5Pt-NWT, 1.0Cu-NWT and 0.1Ag/1.0Cu-NWT are around 390 nm, 408 nm, 420 nm and 435 nm respectively. The DR spectra of the modified NWT is noteworthy for exhibiting a significant shift in the absorption transition towards longer wavelengths (400 nm < λ < 435 nm). The extended visible light absorption of 0.1Ag/1.0Cu-NWT among all photocatalysts is due to the plasmonic resonance and light harvesting capacity of Ag and Cu respectively which shows absorption above 400 nm (Wang et al. 2022). The absorption of 0.5Pt-NWT and 1.0Cu-NWT was found to be significantly lower compared to 0.1Ag/1.0Cu-NWT, which could be attributed to the lower sensitivity of Pt and Cu to visible light absorption relative to Ag. Ultimately co-decoration of both Ag and Cu on NWT is found to be responsible for improved photocatalytic hydrogen production. Optical band gap studies have been carried out using Tauc Plot (Fig. S9), observed that the absorption shifts are consistent with a reduction in the optical band gap energies, mirroring the trends observed in the absorption spectra. Notably, Ag/Cu-NWT (3.06 eV) exhibits a significantly lower band gap energy compared to bare NWT (3.27 eV), Pt-NWT (3.20 eV), and Cu-NWT (3.15 eV). The reduction in band gap energy in Ag/Cu-NWT is likely due to the synergistic effect of silver and copper decoration, which enhances charge carrier separation and facilitates better light absorption in the visible region, thereby narrowing the band gap (Scarisoreanu et al. 2020). While Pt-NWT and Cu-NWT also exhibit some degree of band gap narrowing, they are not as effective as the Ag/Cu combination. Platinum, although excellent for enhancing photocatalytic activity, is costly and does not significantly affect light absorption in the visible range. Copper alone provides some enhancement in visible light absorption, but it is the addition of silver that amplifies the effect due to the plasmonic resonance (Zhu et al. 2019; Scarisoreanu et al. 2020; Liu et al. 2022).

To gain insight into the growth process of the NWT structure, we monitored the growth steps of the samples using SEM at various NaOH concentrations. Fig. S3 shows the SEM images of synthesised TiO₂ at different NaOH concentrations at the same magnification i.e., at 1µm. Irregular morphology of TiO₂ (TiO₂@5M) was observed with 5M NaOH solution in the synthesis method. Well-organized 3D spheres made up of nanowire TiO₂ (NWT) morphology was obtained with 10M NaOH solution. The rod like (TiO₂@15M) and scale like (TiO₂@20M) morphologies of TiO₂ were obtained when 15M and 20M NaOH solution was used in the synthesis method. It is obvious from the above data that the NaOH concentration in the reaction mixture plays crucial role in morphology elucidation. It can also be inferred that the morphology of the product can be controlled by the choice of solvent (Lu et al. 2021). Solvent provides sufficient space to grow the titanium oxide into nano-wire ball like morphology. The observed variations in morphologies can be attributed to the polarity and coordinating ability of the co-solvent, which significantly affects the solubility, reactivity, and diffusion behaviour of the reactants, thereby influencing the structural and morphological characteristics of the final products (Das et al. 2008; Lu et al. 2021). The optimized NWT morphology not only enhanced light harvesting ability due to its unique nanowire ball like structure but also improved its surface area due to high porosity for enhanced photocatalytic hydrogen production.

The efficient 0.1Ag/1.0Cu-NWT photocatalyst's microscopic nature was first characterized by SEM (Fig. 3a-c), and then by low-resolution transmission electron microscopy (LRTEM) and high-resolution transmission electron microscopy (HRTEM) (Fig. 3d-h). Uniformly dispersed ball like structure made up of nanowire is shown in Fig. 3a having diameter of nanowire as 10–20 nm and the whole NWT structure is around 3–4 µm as confirmed by SEM images (Fig. S4). Figure 3d-h shows LRTEM and HRTEM images of optimized 0.1Ag/1.0Cu-NWT photocatalyst. The lattice fringes observed in the HRTEM image (Fig. 3g-h) of 0.1Ag/1.0Cu-NWT correspond to the (111) plane of Ag (d = 0.24 nm) (Li et al. 2017), (111) plane of Cu (d = 0.211 nm) (Kainthla et al. 2018) and (101) plane of anatase TiO₂ (d-space value of 0.326 nm) (Reddy et al. 2017). These results provide clear evidence of homogeneously dispersed Ag and Cu in their metallic forms on the NWT surface, indicating a close association between them. The crystalline quality of the 0.1Ag/1.0Cu-NWT photocatalyst was further demonstrated by the Selected Area Diffraction (SAED) pattern shown in Fig. 3i, which reveals a well-defined diffraction pattern with a lattice spacing of 0.326 nm, matching the (101) plane of anatase TiO₂. SAED pattern of the TiO₂ nanowires reveal their single crystallinity and conclude the growth of the single-crystal TiO₂ nanowire/nanorod along the [0 0 1] direction (Sun et al. 2013). Corresponding LR and HRTEM along with SAED pattern for 0.5Pt-NWT and 1.0Cu-NWT photocatalysts are shown in Fig. S5a-d and S5e-h respectively.

Figure 4 presents the elemental mapping and TEM-EDAX analysis to visualize the distribution of elements and determine the atomic percentage of each element across the 0.1Ag/1.0Cu-NWT photocatalyst. Figure 4a-f shows elemental mapping of 0.1Ag/1.0Cu-NWT photocatalyst depicting the homogeneous dispersion of elements throughout the sample. Figure 4g shows TEM-EDAX spectra of 0.1Ag/1.0Cu-NWT photocatalyst which confirms the presence of 0.13% Ag, 6.72%Cu, 53.56% O and 39.58% Ti atomic percentages. Cu atomic percentage has been found to be higher than the actual percentage of synthesized 1%Cu-NWT which might be due to the copper gride used for the sample analysis. Overall observation confirmed the presence of Ag and Cu in the 0.1Ag/1.0Cu-NWT photocatalyst.

The impressive organization of nanowires in TiO₂ serves as a driving force for our in-depth examination for its Brunauer–Emmett–Teller (BET) surface area and pore volume. Consequently, we conducted an investigation onto the N₂ sorption isotherm and corresponding BET surface area plot of both P25 TiO₂ and NWT, aiming to validate the superior photocatalytic performance of NWT compared to P25 TiO₂ (as shown in Fig. S6). TiO₂ being the main photocatalyst which would contribute to the photocatalytic efficiency majorly due to its surface area and other properties, therefore BET surface area, pore volume, and pore size diameter of the P25 and NWT has been reported in Table 1. Metal decoration or co-decoration did not contribute much to the surface area. As expected, the BET surface area of NWT (123 m² g^− 1) was found to be significantly higher than that of P25 TiO₂ (52.90 m² g^− 1). The well-organized porous morphology of NWT exhibits a 2.5 times greater surface area than P25 TiO₂, leading to improved light harvesting ability through repeated reflection and refraction. Additionally, the increased surface area provides more active reaction sites and facilitates mass transfer, allowing for efficient transport of reactant and product molecules.

Table 1

BET surface area, pore volume and pore size diameter values of the P25 TiO₂, and NWT samples.
Sample	BET surface area (m²/g)	Pore volume (cm³/g)	Pore size diameter (nm)
P25	52.90	0.354	25.94
NWT	123.00	0.636	16.20

Since 0.1Ag/1.0Cu-NWT display high crystallinity and well optical response ability, it is very necessary to investigate its surface compositions and electronic status, X-ray photoelectron spectroscopy (XPS) analysis was also performed. Figure 5a illustrates the survey spectrum of the 0.1Ag/1.0Cu-NWT photocatalyst, revealing the presence of elements Ti, O, Cu, and Ag. Figure 5b shows the peaks corresponding to Ti 2p at 458.80 eV and 464.55 eV, and 530.23 eV and observed data of O1s peak (Fig. 5c ) includes one major peak at 530.23 eV and small shoulder also appeared at 532.22 eV corresponding to the binding energies of O1s, which was attributed to O 1s electron binding energy for TiO₂ and the H–O from the absorbed H₂O on their surface respectively (Erdem et al. 2001; Kruse and Chenakin 2011; Zhu et al. 2017). The Ti 2p region displays an asymmetric peak at 458.80 eV and 464.55 eV, corresponding to the binding energies of Ti 2p_3/2 and Ti 2p_1/2, respectively (Erdem et al. 2001; Su et al. 2013). The binding energy values at 367.85 and 373.82 eV refers to Ag 3d_5/2 and Ag 3d_3/2, respectively (Fan et al. 2014). This observation confirms the presence of metallic silver (Ag⁰) in the sample. The peaks at 932.75 and 952.56 eV are identified as Cu 2p_3/2 and Cu 2p_1/2, respectively, indicating the presence of Cu in reduced form (Cu⁰) (Zhang et al. 2015a; Reddy et al. 2017). High resolution spectra of Cu 2p and Ag 3d serving as proof of Cu and Ag in their metallic states. The XPS spectra of 0.5Pt-NWT photocatalyst is depicted in Fig. S8, and the presence of Platinum species was confirmed through deconvolution of Pt 4f peak into two components, Pt 4f_7/2 and Pt 4f_5/2, at binding energies 68.97 and 72.52 eV, respectively (Fig. S8d) (Shinde et al. 2018b; Sravani et al. 2020).

3.2. Photocatalytic hydrogen production

A series of methodical experiments on photocatalytic hydrogen production were conducted, employing simulated solar light irradiation, and utilizing methanol as a sacrificial agent across all the prepared photocatalysts. A comparison was made with commercially available P25 TiO₂. P25 TiO₂, being the widely recognized benchmark photocatalyst in commercial use, is the preferred starting point for further development and modifications aimed at overcoming its limitations. The objective is to augment the efficiency while maintaining the cost-effectiveness of both the photocatalyst and the experimental configuration for solar-driven photocatalytic hydrogen generation. Thus, motivated by this idea, focused attempts were made to obtain and optimize the morphology of synthesized TiO₂ which would deliver much more superior results as compared to commercially available P25 TiO₂ in terms of efficiency.

Thus initially, the effect of morphology of TiO₂ on the hydrogen production capacity was studied. Moreover, the optimized morphology was further improved by decorating it with noble metal Pt, non-noble metal Cu, and a combination of non-noble Cu and a minimal amount of noble metal Ag to further enhance hydrogen production. The trend observed for photocatalytic hydrogen production, as depicted in Fig. 6a, is as follows: P25 TiO₂ < NWT < Cu-NWT < Pt-NWT ≤ Ag/Cu-NWT. Optimized low-cost 0.1Ag/1.0Cu-NWT (10,184 µmol/g) photocatalyst showed the highest hydrogen production as compared to all P25 TiO₂, NWT, Cu-NWT photocatalysts and at par hydrogen production with 0.5Pt-NWT (10,050 µmol/g) photocatalyst.

Morphology obtained by using 10 M NaOH concentration i.e., NWT showed a dramatic increment in hydrogen production than other morphologies obtained at 5 M, 15 M and 20 M respectively (Fig. 6b). In comparison to the commercially available P25 TiO₂ (99 µmol/g), NWT exhibited a remarkable nearly 7-fold increase in hydrogen production capacity (678 µmol/g). This significant improvement in photocatalytic efficiency can be attributed to the favourable morphology achieved through the specific arrangement of atoms. This morphology enhances the absorption of light energy (as shown in Fig. 2d and S7) and facilitates mass transfer through the increased specific surface area (see BET explanation).

To achieve the highest possible activity, NWT was modified by decorating with the popular noble metal Pt since Pt has more work function as well as better reduction potential than other noble metals. Various % Pt-NWT photocatalysts were synthesized and checked for photocatalytic hydrogen production. Optimized 0.5Pt-NWT (10,050 µmol/g) showed a better increment in hydrogen production than other Pt percentages. 0.2 to 0.5 percentage of Pt with NWT i.e., 0.2Pt-NWT, 0.3Pt-NWT, 0.4Pt-NWT and 0.5Pt-NWT shows gradual increment in hydrogen production (Fig. 6c). The photocatalytic performance of nanowire-tethered (NWT) materials decreases when the Pt content exceeds 0.5%, a result of the light-blocking and scattering effects caused by the accumulation of Pt on the NWT surface (Sun et al. 2012; Lee and Chang 2019). However, the optimized 0.5Pt-NWT exhibited significantly enhanced hydrogen production, reaching nearly a 15-fold increase compared to NWT (678 µmol/g) and a remarkable 102-fold increase compared to P25 (99 µmol/g). The reason behind the betterment is the Pt decoration on NWT which reduces the electron hole recombination and increases the absorption of light energy (Fig. 2b-c and 7).

Further, as Pt-TiO₂, is a very exhaustively studied system due to the very high work function of Pt to reduce H⁺ to H₂ for enhanced efficiency, it is always a challenge to work out a system or photocatalyst which would be a cost-effective alternative to beat Pt-TiO₂. Therefore, the study was further directed to modify the optimized NWT photocatalyst with bimetallic non-noble metal Cu and low-cost noble metal Ag to explore and exploit their very fascinating properties of SPR effect and extended solar light absorption for improved hydrogen production activity at par with Pt-TiO₂ systems.

Firstly, TiO2 decoration with varying Cu concentration in the range of 0.5 to 2% has been studied. 1% Cu-NWT was found to show excellent improvement in hydrogen production sample among other concentrations; however these results were still lower than the Pt-NWT system.

In this attempt, initially, NWT was decorated with an inexpensive non-noble metal Cu with its percentage varying from 0.5 to 2%. (Fig. 6d). The increasing hydrogen production trend was observed from 0.5%to1.0%, and on further increase in Cu % the trend was found to decrease. The optimized 1.0Cu-NWT photocatalyst (3,907 µmol/g) demonstrated a substantial increase in hydrogen production, achieving nearly 6-fold improvement compared to NWT (678 µmol/g) and a remarkable 40-fold increase compared to P25 TiO₂ (99 µmol/g). This enhancement in hydrogen production is due to the presence of Cu which absorbs plasmon-induced irradiation to generate hot electrons which are partly ejected to CB of TiO₂ overcoming the Schottky barrier and separating the photogenerated charge carriers significantly (Zhang et al. 2015a; Kumaravel et al. 2019).

Further, optimized 1.0Cu-NWT was decorated with a minimal quantity of Ag to improve the efficiency of the photocatalyst and to get comparable results at par with Pt-NWT photocatalyst. A very small quantity of Ag i.e., 0.1% Ag was decorated on an optimized 1.0Cu-NWT photocatalyst which resulted in a dramatic increment in photocatalytic hydrogen production capacity as compared to Cu-NWT, NWT as well as P25 TiO₂. 0.1Ag/1.0Cu-NWT (10,184 µmol/g) photocatalyst showed almost 2.6-fold increment in hydrogen production than 1.0Cu-NWT (3,907 µmol/g) photocatalyst.

This improvement also showed comparable hydrogen production as of optimized 0.5Pt-NWT (10,050 µmol/g) photocatalyst (Fig. 6a). The main reason for choosing 0.1%Ag on 1.0 Cu-NWT system was to highlight that even the very small amount viz. 0.1% of Ag along with 1.0Cu-NWT will be sufficient to replace the 0.5Pt-NWT. It is indeed interesting to study the effect of variable concentration of Ag with 1.0 Cu-NWT system for further enhancement in hydrogen production. The detail study and optimization of variable metal concentrations and combinations of different metals will surely replace the Pt in future for cost-effective catalysts for cheap hydrogen production. The remarkable photocatalytic performance exhibited by the 0.1Ag/1.0Cu-NWT photocatalyst can be attributed to several key factors. Firstly, its unique morphology and bimetallic decoration play a crucial role. These features enable enhanced absorption of visible light, resulting in more efficient utilization of solar energy. Additionally, the modified structure of the photocatalyst leads to an increased surface area, providing more active sites for the photocatalytic reaction. Moreover, the bimetallic decoration facilitates accelerated charge transfer processes, enhancing the overall efficiency of the photocatalyst. Lastly, the presence of silver and copper on the nanowires effectively suppresses the recombination of photogenerated electron-hole pairs, leading to an extended lifetime of the charge carriers and further enhancing the photocatalytic performance. These attributes ultimately result in excellent photocatalytic hydrogen production, as explained in the mechanism section.

To evaluate the reusability of the optimized 0.1Ag/1.0Cu-NWT photocatalyst, cyclic experiments were performed under identical experimental conditions. As depicted in Fig. 6e, the amount of hydrogen production remained consistent throughout the five cycles. This observation indicates that the photocatalyst maintained its activity without any noticeable decrease, demonstrating its remarkable stability.

3.3. Mechanism of photoinduced charge transfer in Ag/Cu-NWT

The results obtained indicate that the 0.1Ag/1.0Cu-NWT photocatalyst possesses remarkable photocatalytic hydrogen production ability under simulated solar light irradiation. In order to elucidate the mechanism responsible for enhanced photocatalytic hydrogen production, a plausible charge transfer pathway is proposed and visually illustrated in Fig. 7.

The achieved nanowire ball-like morphology of the photocatalyst through the use of 10M NaOH resulted in a significant improvement in hydrogen production. This enhancement can be attributed to the unique structure of the photocatalyst, which plays a vital role in efficiently harnessing light energy through repeated reflection and refraction from the light source. This ball-like arrangement of nanowires also has been found to improve the porosity and surface area of the photocatalyst which ultimately facilitates more reaction sites and mass transfer. The 2.5 times improved BET surface area and UV-visible DRS (Fig. 2d) spectral data of NWT clearly demonstrates probable reasons for 7-fold improved photocatalytic hydrogen production as compared to P25 TiO₂.

To achieve the highest possible activity at par or better than Pt-TiO₂ with an added advantage of cost effectivity, the NWT was decorated with non-noble metal Cu and a minimal quantity of noble metal Ag. 0.1Ag/1.0Cu-NWT. The unique structure of this photocatalyst exhibited superior hydrogen production compared to all other prepared photocatalysts, and similar hydrogen production to that of 0.5Pt-NWT. This can be attributed to the proposed charge transfer mechanism, where upon light irradiation, electrons are excited from the valence band to the conduction band of NWT, and hot electrons are continuously supplied to the conduction band of TiO₂ due to the collective oscillation of conduction band electrons of decorated Ag nanoparticles. The surface plasmon resonance (SPR) effect of the decorated Ag nanoparticles enhances the absorption of visible light, resulting in more photoelectrons being available in the conduction band of NWT. (Nyamukamba et al. 2017). Further, these photoexcited electrons are captured by Cu which acts as an electron sink for the effective H⁺ to H₂ reduction reaction. Thus, Cu decoration not only acts as a co-catalyst but also plays an important role in charge separation (Reddy et al. 2017; Wang et al. 2022). The VB holes are trapped by methanol which acts as a hole scavenger or used for the oxidation process of H₂O (Denisov et al. 2019). Hence, notwithstanding the significance of the SPR effect, the catalytic activity of co-metallic decorated Ag and Cu on NWT exerts a more influential effect by mitigating electron-hole recombination and promoting the charge transfer rate. In conclusion, the combination of the tailored morphology of TiO₂ (NWT) and the co-metallic decoration (Ag/Cu-NWT) bestows exceptional characteristics upon the photocatalyst. These features encompass amplified light absorption in the visible spectrum, augmented surface area, expedited efficiency in charge transfer, and subdued recombination of photogenerated electron-hole pairs. Collectively, these factors contribute to the remarkable photocatalytic hydrogen production achieved by the catalyst.

We have successfully synthesized NWT using the solvothermal method, as well as Pt-NWT, Cu-NWT, and Ag/Cu-NWT using the photo-deposition method. These photocatalysts have been thoroughly characterized using various techniques including XRD, FTIR, Raman, UV-visible DRS, BET, SEM, LRTEM-HRTEM, and XPS. TiO₂ with different morphologies were obtained at different NaOH concentrations as confirmed by the SEM analysis. 3D self-assembled nanowire TiO₂ (NWT) morphology was found to have greater potential to absorb more light energy due to its structure and higher surface area thereby providing more active sites for improved photocatalytic hydrogen production than other morphologies. Further, NWT was successfully decorated with traditional noble metal Pt, non-noble metal Cu and a bimetallic combination of Ag/Cu respectively to compare their efficiencies. The deposition of Cu and Ag particles on NWT with homogeneous dispersion of these elements throughout 0.1Ag/1.0Cu-NWT photocatalyst has been confirmed by elemental mapping, TEM-EDAX and XPS analysis. The analysis of optical properties of the 0.1Ag/1.0Cu-NWT photocatalyst has unveiled its remarkable light absorption capabilities, primarily attributed to the surface plasmon resonance (SPR) effect induced by the presence of Ag and Cu. The finely structured morphology, along with the cooperative influences of bimetallic Ag and Cu embellishment on the surface of nanowire-TiO₂ (NWT) materials, facilitates augmented absorption of visible light and expanded surface area of the photocatalyst. Consequently, these factors engender heightened efficacy in charge transfer and inhibition of recombination of electron-hole pairs generated through photon absorption. The cumulative outcome is an outstanding level of activity in the process of photocatalytic hydrogen production. Importantly, this study emphasizes the facile fabrication of a cost-effective Ag/Cu-NWT solar photocatalyst as a viable alternative to the widely recognized Pt-TiO₂ system, while maintaining high photocatalytic hydrogen production capacity.

Acknowledgements

SW is thankful to UGC (Grant No.2061610320), New Delhi for Junior Research Fellowship and to P. E. S. Modern College of Engineering, Pune. PT is thankful to the Board of Research in Nuclear Sciences (BRNS) (Grant No. 2013/37C/52/BRNS/2464 Dated December 4, 2013), UGC-DAE Consortium for Scientific Research, University Grants Commission (Grant No. CSIR–IC–/MSRSR-15/CSR- 223/2017–18/1304) and UPE (II) grant under UGC, for financial support. The authors express their gratitude to the Central Instrumentation Facility (CIF) at the Department of Chemistry and the Department of Physics of Savitribai Phule Pune University, Pune, India for providing characterization facilities. The authors would like to express their gratitude to the National Chemical Laboratory (NCL), Pune, for granting access to the HRTEM characterization facility.

Acknowledgements

Data availability: The data can be requested from the authors.

Ethical approval: Not applicable.

Consent to participate: Not applicable.

Consent to publish: Not applicable.

Authors contributions: SW was involved in designing the methodology, material synthesis, conducting experiments, and original manuscript preparation. PT was involved in conceptualizing the research, providing the resources, guidance, reviewing, revising, and editing the manuscript, and supervising the research work.

Funding: SW is thankful to UGC (Grant No.2061610320), New Delhi for Junior Research Fellowship. PT is thankful to the Board of Research in Nuclear Sciences (BRNS) (Grant No. 2013/37C/52/BRNS/2464 Dated December 4, 2013) and UGC-DAE Consortium for Scientific Research for financial support.

Competing interests: The authors declare no competing interests.

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23 Sep, 2024
Editor invited by journal
23 Sep, 2024
First submitted to journal
20 Sep, 2024

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Synthesis of Ag/Cu decorated 3D self-assembled nanowire TiO2 Photocatalyst for Hydrogen Production: A Promising Pathway towards Sustainable Energy Generation

Status:

Journal Publication

Version 1

Abstract

Figures

Highlights

1. Introduction

2. Experimental

2.1. Materials

2.2. Synthesis of catalyst materials

2.3. Photocatalytic hydrogen production setup

2.4. Characterization of the photocatalysts

3. Results and discussion

3.1. Catalyst characterization

3.2. Photocatalytic hydrogen production

3.3. Mechanism of photoinduced charge transfer in Ag/Cu-NWT

4. Conclusions

Declarations

References

Supplementary Files

Status:

Journal Publication

Version 1