BiOCOOH Microflowers Decorated with Ag/Ag 2 CrO 4 Nanoparticles as Highly Efficient Photocatalyst for the Treatment of Toxic Wastewater

: A novel flower ‐ like Ag/Ag 2 CrO 4 /BiOCOOH heterojunction photocatalyst was synthesized by a facile in ‐ situ precipitation strategy combined with photoreduction treatment. Morphological studies revealed that numerous Ag/Ag 2 CrO 4 nanoparticles were evenly anchored on BiOCOOH microflowers, producing a novel heterojunction with the compactly interfacial contact. Optical absorption characterization demonstrated that Ag/Ag 2 CrO 4 /BiOCOOH possessed much better sunlight harvesting ability than Ag 2 CrO 4 /BiOCOOH and BiOCOOH. Photocatalytic experiments verified that compared with BiOCOOH, Ag 2 CrO 4, Ag/Ag 2 CrO 4, and Ag 2 CrO 4 /BiOCOOH, Ag/Ag 2 CrO 4 /BiOCOOH achieved remarkable efficiency by eliminating 100% of rhodamine B (RhB), 82.6% of methyl orange (MO) or 69.4% of ciprofloxacin (CIP) within 50 min at a catalyst dosage of 0.4 g/L. The high photocatalytic performance is likely owing to the improved sunlight response and the distinctly suppressed recombination of charge carriers arising from the formation of the novel 3D hierarchical heterostructure. The quenching test signified that h + , and •O 2 − were detected as the prevailing active species in wastewater treatment. This study may provide a viable strategy for enhancing the photocatalytic performance of wide band gap semiconductors.

Silver chromate (Ag2CrO4, Eg = ~1.8 V) is proved a promising photosensitizer since its narrow band gap could benefit the sunlight absorption of the semiconductor photocatalysts [33][34][35][36][37][38]. More importantly, the obtained Ag/Ag2CrO4 through the reduction of Ag + to Ag 0 , has been renowned as a fascinating couple to modify semiconductors for triggering robust catalytic ability because it is capable of effectively reinforcing visible-light absorption by surface plasmon resonance effect of metallic Ag and expediting the spatial separation of carriers [39]. Enlighten by the above analyses, the flowerlike heterostructure of BiOCOOH modified by Ag/Ag2CrO4 nanoparticles (NPs) could be anticipated to be an outstanding sunlight-driven photocatalyst for pollutant removal. However, to our knowledge, no pioneering study has been done on the exploration of Ag/Ag2CrO4/BiOCOOH, inspiring comprehensive research.
This research demonstrates the fabrication of an excellent heterojunction photocatalyst with Ag/Ag2CrO4 NPs anchored on flowerlike BiOCOOH through a facile in-situ precipitationphotoreduction strategy for the first time. The in-situ growth of Ag on Ag2CrO4 through photoreduction is a significant route to realize the robust bonding between Ag2CrO4 and Ag, which is beneficial for the efficient charge transfer. Markedly, Ag can scavenge photo-induced electrons efficiently from Ag2CrO4 and then migrate fast to take part in the further reaction due to its excellent conductivity. Benefiting from the novel hierarchical architecture, the Ag/Ag2CrO4/BiOCOOH was applied to degrade rhodamine B (RhB), ciprofloxacin (CIP) and methyl orange (MO) in aqueous solution under simulated solar light. The enhancement mechanism for the Ag/Ag2CrO4/BiOCOOH photocatalyst was explored.

Structure and Morphology
Typical XRD patterns of BiOCOOH, Ag2CrO4, Ag2CrO4/BiOCOOH, and Ag/Ag2CrO4/BiOCOOH are shown in Figure 1. BiOCOOH and Ag2CrO4 were in tetragonal structure (JCPDS 35-0939) [27,30] and orthorhombic structure (JCPDS 26-0952) [35], respectively. For Ag2CrO4/BiOCOOH, the diffraction peaks of BiOCOOH and Ag2CrO4 were detected simultaneously, verifying the successful integration of BiOCOOH and Ag2CrO4. As to Ag/Ag2CrO4/BiOCOOH, apart from the peaks from Ag2CrO4/BiOCOOH, one weak peak belonging to the (111) facet of cubic Ag (JCPDS No. 04-0783) was also observed, implying the successful construction of Ag/Ag2CrO4/BiOCOOH. Further, the decoration of Ag/Ag2CrO4 NPs did not change the crystalline phase of BiOCOOH, reflecting that the Ag/Ag2CrO4 NPs could be just deposited on the BiOCOOH rather than covalently incorporated into the crystalline phase of BiOCOOH. The elemental constituents and valence states of Ag/Ag2CrO4/BiOCOOH were investigated using the X-ray photoelectron spectroscopy (XPS) (Figure 2). The full-survey XPS spectrum ( Figure  2a) evidenced the co-existence of Bi, O, Ag, Cr, and C elements, in agreement with the constituents of the sample. High-resolution Bi 4f XPS spectrum (Figure 2b) showed that two peaks located at 159.2 and 164.7 eV were attributed to the Bi 4f7/2 and Bi 4f5/2 of the Bi 3+ in BiOCOOH. As to C 1s ( Figure  2c), two peaks at 284.8 and 288.1 eV corresponded to the C-OH and C-O bonding, respectively. From Figure 2d, the peaks centered at 367.8 and 373.8 eV corresponded to Ag + of Ag2CrO4, and those situated at 368.4 and 374.6 eV were associated with metallic Ag 0 . This fact validated that partial Ag + ions were reduced to Ag 0 through irradiation. Figure 2e showed the two characteristic peaks at 578.8 and 588.1 eV, which could link to Cr 2p3/2 and Cr 2p1/2, confirming the presence of Cr 6+ species. The peaks of O 1 s at 530.2 eV and 531.8 eV could be indexed to the lattice oxygen of Ag/Ag2CrO4/BiOCOOH and hydroxyl oxygen ( Figure 2f). Moreover, the atomic ratio of Ag:Cr was determined to be 5.92:2.54, reflecting that the molar ratio of Ag 0 :Ag2CrO4 was about 0.33:1. These results were in line with the XRD characterization, signifying that both BiOCOOH and Ag/Ag2CrO4 existed in Ag/Ag2CrO4/BiOCOOH. The morphologies of BiOCOOH and Ag/Ag2CrO4/BiOCOOH were observed based on SEM and TEM images (Figures 3 and 4). Clearly, 3D flower-like BiOCOOH (diameter: ~1.7-3 μm) microspheres were assembled by 2D nanosheets with smooth surfaces (Figure 3a,b). After its decoration with the Ag/Ag2CrO4 NPs, numerous Ag/Ag2CrO4 NPs were found on the surface of BiOCOOH microspheres (Figure 3c  The morphological features of Ag/Ag2CrO4/BiOCOOH were further studied by TEM. TEM images (Figure 4a,b) further reflects that the Ag/Ag2CrO4 NPs (size: ~20-70 nm) were intimately anchored on BiOCOOH microspheres. Of note, the in-situ deposition of Ag/Ag2CrO4 NPs on BiOCOOH can lead to the close contact of interfaces between them, which is conducive to the efficient separation and transformation of photo-excited carriers and therefore probably results in a prominent enhancement in the photocatalytic capability of the as-fabricated Ag/Ag2CrO4/BiOCOOH ternary heterojunction.

Optical Properties
UV-vis spectra of BiOCOOH, Ag2CrO4, and Ag/Ag2CrO4/BiOCOOH samples were examined to analyze the optical characteristics ( Figure 5). The optical absorption verges (λg) of BiOCOOH and Ag2CrO4 are ~370 [29] and 750 nm [39], respectively. When Ag/Ag2CrO4 was in-situ grown on BiOCOOH, the as-prepared Ag/Ag2CrO4/BiOCOOH heterojunction presents a broader absorption region and stronger absorption intensity [39]. In fact, Ag NPs possess darkened color to improve the visible-light absorbance. Meanwhile, they could induce the surface plasmon resonance (SPR) absorption. This fact implies that Ag/Ag2CrO4/BiOCOOH could efficiently harvest solar energy for the fast elimination of pollutants.
Furthermore, the band gap energy (Eg) of Ag2CrO4 and BiOCOOH are estimated using the following equation: αhν = A(hν − Eg) n/2 , where α, A, and ν are absorption coefficient, a constant and light frequency, respectively. And n equals to 4 for BiOCOOH and Ag2CrO4. Accordingly, the Eg of Ag2CrO4 and BiOCOOH can be determined to be 3.7 [26,31] and 1.8 eV [35,39] from the plot of (αhν) 1

Photocatalytic Activity
To demonstrate the potential application of the as-prepared samples, the photocatalytic degradation of RhB was first performed under simulated solar illumination ( Figure S1 and Figure 6). Prior to photocatalytic reactions, all the as-fabricated photocatalysts were vigorously stirred for 1h to realize saturation adsorption ( Figure S1). Apparently, BiOCOOH possessed a relatively stronger ability to adsorb RhB compared to other samples. After 60 min in the dark, 21.2% of RhB was adsorpted by BiOCOOH ( Figure S1). RhB is stable and nearly no photolysis of RhB happened after 50 min of simulated solar irradiation. When employing BiOCOOH, Ag2CrO4, Ag2CrO4/BiOCOOH, or Ag/Ag2CrO4/BiOCOOH as the photocatalyst, after illumination for 50 min, about 25.7%, 41.3%, 50.9%, 78.3% or 100% of RhB was photo-catalytically eliminated. Clearly, Ag/Ag2CrO4/BiOCOOH presented the best catalytic capability for degrading RhB (Figure 6a). Furthermore, the photocatalytic capability was superior to the previously reported BiOCOOH-based samples (e.g., Ag2CO3/BiOCOOH [31] and BiOBr/BiOCOOH [30]) due to the formation of BiOCOOH-based three-component heterojunction (Table 1). Moreover, the BET surface areas of BiOCOOH, Ag2CrO4/BiOCOOH, and Ag/Ag2CrO4/BiOCOOH were determined as 27.35, 25.76, and 25.11 m 2 g −1 , respectively (Table S1). Clearly, the BET surface area of Ag/Ag2CrO4/BiOCOOH is not the largest. This fact demonstrates that the decisive role of Ag/Ag2CrO4 rather than the BET surface area in enhancing the photocatalytic activity of Ag/Ag2CrO4/BiOCOOH. Of note, the degradation efficiency of RhB by Ag/Ag2CrO4/BiOCOOH was substantially improved relative to a mechanical mixture of Ag/Ag2CrO4 and BiOCOOH (59.6%), which might be ascribed to the effective separation of photo-induced carriers at the heterojunction interface between Ag/Ag2CrO4 and BiOCOOH (Figure 6a).  Figure 6b shows the apparent photo-degradation rate constants (k) for as-prepared samples. Apparently, Ag/Ag2CrO4/BiOCOOH achieved the largest k value of 0.0870 min −1 , which was pronouncedly greater than that of BiOCOOH (0.0052 min −1 ), Ag2CrO4 (0.0101 min −1 ), Ag2CrO4/BiOCOOH (0.0300 min −1 ), or Ag/Ag2CrO4 (0.0146 min −1 ).
The photochemical stability is also a crucial parameter in industrial applications [24,40,41]. Figure 6d displays the cycling performance of Ag/Ag2CrO4/BiOCOOH. Ag/Ag2CrO4/BiOCOOH has no dramatic decline in the photodegradation behavior of RhB during five successive runs. Besides, the Ag+ leaching after the photocatalytic reaction was investigated and the leaching quantity of the Ag ion was tested to be 0.013 ppm. Further, there is no distinct change in its crystalline phases and microstructures through the analysis of the XRD pattern and TEM image of the Ag/Ag2CrO4/BiOCOOH heterojunction after the recycling runs (Figure 7 and Figure S2), verifying the superior stability of Ag/Ag2CrO4/BiOCOOH.  In addition, industrial dye MO and antibiotic CIP were chosen as the model pollutants to further manifest the remarkable photocatalytic activity of Ag/Ag2CrO4/BiOCOOH. As shown in Figure 8, 82.6% of MO and 69.4% of CIP can be efficiently eliminated within 50 min of simulated sunlight irradiation. Therefore, it can be inferred that Ag/Ag2CrO4/BiOCOOH possesses the superior degradation ability for industrial dyes (RhB and MO) and antibiotic (CIP).

Photocatalytic Mechanism
In order to speculate the mechanism of photocatalytic decomposition of contaminants over the Ag/Ag2CrO4/BiOCOOH, the exact effects of probably generated reactive species on the photodegradation of RhB was investigated by the radical quenching experiment. 1 mM of ammonium oxalate (AO), isopropyl alcohol (IPA), or benzoquinone (BQ) was adopted to consume h + , •OH, or O2 •− species during the photocatalytic reaction. From Figure 9, upon the addition of AO and BQ into the photocatalytic system, the elimination efficiency of RhB distinctly decreased from 100% to 43.4% and 23.2%, respectively, evidencing the premier role of h + and •O2 − in RhB removal. However, IPA exerts little influence on the RhB degradation, implying the subordinate role of •OH radicals. Photoluminescence (PL) spectroscopy was adopted to investigate the charge separation rate, which plays an essential role in determining the photocatalytic capability of a photocatalyst [41][42][43]. Generally, a weakened PL signal corroborates an improvement in separation efficiency of photo-induced carriers [22,29,30,41,42,44]. As exhibited in Figure 10, compared with bare BiOCOOH, Ag/Ag2CrO4/BiOCOOH emerges with a distinctly lower emission intensity, demonstrating the remarkable improvement of charge separation efficiency for Ag/Ag2CrO4/BiOCOOH. Thus, it can be inferred that the construction of Ag/Ag2CrO4/BiOCOOH ternary heterojunction can effectively impede the reunion of carriers, probably leading to a superior photocatalytic activity. On account of the above characterization and analysis, a probable mechanism for the drastically elevated photocatalytic ability of Ag/Ag2CrO4/BiOCOOH is put forward (Figure 11). The remarkable photocatalytic activity of Ag/Ag2CrO4/BiOCOOH principally arises from the novel hierarchical heterostructure, which achieves an appreciable improvement of light absorption ( Figure 5) and carrier separation ( Figure 10) [2,[45][46][47][48]. Upon simulated solar illumination, both BiOCOOH and Ag2CrO4 are excited to trigger the production of electrons and holes on the corresponding CB and VB, respectively. Both the CB and VB potentials of Ag2CrO4 are lower than those of BiOCOOH. Hence, partial photo-induced electrons from the CB of BiOCOOH can preferably drift into that of Ag2CrO4, and then the accumulated electrons rapidly flow into the metallic Ag NPs that act as electron scavenging centers [39]. Simultaneously, part of photo-induced electrons the CB of BiOCOOH are involved in the reduction of O2 to yield •O2 − since the CB potential of BiOCOOH is more negative than the φ(O2/•O2 − ) (−0.33 eV versus NHE). This result is in good agreement with that of the trapping experiments ( Figure 9). On the other hand, the photo-induced holes on the VB of BiOCOOH are injected into that of Ag2CrO4. Such carrier movement makes charge separation more effective, which is beneficial for the efficient production of reactive species [49]. Under the circumstances, enriched holes on the VB of Ag2CrO4 and plenty of O2 •− radicals are engaged in the elimination of RhB and CIP. The radical quenching experiment has also verified that h + and •O2 − , not •OH radicals, principally accounts for the photocatalytic destruction of contaminants (Figure 9). In a word, such a novel hierarchical heterostructure not only could appreciably facilitate the separation of photoexcited holes and electrons but also pronouncedly ameliorate the optical absorption to realize the efficient decomposition and mineralization of harmful contaminants under simulated solar light.

Chemicals
All reagents of analytical grade were obtained from Chinese Sinopharm.

Photocatalysts Fabrication
BiOCOOH was fabricated basing on a reported route [30,50]. Typically, 0.96 g of Bi(NO3)3•5H2O was taken into 50 mL of glycerol and sonicated for 15 min, followed by the addition of 20 mL of DMF and 10 mL of deionized water under vigorously stirring. The stirring further lasted for 60 min. After that, the solution was transferred into an autoclave heated at 160 °C for 20 h in an electric oven, and subsequently cooled down naturally. The BiOCOOH sample was washed three times, and then dried at 70 °C overnight. Ag2CrO4/BiOCOOH was constructed via a simple in-situ deposition route. First, 3 mmol of BiOCOOH was uniformly dispersed in 100 mL of H2O via ultra-sonication treatment. Then, 1 mmol of AgNO3 and 0.2 g of Polyvinylpyrrolidone (PVP, Mw = 40,000) were added to the above solution to form solution A, followed by vigorously stirring at a stirring speed of 1000 rpm (Revolutions Per minute) for 1 h. After that, 0.5 mmol of K2CrO4 was ultrasonically dissolved in 10 mL of H2O to form solution B. Afterward, solution B was dropwise added into solution A by using a syringe pump at a speed of 5 mL/h under constant stirring (1000 r/min). The resultant solution was further stirred for another 2 h. Finally, the Ag2CrO4/BiOCOOH sample was rinsed more than four times and then dried at 70 °C overnight. The Ag2CrO4/BiOCOOH sample was collected by centrifugation (5000 r/min) for 5 min, washed thoroughly with water five times, and dried at 70 °C overnight. Ag/Ag2CrO4/BiOCOOH was fabricated via the photo-reduction of Ag2CrO4/BiOCOOH. Typically, 0.3 g of Ag2CrO4/BiOCOOH was mixed with the solution of 40 mL H2O and 20 mL methanol under stirring for half an hour. Afterwards, the suspension was irradiated by the light (Light intensity = ~170 mW/cm 2 ; The distance between the Xe lamp and the top of the suspension = ~25 cm.) from a 300 W Xe lamp for 2 h with constant agitation. Lastly, the as-fabricated Ag/Ag2CrO4/BiOCOOH was collected after centrifugation (5000 r/min) for 5 min.

Characterization
The details about characterization methods are supplied in the Supporting Information (Experimental Section).

Photocatalytic Activity Evaluation
The application of Ag/Ag2CrO4/BiOCOOH to the elimination of RhB (10 mg/L, 100 mL) or CIP (10 mg/L, 100 mL) was evaluated under simulated solar light by utilizing a 300 W Xe lamp (CEL-PF300, Beijing China Education Au-light Co., Ltd., Beijing, China) without any light filter as the simulated solar light source [51]. The light intensity was determined as ~170 mW/cm 2 . The distance between the Xe lamp and the top of the reactor mouth was about 20 cm. The temperature of the photocatalytic system was maintained at about 22 °C. Typically, 40 mg of photocatalyst and 100 mL of RhB (10 mg/L), CIP (10 mg/L) or MO (10 mg/L) were mixed in a reactor under magnetically stirring (800 r/min) in the dark for 1 h to reach adsorptive equilibrium. During the photocatalytic reaction, a small portion of the solution (2 mL) was drawn every 10 min and subsequently centrifuged (speed: 9000 r/min) for 6 min to get rid of the catalysts. The obtained supernatant was analyzed using a UVvis spectrophotometer (Shimadzu UV-2600, Tokyo, Japan) at the corresponding maximum absorption wavelength (554 nm for RhB, 276 nm for CIP, 464 nm for MO). The degradation efficiency (η) of RhB, CIP or MO was calculated by using the equation: η = Ct/C0 × 100%, here C0 and Ct are the absorbance of the contaminant solution after 1 h of stirring in dark and the absorbance of the contaminant solution at time t, respectively. The stability of Ag/Ag2CrO4/BiOCOOH was tested for five consecutive runs. After each round, the photocatalyst was collected, centrifuged, washed with water, and subsequently dried at 60 °C overnight. After that, the photocatalyst was subjected to the next run. Due to the inevitable loss of photocatalysts during the recycling process, some parallel tests were carried out to guarantee that the dosage of photocatalyst utilized in each round was the same (40 mg). Total organic carbon (TOC) of RhB solutions during the reaction was monitored by using a Shimadzu TOC-LCSH/CPH analyzer (Shimadzu TOC-LCSH/CPH, Tokyo, Japan).

Conclusions
We have developed an efficient photocatalytic system of a novel 3D flower-like Ag/Ag2CrO4/BiOCOOH heterojunction by a facile in-situ route. Such a hierarchical heterostructure not only could remarkably boost the separation of photo-excited holes and electrons but also pronouncedly strengthen the sunlight absorption to realize the efficient utilization of solar light. Compared to BiOCOOH, Ag2CrO4, Ag/Ag2CrO4, and Ag2CrO4/BiOCOOH, Ag/Ag2CrO4/BiOCOOH exhibits a remarkable enhancement in photocatalytic elimination of harmful pollutants under simulated solar irradiation. The premier role of Ag/Ag2CrO4 for the upgrading the photocatalytic capability could endow Ag/Ag2CrO4/BiOCOOH with sufficient light absorption and efficient separation of carriers. The h + and •O2 − radicals predominantly account for the destruction of contaminants. Further, Ag/Ag2CrO4/BiOCOOH has high stability and powerful mineralizing ability, making it favorable for real wastewater treatment. This study offers a simple route for improving the photocatalytic performance of the wide band-gap semiconductor photocatalysts and constructing a highly efficient hierarchical heterojunction for environmental remediation.