High performance sol–gel synthesized Ce0.9Sr0.1(Zr0.53Ti0.47)O4 sensing membrane for a solid-state pH sensor

In this paper, we developed a high-performance solid-state pH sensor using a Ce0.9Sr0.1(Zr0.53Ti0.47)O4 (CSZT) membrane through a very simple sol–gel spin-coating process. The structural properties of the CSZT membrane are correlated with its sensing characteristics. The CSZT based EIS sensor exhibited a high pH sensitivity of 92.48 mV pH−1, which is beyond the Nernst limit (59.4 mV pH−1), and good reliability in terms of a low hysteresis voltage of 1 mV and a small drift rate of 0.15 mV h−1. This behaviour is attributed to the incorporation of Sr in the CSZT sensing membrane, which promotes change in the Ce oxidation state from Ce4+ to Ce3+.


Introduction
Determination of pH value is one of the most important measurements in all pH dependent chemical processes, especially in agricultural development, biochemical technology, the pharmaceutical industry, and corrosion control areas. 1 Currently, the most oen used electrode is a traditional glassmembrane pH electrode due to its Nernstian response and low sensitivity to interfering species. 2 However glass electrodes have two main problems, namely the fragility of the glass membrane and easy fouling in aggressive electrolytes. Substantial progress has been recently made towards integrating high-k dielectric materials into electrolyte-insulatorsemiconductor (EIS) sensors due to their high sensing performance. [3][4][5] EIS-based solid-state sensors, e.g. ion-sensitive eldeffect transistors (ISFETs), 3 light-addressable potentiometric sensors (LAPs), 6 and organic thin-lm transistors (OTFTs), 7 have attracted intense interest in recent decades due to their simple structure, low cost, and easy fabrication process. The gate insulator plays the most important role in an EIS device because this insulating membrane is directly placed in an aqueous solution. The accumulated charges at the surface of the gate oxide lm that arise from the electrolyte solution cannot be passed through the lm, thus leading to the change in the channel conductance and current modulation. 4 Different kinds of high-k dielectric material, such as Al 2 O 3 , ZrO 2 , HfO 2 , Ta 2 O 5 , TiO 2 , and Y 2 O 3 , 5, [8][9][10][11] have been recently studied as sensing lms in ISFET or EIS devices. Nevertheless, these materials can easily be reacted with Si substrates to form a silicate layer at the interface of the oxide lm-Si substrate, thus degrading their electrical performance. 12 To improve the electrical properties, rare-earth (RE) oxide lm materials, including CeO 2 , Pr 2 O 3 , Gd 2 O 3 , and Yb 2 O 3 , have been investigated for use in complementary metal-oxide-semiconductor (CMOS), non-volatile memory, and EIS devices. [13][14][15] However, moisture absorption is a major issue when RE oxides are used as gate dielectrics in CMOS devices, which degrades their electrical performance due to the formation of hydroxides. 16 To avoid the unwanted hydroxide lm, the Zr/Ti ratio of 0.53/0.47 was previously successfully adopted in a sensing lm by our group. 17 In this letter, we report the development of a highperformance Ce 0.9 Sr 0.1 (Zr 0.53 Ti 0.47 )O 4 (CSZT) sensing membrane through a spin-coating process for a solid-state pH sensor, which is far beyond the Nernst limit of 59.4 mV pH À1 at 25 C.

Experimental
The CSZT mixed oxide membrane was synthesized with 1 N HNO 3 and CH 3 COOH via a simple sol-gel method. The cerium acetate hydrate, strontium nitrate, zirconium propoxide, and titanium isopropoxide were mixed according to the molar ratio of Ce : Sr : Zr : Ti ¼ 0.9 : 0.1 : 0.53 : 0.47. To adjust the concentration to 0.2 M with a total volume of 20 ml, acetic acid was used. Aer cleaning the 4-in p-type (100) Si wafer through a standard RCA process, the CSZT sensing membrane was deposited on a Si substrate using a spin-coating technique. Aer the spin-coating, the sample was placed on the hot plate at 150 C for 5 min for solvent removal and then baked at 350 C for 10 min for organic removal. To achieve good lm quality, the membrane was annealed using a conventional furnace at 800 C for 20 min in an oxygen atmosphere. A 100 nm-thick Al lm was deposited on the backside of the wafer using a thermal evaporator to form good ohmic contact. To dene the sensing area of the deposited CSZT, an automatic robotic dispenser was used through an adhesive silicone gel (S181) acting as a segregating layer. The EIS capacitive device was assembled on the Cu lines of a custom-made printed circuit board (PCB) by silver glue. To avoid the leakage from the electrolyte, an adhesive epoxy was deployed to encapsulate an EIS device and Cu. The pH sensitivity, hysteresis voltage, and dri rate of the CSZT EIS sensor were measured using an Agilent 4284A Precision LCR Meter with a Ag/AgCl reference and depicted with capacitance-voltage (C-V) curves. A reference electrode was employed to control and x the potential between the electrolyte solution and EIS sensor. Fig. 1(a) shows the X-ray diffraction (XRD) data of the CSZT membrane. The well-dened plane of (101) at 2q ¼ 29.15 is found in the XRD pattern, and is indicative of the uorite-type tetragonal structure. In addition, the (101) peak position of the CSZT membrane was shied to a lower 2q value and the d spacing became higher (3.06Å) relative to those of the Ce 0.5 Zr 0.5 O 2 reference (JCPDS card no. 00-038-1436). This behaviour is mainly due to the higher ionic radii of the Ti and Sr incorporated into the CSZT membrane. Fig. 1(b) displays the atomic force microscopy (AFM) surface morphology image of the CSZT membrane. The surface roughness was estimated to be 0.39 nm. Fig. 2 displays the XPS spectra of (a) Ce 3d, (b) Sr 3d, (c) Zr 3d, (d) Ti 2p, and (e) O 1s for the CSZT membrane. The Ce 3d, Sr 3d, Zr 3d, Ti 2p, and O 1s element peaks were tted using a combined symmetric Gaussian-Lorentzian line shape function aer a Shirley background subtraction, except for the Ce 3d peak (linear background). Fig. 2(a) 17 This is ascribed to the Sr incorporated into the CSZT membrane enhancing the change from Ce 4+ to Ce 3+ in the Ce oxidation state. Fig. 2(b) depicts that the Sr 3d 3/2 and 3d 5/2 double peaks at 134.8 eV and 133.1 eV, respectively, for the CSZT membrane are shied toward higher binding energies compared with those of the SrTiO 3 reference. 19 The higher Sr 3d double binding energies of the CSZT lm may be attributed to a mixture of Sr 2+ ions in the CeO 2 lattice. In addition, the ionic radius of Ce 4+ (0.99Å) is larger than that of Ti 4+ (0.74Å). 20 Moreover, there is a shi in the Zr 3d 3/2 and 3d 5/2 split peaks (184.1 and 181.8 eV, respectively) to binding energies that are lower relative to those of ZrO 2 lm (185.8 and 182.2 eV, respectively), 21 as shown in Fig. 2(c). The lower Zr 3d binding energy values for the CSZT lm are presumably due to the alloy effect because Ce 4+ is the predominant nearest neighboring cation. Fig. 2(d) shows that the Ti 2p 1/2 and 2p 3/2 split peaks located at 463.6 and 458.1 eV, respectively, are shied toward binding energies that are lower relative to those of the TiO 2 reference (464.3 and 458.7 eV, respectively), 22 indicating the formation of Ce-Sr-O-Zr-Ti bonds. Fig. 2(e) demonstrates that the O 1s spectra of the CSZT membrane were convoluted with three appropriate curve-tting lines. For the three spectra, the rst peak at 531.4 eV represents Ce(OH) x , the second peak at 530 eV indicates the Ce 3+ state, and  the third peak at 529.1 eV indicates the Ce 4+ state. 23,24 The intensity of the O 1s peak corresponding to the Ce 2 O 3 component is lower than that of CeO 2 , but is higher relative to our previous reports (the CZT lm). 17 Fig. 2(f) shows the HR-TEM image of the CSZT membrane. The oxide thickness of the CSZT membrane was evaluated to be $47 nm. Fig. 3(a) shows the C-V plots of the CSZT EIS sensor annealed at 800 C for standard buffer solutions. To measure the sensitivity of the EIS device the shi in reference voltage (V REF ), as shown in the C-V plots, was measured as it changes with the pH of the buffer solution due to mainly protonation or deprotonation which modies the surface potential through dipole formation on the sensing membrane. Fig. 3(b) presents the V REF of the CSZT EIS sensor as a function of pH. The CSZT EIS sensor exhibited a super-Nernstian pH sensitivity of 92.48 mV pH À1 with a linear response in the range of pH 2-12, which is far larger than the theoretical Nernstian value (59.4 mV pH À1 at 25 C). This super-Nernstian response may be attributed to the incorporation of Sr into the CSZT lm, which enhances the change in the Ce oxidation state from Ce 4+ to Ce 3+ . In this case, we could suspect that in a mild solution, the oxidized Ce 4+ ion and reduced Ce 3+ ion participate in the redox reactions below:

Results and discussion
Ce(OH) 3 Substituting eqn (1) and (2) From this reaction, only two electrons are transferred per four protons (proton/electron ratio of 2), hence a sensitivity of 118.8 mV pH À1 was achieved because of the mixing of the oxidized CeO 2 and reduced Ce 2 O 3 states in the CSZT membrane. The empirical result of the pH sensitivity being over 59.4 mV pH À1 can be explained by there being less than one electron per proton transferred in the redox reaction. In contrast, the pH sensitivity being below 59.4 mV pH À1 might be due to there being more than one electron per proton transferred in this reaction.

Conclusions
In summary, we have demonstrated a high-performance CSZT membrane deposited on a Si substrate through a simple sol-gel spin-coating process. A high pH sensitivity of 92.48 mV pH À1 , a small hysteresis voltage of 1 mV, and a low dri rate of 0.15 mV h À1 were achieved by the CSZT EIS sensor. These results are attributed to the incorporation of Sr in the CSZT, which enhances the change in the Ce oxidation state from Ce 4+ to Ce 3+ , resulting in a rise in the ratio of protons to electrons transferred in the redox reaction. This CSZT membrane EIS sensor can be used in future solid-state biosensor devices.

Conflicts of interest
There no conicts to declare.