Enhanced sunlight-driven photocatalysis owing to synergetic effect of gold nanoparticles-incorporated ZnO/rGO ternary heterostructures

Neha Athar; Gul Naz; Muhammad Ramzan; Muhammad Shahid Sadiq; Muhammad Arshad; Hafiz Muhammad Adeel Sharif; Awatif A. Hendi; Maha M. Almoneef; Manal A. Awad

doi:10.1016/j.jksus.2024.103104

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03 2024

:36;

103104

doi:

10.1016/j.jksus.2024.103104

Enhanced sunlight-driven photocatalysis owing to synergetic effect of gold nanoparticles-incorporated ZnO/rGO ternary heterostructures

Neha Athar^{^a}, Gul Naz^{^a,⁎}, Muhammad Ramzan^{^a}, Muhammad Shahid Sadiq^{^a}, Muhammad Arshad^{^b}, Hafiz Muhammad Adeel Sharif^{^c}, Awatif A. Hendi^{^d}, Maha M. Almoneef^{^d,⁎}, Manal A. Awad^{^e}

a Institute of Physics, Faculty of Physical and Mathematical Sciences, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan

b Nanosciences and Nanotechnology Department, National Centre for Physics, Quaid-i-Azam University, Islamabad, Pakistan

c Research Center for Eco-Environmental Engineering, Dongguan University of Technology, Dongguan 523808, China

d Department of Physics, College of Science, Princess Nourah Bint Abdulrahman University, P.O. Box 84428, Riyadh 11671, Saudi Arabia

e King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia

⁎Corresponding authors. gul.naz@iub.edu.pk (Gul Naz), mmalmoneef@pnu.edu.sa (Maha M. Almoneef)

Received: 2023-6-12, Accepted: 2024-1-16,

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Abstract

Hydrothermal synthesis of pristine ZnO, ZnO/rGO binary nanocomposites (BNCs) and ZnO/rGO/Au ternary nanocomposites (TNCs).
Small Au nanocubes and multifaceted pentagonal ZnO nanostructures wrinkled on the sheet of rGO in ternary nanocomposites.
Investigation of lattice parameters of pristine ZnO nanostructures, ZnO/rGO, and ZnO/rGO/Au nanocomposites.
Enhanced sunlight-driven photocatalysis by ZnO/rGO/Au TNCs as nanocatalysts.
Photoexcited electrons of Au NPs surface migrate to the conductive rGO layers through ZnO conduction band.

Abstract

The combination of gold nanoparticles (Au NPs) incorporated ZnO/rGO heterostructures is unique for sunlight-driven photocatalysis (SPC). Here, hydrothermally synthesized pristine ZnO nanostructures (NSs), ZnO/rGO binary nanocomposites (BNCs) and ZnO/rGO/Au ternary nanocomposites (TNCs) of bandgap (E_g) values 3.56 eV, 3.37 eV and 3.17 eV, respectively are reported. These NSs are analyzed optically, structurally and morphologically via UV–Vis absorption spectroscopy, X-ray diffraction and SEM techniques, respectively. The SPC of methylene blue (MB) organic dye was considered using all three samples. The slow charge transfer over grain boundaries of ZnO results in the prohibition of photogenerated electron-hole pairs, exhibiting low photodegradation efficiency (87%). However, in BNCs the creation of alternative pathway of the carriers due to low work function level of rGO as compared to ZnO enhances the charge separation with an increased photocatalytic efficiency (92%). In contrast, TNCs show rather fast photodegradation efficiency (96%) as the photoexcited electrons on Au NPs surface migrate to the CB of ZnO because of the SPR stimulation of Au. These electrons easily shift to the conductive rGO layers and react with oxygen species (O₂) producing superoxide radicals (•O₂^–). Thus, TNCs sample has appeared to be auspicious candidate for the photodegradation of organic pollutants in untreated water.

Keywords

ZnO/rGO/Au nanocomposites

Band gap

Surface plasmon resonance

Sunlight-driven photocatalysis

Methylene blue dye

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1 Introduction

With the growing era, water pollution has been appearing as a major issue due to the raw excrete ejected mainly from industries (Lin et al., 2020). Surveys show that water usually gets contaminated by 70% of the raw excrete (Rafiq et al., 2021), mainly including organic pollutants (Juneja et al., 2018). To find a source that should be able to conserve water, particularly efficient along with economical rate, is of major concern. For example, sunlight can be considered over water sanitization to eliminate organic impurities with the aid of photocatalysts. Due to 5% ultraviolet (UV) and 95% visible energy ranges (Chaudhary et al., 2017), the sunlight-driven photocatalysis (SPC) is sprouting (Asgharian et al., 2019, Koe et al., 2020) and as a photocatalyst, semiconducting metal oxides (MOs) are being in use for many years (Ameta et al., 2018, Tahir et al., 2021, Tahir et al., 2022 ). In this regard, pentavalent optoelectronic zinc oxide (ZnO) of band gap (E_g) ∼ 3.37 eV and with a high exciton binding energy value of 60 meV at ambient temperature can play a significant role (Lee et al., 2016). Due to its thermal stability, high optical performance, low price, and electrochemical properties, ZnO can be used in energy storage, water treatment and biomedical fields (Mahajan et al., 2018).

Nevertheless, the wide E_g of ZnO limits the photodegradation reaction (Chen et al., 2017). Thus, to improve the photocatalytic activity, graphene (Gr) or its derivatives are combined with ZnO to improve its photodegradation performance (Chandrakalavathi et al., 2018). The Gr due to its large surface area has potential usage in many semiconducting devices (Seabra et al., 2014). Additionally, reduced graphene oxide (rGO), being more stable than pristine Gr, has higher surface area, high thermal conductivity (Mei et al., 2015), and it can be functionalized easily like Gr (Minta et al., 2020). Few parameters are considered for improving the photocatalytic activities e.g., light harvest performance and photogenerated electron-hole pairs (Han et al., 2016). Additionally, doping of noble metal nanoparticles (NPs) into ZnO/Gr-based nanocomposites has been demonstrated as beneficial for many uses due to their unique plasmonic features (Hsieh and Ting, 2018, Rajeswari and Prabu, 2020).

Subsequently, the incorporation of gold (Au) into ZnO/rGO-based nanocomposites (NCs) has resulted in improved photocatalytic activity (She et al., 2017) due to its surface plasmon mediated effect (Saber et al., 2021). Following a two-step process i.e. pulsed laser and wet chemical synthesis, a ternary nanocomposites of ZnO/Au/rGO heterostructures have shown remarkable photodecomposition efficiency of 95% for methylene blue (MB) dye after 120 min solar irradiation (Naik et al., 2020). Through the mediation of a cationic ligand, Au NPs-decorated rGO wrapped on ZnO hollow heterostructures, have exhibited significant photocatalytic degradation (Amendola et al., 2017).

In this work, sunlight-driven photocatalytic degradation of MB using hydrothermally synthesized pristine ZnO NSs, binary nanocomposites (BNCs) of ZnO/rGO and ternary nanocomposites (TNCs) of ZnO/rGO/Au is described. Among various multi-steps methods, the hydrothermal way is environment friendly, modest, low temperature operated (Low et al., 2016) and producing high yield at controlled rate (Polsongkram et al., 2008). The reusability of all three photocatalysts is also examined through photocatalytic recycling tests over certain runs.

2 Experimental

2.1

2.1 Ternary nanocomposites synthesis

The required chemicals, synthesis of Gr, GO, pristine ZnO NSs and BNCs along with their characterizing techniques is provided in Supporting Information (SI). Here in hydrothermal synthesis of TNCs, 200 mg of GO was added in 150 mL of DI water and stirred for 1 h. Then, 20 mL of 70 mM ZnC₄H₆O₄ solution and 29.4 mM HAuCl₄·3H₂O stock solution each were alternatively mixed into GO solution. Then, NaOH solution was added dropwise to bend the pH to 11. Under 30 min stirring, 400 mg of NaBH₄ was added. Using an autoclave, a hydrothermal treatment was given for 30 min at 150 ℃. Finally, the solution was filtered and washed with DI water. The yield was calcined at 250 ℃ for 4 h to obtain a fine powder of TNCs (Fig. 1).

Schematic representation of ZnO/rGO/Au ternary nanocomposites synthesis.

2.2

2.2 Photocatalytic testing

The SPC of MB using all three photocatalysts was performed on clear days of July 2021 (date: 08–13 July) during 12:00 $-$ 2:00 pm at The Islamia University of Bahawalpur, Pakistan. Using SR30D1-pyranometer by Hukseflux, the sunlight intensity was $\approx$ 1045–1046 W/m². First 25 mg of TNCs was added in 50 mL of MB (10 mg/L) solution. The mixture was kept in dark for 50 min until it achieved the absorption–desorption equilibrium. Then, the solution was taken to sunlight under stirring. After every 15 min, 2 mL solution was collected to perform optical dispersion (OD). For pristine ZnO NSs and BNCs, the dark room time was 15 min and 50 min, respectively. The photodegradation efficiency of MB is evaluated by;

(1)

% o f d e g r a d a t i o n (η) = \frac{A_{o} - A_{t}}{A_{o}} \times 100 %

where ‘A_o’ is the initial absorbance of MB and ‘A_t’ is the absorbance at time ‘t’ (Naik et al., 2020). The photodecolorization can be indicated by the Langmuir-Hinshelwood equation (Juneja et al., 2018);

(2)

\ln (\frac{A_{t}}{A_{o}}) = k \times t

3 Results and discussion

3.1

3.1 UV–visible spectroscopic analyses

Fig. 2(a) shows the absorption peak of Gr at 251 nm with $π - π$ * transition of C = C bonding, while due to C = O in GO the peak at 290 nm shows the $n - π$ * transition (Singh et al., 2016). Fig. 2(b) shows the dense absorption of visible light at 525 nm for Au NPs. Fig. 2(c) shows formation of crystalline ZnO NSs with wide E_g value of 3.56 eV. Fig. 2(d) shows that BNCs may cause enhanced charge separation by photogenerated electron-hole pairs due to reduction of the E_g to 3.37 eV. In Fig. 2(e), a bathochromatic shift in the absorption edge of Au NPs is apparent in TNCs spectrum along with the traces of rGO peak below 380 nm. The attachment of Au⁺ ions into ZnO lattice may lead to a narrowing E_g while stablishing a p-type conductivity (Kang et al., 2016). Such plasmonic Au attachment leads to further lowering of the E_g to 3.17 eV. By Equation (3), the absorption coefficient of pristine ZnO NSs, BNCs, and TNCs is determined for Tauc plot.

(3)

{(α h ν)}^{n} = B (h ν - E_{g})

Optical analyses of (a) Gr and GO, (b) Au NPs, (c) ZnO NSs, (d) ZnO/rGO binary nanocomposites and (e) ZnO/rGO/Au ternary nanocomposites.

Where, ‘ $α$ ’ is the coefficient of absorption, ‘B’ is a tailing parameter, ‘hν’ is the photon energy and ‘n’ is the index i.e. 2, 1/2, 3 depending on intraband transitions. Briefly, a good linear fit arises on extrapolating Tauc plot for n = 2 (indirect transition) (Kumar et al., 2016).

3.2

3.2 X-ray diffraction analyses

In Fig. 3, the structural analyses for all the samples are shown. Fig. 3(a) shows a sharp peak of GO at 42.13° having (1 0 0) plane (JCPDS No. 01–089-8487) (Stobinski et al., 2014). Fig. 3(b) shows the diffraction peaks of Au NPs at 38.29° and 44.03° representing (1 1 1) and (2 0 0) planes, respectively. Small peaks at 64.46° and 77.41° correspond to (2 2 0) and (3 1 1) planes, respectively (JCPDS No. 00–001-1174). Fig. 3(c) shows the diffraction peaks of hexagonal wurtzite ZnO NSs as (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 1), (2 0 1), and (0 0 4) planes at 31.74°, 34.45°, 36.17°, 47.64°, 56.59°, 62.83°, 68.06°, 69.27°, and 72.49°, respectively (JCPDS No. 00–005-0664). In BNCs, the shifting of diffraction peaks due to incorporation of multifaceted ZnO NSs onto rGO lattices is shown in Fig. 3(d).

Stacking of X-ray diffraction pattern of (a) GO, (b) Au NPs, (c) ZnO NSs, (d) ZnO/rGO BNCs and (e) ZnO/rGO/Au TNCs.

In Fig. 3(e), the XRD pattern of TNCs shows a minor shift in longer 2 $θ$ value, which confirms the larger particle size due to an increase in grain boundaries under hydrothermal condition. Further, the addition of rGO causes the formation of aggregates in TNCs sample as clear from the SEM image in Fig. 4(c). Such parameters may result in an increase in the crystallite size (Głuchowski et al., 2022). The average crystallite size (D) and inter-planner spacing (d) of all samples are tabulated in Table 1.

Morphological analyses of (a) pristine ZnO NSs, (b) binary nanocomposites of ZnO/rGO, and (c) ternary nanocomposites of ZnO/rGO/Au along with (d) EDX spectrum of ternary nanocomposites of ZnO/rGO/Au.

Table 1 Lattice parameters of pristine ZnO NSs, ZnO/rGO, and ZnO/rGO/Au nanocomposites.

Sample ID	$c / a$	D (nm)	d (Å)	Lattice Strain (ε)	Dislocation density/ $δ \times 10^{- 3}$ (nm⁻²)	Bandgap E_g (eV)
Pristine ZnO	1.596	30.63	0.21	0.065	0.578	3.56
ZnO/rGO	1.818	84.46	2.23	0.001	0.562	3.37
ZnO/rGO/Au	1.144	92.76	2.24	0.001	0.14	3.17

The granular magnitude is evaluated by Debye-Scherer formula;

(4)

D = \frac{k λ}{β C O S θ}

where ‘k’ is the Sherrer’s constant, a crystallite shape factor $\approx$ 0.9. The ‘ $β$ ’ is the half-wave full maxima, ‘ $θ$ ’ is the Bragg’s angle and ‘λ’ is the wavelength (1.54 Å) (Koe et al., 2020). The lattice parameters are calculated by;

(5)

\frac{1}{d_{{hkl}^{2}}} = \frac{4}{3} \frac{h^{2} + h k + k^{2}}{a^{2}} + \frac{l^{2}}{c^{2}}

where a = b and c are the lattice parameters, ‘hkl’ is the Miller indices. The dislocation density $(δ)$ which is defined as the number of dislocations per unit volume of the crystalline material is calculated by

(6)

δ = 1 / D^{2}

Table 1 lists the lattice parameters of ZnO NSs, ZnO/rGO, and ZnO/rGO/Au by using Equation (7). According to Williamson-Hall relation, the decreased ε and δ values results due to an increased crystallite size (Naz et al., 2022).

(7)

ε = \frac{β}{4 t a n θ}

3.3

3.3 SEM analyses

Fig. 4(a) shows the micrograph of multifaceted pentagonal rod-like pristine ZnO NSs of varying sizes above 100 nm. Fig. 4(b) shows the ZnO NSs appear as conjugates on rGO sheets. Fig. 4(c) shows that the multifaceted ZnO NSs and small spherical Au NPs with sizes <100 nm are wrinkled on rGO sheets. Average particle size of ZnO NSs falls in the range 300–800 nm along lateral dimension (excluding rod-like ZnO NSs). In Fig. 4(d), the EDX spectrum confirms the presence of carbon, oxygen, and small traces of Zn and Au in TNCs in terms of elemental purity..

3.4

3.4 Photocatalytic activities

The foundation of photocatalysis (PC) is based on the generation of electron-hole pairs in the nanocatalyst upon light illumination (Rafique et al., 2020). Due to broad E_g of ZnO NSs (3.56 eV), the photocatalytic performance is limited to 87%. To increase the degradation efficiency, the BNCs and TNCs are synthesized and then examined. The detail of photocatalytic activities by pristine ZnO, and ZnO/rGO can be found in SI. The SPC activities of TNCs were performed and compared with the available literature in order to investigate their photodegradation efficiencies (Table 2).

Table 2 Parameters of photodegradation of MB using ternary ZnO/rGO/Au nanocomposites, comparing with the available literature.

Method of synthesis	Morphology	Catalyst loading (mg/L)	Dye conc. (mg/L)	Light Int. (W/m⁻²)	Reaction Time (min)	Photocatalytic efficiency (%)	Light source	Ref.
Combustion	Spherical ZnO NPs, nanosheet of rGO	500	10	1000	120	95	Solar	(Naik et al., 2020)
Electrochemical deposition	NRs ZnO, Au NPs, nanosheet rGO	500	10	300 W	120	82	Visible	(Kang et al., 2016)
Low temperature Hydrothermal	Au/graphene-wrapped ZnO flower-NSs	200	5 µM/ 10 µM	900 ± 30	30	93/100	Solar	(Juneja et al., 2018)
Chemical	Au/graphene-wrapped ZnO hollow spheres	Quartz film deposited (2 × 2 cm²)	10 µM	100 W	150	68	UV	(Khoa et al., 2015)
Electrochemical deposition	Au/rGO-ZnO nanorod arrays	Film deposited (3 × 3 cm²)	10	250 W	120	82	Visible	(Wang et al., 2016)
Hydrothermal	Au/ZnO NSs wrinkled on rGO sheets	500	10	1045–1046	135	96	Solar	Present

3.4.1

3.4.1 Photocatalysis by ZnO/rGO/Au ternary nanocomposites

The SPC activity of TNCs of ZnO/rGO/Au was performed and the effect of rGO and Au loading on ZnO photodegradation efficiency was examined. Their photodegradation rate was observed to be very fast with high efficiency and completed within 105 min. Fig. 5 is showing the schematic of basic mechanism of SPC by TNCs sample. Initially, on the exposure of sunlight, the ZnO NSs absorb light producing holes in the VB and electrons in the CB. The oxidation ability of the VB holes may allow react with the hydroxyl anions, resulting in hydroxyl (•OH) radicals. As Au NPs can produce hot electrons on their surfaces stimulated by their SPR phenomenon, they easily migrate to the CB of ZnO where they are trapped by the conductive rGO sheets because of lower work function of rGO (Φ_rGO = 4.42 eV) than ZnO (Φ_ZnO = 4.05 eV). These holes then react with oxygen species (O₂) to produce superoxide radicals (•O₂^–) (Rodwihok et al., 2019). On the other hand, photogenerated holes in of ZnO VB react with water molecules to produce •OH radicals. The formation of these two radicals is more auspicious for the photodegradation of MB aqueous solution than either alone (Naik et al., 2020). Hence, the addition of Au NPs in BNCs of ZnO/rGO significantly improves the photocatalytic activity by enhanced generation of electron-hole pairs.

Mechanism of sunlight-driven photocatalysis by ZnO/rGO/Au NCs.

In another words, the reduction of E_g of TNCs leads to delayed recombination of electron-hole pairs and improved O₂ and OH radicals as ROS (reactive oxygen species) that cause the decomposition of the organic dye. The efficiency of photodegradation increased to 96% with the k and R² values being 0.019 min⁻¹ and 0.992, respectively. Below given Fig. 6(a) is exhibiting the dye photodegradation while Fig. 6(b-d) is representing the reusability of TNCs nanocatalyst sample.

(a) UV-vis spectra of MB with ZnO/rGO/Au TNCs as photocatalysts during the degradation process under sunlight exposure and (b-d) their reusability tests for SPC activity.

Fig. 7(a) and (b) show the comparison of photodegradation activities (A_t/A_o) and pseudo-first-order kinetics of the reactions by pristine ZnO NSs, BNCs and TNCs nanocatalysts for MB dye degradation as a function of time. The photodegradation efficiency is excellent in the presence of TNCs sample.

Plots showing (a) MB photodegradation (At/Ao) versus time by ZnO NSs, BNCs and TNCs samples and (b) pseudo-first-order kinetics of the reaction, where Ao represents initial concentration and At represents concentration of the photoreaction at a particular time t after photo-irradiation.

From Fig. 7, it is clear that TNCs sample has exhibited maximum photodegradation of MB dye under 135 min of sunlight exposure while ZnO/rGO suppressed the photocatalytic performance slightly in comparison to ZnO/rGO/Au TNCs. The given Table T1 in SI shows the first-order kinetic constants, relative coefficients along with their photocatalytic efficiencies for SPC degradation of MB dye using pristine ZnO NSs, BNCs and TNCs nanocatalysts.

3.4.2

3.4.2 Reusability tests

In order to carry out heterogeneous photocatalysis into wastewater treatment applications, the general price of the product and/or procedure should be minimized. To investigate the cost of the photocatalytic activity, the reusability of all three photocatalysts is employed. After first round of photodegradation, the remaining solutions were centrifuged to recover the samples. Their photocatalytic activities were repeated three times to check the photocatalytic efficiency of reused pristine ZnO NSs, BNCs and TNCs samples. Fig. 8 is showing the comparison of photo-effectiveness of TNCs photocatalysts through reusability efficiency bar graph. Thus, coupling semiconducting materials with plasmonic Au and graphene produces advantageous photocatalysts for organic remediation.

A histogram showing the comparison of recycled photodegradation efficiencies of ZnO/rGO/Au NCs.

4 Conclusion

In this study, the crystalline phases of pristine ZnO NSs, ZnO/rGO (BNCs) and ZnO/rGO/Au (TNCs) were synthesized under hydrothermal conditions. The bandgap (E_g) values of pristine ZnO NSs, BNCs and TNCs followed the scheme 3.56 eV > 3.37 eV > 3.17 eV, respectively. All synthesized powder samples were examined for sunlight-driven photocatalytic (SPC) degradation of methylene blue (MB) dye. The higher work function of rGO as compared to ZnO caused creation of alternative pathway of the carriers, thus enhancing the charge separation and resulting in increased photocatalytic efficiency of BNCs than pristine ZnO NSs. On contrary, TNCs showed fastest photodegradation efficiency as the photogenerated electrons on Au NPs surface under SPR stimulation migrate to CB of ZnO where they are trapped by the conductive rGO sheets and react with oxygen species, producing superoxide radicals (•O₂^–). The photoeffectiveness for pristine ZnO, BNCs and TNCs were 87%, 92% and 96%, respectively.

Acknowledgments

Princess Nourah bint Abdulrahman University Researchers Supporting Project Number (PNURSP2024R56), Princess Nourah bint Abdulrahman University (Riyadh/Saudi Arabia) funded this research work.

Declaration of competing interest

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

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Appendix A

Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jksus.2024.103104.

Appendix A

Supplementary data

The following are the Supplementary data to this article:

Supplementary Data 1

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Introduction

Experimental

Ternary nanocomposites synthesis
Photocatalytic testing

Results and discussion

UV–visible spectroscopic analyses
X-ray diffraction analyses
SEM analyses
Photocatalytic activities

Conclusion

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Enhanced sunlight-driven photocatalysis owing to synergetic effect of gold nanoparticles-incorporated ZnO/rGO ternary heterostructures

Abstract

Abstract

Keywords

ZnO/rGO/Au nanocomposites

Band gap

Surface plasmon resonance

Sunlight-driven photocatalysis

Methylene blue dye

1 Introduction

2 Experimental

2.1 Ternary nanocomposites synthesis

2.2 Photocatalytic testing

3 Results and discussion

3.1 UV–visible spectroscopic analyses

3.2 X-ray diffraction analyses

3.3 SEM analyses

3.4 Photocatalytic activities

3.4.1 Photocatalysis by ZnO/rGO/Au ternary nanocomposites

3.4.2 Reusability tests

4 Conclusion

Acknowledgments

Declaration of competing interest

References

Appendix A

Supplementary data

Appendix A

Supplementary data

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