Introduction

Solar energy is the most abundant and clean energy source. It can be used to generate clean energy in the form of electricity as well as solar fuels (hydrogen and oxygen) [1,2,3]. Plants produce their food using solar energy through photosynthesis [3, 4]. Artificial photosynthesis is promising to produce solar fuels. Currently, solar photovoltaic [4] and solar thermal [5] are main energy conversion technologies in market [6]. Light-matter interactions are fundamental reactions for theoretically [7] and practical applications, for example energy conversion devices [8, 9].

Dye-sensitized photoelectrochemical cells (PECs) offer a promising method to generate solar fuel and solar power. This technology has the potential for zero greenhouse gas emissions [10,11,12,13]. A PEC device is made up of a photoanode, photocathode and an electrolyte, connected in an external circuit [14, 15]. Light-absorbing semiconductor electrode coated with green pigment is a key component in PEC device [16, 17]. During the operation of PEC device, photons are absorbed by the dye molecule and create exciton that is quickly split at the surface of semiconducting film with injecting electrons into film and leaving holes at the opposite side with electrons injected into the thin film [18, 19]. The understandings of the semiconductor–dye and dye-electrolyte interfaces are important to optimize the PEC devices [20,21,22]. The performance of PEC devices can be improved through enhanced absorption of sunlight, novel materials and protective surface coatings [23, 24].

Nanotechnology research and development has created the engineered nanomaterials those have achieved better performance and multifunctional nanohybrids [25, 26]. The surface of nanocrystalline films adsorbs more dye molecule because of higher surface to volume ratio [27]. Metal oxides, such as ZnO and TiO2, are used widely as photoelectrodes in PECs due to their band edge alignments relative to the redox potentials [28, 29]. ZnO has direct wide bandgap of about 3.37 eV [30]. ZnO absorbs mostly ultraviolet light of solar spectrum. The high electron mobility of n-type ZnO layer has increased its applications as electron collector in solar cells [31,32,33]. There is a need to understand semiconductor-dye interface to improve the device performance.

Natural dyes are cheap and environment friendly. They can be extracted from plant leaves, flowers, seeds, and fruits, etc. [34, 35]. The dye absorbs a larger fraction of the visible light from solar spectrum. The lowest unoccupied molecular orbit (LUMO) and highest occupied molecular orbit (HOMO) of the dye plays a important role in photo excitation process of PEC. The pigments of natural dyes such as chlorophyll, quinine, and carotene, etc. have been used with semiconductors for photo conversion applications [36, 37].

Here, we have fabricated and utilized basil sensitized biohybrid photoelectrodes for solar photovoltaic and photoelectrochemical energy conversion applications.

Experimental details

Preparation of biohybrid photoelectrodes

The basil (oscimum) leaves were washed, crushed into fine powder, and immersed in distilled water to extract basil dye at room temperature for 24 h. The extraction was prepared from the solution using filter paper [38, 39].

ZnO thin films were deposited on FTO ubstrate (size of 1 × 1 cm2, resistivity 7 Ωcm, Sigma Aldrich) using magnetron sputtering method with thickness about 500 nm. The commercially purchased ZnO bulk target was used for deposition. A basil natural dye layer was prepared on ZnO using drop casting technique [40]. These biohybrid electrodes were used for preparing PV and PEC devices.

Preparation of photoelectrochemical (PEC) device

PEC device configuration was prepared in a three electrode system. The hybrid electrodes (ZnO/FTO and Basil/ZnO/FTO), platinum (Pt) wire and saturated calomel electrode (SCE) were used as working, counter and reference electrode, respectively. The aqueous solution of 0.1 M Na2SO4 was used as an electrolyte [41].

Characterization methods

The optical properties of photoelectrodes were studied using UV–Visible spectrometer (Shimadzu, model 6210) in the wavelength range from 200–1100 nm. An X-ray diffractometer (Rigaku, D8) was used to record X-ray diffraction (XRD) pattern of ZnO thin film with CuKα radiation (λ = 0.17890 nm). Photoelectrochemical (PEC) properties were studied using Scanning electrochemical microscopy techniques (CH Instrument, CHI920D) in three electrode testing configuration under white LED light (56 mW/cm2) and dark as shown in Fig. 1. The linear sweep voltammetry and photoamperometry techniques were used for measuring current–voltage (I-V) and time dependent current flow behavior, respectively [42,43,44].

Fig. 1
figure 1

Fabrication process for basil sensitized PEC device

Full size image

Result and discussion

Structural properties of ZnO thin film

XRD pattern of ZnO thin film was recorded with θ-2θ configuration, which is shown in Fig. 2. This pattern was used to identify the crystal phase and plane orientation in the sample. The main peak at 2θ = 33.58° corresponds to ZnO (002) according to JCPDS (No. 036–1451). This pattern confirms the growth of ZnO thin film with high crystalline structure. The crystallite size was determined about 7 nm using Debye-Scherer’s Eq. (1) [45, 46].

$$D = {\text{K}}\lambda /(\beta \cos \theta ),$$
(1)

where D, K, λ, β, θ are crystal size, dimensionless shape factor (0.94), wavelength of X-ray, FWHM (full width at half maximum intensity of the peak, and the Braggs angle, respectively.

Fig. 2
figure 2

XRD pattern of ZnO/FTO film

Full size image

Optical properties of photoelectrodes

The absorption spectra of Basil/ZnO/FTO electrode, ZnO/FTO photoelectrodes and basil dye are shown in Fig. 3a, b and c, respectively. The absorption peak for ZnO and basil dye are observed at 349 nm 675 nm those are in ultraviolet and visible range, respectively. The observed peak in basil dye at 675 nm and 286 nm corresponds to chlorophyll and quinine, respectively [47, 48]. The band gap energy were determined at 3.98 eV for ZnO and 5 eV for basil dye using Eq. (2), which is expressed as Tauc plot in inset of Fig. 3.

$$\left( {\alpha hv} \right)^{1/m} = \, k\left( {hv - E_{g} } \right)$$
(2)

where, m = 1/2, 3/2 for direct and or indirect semiconductor, respectively. α, h, v, Eg are absorption co-efficient, plank constant, frequency of light, band gap.

Fig. 3
figure 3

Absorption curve of a basil/ZnO/FTO, b ZnO/FTO and c basil dye. Tauc plots are shown in inset curve

Full size image

Figure 4a and b shows the Urbach plot for ZnO film and basil dye, respectively. Urbach relation is expressed in Eq. (3). The Urbach energy (Eu) is determined from Urbach plot and calculated about 0.0854 eV for ZnO film and 0.6751 for basil dye [49].

Fig. 4
figure 4

Urbach plot for a ZnO film, and b basil natural dye

Full size image

The optical parameters are listed in Table 1.

$$\alpha \left( \upsilon \right) \, = \, \alpha_{0} \exp \left( {h\upsilon /E_{u} } \right),$$
(3)

where α0, Eu, ʋ, and ℎ are for a constant, Urbach energy, frequency, and Planck’s constant.

Table 1 Optical parameters of photoelectrodes
Full size table

Solar energy conversion properties

Photo electrochemical photocurrents generation

The energy diagram of prepared device is shown in Fig. 5 that gives the explanation of charge transfer mechanism and energy conversion in PEC [50]. The basil-ZnO interface plays important role in operation and characteristics of PEC. The surface to volume ratio is large in nanostructured ZnO particles and hence dye molecules adsorbed in large numbers on the surface [51]. The dye molecules excite upon the illumination of light on basil layer. This photo excitation happens because of electron charge transfer between HOMO to LUMO in basil. The photo excited electron enters into the area of ZnO film and diffuses at backside contact of device. Hence, the photocurrents are generated as a result and can be collected through complete a circuit. The separation energy of HOMO–LUMO and interfaces controls PEC response in device. More interactions between dye molecule and ZnO particle can improve the charge transfer characteristics and performance of the device [52].

Fig. 5
figure 5

Energy band diagram of hybrid device

Full size image

The photoelectrochemical current–voltage characteristics of basil/ZnO/FTO and ZnO/FTO are shown in Fig. 6. The transient photocurrent behaviors are shown in 6c, and 6d. The basil sensitized ZnO photoelectrode has shown enhanced PEC properties. The open circuit potential (Voc) was increased from -0.46 V to 0.6 V whereas short circuit current (Isc) was increased from -0.05 mA to 0.14 mA due to usage of basil layer, respectively. The basil sensitized photoelectrode has shown fast switching properties and higher photoresponsivity compared to bare ZnO photoelectrode. The transient photocurrent characteristics of basil sensitized device show the slow degrading nature of natural basil dye due to the reason that photo voltage is decreasing with time. The photoelectrochemical parameters are listed in Table 2.

Fig. 6
figure 6

Linear sweep voltametry of a ZnO/FTO, b basil/ZnO/FTO. Photoamperometric of (c) ZnO/FTO, (d) basil/ZnO/FTO under dark (black) and light (red)

Full size image
Table 2 Photoelectrochemical parameters of PEC devices
Full size table

Tafel curve is shown in Fig. 7 that consists of three different zones as the polarization, Tafel and diffusion zone at low, middle (with a sharp decrease), and high potential (horizontal region) [53, 54]. The current density decides the electrocatalytic ability of photoelectrode, which can be determined by extrapolating the intercept of cathodic and anodic regions in Tafel zone. A sharp slope can be seen in Tafel zone for ZnO/FTO and basil/ZnO/FTO device. A larger exchange current density (J0) in basil/ZnO/FTO than ZnO/FTO device shows the lower charge transfer resistance (Rct) and higher electrocatalytic activity at the interface of photoelectrode and electrolyte. J0 is inversely related with Rct according to Eq. (4). Basil sensitized photo electrode has shown positively shifted equilibrium corrosion potential (Ecorr) compared to that of bare ZnO device, which is indicating an improvement in the corrosion protection.

$$J_{0} = RTnFR_{{{\text{ct}}}}$$
(4)

where R is gas constant, T is temperature, n is number of electrons, F is faraday constant and Rct is charge transfer resistance.

Fig. 7
figure 7

Tafel plot of bare and hybrid device

Full size image

Mott-Schottky plots for bare and hybrid electrodes are shown in Fig. 8. A representation of the reciprocal square capacitance is a linear function of the potential, which is known as Mott-Schottky plot. These plots give information on doping density and flatband potential (Vfb). The intercept to the x-axis determines the value of Vfb. The positive slop confirms the n-type behaviour of semiconductor. The Mott-Schottky plot is sensitive to the surface of the electrode and electrode–electrolyte interface. Vfb shifts about 0.99 V (vs. SCE) toward cathodic potential in basil sensitized ZnO that attributes for higher Voc value. The decline in potential show the higher stability in basil/ZnO/FTO than ZnO/FTO [55,56,57].

Fig. 8
figure 8

Mott-Schottky plots of bare and hybrid devices

Full size image

Photoelectrochemical fuel generation

Photoelectrochemical water splitting process gives hydrogen and oxygen using light energy [58,59,60]. We have demonstrated the PEC device in which a ZnO electrode is modified with basil layer and connected with a platinum counter electrode through a circuit. The route of the photocurrent tells the reduction (hydrogen evolution) at counter electrode and the oxidation (oxygen evolution) at working electrode. Many gas bubbles could be observed at the basil/ZnO/FTO photoelectrode surface that continuously evolved from the surface as shown in Fig. 9 [61,62,63]. In PEC water splitting, the photo excited electron leaves a hole in semiconductor and leaves positive charge carriers (H+ ions or protons) in the solution. This unites with other proton and two electrons to make hydrogen (H2) gas. The generation of solar fuel using visible light is expressed in below reactions.

$${\text{ZnO }} + { 2}hv^{ - } + \, 2{\text{e}}^{ - } + {\text{ 2p}}^{ + } \left( {\text{excitation of ZnO by light}} \right)$$
$${\text{2p}}^{ + } + {\text{ H}}_{{2}} {\text{O}}^{ - } + \, \raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} {\text{O}}_{{2}} + {\text{ 2H}}^{ + } \left( {{\text{at}}\,{\text{ the}}\,{\text{ ZnO}}\,{\text{ electrode}}} \right)$$
$${\text{2e}}^{ - } + {\text{ 2H}}^{ + } + {\text{ H}}_{{2}} \left( {\text{at the platinum electrode}} \right)$$
$${\text{The}}\,{\text{overall}}\,{\text{ reaction}}\,{\text{ is}}\,{\text{H}}_{{2}} {\text{O }} + {\text{ 2 hv }} = \, + \raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} {\text{ O}}_{{2}} + {\text{ H}}_{{2}}$$
Fig. 9
figure 9

Photoelectrochemical setup (left) and PEC device (right) for solar fuel generation

Full size image

Conclusion

The photoelectrochemical cells were demonstrated for solar energy conversion applications. PECs were prepared using ZnO, and basil dye. Natural dyes play a significance role to improve the light absorption in device. The ultra-small sized particles and thin layers of hybrid nanostructures at interface in PEC device can enhance the values of photovoltage and photocurrent with a significant percentage. We achieved Voc of 0.6 V and Isc of 0.14 mA. The basil coated photoelectrode has shown better photo switching and photo responsive properties. The shifting in flat band potential is also an important reason behind higher Voc in hybrid PEC. The hybrid PEC also has been studied for production solar fuel as hydrogen and oxygen through water splitting. The hybrid PEC can solve the challenging problems in renewable energy and solar fuel in future.