Comprehensive Study on Lemon Juice-Based Green Synthesis and Catalytic Activity of Bismuth Nanoparticles.

Bismuth nanoparticles have gained considerable interest in catalysis because of their small size, large surface-to-volume ratio, and low toxicity. In spite of these advantages, the toxic reagents and solvents used in the synthetic process are significant limitations to their development and utilization. In this study, a green approach employing easily accessible lemon juice was applied for the synthesis of bismuth nanoparticles (BiNPs) as a green alternative to conventional chemical ones. This study clarified the formation and growing process of green-synthesized BiNPs using lemon juice as a reducing and capping agent. The reaction time and amounts of lemon juice significantly affect the growth, morphology, and stability of BiNPs, as confirmed from XRD, DLS, SEM, and TEM analyses. The synthesized BiNPs effectively catalyzed the reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH4, and the reduction was significantly accelerated by sunlight and the removal of the fibrous coating layer around BiNPs. Moreover, the synthesized BiNPs also show excellent catalytic efficacy toward the reduction of organic dyes, namely, methyl orange, methylene blue, and rhodamine B. All catalytic reductions followed the pseudo-first-order kinetics, and the rate constants are in the order of k MB > k RhB > k MO > k 4-NP. The stated biogenic synthetic route paves the way for the green industrial fabrication of BiNPs and their uses in catalysis for wastewater treatment.

Introduction

The development of nanostructured materials has garnered much attention due to their fascinating physical and chemical properties and wide range of applications. Bismuth nanoparticles (BiNPs) are promising individuals with unique properties, including low toxicity, high stability, high X-ray attenuation coefficient, strong near-infrared absorbance, and high photothermal conversion efficiency.[1−9] Research on BiNPs has been rapidly increasing this decade and has been successfully utilized in bio-applications,[10,11] catalysis,[12−16] radiation shielding,[17] and sensing.[18−21] Specifically, BiNPs have attained a growing research interest in catalysis, such as the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) using NaBH4,[14,22] efficient electrochemical CO2 reduction,[16] solution–liquid–solid growth of CdSe quantum wires,[12] and reductive coupling of nitroarenes to azo compounds.[15] The photocatalytic performance of BiNPs was also reported by Dong et al. and Qin et al. for the removal of NO[13] and the reduction of Cr(VI) to Cr(III),[23] respectively. Moreover, BiNPs are also used for the efficient removal of Cr(VI).[24]

In order to synthesize high-quality BiNPs, a variety of synthetic approaches have been employed. The solution-phase chemical reduction method is the most popular due to its facile nature,[4,7,14,20,22,25−29] but this approach is not environmentally benign enough due to the use of toxic reducing agents/organic solvents and/or auxiliary stabilizers that are still unavoidable. Therefore, to overcome the challenges of this method, green synthesis could be an expectative alternative, enabling eco-friendly and cost-effective protocols using green and renewable materials.

The green approaches utilize materials derived from animals, microorganisms, and plants as reducing and capping agents.[30] Plant sources are advantageous in their wide variety and easy availability allowing simple, rapid, and cost-effective procedures suitable for large-scale production.[31−33] However, to the best of our knowledge, very few reports are available,[34−36] and only one report deals with plant sources-based synthesis, which produced amorphous BiNPs using hydroalcoholic extract of Moringa oleifera leaves.[36] We recently reported the first biogenic synthesis of crystalline BiNPs using lemon juice as a reducing and capping agent.[37] Lemon, which is commonly used for cooking all over the world, has been employed for the synthesis of various nanomaterials owing to the high contents of bioactive compounds like ascorbic acid, citric acid, sugars, and polyphenols serving as reducing agents.[38−41]

Nitroaromatic compounds are widely utilized in various industries including pharmaceuticals, agricultural chemicals, and dyes.[42] These compounds are highly hazardous to the ecological system when released into the environment. Due to their stable nature in the environment and resistivity toward biodegradation, they are listed as priority pollutants by US Environmental Protection Agency.[43,44] Similarly, organic dyes are also responsible for substantial damage to the ecological system. These two are the most hazardous pollutants in the universe.[45] Thus, the removal of these hazardous substances is an essential environmental subject. Nanocatalysis has received intensive attention to accelerate the removal of such harmful substances.[43,46,47] Currently, several metallic nanoparticles, including those of silver (AgNPs) and gold (AuNPs), are widely and effectively utilized to accelerate the removal of various nitroaromatic compounds and organic dyes through their catalytic reduction using sodium borohydride (NaBH4).[42,48−56] BiNPs are advantageous over AgNPs and AuNPs because of the higher abundance and lower price. Thus, BiNPs could be a suitable alternative catalyst for removing the above-mentioned hazardous substances. However, only two works are reported for the catalytic applications of chemically reduced BiNPs for the reduction of nitroaromatic compounds.[14,22] Furthermore, no report represents the catalytic reduction of organic dyes using BiNPs as a catalyst.

In this work, we demonstrate a detailed study on the lemon juice-based green synthesis of BiNPs and their catalytic performance toward the reduction of 4-NP as a model of nitroaromatics and organic dyes, namely, methyl orange (MO), methylene blue (MB), and rhodamine B (RhB), in the presence of NaBH4.

Results and Discussion

Based on the previous preliminary work revealing that the reaction of Bi(NO3)3 and lemon juice produces crystalline BiNPs stabilized by phytochemicals capping on the surface,[37] we investigated the effect of time and the amount of lemon juice on the growing process and morphology of BiNPs.

Effect of Reaction Time

First, the time course of the formation of BiNPs was examined using 15 mL of lemon juice to 121.2 mg of Bi(NO3)3·5H2O (Table ). The yield after 1 h of the reaction was 67.6%. After 2 h, the yields of BiNPs increased and became almost identical, indicating the completion of the reduction of Bi3+ ions. Plausible phytochemicals for the reduction are ascorbic acid, citric acid, and sugars.

Table 1

Summary of the BiNPs Obtained at Different Reaction Timesa

run	reaction time (h)	yield (%)b	crystallite size (nm)c	residual weight at 500 °C (%)d
1	1	67.6	19	89
2	2	84.1	20	84
3	4	85.3	21	82
4	6	85.8	29	80

Conditions: Bi(NO3)3·5H2O = 121.2 mg; lemon juice = 15 mL, 80 °C, pH = 12.3–12.4.

Isolated yield after centrifugation assuming as the theoretical yield.

Calculated using Scherrer’s formula applied to the peaks at 27.1, 37.9, and 39.6°.

Residual weight after TGA measurement (10 °C/min, N2).

The growth of the crystal in BiNPs was evaluated by XRD measurements. The XRD patterns (Figure ) of the obtained product synthesized using 15 mL of lemon juice at different times were indexed to the pure rhombohedral phase of elemental bismuth (JCPDS no. 44-1246),[57,58] indicating that the obtained products are BiNPs without detectable oxide phases. The time-dependent XRD patterns of the obtained BiNPs show that the peaks became sharper with the reaction time, indicating the growth of the crystallites. The average sizes of nanocrystallites were estimated by Scherrer’s formula as followswhere D is the diameter of the coherent diffraction domain, k is the shape constant (k = 1 for spherical domains), λ is the wavelength of the X-ray source (0.1541 nm), β is the full width at half-maximum, and θ is the diffraction angle corresponding to the lattice planes (012), (104), and (110).

Figure 1

XRD patterns of BiNPs synthesized using 15 mL of lemon juice at different reaction times (1–6 h).

The average crystallite sizes calculated are 19, 20, 21, and 29 nm for the reaction times of 1, 2, 4, and 6 h, respectively, quantitatively indicating the gradual growth of the crystallites.

The morphology and growing process of the green synthesized BiNPs were evaluated using electron microscopy. Figure S1 shows the scanning electron microscopy (SEM) images of the BiNPs synthesized using 15 mL of lemon juice at different reaction times. BiNPs with irregular morphologies were observed in the SEM images. Amorphous substances coating the solid substances became apparent with the increase of the reaction time. Specifically, the SEM image of the product obtained at 6 h did not show clear particles. The agglomeration occurred during drying, as confirmed by the stable water dispersibility of BiNPs described later. Although the amorphous substance originating from the phytochemicals of lemon juice prevents the measurement of the particle sizes from the SEM images, we predicted the approximate size of the particles in the range of 50–100 nm from the image of the product obtained at 2 h.

To understand the process of the growth of Bi core, we carried out a transmission electron microscopy (TEM) analysis for the BiNPs synthesized using 15 mL of lemon juice at different reaction times (Figure ). The TEM image of BiNPs obtained at 1 h shows that primary particles smaller than 10 nm were predominantly formed and that larger particles were also formed presumably by the fusion of the primary particles. As the reaction proceeded, the seed particles decreased, and, concurrently, larger spherical particles increased. The size of the grown particles becomes larger with time. The TEM image of BiNPs obtained at 6 h shows that seed particles are attached around the edge of larger spheres, implying that the fusion of primary particles is the major factor in the growth of BiNPs rather than the growth of the primary crystals.

Figure 2

TEM images of the BiNPs synthesized using lemon juice at different reaction times (1–6 h).

Figure depicts the thermogravimetric analysis (TGA) curves of obtained BiNPs synthesized using 15 mL of lemon juice at different reaction times. These TGA curves show two stages of weight loss. The first one below 120 °C mainly originates from the evaporation of adsorbed water and light phytochemicals, and the second one from 170 to 430 °C is associated with the decomposition of phytochemicals capping on the surface of BiNPs. In addition, the weight loss gradually increases from 9 to 17% as the reaction time becomes longer, indicating the increase of the capping phytochemicals on the surface of the BiNPs with the progress of the reaction.

Figure 3

TGA curves of obtained BiNPs synthesized using lemon juice at different reaction times (10 °C/min, N2 flow).

The dispersion stability of BiNPs was confirmed by dynamic light scattering (DLS) measurements conducted on different days to measure the hydrodynamic sizes of the BiNPs obtained at different reaction times using 15 mL of lemon juice (Figure S2). The higher average hydrodynamic diameter (Dh) and the polydispersity index (PDI) of the BiNPs obtained at 1 h are attributed to aggregation due to insufficient surface stabilization. Beyond 2 h, the Dh and PDI are almost stable, indicating that proper surface stabilization was achieved within 2 h. A similar tendency was observed for the dispersion stored for 7 days, namely, the size of BiNPs obtained at 1 h is larger than others. While the Dh values of BiNPs obtained at 1 and 2 h were almost identical, those for 4 and 6 h became slightly larger than those at day 1, probably due to slight aggregation. However, no precipitation was observable, and the relatively small sizes indicate the good water dispersibility of BiNPs maintained for 7 days.

Overall, the investigation on the effect of the reaction time suggests that 2 h is the optimum reaction time to produce BiNPs with good quality in this synthesis using lemon juice as a reducing and capping agent.

Effect of Amount of Lemon Juice

The effect of the amount of lemon juice on the formation of BiNPs was examined using different amounts of lemon juice to 121.2 mg of Bi(NO3)3·5H2O at 2 h (Table ).

Table 2

Summary of the BiNPs Obtained Using Different Amounts of Lemon Juicea

run	lemon juice (mL)	yield (%)b	crystallite size (nm)c	residual weight at 500 °C (%)d
1	10	70.3	14	74
2	15	84.1	19	83
3	20	85.7	21	85
4	25	88.4	21	87

Conditions: Bi(NO3)3·5H2O = 121.2 mg; lemon juice = 15 mL, 80 °C, pH = 12.3–12.4.

Isolated yield after centrifugation assuming as the theoretical yield.

Calculated using Scherrer’s formula applied to the peaks at 27.1, 37.9, and 39.6°.

Residual weight after TGA measurement (10 °C/min, N2).

Similar to the reaction time, the amount of lemon juice also affects the X-ray diffraction (XRD) profiles of BiNPs (Figure ). The peaks became sharper as the amount of lemon juice increased, indicating the formation of larger crystals under the conditions with higher amounts of lemon juice. The average crystallite sizes are 14, 20, 21, and 21 nm for 10, 15, 20, and 25 mL of lemon juice, respectively. A probable reason is enhanced reduction by the increase of reducing species in lemon juice.

Figure 4

XRD patterns of the BiNPs synthesized using different amounts of lemon juice for 2 h.

Figure S3 shows the representative SEM images of BiNPs synthesized using different amounts of lemon juice at 2 h. BiNPs with irregular morphologies were observed in the SEM image of the product obtained using 10 mL of lemon juice. The shape of the particles became regular with the increase of the amount of lemon juice. Specifically, the SEM image of the product obtained using 25 mL of lemon juice shows the presence of almost spherical particles with the approximate size of 50–100 nm. In addition, fibrous structures probably consisting of phytochemicals were also observed. In our previous report, we clarified that the fibrous coating layer around BiNPs originated from the phytochemicals present in the lemon juice. We also clarified that ethanol and chloroform can partially remove the coating layer and found that these are mainly polysaccharides and fatty acid derivatives.[37]

The TEM images of BiNPs synthesized using different amounts of lemon juice (Figure ) show the presence of predominantly formed primary crystals smaller than 10 nm with some larger particles. As the amount of lemon juice increased, the number of BiNPs increased, which is attributable to the enhanced reduction with the increase of reducing species in lemon juice.

Figure 5

TEM images of the BiNPs synthesized using different amounts of lemon juice for 2 h.

TGA curves of the BiNPs synthesized using different amounts of lemon juice (Figure ) show that the weight loss occurred at the same temperature range as the aforementioned TGA curves for BiNPs obtained at different times. The weight loss gradually decreases from 16 to 9% as the volume of lemon juice increases from 10 to 25 mL. The highest weight loss of BiNPs obtained using 10 mL of lemon juice could be associated with the incomplete reduction of Bi3+ ions.

Figure 6

TGA curves of the BiNPs synthesized using different amounts of lemon juice for 2 h.

The dispersion stability of BiNPs synthesized using different amounts of lemon juice at 2 h was evaluated by DLS measurement conducted after storage for 1 and 7 days (Figure S4). The higher Dh and PDI values of the BiNPs obtained with 10 mL of lemon juice are attributed to aggregation due to the incomplete reduction and insufficient surface stabilization. The Dh and PDI values of BiNPs using more than 15 mL are almost stable, indicating that the surface was properly stabilized using higher amounts of lemon juice. A similar tendency was observed for the dispersion stored for 7 days, namely, the size of BiNPs obtained with 10 mL of lemon juice is larger than others. While the Dh values of BiNPs obtained using 10 mL of lemon juice stayed almost identical, others became slightly larger than those on day 1, probably due to slight aggregation. As observed for the BiNPs obtained at different reaction times, BiNPs obtained using different amounts of lemon juice also showed good water dispersibility for 7 days. The high colloidal dispersibility of the BiNPs originates from the coating layer observed in the SEM images consisting of phytochemicals in the lemon juice. The larger Dh measured by DLS than the size observed by SEM can be attributed to the surrounding hydration layer and swelled phytochemicals attached to the surface of the BiNPs.

Overall, the analysis of BiNPs synthesized using different amounts of lemon juice for 2 h revealed that higher amounts of lemon juice produce BiNPs with better quality. Therefore, for the investigation of catalytic activity, we employed BiNPs synthesized using 25 mL of lemon juice for 2 h because of the sufficient yield, quality of the crystal, and high Bi content.

Catalytic Reduction of 4-NP

According to the previous reports,[14,59] the reduction of 4-NP only with aqueous NaBH4 is unachievable due to the presence of the kinetic barrier and the potential difference between the donor (borohydride) and the acceptor (4-nitrophenolate ions). The inclusion of metal nanoparticles can diminish the kinetic barrier by decreasing the activation energy, and the reduction proceeded by facilitating the relay of the electron from the donor to the acceptor entities. In addition, the reduction follows pseudo-first-order kinetics with respect to the substrate 4-NP.[14,22,42,59] In this study, an extended investigation on the catalytic reduction of 4-NP is demonstrated, including the acceleration by sunlight during the catalytic reduction process. Because sunlight is a freely available and eternal light source, its utilization can afford an additional advantage in the development of sustainable wastewater treatment.[60]

Thus, the catalytic reduction of 4-NP was conducted under room light, in the dark, and under sunlight in the presence of BiNPs and NaBH4. The progress of the catalytic reduction of 4-NP was monitored by UV–vis spectroscopy by measuring the time-dependent absorption spectra of the reaction mixture based on previous reports.[14,22,37] Briefly, the maximum absorption peak (λmax) of aqueous 4-NP was observed at 318 nm in the UV–vis spectra. Upon the addition of freshly prepared NaBH4 solution, the peak position shifted from 318 to 402 nm, indicating the formation of 4-nitrophenolate ions. In the absence of BiNPs, the peak intensity at 402 nm and color of the mixtures remained constant even after 12 h, irrespective of the reaction environment. With the addition of BiNP dispersion, the peak intensity at 402 nm gradually decreased. Meanwhile, a new peak grew at 300 nm (Figure a–c). The decrease of the peak intensity at 402 nm and the increase at 300 nm took place simultaneously (Figure a–c) by the formation of 4-aminophenol (4-AP) as the reduced product of 4-NP. Similar to previous reports,[14,22,59] the reaction rate follows pseudo-first-order kinetics agreeing with the Langmuir–Hinshelwood mechanism, schematically displayed in Figure , in which the catalytic reduction proceeds on the surface of BiNPs according to the following step. First, borohydride ions react with BiNPs and transfer a hydride to the surface of BiNPs to form the Bi–H covalent bond; second, the reaction of 4-NP concomitantly adsorbed on BiNPs and surface-bound hydride forms 4-AP; and finally, 4-AP desorbs from the surface of BiNPs. The rate constants (k) of the reduction under room light, in the dark, and under sunlight were calculated from the linear plots of ln(A/A0) versus time (Table and Figure d) under pseudo-first-order kinetics and found to be 9.87, 8.13, and 36.40 g–1 min–1, respectively. The catalytic rates under room light and dark are comparable, whereas the rate under sunlight is 3.5 times higher than others. We presume that the sunlight illuminations did not lead to the change of the catalytic mechanism, rather, the UV-light present in sunlight accelerated the electron transfer to enhance the catalytic conversion. The presumption was confirmed from a comparative analysis of the catalytic reduction of 4-NP with and without UV irradiation, where 4-NP was completely reduced within 20 min under UV irradiation (365 nm), whereas it took 50 min without UV irradiation. In the absence of BiNPs, UV irradiation does not show any changes in the color of the mixture even after 50 min.

Figure 7

Optical images and time-dependent UV–vis absorption spectra for the catalytic reduction of 4-NP by NaBH4 in the presence of BiNPs (a) under room light, (b) in the dark, and (c) under sunlight. (d) Pseudo-first-order kinetic plot of the catalytic reduction.

Figure 8

Mechanistic model (Langmuir–Hinshelwood mechanism) of the reduction of 4-NP by sodium borohydride in the presence of BiNPs.

Table 3

Corresponding Data of the Catalytic Reduction of 4-NP at Different Conditions

catalysts	conditions	reduction time (min)	rate constant, k (g–1 min–1)
BiNPs	under room light	50	9.87
	in the dark	50	8.13
	under sunlight	25	36.40
EtOH washed-BiNPs	under room light	30	16.67

Moreover, we investigated the catalytic efficacy of BiNPs after being washed with ethanol under room light (Figure S5 and Table ). In the presence of BiNPs after being washed with ethanol, complete reduction took place within 30 min (Figure S5a), whereas it took 50 min using original BiNPs (Figure a). The k value for the washed-BiNPs (16.67 g–1 min–1) was 1.7 times higher than the k value of the original BiNPs (9.87 g–1 min–1). The higher catalytic efficiency could originate from the disclosure of active sites for the catalytic reduction upon the partial removal of coating substances and the weakened interaction between the substrate and the surface of BiNPs. Although coating substances were partially removed, BiNPs still maintained excellent water dispersibility, as confirmed by the DLS profile with Dh of 180 nm showing a sharp peak at the thirties of nanometers (Figure S6). The size smaller than the original one suggests that the primary particles departed from aggregates connected by the capping layer. The departure of primary particles can also be confirmed from the TEM image of BiNPs after being washed with ethanol without fibrous structures (Figure ).

Figure 9

TEM images of BiNPs (a) before and (b) after being washed with ethanol.

Catalytic Reduction of Organic Dyes

The catalytic efficacy of lemon-juice-based BiNPs was also investigated toward the reduction of organic dyes. The catalytic reduction of MO, MB, and RhB was carried out using BiNPs under room light, and the progress of the reduction was conveniently monitored by UV–vis spectroscopy as the reduction of 4-NP.

Catalytic Reduction of MO

The poor biodegradability of azo dyes often causes water pollution and irremediable environmental problems.[61] Therefore, the elimination and degradation of azo dyes are essential to save the environment. Nanocatalyst-based catalytic reduction is a common technique to decompose azo dyes into non or weakly toxic constituents. We employed MO as an example of azo dyes. The aqueous solution of MO showed a strong absorption peak at 464 nm. The intensity of the peak negligibly changed in the presence of aqueous NaBH4 even after 2 h, and the color of the mixture remained unchanged. In contrast, with the addition of BiNPs, the absorption peak at 464 nm gradually became weaker and completely disappeared after 22 min, and the color of the reaction mixture changed from orange to colorless (Figure a). This discoloration is attributable to the reduction of azo moieties of MO, producing hydrazine derivatives in the same manner as with other metal nanoparticles.[53] The catalytic reduction of MO also follows the Langmuir–Hinshelwood mechanism, in which two reactants react after adsorption on the solid surface, and then the product desorbs. The catalytic reduction of MO by NaBH4 is known to follow pseudo-first-order kinetics,[45,62] and the rate constant (k) of this reduction catalyzed by BiNPs was calculated to be 181.3 g–1 min–1 from the plot of ln(A/A0) versus time (Table and Figure S8a). BiNPs washed with ethanol were more active than pristine BiNPs in a similar manner with the reduction of 4-NP. The reduction finished within 12 min with the k value of 356.0 g–1 min–1 (Figures S7a and S8a), whereas it took 22 min using original BiNPs (Figure a).

Figure 10

Optical images and time-dependent UV–vis absorption spectra for the catalytic reduction of (a) MO, (b) MB, and (c) RhB by NaBH4 in the presence of BiNPs.

Table 4

Data of the Catalytic Reduction of Organic Dyes with NaBH4 Catalyzed by Original and EtOH-Washed BiNPs

dyes	catalyst status	reduction time (min)	rate constant, k (g–1 min–1)
MO	original	22	181.3
	EtOH washed	12	356.0
MB	original	14	434.7
	EtOH washed	8	1096.0
RhB	original	24	302.7
	EtOH washed	16	441.3

Catalytic Reduction of MB

MB is a cationic water-soluble thiazine dye widely used in chemical and medicinal industries.[48,53] Elimination of MB by degradation can be achieved easily by catalytic reduction. The aqueous solution of MB showed a strong absorption peak at 663 nm with a shoulder at 612 nm, and negligible change was detected in the intensity of the peaks in the presence of NaBH4 even after 2 h, and the color of the mixture remained unchanged. With the addition of BiNPs, the absorption peak at 663 nm gradually became weaker and completely disappeared after 14 min, and the color of the reaction mixture changed from deep blue to colorless (Figure b). This discoloration is attributable to the reduction at the heterocyclic ring of MB, producing colorless leucomethylene blue in the same manner as other metal nanoparticles.[54,62,63]

Similar to MO, the catalytic reduction of MB also follows the Langmuir–Hinshelwood mechanism. The catalytic reduction of MB by NaBH4 is known to follow pseudo-first-order kinetics.[49,64,65] The rate constant (k) of this reduction catalyzed by BiNPs was calculated to be 434.7 g–1 min–1 from the plot of ln(A/A0) versus time (Table and Figure S8b). BiNPs washed with ethanol were more active than pristine BiNPs in a similar manner with the reduction of 4-NP and MO. The reduction finished within 8 min with the k value of 1096.0 g–1 min–1 (Figures S7b and S8b), whereas it took 14 min using original BiNPs (Figure b).

Catalytic Reduction of RhB

RhB is a xanthene-based water-soluble cationic dye extensively used in paint, textiles, and food industries. It is also known as a water tracer fluorescent dye.[42,48,66] Analogous to MO and MB, elimination of RhB by degradation was also achieved by catalytic reduction. The aqueous solution of RhB showed a strong absorption peak at 553 nm, and negligible change was detected in the intensity of the peaks in the presence of NaBH4 even after 2 h, and the color of the mixture remained unchanged. With the addition of BiNPs, the absorption peak at 553 nm gradually became weaker and completely disappeared after 24 min, and the color of the reaction mixture changed from deep pink to colorless (Figure c). This discoloration is attributable to the de-ethylation of the diethylamino group of RhB followed by degradation of the chromophore in the same manner as with other nanomaterials.[42,67] As mentioned for MO and MB, the catalytic reduction of RhB also follows the Langmuir–Hinshelwood mechanism. The catalytic reduction of RhB by NaBH4 is also known to follow pseudo-first-order kinetics,[42,66] and the rate constant (k) of this reduction catalyzed by BiNPs was calculated to be 302.7 g–1 min–1 from the plot of ln(A/A0) versus time (Table and Figure S8c). BiNPs washed with ethanol were more active than pristine BiNPs in a similar manner with the reduction of 4-NP, MO, and MB. The reduction finished within 16 min with the k value of 441.3 g–1 min–1 (Figures S7c and S8c), whereas it took 24 min using original BiNPs (Figure c). In addition, the catalytic reduction of MB took the shortest time among others. In the case of 4-NP and MO, the results complied with the literature, but not for RhB.[48,49,56,68] The exact reason is unknown, but presumably, the phytochemicals present on the surface of BiNPs and the relatively bulky groups of RhB hinder the physical contact of BiNPs with RhB.

Moreover, to evaluate the repeatability, the catalytic reaction using 4-NP with new BiNPs each time was conducted, and it was found that the reduction times were identical (Figure S9).

Conclusions

Stably dispersible rhombohedral crystalline BiNPs are the outcome of the lemon-juice-based green synthetic approach. Utilization of safe and easily accessible lemon juice framed this approach as an economical, efficient, and eco-friendly process, potentially substituting conventional chemical methods employing hazardous reducing agents like sodium borohydride, hydrazine, sodium-hypophosphite, and borane morpholine. The reaction time and the amount of lemon juice have pivotal roles in the growing process and morphology of BiNPs. These green-synthesized BiNPs efficiently catalyzed the reduction of 4-NP, and the efficacy improved under sunlight. Furthermore, BiNPs catalyzed the reduction of organic dyes, namely MO, MB, and RhB. All catalytic reductions followed the pseudo-first-order kinetics. The rates of the catalytic reduction were enhanced by using BiNPs washed with ethanol. This work will contribute to the development of the employment of plant sources for the facile green synthesis of crystalline bismuth nanomaterials and serve a wide range of applications to achieve a better ecological system.

Experimental Section

Collection of Lemon Juice

Yellow lemon was purchased from a supermarket in Yonezawa, Yamagata, Japan, and thoroughly washed with deionized distilled water. The juice was collected by squeezing the lemon followed by centrifugation (13,000 rpm, 10 min) and filtration and finally stored at 4 °C for further use.

Materials

Bi(NO3)3·5H2O, MO, MB, NaBH4, and NaOH were purchased from Kanto Chemical Co. Inc. (Tokyo, Japan). RhB and 4-NP were obtained from Tokyo Chemical Industry Co. Ltd. (Tokyo, Japan). All reagents were of analytical grade and were used directly without further purification. In all experiments, deionized water was employed.

Green Synthesis of Bismuth Nanoparticles

BiNPs were synthesized by a green synthetic protocol (Scheme ) as reported previously.[37] In a typical procedure, Bi(NO3)3·5H2O (121.2 mg, 0.250 mmol) was mixed with lemon juice (10–25 mL) by sonication. Then, the volume was adjusted to 30 mL using deionized distilled water, and the pH of the mixture was adjusted to 12.3–12.4 using 4 M aqueous NaOH. The mixture was stirred in a thermostat bath at 80 °C for 1–6 h under an aerobic condition. The resulting reaction mixture was cooled to room temperature with constant stirring and then centrifuged to precipitate the product from the suspension. The precipitate was washed with water four times. The product was vacuum dried for 12 h at 60 °C.

Scheme 1

Green Synthesis of Bismuth Nanoparticles Using Lemon Juice

Measurements

XRD analysis was conducted on a Rigaku (Tokyo, Japan) MiniFlex 600 diffractometer with Cu-Kα radiation. SEM measurement was conducted on a Hitachi (Tokyo, Japan) SU-8000 microscope at accelerating voltages of 10 and 15 kV. TEM measurement was conducted on a JEOL TEM-2100F field emission electron microscope. TGA was carried out on a Seiko Instruments (Tokyo, Japan) TG/DTA 6200 (EXSTER6000) at a heating rate of 10 °C min–1 under N2. Hydrodynamic diameter and zeta potential were measured by DLS analysis conducted on a Malvern (Malvern, UK) Zetasizer Nano ZS instrument. UV–vis spectroscopic analysis was carried out on a HACH DR 5000 (Loveland, CO, USA) UV–visible spectrometer. Absorption spectra were recorded at a resolution of 1 nm within 200–1100 nm. Centrifugation was performed on a KUBOTA 3700 (Osaka, Japan) microrefrigerated centrifuge machine.

Catalytic Activity

The catalytic reduction of 4-NP was performed by adding a freshly prepared aqueous solution of NaBH4 (20 mM, 15 mL) to an aqueous solution of 4-NP (22 ppm, 50 mL) and 5 mL (1.5 mg/mL) of the colloidal dispersion of BiNPs under room light (at ca. 140 lux lit with fluorescent light), in the dark, or under sunlight. The progress of the reductions was monitored by measuring the UV–vis absorption spectra over time at 25 °C. The control experiments were also conducted using an identical procedure without BiNPs. The catalytic reduction of MO, MB, and RhB was performed similarly by mixing the aqueous solution of MO (15 ppm, 20 mL) or MB (10 ppm, 20 mL) or RhB (15 ppm, 20 mL) with freshly prepared aqueous NaBH4 solution (20 mM, 2.5 mL) and colloidal dispersion of BiNPs (1.5 mg/mL, 0.5 mL) under room light.