Structural and optical characteristics of α-Bi2O3/ Bi2O(3-x):Ho3+ thin films deposited by pulsed laser deposition for improved green and near-infrared emissions and photocatalytic activity

Pulsed laser deposited films on glass substrate deposited at different substrate temperatures (Ts) and partial pressures of oxygen, Ho3+-doped Bi2O3 films were produced. The degradation capability of the Rhodamine B dye using the Bi2O3:Ho3+ films was explored. The impact of the Bi2O3:Ho3+ content on the dye degradation performance was analyzed. The X-ray powder diffraction patterns showed that the films deposited at 400 °C had an α-Bi2O3 phase. The impacts of various Ts and O2 partial pressures were correlated with the surface morphology and the thickness of the films using results of field emission scanning electron microscope. The thin films deposited at a low O2 partial pressure of 5–20 mTorr at Ts = 400 °C exhibited nano-needles with an average size of 80 nm and a length of ∼750 nm. The estimated band gap of the prepared films was found to vary between 2.6 and 3.0 eV. The photoluminescence (PL) of the Bi2O3:Ho3+ thin films excited at 450 nm showed an intense green band emission observed at 548 nm, and the feeble emissions at 654 and 753 nm were ascribed to the transitions of Ho3+. The nano-needle particles of the α-Bi2O3:Ho3+ exhibited a maximum PL intensity for the 20 mTorr O2 partial pressure thin film. The films prepared in vacuum and with an O2 partial pressure of 5 mTorr exhibited a 41 % dye degradation efficiency during a duration of 270 min of the photocatalysis experiment.


Introduction
In recent years, bismuth oxide (Bi 2 O 3 ) thin films have gained wide interest owing to their good physical properties, which consist of a broad optical bandgap that varies from 2.0 to 3.9 eV, efficient photocatalytic activity, oxide ion conductivity, good photoluminescence (PL), high refractive index and dielectric permittivity [1][2][3][4].It has mainly four crystal polymorphisms such as α-, β-, γand δ-Bi 2 O 3 .Among all forms α-Bi 2 O 3 is stable at room temperature (RT) and δ-Bi 2 O 3 is stable at higher temperatures, and the remaining forms are metastable [5,6].Bi 2 O 3 thin films are widely studied and are involved in a multidirectional application, such as solar cells, gas, and humidity sensors, optical coatings, components in electronic circuitry, dye-sensitized photovoltaic cells (DSSCs) [7] and for heterogeneous photocatalysis [8,9].Bi 2 O 3 thin films were successfully grown by various methods namely spin-coating [9], radio-frequency magnetron sputtering [10], combination of a sol-gel process and electrospinning methods [7], thermal-and plasma-enhanced atomic layer deposition [11], photo-chemical solution deposition [12] and pulsed laser deposition (PLD) [6].Paulo H.E. Falsetti et al. studied on the degradation of Rhodamine B (RhB) using β-Bi 2 O 3 thin films under UV radiation and their results suggested that the thin films had good catalytic activity for the removal of RhB from water [9].Juan C. Medina et al. showed the photocatalysis of δ-Bi 2 O 3 thin films for the degradation of methyl orange (MO) under illumination of sunlight, white light, and UV radiation.The percentage of the photocatalytic degradation was considerably higher for UV radiation compared to sunlight and white light [13].There are only limited reports on the photoluminescence of trivalent rare earth (RE 3+ ) ion doped Bi 2 O 3 thin films.Housei Akazawa, studied the PL properties of Er 3+ -doped Bi 2 O 3 prepared by a sputtering technique on a SiO 2 substrate.And his results showed very strong emission peaks at 1530 and 1560 nm, at a higher Er content (> 4 at%) [14].He also investigated the band gap and PL of the α-Bi 2 O 3 film under different deposition temperature and O 2 flow rate [15].The Er 3+ -doped Bi 2 O 3 showed various crystal polymorphs, namely α, β, γ, δ-Bi 2 O 3 .However, the α-Bi 2 O 3 :Er 3+ showed stronger PL intensities compared to the other polymorphs [16].
The study of the PL properties revealed that the RE-doped Bi 2 O 3 displayed significantly higher luminescence intensity compared to the non-doped Bi 2 O 3 [15].Also, we expect that RE 3+ sites can be easily substituted at Bi 3+ sites because of similar ionic radii [1,3].The ionic radii of Ho 3+ ions (0.90 Å) are similar to the ionic radii of Bi 3+ (1.03 Å) for coordination number six.And Ho 3+ have higher luminescence from the visible to infrared regions, which belongs to their 4f-4f transitions [3].The holmium element was rarely reported as a catalyst.There are a few reports on Ho 3+ as a catalyst.Further, Ho 3+ doping led to distortion in the host lattice, which reduces the electron hole recombination rate and generation of highly reactive hydroxyl radical, which helps to improve the photocatalysis process [3,17].Therefore, in this study the structural, surface morphology, surface chemical, optical properties, and photocatalytic activity of Ho 3+ activated Bi 2 O 3 films deposited by PLD on glass substrate at different deposited substrate temperatures (T s ) and oxygen partial pressures (PO 2 ) were investigated.This study is unique in the sense that several surface characterization techniques in combination with optical measurements were used to explain the photocatalyst behaviour of Ho 3+ activated Bi 2 O 3 films for degradation of RhB under UV-vis light.

Materials and methods
Bi 2 O 3 :Ho 3+ (1 mol%) powder was synthesized by the co-precipitation technique [1].The cleaned microscope glass substrates were positioned in the PLD chamber.The substrate temperature was maintained at RT, 200, and 400 • C during the deposition of the films in a vacuum (2.6 × 10 − 5 Torr).The films grown using the Nd:YAG laser (266 nm) and the target to substrate distance was fixed to 5 cm for all films.The time of deposition, laser energy and area of ablation were maintained at 15 min, 50 mJ and 1.5 mm 2 , respectively.The films were deposited with PO 2 that varied between 5 and 200 mTorr, while the optimum substrate temperature of 400 • C was kept constant.
An Advance diffractometer (Bruker AXS D8) equipped with the CuK α source were performed to obtain the crystal structure information of the Bi 2 O 3 :Ho 3+ PLD films.The surface morphologies, film thickness and elemental maps of the films were examined using a JEOL model JSM-7800 F field emission scanning electron microscopy (FESEM) (Tokyo, Japan) connected with an energy dispersive X-ray spectrometer (EDS).A Shimadzu atomic force microscopy (AFM) SPM-9600 model, a PHI 5400 XPS spectrometer and a Per-kinElmer Lambda 950 UV-vis-NIR spectrophotometer were utilized for characterization.Complete information of the characterization methods can be obtained in Refs.[1,3].The XPS spectra were fitted utilizing the XPS PEAK 4.1 programme.A FLS980 Edinburgh Instruments (Livingston, UK) was used to obtain PL spectra of the deposited films equipped with a 450 W Xe lamp.
The photocatalytic activity of the 1 mol% Ho 3+ doped Bi 2 O 3 films was evaluated for the RhB degradation in the UV-vis light at RT.The detail setup of the photocatalytic reactor was given in previous work [18].The photocatalysis was carried out using a 100 mL of RhB solution with a 10 ppm dye concentration and a catalyst deposited film with a 2 × 2 cm 2 size.The RhB solution was stirred for 60 min in the dark to attain an equilibrium.The suspensions were then transferred to a photoreactor, and the films were placed in the dye solution about 3 mm from the surface of the dye solution with the help of a stainless-steel wire.And irradiated with UV-vis light (350-800 nm) with constant stirring at a speed of 450 rpm for up to 270 min.During the irradiation, 3 mL samples were collected at 30 min intervals.The absorbance spectra of the dye were then collected with an UV-vis spectrophotometer.

X-ray powder diffraction
As described in Fig. 1(a), the X-ray powder diffraction (XRPD) patterns of the Bi 2 O 3 :Ho 3+ film grown at substrate temperatures ranging from RT-400 • C in vacuum (2.6 × 10 5 Torr).The film deposited in vacuum at a substrate temperature of 400 • C displayed the Bragg reflections corresponding to the (102), (002), (1 11), (120), (012), (1 21), ( 031) and (304) planes of monoclinic Bi 2 O 3 (COD #1010004) with a space group P2 1 /C as observed in the range of 15-65 • .No other Bragg reflections, which could indicate the presence of any other crystallographic orientations of Bi 2 O 3 , Ho, or impurities phases, were detected.It was noticed that all the films have polycrystalline natures, and this indicates that single phase monoclinic Bi 2 O 3 film were grown successfully on the glass, suggesting that Ho with 1 mol% doping has not altered the crystal structure of the Bi 2 O 3 and will evenly replace the sites of the Bi 3+ in the Bi 2 O 3 matrix.However, considering the ionic radii for the 6-coordinated Bi 3+ (1.03 Å) and the 6-coordinated Ho 3+ (0.90 Å) [19] ions, there is a possibility that the Bi 3+ ions be replaced by the Ho 3+ ions during the doping process due to the same oxidation state and small ionic radii difference between the Bi 3+ and Ho 3+ ions.No secondary Ho phase was detected in the XRPD pattern due to low concentration of the Ho ions (1 mol%).In a previous report [3] on crystal structure of Bi 2 O 3 :Ho 3+ powders there were also no secondary phases of Ho detected in the XRPD patterns aside from a small change in the unit cell volume of the crystal even dopant concentration of 5 mol% due to a slight variance in the ionic radii between Bi 3+ and Ho 3+ (6-coordinates).Additionally, the presence of Ho 3+ in the Bi 2 O 3 was confirmed by UV-Vis diffuse reflectance and PL results [3].Further the confirmation of substitution of Bi 3+ by Sm 3+ (0.96 Å) ions in the Bi 2 O 3 powders were confirmed by XPS results [20].Similar XRPD patterns were recorded for the Bi 2 O 3 :Ho 3+ film grown in the various PO 2 between 5 and 200 mTorr (Fig. 1(b)).Due to an increase in PO 2 the film with a preferential orientation was found to alter from the α-Bi 2 O 3 (120) direction to non-stoichiometric Bi 2 O 2 .3 (JCPDS Card 76-2477) of plane (107) in the PO 2 range 5-200 mTorr.
When the PO 2 increases the intensity of the reflection corresponding to the (107) plane was increased.At low PO 2 , the difference in the orientation could be described by the reduction of the deposition rate due to rearrangement of atoms to obtain (107) reflection is low at a high deposition rate.The high O 2 partial pressure (200 mTorr) induced a decreasing of the deposition rate due to collisions among the ejected species and the O 2 molecules.This is due to the mean free path of oxygen molecules at high pressure is shorter than the target-substrate distance.Similarly growth rate dependence on PO 2 has been noticed with other oxide materials [21,22].The films deposited in the lower PO 2 (5-20 mTorr) showed dominant α-Bi 2 O 3 with small non-stoichiometric Bi 2 O 2 .3 reflections.The films deposited at higher PO 2 (100-200 mTorr) showed both α-Bi 2 O 3 and non-stoichiometric Bi 2 O 2 .3 phases and the non-stoichiometric Bi 2 O 2 .3 phase was dominated.It indicated that the α-Bi 2 O 3 phase was obtained at lower PO 2 and the non-stoichiometric Bi 2 O 2 .3 phase formed at higher PO 2 .The formation of the non-stoichiometric Bi 2 O 2 .3 phase was since, a light O ablated atom was scattered with background oxygen molecules more strongly than the heavy Bi and Ho ablated atoms in the high O 2 pressure.A relatively large number of oxygen therefore fails to arrive on the substrate surface, and this reductions of the O/Bi ratio of the film from its stoichiometric Bi 2 O 3 [22,23].
The crystallite size (D) was determined from the XRPD data using the Scherrer formula.The reflection (120) of α-Bi 2 O 3 was considered for the determination of the crystallite size and decrease with an increasing O 2 partial pressure and its value varied in the range of 55-49 nm.While the D of the reflection (107) of the Bi 2 O 2 .3 has increased between 22 and 44 nm.This can be ascribed to an increase mobility of the adatoms and coalescence of small crystallites to form bigger crystallites.From the XRPD analysis, the nonstoichiometric Bi 2 O 2 .3 phase formation was influenced by the O 2 partial pressures.It clearly indicates that the high PO 2 induced the non-stoichiometric Bi 2 O 2 .3 phase as more dominate than the α-Bi 2 O 3 .

Surface morphology and elemental analysis
In Fig. 2(a and b) the top and cross-sectional views of the PLD deposited films at a substrate temperature of 400 • C in vacuum (2.6 × 10 − 5 Torr) showed a good solid crystalline film with some features on the surface.The good crystallinity of the film is ascribed to the heat emerging during coalescence process at high temperatures.The higher substrate temperatures resulted in a higher mobility of the particles and hence resulting in the larger sized particles.This result good agreement with the XRPD patterns [24,25].Fig. 2(c-j) also show SEM images and cross-sectional views of the PLD films surfaces deposited on the microscope glass at the selected PO 2 between 5 and 200 mTorr at a Ts of 400 • C. At the lower PO 2 (5 and 20 mTorr) the films showed a less dense needle like morphology.Specially, the 5 mTorr sample contained particles that have a lot of exposed surface area throughout the thin film.At the lower oxygen pressures (≤ 20 mTorr), the ablated species with a high kinetic energy avoid condensation because of the small possibility of these species collision with the O 2 partial pressure gas species and thus reach to the substrate surface more easily to form the distinct grains [26,27].The 20 mTorr thin film, however, is denser than the 5 mTorr film.This may lead to a higher photon absorption probability for the 20 mTorr film, but the 5 mTorr film may act as a better catalyst due to higher surface area available to react.The thin films deposited at low PO 2 of 5-20 mTorr exhibited nano-needles with an average size of ~80 nm and length of ~750 nm.As the O 2 partial pressure was increased the surface morphology changed with an increase in the particles size.Which led to the decrease of the kinetic energy, and the mean free path of the ejected species from the target owing to collisions between the ablated atoms and O 2 at higher partial pressures during the film deposition.The ablated species with a lower kinetic energy that appeared at the substrate led to the reduction in surface diffusion.This condition favoured the grain growth and led to more dense grains with larger grain size due to agglomeration [26,27].Please note: In contrast to the lower O 2 partial pressure films, which only have a very thin solid component and instead consist of a less dense needle-like structure, the higher O 2 partial pressure films all featured a clearly defined solid section with predicted features on top of the films.The thickness of the solid films varied from 100 to 850 nm and the thickness of the parts that are rougher compared to the solid films (surface roughness) approximated 205-1460 nm for the PLD deposited films, Fig. 2 pressure (200 mTorr).The reduction of the solid thickness of the film at the high O 2 partial pressure (200 mTorr) is ascribed to the decrease in the mean-free path of the vaporized species.A similar result was reported on ZnO:Eu films deposited by PLD [21].The film deposited in vacuum exhibited high solid film thickness compared to the films deposited at O 2 partial pressures, which might be due to a relatively large number of ablated atoms therefore more easily reach the substrate surface.The EDS spectra of Bi 2 O 3 :Ho 3+ (1 mol%) films are depicted in Fig. 3 (a,b) and the apparent peaks in the EDS spectra indicate the presence of Bi, O, and Ho.Moreover, elemental mapping confirms the uniform distribution of the elements on the surface, as well as the homogeneous doping of Ho (Fig. 4(a-l)).

Atomic force microscopy analysis
Fig. 5 (a− j) displays the two-dimensional (2D) and the three-dimensional (3D) AFM images of the Ho 3+ -doped Bi 2 O 3 films deposited by the PLD method in vacuum and selected PO 2 .The AFM micrographs of the deposited films show nonuniform distribution of grains.The AFM micrographs additionally show that the particles were greatly unified with each other throughout the surface of the films.The film deposited in vacuum with substrate temperature of 400 • C revealed the particles were agglomerated.Less particles agglomeration of the deposited films was obtained at the low PO 2 (5 and 20 mTorr) with the same substrate temperature.Increasing the PO 2 from 100 to 200 mTorr led to more particle agglomeration, moreover the film deposited at the 100 mTorr O 2 partial pressure revealed well defined spherical shape particles distributed at the film surface.The film surface roughness was analyzed by the arithmetic average roughness (R a ), the root mean square (rms) roughness (R q ) and ten-point average roughness (R z. ) The R a , rms and R z roughness increased at 5 mTorr O 2 partial pressure and decreased at 20 mTorr.Again, it further gradually increased with increasing O 2 partial pressure in the range from 20 to 200 mTorr, which indicates the modified surface topography of the films.The surface topography can be tuned by increasing the O 2 partial pressure during the film grown, which plays a vital role in the PL and photocatalytic activities.O 2 partial pressure plays a very substantial role in the increase of the roughness of the film surfaces, which enhances the surface area and therefore increases the dye molecules adsorption, which in turn enhances the photocatalysis efficiency of the film.Fig. 6 confirmed that the film deposited at the 20 mTorr O 2 partial pressure exhibited a smoother surface, while films deposited at high PO 2 displayed a higher roughness due to the reflecting nucleation, coalescence and unceasing film growth processes.These results are comparable with those of Raoufi et al. [28,29].It is clear that the AFM results cannot be used stand alone but must be used in conjuction with other techniques such as SEM in this case to obtain the full picture.

X-ray photoelectron spectroscopy study
The deposited films were characterized via the Bi 4f and O 1s core levels as shown in Fig. 7. XPS fittings revealed a component at 156.6 eV, which clearly designated the presence of Bi in the metallic state (see Fig. 7   films [18,30].A third component at higher BE (158.8/159eV) has been assigned to Bi 2 O 3 -containing Bi 3+ species [3,31].The Bi 4f XPS region (Fig. 7 (a,b)) shows the doublet splitting, with the lower BE doublet assigned to Bi 4f 5/2 and the higher BE, Bi 4f 3/2 which is further confirmed by the spin-orbit coupling values of 5.3 eV for Bi 4f 5/2 and Bi 4f 3/2 [28].The peaks area's ratio of the Bi 2+ to Bi 3+ of the film deposited at 200 mTorr O 2 partial pressure is higher than the film grown in the vacuum in consistence with the XRPD patterns.The oxygen vacancies (V o ) in the film can be confirmed using the O 1s core level XPS spectra [32].The O 1s region was fitted with four components (Fig. 7(c and d)), the first component at low BE (529.1 eV) was assigned to the lattice oxygen (O L ) in the Bi 2 O 3 .The second high BE component at 529.9 eV is ascribed to the O L in the Bi 2 O 2.3 (Bi 2+ ).The third and fourth higher BE's components were assigned to V o and surface impurities of hydroxide and O-C --O [1] respectively.The surface impurities occurred due to the exposure of the films to the air atmosphere prior of transferring them into the XPS vacuum chamber.The relative oxygen peak intensity of the oxygen in the Bi 2 O 2.3 (Bi 2+ ) increased at 200 mTorr.The film deposited under vacuum showed better stoichiometry (Bi 2 O 3 ) than the film deposited under different O 2 partial pressures.

UV-vis-DRS study
The diffuse reflectance spectra (DRS) were measured for the Bi 2 O 3 :Ho 3+ (1 mol%) films deposited by PLD are shown in Fig. 8(a).The spectrum displays an absorption edge at around 415 nm corresponding to the bandgap of the films.All the films begin to absorb radiation in the UV to near visible spectral range between 300 and 430 nm and were reflective in the visible spectral region, resulted in a light-yellow colour for the Bi 2 O 3 :Ho 3+ films.An increase in the O 2 partial pressure in the PLD chamber resulted in the reduction of the reflective intensity as displayed in Fig. 8(a).In addition, the film deposited in vacuum condition showed a low reflective intensity.It is clearly shown that a low O 2 partial pressure amount was helpful to improve the reflectivity of the films.The DRS of the films deposited in vacuum and PO 2 of 100 mTorr, 200 mTorr contained an absorb band centered at approximately 498 nm and can be ascribed to the structural defects.A similar result was observed in pervious results of Bi 2 O 3 :Sm 3+ , and Bi 2 O 3 :Ho 3+ powder.The UV-vis diffuse reflectance (DR) spectrum of the Bi 2 O 3 :Ho 3+ film mainly originated from direct transitions.Please note that the absorption of the 20 mTorr film is higher than that of the 5 mTorr film as speculated from the SEM images.
From the DR spectrum, the bandgap calculations were done by Kubelka-Munk (K-M) function.The reflectance spectrum of the film was converted to the K-M function [18].The bandgap (E g ) of the deposited films were determined from the plot of [F (R ∞ ) hν] 2 versus the photon energy (in eV) is extrapolated up to [F (R ∞ ) hν] 2 = 0 as presented in Fig. 8(b-f).The estimated value of E g is 3.01 eV for the 5 mTorr O 2 partial pressure.The optical bandgap decreased with the rise in O 2 partial pressure; when the pressure was 20, 100 and 200 mTorr, the E g was 2.90, 2.85, and 2.82 eV, respectively.Whereas the film deposited in vacuum show a value of E g = 2.60 eV.The obtained optical bandgap are well consistent with our previous results [3,18] as well as others reports [11,15].

Photoluminescence
Fig. 9(a and b) shows the PLE and PL spectra of the PLD films.Fig. 9(a) present the PLE spectra of the films recorded at an emission of 548 nm, and all the excited bands could be attributed to the intra-4f 10 transitions of the Ho 3+ [3,33].All the excited bands derived from the ground 5 I 8 level to the excited state of the Ho 3+ .The strong intense excitation bands centered at 450 and 462 nm originated from the 5 I 8 → 5 G 6 , 5 F 1 and 5 I 8 → 3 K 8 , transitions, respectively.Similarly, relatively feeble excitation bands centered at 360 nm ( 5 I 8 → 3 H 5 , 3 H 6 ), 425 nm ( 5 I 8 → 5 G 5 ), 473 nm ( 5 I 8 → 5 F 2 ) and 485 nm ( 5 I 8 → 5 F 3 ) [3,32].From Fig. 9(a), the prominent intense PLE band (450 nm) has enhanced up to 20 mTorr of O 2 partial pressure and then its intensity reduced.
For an excitation at 450 nm, the emission of the Bi 2 O 3 :Ho 3+ film consisted of a prominent green emission from the 5 F 4 / 5 S 2 → 5 I 8 transition at 548 nm and a near infrared (NIR) emission from 5 F 2 → 5 I 8 at 753 nm, and an additional feeble blue emission from 5 F 3 → 5 I 8 at 490 nm and a red emission from the 5 F 5 → 5 I 8 transition of the Ho 3+ centered at 655 nm (see Fig. 9(b)).To determine the optimal deposition conditions of the film, an increase of the O 2 partial pressure the intensities of the green and NIR bands became stronger as a function of O 2 partial pressure and reached a maximum at 20 mTorr.Which was probably due to the less dense needle like structures of the PLD films at the lower PO 2 .Smooth films may experience lower emission due to the internal reflections produced by smoother surfaces of the films [34] in the case of the solid more dense films at the higher O 2 pressures.The luminescence quenching effect appeared with the O 2 partial pressure over 20 mTorr, resulting in a decrease in the green and NIR emissions.Similar, results were observed in Sr 2 SiO 4 :Eu 3+ thin film grown by PLD with different PO 2 and reported by Jeong et al. [34].The rms roughness and PL intensity of Sr 2 SiO 4 :Eu 3+ films increased evenly with an increasing O 2 pressure up to 150 mTorr and decreased evenly at higher O 2 pressures 150-200 mTorr [34].Furthermore, the Bi 2 O 2.3 phase was stronger at higher deposited PO 2 (> 20 mTorr).As a result, the Ho 3+ ions occupied the distorted sites of the Bi 3+ in the matrix, thus the probability of a non-radiative transition is stronger.The luminescence obtained from the film deposited in an O 2 partial pressure condition is 10 times higher than the film deposited in vacuum due to aggregated particles in the vacuum.Indicating, that the films deposited in moderate PO 2 are favourable for enhancing the luminescence of the Bi 2 O 3 :Ho 3+ films [3,33].
Fig. 10 (a) shows the PLE spectra of the Bi 2 O 3 :Ho 3+ films obtained at an emission wavelength of 1202 nm.Several excitation bands were observed in the excitation spectrum with maxima at 448, 462, 473, 485, 536, and 644 nm, and these bands were assigned to the corresponding electronic transitions as 5 I 8 → 5 G 6 , 5 F 1 , 5 I 8 → 3 K 8 , 5 I 8 → 5 F 2 , 5 I 8 → 5 F 3 , 5 I 8 → 5 F 4 , 5 S 2 and 5 I 8 → 5 F 5 of Ho 3+ ions, respectively.In addition, a broad band centered at 408 nm was observed, and this band was assigned to the Bi 2+ transition as 2 P 1/2 → 2 P 3/2 (2) from monitoring the NIR emission wavelength at 1202 nm, [35].Liu et al. [31] found that Bi-doped BaBPO 5 polycrystalline displayed broad excitation bands centered at 478 and 710 nm from monitoring the NIR emission wavelength at 1164 nm and these bands were assigned to the Bi 2+ [32].Other reports showed that the Bi-doped SrAl 4 O 7 phosphor displayed four excitation bands centered at ~277, 325, 416, and 590 nm if monitored at 710 nm.Three of them, positioned at 277, 416, and 590 nm were correspond to the transitions from ground 2 P 1/2 level to the 2 S 1/2 , 2 P 3/2 (2) and 2 P 3/2 (1) of the Bi 2+ ions, respectively [36].Furthermore, an excitation peak was observed at 601 nm for all films due to the second harmonic oscillations.
The integrated PLE intensity of the prominent Ho 3+ : 5 I 8 → 5 G 6 , 5 F 1 transition has increased up to the 100 mTorr O 2 partial pressure and thereafter decreased (Fig. 10 (b)).Moreover, the integrated PLE intensity of Bi 2+ : 2 P 1/2 → 2 P 3/2 (2) has exponentially grown with O 2 partial pressure and is shown in Fig. 10 (b).The PL intensity of the Ho 3+ doped Bi 2 O 3 films has been monitored on excitation with 450 nm in the 700-1500 nm region, and the spectra thus obtained are shown in Fig. 10 (c).The emission spectra contain several emission peaks centered at 753, 1023, 1202, and 1385 nm and these peaks were assigned to arise due to 5 S 2 + 5 F 4 → 5 I 7 , 5 S 2 + 5 F 4 → 5 I 6 , 5 I 6 → 5 I 8 and 5 S 2 + 5 F 4 → 5 I 5 , transitions, respectively [37,38].The variation in PL intensities of all NIR emissions with PO 2 reflected the same as the PLE spectra.The decreased PL intensities after 100 mTorr were due to changes in the local symmetry around Ho 3+ [35,37,38].

Photoluminescence lifetimes
It is important to know the recombination processes of charge carriers in trap defect states.The quality of deposited Bi 2 O 3 :Ho 3+ films and their role in photocatalytic performance have been analyzed by estimating the activator and defect lifetimes using PL lifetime spectroscopy.PL lifetime decay curves were measured for the most intense transition of the green emission from the 5 F 4 / 5 S 2 → 5 I 8 level of Ho 3+ with an excitation of 450 nm.The PL lifetime decay for the emission at 548 nm, with an excitation of 450 nm in the doped Bi 2 O 3 films deposited in vacuum and selected PO 2 (5-200 mTorr), at the optimized substrate temperature (400 • C) is shown in Fig. 11 (a), respectively.The lifetime decay in all deposited films was fitted with a three-exponential function.The summary of the PL lifetime data of Ho 3+ doped Bi 2 O 3 films at the 450 nm excitation wavelength is displayed in Fig. 11(b-f).
The tri-exponential fits that yielded the different decay times are summarized in Table 1.For the Bi 2 O 3 :Ho 3+ vacuum films, the decay is based on three decay times: 7 ± 1 μs fast decay, 57 ± 2 μs a modest decay and 115 ± 20 μs a slow decay [38].The fast decay time (τ 1 ), for the Bi 2 O 3 :Ho 3+ films is ascribed to Ho 3+ ions close to the surface [39] and its decay time has not altered a lot in all the deposited films.The modest decay time (τ 2 ) is attributed to Ho 3+ ions in the bulk.The major contributing was from the slow decaying component (τ 3 ) that might arise from defects.Both τ 2 and τ 3 have decreased in the films deposited in the PO 2 compared with the vacuum deposited film, which was probably due to a combination effect of the Ho 3+ ions occupying the stoichiometric Bi 2 O 3 and non-stoichiometric Bi 2 O 2.3 , V o , and surface morphologies.As seen from Fig. 12, the average lifetime was shorter in the vacuum as compared to those of the different O 2 partial pressures.An optimal oxygen vacancy formation in the Bi 2 O 3 host might facilitate the energy transfer and relaxation processes better.

Photocatalysis
To know the relationship between the photocatalytic property of the PLD films deposited at 400 • C (vacuum: 2.6 × 10 − 5 Torr) and the different PO 2 5, 20, 100 and 200 mTorr (T s = 400 • C) the photocatalytic activity of the Bi 2 O 3 :Ho 3+ (1 mol%) films has been examined by selecting the degradation of RhB.Earlier to the UV-vis light irradiation adsorption-desorption measurement was done in the dark for 60 min and the photolysis of RhB (without catalyst) and with catalysis in the presence of a microscope glass was done and the results are tabulated in Table 2. Fig. 13 Shows the photocatalytic absorbance spectra of the film deposited at a T s of 400 • C in vacuum (2.6 × 10 − 5 Torr) (size 2 ×2 cm) from 0 to 270 min (with time intervals of 30 min), under the UV-vis light irradiation.These results display the maximum intensity of the RhB absorption spectra at 552 nm which decreased with an increase in the UV-vis light irradiation time.It represents the destruction of the RhB and the formation of some intermediates [1].
Fig. 14 presents the changes in the RhB concentration due to the UV-vis light irradiation in the presence of the different thin film catalysts.Where C t and C o , in Fig. 14 are the reaction concentration of the RhB at different times and the initial concentration of the RhB after the equilibrium adsorption.All the Bi 2 O 3 :Ho 3+ (1 mol%) films showed improved photocatalytic activity in comparison with the photolysis of RhB, without a catalyst.Which indicated that the Bi 2 O 3 :Ho 3+ (1 mol%) films acted as an efficient catalyst for the degradation of the RhB.And the order of degradation efficiency of the RhB for the different samples could be as follows: 5 mTorr (41 %), vacuum (41 %), 200 mTorr (40 %), 100 mTorr (35 %) and 20 mTorr (33 %).The degradation efficiency % θ deg was determined by using the formula below (1): Where, C deg = C t /C o , Co is the concentration of the RhB at t = 0 after the equilibrium adsorption and C t the reaction concentration of the RhB after different time intervals (t).A clear relationship between the surface morphology and the photocatalytic activity was observed for the PLD Bi 2 O 3 :Ho 3+ (1 mol %) films.The best photocatalytic activity and adsorption for the RhB was obtained for the film deposited at 5 mTorr (41 %), and was attributed to its surface morphology (see FESEM image and cross section of the 5 mTorr film as shown in Fig. 2).The film contained needle shape particles all-over the film surface, and supported a higher photocatalytic activity as compared to the other Bi 2 O 3 :Ho 3+ (1 mol%) films [9].AFM study helped to show that there is a relation between the photodegrading process and the surface roughness, the films deposited at 5 mTorr and 200 mTorr have higher surface roughness that led to high surface area and hence high adsorption and photocatalytic activity.The photocatalytic activity increased with an increase in surface roughness [40].Fig. 15 clearly shows that the rate constants (k) for the Bi 2 O 3 :Ho 3+ (1 mol%) films under UV-vis light followed a pseudo-first-order reaction and the obtained rate constant values of the various Bi 2 O 3 :Ho 3+ (1 mol%) films are tabulated in Table 3 [1].

The proposed mechanism for the photocatalytic activity of RhB
When a photon with energy of hν (UV-vis irradiation) is incident on the film surface, the photons with their energy equal to or bigger than the band gap of the film is absorbed by the Bi 2 O 3 :Ho 3+ (1 mol%).This leads to the generation of electrons (e − CB ) and holes (h + VB ) at the conduction band (CB) and the valence band (VB) (Eq.( 2)), respectively.These photon-generated h + VB act as oxidation sites and contribute to the generation of hydroxyl radicals (OH • ) (Eqs.(3) and ( 4)).The e − CB acts as reduction sites, which react with the surface oxygen (O 2 ) that leads to the generation of superoxide anion radicles (O 2

••
) help to increase the surface OH groups.The V o •• might absorb more O 2 reacting with the captured electrons (Eqs.( 6) and ( 7)).Or V o •• may become traps for the ejected electrons (Eqs.( 8) and ( 9)), which can control the photo-induced electrons and holes recombination that leads to the better catalytic activity of the RhB and the schematic representation is given in Fig. 16.Both OH • and O 2 •serve as an oxidant to degrade the organic pollutant [10] (Eq.( 10)).The reactions are given below [1,9,10 or or where, oxygen vacancies are represented by (V o

Conclusions
α-Bi 2 O 3 :Ho 3+ films were deposited on microscope glass at a temperature of 400 • C using the PLD technique.The Bi 2 O 2.3 phase stronger at the higher O 2 partial pressures, which indicated that oxygen vacancies occurred in the films.The surface morphologies of the 5 and 20 mTorr PO 2 showed needle shape particles.The film deposited at an O 2 partial pressure of 20 mTorr exhibited smoother surfaces, while the films deposited at lower and higher than the 20 mTorr PO 2 displayed higher roughness values.The green and NIR emissions obtained from the film deposited at an O 2 partial pressure of 20 mTorr showed an intensity that was more than 10 times higher than the film deposited in a vacuum.The best photocatalytic activity of the RhB degradation was attained for the films deposited at PO 2 of 5 and 200 mTorr, owing to the modified surface morphologies and due to the high surface roughness.The PL and photocatalytic activities depend strongly on the surface morphology of the Bi 2 O 3 :Ho 3+ films.

Fig. 2 .
Fig. 2. Top and cross-sectional views (FESEM) of Bi 2 O 3 :Ho 3+ PLD films deposited at a substrate temperature of 400 • C (vacuum: 2.6 × 10 − 5 Torr) (a,b) and the selected PO 2 (T S = 400 • C) (c-j) (The measured solid film thickness were marked in the red arrow), and (k) the thickness of the solid part and uneven (rough) part of the thin films with different PO 2 .

Fig. 10 .
Fig. 10.(a) The PLE spectra, (b) PLE intensity as a function of PO 2 and (c) NIR-PL spectra of the Bi 2 O 3 :Ho 3+ (1 mol%) PLD films grown in vacuum and different PO 2 and films excited at 450 nm @ RT.

Fig. 11 .
Fig. 11.PL lifetime decay curves (a) and (b-f) fitted decay curves for the PLD films deposited in vacuum and different PO 2 and films excited at 450 nm and measured at 548 nm @ RT.

Fig. 14 .
Fig. 14.The variation of the RhB degradation in the presence of the Bi 2 O 3 :Ho 3+ (1 mol%) PLD films at different UV-vis light exposure.

Fig. 15 .
Fig. 15.Plots of ln (C 0 /C t ) vs irradiation time for the degradation of RhB in the presence of Bi 2 O 3 :Ho 3+ (1 mol%) films under UV-vis light exposure.
J.Divya et al.

Table 2
The concentrations and fractions of the RhB adsorbed in dark of the Bi 2 O 3 :Ho 3+ (1 mol%) films.

Table 3
Parameters of photocatalytic activity in degradation of the RhB by Bi 2 O 3 :Ho 3+ (1 mol%) films under UV-visible irradiation.