Enhanced visible-light-driven photocatalytic performance for degradation of organic contaminants using PbWO4 nanostructure fabricated by a new, simple and green sonochemical approach

Highlights • Spindle shaped PbWO4 nanostructure with enhanced photocatalytic activity was obtained by facile and green sonochemical way for the first time.• Maltose as a new capping agent was utilized.• Sonication time, dose of precursors, power of ultrasound waves and kind of capping agent were optimized.• The made lead tungstate samples in role of visible-light photocatalyst were applied to remove organic pollution in water.• Usage of ultrasonic irradiation has remarkable effect on performance of PbWO4 photocatalyst for decomposition.


Introduction
As a proper and environmentally friendly solution to address a critical global concern that could affect the entire globe, visible-light photocatalysis technology has been proposed to eliminate water contamination [1]. In this approach, the creation of reactive oxygen species to remove water contaminants can happen with the aid of photocatalyst possessing desirable band gap through advanced oxidation processes [2][3][4][5]. One of the most notable areas of research pursued by scientists is the design of efficacious catalytic materials that are activated with the aid of visible light [6][7][8]. The reason for this is the maximal usage of free and plentiful solar energy for water contamination removal.
Lead tungstate has aroused very notable interest due to its wide range of uses in scintillators, amplifiers, photocatalysis and lasers [9,10]. Lead tungstate structures have been prepared applying various approaches such as co-precipitation, hydrothermal, solid-state reaction and Bridgman route [11][12][13][14]. Today, the sonochemical pathway is applied as a helpful synthesis procedure to create a range of nanoscale compounds with various shapes [15][16][17]. Ultrasound is employed as a swift procedure for various tasks such as architectural control of nanostructures and dispersion of materials [18]. Cavitation created with the aid of ultrasound waves may bring to favorable and specific structures with high uniformity at the nanoscale. Owing to hot-spot theory, creation of excessively high temperatures and release of immense energies can be happened within sonochemical cavitation that can be favorable for a wide range of chemical reactions.
Nowadays, the architectural control of nanoscale compounds with very favorable shape and dimension is a momentous aim for researches because their features and yield are dependent on shape and dimension [19][20][21][22]. Green chemistry-based pathway defining as the design and usage of procedures to create a range of nanoscale compounds with the help of bioproducts, has aroused very notable interest [23,24]. The reason for this is the wide availability, non-toxicity and renewability of substances that are utilized to create nanoscale compounds with green chemistry-based pathway, being safe for environment and humans [25,26]. This paper offers a new pathway through ultrasound-aided fabrication of spindle shaped PbWO 4 nanostructure with the help of an environmentally friendly capping agent (maltose) for the first time.
To the best of our knowledge, usage of maltose has never been reported for the creation of PbWO 4 nanostructure under ultrasound waves. We decided to apply maltose due to its non-toxicity and wide availability. We explored the role of various effective factors such as time, dose of precursors, power of ultrasound waves and kind of capping agents on the uniformity, efficiency, dimension and structure of lead tungstate. The attributes of PbWO 4 samples were examined with the aid of diverse identification techniques. Further, we applied the prepared lead tungstate samples in role of visible-light photocatalyst to remove organic pollution in water. The yield of lead tungstate in role of visible-light photocatalyst rarely was examined. We examined the role of kind of

Characterization
TEM analysis was utilized to check the shape and dimension of the created PbWO 4 sample on a Philips CM30 TEM. Optical features of the spindle shaped PbWO 4 nanostructure were checked applying spectrophotometer (Shimadzu, UV-2550, Japan). The shape, structure and elemental composition of the created PbWO 4 samples were explored utilizing MIRA3-TESCAN FESEM. The phase purity of the spindle shaped PbWO 4 nanostructure was explored with a diffractometer of Philips Company. A vibrating sample magnetometer (VSM, Meghnatis Kavir Kashan Co., Kashan, Iran) was applied to explore the magnetism of the spindle shaped PbWO 4 nanostructure. FT-IR spectrometer (Magna-IR) was applied to explore the surface of the spindle shaped PbWO 4 nanostructure.

Photocatalytic activity
The prepared lead tungstate samples in role of visible-light photocatalyst were applied to remove organic pollution in water. The contaminant (1 mg of erythrosine or Acid Black 1 or methyl violet) solution with defined quantity of lead tungstate sample in role of visiblelight photocatalyst were mixed and aerated within 1/2h in darkness for the equilibrium of the adsorption of contaminant upon the surface of lead tungstate sample. Subsequent, each mixture was irradiated with 125 W Osram lamp [8]. From following relation was employed to specify the contaminant (erythrosine or Acid Black 1 or methyl violet) decomposition rate: A 0 and A t mention the initial and ultimate absorbance for contaminant (erythrosine or Acid Black 1 or methyl violet) [8].

Results and discussion
Here, we offer a new pathway through ultrasound-aided fabrication of PbWO 4 nanostructure with the help of an environmentally friendly capping agent. We explored the role of different effective factors such as time, dose of precursors, power of ultrasound waves and kind of capping agents on the uniformity, efficiency, dimension and structure of lead tungstate.

Morphology investigation
In sonochemistry, the power and time of ultrasound are examined as two efficient instrumental variables that can affect the features of products in terms of size and morphology [27]. Additionally, experimental variables such as the precursor dose and the kind of capping agent can affect the features of the products. Thus, optimization of these two kinds of variables seems necessary to achieve a product with very desirable features.
To explore the role of power of ultrasound waves on lead tungstate, samples 1, 2 and 3 were fabricated with 60, 30 and 90 W, correspondingly (see Fig. 1a-f). We observed that the alteration in power of ultrasound waves from 60 W to 30 and 90 W can bring to the change in features of lead tungstate in terms of size and uniformity. Very proper power to create the regular and suitable lead tungstate nanostructure can be 60 W ( Fig. 1a and b). It is found in Fig. 1c-f, the irregular agglomerated structures could be created with usage of other powers (90 and 30 W). It seems that enhancing the power of ultrasound from 30 to 60 W can be effective in further collapse of the cavitation bubble, resulting in a stronger shock wave, which can hinder the agglomeration [28]. On the other hand, increasing the power of ultrasound can accelerate thermodynamic stability via the growth of primary nuclei, and thus particles can agglomerate owing to the release of more energy in less time [27]. Hence, very desirable ultrasonic power to achieve lead tungstate nanostructure with a high uniformity, 60 W was selected.
Further, to explore the role of ultrasonic time on lead tungstate features, samples 4 and 5 were fabricated with 60 W for 5 and 20 min (see Fig. 2a-d). The shapeless structures can be created within 5 min. By enhancing the ultrasound time to 10 min, the regular and suitable lead tungstate nanostructure was created, which could indicate the positive effect of enhancing the ultrasound time ( Fig. 1a and b). Thus, it was assumed that enhancing this variable for up to 20 min could reduce the particle size and improve the uniformity, but the outcomes demonstrated that irregular agglomerated micro/nanostructures were created during this time ( Fig. 2c and d). By changing the ultrasound time from 5 to 10 min, energy is continuously added to the reaction system and can   inhibit the growth of lead tungstate nanostructures [29]. It has been shown that by applying sonication, the primary nanoparticles can dissolve and grow into larger crystals. Crystal dissolution and growth are two parallel processes that occur [30]. Thus, by changing the ultrasound time from 5 to 10 min, the growth of the lead tungstate nanostructure may not be optimal in terms of energy. Instead, dissolving the lead tungstate nanostructure created over 5 min may be energetically desirable [31]. Ostwald ripening process may be reason for creation of the irregular agglomerated micro/nanostructures with the alteration in time from10 to 20 min. It seems that lead tungstate nanostructures created owing to high surface energy (induced with reduction in size to nanometer scales) [27,32] can act as primary nuclei and cause the growth process to occur via Ostwald ripening process, resulting in the irregular agglomerated micro/nanostructures produced by applying ultrasound in more time [27]. Evidently, very convenient time to create the regular and suitable lead tungstate nanoparticles with tiny size can be 10 min ( Fig. 1a and b).
Also, we explored the role of dose of precursors on the uniformity and structure of lead tungstate and applied 0.04, 0.08, 0.16 mol of lead precursor (molar ratio of Pb:W = 1:1) for creation of samples 6, 7 and 8, correspondingly, with 60 W for 10 min. Less regular nanostructures, relatively homogeneous polygon nanostructures and scale-like nanostructures can be created with usage of 0.04 ( Fig. 3a and b), 0.08 (Fig. 3c  and d), 0.16 mol (Fig. 3e and f) of lead precursor. Obviously, very proper dose of lead precursor to fabricate the regular and suitable lead tungstate nanoparticles with tiny size can be 0.01 mol ( Fig. 1a and b).
To explore the role of kind of capping agent on lead tungstate features, samples 9, 10, 11 and 12 were fabricated with fructose, glucose, maltose and amylum (see Fig. 4a-h). Cluster-like nanostructures ( Fig. 4a  and b), polygon-like nanostructures ( Fig. 4c and d), very homogeneous spindle shaped PbWO 4 nanostructures ( Fig. 4e and f) and star-like micro/nanostructures ( Fig. 4g and h) can be created with usage of fructose, glucose, maltose and amylum. It seems that the environmentally friendly capping agents (fructose, glucose, maltose and amylum) are able to the inducement of initial nanoparticles for assembly in the specified directions, being advantageous for creation of various structures. Evidently, usage of maltose as environmentally friendly capping agent brings to creation of spindle shaped PbWO 4 nanostructure with great uniformity under ultrasound irradiation. Further, we can conclude that architectural control of lead tungstate can be made possible with usage of diverse capping agents under ultrasound irradiation.
It is found in Fig. 5a-d, the successful fabrication of spindle shaped PbWO 4 nanostructures has been corroborated with the aid of TEM data. Sample 11 displays spindle-like morphology. We considered it as the optimal structure having very proper uniformity.

Formation mechanism of spindle shaped PbWO 4 nanostructure
To explore the role of ultrasonic irradiation, sample 13 was produced through vigorous stirring within 40 min in presence of maltose. Sample 13 displays the inhomogeneous and unshapen structures (see Fig. 6a and b). Ultrasound is employed as a swift tool for various tasks like architectural control of nanostructures [18]. Cavitation created with the aid of ultrasound waves may bring to favorable and specific nanoscale structures with high uniformity. Owing to hot-spot theory, creation of excessively high temperatures and release of immense energies can be happened within bubbles collapse that can be favorable to conversion of massive structures to tiny particles [33]. Thus, we can conclude that ultrasound irradiation can be very advantageous in architectural control of lead tungstate (see Scheme 1).
As noted above, spindle shaped PbWO 4 nanostructure with great uniformity was sonochemically fabricated with usage of maltose. The simultaneous impact of ultrasonic ultrasound waves and maltose as an environmentally friendly capping agent may be reason to create very homogeneous spindle shaped PbWO 4 nanostructure. Thus, special propulsion conditions resulting from sonochemical cavitation can be favorable to simultaneous gelatinization of maltose and creation of spindle shaped PbWO 4 nanostructure. The possible mechanism to create the spindle shaped PbWO 4 nanostructure may be as: The sonochemical formation of the spindle shaped PbWO 4 nanostructure may be taken place in two stages: the ultrasound-induced creation of the initial nuclei and afterwards ultrasound-induced assembly of the formed nuclei to make the spindle shaped PbWO 4 nanostructure.

Structure and purity of spindle shaped PbWO 4 nanostructure
The successful formation of pure PbWO 4 nanostructure (sample 11) was corroborated with XRD outcome (see Fig. 7a). The outcome is in full compliance with tetragonal phase lead tungsten oxide (JCPDS No. 08-0108). The crystallite size for the spindle-shaped PbWO 4 nanostructure was estimated applying the Scherrer equation [5] to be near 38 nm. We found no characteristic diffraction band for impurity in XRD outcome, signifying purity of the spindle-shaped nanostructure.  The successful formation of pure PbWO 4 nanostructure (sample 11) was also corroborated with FT-IR outcome (see Fig. 8a). The band near 780 cm − 1 may be assigned to lead tungsten oxide [34]. The bands around 3433 and 1539 cm − 1 may be ascribed to the physisorbed water [3]. Fig. 8b gives VSM outcome for checking the magnetism of the spindle-shaped PbWO 4 nanostructure. Obviously, sample 11 possesses proper magnetism (saturation magnetization = 0.0042 emu g − 1 ) for simple recovery that is momentous in practical utilizations.

Magnetism and optical property of spindle-shaped PbWO 4 nanostructure
The optical features of the spindl-shaped PbWO 4 nanostructure (sample 11) were examined with DRS (see Fig. 9). An absorption band nearly 417 nm is seen. It is accepted that the energy gap is the notable parameter impacting photocatalytic yield. The determined energy gap for the spindle-shaped PbWO 4 nanostructure from Tauc's plot [8] is of about 2.7 eV. Accordingly, this outcome corroborates easy activation of the spindle-shaped PbWO 4 in the role of the nano-sized photocatalyst (by visible illumination) to remove water contamination.

Photocatalytic activity
The produced lead tungstate samples in role of visible-light photocatalyst were applied to remove organic pollution such as Acid Black 1 in water. The role of utilization of ultrasound waves as well as kind of capping agent on lead tungstate efficiency (samples 9-13) was explored (see Figs. 10 and 11). Insignificant catalytic decomposition of Acid Black 1 was occurred without usage of any lead tungstate samples. We found that, after 60 min of illumination, the decomposition amount of Acid Black 1 for the spindle-shaped PbWO 4 nanostructure (sample 11) is about 93% and it denoted the most proper photocatalytic yield. In contrast, about 81, 86, and 77% of Acid Black 1 could be destructed with the help of the cluster-like nanostructures (sample 9), polygon-like nanostructures (sample 10), and star-like micro/nanostructures (sample 12). The yield of the spindle-shaped PbWO 4 nanostructure (sample 11) is the more proper than the yield of the inhomogeneous and unshapen structures (sample 13). Sample 13 could eliminate about 40% of Acid Black 1 under the same condition. Additionally, the reaction kinetics of the produced lead tungstate samples were checked utilizing the pseudo-first-order mode [35]. It is found in Fig. 11, the rate constants (K) of 0.0301, 0.0345, 0.0471, 0.0264 and 0.0089 min − 1 are for samples 9, 10, 11, 12 and 13. Evidently, the lead tungstate samples fabricated with utilization of ultrasound waves denotes a much proper photocatalytic yield than the sample 13 produced without utilization of ultrasound waves. Usage of ultrasonic irradiation could bring to improvement of catalytic yield of PbWO 4 to 93%. From FESEM outcomes (see Figs. 4 and 6), we can conclude that utilization of ultrasound waves and maltose as an environmentally friendly capping agent has very notable and efficacious impact on creation of very homogeneous spindle-shaped PbWO 4 nanostructure with specific architecture as the most efficient photocatalyst for water contamination removal.
After type of catalyst, we explored the role of its dose as notable factor in contaminant removal capability (see Fig. 12). We found that alteration in catalyst dose from 20 to 40 mg, could bring to increment in decomposition yield (33 to 93%), but by utilization of 60 mg catalyst, 53% of Acid Black 1 could be destructed. The enhancement of surface area and thus improvement of the absorption of Acid Black 1 on the spindle-shaped PbWO 4 nanostructure surface may be reason for increment in decomposition yield. In contrast, the nanostructure thickness as well as saturation of PbWO 4 nanostructure layers may be reason for decrement of catalytic yield by utilization 60 mg catalyst. Consequently, very proper dose of the spindle-shaped PbWO 4 nanostructure in role of catalyst can be 40 mg.
The spectral changes within the decomposition of Acid Black 1 with utilization of the spindle-shaped PbWO 4 nanostructure as visible-light photocatalyt are given in Fig. 13a. Evidently, the elimination of Acid Black 1 is continuous. It is accepted that as a momentous attribute of nano-sized catalyst can be stability of it over several catalytic runs. Upon repeating the visible-light photocatalysis experiments, 82% of visiblelight photocatalysis yield was achieved after eleven times of the spindle-shaped PbWO 4 nanostructure reuse (see Fig. 13b). This outcome corroborates very proper durability of PbWO 4 nanostructure. Fig. 13c gives XRD outcome for checking the consistency of the spindle-shaped PbWO 4 nanostructure after catalytic experiment to eliminate Acid Black 1. It is obvious that XRD outcomes of the spindle shaped PbWO 4 nanostructure before and after visible-light photocatalysis experiments are identical (see Fig. 7a and 13c).
The possible mechanism to eliminate Acid Black 1 with the help of the spindle-shaped PbWO 4 nanostructure may be expressed as [17]: Spindle shaped PbWO 4 nanostructure + hν → Spindle shaped PbWO 4 nanostructure* + e − + h +  Visible-light photocatalytic ability of the spindle-shaped PbWO 4 nanostructure was also explored to eliminate organic pollution like erythrosine and methyl violet in water (see Fig. 14). Yield of the spindleshaped PbWO 4 nanostructure to eliminate erythrosine and methyl violet is about 99 and 98%. Overall, the outcomes could introduce the spindleshaped PbWO 4 nanostructure (produced at power of 60 W for 10 min and with usage of maltose) as an efficacious substance for water contamination removal under visible light.

Conclusions
Shortly, a facile and swift sonochemical route was employed for creation of spindle-shaped PbWO 4 nanostructure (a highly efficient photocatalyst for removal of organic pollution in water) with the aid of an environmentally friendly capping agent (maltose) for the first time. To optimize efficiency, dimension and structure of lead tungstate, the diverse effective factors were altered such as time, dose of precursors, power of ultrasound waves and kind of capping agents. The attributes of PbWO 4 samples were checked with the aid of diverse identification techniques. The synthesized lead tungstate samples in role of visiblelight photocatalyst were applied to remove organic pollution in water. Kind of pollutants, dose and type of catalyst as notable factors were examined in contaminant removal capability. Very favorable catalytic yield and durability demonstrated by spindle-shaped PbWO 4 nanostructure (made at power of 60 W for 10 min and with usage of maltose). Usage of ultrasonic irradiation could bring to improvement of catalytic yield of PbWO 4 to 93% in elimination of Acid Black 1. Overall, the outcomes could offer the spindle-shaped PbWO 4 nanostructure as an efficacious substance to eliminate water contamination under visible light.