Preparation of hexagonal nanoporous Al2O3/TiO2/TiN as a novel photodetector with high efficiency

The unique optical properties of metal nitrides enhance many photoelectrical applications. In this work, a novel photodetector based on TiO2/TiN nanotubes was deposited on a porous aluminum oxide template (PAOT) for light power intensity and wavelength detection. The PAOT was fabricated by the Ni-imprinting technique through a two-step anodization method. The TiO2/TiN layers were deposited by using atomic layer deposition and magnetron sputtering, respectively. The PAOT and PAOT/TiO2/TiN were characterized by several techniques such as X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive X-ray (EDX). The PAOT has high-ordered hexagonal nanopores with dimensions ~ 320 nm pore diameter and ~ 61 nm interpore distance. The bandgap of PAOT/TiO2 decreased from 3.1 to 2.2 eV with enhancing absorption of visible light after deposition of TiN on the PAOT/TiO2. The PAOT/TiO2/TiN as photodetector has a responsivity (R) and detectivity (D) of 450 mAW-1 and 8.0 × 1012 Jones, respectively. Moreover, the external quantum efficiency (EQE) was 9.64% at 62.5 mW.cm−2 and 400 nm. Hence, the fabricated photodetector (PD) has a very high photoelectrical response due to hot electrons from the TiN layer, which makes it very hopeful as a broadband photodetector.

used the TiN layer for enhancing the properties of TiON photoactive material for solar cell using the good TiN properties; hardness, nontoxicity, high thermal conductivity, high melting point, good photochemical stability, and high UV-Vis light absorbance 19 .
Besides, TiN is characterized by a catalytic effect that enables it to be used in self-cleaning. Awad et al. used triple layers from TiO 2 /TiN/TiO 2 for self-cleaning 20 . The optical properties of the prepared layers are enhanced with increasing TiN thickness which facilitates the degradation of organic dye. Moreover, the TiN was used in devices such as fuel cells, supercapacitors, and solar cells [21][22][23] .
On the other side, TiO 2 nanomaterial with a high surface area has a special interest in photocatalytic application 24 . Nanotubes accept a more active site from the internal or external tube with a large surface area per volume. Also, TiO 2 has additional good properties such as biocomPAOTibility, low-cost, easy preparation, and high stability 23 . These properties qualify TiO 2 materials for applying in sensors, supercapacitor, photodegradation, solar cell, and light absorbance 25 . Kunwar et al. studied the GaN/TiO 2 /Au layers to increase the light detection region to reach the visible region 1 . Another study based on TiO 2 -graphene for enhancement of the UV photodetection was carried out, the incorporation of graphene to increase the light absorbance region 24 .
On the other hand, PAOT has a highly ordered two-dimensional hexagonal porous structure with high surface area which improves the interaction with light. Also, PAOT has high stability (chemically, thermally, and mechanically), good optical properties, biocomPAOTible, abundant, and inexpensive. PAOT is an attractive material as template for fabricate of nanotubes, nanowires, and nonodotes arrays 26 . This because its unique nanometric properties which are highly required in potential broad applications including biosensors, catalysts, magnetic storages, solar cells, optoelectronics, photonic crystals, and drug delivery systems 27 . PAOT structure is a typical self-ordered nanoporous material composed of hexagonally arranged cells with cylindrical pores in the centers that are aligned perpendicular to Al surface 28,29 . These hexagonal PAOT shapes can be easily transferred to other materials by depositing the material into the pores of the PAOT using some techniques such as chemical vapor deposition, atomic layer deposition, or magnetron sputtering deposition. The used technique can fabricate arrays of nanophotonic structures. The dimensions of the nanophotonic structures can be controlled by changing the geometrical structure of the PAOT.
In this study, we have prepared a novel PAOT/TiO 2 /TiN PD with high stability and efficiency. All the materials of the photodetector were prepared with high controlled technique. PAOT was prepped by the Ni-imprinting method, then TiO 2 /TiN deposition occurred using atomic layer deposition and direct current sputtering techniques, respectively. The application of the PAOT/TiO 2 /TiN as a photodetector occurred by study the effect of light power intensity and wavelengths. The photodetector parameters R, D, and EQE were determined. The prepared photodetector can be applicable in the industrial field with high stability and low cost.

Experimental part
Aluminum oxide template synthesis. The pores aluminum oxide template (PAOT) was prepared using the imprinting technique using Ni-mold through two step anodization method 23 . Ni mold has a hexagonal nanopillar order with 400 nm space shallow array. An electropolishing process for the Al foil was completed using solution C 2 H 5 OH and HClO 4 (1:1). The imprinted Al foil (99.99%) was obtained by applying 10 kN/cm 2 using an oil pressing system for 3 min. After that, the two-steps anodization process occurred at 160 V in ethylene glycol:H 3 PO 4 :H 2 O (100:200:1) electrolyte at 2 °C for 15 and 120 min, respectively. After the first anodization, the chemical etching process was carried out using H 3 PO 4 (6 wt%) and H 2 CrO 4 (1.5 wt%) mixture at 60 °C for 12 h. Finally, the pores widening process occurred through immersing the PAOT into H 3 PO 4 (6 wt%) solutions for complete synthesis of the Al 2 O 3 template with hexagonal pores.
Synthesis of TiO 2 /TiN. TiO 2 /TiN nanotube composite was prepared inside the PAOT by using atomic layer deposition and magnetron sputtering physical devices, respectively. TiO 2 nanotube composite was prepared inside the PAOT by using atomic layer deposition (ALD, Picosun SUNALE R150) at 300 °C. TiCl 4 and H 2 O were used as sources for Ti and O, respectively.
TiN nanotube was fabricated inside the PAOT/TiO 2 by using magnetron sputtering (DC sputtering, LA440S Ardenne). The sputtering was carried out in a mixture of N 2 and Ar gases with a rate of 75 and 25 sccm, respectively. High purity Ti metallic (99.9%) was used as a target at working pressure was 13 × 10 -3 mbar. The PAOT/ TiO 2 substrate was about 6 cm from the Ti-target at 250 °C.
Characterization. The characterization of prepared ample was carried out by different analytical tools, in which the material morphology was proved using SEM (SEMAuriga Zeiss FIB). Moreover, the SEM device has another unit tool for energy dispersive X-ray (EDX) analysis. The chemical structure was confirmed using an X-ray diffractometer (Bruker/Siemens D5000, XRD). In addition to that, a double beam spectrophotometer device (Perkin Elmer, Lamba 950) used for the optical analyses.
Photodetector fabrication process. The photodetector measurements of the prepared PAOT/TiO 2 /TiN sample were carried out using the Keithley device (model 2500, Tektronix Company) as seen in schematic Fig. 1. The measurements investigated though − 1 to + 1 V under Xe lamp (Newport) as a light source. Two electrodes were connected over the sample using a silver paste, the final size was 1 cm 2 . The effects of light power intensity and light wavelengths on the photodetector were studied. Also, the effect of sample stability under light was investigated. All measurements for the fabricated device carried out at room temperature and normal atmosphere.

Results and discussion
SEM and XRD analyses. The SEM of PAOT after pore widening using H 3 PO 4 solution is shown in Fig. 2a   The peaks observed for PAOT in Fig. 3a agree with previous works 31,32 . These peaks indicate that the PAOT is polycrystalline structure. The high intensity of (119) is indicating good crystallization for growth crystal in this orientation. Also, the porous alumina was well aligned perpendicular relatively to the surface of Al layer. Therefore, there was porous Al 2 O 3 layer formed on the Al substrate surface during anodizing process.
After deposition of thin TiN over TiO 2 , no phase is created. The thin thickness of the TiN layer (8 nm) can be produced out of phase diffraction for X-ray interface between TiO2/TiN layers. This due to the N 2 atoms may occupy the locations of O 2 atoms in anatase TiO 2 crystal or they are located at the grain boundaries and form amorphous portions. Also, the scattering effect of X-ray radiations can be produced out of phase diffraction at the interface between TiO 2 /TiN layers 33,34 , hence don't verified the Bragg condition. Moreover, the higher reactivity of oxygen towards titanium lead to prefer the formation of Ti-O bonds compared than Ti-N bonds and hence helps the formation of TiO 2 phase which is thermodynamically stable phase 35 .  Fig. 4a. In UV and visible regions, the average PAOT/TiO2/TiN reflectivity compared to PAOT is relatively low. This means that absorbance increases in the visible region after the deposition of TiO 2 / TiN on the template. All the spectra exhibit PAOTterns oscillations of interference fringes 30 . There are very small interference fringes oscillations with the PAOT reflectance due to interference between the reflected light from the bottom (PAOT/Al) and top (PAOT/air) interfaces. For TiO 2 /TiN-coated PAOT, the oscillation strength of the interference is stronger than PAOT. This is ascribed to the strong light modulation reflected from the top interfaces of the TiO 2 /TiN layer 36 . Therefore, the amplitudes of the reflected beams are improved.
The optical absorbance of TiO 2 and TiO 2 /TiN is shown in Fig. 4b. From the figure, the TiO 2 nanotube has a strong absorbance in the UV that is related to Π-Π* of the titanium ions 37,38 .
Then, the absorption decreases sharply with increase wavelengths and becomes nearly constant above 600 nm. This suggested that the TiO 2 showed a low spectral response to the visible light. For TiO 2 /TiN film, the right absorption edge displays redshift towards a higher wavelength at the visible region compared with that of TiO 2 . Also, the absorbance is enhanced by coating these TiO 2 nanotubes with a very thin film of TiN. Also, the absorbance is enhanced by coating these nanotubes with a very thin film of TiN.
In general, the bandgaps of TiO 2 and TiO 2 /TiN are calculated based on the Tauc equation for direct optical band gaps of semiconductor, Eqs. 1, 2 39,40 . www.nature.com/scientificreports/ where Eg is the energy bandgap, h is the Planck constant, ν is the light frequency, K is the constant, A is the optical absorbance, d is the material thickness, and α is absorption coefficient. From Fig. 4b, the energy gap value of TiO 2 is decreased from 3.1 to 2.3 eV after sputtering TiN which is agreeing with the redshift of the absorption edge. This decrease is due in Eg due to increasing free carriers 20 . The TiN covalent bond enables one electron to leave the Ti atom. The barrier at the TiN/TiO 2 interface permits free electrons to transfer from TiN to TiO2. The boundless electron needs little energy for its freedom. So, the prepared PAOT/TiO 2 /TiN materials can be applied as a photodetector, in which they have a good absorbance in the UV and initial part of Vis regions. Fig. 5. The value of this current is very small that changed from − 1.8 to 1.8 µA cm −2 under an applied potential from − 1 to + 1 V. This indicates the PAOT/TiO 2 /TiN has very small charge electrons that respond to the applied potential.

Testing PAOT/TiO 2 /TiN as a photodetector. Effect of light power intensity. The dark current (J d ) is measured and presented in
Under the dark condition, On the TiO2/TiN surface, oxygen molecules are adsorbed and free electrons from the conduction band are captured as follows, Eq. (3) 41,42 ; This results in enhanced resistivity and consequently lowers the current density. Under light illumination, electron-hole pairs are generating as a result of the electrons that will transfer from the valence band to the conduction band. The holes drift to the surface and desorb the oxygen ions according to Eqs. 4 and 5.
The remaining electrons increase the conductivity of TiO 2 /TiN and therefore the photocurrent increases. When the light source is switched off (dark), the adsorbed oxygen molecules on the TiO 2 /TiN surface are desorption induces the release of the electrons. Thus, the sensor reverts to its initial mode.
The behavior of oxygen atoms and molecules on the TiN surface was previously revealed in many works. Rodriguez et al. experimental studied the oxidation of TiN using synchrotron-based photoemission 43 . There was chemisorption of oxygen without significant surface oxidation at the lowest temperature. The adsorption due to van der Waals forces between the TiN adsorbing material and the adsorbed O 2 molecules. Seifitokaldani et al. investigated the interaction between oxygen molecule and TiN surface using density functional theory (DFT) 44 . The calculated of oxygen adsorption energy proved oxygen adsorption on the TiN. Sinnott et al. examined computationally the TiN surface by using third-generation charge-optimized many-body (COMB3) potential www.nature.com/scientificreports/ formalism and compared with available experimental data 45 . The simulation results predict that the oxygen molecule binds initially to a Ti atom in the TiN surface. Subsequently, it moves to a bridge position over two Ti atoms and then dissociates. The dissociation of oxygen molecules is adsorbed on the TiN surface. The bridge Ti atoms is the preferred adsorption site for the oxygen molecule 46 .
The effect of light power intensity on the PAOT/TiO 2 /TiN photodetector from 37.5 to 100 mW/cm 2 at room temperature is shown in Fig. 5. The TiN/TiO 2 exhibit a linear I-V curve when at low voltage, which agrees with the previous work 47 .
The values of photocurrent density change greatly. Jph with the applied light power intensity. The Jph values are increased from 0.1 to 10.73 mA.cm −2 with increasing the light power from 37.5 to 100 mW/cm 2 . The plasmonic resonance occurring in TiN nanostructures and generating this photocurrent. The relation between the light power intensity and the produced photocurrent density at 1.0 V is mentioned in Fig. 5b. The nonlinear relation between light intensity and photocurrent density indicates the complex electron-hole transportation reaction 48 . This confirms the generation of more carriers on the material surface with increasing the light power intensity as a result of increasing excitation of electrons from VB to CB 49 .
The relation between photocurrent and light intensity can be described by a simple power-law as Eq. (6).
where B is a wavelength constant, P is the incident light power. y is the exponent parameter that determines the sensitivity of photodetector (photocurrent) to the incident light intensity. This parameter refers to the complex process of electron-hole generation, recombination, and trapping of the carriers in photodetectors 50 . It can determine the response rate. By fitting the experimental results using the previous equation, red line in Fig. 5b, y is estimated to be nearly integer exponent (0.93), which suggesting highly photosensing ability 51 . These results indicate the prepared PAOT/TiO 2 /TiN can work as a good photodetector for the light power intensity. Peng gives the linear range of dynamic (LDR, usually quoted in dB), Eq. (7) 52 : At 100 mW/cm 2 , the LDR value is estimated to be 68.8 dB. The relatively large LDR indicates that the device is linearly responsive.
Light wavelength. Photocurrent-voltage characteristics (Jph-V) have been recorded with a series of monochromatic wavelengths to obtain the spectral response of the device PAOT/TiO 2 /TiN. Figure 6 shows J ph -V characteristics under monochromatic wavelength illumination ranging from UV to NIR (395-636 nm). From this figure, the Photocurrent density values decrease as the wavelengths increase from 395 to 508 nm, then increase again at 588 nm, then continue in decreasing till reach 636 nm. Figure 6b gives the Jph values at + 1 V under illumination with different monochromator light. The increase of Jph values produced by the increase in wavelengths is related to Jph unsaturated phenomena, in which increases the wavelengths causes an increase in the released photo-generated current 53,54 . This matches the optical analyses of the prepared materials, Fig. 4.
The photodetector performance is determined from the calculation of some parameters such as the photoresponsivity (R), specific detectivity (D), and external quantum efficiency (EQE) 55 . The R-value represents the relationship between the photocurrent density and the intensity of the light 56 . It can be estimated from the I-V data at + 1 V according to the following equation, Eq. (8) 1 .  where A is the effective photodetector surface area and e is the electron charge.
The responsivity of the photodetector versus the applied wavelengths at 62.5 mW is shown in Fig. 7a. The maximum photoresponse is R = 450 mAW −1 at about 400 nm. This agrees well with the J ph values as shown in Fig. 6.
The device has significant performance in the region of visible light due to the TiN plasmonic contribution in this region.
The device's specific detectivity reaches its peak value, D = 8.0 × 10 12 Jones, at about 400 nm. The external quantum efficiency (EQE) is the relation between the incident light photon flux and produced electrons 57 . The photon flux is directly proportional to the light intensity. The EQE value is determined from the R-value depending on the light wavelength (λ) according to Eq. (10) 58 .
The EQE for PAOT/TiO 2 /TiN is changed from 0.42 to 9.64% with changing the light intensity from 25 to 62.5 mW cm -2 , respectively, and then it decreases to 8.07% as light intensity increases to 100 mW cm -2 as mentioned in Fig. 7b.
Based on the above results, the fabricated photodetector exhibits better performance in terms of photoresponsivity and quantum efficiency. Therefore, the prepared PAOT/TiO 2 /TiN works well as a novel photoreactor for detecting the light power intensity and wavelengths with high efficiency.
Stability and reproducibility. The stability and reproducibility of the prepared PAOT/TiO 2 /TiN photodetector are studied as shown in Fig. 8. The study of the photoelectrode stability was carried out by applying of a 1.0 V potential on the photodetector and measuring the produced J ph . From Fig. 8a, the electrode has high stability till 2000s. The value of Jph is nearly constant for a long period indicating that the fabricated PDs have an acceptable stability. A very small photocurrent change over time due to adsorption of O 2 molecules on PD surface. This high stability comes from the construction of the prepared photodetector that is based on inorganic stable materials 59 .
The reproducibility measurements were carried out by testing the photoelectrode many times under light intensity 37.5 and 100 mW/cm 2 . From Fig. 8b, all the runs have almost the same values. This refers to a good reproducibility of the prepared detector and simultaneously confirms the stability of the detector 60 .
Finally, the values of R, D, and EQE of the proposed PD are higher than those previously reported as summarized in Table 1.

Mechanism
The working principle of PAOT/TiO 2 /TiN photodetector is mentioned through interaction between the photon incident, NPs and energy band theory as seen in the schematic in Fig. 9. The energy bandgap of TiO 2 is about 3.1 eV as mentioned before in some literature with the energy diagram 50,74 . Under the light, illumination, the electron-hole pairs in TiO 2 are generated, which contributes to the generation of photo current in the PD. Meanwhile, the electromagnetic fields in TiN can also be increased due to the electron collective oscillation as For the TiN/TiO 2 junction, the carriers' diffusion (electrons and holes) continues until the Fermi energy (E F ) becomes the same in both materials. This leads to band bending at the TiN/TiO 2 interface and formed a depletion region (or built-in electric field).
The built-in electric field can efficiently separate electron-hole pairs that generated. When the TiN layer is exposed to the light, hot electrons and holes are generated at the Fermi level of TiN as a result of surface plasmon resonance. The work function of TiN (4.3 eV) is lower than that of TiO 2 (4.9-5.5 eV). The energy difference between TiO 2 valence band and TiN Fermi level is too high, and this prevents injection of holes from TiN to TiO 2 .
On the other hand, the barrier height between TiO 2 conduction band and TiN Fermi level is small, so the hot electrons can be transferred from TiN to the conduction band of the TiO 2 after passing the barrier 75 . This way prevents the carrier's recombination which helps in the continuous electron flow into the device and leads to photocurrent generation. Also, the good compact synthesis of these two layers forms another factor for electrons flow.

Conclusion
We have prepared a novel photodetector PAOT/TiO 2 /TiN with high stability and low cost and high efficiency for industrial application. The photodetector was tested under different light intensity (37.5 to 100 mW/cm 2 ) and wavelengths (395-636 nm). The photodetector has R, D, and EQE of 450 mAW -1 , 8.0 × 10 12 Jones, and 9.64%, respectively. All the materials of the photodetector were prepared with highly controlled techniques. Different characterization analyses were used to confirm the morphology, chemical structure, and optical properties. We