Does noble metal modification improve the photocatalytic activity of BiOCl?

Progress in Natural Science: Materials International 2013;23(3):286–293 Chinese Materials Research Society

Progress in Natural Science: Materials International www.elsevier.com/locate/pnsmi www.sciencedirect.com

ORIGINAL RESEARCH

Does noble metal modification improve the photocatalytic activity of BiOCl? Liang Konga, Zheng Jianga,b,n, Henry H.-C. Laia, Tiancun Xiaoa, Peter P. Edwardsa,nn a

Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, UK Environment and Sustainability Institute, University of Exeter, Cornwall Campus, Penryn, Cornwall TR10 9EZ, UK

b

Received 8 October 2012; accepted 21 February 2013 Available online 31 May 2013

KEYWORDS Photocatalysis; BiOCl; Noble metal; Rhodamine B; Plasmonic effect

Abstract Noble metal-surface-deposited BiOCl photocatalysts were prepared through photo-deposition and used for photodecomposition of Rhodamine B (RhB). The received materials were characterised using X-ray photoemission spectroscopy (XPS), UV–vis diffuse reflectance spectroscopy (UV–vis DRS), and X-ray diffraction (XRD) to understand the influence of surface deposited noble metals. The results showed that the noble metal species on the surface of BiOCl are in metallic state, which also brought about enhanced light absorption in broad UV–vis region due to plasmonic effects induced by the surfacedeposited noble metal species. All the samples showed good activity in photodecomposition of RhB under UV-light irradiation, but only Ag/BiOCl was more active than bulk BiOCl. The mechanism of the different reactivity of these noble-metal modified BiOCl was tentatively proposed based on the band structure and the interactions between noble metals and the BiOCl. & 2013 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.

1.

Introduction

n

Corresponding author at: University of Exeter, Environment and Sustainability Institute, Cornwall Campus, Penryn, Cornwall TR10 9EZ, UK. nn Corresponding author at: University of Oxford, Inorganic Chemistry Laboratory, OX1 3QR, UK. E-mail addresses: [email protected] (Z. Jiang), [email protected] (P.P. Edwards). Peer review under responsibility of Chinese Materials Research Society.

The increasing applications of solar energy have been stimulating the increasing demand of semiconductor photocatalysts, which play vital roles in solar cells, artificial photosynthesis (photocatalytic water splitting, CO2 photo-reduction) and environmental cleanup [1–3]. Besides the extensively investigated TiO2 photocatalyst, great efforts have been made to explore more efficient semiconductor photocatalysts as alternatives to TiO2 [4]. As a typical p-type main group V–VI–VII ternary semiconductor, BiOCl is a stable but more active photocatalyst than TiO2 due to the unique optical, electrical and catalytic properties of the

1002-0071 & 2013 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.pnsc.2013.05.002

Does noble metal modification improve the photocatalytic activity of BiOCl? layered structure of BiOCl [4]. However, BiOCl is still facing the same challenges encountered by most photocatalysts, such as limited light absorption, and great energy loss associated with the fast recombination of photo-generated charge carriers (electron– hole) [5]. A variety of strategies, such as formation of heterojunctions [6], impurity doping [7], and surface metallisation [8], have been employed to improve the light absorption and reduce the recombination of charge carriers. Semiconductor heterojunctions require specific band positions of the dual semiconductors and impurity doping highly depends on the doping level; in contrast, surface metallisation takes great advantage of enhanced trapping ability of photo-generated electrons because of the high conductivity and additional plasmonic effect of the surface noble metals [9]. So far, Ag/TiO2 [10] and Ag/AgCl [11] have been proved as efficient photocatalysts under visible-light irradiation, which has arisen from the fact that surface noble-metal nano-particles (i.e., Au and Ag) can strongly absorb visible-light due to their localised surface plasmonic resonance (SPR) [12,13]. To date, research regarding semiconductor surface metallisation mainly concentrated on the n-type semiconductors (i.e., TiO2, AgX, etc.) rather than ptype semiconductor photocatalysts [14]. Though Ag/BiOBr and Pt/ BiOI showed enhanced photocatalytic performance in previous reports [15,16], the effects of various noble metals deposited on the photocatalyst of BiOCl have not been well explored. In this work, we deposited a small amount of various noble metals (Ag, Pt, Pd, and Rh) on BiOCl via a simple photodeposition method and found that those noble metals have different influences on the structure, property and photocatalytic performance of BiOCl. The interaction between noble metal and BiOCl has also been discussed to interpret the effects of surfacedeposited noble metal. 2. 2.1.

Experimental Preparation of the photocatalysts

All the reagents were of analytic grade and were used without further purification. The BiOCl employed was prepared as follows: stoichiometric amount of bismuth nitrate pentahydrate (Riedel-de Haën, 98.5%) was dissolved in 50 mL aqueous solution containing 5 mL acetic acid (HAc) with magnetic stirring. The acidic Bi (NO3)3
5H2O solution was vigorously stirred for 30 min prior to adding to 30 mL distilled water containing stoichiometric amounts of KCl (BDH AnalaR, 99.5%). White precipitates were immediately observed upon mixing the solutions. After stirring for another 30 min at room temperature, the suspension was aged for 3 h. The resulting precipitate was filtered, washed thoroughly with distilled water, and then dried at 65 1C overnight. 2.2.

Modification of BiOCl with noble metals

Metal modification of BiOCl samples was performed by photodeposition of Pt, Pd, Ag and Rh from the stocked solution using noble metal chloride (all purchased from Sigma-Aldrich, 98+%) as precursors. 1.2 g BiOCl sample was added into 100 mL distilled water with magnetic stirring and ultrasonication treatment for 20 min to ensure good dispersion of BiOCl particles. Stocked solutions of the noble metal chloride (corresponding to a 0.5 wt% metal loading) in 0.02 mol/L HCl were prepared and mixed with suspensions of BiOCl in distilled water (RhCl3 was dissolved in 0.02 mol/L NaOH solution, and then acetic acid was added to maintain the pH of the solution).

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20 mL of ethanol was added into the mixture as a sacrificial donor. Photo-deposition was performed by illuminating the suspensions for 2 h with an overhead 300 W Xenon lamp (PLS-SXE300, Beijing TrustTech). The product was filtered, washed with distilled water and ethanol, and dried at 80 1C overnight. 2.3.

Characterisation of the catalysts

The bulk and surface characteristics of all the samples were investigated by XRD, XPS and UV–vis DRS. Powder X-ray diffraction (PXRD) measurements were carried on a PANalytical X'Pert PRO diffractometer using Cu Kα1 (λ¼0.15418 nm) radiation under 45 kV, 40 mA with scanning in the range of 10–601 2-θ. Crystallite sizes of the samples were estimated from the corresponding X-ray diffraction peaks on (110) diffraction of BiOCl by using the Scherrer Equation: τ ¼ Kλ=βcos θ, where τ is the mean crystallite size of the ordered domains, K is the shape factor, which has a typical value of about 0.9, λ is the incident X-ray wavelength, β is the line width at half the maximum intensity (FWHM), and θ is the Bragg angle. Surface elemental analysis by X-ray photoelectron spectroscopy (XPS) was conducted on a VG ESCALAB2 XPS spectrometer with an Al Kα monochromatic source and a charge neutraliser. All the binding energies are referenced to C 1s peak at 284.5 eV. Core levels of Bi 4f, O 1s, Cl 2p, Ag 3d, Pd 3d, Pt 4f and Rh 3d are identified individually. Optical properties of the samples were studied by UV–vis spectroscopy. The diffuse reflectance spectra (DRS) were recorded on a Varian Cary 5000 UV–vis spectrometer with scan rate of 600 nm min−1. Both absorbance and diffuse reflectance spectra were recorded for all samples. The band gap was derived from the formula Eg ¼1239.8/λg, in which λg represents the absorption edge. The optical absorption near the band gap edge follows the equation αhv¼A(hv−Eg)n/2 which has been reported in the previous literature [17]. For a specific semiconductor, the square of absorption coefficient (n¼4) is linear with energy for direct optical transitions in the absorption edge region, whereas the square root of absorption coefficient (n¼1) is linear with energy for indirect transitions. 2.4.

Photocatalytic activity measurement

The photocatalytic activity of the samples was determined by degradation of Rhodamine B (RhB) in an aqueous solution under UV light irradiation with an overhead 300 W Xenon lamp (PLSSXE300, Beijing Trust Tech) as the light source. The intensity of the incident UVA light on the solution was detected to be 4.98 mW cm−2 by using a radiometer (Beijing). In each experiment, 0.1 g photocatalyst was dispersed into 100 mL RhB aqueous solution (20 mg/L) in a 500 mL beaker. Prior to irradiation, the suspension was magnetically stirred in dark for 60 min to establish the adsorption–desorption equilibrium between catalyst and RhB. During the photocatalytic reaction, approximately 3 mL suspension was collected at given time intervals and centrifuged (14,000 rpm, 3 min) to remove photocatalyst particles. The collected solution was tested by a Perkin-Elmer Lamda 750S UV– visible spectrophotometer. The characteristic adsorption peak of RhB at 553 nm is used to determine its degradation. 2.5.

Band structure estimation

The activity of a photocatalyst closely relates to its electron band structures in terms of the excitation, transportation and fate of the

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photo-generated charge carriers; especially, the conduction and valence band positions (CB and VB) determine the redox reactivity in photodecomposition of organic contaminants. The VB edge of a ground-state semiconductor can be expressed empirically via Mulliken electronegativity theory [18–21]: EVB ¼ χSemiconductor −E e þ 0:5Eg where EVB is the VB edge potential, χSemiconductor is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms as quoted from the Handbook of Chemistry and Physics, Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV), and Eg is the band gap energy of the semiconductor [22,23]. The Eg of BiOCl is about 3.31 eV. The estimated VB position of BiOCl in the system is located at 3.58 eV, and the corresponding CB position is 0.28 eV. However, once BiOCl is modified by noble metal, the band positions of the surface metallised BiOCl will be influenced by the work function of the metals, which will be discussed in the following mechanism section. 3. 3.1.

Results and discussion Crystallinity of noble metal/BiOCl

The XRD patterns (Fig. 1) of the noble metal/BiOCl samples are well consistent with those of the tetragonal BiOCl phase (space group P4/ nmm, JCPDS: 82-0485) without detectable change of the characteristic diffraction peaks, suggesting that the surface deposition of noble metals does not significantly affect the crystal structure of BiOCl [24]. There are no diffraction peaks of noble metals observed due to the low loading (0.5 wt%) and the high dispersion of noble metals on BiOCl. Although the lattice parameters of the BiOCl are not affected significantly, as shown in Table 1, the deposited noble metal species drastically decrease the crystallite size in the following order: BiOCl4Rh/BiOCl4Pt/BiOCl4Pd/BiOCl4Ag/BiOCl. Moreover, slight changes of relative peak intensities are observed from the XRD pattern, i.e. increase of ratio of (110)/(102), which could be caused by the change of preferred orientation during the process of ultrasonication treatment [25]. 3.2.

Optical absorption property of noble metal/BiOCl

UV–vis absorption spectra were recorded to detect the influence of metal species on BiOCl [26,27]. Fig. 2a shows the UV–vis diffuse

Intensity (a.u.)

Pt/BiOCl Pd/BiOCl Ag/BiOCl

10

20

30

2θ (°)

40

50

(212)

(202) (211) (104)

(200) (201) (113)

(112)

BiOCl

(103)

(110) (102) (111) (003)

(002)

(001)

(101)

Rh/BiOCl

60

Fig. 1 XRD patterns of as-synthesised BiOCl and samples with 0.5 wt% photo-deposited Pt, Pd, Ag and Rh.

reflectance spectra of BiOCl and noble metal/BiOCl samples in the range of 200–800 nm. Pure BiOCl can absorb UV light only with wavelength below 360 nm, while all the noble metal/BiOCl samples can absorb incident light across the whole UV–vis region even though such a small amount of noble metals can be supported on the unadulterated BiOCl. The enhanced absorption of noble metal/BiOCl samples in both UV and visible-light regions may be a result of the close field surface plasmonic effect [15]. There is no obvious change in the absorption edges of noble metal/BiOCl samples in comparison to the pure BiOCl (as shown in Fig. 2a), suggesting that the noble metals were not incorporated into the BiOCl matrix but deposited well on the surface of BiOCl. It is clearly observed that the Ag/BiOCl sample exhibits the strongest absorption ability and also the most prominent plasmonic effect (inset of Fig. 2a) among the noble metal/BiOCl samples. Generally, an intensified absorption peak located at around 500 nm could be observed and attributed to the surface plasmon excitation for the Ag particles [28], as demonstrated in Fig. 2a and its inset. It is well known that the surface plasmon band of a metal is affected by several factors, such as particle size, shape, reflective index, interaction between the metal particles and the support, etc. [29]. In general, the position of the surface plasmon band hardly depends on the particle size of the semiconductor support, in particular size greater than 20 nm [28]. In the present work, the crystallite size of BiOCl is around 36.1 nm in average; hence the size effect of BiOCl on the surface plasmon bands of noble metal particles is negligible. The indirect and direct band-gap energy of noble metal/BiOCl can thus be estimated from the data plot of (αhv)1/2 and (αhv)2 versus photo-energy (hv; Fig. 2b and c); to distinguish the transition character (direct or indirect) of the sharp absorption edge, the shapes of the lines are analysed. As shown in Fig. 2b, the optic transition plots of pure BiOCl display a linear trend for the (αhv)1/2 while the (αhv)2 versus photo-energy deviates from the straight line. This feature suggests that the absorption edge of BiOCl is caused by indirect transition of photogenerated charge carriers [4]. However, the addition of a noble metal may have changed the optic transition of BiOCl semiconductor: in the band gap region, the plots of (αhv)1/2 and (αhv)2 versus photo energy (Fig. 2c) are not in linear trends, suggesting that the noble metal/BiOCl samples are neither indirect nor direct semiconductors, and some interaction would occur between the noble metals and BiOCl.

3.3.

Surface elemental analysis

In order to further investigate the electronic interactions between noble metal particles and BiOCl support semiconductor, XPS studies were carried out. The characteristic binding energies (B.E.) of Bi, O, Cl and noble metal elements can be well identified from the XPS measurements. The overall spectra show the presence of Bi, O, Cl, noble metals and adventitious carbon in the noble metal/ BiOCl materials. The surface atomic ratios of noble metal, Cl, O and Bi elements of the samples calculated on the basis of XPS data are listed in Table 2. It can be seen from the atomic ratio that the composition for each sample is varying depending on the involved noble metals, i.e., the concentration of oxygen is relatively higher than those of the Bi and Cl species; theoretically the atomic ratio of O should be 33%. This is probably due to the surface adsorbed hydroxyl group (–OH) [30]. The noble metals in the study are all in metallic states without any signals associated with noble metal cations observed (Fig. 3a), suggesting that the photo-reduction is a

Does noble metal modification improve the photocatalytic activity of BiOCl?

289

Crystal parameters, photodegradation rate constant and band gap energy (Eg) of noble metal/BiOCl photocatalysts.

Sample

Crystallite size (Å)

BiOCl Pt/BiOCl Pd/BiOCl Rh/BiOCl Ag/BiOCl

Lattice parameters (Å)

451 355 325 384 292

3.892 3.889 3.887 3.893 3.888

7.376 7.361 7.365 7.383 7.387

500 nm

0.247 0.169 0.194 0.14 0.253

3.322 3.277 3.189 3.281 3.256

6

1.4

1.0

5

1.2

4

1.0

3 2

0.8

0.5 0.0 200

Eg (eV)

1.6

(αhν)1/2

Absorbance (a.u.)

c

Ag/BiOCl Rh/BiOCl Pd/BiOCl Pt/BiOCl BiOCl

2.0 1.5

a

kRhB (−1 min)

(αhν)2

Table 1

1 0.6 300

400

500

600

700

800

3.3

3.4

3.5

3.6

hν (eV)

Wavelength (nm)

Fig. 2 (a) UV–vis diffuse reflectance spectra of noble metal/BiOCl photocatalysts, (b) the plots of (αhv)1/2 and (αhv)2 versus photo energy of pure BiOCl and (c) Ag/BiOCl.

good strategy in deposition of atomic noble metals on the BiOCl to form new photocatalysts. Fig. 3b shows the Bi 4f5/2 and Bi 4f7/2 XPS core lines with B.E. locating around 164.3 eV and 159.0 eV, which are assigned to Bi3+ in BiOCl [31]. The Bi 4f core lines shifted towards lower energies for the noble metal/BiOCl samples except for Ag/BiOCl whose Bi 4f core lines blue-shifted by 0.2 eV. The most significant red-shift of Bi 4f B.E. is observed on the Pd/BiOCl sample. The detailed B.E. changes may be found in Table 2. There is no obvious change observed from Cl 2p1/2 (199.4 eV) and Cl 2p3/2 (197.8 eV) peaks of Cl− in both BiOCl and noble metal/BiOCl samples (as shown in Fig. 3d), suggesting that the supported noble metal did not affect the inter-layered Cl− [32,33]. There are two different oxygen species presented on the surface of the series samples (Fig. 3c). The O 1s B. E. around 532.0 eV is due to the Bi–O bond in BiOCl, and the O 1s peak at 529.9 eV due to the surface hydroxyl groups on the sample [34–36]. Interestingly, the O 1s of Bi–O shifts towards higher energy,

against the trend of Bi 4f on the noble metal/BiOCl samples except for that of Ag/BiOCl, which further confirms that surface metallisation induces strong interaction of the composite. The valence band maximum (VBM) XPS spectra of the samples are compared in Fig. 3f. The position of VBM with respect to the Fermi level was determined by the intersection of linear fits to the leading edge of the valence band photoemission and the background [37]. It can be clearly observed that the shifts of VB edge towards lower energy on the noble metal/BiOCl than that of BiOCl and the shifts highly depend on the noble metals, suggesting that the interaction between noble metal and BiOCl is varying as well.

3.4.

Photocatalytic activity

Photocatalytic decomposition of RhB, a chemically stable organic dye with a characteristic absorption peak at 554 nm, was chosen as

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Table 2

XPS peak position and element concentration (atomic %) in noble metal/BiOCl samples.

Sample

BiOCl Rh/BiOCl Pd/BiOCl Pt/BiOCl Pt/BiOClafter Ag/BiOCl

Bi 4f (eV)

O 1s (eV)

Cl 2p (eV)

4f5/2

4f7/2

at%

1s

at%

2p1/2

2p3/2

at%

Position

at%

164.4 164.3 164.2 164.2 164.3 164.4

159 159 158.9 158.9 159 159.1

39.2 36.3 37.4 38.3 33.4 36.8

530.1 529.8 529.7 529.8 529.9 530.1

38.2 52.4 36.1 34.2 43.8 41.9

199.5 199.3 199.3 199.3 199.4 199.6

197.8 197.7 197.7 197.7 197.8 197.9

22.6 10.9 25.9 27.3 22.7 20.9

– 308.72 340.8 74.23 74.17 374.0

– 0.49 0.56 0.18 0.19 0.46

a probe reaction to clarify the effects of deposited noble metals on the performance of BiOCl. The RhB photodegradation on each noble metal/BiOCl sample follows the pseudo-first-order kinetics model, ln(C/C0) ¼ −kt, where C0 and C are the initial concentration of the dye solution and the concentration at time t, respectively, and k is the pseudo-first-order rate constant [8]. The rate constant, k, of RhB photodegradation was derived from the ln(C/C0)–t plots (Fig. 4b), and is listed in Table 2. As shown in Fig. 4a, under UV light irradiation under the same reaction conditions, all the samples can completely decompose RhB within 30 min. The photocatalytic activity sequence is in the order Ag/BiOCl4pure BiOCl4Pd/BiOCl4Pt/BiOCl4Rh/ BiOCl. The results reveal that the surface metallisation by noble metals did not significantly enhance the photocatalytic activity of the noble metal/BiOCl samples despite that enhanced light absorption in broad UV–vis region, and in some cases, the surface deposition even deteriorated the performance of BiOCl. On one hand, this phenomenon may be attributed to the coverage of noble metal to the surface reactive sites of BiOCl under UV light; on the other hand, the results also suggest that the plasmonic effect can have positive effect on lots of visible-light-responsive photocatalysts but is less effective for the UV-responsive BiOCl. The possible reason for such unusual phenomena will be detailed in the next section (Working mechanism). Although most of the noble metal/BiOCl samples are less active than pure BiOCl except for Ag/BiOCl, the noble metal/BiOCl samples showed high stability with UV-light irradiation. For example, the fresh and used 0.5 wt% Pt/BiOCl samples show very similar XRD patterns (Fig. 4c) except for the stronger Bragg diffraction intensity for the sample after photocatalysis. The enhanced intensity may be attributed to the growth of the crystallite during the photocatalytic reaction. The XPS spectra of fresh and used Pt/BiOCl samples are also compared in Table 2, where no obvious difference in composition is identified between the fresh and used samples. Both the XRD pattern and XPS spectra of the samples after reaction revealed that the metal-modified samples are considerably stable under UV light irradiation.

3.5.

Noble metal (eV)

Working mechanism

It is essential to understand the mechanism of the unusual photocatalytic activity of the noble metal/BiOCl photocatalysts. In principle, the Schottky barriers, formed on the contact interface between the metal and semiconductor, would equalise the Fermi edges of the metal and the BiOCl. Correspondingly, conduction band (CB) or valence band (VB) of the p-type semiconductor bends upwards (or downwards) to the metal with respect to their relative work functions (schematically shown in Fig. 5b) [14];

335.5 70.86 70.96 368.0

such band bending results from the interface electrons transferring between the semiconductor and the supported metal [20,39]. In the present research, the activity of the noble metal/BiOCl photocatalysts with similar loading amount either decreases or increases compared to BiOCl, suggesting that the activity difference arises from the band bending due to charge transfer on the interface. Essentially, the potentials of the metallic noble metals with respect to the standard hydrogen electrode (EM) can be calculated as in the following: EM ¼φM−Ee, where φM is the working function with respect to vacuum level and Ee is the energy of free electrons relative to the hydrogen scale (ca. 4.5 eV) [40]. The flat-band potential of BiOCl can be calculated from the Mulliken electronegativity theory: EVB ¼ χBiOCl−Ee+0.5Eg, where EVB is the potential of VB top, χBiOCl is the electronegativity of BiOCl (6.43 eV) and Eg the band gap energy of BiOCl. As can be seen from Fig. 5a, with the potential scale relative to the standard hydrogen electrode (NHE), we may compared work functions of the noble metals and the band structure of pure BiOCl as follows: EAg (Ef ¼ 0.24 eV)oECB(BiOCl) (0.28 eV)oERh (0.48 eV)oEPd (1.1 eV)oEPt (1.43 eV)oEVB(BiOCl) (3.58 eV). Because BiOCl is an intrinsic p-type semiconductor (Eg ¼3.31 eV), its Fermi level should be close to valence band edge (3.58 eV) and thus all the noble metals possess lower work function (position more negatively) than that of BiOCl [22]. The higher work function of BiOCl will result in transfer of photo-generated holes (positively charged) to the noble metals at their interfaces; however, such transfers will not significantly affect the band structure of the metals because they have enormous number of electrons, but influence the interface state of the VB and CB of the p-type BiOCl. As a result, the CB and VB of BiOCl would bend positively at the noble metal/ BiOCl interface (Fig. 5b) due to the holes transferring to the noble metals. Accordingly, the interface bending depleted some positive charges at the VB of BiOCl (less photo-generated holes) and thus deteriorated the oxidative ability of the VB of the BiOCl. Such interpretation may partially explain why the noble metal/BiOCl photocatalysts, except for Ag/BiOCl, showed worse activity in photodegradation reaction but cannot depict the slightly enhanced activity of Ag/BiOCl. It is also of great significance to understand the fate of the photo-generated electrons during the photocatalytic reactions. The previous research has proven that the photo-generated electrons from BiOCl will attack soluble oxygen to give rise to active − oxygen radical (O− 2 d) [41]. The O2 d is a very active oxidising agent which can break down organic molecules. Among the noble metals, only Ag has a more negative work function than the CB of BiOCl, suggesting that Ag can offer electrons more easily than other noble metals to BiOCl through their interface. In contrast, the other noble metals would accept electrons from CB of BiOCl, which would reduce the possibility of transferring electrons on CB

Does noble metal modification improve the photocatalytic activity of BiOCl?

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Fig. 3 XPS spectra of (a) Pt 4f (before and after photocatalytic reaction), Ag 3d, Pd 3d and Rh 3d; (b) Bi 4f; (c) O 1s; and (d) Cl 2p. (e) Full scan spectrum of Pt/BiOCl sample and (f) VBM of all the noble metal/BiOCl samples.

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1.0

0 Pt/BiOCl Rh/BiOCl Pd/hBiOCl BiOCl Ag/BiOCl blank

C/C0

0.6

-2

ln(C/C0)

0.8

-1

0.4

-3 -4 Pt/BiOCl Rh/BiOCl Pd/BiOCl BiOCl Ag/BiOCl blank

-5 0.2 -6 0.0

-7 0

10

20

30

40

0

5

10

Time (min)

15

20

25

30

Intensity (a.u.)

Time (min)

after irradiation

before irradiation

10

20

30

40

50

60

2θ (°)

Fig. 4 (a) Temporal course of the degradation of RhB in the presence of different noble metal/BiOCl; (b) kinetic linear plots of RhB photo-degradation on noble metal/BiOCl and (c) XRD patterns of 0.5 wt% Pt/BiOCl sample before and after photoreaction.

Fig. 5 Diagrams of (a) the working functions [38] at a pH of 0 and (b) illustrative band bending at the interface between noble metals and BiOCl, where EF and FS are the Fermi energy of the metal and semiconductor, respectively, and φM and φS are the work function of metal and semiconductor.

to O− 2 d, i.e., reduced oxidising ability. Therefore, the overall effects of the interface Schottky barriers of noble metal/BiOCl highly depend on the work functions of the noble metals and thus lead to different activities of the noble metal/BiOCl photocatalysts.

4.

Conclusions

Noble metal-modified BiOCl photocatalysts were synthesised via the photo-deposition method. The metal surface modification of

BiOCl did not change the crystallite structure of the BiOCl but enhanced the light absorbance in the visible-light region. Most of the noble metal modified BiOCl did not show improved photocatalytic activity, except for Ag/BiOCl, in comparison to the unadulterated BiOCl in photo-decomposition of RhB under UVlight irradiation; even somehow decreased photocatalytic activities were observed on certain noble metal species. The unusual photocatalytic behaviour of the series of noble metal/BiOCl photocatalysts can be mainly attributed to the photo-generated charge transfer at the interfaces of the noble metal/BiOCl

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