Significance of Crystal Morphology Controlling in Semiconductor

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The Significance of Crystal Morphology Controlling in Semiconductorbased Photocatalysis: A Case Study on BiVO4 Photocatalyst Yue Zhao, Rengui Li, Linchao Mu, and Can Li Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 12, 2017

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Crystal Growth & Design

The Significance of Crystal Morphology Controlling in Semiconductorbased Photocatalysis: A Case Study on BiVO4 Photocatalyst Yue Zhao#†‡, Rengui Li#†, Linchao Mu†‡, and Can Li*† †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, Zhongshan Road 457, Dalian ‡ University of Chinese Academy of Sciences, Beijing, 100049, China KEYWORDS: semiconductor-based photocatalysis, solar energy conversion, morphology control, bismuth vanadate ABSTRACT: Precise control of the morphology and crystalline structure of semiconductor-based photocatalyst is crucial for improving the efficiency of solar energy conversion system. In this work, taking BiVO4 semiconductor photocatalyst as an example, we investigated the formation process for the regular decahedron BiVO4 crystals prepared by a convenient hydrothermal method, and found that the synthesis is undergoing a dissolution-recrystallization process, concomitantly, the phase was transformed from tetragonal zircon type to monoclinic sheelite type. By controlling the kinetics of crystal growth for BiVO4 through regulating acidity of the reaction solution, we can control the morphology of monoclinic BiVO4 from regular decahedron crystals to short rod-like particles, particularly can precisely modulate the proportion of {010}/{011} facets for the final regular decahedron BiVO4. By tuning the crystalline phase and morphologies of BiVO4 crystal, we found that the photocatalytic water oxidation activity for the well-defined BiVO4 crystal with specific configuration of {010} and {011} exposed facets can be 50 times of their regular tetragonal BiVO4 particles. Our work shows a convenient strategy for precise control of the growth process of semiconductor-based photocatalyst, based on the understanding of the crystal morphology evolution mechanism, which will be instructive for constructing semiconductor-based photocatalysts for solar energy conversion.

Photocatalytic solar energy conversion is regarded as one of the most promising solutions to convert solar energy to chemical fuels, such as water splitting to produce hydrogen.1-5 Semiconductor-based photocatalysts are widely-used in photocatalysis. For semiconductor-based photocatalysts, morphology and crystalline structure have been demonstrated to be crucial factors in a complex photo-to-chemical conversion process including light absorption, charge separation and surface catalytic reactions.4-12 Different morphologies of photocatalyst possess different surface structures and exposed facets, which show different photocatalytic activities in many photocatalyst systems. For example, Jaroniec and his coworkers think TiO2 nanosheet predominantly exposed {001} facets are more active in photocatalytic hydrogen production than other morphologies.13 But Murray and his coworkers reported that the {101} facets of TiO2 shows better photoactivity for hydrogen production than other facets.14 Recently, Selcuk and Selloni demonstrated that optimization of the ratio of {101} and {001} facets could provide a way to enhance the photocatalytic activity of anatase.15 And the effect of facet has aslo been found on Ag3PO4 photocatalyst. Tang and his coworkers have indicated that Ag3PO4 with tetrahedron morphology exposed with {111} facets was also reported to exhibit a much higher quantum efficiency in photocatalytic water oxidation than rhombic dodecahedron and cubic ones.16

Meanwhile, the crystalline phase of semiconductors has also been found to be an important affecting factor in many kinds of photocatalysts (Ga2O3,10 WO3,17 Bi2O3,18 BiPO419 etc.). However, most of these examples were reported to solely discuss the effect of morphology or crystalline structure as it was, especially precise controlling the morphology together considering the crystalline structure and the connection with photocatalytic performance are rarely reported. BiVO4 is one of the attractive semiconductors with three crystalline phases, tetragonal zircon-type (z-t), tetragonal sheelite-type (s-t) and monoclinic sheelite-type (s-m), and monoclinic BiVO4 is generally considered to be more active than others.20-24 Different morphologies of monoclinic BiVO4, such as mesoporous,25 ellipsoidal,26 nanosheet,27 nanoflower28 etc., have been reported for photocatalytic reactions and show superior photoactivities in different photocatalytic reactions. It should be noted that most of these reported samples were synthesized by using some templates or surfactants, some of which are difficult to be removed and the post-treating may destroy the morphologies or the unique surface structures for investigating the correlation between photocatalytic performance and morphology. In this work, taking BiVO 4 as an example, we try to precisely control its morphology via further comprehending the transformation of the crystalline

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Figure 1. (a) XRD patterns of BiVO4 samples prepared by hydrothermal method at 473 K for different time; (b) Raman shift of different BiVO4 samples; (c) The percentage of monoclinic and tetrahedron phase for different BiVO4 samples and (d) UV-vis diffuse reflectance spectra of different BiVO4 samples. αmono: The αmono=

percentage of monoclinic BiVO4 samples. (121)

121+ (200)

， Imono(121)

and Itetra(200) are the

intensity of BiVO4 samples’ diffraction peak obtained from XRD patterns.

structure, and further reveal the correlation between the photocatalytic activity with the morphology, and especially the crystal facets exposed. Our work introduces a hydrothermal method aiming to control the morphology of BiVO4 photocatalyst via controlling the kinetics of crystal growth for BiVO4 photocatalyst without using any additional surfactants. The investigations show that the formation of regular decahedron morphology BiVO4 crystals and its phase transformation from tetragonal zircon to monoclinic scheelite are closely associated with each other. According to the understanding of the formation process of regular decahedron BiVO4 crystals, we achieved precise control of the proportion of exposed facets for final regular decahedron BiVO4 crystals. Photocatalytic water oxidation performance of BiVO4 photocatalyst reveals that the photoactivity not only rely on its crystalline structure but also strongly depend on the morphology and proportion of exposed facets. Traditionally, monoclinic sheelite BiVO4 can be prepared by solid state reaction or calcination method from tetragonal zircon BiVO4 at high temperatures.20 However, the morphology is difficult to be controlled using such drastic and fast processes. Oppositely, hydrothermal synthesis is a mild process, and it is a commonly-used method to synthesize well-crystallized and chemically stoichiometric materials under relatively mild conditions, which exploits the solubility of almost all inorganic substances in water at elevated temperatures and pressures, and subsequent crystallization of the dissolved material from the fluid.29 So we chose hydrothermal

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method to control the growth kinetics of BiVO4 photocatalyst aiming to precisely control the morphology of BiVO4 without using additional surfactants. First a series of BiVO4 samples were prepared by varying the reaction time of BiVO4 via precipitation followed hydrothermal treatment. XRD measurements (Figure1a) show that the as-prepared BiVO4 precipitate without hydrothermal treatment is pure tetragonal zircon BiVO4 (z-t) (standard card of No. 14-0133, space group: I2/a, a=7.3, b=7.3, c=6.457, β=90°). When hydrothermal time is short enough (less than 0.5 h), the crystalline structure with tetragonal zircon does not change. Then when the reaction time extends to 1.0 h, monoclinic scheelite BiVO4 (s-m) (standard card of No. 14-0688, space group: I2/a, a=5.159, b=11.701, c=5.092, β=90.38°) with characteristic diffraction peaks at 15.1, 18.6, 18.9, 28.6, 28.8, 28.9, and 30.6 are observed. Further prolonging the hydrothermal time, the characteristic peaks corresponding to monoclinic BiVO4 increase and become dominant, which indicates that tetragonal zircon BiVO4 was gradually converted to monoclinic scheelite BiVO4 during the hydrothermal process. Raman spectra of all these BiVO4 samples were recorded to confirm above results (Figure 1b). It is found that an intense band present at around 850 cm-1, which is due to the symmetric V-O stretching mode of tetragonal zircon BiVO4 (z-t), can be observed for BiVO4 samples with short reaction time (a-d). However, this band is shifted to around 826 cm-1 when prolong the hydrothermal time, the shifted band is the characteristic symmetric V-O stretching mode of monoclinic scheelite BiVO4 (s-m).30 It has been reported that the V-O bond length of BiVO4 becomes longer when the phase transfers from tetragonal to monoclinic BiVO4.31,32 The shifted Raman bands for different BiVO4 samples also provide further evidence for that the local structure transition of BiVO4 sample takes place during the hydrothermal process, which is in good agreement with the XRD analysis. To quantize the phase components for these samples, we introduced an intensity ratio of diffraction peak, Imono (121)/(I mono (121)+I tetra (200)) to estimate the phase composition of tetragonal zircon and monoclinic scheelite for BiVO4 samples as reported.33 The relation between hydrothermal time and the calculated fraction of monoclinic and tetragonal phases for BiVO4 samples is shown in Figure 1c. We found that the as-synthesized BiVO4 undergoes a process of phase transition during the hydrothermal treatment, which is converted from tetragonal zircon phase to monoclinic scheelite phase. From these phenomena, it could be speculated that the BiVO 4 precursor first existed as tetragonal zircon BiVO 4 powders, then gradually transformed to monoclinic scheelite BiVO4 during hydrothermal process. Figure 1d shows the UV-vis diffuse reflectance spectra of BiVO4 samples synthesized at different reaction stages. It is found that during the hydrothermal process, a shoulder absorbance around 470 nm become obvious, and the


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Figure 3.Schematic model of the formation for monoclinic BiVO4 particles prepared through hydrothermal treatment.

Figure 2. SEM images for the BiVO4 samples obtained by hydrothermal treating for different time. (a) 0 h, (b) 0.5 h, (c) 1.0 h, (d) 2.0 h, (e) 4.0 h, (f) 6.0 h, (g) 10 h and (h) 16 h.

absortption edge transform from 430 nm to 535 nm. It was reported that the absorption edge of ~430 nm for BiVO4 (z-t) is attributed to the electron transition from a valence band formed by O2p orbital to a conduction band formed by V3d orbital.34 For BiVO4 (s-m), its valence bands were contributed of both Bi6s and O2p, named hybrid orbital of Bi6s and O2p (Bi 6s-O2p), because of the distortion of BiVO4 crystal lattice.24 In this figure, the absorption edges of BiVO4 samples (0 h–16 h) did not vary in an orderly fashion as reported before.33 So the change of absorption edge is not due to the in situ continuous distortion of zircon tetragonal BiVO4. It is just the mixture of tetragonal zircon BiVO4 and sheelite monoclinic BiVO4 that transformed from tetragonal zircon BiVO4. The morphologies of as-prepared BiVO4 samples were also characterized by Scan Electronic Microscopy (SEM). As s hown i n Fi gure 2, BiVO 4 pre c urs or before hydrothermal treatment is composed of microspheres with a dimension in 5~10 µm. The surface of microspheres be c a m e r o u gh g ra d ua l l y a t t he i ni t i a l s t a ge o f hydrothermal reaction. When the reaction time was longer than 2.0 h, the individual decahedron BiVO4 particles

with dimensions of 1~2 µm are observed. Meanwhile, the portion of the decahedron particles increased along with the increase of hydrothermal pretreatment time. Almost all particles show decahedron crystals with dimensions of 1~2 µm after a hydrothermal treatment for 10 h and the particle size and the morphology of BiVO4 samples did not change anymore even further increasing the pretreatment time. Hence, the crystals transformed from a bigger particles to a smaller particles with regular morphology during hydrothermal process. And HRTEM and SAED characterizations (Figure S1) show that small tetragonal and monoclinic particles existed at different hydrothermal stages. Based on our results and related references,35-37 we can speculate that the hydrothermal treatment for the preparation of sheelite monoclinic BiVO4 with decahedron shape is a dissolutionrecrystallization process. As described above, we can simulate the formation process of monoclinic sheelite BiVO4 crystals with regular decahedron shape, prepared by precipitation followed hydrothermal treatment. Initially, The bismuth precursor (Bi(NO3)3·5H2O) and vanadate precursor (NH4VO3) were dissolved in nitric acid solution and then Bi(NO3)3 reacted with water to form BiONO3 (Eq.(1)) as the pH slowly increased by adding ammonia solution. At the same time, BiONO3 reacted with VO3- to form tetragonal zircon BiVO4 microspheres with a diameter of 5~10 µm (Eq.(2)).24 Under the condition of hydrothermal treatment, the surface of BiVO4 microspheres dissolved to growth units in the solution gradually, and meanwhile, some high-energy sites appeared at the surface of the sphere-like BiVO4 particles, corresponding to the fact that microsphere surface became rough at the initial stage. When the growth units transferred on the surface of BiVO4 microspheres, they would nucleate on these high energy sites as original seeds. And then as the hydrothermal treatment went on, more BiVO4 growth units were fed to these seeds, the seeds grew along the energetically favorable directions to form the decahedron particles. Finally, decahedron BiVO4 crystals were obtained when the reaction time is long enough.(Figure 3) At the same time, the transformation of crystalline phase from zircon tetragonalphase to sheelite monoclinic phase took place simultaneously in this process. BiNO H2 O⇌2HNO3 BiONO3 (1) BiONO3 VO ⇌BiVO4 NO (2)



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Figure 4. SEM images of the BiVO4 samples prepared by hydrothermal method under different pH values. (a) pH=0.25, (b) pH=0.5, (c) pH=0.75, (d) pH=1.0, (e) pH=1.5, (f) pH=2.5, (g) pH=3.0, (h) pH=3.5 and (i) pH=4.0.

According to the understanding as discussed above, the key process for the formation of different facets is recrystallization stage. So controlling the kinetics of recystallization is a feasible way to control the formation of different facets. It has been reported that the monoclinic BiVO4 with regular decahedron crystal was previously synthesized and exposed with different facets using TiCl3 as a directing agent, which can significantly influence the growth kinetics of growth units on the surface of crystals.38 But this directing agent will leave behind on the BiVO4 surface as a residue, which may affect some surface properties. So it is strongly desired to control the morphology and exposed facet of BiVO4 without using additional templates or surfactants. As we all know that during hydrothermal process, the acidity of hydrothermal solution can affect the kinetics of recrystallization reaction via controlling the saturability of the reactants and the structure of growth units, and furter influence final morphology of the crystals. Kudo and his coworkers have reported that the acidity during the synthesis process can influence the final morphology of the prepared BiVO4 crystals.30 However, it can be seen that the final products are not in a regular shape. In this work, we want to achieve precise control for the proportion of exposed facets based on the regular decahedron BiVO4. In addition,we found that Kudo and his coworkers used amorphous BiVO4 as precusor for

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hydrothermal treatment. It is different from us, using crystalline tetragonal zircon BiVO4 as precusor. So there maybe some differences between these two methods for the kinetics of recrystalline during hydrothermal process. So in this work we try to investigate the influence of acidity in our system, and interestingly, we found that by tuning the acidity of solution for the above-mentioned hydrothermal process, the regular morphology of BiVO4 crystals with smooth surface could be successfully obtained. As shown in Figure 4, the morphology of BiVO4 varied from decahedron crystals to short-rod like particles. A surprising result is that the thickness of decahedron crystals gradually changed under pH conditions between 0.25 and 1.0, indicating that the proportion of exposed {101} and {010} facets for BiVO4 crystals can be precisely controlled by changing the acidity of the hydrothermal solution. When pH value was increased to 1.5, some decahedron BiVO4 crystals evolved to irregular particles. Further increasing the pH value, the regular decahedron crystals disappeared completely and short rod-like BiVO4 particles with length of ~1 µm were observed. And the XRD patterns (Figure S2) show that all of these samples are pure monoclinic sheelite phase. The results indicate that all of these samples underwent a phase transformation process, and the morphologies of BiVO4 can be rationally controlled from regular decahedron crystals to rod-like particles, especially the varied proportion of regular decahedron crystals, only by slightly changing the acidity of solutions without using additional agent. Finally, Photocatalytic activities for all the as-prepared BiVO 4 sam ples we re c onducte d i n terms of the photocatalytic water oxidation performance under visiblelight irradiation (λ ≥ 420 nm). First the samples prepared by controlling the hydrothermal time were tested as shown in Figure 5a, and it can be clearly observed that the photoactivities of the BiVO4 samples are closely related to reaction time for hydrothermal treatment. The photoactivity was enhanced almost linearly with the increase of reaction time, corresponding to the increase of monoclinic phase percentage for BiVO4 samples. The BiVO4 sample after 10 h hydrothermal treatment (existing as pure monoclinic phase) exhibits the best photocatalytic activity, which is over 30 times of the sample before

Figure 5. (a) Photocatalytic water oxidation of BiVO4 samples prepared under different hydrothermal time. (b) Photocatalytic water oxidation of BiVO4 samples prepared under different pH conditions. Reaction condition: catalyst, 100 mg; 10 mMFe(NO3)3, 150 mL; light source, Xe lamp (300 W, λ ≥ 420 nm).


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pretreatment. It should be pointed out that all these BiVO4 samples have comparable surface areas, indicating that the surface area is not the main factor for the variety of photocatalyitc performance (Table S1). As the crystalline phase and morphology of BiVO4 are all varied by hydrothermal pretreatment, a controllable experiment was then done to investigate which one is the key factor. Monoclinic phase BiVO4 sample was prepared by calcinating tetragonal BiVO4 precursor at 773 K. The crystalline phase for the as-prepared BiVO4 sample could be confirmed by XRD and Raman characterizations (Figure S3). However, the calcinated BiVO4 sample (BET: 3.381 m2 g-1) revealed the same microsphere morphology with tetragonal phase BiVO4 precursor (Figure S4). The photocatalytic performances of these samples showed that the photoactivity of pure monoclinic phase calcinated at high temperature is about 2 times of pure tetragonal phase without calcination (Figure S5), nevertheless, the photoactivity of monoclinic BiVO4 with microsphere morphology is only less than 10% of the regular monoclinic BiVO4 crystals prepared by hydrothermal treatment (Figure S5). This result leads us to draw a conclusion that the photocatalytic performance of BiVO4 photocatalyst is closely related to both crystalline phase and morphology. To further confirm the effect of morphology for BiVO4 on photocatalytic activity, further investigations of BiVO4 crystals with precisely control the morphology without changing the crystalline phase have been conducted. From the results (Figure 5b), it can be seen that the photoactivity for regular decahedroncrystal can be greatly improved to be more than 3 times of the case for irregular crystal, although all these BiVO4 samples are monoclinic phase. And interestingly, different performance can be observed between these regular decahedron crystal with different proportion of {010}/{011} facets. Therefore, we can conclude that the morphologies and exposed facets of BiVO4 play significant roles in photocatalytic performance for BiVO4 photocatalyst. The stability test (Figure S6) shows that there is no obvious decrease of photocatalytic activity for BiVO4 photocatalyst after more than 3 cycles, indicating a good stability of photocatalyst in our system. For decahedron BiVO4 crystals, the energy levels in the conduction bands (∆Eg (CB)) and the valence bands (∆Eg (VB) of {010} facet were calculated to be higher than {011} facet.11, 39 This difference makes it feasible that electrons transfer from {011} to {010} facets, and holes transfer from {010} to {011} facets. Therefore, {010} facets provide the active sites for reduction reaction and {011} facets provide the active sites for oxidation reaction based on the charge separation between different facets of decahedron BiVO4. And for photocatalysis reaction, the apparent reaction rate is limited by the slower rate of the oxidation reaction and reduction reaction. According to our results and discussion above, it can be speculate that the reaction rates of oxidation and reduction reaction can be optimized via controlling the ratio of {010} and

{011} facets for BiVO4 crystals. So the best performance can be achieved when the surface for both reduction and oxidation reactions are optimized. In summary, BiVO4 crystals were synthesized by a hydrothermal method, and based on the investigations of the different stages for the hydrothermal process, we found that the formation of BiVO4 regular morphology is a dissolution-recrystallization process, which is experiencing the transformation of structure from tetragonal zircon-type to monoclinic sheelite-type. And then based on the understanding of the hydrothermal synthesis kinetics, BiVO4 with different morphology from regular decahedron crystals to short rod-like particles can be synthesized, especially, the proportion of {010}/{011} facets for the final regular decahedron BiVO4 crystals can be precisely controlled. The performance of photocatalytic water oxidation of BiVO4 photocatalyst is demonstrated to be strongly dependent on the crystal structure and the morphology, even the proportion of the exposed facets. Efficient photocatalytic water oxidation performance was achieved by optimizing BiVO4 crystals to final monoclinic decahedron crystals with specific configuration of {010} and {011} exposed facets, which is more than 50 times of the irregular tetragonal BiVO4 particles.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. brief description (file type, i.e., PDF)

AUTHOR INFORMATION Corresponding Author * Can Li. Email:[email protected]

Author Contributions #

These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by 973 National Basic Research Program of the Ministry of Science and Technology (2014CB239400), National Natural Science Foundation of China (21501236, 21673230, 21633010), Youth Innovation Promotion Association of Chinese Academy of Sciences (2016167), Dalian Institute of Chemical Physics (DICP ZZBS201610) and National Natural Science Foundation of Liaoning province (201602739).

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For Table of Contents Use Only

The Significance of Crystal Morphology Controlling in Semiconductorbased Photocatalysis: A Case Study on BiVO4 Photocatalyst Yue Zhao#†‡, Rengui Li#†, Linchao Mu†‡, and Can Li*†

Synopsis:

BiVO4 with different morphology from regular decahedron crystals to short rod-like particles can be synthesized, especially, the proportion of {010}/{011} facets for the final regular decahedron BiVO4 crystals can be precisely controlled. The performance of photocatalytic water oxidation of BiVO4 photocatalyst is strongly dependent on the crystal structure and the morphology, even the proportion of the exposed facets.

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Significance of Crystal Morphology Controlling in Semiconductor

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