Controllable fabrication of Bi2O3 nanoparticles by atomic layer deposition on TiO2 films and application in photodegradation

Solar Energy Materials & Solar Cells 204 (2020) 110218

Contents lists available at ScienceDirect

Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat

Controllable fabrication of Bi2O3 nanoparticles by atomic layer deposition on TiO2 films and application in photodegradation Jiaze Li a, Niefang Mao a, Xin Li a, Fangfang Chen a, Yawei Li a, *, Kai Jiang a, Zhigao Hu a, Junhao Chu a, b a b

Key Laboratory for Polar Materials and Devices of Ministry of Education, East China Normal University, Shanghai, 200241, People’s Republic of China National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai, 200083, People’s Republic of China

A R T I C L E I N F O

A B S T R A C T

Keywords: Bi2O3 nanoparticles TiO2 Heterojunction Atomic layer deposition Photodegradation

Bi2O3 nanoparticles prepared by atomic layer deposition (ALD) are growth on TiO2 films. As catalyst, It is found that the photocatalytic effective is associated with the cycle number of loading Bi2O3 nanoparticles. The pho­ tocatalytic activity of various samples may due to the different size of deposited Bi2O3, which can be observed by the field-emission scanning electron microscope (FE-SEM). The morphology may attribute to the growth con­ trolling of ALD method. Then, a mechanism is proposed to explain why the compound with 10 cycle Bi2O3 shows the best behavior. Because that the Bi2O3 grown by ALD in several cycles is island, the photocatalytic was further improved by the structure of Bi2O3 nanoparticles/TiO2 films. What’s more, the sample had a good stability by keeping a high activity without obvious deactivation after four recycles of the degradation. The result shows that ALD technology has a broad potential application in the field of photocatalytic because its mass industrial production and the achievement of surface improvement in a short time.

1. Introduction Nowadays, global environmental pollution and ecological destruc­ tion are great challenges. In particular, industrial wastewater is increasing by the process of chemical industry. Dye wastewater is the main harmful industrial wastewater. The basic materials of dye pro­ duction are benzene, naphthalene, anthraquinone, aniline and benzi­ dine compounds. Organic dye wastewater not only has high chroma, but also high chemical oxygen consumption concentration in the water, and most of the dyes have the effect of toxic, carcinogenic, teratogenic and mutagenic. This makes people pay great attention to the new pollutionfree technology [1,2]. Rational developments of new energy and mate­ rial are key to environmental protection. To improve the wastewater pollution, Photocatalytic is a new energy saving and environmental protection technology. It is a process which the organic pollutants, such as organic dyes, can be converted into non-toxic or less toxic substances under the action of catalyst [3–6]. The combination of two semi­ conductors is an effective method to improve photocatalytic efficiency. It can enhance the effect of charge separation and change the absorption range of the spectrum. For example, Das and co-workers fabricated a series of CuS/Bi4Ti3O12 materials. Compared with Bi4Ti3O12 materials,

the CuS/Bi4Ti3O12 heterojunctions had a new visible light absorption range from 400 nm to 800 nm and significantly improved the charge carrier separation [7]. The heterostructure can effectively inhibit carrier recombination due to the different positions of valence band and con­ duction band in two semiconductors [8–10]. When two semiconductors are combined, an electric field is built up inside the heterogeneous structure, because of the diffusion of majority carrier in two semi­ conductors. As a result, it restrains the recombination of photogenerated electron-hole pairs. To improve photocatalytic efficiency, the selection of semiconductor materials is particularly important. Titanium dioxide (TiO2) is the most widely investigated photo­ catalyst due to its good photocatalytic activity, high thermal stability, nontoxicity, low cost, and excellent degradation capacity [11,12]. However, a large band gap (3.2 eV) of TiO2 can be only activated by the illumination of ultra-violet light, which merely makes up 4% of the solar spectrum [13]. The low photogenic charge separation efficiency of TiO2 limited its application in photocatalytic field. Some studies have shown that TiO2 with other materials formed as heterojunctions can improve the photocatalytic performance. For example, Sarkar et al. have syn­ thesized a p-n junction by assembling p-type Ag2O nanoparticles on n-type TiO2. The heterostructure with a 4:1 M ratio of TiO2 and AgNO3

* Corresponding author. E-mail address: [email protected] (Y. Li). https://doi.org/10.1016/j.solmat.2019.110218 Received 14 June 2019; Received in revised form 12 September 2019; Accepted 8 October 2019 Available online 12 October 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.

J. Li et al.

Solar Energy Materials and Solar Cells 204 (2020) 110218

could exhibit best photocatalytic activity [14]; Sun et al. discovered a high performance photocatalyst based on a heterostructure of ZnO nanoparticles and the mesoporous biphase TiO2, which was fabricated by a facile water bath reflux method. When the content of ZnO was 20 wt %, the highest photocatalytic activity could be achieved [15]; the het­ erostructure can improve the charge separation efficiency of TiO2 then enhance the performance of photocatalytic. Bi2O3 is a significant p-type metal-oxide semiconductor. However, it has a low photocatalytic activity because of its fast recombination of photogenerated electro-hole pairs [16–18]. When it was combined with n-type TiO2 as a p-n heterogeneous structure [19], the range of the light response of the heterojunction can be extend due to the direct band gap of Bi2O3 is 2.8 eV, which can be excited by visible light [20]. Thus, the photocatalytic activity of Bi2O3/TiO2 can be significantly improved. In recent years, the development of Bi2O3/TiO2 composition nanostructure gets widely concerns in the photocatalytic filed. For instance, Huang et al. have reported hierarchical composites of three-dimensional Bi2O3 nanoparticles and TiO2 nanorods prepared by a two-step hydrothermal method [21]. Liu et al. have prepared a nanoheterojunction composite photocatalyst of Bi2O3 and TiO2 by an organic binder of Maleic acid [16]. Through controlling the growth of nanostructures, the above research works have achieved better photocatalytic performance. Atomic layer deposition (ALD) is a powerful manufacturing technology [22–24]. On account of its better control of film thickness, it is widely used on the growth of film. In recent years, ALD has arisen great concern in synthesis of complex nanostructures and the application of surface engineering. The surface modification by ALD is an extremely promising route to prepare visible light active photocatalysts. The size of the grains and the clusters can be accurately dominated by controlling the cycles of deposition. Some research has found that ALD can also use for nano­ particle growth. For example, Liang et al. used Ag nanoparticles to decorate TiO2 tubes by ALD. It slowed the recombination of electrons and holes and enhanced the efficiency of photodecomposition [25]; Akbari et al. have deposited TiO2 by ALD on Si/SiO2–Au substrate to improve the visible light photocatalytic [26]; Chen et al. have produced a Pt@TiO2@anodic aluminum oxide (AAO) membrane nanoreactor by ALD, it showed efficient photocatalysis performance [27]. The noble metals deposit on a semiconductor surface can change the photo­ catalytic process by modifying semiconductor surface morphology. However, there are few studies on surface modification of metal oxides prepared by ALD. In this work, Bi2O3 nanoparticles were fabricated as island not a film via ALD, which the photocatalytic can be further enhanced by the heterogeneous structure. Methyl blue (MB), as one of the typical dyes, was used to simulate the catalytic degradation of dye wastewater. The photocatalytic performance of these composite struc­ tures is studied and a model is proposed to explain the phenomenon of the optimal value achieved best photocatalytic.

each stage was 300s, 300s and 600s. After repeating the above process 6 times, the TiO2 film was obtained (Denoted as T0). 2.2. Bi2O3/TiO2 heterostructure preparation The Bi2O3 nanoparticles were synthesized on the surface of TiO2 film (T0) by ALD (Sunale r-75, Picosun Oy) with the cycles of 2, 5, 10 and 20, denoted as T2, T5, T10 and T20. As a contrast, we deposited 2000 cycles of Bi2O3 by ALD on Si substrates without TiO2, marked as pure Bi2O3. Bi (thd)3 and deionized water were used as bismuth and oxygen precursor, respectively. High-purity nitrogen (99.9999%) was used as the sources carrier and purging gas. In the reaction chamber, the Bi2O3 were deposited on the TiO2 film at 300 � C. The sample was alternatively exposed to the precursors of Bi(thd)3 with 3s pulse time and H2O with 0.2s pulse time. Purge times of 5s for N2 gas were used for Bi(thd)3 pulses and 4s for H2O pulses. The rate of carrier gas flow 200 sccm (standard cubic centimeters per minute) was used for all precursors. Bi(thd)3 was evaporated from a booster bottle held at 190 � C. The H2O vapor was generated in a liquid-source bottle at room temperature. 2.3. Characterization X-ray diffraction (XRD) patterns were recorded on a Brucker AXS D8 powder diffractometer unit by using Cu Ka radiation (λ ¼ 0.154 nm). The morphology and composition of samples were observed by a fieldemission scanning electron microscope (FE-SEM) and the energy dispersive X-ray spectrometer (EDS, NERCN-TC-006, S-4800). Photo­ luminescence (PL) spectra were measured at room temperature using a fluorescence spectrophotometer (Lab RAM HR 800 UV) with an exci­ tation wavelength of 325 nm. PL spectra of all samples were examined in the range of 350–750 nm. The optical properties of the samples were measured by a UV–vis spectrophotometer (PerkinElmer Lambda 950). 2.4. Photocatalytic test Photocatalytic activity of the as-prepared samples was evaluated by the degradation of methyl blue (MB) in a double-layer cold trap pho­ tocatalytic reactor under visible-light irradiation of a 300 W Xe lamp (PLS-SXE300, Emission wavelength from 320 nm to 780 nm). Prior to light irradiation, the catalyst was placed in methyl blue solution and stood for 30min to ensure adsorption-desorption equilibrium between the organic molecules and the catalyst surface. Moreover, the degrada­ tion of MB was also carried out without catalyst to deduct light irradi­ ation effect. Before each illumination about 60 min, MB were sampled and analyzed by recording the variations of the absorption-band maximum (628 nm) in the UV–vis spectrum. The photocatalytic activ­ ity of TiO2 thin films with different cycles of Bi2O3 and pure Bi2O3 were measured for 8 h (T0, T2, T5, T10, T20 and pure Bi2O3). Each of the samples was measured for several times to ensure its precision. The sample T10 was used to identify the stability of photocatalyst by recy­ cling fourth degradation, after each 8 h, the degraded MB solution was removed and the MB solution with initial concentration was poured into the quartz glass photochemical reactor to begin the next run.

2. Experimental sections TiO2 films processed by sol-gel method and Bi2O3 nanoparticles deposited by ALD were formed as Bi2O3/TiO2 photocatalysts. 2.1. Hierarchical TiO2 film preparation

3. Discussion

0.01 mol tetrabutyl titanate was injected into the co-solvent mixture containing 25.4 ml absolute ethyl alcohol and 10.5 ml acetic acid. After 30 min of magnetic stirring at 60 � C, the acetone with the same content as tetrabutyl titanate was added as a stabilizer. After reheating and stirring for 4 h and aging for 24 h, the stable solvent was obtained. TiO2 film was formed on heavily doped Si substrates. The substrates were cleaned by deionized water and alcohol, then annealed 180 s at 800 � C. TiO2 solution was dropped vertically off the substrate. The volume of each drop was about 23–25 milli ml. The solution was spin-coated on Si substrate by the speed of 1000r/s for 5s, then 5000r/s for 30s.There was a three-stage annealing at 200 � C,400 � C and 670 � C, the holding time of

3.1. Crystal structure analysis Fig. 1 shows the XRD patterns of pure TiO2 (a), pure Bi2O3 (f) and TiO2 thin films with different cycles of Bi2O3 (b)-(e). The diffraction peaks in curve (a)-(e) can be indexed to the anatase phase of TiO2. The diffraction peaks at 25.33� and 48.01� corresponded to crystal plane (120) and (231), respectively, according to the standard data (JCPDS No. 29–1360). The diffraction peak in curve (f) at 27.94� was indexed to the(201)plane to the pure Bi2O3 (JCPDS No.65–1209). No observation of Bi2O3 diffraction peaks were found in the composite of Bi2O3/TiO2. It 2

J. Li et al.

Solar Energy Materials and Solar Cells 204 (2020) 110218

be observed in Fig. 2(c), which the deposition of Bi2O3 was 5 cycles (sample T5). Those clusters were irregularly shaped which was similar to that of sample T2; When the Bi2O3 cycles were 10(sample T10), it could be observed in Fig. 2(d) that the size of clusters ranged from 27.9 nm to 42.7 nm irregularity, the large numbers of nanoparticles were still existed in sample T10. When the cycles achieved to 20 cycles (sample T20), there were large quality of spherical clusters, only a few numbers of nanoparticles were observed in the Fig. 2(e). As few cycles were deposited on TiO2 films, Bi2O3 nanoparticles formed as small spherical particles and clusters. This phenomenon was confirmed by other researchers. Baker and co-workers had deposited Pt by ALD on Al2O3 from 38 to 150 cycles. When there are few numbers of the deposited cycles, the Pt nucleated on the Al2O3 substrate as discrete nanoclusters [30]. A cross-sectional SEM image of sample T20 is shown in Fig. 2(f). There was a clear boundary between TiO2 film and Si substrate, there are a few islands can be observed on the TiO2 film. It is consistent with the results of Fig. 2(e), the islands are Bi2O3 in the form of clusters and nanoparticles, and the thickness of TiO2 film was about 280 nm. Theo­ retically, it is generally believed that ALD growth is mostly framed in terms of sequential self-limiting chemisorption of precursors, leading to a layer-by-layer deposition [31]. However, the nucleation and island growth of materials are certain to be determined by atomistic processes, such as surface diffusion of adatoms, nucleation, diffusion and coales­ cence of nanoparticles, and the atomic bonding and separation of nanoparticles [32]. Up to now, the mechanism has been inferred by many scholars. For example, Simonsen and co-workers had revealed via series time-resolved image that the sintering of Pt nanoparticles was mediated by an Ostwald ripening process. It means that the larger par­ ticles were formed at the expense of the smaller particles [33]. Grillo et al. explained the experiments data by their atomistic modeling and conclusions were drown that the nanoparticles grow mostly by the diffusion and coalescence of nanoparticles [32]. Based on the particle size distribution. Jak et al. found that the main growth mechanism for palladium clusters on TiO2(110) was cluster diffusion and coalescence [34]. Woehl and co-workers have a conclusion that single nanoparticles via atom attachment and ensemble-scale growth is dominated by ag­ gregation [35]. Meanwhile, Jin et al. had found that the diffusion and coalescence of atomistic processes were accompanied by the generation and enlargement of the clusters in the first 80 cycles. Furthermore, the steric hindrance enables the bulky precursors not covered all the surface [36]. The chemical composition of Bi2O3/TiO2 was further studied by using EDS technique attached on the SEM. Fig. 3 clearly shows the EDS results of sample T20 with its relevant elemental mapping. As shown in

Fig. 1. XRD patterns of pure TiO2, pure Bi2O3 and the Bi2O3/TiO2 hetero­ junction with different cycles of Bi2O3.

may be caused by the low content of Bi2O3 and the extremely high dispersion of Bi2O3 particles. This phenomenon was also reported by other researches. For instance, You et al. have produced a highly effi­ cient Bi2O3–TiO2 composite, prepared by a facile hydrothermal method. There was no obvious Bi2O3 diffraction peaks observed in XRD image [28]; María et al. have reported a series of Bi2O3–TiO2 composite cata­ lysts with variable quantities of bismuth, which has a lack of identifiable signals coming from the bismuth component shown by the XRD and Raman [29]. The Bi2O3/TiO2 samples prepared by all the methods above were in accordance with our results. 3.2. Morphology characterization Fig. 2(a)-(f) are the SEM top and cross-section views of Bi2O3/TiO2 heterostructure with different cycles of Bi2O3. Fig. 2(a) shows FESEM top-view image of the synthesized TiO2 films (sample T0) on the Si substrate, the surface of TiO2 film was dense and the size of grains were uniformed. As shown in Fig. 2(b)–(e), there were some white nano­ particles formed as small spherical particles or assembled as clusters which were randomly distributed on TiO2 films. With the growth of Bi2O3, the increasing of small spherical particles given rise to the number of clusters. As shown in Fig. 2(b) (sample T2), there were some clusters and a few spherical particles; More clusters and particles could

Fig. 2. (a)–(e) is the SEM top view of low magnification T0, T2, T5, T10 and T20 respectively, and the top right corner of the figure is the SEM top view of high magnification; F is the cross-section of T20. 3

J. Li et al.

Solar Energy Materials and Solar Cells 204 (2020) 110218

Fig. 3(a), the EDS spectrum showed strong signals of O, Bi and Ti ele­ ments, whereas no impurity peaks were observed. Fig. 3(b) is a scanning SEM image of a nanostructure of Bi2O3/TiO2, on which elemental mapping analysis was induced. Fig. 3(c), (d) show EDS elemental mapping of Ti and Bi, respectively. It can be observed that Ti element was evenly distributed on the surface of the sample. However, the dis­ tribution of Bi element was dispersed and existed in the form of clusters and small spherical particles. It is consistent with the result of Fig. 3(b). EDS mapping result further confirmed Bi2O3 nanoparticles and clusters coexisted on the TiO2 films.

� � C0 ¼ kt In C

(1)

Where C0 is the initial concentration of MB, C is the concentration which changes over time, and k is the pseudo first-order kinetics rate constant. Clearly, T2, T5, T10 sample gave a rate constant of dye degradation about 1.6 times higher than pure TiO2 films (T0) in the Fig. 4(b). 3.4. Photoluminescence spectra For semiconductors, the photoluminescence (PL) spectra are related to the transfer behavior of the photogenerated electrons and holes, it can reflect the separation and recombination of photogenerated charge carriers. The PL spectra of the sample T0, T2, T5, T10 and T20 are shown in Fig. 5. The PL spectrum of pure TiO2 has a strong emission peak at about 552 nm, which came from the electron capture emission by sur­ face states [39], and the PL intensities of Bi2O3/TiO2 composites was lower than that of pure TiO2. This indicates that TiO2 incorporated with an appropriate amount of Bi may slow the radiative recombination process of photogenerated electro-hole pairs in TiO2 [19]. It is consistent with the results of photocatalytic. Comparing with T0, a blue-shift of the peaks can be observed with increasing deposition of Bi2O3. The PL emission bands at 2.4eV (about 533 nm) associated with complexes containing closely packed bismuth and oxygen in the crystal structure [40]. What’s more, there is a weaker peak at about 575 nm. The similar peak position at 589 nm and 528 nm of Bi2O3 was reported by Kumari et al., which was attributed to the emission corresponding to the Bi3þ ions [41].

3.3. Photocatalytic performance Photocatalytic performance of the series nanocomposites of Bi2O3/ TiO2 were evaluated by degradation experiments of MB under ultravi­ olet and visible light irradiation, as indicated in Fig. 4. The absorbance of MB was detected at the maximum absorption edge of 628 nm in the ultraviolet visible spectrum. As a comparison, two experiments were conducted. One, only MB degradation was performed under light irra­ diation in the absence of photocatalysts, the results showed that there was a little degradation of MB solution after 8 h, recorded as MB0; Another, MB degradation with photocatalysts was performed without light, the results showed that there was no appreciable degradation of MB after 8 h, recorded as Dark. Therefore, both illumination and pho­ tocatalysts were essential for the highly efficient degradation. All data of the samples were obtained from the normalization of the Dark data. All the experimental data were shown in Fig. 4(a), the pure Bi2O3 showed a lower photocatalytic activity than pure TiO2 (T0). Clearly, there was an optimal cycle of Bi2O3 about 10(T10) to exhibit the highest photo­ catalytic activity, and the degradation rate of sample T20 was even lower than T0. The similar phenomenon was also found by Hou et al. who produced various atomic ratio of Bi/Ti via a facile nonaqueous sol–gel method. There was an optimal Bi/Ti proportion of 0.0175 to exhibit the highest photocatalytic activity [37]. When the Bi/Ti pro­ portion was higher than its optimum amount, higher concentration of Bi caused a decrease in the photocatalytic activities. Generally, the decomposition of the dye could be seen as a pseudo-first-order kinetics reaction with a simplified Langmuir Hinshelwood model when C0 is very small [22,38].

3.5. Photocatalytic stability in degradation reaction To test the stability of the photocatalysts, T10 as a representative photocatalyst, was used for repeating photodegradation experiments of MB. As shown in Fig. 6, the photodegradation of MB was monitored by sample T10 over four cycles, and each of cycle held 8 h. As a result, the photocatalytic efficiency exhibited no decrease and no significant loss of activity, it indicates that T10 composite was an efficient and stable visible-light photocatalyst.

Fig. 3. Sample T20 (a) SEM-EDS, (b) FE-SEM images, and (c)–(d) EDS mapping of Ti and Bi. 4

J. Li et al.

Solar Energy Materials and Solar Cells 204 (2020) 110218

Fig. 4. Evaluation of photocatalytic activities (a) and the relative kinetic rates (b) in MB degradation with pure TiO2 films(T0), pure Bi2O3 films (Bi2O3) and TiO2 films decorated with different cycles of Bi2O3 (T0, T2, T5, T10 andT20) under visible-light irradiation.

Fig. 5. The PL spectra of the pure TiO2 and Bi2O3/TiO2 nanocomposites.

3.6. Mechanism of enhanced photocatalytic process Pure Bi2O3 and pure TiO2 demonstrated lower photocatalytic activ­ ity than Bi2O3/TiO2 nanocomposite under visible-light irradiation. It may due to the contribution of Bi2O3/TiO2 heterostructure. The heter­ ojunction can efficiently promote the generation, transfer, and separa­ tion process of electron and hole pairs under ultraviolet and visible light irradiation. The unusual high photocatalytic activity of Bi2O3/TiO2 nanocomposites originates from the different position of energy band of two semiconductors. The positions of the valence band maximum (VBM) and the conduction band minimum (CBM) are critical parameters in determining the feasibility of photocatalytic activity. According to the following empirical equation [17,21]. ECB ¼ X

Ee

0:5Eg

(2)

EVB ¼ X

Ee þ 0:5Eg

(3)

Fig. 6. Recycling stability of T10 sample for photocatalytic degradation of MB under visible light irradiation. Table 1 Absolute electronegativity, energy band gap, calculated conduction band and valance band edge for TiO2 and Bi2O3 semiconductors.

Where X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms; Ee is the energy of free electrons on the hydrogen scale (about4.5 eV); and Eg is the band gap of the semiconductor. The values of X, EVB, ECB, and Eg are listed in Table 1. A proposed mechanism for the enhanced photoactivity over the Bi2O3/TiO2 nanocomposite is shown schematically in Fig. 7. There is an electric field formed in the boundary interface of Bi2O3/TiO2. Owing to the electric field, the photogenerated electron hole pairs can be

Semiconductors

Absolute electronegativity X(eV)

Energy Band gap Eg(eV)

Calculated conduction band edge (eV)

Calculated Valance band edge(eV)

TiO2 Bi2O3

6.33 [42] 5.98 [17]

3.20 [13] 2.80 [20]

0.23 0.08

3.43 2.88

effectively separated. When the photoexcitation energy is greater than the Eg of p-type Bi2O3 under visible light irradiation, the electrons of ptype Bi2O3 in the valence band can be excited to the conduction band. Since the conduction band edge potential of p-type Bi2O3 is significantly higher than that of n-type TiO2, the electrons can migrate from the surface of Bi2O3 to TiO2. As the internal field in p-n heterostructure can 5

J. Li et al.

Solar Energy Materials and Solar Cells 204 (2020) 110218

finished in a short time. It is suitable for the large-scale production. Therefore, the method reported in our work has potential application in many kinds of photocatalytic fields, such as the elimination of organic pollutants, the photocatalytic water splitting, and the photocatalytic reduction of CO2. 4. Conclusions In summary, we have demonstrated an ALD/sol-gel method to syn­ thesize a series Bi2O3/TiO2 photocatalysts, which exhibited better photocatalytic activities and stability in degradation of MB under visible light irradiation. The deposited Bi2O3 of 10 cycles showed the best performance in the experimental conditions. Furthermore, the catalyst of T10 was recoverable and reusable after four cycling runs, demon­ strating the good stability of Bi2O3/TiO2 photocatalysts, which will greatly promote their industrial application to eliminate the organic pollutants from wastewater. Acknowledgement Fig. 7. Schematic illustration of the charge separation and transfer in the Bi2O3/TiO2 heterojunction under visible-light. The photogenerated electrons transfer from the conduction band of semiconductor Bi2O3/TiO2 composite.

This work was financially supported by the National Key R&D Pro­ gram of China (grant Nos. 2017YFA0303403 and 2018YFB0406500), the National Natural Science Foundation of China (grant Nos. 61674057 and 91833303), Projects of Science and Technology Commission of Shanghai Municipality (grant No.18JC1412400).

effectively separate photogenerated electro-hole pairs, the recombina­ tion of electron and hole pairs will be greatly reduced by p-n hetero­ geneous structure. Electrons on the surface of the photocatalysts can be captured by dissolved O2 in solution to produce the superoxide radical O-2, which can degrade MB into CO2, H2O and other small inorganic molecular substances. Holes (hþ) on the surface of the photocatalysts can react with OH- to form the strongly oxidizing hydroxyl radical HO⋅. The photocatalytic oxidation is carried out at the surface of p-type Bi2O3 and the photocatalytic reduction is carried out at the surface of n-type TiO2, thus, photocatalytic reaction can be greatly improved. Based on this mechanism of photocatalytic process, the results of photocatalytic can be well explained. From SEM in Fig. 2(b)–(e), when the cycles of Bi2O3 was 10, there are still a large number of small spherical particles. The internal field between Bi2O3 and TiO2 can be used to inhibit the recombination of electro-hole pairs. However, the sample T20 has numerous clusters and a few small spherical particles. When visible light is irradiated on the surface of Bi2O3, the photo­ generated electro-hole pairs on the surface of large size clusters take more time transport to the interface between Bi2O3 and TiO2 than that of particles. As mentioned above, because of the fast recombination of photogenerated electro-hole pairs in Bi2O3 films, most of the carriers recombine before moving to the surface so that only a few numbers of carriers reached in the interface between Bi2O3 and TiO2 then react with MB dye molecule. As a result, the photocatalytic activity of T20 is lower than T10. Furthermore, the samples with more small spherical particles can form more p-n heterojunctions. The photogenerated electro-hole pairs are so fast to reach the surface between Bi2O3 and TiO2 that they cannot recombined in particles due to the low content of Bi. A large number of photogenic carriers reached in the surface and react with MB dye molecule caused high photocatalytic activities. ALD technology is based on self-limiting and self-saturating adsorption reaction on the surface. The reaction was happened on the surface of substrates. It is suitable for depositing materials on the surface of substrates with complex aspect ratio. In our research, the ALD tech­ nology was used to deposit on the surface of planar films. The photo­ catalytic activity can be further improved if the surface morphology of the substrates is complex structure, such as nanowires [43], nanotubes [44], nanoparticles [45]. Although ALD was widely used as one of the solutions for the ultrathin films, it still limited by the lower reaction rate. However, according to our work, the fabrication of nanoparticles can be achieved in several ALD cycles. In other words, the deposition can be

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110218. References [1] F. Chen, H. Huang, L. Ye, T. Zhang, Y. Zhang, X. Han, T. Ma, Thickness-dependent facet junction control of layered BiOIO3 single crystals for highly efficient CO2 photoreduction, Adv. Funct. Mater. 28 (2018) 1–11, https://doi.org/10.1002/ adfm.201804284. [2] Q. Zou, H. Li, Y. Yang, Y. Miao, Y. Huo, Bi2O3/TiO2 photocatalytic film coated on floated glass balls for efficient removal of organic pollutant, Appl. Surf. Sci. 467–468 (2019) 354–360, https://doi.org/10.1016/j.apsusc.2018.10.182. [3] X. Dong, J. Li, Q. Xing, Y. Zhou, H. Huang, F. Dong, The activation of reactants and intermediates promotes the selective photocatalytic NO conversion on electronlocalized Sr-intercalated g-C3N4, Appl. Catal. B Environ. 232 (2018) 69–76, https://doi.org/10.1016/j.apcatb.2018.03.054. [4] M. Cheng, C. Lai, Y. Liu, G. Zeng, D. Huang, C. Zhang, L. Qin, L. Hu, C. Zhou, W. Xiong, Metal-organic frameworks for highly efficient heterogeneous Fenton-like catalysis, Coord. Chem. Rev. 368 (2018) 80–92, https://doi.org/10.1016/j. ccr.2018.04.012. [5] S. Natarajan, H.C. Bajaj, R.J. Tayade, Recent advances based on the synergetic effect of adsorption for removal of dyes from waste water using photocatalytic process, J. Environ. Sci. (China) 65 (2018) 201–222, https://doi.org/10.1016/j. jes.2017.03.011. [6] J. Xiong, J. Di, J. Xia, W. Zhu, H. Li, Surface defect engineering in 2D nanomaterials for photocatalysis, Adv. Funct. Mater. 28 (2018) 1–19, https://doi. org/10.1002/adfm.201801983. [7] K. Das, D. Majhi, Y.P. Bhoi, B.G. Mishra, Combustion synthesis, characterization and photocatalytic application of CuS/Bi4Ti3O12 p-n heterojunction materials towards efficient degradation of 2-methyl-4-chlorophenoxyacetic acid herbicide under visible light, Chem. Eng. J. 362 (2019) 588–599, https://doi.org/10.1016/j. cej.2019.01.060. [8] C. Wang, X. Zhang, B. Yuan, Y. Wang, P. Sun, D. Wang, Y. Wei, Y. Liu, Multiheterojunction photocatalysts based on WO3 nanorods: structural design and optimization for enhanced photocatalytic activity under visible light, Chem. Eng. J. 237 (2014) 29–37, https://doi.org/10.1016/j.cej.2013.10.003. [9] C. Xu, P. Chen, J. Liu, H. Yin, X. Gao, X. Mei, Fabrication of visible-light-driven Ag/ TiO2 heterojunction composites induced by shock wave, J. Alloy. Comp. 679 (2016) 463–469, https://doi.org/10.1016/j.jallcom.2016.04.048. [10] G. Yang, B. Yang, T. Xiao, Z. Yan, One-step solvothermal synthesis of hierarchically porous nanostructured CdS/TiO2 heterojunction with higher visible light photocatalytic activity, Appl. Surf. Sci. 283 (2013) 402–410, https://doi.org/ 10.1016/j.apsusc.2013.06.122. [11] Y.Q. Cao, X.R. Zhao, J. Chen, W. Zhang, M. Li, L. Zhu, X.J. Zhang, D. Wu, A.D. Li, TiOxNy modified TiO2 powders prepared by plasma enhanced atomic layer deposition for highly visible light photocatalysis, Sci. Rep. 8 (2018) 1–9, https:// doi.org/10.1038/s41598-018-30726-w.

6

J. Li et al.

Solar Energy Materials and Solar Cells 204 (2020) 110218

[12] W. Ouyang, F. Teng, X. Fang, High performance BiOCl nanosheets/TiO2 nanotube Arrays heterojunction UV photodetector: the influences of self-induced inner electric fields in the BiOCl nanosheets, Adv. Funct. Mater. 28 (2018) 1–12, https:// doi.org/10.1002/adfm.201707178. [13] D. Sud, A. Syal, Investigations on the phase transformation, optical characteristics, and photocatalytic activity of synthesized heterostructured nanoporous Bi2O3–TiO2, J. Chin. Chem. Soc. 63 (2016) 776–783, https://doi.org/10.1002/ jccs.201600099. [14] D. Sarkar, C.K. Ghosh, S. Mukherjee, K.K. Chattopadhyay, Three dimensional Ag2O/TiO2 type-II (p-n) nanoheterojunctions for superior photocatalytic activity, ACS Appl. Mater. Interfaces 5 (2013) 331–337, https://doi.org/10.1021/ am302136y. [15] C. Sun, Q. Xu, Y. Xie, Y. Ling, Y. Hou, Designed synthesis of anatase-TiO2(B) biphase nanowire/ZnO nanoparticle heterojunction for enhanced photocatalysis, J. Mater. Chem. A. 6 (2018) 8289–8298, https://doi.org/10.1039/c7ta10274g. [16] A.K. Chakraborty, M.E. Hossain, M.M. Rhaman, K.M.A. Sobahan, Fabrication of Bi2O3/TiO2 nanocomposites and their applications to the degradation of pollutants in air and water under visible-light, J. Environ. Sci. (China) 26 (2014) 458–465, https://doi.org/10.1016/S1001-0742(13)60428-3. [17] J. Hou, C. Yang, Z. Wang, S. Jiao, H. Zhu, Bi2O3 quantum dots decorated anatase TiO2 nanocrystals with exposed {001} facets on graphene sheets for enhanced visible-light photocatalytic performance, Appl. Catal. B Environ. 129 (2013) 333–341, https://doi.org/10.1016/j.apcatb.2012.09.009. [18] Q. Peng, G. Peng, L. Wu, X. Wang, X. Yang, X. Li, Entrapment of Bi2O3 nanoparticles in TiO2 nanotubes for visible light-driven photocatalysis, Res. Chem. Intermed. 44 (2018) 6753–6763, https://doi.org/10.1007/s11164-018-3520-z. [19] N. Wei, H. Cui, C. Wang, G. Zhang, Q. Song, W. Sun, X. Song, M. Sun, J. Tian, Bi2O3 nanoparticles incorporated porous TiO2 films as an effective p-n junction with enhanced photocatalytic activity, J. Am. Ceram. Soc. 100 (2017) 1339–1349, https://doi.org/10.1111/jace.14667. [20] H.Y. Chung, W. Chen, X. Wen, J.N. Hart, H. Wu, Y. Lai, R. Amal, Y.H. Ng, Oxygendeficient bismuth tungstate and bismuth oxide composite photoanode with improved photostability, Sci. Bull. 63 (2018) 990–996, https://doi.org/10.1016/j. scib.2018.06.012. [21] Y. Huang, Y. Wei, J. Wang, D. Luo, L. Fan, J. Wu, Controllable fabrication of Bi2O3/ TiO2 heterojunction with excellent visible-light responsive photocatalytic performance, Appl. Surf. Sci. 423 (2017) 119–130, https://doi.org/10.1016/j. apsusc.2017.06.158. [22] C.Y. Su, L.C. Wang, W.S. Liu, C.C. Wang, T.P. Perng, Photocatalysis and hydrogen evolution of Al- and Zn-doped TiO2 nanotubes fabricated by atomic layer deposition, ACS Appl. Mater. Interfaces 10 (2018) 33287–33295, https://doi.org/ 10.1021/acsami.8b12299. [23] L. Xu, G. Fang, Y. Cao, A. Li, Interfacial catalysis in and initial reaction mechanism of Al2O3 films fabricated by atomic layer deposition using non-hydrolytic sol-gel chemistry, Phys. Chem. Chem. Phys. 18 (2016) 31223–31229, https://doi.org/ 10.1039/c6cp05842f. [24] J. Camacho-Bunquin, P. Aich, M. Ferrandon, A.“Bean” Getsoian, U. Das, F. Dogan, L.A. Curtiss, J.T. Miller, C.L. Marshall, A.S. Hock, P.C. Stair, Single-site zinc on silica catalysts for propylene hydrogenation and propane dehydrogenation: synthesis and reactivity evaluation using an integrated atomic layer depositioncatalysis instrument, J. Catal. 345 (2017) 170–182, https://doi.org/10.1016/j. jcat.2016.10.017. [25] Y.C. Liang, C.C. Wang, C.C. Kei, Y.C. Hsueh, W.H. Cho, T.P. Perng, Photocatalysis of Ag-loaded TiO2 nanotube arrays formed by atomic layer deposition, J. Phys. Chem. C 115 (2011) 9498–9502, https://doi.org/10.1021/jp202111p. [26] M. Karbalaei Akbari, Z. Hai, Z. Wei, J. Hu, S. Zhuiykov, Wafer-scale twodimensional Au-TiO2 bilayer films for photocatalytic degradation of Palmitic acid under UV and visible light illumination, Mater. Res. Bull. 95 (2017) 380–391, https://doi.org/10.1016/j.materresbull.2017.08.001. [27] H.S. Chen, P.H. Chen, S.H. Huang, T.P. Perng, Toward highly efficient photocatalysis: a flow-through Pt@TiO2@AAO membrane nanoreactor prepared by atomic layer deposition, Chem. Commun. 50 (2014) 4379–4382, https://doi. org/10.1039/c4cc01166j.

[28] S. You, Y. Hu, X. Liu, C. Wei, Synergetic removal of Pb (II) and dibutyl phthalate mixed pollutants on Bi2O3-TiO2 composite photocatalyst under visible light, Appl. Catal. B Environ. 232 (2018) 288–298, https://doi.org/10.1016/j. apcatb.2018.03.025. [29] M.N. G� omez-Cerezo, M.J. Mu~ noz-Batista, D. Tudela, M. Fern� andez-García, A. Kubacka, Composite Bi2O3-TiO2 catalysts for toluene photo-degradation: ultraviolet and visible light performances, Appl. Catal. B Environ. 156–157 (2014) 307–313, https://doi.org/10.1016/j.apcatb.2014.03.024. [30] L. Baker, A.S. Cavanagh, D. Seghete, S.M. George, A.J.M. MacKus, W.M.M. Kessels, Z.Y. Liu, F.T. Wagner, Nucleation and growth of Pt atomic layer deposition on Al2O3 substrates using (methylcyclopentadienyl)-trimethyl platinum and O2 plasma, J. Appl. Phys. 109 (2011), https://doi.org/10.1063/1.3555091. [31] J. Shi, Z. Li, A. Kvit, S. Krylyuk, A.V. Davydov, X. Wang, Electron microscopy observation of TiO2 nanocrystal evolution in high-temperature atomic layer deposition, Nano Lett. (2013), https://doi.org/10.1021/nl403566u. [32] F. Grillo, H. Van Bui, J.A. Moulijn, M.T. Kreutzer, Understanding and controlling the aggregative growth of platinum nanoparticles in atomic layer deposition: an avenue to size selection, J. Phys. Chem. Lett. (2017), https://doi.org/10.1021/acs. jpclett.6b02978. [33] S.B. Simonsen, I. Chorkendorff, S. Dahl, M. Skoglundh, Direct observations of oxygen-induced platinum nanoparticle ripening studied by in situ TEM, J. Am. Chem. Soc. (2010) 7968–7975. [34] J.W.M. Frenken, Scanning tunnelling microscopy study of the growth of small palladium particles on TiO (110), Surf. Sci. 457 (2000) 295–310. [35] T.J. Woehl, C. Park, J.E. Evans, I. Arslan, W.D. Ristenpart, N.D. Browning, Direct observation of aggregative nanoparticle growth: kinetic, Nano Lett. (2014), https://doi.org/10.1021/nl4043328. [36] L. Jin, Y. Li, Z. Hu, J. Chu, L. Jin, Y. Li, Z. Hu, J. Chu, Full three-dimensional morphology evolution of amorphous thin films for atomic layer deposition Full three-dimensional morphology evolution of amorphous thin films for atomic layer deposition, AIP Adv. (2018), 045304, https://doi.org/10.1063/1.5025008. [37] Y. Liu, F. Xin, F. Wang, S. Luo, X. Yin, Synthesis, characterization, and activities of visible light-driven Bi2O3-TiO2 composite photocatalysts, J. Alloy. Comp. 498 (2010) 179–184, https://doi.org/10.1016/j.jallcom.2010.03.151. [38] R. Malik, V.K. Tomer, N. Joshi, T. Dankwort, L. Lin, L. Kienle, Au-TiO2 -loaded cubic g-C3N4 nanohybrids for photocatalytic and volatile organic amine sensing applications, ACS Appl. Mater. Interfaces 10 (2018) 34087–34097, https://doi. org/10.1021/acsami.8b08091. [39] Z. Guan, H. Wang, X. Wang, J. Hu, R. Du, Fabrication of heterostructured β-Bi2O3TiO2 nanotube array composite film for photoelectrochemical cathodic protection applications, Corros. Sci. 136 (2018) 60–69, https://doi.org/10.1016/j. corsci.2018.02.048. [40] S. Schmidt, E.T. Kubaski, D.P. Volanti, T. Sequinel, V.D.N. Bezzon, S. M. Tebcherani, J.A. Varela, Effect of pressure-assisted heat treatment on photoluminescence emission of α-Bi2O3 needles, Inorg. Chem. (2015), https://doi. org/10.1021/acs.inorgchem.5b01237. [41] L. Kumari, J.H. Lin, Y.R. Ma, One-dimensional Bi2O3 nanohooks: synthesis, characterization and optical properties, J. Phys. Condens. Matter 19 (2007), https://doi.org/10.1088/0953-8984/19/40/406204. [42] S. Sood, S.K. Mehta, A.S.K. Sinha, S.K. Kansal, Bi2O3/TiO2 heterostructures: synthesis, characterization and their application in solar light mediated photocatalyzed degradation of an antibiotic, ofloxacin, Chem. Eng. J. 290 (2016) 45–52, https://doi.org/10.1016/j.cej.2016.01.017. [43] J. Resasco, H. Zhang, N. Kornienko, N. Becknell, H. Lee, J. Guo, A.L. Briseno, P. Yang, TiO2/BiVO4 nanowire heterostructure photoanodes based on type II band alignment, ACS Cent. Sci. 2 (2016) 80–88, https://doi.org/10.1021/ acscentsci.5b00402. [44] H. Zou, M. Song, F. Yi, L. Bian, P. Liu, S. Zhang, Simulated-sunlight-activated photocatalysis of Methyl Orange using carbon and lanthanum co-doped Bi2O3-TiO2 composite, J. Alloy. Comp. 680 (2016) 54–59, https://doi.org/10.1016/j.jallcom. 2016.04.094. [45] W. Zhao, X. Wang, H. Sang, K. Wang, Synthesis of Bi-doped TiO2 nanotubes and enhanced photocatalytic activity for hydrogen evolution from glycerol solution, Chin. J. Chem. 31 (2013) 415–420, https://doi.org/10.1002/cjoc.201201151.

7