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. 2021 Nov 5;6(45):30534–30541. doi: 10.1021/acsomega.1c04143

SrSnO3-Assembled MWCNT Heterojunctions for Superior Hydrogen Production under Visible Light

Mohammad W Kadi †, Reda M Mohamed †,‡,*
PMCID: PMC8600627  PMID: 34805682

Abstract

graphic file with name ao1c04143_0013.jpg

A one-step sol–gel method for SrSnO3 nanoparticle synthesis and the incorporation of multi-walled carbon nanotubes (MWCNTs) to produce a SrSnO3@MWCNT photocatalyst is presented. The incorporation of MWCNTs results in enhancement of structural, optical, and optoelectrical properties of SrSnO3. The optimized 3.0% addition of MWCNTs results in light absorption enhancement and a reduction of the band gap from 3.68 to 2.85 eV. Upon application of the photocatalyst in the photocatalytic hydrogen production reaction, SrSnO3@MWCNT-3.0% yields 4200 μmol g–1 of H2 in just 9 h with the use of 1.6 g L–1 of the photocatalyst. SrSnO3@MWCNT exhibits remarkable chemical and photocatalytic stability upon regeneration. Enhanced photocatalytic ability is attributed to improved surface properties and charge-carrier recombination suppression induced by the MWCNT addition. This study highlights the remarkable improvements in chemical and physical properties of semiconductors with MWCNT incorporation.

1. Introduction

The ever-increasing energy demand and environmental concerns led researchers to increase their efforts for a search of clean and sustainable energy sources.1−5 Harvesting the energy emitted by the Sun is a major point of interest. One important aspect in utilizing Sun’s energy is its storage as chemical energy in the form of hydrogen. Photocatalytic hydrogen production has received considerable attention and is still an active area of research.6 Photocatalytic hydrogen production can be achieved by constructing a material that produces electron–hole pairs upon irradiation. By controlling the flow of the charge carriers, the electrochemical reaction between photoelectrons and H+ species in the aqueous media will result in H2 generatation.7 Honda and Fujishima used TiO2 and UV light to split water to produce hydrogen for the first time.8 Inspired by this breakthrough, many researchers explored all aspects that can improve hydrogen production via this route. A major aspect being the use of visible light to induce photocatalysis as visible light constitutes more than 40% of the emitted energy of the Sun.4,9−14 In constructing a photocatalyst, one must consider many factors that are detrimental to the efficiency of materials. These factors include the crystal structure, the energy of the band gap, and the surface area of the materials. Many strategies have been applied to achieve desirable band energies, morphology, and high surface areas. Examples of applied strategies include heteroatom doping, control of the nanometer size of the materials, and the control of pore volumes and numbers.15,16

In recent years, perovskite semiconductors have been shown to possess properties that can be utilized in wide range applications in renewable energy and environmental remediation.17−21 Perovskites have been applied in water splitting and photocatalytic degradation of toxic compounds.22−25 For example, alkaline-earth stannates are ferromagnetic perovskites with distinctive properties as they are piezo- and pyroelectric materials. These properties allow their utilization in capacitors, photoluminescent devices, sensors, many electronic devices, and as photocatalysts.26−30 SrSnO3 has a three dimensional crystal structure in which SnO6 octahedral and SrSnO3 octahedral distortions promote electron/hole mobility and separation, yielding effective photocatalysis as has been shown in several studies of organic pollutant degradations.31−35 SrSnO3 was also used in humidity sensors,36 lithium ion batteries,37 thermal capacitors,38 and in electronic devices.39

To improve the photocatalytic activity of a material, consistent separation of electron hole pairs must be achieved.40−46 This can be done by incorporating noble metals or nanostructured carbon materials as modifiers or cocatalysts.47−51 Multiwalled carbon nanotubes (MWCNTs) have high electron storage capacity, have a very large specific surface area, superior metallic conductivity, and are mechanically and chemically very stable.52−54 Their use, for example, with anatase TiO2 resulted in enhanced photocatalytic performance through the creation of a heterojunction on TiO2 surfaces.55,56 They can also act as electron sinks separating them from holes and allowing electrons to create reactive radicals such as superoxide (O2•−) and hydroxyl radicals that facilitate photocatalysis.57,58 In this paper, we report the synthesis of mesoporous SrSnO3 using a simple sol–gel method in the presence of nonionic surfactant Pluronic F-127. The effect of addition of different weight percentages of MWCNTs on the physical and photocatalytic performances of mesoporous SrSnO3 for production of hydrogen under visible light is studied.

2. Experimental Section

2.1. Materials

Polyethylene oxide–propylene oxide triblock copolymer (PEO/PPO/PEO), average Mn ∼12,600, known as the Pluronic F-127 surfactant, Sr(NO3)2, SnCl4·5H2O, CH3COOH, HCl, and C2H5OH were purchased from Sigma-Aldrich. MWCNTs (>95%) were also obtained from Sigma-Aldrich.

2.2. Synthesis of Mesoporous SrSnO3

0.3 g Pluronic F-127 was dissolved in 30 mL of C2H5OH, followed by the addition of 2.1 g Sr(NO3)2 and 2.6 g of SnCl4·5H2O gradually with continuous stirring. 0.82 mL of HCl and 2.7 mL of CH3COOH were added to the solution with stirring for additional 1 h allowing the sol to develop. The sol was dried at 40 °C in 40–80% humidity, followed by 12 h drying at 65 °C. The product was calcined at 700 °C.

2.3. MWCNT Activation

30–40 nm diameter MWCNTs were used in the synthesis process. MWCNTs were activated using concentrated HNO3. In this process, 0.25 g of the nanotubes were added to 100 mL of 6 M HNO3 in a three-neck flask and refluxed at 120 °C for 6 h. This step will eliminate amorphous carbon and other contaminants and will functionalize the outer walls of tubes with carboxylic groups, which will allow SrSnO3 crystal incorporation. After refluxing, activated MWCNTs were sieved and rinsed several times with H2O. The water was evaporated overnight at 110 °C.

2.4. Preparation of SrSnO3-MWCNT Nanocomposites

A desired amount of the activated MWCNT was dispersed in a 2:1 H2O and ethanol mixture and sonicated for 30 min, followed by SrSnO3 addition. The mixture was dried at 60 °C for 12 h resulting in mesoporous SrSnO3@MWCNT nanocomposites with the SrSnO3 nanoparticles attached at the MWCNT surface. MWCNT/SrSnO3 nanocomposites with MWCNT weight percentages of 1, 2, 3 and 4 wt % were prepared by adding 0.01, 0.02, 0.03, and 0.04 g separately to 1.0 g of SrSnO3 in four independent samples.

2.5. Characterizations of Photocatalysts

X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were utilized to investigate the crystal structure and elemental oxidation states of the nanocomposites For this Bruker axis D8, Cu Kα radiation, and XPS, Thermo Scientific, were utilized. A transmission electron microscope (JEOL-JEM-1230) was used to obtain transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) images of the dispersed materials on carbon-coated TEM grids. Adsorption/desorption isotherms and consequently the specific surface area (SBET) were determined utilizing a Chromatech device (Nova 2000). Light absorption capacity of the photocatalyst was estimated using a V-570 (JASCO) spectrophotometer in the range of 200–800 nm. Diffusive reflectance (DR) spectra and the Kubelka–Munk formula were used to calculate Eg of the semiconductors. The photoluminescence (PL) response of all the samples was obtained using a Shimadzu RF-5301 fluorescence spectrometer. The Zennium-Zahner electrochemical system was used to measure the photocurrent response of all samples blended on conductive substrates of 0.5 cm–2 size.

2.6. Photocatalytic Activity

The photocatalytic efficiency of the SrSnO3 and SrSnO3@MWCNT nanocomposites was measured from H2 yield from glycerol solutions upon visible light irradiation using a Pyrex photoreactor. SrSnO3 and SrSnO3@MWCNT nanocomposites (50 mg) were suspended separately in glycerol (10 vol % in 200 mL of water) by stirring and loaded to the photoreactor. Prior to the photocatalytic reaction, the photoreactor was bubbled with Argon flow (300 mL/min) for 10 min to purge the oxygen in the photocatalyst suspension. A 500 W Xe lamp (UV cutoff filter > 420 nm) located vertically on the top of the photoreactor was used for illuminating the solution. An Agilent GC 7890A system was utilized to quantify hydrogen evolution during the course of the photocatalytic reaction for 9 h with gas sampling at 60 min intervals.

3. Results and Discussion

3.1. Photocatalyst Characterizations

The XRD patterns of pure SrSnO3, pure MWCNT, and the SrSnO3@MWCNT nanocomposite samples are shown in Figure 1. The peak at 25.9° confirms the presence of (002) of MWCNTs in the pure sample (JCPDS card no. 26-1079). Both pure SrSnO3 and SrSnO3@MWCNT samples show diffraction peaks consistent with the crystalline planes of the pure SrSnO3 (JCPDS no. 77-1798).33,35 However, no MWCNT peaks appear in all SrSnO3@MWCNT samples, most probably because of low concentrations. It is also notable that crystallite sizes were smaller in all SrSnO3@MWCNT samples as the addition of the nanotubes hinders SrSnO3 particle agglomeration.

Figure 1.

Figure 1

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XRD patterns of SrSnO3, MWCNTs, and SrSnO3@MWCNT.

The SrSnO3@MWCNT-3% sample was subjected to high-resolution XPS, and the spectra are shown in Figure 2. The XPS signals were calibrated with the standard C 1s peak at 284.8 eV. Two overlapping peaks for the two 3d orbitals of the Sr atom are shown in Figure 2A with Sr3d5/2 at 132.7 eV and Sr3d3/2 at 134.5 eV, and the separation between the two is ∼1.8 eV hence the overlap at this resolution.34 On the other hand, for the Sn atom the same two energy levels are well separated, 3d5/2 at 486.3 eV and 3d3/2 at 494.8 eV as shown in Figure 2B. The 1s orbital of oxygen shows at slightly different positions depending on the chemical state of the atom in the crystal structure (Figure 2C). The 529.4 eV peak is attributed to the O2– ions within the lattice structure, the 530.9 eV peak is attributed to the O2– ions in the O-deprived areas in the structure, and the 532.4 eV peak can be attributed to the loosely bound O atoms of O2 and H2O molecules adsorbed on the surface of the lattice structure.33 Orbitals of the three atoms confirm the crystal structure of SrSnO3. As for MWCNTs, the energy of the 1s of carbon atoms at 284.8 eV indicates the presence of the C–C bond in the nanotubes, while C–O band shows at 286.8 eV.

Figure 2.

Figure 2

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High-resolution XPS spectra of Sr 3d (A), Sn 3d (B), O 1s (C), and C 1s (D) of SrSnO3@MWCNT-3 wt %.

The TEM images of pure SrSnO3, pure MWCNT, and SrSnO3@MWCNT nanocomposite samples are presented in Figure 3A–C, respectively. SrSnO3 appears as nanoparticles with an average size of 20 nm. As shown in Figure 3B, the nanotubes are pure with no metal or carbon impurities. The nanocomposites in Figure 3C show scattered SrSnO3 particles on the nanotube surface. In HRTEM images, for SrSnO3@MWCNT-3 wt %, the 0.284 nm d-spacing shows the incorporation of SrSnO3 into the nanocomposite, while the (002) at 0.330 nm confirms the incorporation of MWCNTs into the nanocomposite (Figure 3D).

Figure 3.

Figure 3

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TEM images of (A) SrSnO3, (B) MWCNTs, and (C) SrSnO3@MWCNT, and (D) HRTEM image of SrSnO3@MWCNT-3 wt %.

Type IV isotherm as is evident from Figure 4 of the adsorption–desorption data for the SrSnO3@MWCNT-3 wt % sample reveals the mesoporous nature of the material. Surface areas deduced from the data are shown in Table 1. SrSnO3, SrSnO3@MWCNT-1 wt %, SrSnO3@MWCNT-2 wt %, SrSnO3@MWCNT-3 wt %, and SrSnO3@MWCNT-4 wt % have BET surface areas of 120, 126, 131, 138, and 139 m2/g, respectively. SrSnO3 has a surface area of 120 m2/g as the synthesis procedure produces a strong mesoporous structure. With the incorporation of a higher surface area MWCNT, the resultant SrSnO3@MWCNT nanocomposite has a BET surface area larger than pure SrSnO3.

Figure 4.

Figure 4

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Adsorption–desorption isotherm of SrSnO3@MWCNT-3 wt %.

Table 1. SrSnO3 and SrSnO3-MWCNT Surface Areas.

material BET surface area, m2/g
SrSnO3 120.00
SrSnO3-MWCNT-1 wt % 126.00
SrSnO3-MWCNT-2 wt % 131.00
SrSnO3-MWCNT-3 wt % 138.00
SrSnO3-MWCNT-4 wt % 139.00
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SrSnO3 absorbs light in the ultraviolet region as can be seen in Figure 5. Upon incorporation of MWCNTs, absorption of all the SrSnO3@MWCNT samples occurs in the visible region, this will result in the enhancement of light-harvesting ability of the material in a photocatalytic reaction. The UV–vis spectra can also be used to deduce the band gap energies of the material under investigation. Table 2 shows a gradual reduction in band gap energy on going from pure SrSnO3 to an increasing MWCNT content, and the band gap values of 3.68, 3.24, 3.00, 2.86, and 2.85 eV for SrSnO3, SrSnO3@MWCNT-1 wt %, SrSnO3@MWCNT-2 wt %, SrSnO3@MWCNT-3 wt %, and SrSnO3@MWCNT-4 wt %, respectively, clearly show this trend, which favors absorption of visible light thus increasing the photocatalytic performance of the nanocomposite. The PL spectra also confirm absorption in the visible region and a reduction of band gap energies upon addition of the MWCNTs to SrSnO3. As shown in Figure 6, all SrSnO3@MWCNT samples have visible region emission showing that addition of MWCNTs to SrSnO3 shifts the emission peak of SrSnO3 from the ultraviolet to the visible region. Compatible bandgap energies of 3.69, 3.25, 3.01, 2.87, and 2.86 eV for SrSnO3, SrSnO3@MWCNT-1 wt %, SrSnO3@MWCNT-2 wt %, SrSnO3@MWCNT-3 wt %, and SrSnO3@MWCNT-4 wt % were also deduced from the PL spectra. The reduction of band gap energy is a direct result of adjusting the weight percent of MWCNTs that significantly red-shifted the PL spectra of the nanocomposite photocatalysts. The SrSnO3@MWCNT samples have photocurrent response values greater than pure SrSnO3 (Figure 7), this translates to lower electron–hole recombination rates, which directly manifest in enhanced photocatalytic activity.

Figure 5.

Figure 5

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UV–Vis spectra of SrSnO3 and SrSnO3@MWCNT.

Table 2. Band gap Energy Values.

sample band gap energy, eV
SrSnO3 3.68
SrSnO3-MWCNT-1 wt % 3.24
SrSnO3-MWCNT-2 wt % 3.00
SrSnO3-MWCNT-3 wt % 2.86
SrSnO3-MWCNT-4 wt % 2.85
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Figure 6.

Figure 6

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PL spectra of SrSnO3 and SrSnO3@MWCNT.

Figure 7.

Figure 7

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Photocurrent response of SrSnO3 and SrSnO3@MWCNT.

3.2. Hydrogen Production Using SrSnO3@MWCNT Photocatalysts

Using 1.2 g L–1 of pure SrSnO3 and SrSnO3@MWCNT photocatalysts, efficiency of the water splitting reaction was studied with 9 h visible light illumination as shown in Figure 8. The hydrogen production efficiency was very small when SrSnO3 was used, the main reason being its limited light absorption in the UV region. The hydrogen production efficiency for SrSnO3 @MWCNT increased over time (Figure 8). The hydrogen production efficiency was enhanced for the MWCNT-supported SrSnO3 to 360 and 2700 μmol g–1 for 1.0 and 2.0 wt % MWCNT loadings and reached 3592 μmol g–1 for SrSnO3@MWCNT-3 wt % (Figure 8). An important factor that contributed to the enhancement of the photocatalytic efficiency of SrSnO3 is the incorporation of the MWCNT content, which improves the light absorption efficiency, increases the surface area and hence the active sites available for the reaction, effective charge carrier separation ability, and improves the surface texture. The 3% MWCNT content was proved to be the optimal content that produces highest desired improvements.

Figure 8.

Figure 8

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Hydrogen production efficiency employing 1.2 g L–1 of SrSnO3 and SrSnO3@MWCNT and 9 h visible light illumination.

The effect of varying the quantity of SrSnO3@MWCNT-3 wt % added to the photocatalytic reaction was studied, and the trends are shown in Figure 9. A range of concentrations between 0.4 and 2.4 g L–1 were utilized. A photocatalyst load range starting from 0.4 to 1.6 g L–1 results in an enhancement of the yield from 1350 to 4200 μmol g–1. However, a further increase of doses to 2.0 and 2.4 g L–1 results in a drop of hydrogen production efficiency to 3000 and 1500 μmol g–1, respectively. Obstruction of light penetration to reach active species may be a reason for a high number of photocatalyst particles at high concentrations. The photocatalytic hydrogen evolution utilizing the adjusted photocatalyst dose is superior than the current published results of similar nanocomposites as shown in Table S1.

Figure 9.

Figure 9

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Effect of SrSnO3@MWCNT-3 wt % photocatalyst quantity on the hydrogen production efficiency.

Consistent photocatalytic efficiency was achieved even when the SrSnO3@MWCNT-3 wt % heterojunction was used in five consecutive experiments (Figure 10). After five-time regeneration, the structural properties show no significant change. This was asserted by XRD and XPS investigations (Figure S2, Supporting Information). Furthermore, no loss of the Sr or Sn content was detected in the residual reaction solutions proving that the photocatalyst material remained intact.

Figure 10.

Figure 10

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Effect of SrSnO3@MWCNT-3 wt % reuse on its efficiency.

3.3. Proposed Photocatalytic Action

A schematic representation of a viable mechanism of the hydrogen production reaction is presented in Figure 11. In the first step, photons with energy greater than the SrSnO3 band gap energy produce electrons at the conduction band and positive holes at the valence band

3.3. 1

Figure 11.

Figure 11

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A schematic of the proposed hydrogen production mechanism employing SrSnO3@MWCNT.

Upon production, the electrons get scavenged by the MWCNT, which plays a sink-like role, resulting in the much-desired charge-carrier separation

3.3. 2

In the following step, radicals are readily produced resulting in the completion of hydrogen production

3.3. 3
3.3. 4

4. Conclusions

A facile one-step sol–gel route for the synthesis of SrSnO3 nanoparticles is introduced in this paper. The synthesized SrSnO3 is then utilized to produce a SrSnO3@MWCNT photocatalyst. The incorporation of MWCNTs has a great effect on several properties of SrSnO3 nanoparticles that include structural, optical, and optoelectronic properties. In the synthesized heterojunction, MWCNTs were consistently dispersed and led to a higher surface area of the material. The band gap energy of SrSnO3@MWCNT was close to the desired value of ∼2.86 eV. Light absorption ability was significantly improved with exceptional charge-carrier separation ability. Using a 1.6 g L–1 photocatalyst dose and a 9 h reaction time, a hydrogen yield of 4200 μmol g–1 was achieved. SrSnO3@MWCNT shows remarkable stability upon reuse and its structural integrity does not get compromised with regeneration.

Acknowledgments

This project was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, Saudi Arabia, under grant no. G: 38-130-1441. The authors, therefore, acknowledge with thanks DSR for technical and financial support.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c04143.

  • Comparison of the recently published results for photocatalytic evolution of H2 with the results of this present study; XRD pattern; and high-resolution XPS spectra (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c04143_si_001.pdf (175.7KB, pdf)

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