1 Introduction

Nowadays, the demand for research drives the significant focus on materials with unique properties for wide applications. Therefore, the prime aim of the research fraternity is to address these issues effectively. In such a scenario, zinc oxide (ZnO) can be foreseen as an efficient electronic, optical and energy material with negligible environmental contamination [1]. It is well-known that ZnO is a direct bandgap (3.4 eV) semiconductor, with a large exciton binding energy of 60 meV and excellent thermal/mechanical stability. It has grabbed the wide attention of the scientific community due to its versatile electronic/optical properties towards the various applications. Particularly, optical studies on ZnO thin film are essential for the utilization in optoelectronic devices. In general, optical properties of ZnO can be tuned by varying microstructure, thickness, bandgap energy, optically active defects and growth/deposition conditions [2]. So far ZnO has been reported as one of the efficient materials for photocatalysis and photovoltaic because of its tuneable properties with low cost and non-toxicity [3, 4].

Tremendous research work has been focussing on tuning the properties of ZnO thin films; thereby preparation methods are playing a crucial role to tune the properties of the material to attain the utilization of ZnO thin film in the desired applications. Saravanan et al. compared the photocatalytic activity of nanosized ZnO prepared by different methods such as thermal decomposition, precipitation and sol–gel process [5]. They have observed that the ZnO prepared via the sol–gel process exhibited a higher efficiency of 99% in 120 min of UV illumination for the degradation of methylene blue (MB) dye in comparison with the ZnO synthesized through other methods. Recently in 2021, Uribe-Lopez et al.  compared the photocatalytic activity of ZnO nanoparticles prepared via precipitation and sol–gel method [6]. ZnO prepared via precipitation method exhibited an excellent efficiency of 100% in 120 min of UV illumination for phenol degradation when compared with that of ZnO prepared via sol–gel technique (80%). Kaneva et al. compared the photocatalytic activity of ZnO thin film fabricated via spray pyrolysis and sol–gel technique [7]. The ZnO film fabricated via spray pyrolysis at 350 °C exhibited an improved efficiency of 68.8% in 180 min of UV irradiation than that for the sol–gel prepared ZnO film (59.8%) for the degradation of malachite green (MG) dye. Furthermore, Klauson et al. fabricated ZnO nanostructured layers with various morphologies via the spray pyrolysis technique and the needle-like ZnO structure was observed to be more efficient for photocatalytic degradation of humic acid (HA) [8]. Nowadays, new methods/technology involves in developing cheaper and flexible devices. Indeed, thin film technology is playing a prime role to obtain the enhanced properties with defect-free thin films suitable to the electronic and optoelectronic devices. The existing ZnO thin films have been fabricated by various deposition techniques such as spin coating, dip coating, spray pyrolysis, ultrasonic spray pyrolysis, sputtering, pulsed laser deposition, hydrothermal method, electrochemical deposition, etc. [9,10,11,12,13]. Among these, the ultrasonic spray pyrolysis technique can be seen as a low-cost as well as an effective technique for the fabrication of uniform and large-area film coatings with minimal chemical wastage [11, 14].

It is known that film thickness critically affects the crystal structure as well as the morphology of the photocatalysts. Recently, Bakhtiarnia and co-workers studied the effect of the film thickness of sputtered BiVO4 films on their structural, morphological, optical and photocatalytic properties and found that the efficiency was optimum at a thickness of 715 nm [15]. The film thickness can play a vital role in the generation and transport of charge carriers, which in turn affect the photovoltaic and photocatalytic performances. Pianaud et al. studied that the film thickness of Ta3N5 films has a crucial impact on the charge generation and transport mechanism [16]. The photoactivity and incident photon-to-current efficiency (IPCE) is found to be higher for thicker films than for thinner ones. Various works have studied the photocatalytic and photocurrent properties of ZnO separately, but only a few works have reported the correlation between the two. Even though various works have reported excellent efficiency of ~ 100% for ZnO, it is essential to decrease the photocatalytic reaction time (illumination time) in order to improve its photocatalytic activity for commercial employability [5, 6]. Hence, in this work we are trying to enhance the photocatalytic activity of ZnO film further. We have reported on ZnO thin films as a function of their thickness (tZnO) to explore the photocatalytic and photocurrent properties in a vivid manner. Thus, this work highlights the multidimensional employability of low-cost ZnO thin films fabricated as a function of their thickness in various optoelectronic device applications.

2 Experimental

2.1 Preparation of ZnO thin films

The precursor solution was prepared as per the stoichiometric ratio as follows. 2.1950 g of zinc acetate dehydrate (ZnCH3COO2)0.2H2O (MERCK) and 4 ml of acetic acid glacial (CH3COOH) (MERCK) were added with 100 ml of deionized water. The mixture was stirred for one hour on a magnetic stirrer at 65 °C at 500 rpm. Meanwhile, the ultrasonic spray deposition nebulizer (OMRON NE-U780) was carefully cleaned and the prepared sol was transferred to the cup of the nebulizer after one hour of stirring. The nebulization rate and airflow volume were set at 3 ml/min and 17 L/min. The average aerosol output rate was 0.14 ml/min. Pre-cleaned glass plates were used as the substrate and were placed on the hot plate at a temperature of 400 °C. ZnO films were deposited on the glass substrates with different spray timings and were annealed at 400 °C for 2 h. The thickness of ZnO films (tZnO) is varied from 80 to 1200 nm.

2.2 Characterization of ZnO thin films

The microstructure of ZnO films was studied using an X-ray diffractometer (Malvern Panalytical system) in the range of 25°–60° with a step size of 0.008° using the Cu Kα radiation with a wavelength of 0.154 nm. The surface morphological studies were done using by SEM instrument (Carl ZEISS). The photoluminescence spectra of the films were recorded using the Perkin Elmer L55 fluorescence spectrometer at an excitation wavelength of 340 nm. To study the current–voltage characteristics (I-V), the ZnO thin films were deposited over conductive ITO substrates in parallel plate electrode configuration with Ag as the top electrode and ITO as the bottom electrode. The Ag electrodes were deposited via DC sputtering technique.The photocurrent of the ZnO thin films was measured using a white light LED source with wavelength range of 400–800 nm with a measured power density of 15 W/m2. Similarly, the incident photon-to-electron conversion efficiency (IPCE) factor was studied using a monochromatic red-light source with a measured power density of 20 W/m2. The photocatalytic activity of the ZnO thin films was studied using 0.5 mM Rhodamine B (RhB) aqueous dye solution as a template under halogen lamp illumination (wavelength range: 350–1000 nm) of 15 min. Furthermore, the time dependence, as well as the reusability of the ZnO thin film photocatalysts, were also determined.

3 Results and discussions

3.1 Structural studies

The X-ray diffraction patterns of ZnO thin films deposited at different ZnO thicknesses (tZnO) are shown in Fig. 1. The diffraction peaks (100), (002), (101) and (110) indicated the formation of polycrystalline wurtzite phase of ZnO (ICSD. 01-078-4603) without any secondary or impurity phase [29]. It is observed that the intensity of (100), (002) and (101) diffraction peaks increase with the increase of tZnO indicating the continuous growth of ZnO layers. The growth rate along the (002) orientation is observed to be lower than that of the (100) and (101) orientations because of its higher surface energy [17, 18].

Fig. 1
figure 1

XRD pattern of ZnO thin films deposited at different tZnO

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Usually, the exposed (002) plane has a polar surface and thus induces dipole nature with the positive Zn-terminated (002) facets and negative O-terminated (00-2) facets. The Zn+2 ionic sites on the (002) facets have a great affinity towards oxygen adsorption and results in the capture of electrons near the surface and hence forms a thin layer of O2− on the ZnO surface. This surface space charge region can act as a hole trapping center and thus enhance the charge carrier separation [18, 19].The surface polarity of ZnO films can be estimated by taking the intensity ratio (I002/I100) and is tabulated in Table 1. But, in this work, it is observed that this ratio is decreased as tZnO increased from 240 to 1200 nm and it can be attributed to the lower growth rate along (002) orientation [18, 20]. Furthermore, the higher amount of unsaturated Zn2+ along (100) facet improves the adsorption of oxygen-containing species and favours the formation of reactive radicals which aids in the photocatalytic mechanism [21].The crystal lattice parameters and crystallite size of the ZnO films were also estimated and are tabulated in Table 1. The lattice parameters were estimated by using the standard formula for the hexagonal crystal lattice given as;

$${\text{d}}_{{({\text{hkl}})}} = \frac{1}{{\sqrt {\left( {\frac{{\frac{4}{2}\left( {h^{2} + hk + k^{2} } \right)}}{{a^{2} }} + \frac{{l^{2} }}{{c^{2} }}} \right)} }}$$
(1)
Table 1 Lattice parameters and crystallite size of ZnO thin films at different tZnO
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where ‘a’, ‘b’ and ‘c’ are the lattice parameters. The crystallite size was estimated using the Debye–Scherrer equation [18],

$$D = \frac{0.9\lambda }{{\beta \cos \theta }}$$
(2)

where ‘λ’ is the wavelength of the X-ray (= 0.154 nm) and ‘β’ is the full width at half maximum in radians. It is found that the crystallite size increases with the increase of tZnO which exhibits the growth of ZnO nanostructures.

3.2 Morphological studies

The SEM images of the ZnO thin films with different tZnO are shown in Fig. 2a–e. SEM images showed the uniform morphology with nanorod structure. It is also seen that the grains are uniformly distributed over the entire film surface. The density of grains is found to be increasing with the increase of tZnO. It is in good agreement with the fact that growth rate (grains) increases as increase of film thickness of ZnO thin films [22, 23]. The increase in the particle size and density of ZnO grains with the increase of tZnO can improve the photogeneration and separation of charge carriers [24]. Moreover, the aspect ratio (length to width ratio) of the ZnO nanostructures is found to be increased with the increase of tZnO as tabulated in Table 2. A higher aspect ratio favours photocatalytic activity and found to be higher at tZnO = 1200 nm [21].

Fig. 2
figure 2

SEM images of ZnO thin films at a tZnO = 80 nm b tZnO = 240 nm c tZnO = 400 nm d tZnO = 800 nm e tZnO = 1200 nm

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Table 2 Dimensions and aspect ratio of the ZnO nanostructures
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3.3 Optical studies

The transmittance spectra of the ZnO films deposited at different tZnO is depicted in Fig. 3a. The red shift observed in the fundamental absorption edge illustrates the decrease in bandgap energy with tZnO. The decrease in bandgap energy with the increase in particle size confirmed the quantum size confinement effect [5]. The optical bandgap energy is estimated using the well-known Tauc plot as shown in Fig. 3b and is tabulated in Table 3. It is found to be minimum at tZnO = 1200 nm. A lower bandgap enables a higher absorption of light and thus can improve the photogeneration of charge carriers [15].

Fig. 3
figure 3

a Transmittance spectra of ZnO thin films deposited at different tZnO, b Tauc plot for the ZnO film with tZnO = 1200 nm

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Table 3 Optical bandgap energy of ZnO thin films deposited at different tZnO
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The photoluminescence spectra of the ZnO films deposited at different tZnO at an excitation wavelength of 340 nm are shown in Fig. 4a.The peak positions in PL spectrum were determined for all films by using the multi-peak Gaussian fit in origin software and the typical one is shown in Fig. 4b. All the films exhibited one emission peak in the UV region and four other emission peaks in the visible region. The observed UV emissions are due to band-to-band transitions and visible emissions are due to the shallow defects and/or deep level defects [25]. Wang et al. reported that the non-radiative and radiative transition probabilities on ZnO which is attributed to crystal imperfection (point defects, grain boundaries and dislocations) for nonradiative transition whereas near band edge excitonic/deep level emission for radiative transition [26]. The observed emissions in the visible regions are due to the suppression of nonradiative defects such as point defects, grain boundaries and dislocations in the ZnO thin film. It also supports the defect-free ZnO film growth observed from SEM images.

Fig. 4
figure 4

a Photoluminescence spectra of ZnO thin films deposited at different tZnO, b Multi-peak Gaussian fitting for the PL spectra of ZnO film with tZnO = 240 nm, and c Ratio of oxygen vacancies to band-to-band transition as a function of tZnO

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The observed peak 1 at around 390 nm (UV emission) in the region can be attributed to the band-to-band transition [27].The observed peak 2 at around 420 nm (violet emission) is assigned to the transitions of electrons from zinc interstitials (Zni) to the valence band (VB). The observed peak 3 at around 460 nm (blue emission) is due to the transfer of an electron from conduction band (CB) to oxygen interstitials (Oi) while the observed peak 4 at around 480 nm (blue emission) arises due to the transition of electrons from Zni to zinc vacancies (VZn).The existence of green emission peak 5 at around 515 nm is due to the transition of electrons from the singly ionized oxygen vacancies (V0*) to VB. Oxygen vacancies play a predominant role than other crystal defects in the photocatalytic process [18]. The ratio of oxygen vacancies to band-to-band transition (I5/I1) is plotted as a function of tZnO and observed to be maximum at tZnO = 1200 nm as shown in Fig. 4c.

3.4 Current–voltage (I-V) characteristics

The current–voltage (I-V) characteristics of the ZnO films deposited at different tZnO from −1 to + 1 V under dark and light illumination conditions are plotted in Fig. 5a–d. The photocurrent values are increasing with the increase of bias voltage ranges observed for both dark and light illumination conditions. The observed increase in current after illumination is due to the increase in the number of mobile charge carriers and consequently the high electrical conductivity [28]. The observed increase in photocurrent value for higher thickness films is due to the increase of grain size and aspect ratio as evident from SEM images [21, 24, 29]. Photocurrent (Iph) is defined as the difference of current under dark and light conditions at a specific voltage, i.e., Iph = Ilight−Idark. The photosensitivity or photoresponse factor (S) as shown in Eq. (3), which is defined as the ratio of photocurrent (Iph) to dark current (Idark), is considered to be a crucial factor in determining the performance of photodetectors [30].

$${\text{Photosensitivity }}\left( {\text{S}} \right) \, = \frac{{{\text{I}}_{{{\text{ph}}}} }}{{{\text{I}}_{{{\text{dark}}}} }}$$
(3)
Fig. 5
figure 5

I-V characteristics of ZnO thin films deposited at different tZnO under dark and light conditions

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The photosensitivity at a voltage of 1 V estimated using Eq. 3 as a function of tZnO is as shown in Fig. 6.It is observed that the ratio of photosensitivity is increasing with an increase in tZnO and found to be maximum at tZnO = 1200 nm. The obtained results are good in accordance with the reported work on thickness-dependent ZnO-based semiconductor materials by various methods [28, 31,32,33,34]. This indicates the enhanced photogeneration of charge carriers at higher tZnO. Even though oxygen vacancies play a role in photocurrent generation, in this work, the thickness effect seems to have a dominant role over oxygen vacancies.The increase in the photogeneration of charge carriers concerning the increase in tZnO is evident from Fig. 6.

Fig. 6
figure 6

Photosensitivity of ZnO thin films as a function of tZnO

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Since the ZnO film with tZnO = 1200 nm exhibited maximum photosensitivity, the incident photon-to-electron conversion efficiency (IPCE) factor of the film was also evaluated for photovoltaic device applications. IPCE is the ratio of the number of photogenerated charge carriers under short-circuit conditions to the number of incident photons at a particular wavelength and is a crucial parameter in the fabrication of photovoltaic devices. The IPCE efficiency is calculated using the following relation [35, 36].

$${\text{IPCE = }}\frac{{{\text{j}}_{{{\text{ph}}}} \left( {\frac{{{\text{mA}}}}{{{\text{cm}}^{2} }}} \right) \times 1239.8\left( {{\text{V}} \times {\text{nm}}} \right)}}{{{\text{P}}_{{{\text{mono}}}} \left( {\frac{{{\text{mW}}}}{{{\text{cm}}^{2} }}} \right) \times \lambda \left( {{\text{nm}}} \right)}} \times 100$$
(4)

where ‘jph’ is the average steady-state photocurrent density, ‘Pmono’ and ‘λ’ is the optical power and wavelength of the incident monochromatic light source respectively. The IPCE for the ZnO film at tZnO = 1200 nm is estimated to be 31.5%. The photosensitivity of ZnO film with tZnO = 1200 nm under white light illumination and red-light illumination is 1.3 and 0.09 respectively. This clearly shows a much higher photocurrent generation under white light illumination than under red light illumination, as expected. The photocurrent under red illumination can be induced by the defects, especially oxygen vacancies, as evident from the PL analysis.

3.5 Photocatalytic studies

The photocatalytic activity of ZnO films was evaluated under UV–visible light illumination by using 0.5 mM RhB aqueous solution as an analyte. The dye solution was dropped uniformly on the surface of the ZnO photocatalyst and then the absorbance spectra were recorded at different illumination times. Electrons and holes are generated in the ZnO photocatalyst under light illumination which participates in the photocatalytic process. The decrease in absorbance after light illumination indicates the photocatalytic activity of ZnO thin films. The enhancement of photocatalytic activity is due to the number of photons absorbed and active centres on the surface of the catalyst [18, 37,38,39]. Generally, the photocatalytic efficiency is estimated using the following equation:

$$\hbox{Photocatalytic}\,\hbox{efficiency} =(\hbox{A}_0-\hbox{A}_{\rm t})/\hbox{A}_0 \ast 100$$
(5)

where ‘A0’ and ‘At’ are the peak values of absorbance before and after illumination. The decrease in the absorbance peak concerning light illumination illustrates the photodecolorization process of RhB molecule due to the destruction of its chromophore aromatic rings while the blue shift in the absorbance peak of RhB dye indicates the photodegradation of the dye molecules due to the de-ethylation process and hence the formation of RhB intermediaries [18, 39]. The variation in the photocatalytic efficiency of ZnO films as a function of tZnO is plotted in Fig. 7a and is found to be maximum at tZnO = 1200 nm.

The photocatalytic efficiency is found to be increased with an increase in tZnO(240–1200 nm). Similarly, the blue shift in the absorbance peak of RhB is depicted as a function of tZnO in Fig. 7a and this shift is also found to be maximum at tZnO = 1200 nm suggesting a higher photodegradation of RhB dye molecules. It is observed that the photodecolourization process dominates at tZnO  ≤ 400 nm while the photodegradation phenomenon dominates at tZnO > 400 nm. The higher photocatalytic activity at tZnO = 1200 nm can be attributed to the synergistic effect due to the higher aspect ratio as observed from SEM analysis, lower bandgap energy, the higher concentration of oxygen vacancies as evident from the PL analysis as well as due to the enhanced photo-generation of charge carriers as evident from the I-V characteristics. In this work, the particle size (both crystallite size from XRD analysis and grain size from SEM analysis) is found to increase with tZnO. According to Tan et al., the formation of a thicker space charge layer (SCL) in larger particles is responsible for an increased band bending which aids the charge carrier separation [24]. A comparatively smaller space layer and lower band bending are observed in small-sized particles, but they exhibit better inter-particle contact leading towards good charge transport. Since the particle size and density increases with tZnO, the photogeneration and separation of charge carriers will also increase as evident from photocurrent studies given in Fig. 5. We have also observed that the band gap energy decreases with tZnO and found to be lower at tZnO = 1200 nm. A smaller band gap favours the photogeneration of charge carriers and hence improves the photocatalytic activity [15].

Fig. 7
figure 7

a Photocatalytic efficiency and shift in absorbance peak of RhB dye as a function of tZnO and b Reusability test of ZnO photocatalyst

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A comparison between Figs. 6 and  7a clearly shows a direct relationship between the photosensitivity and photodegradation of RhB molecules. Since the optimum efficiency is obtained at tZnO = 1200 nm, the time-dependent photocatalytic activity of ZnO film was also studied and is depicted in Table 4. Furthermore, the reusability test of the ZnO thin film photocatalyst is shown in Fig. 7b. It exhibited good reusability and stability up to four cycles. A high stability, reusability and efficiency are essential for a material to be used as a photocatalyst [31]. Thus, this work highlights the photocatalytic and photocurrent properties of ZnO thin films for various device applications.

Table 4 Time dependence study of ZnO photocatalyst
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4 Conclusions

A cost-effective ZnO thin film was fabricated via ultrasonic spray pyrolysis technique. XRD patterns exhibited the formation of polycrystalline wurtzite phase of ZnO. SEM images showed the formation of nanorod structure with uniform distribution of grains. The photocurrent and photocatalytic properties of ZnO thin films were studied as a function of its film thickness (tZnO). The ZnO film with tZnO = 1200 nm showed a higher aspect ratio and lower bandgap energy favouring the photocatalytic and photocurrent properties. Higher generation and separation of photogenerated charge carriers at tZnO = 1200 nm were confirmed by the improved photosensitivity as well as a higher concentration of oxygen vacancies from I-V characteristics and photoluminescence analysis respectively. The incident photon-to-electron conversion efficiency (IPCE) for the ZnO film with tZnO = 1200 nm was estimated to be 31.5%. Furthermore, the photocatalytic efficiency, as well as the photodegradation mechanism, was found to be maximum at tZnO = 1200 nm. The ZnO photocatalyst exhibited 100% efficiency with good stability and reusability. Thus, this work explored the photocatalytic and photocurrent properties of ZnO thin films and observed a direct correlation between the photosensitivity parameter and the photodegradation of RhB dye molecules.